TS3.2 | Integrated approaches to seismotectonic studies
Integrated approaches to seismotectonic studies
Co-organized by NH4
Convener: Filippo Carboni | Co-conveners: Constanza Rodriguez PicedaECSECS, Iris van Zelst, Silvia BrizziECSECS, Niccolò Menegoni, Maurizio Ercoli, Marcel Thielmann
| Tue, 16 Apr, 14:00–15:45 (CEST)
Room K1
Posters on site
| Attendance Wed, 17 Apr, 10:45–12:30 (CEST) | Display Wed, 17 Apr, 08:30–12:30
Hall X2
Orals |
Tue, 14:00
Wed, 10:45
Understanding seismic activity and associated hazard of seismically active regions requires building comprehensive, multiscale models of earthquake deformation. From individual outcrops to regional scales, seismotectonic studies aim to link active faults mapped at the surface down to the base of the seismogenic layer. Despite significant technological advancements in geophysics (seismic reflection, seismology), laboratory and field structural geology (rock mechanics, rare earth elements and cosmogenic nuclides analysis, paleoseismology), remote sensing (SAR, LiDAR, photogrammetry), software and data (GIS, databases, artificial intelligence, big data), and modeling (analogue and numerical modeling, inversion), numerous questions remain about defining fault dimensions, displacements, segmentation, slip rates, and lithologies hosting seismicity. Among different structural settings, a better understanding of intraplate settings, subduction zones and the interplay between megathrust seismicity and earthquakes within both the oceanic slab at various depths and the upper plate, is needed.

This session aims to bring together the broad community interested in seismotectonics, including subduction zone earthquakes and intraplate settings. We invite contributions that integrate structural geological studies with geophysical and geological observations, laboratory experiments, and numerical models to explore the underlying mechanisms of earthquakes at different spatio-temporal scales. Additionally, we specifically encourage contributions that investigate the spatio-temporal relationships and interplay between interplate and intraplate seismicity in subduction zones, as well as their connection with subduction dynamics.

Session assets

Orals: Tue, 16 Apr | Room K1

Chairpersons: Iris van Zelst, Silvia Brizzi, Niccolò Menegoni
On-site presentation
Zoe Mildon, Billy Andrews, Constanza Rodriguez Piceda, and Manuel Diercks

Tectonics and active faults are studied using a broad range of techniques and observations, and each of these datasets have both strengths and limitations. Ideally, a multidisciplinary approach should be used when studying active faults, to mitigate against gaps in data and knowledge, and to span the spatial scales of deformation. Furthermore different approaches can provide insights into how faults and fault networks behave over a wide range of timescales, from annual behaviour (e.g. geodesy, seismology) to millennia (e.g. paleoseismology) and millions of years (e.g. seismic reflection). By using a range of techniques to study fault behaviour over a range of timescales, we gain insights into how faults behave and interact, which ultimately can improve our understanding of the resultant seismic hazard.

For seismic hazard studies, it is important to quantify fault geometry, dimensions and connectivity as these factors influence the magnitude and propagation of earthquakes. However these are typically difficult to constrain from onshore continental faults where  sub-surface information is often limited. Another important aspect to consider for seismic hazard studies is the slip rate of faults, but an aspect that is rarely considered is how slip rates vary spatially and temporally. Using seismic reflection datasets of inactive normal faults, we can study how slip rates vary over far longer timescales than can be considered from field studies alone. While it is challenging to study onshore faults using the same approach, what our findings indicate is that slip rates can vary by more than an order of magnitude over the lifetime of a single fault. Additionally, faults are almost never a single isolated structure, and instead form fault networks, with variable spacing, orientation and lengths. Understanding how a fault network behaves and interacts over time is also important to gain insights into seismic hazard.

Ultimately to gain a comprehensive understanding of fault behaviour in time and space, a range of complementary studies, including observations and modelling, are needed to span the broad range spatial and temporal scales that need to be considered when assessing active faults.


How to cite: Mildon, Z., Andrews, B., Rodriguez Piceda, C., and Diercks, M.: Insights into fault behaviour and seismic hazard from studying active and inactive faults over a range of timescales, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17839, https://doi.org/10.5194/egusphere-egu24-17839, 2024.

Seismicity in Mountain Belts
On-site presentation
Federica Riva, Simone Marzorati, Nicola Piana Agostinetti, Elham Safarzadeh, Diana Latorre, Lauro Chiaraluce, and Massimiliano Rinaldo Barchi

One of the most seismically active areas in Central Italy is located within the Pre-Apennine Umbria region, between the Tiber Valley, the town of Gubbio and the main mountain ridge of the Umbria-Marche Apennines. The fault system that characterizes this region is dominated by a 60 km long low-angle normal fault (Alto Tiberina, ATF), active since the Late Pliocene-Early Pleistocene. This area is mainly monitored and studied though the Alto Tiberina Near Fault Observatory (TABOO-NFO), a multidisciplinary monitoring infrastructure composed of dense arrays of seismic, geodetic, strain , geochemical and electromagnetic sensors deployed both at the surface and on boreholes. This infrastructure is fundamental to investigate the principal geophysical and geochemical processes occurring in this complex geological area (https://www.ont.ingv.it/infrastrutture-di-ricerca/sismologia/taboo). Besides the high rate of micro-seismicity nucleating along the ATF (ML<3.0), there is also a considerable number of synthetic and antithetic faults (e.g., Gubbio Fault, GuF) located in the hanging-wall of the ATF that produced historical and most recent earthquakes of moderate magnitude (e.g., MW 5.1 1984 Gubbio earthquake).

Our study focuses on the most recent seismic sequences, occurred in this area between 2010 and 2023, that produced main shocks of magnitude > 3 (Mw= 3.6 - Pietralunga 2010, Mw= 3.6 - Città di Castello 2013, Mw = 3.9 - Gubbio 2021, Mw = 4.5 - Umbertide 2023).

These seismic events have been registered, located and published in the Database of the Central Eastern Italy by the INGV office in Ancona (https://doi.org/10.13127/resiico/eqs). From these data, all the considered sequences are characterised by dominant normal fault kinematics, coherent with the regional SW-NE active extension. Moreover, they occurred at relatively shallow depth (< 7 km), at the hanging-wall of the ATF, and their location cannot be directly referred to any extensional fault, mapped in the studied area. The aim of our work is to investigate the potential relationship between the cited 2010-2023 seismic sequences and the occurrence of still unknown causative minor faults, at the hanging-wall of ATF. To reach the goal, we propose a revised detailed interpretation of a set of 2D-seismic reflection profiles, calibrated by few deep boreholes, acquired in the 80s for hydrocarbon exploration purposes. Previous studies of these data have been focussed on the ATF and on its major antithetic splay, i.e. the SW-dipping Gubbio normal fault. In this study, we want to explore the presence of other, synthetic and/or antithetic splays, visible at the seismic scale and possibly connected with the 2010-2023 seismic sequences. In order to improve the comparison between the geological structure at depth and the seismicity distribution, we decided to relocate the 2010-2023 catalogue of seismicity for the study area, following two innovative strategies: a 3D velocity model created on purpose for the ATF area and a Markov chain Monte Carlo algorithm for events location.

By combining interpretation of active seismic data with innovative strategies of earthquake re-location, our study proposes as a pivotal experience for seismo-tectonic interpretation of low-magnitude seismic sequences.

How to cite: Riva, F., Marzorati, S., Piana Agostinetti, N., Safarzadeh, E., Latorre, D., Chiaraluce, L., and Barchi, M. R.: Relationships between seismicity and geological structures at the hanging-wall of a low-angle normal fault (Alto-Tiberina fault system, Northern Apennines of Italy)., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4320, https://doi.org/10.5194/egusphere-egu24-4320, 2024.

On-site presentation
Elham Safarzadeh, Massimiliano Rinaldo Barchi, Assel Akimbekova, and Francesco Mirabella

On the 9th of November 2022, a Mw 5.5 earthquake occurred near the Northern Adriatic coast, between Ancona and Pesaro (Marche region, Italy), as part of a complex seismic sequence, including six M>4 events, with NW-SE striking thrust fault focal mechanisms. This study is aimed at reconstructing the subsurface geological setting of the area struck by the seismic sequence, focusing on the active thrusts responsible for the observed seismicity. We interpreted previously unpublished 2D seismic reflection profiles integrating with deep wells, covering approximately 1500 km2.

We analysed the stratigraphic and geophysical log data from eight deep wells. This analysis defined the local stratigraphy, comprising a Late Triassic-Paleogene multilayer of evaporites, carbonates and Tertiary marls, overlain by Pliocene-Quaternary syn-tectonic clastic sediments. Wells data were used to calibrate the strong reflections recognized along the 2D seismic profiles. These profiles include six cross-lines (i.e. SW-NE), connected by three strike-lines (i.e. NW-SE).

Five key-horizons were distinguished: Top Pleistocene unconformity, Base of Pliocene -Pleistocene unconformity, Top of Middle Pliocene, Top of Messinian, and Top of Oligocene. The Messinian's prominent reflection, specifically, played a pivotal role in interpreting and identifying these horizons. These stratigraphic markers are cut and displaced at depth by two major thrust-fault segments, affecting the Mesozoic-Cenozoic carbonate succession.  Along-strike geometry of these major, SW gently dipping thrusts has been identified across four seismic reflection profiles, along a distance of about 24 km. A set of more complex, shallower thrusts, affecting the Tertiary marls and the overlying, syn-tectonic clastic sediments, splays out from these major structures. Time to depth-converted structures were derived by establishing a proper velocity model based on both local and regional log data. The location and depth of the seismic events were plotted along the depth-converted seismic profiles, demonstrating a good correlation with the geometry and kinematics of the deep thrust-fault segments. The interpretation of the deformation observed in the overlying strata suggests a strong Pliocene-Pleistocene contractional phase, up to the end of the Early Pleistocene. In recent times, the increased sedimentation rate masks the continuing tectonic activity.

This study contributes to a deeper understanding of the location, geometry, and kinematics of potentially active buried faults, sheding light on the seismotectonic setting of the study area, leading to a better understanding of the geological structure of the active external thrust in the Northern Adriatic region. The study contributes not only to a better knowledge of the seismotectonic setting of the region, but also plays an important role in formulating effective strategies for seismic hazard assessment and regional seismic risk management.

How to cite: Safarzadeh, E., Barchi, M. R., Akimbekova, A., and Mirabella, F.: Geological and Seismotectonic Analysis of the area struck by the November 2022 Mw 5.5 Offshore Pesaro Earthquake (Northern Adriatic Sea Region, Italy). , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6030, https://doi.org/10.5194/egusphere-egu24-6030, 2024.

On-site presentation
Sandro Truttmann, Tobias Diehl, Giovanni Luca Cardello, and Marco Herwegh

The Alps are a dynamic orogen, as evidenced by recent crustal uplift and seismic activity. Earthquakes are primarily occurring along the many pre-existing Neogene faults formed during the Alpine orogeny, making it challenging to predict which faults are being reactivated. Limited geophysical data, low strain rates, high erosion rates, and widespread faulting complicate the detection of active faults in low-strain regions. Currently there is a lack of knowledge about the abundance, architecture, and properties of active faults in the Alps, which is however critical for evaluating the regional seismic hazard.

This study adopts an interdisciplinary approach to identify and characterize active faults in the Rawil depression and surrounding areas north of the Rhône-Simplon fault system, located in the southwestern Swiss Alps. A comprehensive seismotectonic description of the region is achieved by combining information from recent high-precision earthquake catalogs derived from relative relocations covering about 40 years, new fault maps using remote sensing and field surveys, updated stress inversion from extended focal mechanism catalogs and paleostress inversion from fault slip data, as well as GNSS data. Results from 3D imaging of active faults at depth, based on the high-precision hypocenter catalogs, reveal that subvertical faults, striking E-W, host most of the present-day earthquakes in the region. This imaging also uncovers previously unknown NW-SE striking active faults potentially contributing to the overall strain distribution in this part of the Alps. Compared to principal stress orientations in the upper crust derived from focal mechanisms, faults striking in both E-W and NW-SE directions appear to be optimally oriented for reactivation in the current stress field. Recent crustal stresses, consistent with the results obtained from paleostress inversion indicating NE/SW-directed transtension, suggest a relatively constant stress regime over the last couple of million years. This implies similarities between exhumed and seismically active faults at depth. The agreement between fault geometries exhumed at the surface and reconstructions of active faults at depth, as determined by hypocenter-based 3D imaging of active faults, support these findings. In conclusion, our study demonstrates that such interdisciplinary studies provide valuable insights into the deformation processes in tectonically active regions, contributing to refined seismic hazard assessments.

How to cite: Truttmann, S., Diehl, T., Cardello, G. L., and Herwegh, M.: Seismotectonics of the Southwestern Swiss Alps – Revisiting Faults, Earthquakes, and Crustal Stresses, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12143, https://doi.org/10.5194/egusphere-egu24-12143, 2024.

On-site presentation
Riccardo Lanari, Martina Occhipinti, and Massimiliano Porreca

On 8th September 2023, the Al Haouz province in Morocco was struck by a strong earthquake of 6.8 Mw. The mainshock was generated by a reverse fault with an ENE-WSW orientation, as suggested by the derived focal mechanism.  

To obtain preliminary information of a seismic event, and to characterize the associated seismotectonic framework, in the last decades, the combination of field geology with satellite observations is becoming gradually more frequent. In particular, the DInSAR technique can be a powerful method of analysis to have an initial detailed information on the deformation field produced by the earthquake.  

In the present study, to understand the deformation field induced by the event and the structures involved, the DInSAR technique has been applied to obtain displacement maps in LOS, vertical, and horizontal (E-W) directions. On these maps, the geological meaning of both the vertical and horizontal displacement components is interpreted in the framework of the known tectonic structures of the Western High Atlas Belt. The inferred coseismic deformation along its vertical component shows a wide antiform characterized by an overall E-W trend and a slight southward vergence. On the other side, the horizontal (E-W) component of the deformation seems to be affected by the flexuring of the antiform flanks.  

Integrating the retrieved DInSAR maps with published geological observations and preliminary seismological data, it is possible to demonstrate how the coseismic deformation pattern may be affected not only by a possible activity of the main Tizi n’test fault but also by the far western High Atlas frontal thrust and by possible blind faults, better oriented to the vertical deformation field. 

How to cite: Lanari, R., Occhipinti, M., and Porreca, M.: DInSAR coseismic surface deformation of the 2023 Mw 6.8 Al Haouz earthquake(High Atlas Mountains, Morocco), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19684, https://doi.org/10.5194/egusphere-egu24-19684, 2024.

On-site presentation
Muhammad Tahir Javed, Sylvain Barbot, Farhan Javed, and Carla Braitenberg

Continental convergence of Indian and Eurasian plates produces Himalayas in the north, while tectonically complex transpressional zones of the Sulaiman Fold and Thrust (SFT), and Kirthar Fold and Thrust (KFT) belts in the East. Seismic hazards in the zones are very high and less understood due to complex tectonic settings, and lack of GPS network. Here, we take advantage of spaceborne SAR interferometry and use the Sentinel-1, and ALOS-2 ScanSAR satellite observations to estimate the coseismic deformation caused by the 2021 Mw 6.0 Harnai earthquake in the western zone of the SFT belt. We find the line-of-sight (LOS) displacement of ~80 and ~70 mm from Sentinel-1 descending and ascending interferograms respectively. We find the ~50 mm of LOS displacement from ALOS-2 descending interferogram, but it is majorly biased by lower and upper atmospheric noises even after the GACOS and ionosphere corrections. In order to avoid the major noise components in inversions that may affect the accuracy, we discarded the ALOS-2 LOS displacement and relied only on the ascending and descending interferograms of Sentinel-1data. The deformation has an oblique component, but mostly dominated by thrusting on the NW-SE trending Harnai fault. First, we invert the LOS displacement using geodetic Bayesian Inversions approach, and find two plausible fault plane the NW-SE trending, and the NE-SW trending solutions. The simplified fault parameters have a strike of 327° ± 12, a dip of 31° ± 9, the length of 8.3 ± 2.1 km, and the width of 2.5 ± 2.0 km, which fits well the ISC and USGS fault models. Then, we determine the finite slip distributions on both plausible faults.  The NW-SE trending fault shows the maximum slip is found to be 70 cm at around 8 km depth. The slip distribution along the down dip and strike of the fault shows that 85% of the slip is concentrated in an area of (9 × 9) = 81 km2 at a down dip distance of 3 - 12 km. Furthermore, the results show the earthquake is propagated equally along strike and dip. For the NE-SW trending fault the maximum slip is similar but has higher residuals and scattered slip along depth. Therefeore, we preferred the NW-SE trending fault plane solution because based on the compatibility with fault structures in the region, and higher accuracy in the inversions. We also determine postseismic movement using time series analysis of spaceborne Sentinel-1 SAR data, but no significant afterslip and viscoelastic relaxation signals is found on the fault after the earthquake.

How to cite: Javed, M. T., Barbot, S., Javed, F., and Braitenberg, C.: Insights the transpressioanl deformation patterns in the western zone of Sulaiman Fold-and-Thrust belt from spaceborne geodesy, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19891, https://doi.org/10.5194/egusphere-egu24-19891, 2024.

Seismicity in Subduction Zones
On-site presentation
anne socquet, Audrey Chouli, Blandine Gardonio, Jorge Jara, Sophie Giffard Roisin, David Marsan, and Michel Bouchon

Recent great subduction earthquakes have been preceded by an accelerated rate of both interface seismicity along the megathrust and intermediate depth seismicity within the slab at ~100km depth (e.g. Bouchon et al., 2016, 2018), sometimes along with seismicity lineaments along dip over some hours (Bouchon et al., 2022, 2023). These may also be associated with large-scale gravity and mass changes in the subduction zone (Panet el al. 2018). Such interactions between deep and interface seismicity can last several years and can be associated with deformation within the upper plate (Durand et al., 2014; Jara et al. 2018, Rousset et al. 2023, Mitsui et al. 2021).

However such interactions between deep seismicity and shallow deformation have been observed only on rare occasions. In addition, assessing better how they relate to fluid transfer and slab force balance is key to improved understanding of the driving mechanisms of the plate interface destabilization.

Here we present some intriguing examples of interactions between intraslab seismicity and shallow deformation, and assess their statistical significance. We show that the occurrence of the Tohoku earthquake significantly changed the deep (>150km) seismicity rate, suggesting that this major megathrust event modified the slab equilibrium down to the lower mantle.

We also revisit the interactions between intermediate-depth and shallow seismicity in the Japan trench and the northern Chile subduction zone, during the decade preceding the Tohoku-oki (Mw 9.0, 2011) and Iquique (Mw 8.2, 2014) megathrust events. Cross correlations highlight different periods with significant interactions between intermediate-depth and shallow earthquakes, including the 8 months before the Tohoku-Oki megathrust in Japan, over which multiple bursts of ~7 days are synchronized. In Chile, the 4 months preceding the Iquique megathrust also show strong interactions, with successive bursts of ~4 days. Unlike some other periods, no stress transfer implied by Mw>6 earthquakes can explain the correlations observed before both megathrust events. Clustering of the seismicity allowed to identify along-dip lineament patterns. Their occurrence rate shows a significant increase when approaching the date of the megathrusts. If only a few are observed in Chile, the numerous lineaments downdip Tohoku highlight some structures along which lineaments concentrate. These elongated seismicity features seem to connect intermediate-depth and shallow seismicity and could be explained by fluid migrations.

How to cite: socquet, A., Chouli, A., Gardonio, B., Jara, J., Giffard Roisin, S., Marsan, D., and Bouchon, M.: Interactions between deep seismicity and shallow deformation in the Japan trench and Chile subduction zones , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16945, https://doi.org/10.5194/egusphere-egu24-16945, 2024.

On-site presentation
Yingfeng Ji, Rui Qu, and Weiling Zhu

Due to the steep subduction of a highly concave slab, researchers have characterized megathrusts under the Marianas as among the coldest and curviest plate coupling interfaces in various circum-Pacific subduction zones. Seismic tomography indicates that the heterogeneous underlying plate varies markedly in its subduction angle, velocity, and flexure along the strike and dip, while their effects on the thermal structure and intraslab earthquake occurrence remain enigmatic. By incorporating the 3-D MORVEL velocity and state-of-the-art slab geometry into thermomechanical modeling, we estimated the 3-D subduction thermal state and hydrothermal regime below the Marianas. We find that (1) the concave slab geometry and the complexity of the intraslab velocity variation in the Marianas are associated with a heterogeneous along-strike thermal regime and a cold mantle wedge beneath the central Marianas; (2) amphibolitization and eclogitization of subducted oceanic crust cause variations in fluid pressure and fluid release from the subduction interface, which may influence the distribution of interface seismicity in the Mariana system; (3) the concentration of active hydrothermal vents in the trench > 8 km deep is accompanied by a large temperature gradient and subsequent remarkable slab dehydration in the southern Marianas; and (4) slab dehydration (> 0.02 wt%/km) from 30 to 80 km indicates notable fluid release and potential fluid migration in subduction channels, which may correspond to the large water flux at depth beneath the Marianas.

How to cite: Ji, Y., Qu, R., and Zhu, W.: Subduction thermal structure and megathrust earthquakes under the Mariana arc, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7352, https://doi.org/10.5194/egusphere-egu24-7352, 2024.

On-site presentation
Caroline Chalumeau, Hans Agurto-Detzel, Andreas Rietbrock, Michael Frietsch, Onno Oncken, Monica Segovia, and Audrey Galve

The simplified view of the subduction interface is that of a single plane along which seismic and aseismic deformation occurs. In reality, however, exhumed subduction zones and geophysical imaging have shown that the seismogenic plate interface is a deformed, 100m-1km thick tabular region. Within this region, we currently do not know if seismic slip is localized on a single fault or distributed over several active faults, and how this impacts seismogenesis and the timing of deformation. Here, we use high-resolution earthquake locations to shed light on these questions.

We focus on the aftershock sequence of the March 27th 2022, Mw 5.8 Esmeraldas earthquake which occurred at 19 km depth at the plate interface in Ecuador, and which was recorded by the dense temporary seismic network deployed during the HIPER2 marine campaign. We use machine learning to detect and pick over 1700 earthquakes (Mw 0-3), which we then locate using a double difference algorithm with cross-correlation times and a 3D velocity model. This allows us to obtain an exceptionally detailed image of the seismicity at the plate interface, which falls into a 200-400 m thick zone, comparable to plate interface thicknesses observed in exhumed subduction zones. Using a cross-correlation threshold of 0.75, we extract families of similar earthquakes, whose geometry we investigate using the 3-point method. These families generally occur on subparallel, sometimes superposed planes with a thickness of 0-40 m that is comparable to the thickness of individual fault zones observed within fossil subduction shear zones. These individual fault zones appear to form a network whose geometry impacts the aftershock expansion, itself controlled by afterslip rather than diffusive processes, thus demonstrating the importance of considering the 3D structure of the plate interface when modeling slip.

How to cite: Chalumeau, C., Agurto-Detzel, H., Rietbrock, A., Frietsch, M., Oncken, O., Segovia, M., and Galve, A.: Seismological evidence for a multi-fault network at the Ecuadorian subduction interface, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11125, https://doi.org/10.5194/egusphere-egu24-11125, 2024.

Seismicity in Strike-slip Zones
On-site presentation
Léa Vidil, Elia d'Acremont, Sara Lafuerza, Laurent Emmanuel, Alain Rabaute, and Sylvie Leroy

In the Alboran Sea, oblique convergence between the African and Eurasian plates led to establishing the active Al Idrissi sinistral strike-slip fault system 1 Ma ago. Several moderate magnitude earthquakes (Mw > 6) have been recorded on different segments of this fault system.

The objective of this study is to analyse the dynamics of this nascent plate boundary by studying the seismic events recorded in sedimentary series. We focused on a key transtensive fault transect, namely the Bokkoya fault system, shifting the small Al Idrissi volcano. This fault has a lateral extent of 11km along strike. Sedimentation is strongly affected by the circulation of deep Mediterranean water masses resulting in contouritic deposits, and likely mass movement during seismic events.

We used a panel of geological, geophysical, geotechnical and geochemical tools acquired during the ALBACORE oceanographic campaign (R/V Pourquoi pas? 2021). This work is part of the ANR ALBANEO project, which aims to understand the dynamics of this new plate boundary and to assess the hazards in this area of the western Mediterranean Sea. The data analysed are derived from (i) 4 sediment calypso cores (ALB_CL26, ALB_CL54, ALB_CL53 and ALB_CL52) from 10m to 16m (analysed with a multi-sensor core logger – MSCL and X-Ray Fluorescence-XRF), (ii) piezocone tests (CPTU) with the Ifremer Penfeld as well as (iii) multibeam bathymetry data and (iv) seismic reflection/sub-bottom profiles. This multi-proxy dataset provided detailed lithological and geophysical stratigraphy, calibrated with the picked seismic horizons, and sediment cores dating along a transect perpendicular to the Bokkoya fault system.

Isotopic analysis of 3 cores provided 𝛿18O evolution curves, identifying a thermal anomaly in each of them, and in particular in the one penetrating the fault plane. Oxygen isotopic curves were calibrated using 14C radiocarbon analysis, enabling sedimentary series to be dated up to 40 ka. Accordingly dated, representing the first 16 m of sediment cores: the cold stadials, with the Younger Dryas; the Heinrich Stadials 1 and the Last Glacial Maximum. The sedimentation rate is about 30 cm/kyr in the depression zone whereas on either side of the contourite drift, it is about 20-25 cm/kyr.

The recognition of seismic events in the past is attempted by comparing sedimentary successions in different fault compartments. The active Bokkoya fault appears to offset the sedimentary series with a normal component and a vertical throw of 1 m, evaluated between the seabed and the dated YD reflector.

The results from the different datasets allow us to identify (1) syn-tectonic deposits that may be associated with past co-seismic events (2) intense erosional events that may be associated with localized water masses currents (3) a thermal anomaly whose origin is to be determined. This dataset highlights the complex interaction between tectonics and sedimentation/erosion along this segment of the Bokkoya fault over at least 60 ka.

How to cite: Vidil, L., d'Acremont, E., Lafuerza, S., Emmanuel, L., Rabaute, A., and Leroy, S.: An integrated multi-proxy approach to characterize the southern part of the Al Idrissi strike-slip fault system, Alboran sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15633, https://doi.org/10.5194/egusphere-egu24-15633, 2024.


Posters on site: Wed, 17 Apr, 10:45–12:30 | Hall X2

Display time: Wed, 17 Apr 08:30–Wed, 17 Apr 12:30
Amerigo Corradetti, Stefano Tavani, Marco Mercuri, Lorenzo Bonini, and Thomas Seers

The advancement of computer vision–based photogrammetric image processing pipelines, particularly Structure from Motion–Multi-View Stereophotogrammetry (SfM-MVS), has rapidly evolved. This evolution, coupled with the accessibility of low-cost and portable acquisition tools such as DSLR and mirrorless cameras, Uncrewed Aerial Vehicles (UAVs) and smartphones, has transformed outcrop studies in structural geology, propelling traditional field geology into the digital era. Notably, this revolution has significantly impacted Virtual Outcrop Models (VOMs), elevating them from mere visualization media to fully interrogable quantitative objects. 

Among the various applications of VOMs in structural geology, the extraction of near-planar features, including fracture and bedding surfaces, stands out as crucial. Numerous procedures exist for this purpose, ranging from fully automated segmentation and best-fitting of point clouds to the manual picking of 3D traces on both point clouds and textured meshes.

In this work, we explore the advantages, disadvantages, best practices, and drawbacks associated with the principal procedures for extracting near-planar geological data from VOMs. While automated or supervised recognition and subsequent best-fitting of coplanar patches in point clouds have garnered significant attention, their application is generally limited to specific case studies. Geological outcrops commonly lack patches of sufficiently large near planar surfaces for robust best fitting, necessitating manual picking procedures based on visual and/or structural interpretation. In such cases, the use of textured meshes is preferred over point clouds, and consideration must be given to the accuracy of the textured mesh during digitization, as well as the intrinsic roughness of geological surfaces. 

The analysis of coplanarity and collinearity of picked point sets aids in identifying traces deviating from idealized configurations. However, commonly suggested threshold values often result in small datasets. Nevertheless, relying on the visual inspection of the best-fit plane and real-time computation of best-fit planes from picked point sets generally yields acceptable results, handling coplanarity and collinearity dynamically during the extraction process.

How to cite: Corradetti, A., Tavani, S., Mercuri, M., Bonini, L., and Seers, T.: Virtual outcrop models of geological structures: problems and best practices related to extraction of 3D structural data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3071, https://doi.org/10.5194/egusphere-egu24-3071, 2024.

Seismicity in Mountain Belts
Ada De Matteo, Daniel Barrera, Francesco Maesano, Giovanni Toscani, Silvio Seno, and Roberto Basili

Understanding the recent tectonic activity and seismotectonics of inaccessible buried faults requires the development of feasible and robust approaches. The foredeep deposits of the northern and central Apennines (an offshore area in the central Adriatic Sea, Italy) blanket the active buried frontal thrusts of the Apennines and the Dinarides orogens. Detecting recent-to-ongoing tectonic activity of these thrusts is particularly challenging because sedimentation rates easily exceed the very slow tectonic rates.
In this work, we combine seismic reflection profile interpretation, sediment decompaction, kinematic restoration and balancing to quantitatively analyse the Plio-Pleistocene tectonic activity of the Apennines and Dinarides buried thrusts in the central Adriatic Sea and calculate the slip rates of the major faults. The northern and central Apennines foredeep is filled by a thick Messinian to Quaternary sedimentary wedge, unconformably resting on a Meso- Cenozoic carbonatic and siliciclastic passive margin succession, which is in turn involved in the east-northeast propagation of the fold-and-thrust belt from onshore to offshore (Adriatic Sea). As suggested by previous studies, the region is in a substantial tectonic activity decrease, but local and qualitative observations on specific structures show evidence of recent tectonic activity. The frontal thrusts of both the Apennines and the Dinarides are active, as also demonstrated by the moderate seismic activity historically (few past centuries) recorded in the region and by the recent earthquakes, followed by rather rich aftershock sequences that occurred in this region and nearby (e.g. the Porto San Giorgio earthquake Ml 5.0 in 1987; the Jabuka earthquake Mw 5.5 in 2003, the Pesaro earthquake Ml 5.7 in 2022). We interpreted, depth converted, and restored two northeast-trending regional seismic reflection profiles, thus roughly orthogonal to the main strike of the buried thrusts. We then used the inverse trishear approach to determine the slip necessary to recover the residual tectonic deformation (after decompaction) of four stratigraphic horizons with well-constrained age determinations (Zanclean to Middle Pleistocene). We then calculated and reported the slip rates using probability density functions, considering the uncertainties associated with both horizon ages and the restoration process. All together, our results show a progressive reduction of slip rates over time, with a main slowdown around 1.5 Ma. Reporting slip rates with probabilistic distributions is useful for incorporating epistemic uncertainty on the total seismic moment release in earthquake hazard analyses.

How to cite: De Matteo, A., Barrera, D., Maesano, F., Toscani, G., Seno, S., and Basili, R.: Probabilistic Assessment of Slip Rates Over Time of OffshoreBuried Thrusts: A Case Study in the Central Adriatic Sea(Italy), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2612, https://doi.org/10.5194/egusphere-egu24-2612, 2024.

Frederik Tilmann, Andreas Rietbrock, Bernd Schurr, Ben Heit, Michael Frietsch, Hans Agurto-Detzel, Ya-Jian Gao, Sofia-Katerina Kufner, Edmond Dushi, Besian Rama, Damiano Koxhaj, Dinko Sindija, Gesa Petersen, Efthimios Sokos, Claudio Faccenna, Thomas Meier, and Petr Kolinský

The eastern Adriatic margin with the Dinarides and Hellenides orogens is one of the most hazardous areas in Europe from an earthquake hazard perspective in spite of only moderate shortening rates (e.g. less than 0.5 cm/yr across the Dinarides), as exemplified by the recent highly damaging earthquakes in Durrës, Albania (2019, M6.4) and Petrinja, Croatia (2020, M6.4). Deformation is both fairly localised on shallowly NE dipping thrust faults near the coast, and distributed, with a transition to spatially extended extensional deformation in the southern Dinarides and Northern Hellenides. and a complex regime involving strike-slip and obblique mechanisms in the eastern part of the Northern Dinarides. Making sense of this distributed deformation requires highly accurate locations both horizontally and in depth, which can only be achieved with dense seismic observations.

This region is thus being explored with multi-scale dense seismic deployments. On one hand, the multi-national AdriaArray initiative has combined temporary and permanent broadband stations to achieve a nearly-uniform coverage with typical inter-station distances of 30-40 km. In Albania, where the transition of the Dinarides to the Hellenides is occurring, this regional coverage is complemented by a ultra-dense deployment of nearly 400 stations (mostly geophones with a few broadband sensors) with a nominal spacing of 5 km, which was later rearranged into three orogen-perpendicular profiles, and one along-strike profile with 1 km station spacing. Such large numbers of stations require automated processing approaches leveraging recently developed machine learning-based techniques. The presentation will review the tectonic context for these surveys and share some preliminary results from these deployments as well as an earlier more localised deployment in the area of the Durrës earthquake.

How to cite: Tilmann, F., Rietbrock, A., Schurr, B., Heit, B., Frietsch, M., Agurto-Detzel, H., Gao, Y.-J., Kufner, S.-K., Dushi, E., Rama, B., Koxhaj, D., Sindija, D., Petersen, G., Sokos, E., Faccenna, C., Meier, T., and Kolinský, P.: The potential of high density seismic arrays to elucidate distributed deformation in the Dinarides and Hellenides, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19217, https://doi.org/10.5194/egusphere-egu24-19217, 2024.

Simone Lenci, Derek Keir, Giancarlo Molli, Paola Vannucchi, Chiara del Ventisette, and Carolina Pagli

The Neogene-to-Recent tectonic evolution of the Northern Apennines has been characterized by contraction in the foreland, accompanied by extension in the internal domain. While the architectural and kinematic aspects of these two distinct sectors are better-known, uncertainties persist regarding the transition between them. Understanding the tectonics is made more complex since the width of the internal extensional domain increases south-eastward as the Northern Apennines steps eastward across Italy.

The Sant'Anna Pelago area, located on the Tuscan-Emilian ridge between Alpe di Succiso (NW) and Monte Cimone (SE), exhibits pronounced instrumental seismicity with over a thousand recorded events over the last ~15 years, forming the focus of this study. Sant'Anna Pelago represents a critical zone as the extensional front is situated near the outermost out-of-sequence contractional front affecting the Tuscan Nappe. The region locates close to the orographic divide, and also where the width of the internal extension starts widening significantly eastward. Seismicity in Sant’Anna is expressed through three major seismic sequences over a 10-year period from 2012 to 2022, with events concentrated in 2013, 2018, and 2022. P and S arrival times from 56 evenly distributed stations within 130 km radius from the publicly available INGV database were utilized to perform a preliminary relocation of seismic clusters using NonLinLoc and a local velocity model. Subsequent precise relocations were conducted using differential arrival times through HypoDD.

The relocation revealed three primary deep clusters and several minor aligned ones. The 2013 sequence is 8-km-long, 10-17 km deep, strikes parallel (NW) with the Apenninic trend, and dips 50 degrees towards the SW. This structure aligns prominently with the southern tip of the Zola master fault, surfacing near Pieve Pelago. Earthquakes are particularly dense at the southern tip of this structure. The subsequent clusters are in the footwall of the 2013 sequence, and at similar depths. These show several discrete, sub-vertical structures oriented E-W, each approximately 3 km long. These clusters geometrically resemble synthetic en-echelon faults situated in the footwall of the Zola fault. The western tips of these structures align along a potential envelope segment linking them to the 2013 cluster, a transverse SW-NE structure orthogonal to the Apenninic structure and the Zola Fault. We interpret the 2013 sequence as normal slip on a reactivated NW striking Apenninic contractional structure, and the subsequent en-echelon sequences on E-W faults as mostly normal slip in a dextral stepping zone of local re-orientation of stress. The interpretations will however, be tested with earthquake focal mechanisms and field structural geology. 

How to cite: Lenci, S., Keir, D., Molli, G., Vannucchi, P., del Ventisette, C., and Pagli, C.: Faults segmentation in active external extensional front: insights from seismicity in the Sant'Anna Pelago area, North Apennines, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21217, https://doi.org/10.5194/egusphere-egu24-21217, 2024.

Lorenzo Petracchini, Andrea Billi, Eugenio Carminati, Claudio Chiarabba, Alessia Conti, Roberto Devoti, Mimmo Palano, Giuseppe Pezzo, Laura Scognamiglio, Stefano Tavani, and Elisa Tinti

Identifying seismogenic faults in offshore regions presents significant challenges, particularly in achieving their precise geometry and kinematics. Geological data derived from deep-sea exploration and geophysical surveys are commonly used to characterize offshore active faults together with earthquake hypocentral locations. However, limitations may arise in the quantity and quality of geophysical available data, inhibiting the realization of accurate 3D models. Furthermore, the precise relocation of seismic events is demanding, especially in the depth domain, due to the limited azimuthal coverage and the minimum station-event distance that is well beyond the mean depth of the events. In this context, an interdisciplinary approach becomes imperative to mitigate over-interpretation and over-simplification in defining the seismogenic sources and establishing an all-encompassing rupture model.

By means of an interdisciplinary (geological, seismological, and geodetic) approach, we investigate the outermost Northern Apennines fold-and-thrust belt front in the Adriatic Sea (Italy) involved in the Costa Marchigiana Pesarese seismic sequence started with the 9 November 2022 Mw 5.5 mainshock. Given the proximity of the mainshock and the subsequent seismic sequence to the urbanized coastline, where several cities are situated, characterizing the activated faults and the related estimation of ground displacement becomes crucial for seismic risk assessment and the tsunamigenic potential.

We analysed the geological setting of the area by means of an accurate interpretation of numerous seismic reflection profiles and well data acquired over the past decades, which complemented the publicly available seismic data. The interpretation of this dataset, provided by oil companies, led to an accurate definition of the thrust systems highlighting both the geometry of the activated sector of the thrust front and its relation to potentially active adjacent faults. Moreover, the results show the strong influence of past paleogeography and paleomorphology on the evolution and geometry of this sector of the fold-and-thrust belt, including the buttressing effect of carbonate platforms and inherited highs.

The resulting 3D model was integrated with seismological data and geodetic observations allowing us to well highlight the activated portion of the fault plane: strong motion data and continuous GNSS stations hosted by onshore (storage centers) and offshore (seabed-anchored hydrocarbon platforms) infrastructures were jointly inverted to retrieve the Mw 5.5 coseismic rupture history.

How to cite: Petracchini, L., Billi, A., Carminati, E., Chiarabba, C., Conti, A., Devoti, R., Palano, M., Pezzo, G., Scognamiglio, L., Tavani, S., and Tinti, E.: The active frontal sector of the offshore Northern Apennine thrust belt: insights from an interdisciplinary approach following the 2022 Mw 5.5 Costa Marchigiana Pesarese earthquake (Italy), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17556, https://doi.org/10.5194/egusphere-egu24-17556, 2024.

Alessia Conti, Marco Cuffaro, Eleonora Ficini, Patrizio Petricca, Andrea Billi, Alessandro Bosman, Valentina Ferrante, Filippo Muccini, Valentina Romano, Marco Ligi, Carlo Doglioni, and Sabina Bigi

The Messina Strait and surrounding areas are one of the most interesting regions of the western Mediterranean Sea, characterized by the complex interplay between the Mesozoic-Paleogene Ionian basin, where the Calabrian Arc accretionary prism extends towards the southeast, and the Neogene Tyrrhenian back-arc basin to the northwest.

Complex fault networks with different kinematics, running from the inner side of the Calabrian arc through the Messina Strait and the Ionian coast of Sicily, as far as the Hyblean Plateau, result from the coexistence of different geodynamic settings in the area. Some of these faults are responsible for several of the largest earthquakes occurred in southern Italy and the Mediterranean Sea in recent times. Different works aimed at establishing a relationship between seismogenic sources and mapped faults, defining the location and rupture mechanism of some of these fault lineaments. Even tough, many uncertainties still exist for earthquakes occurred in offshore areas, where the fault kinematics and geometry are in some cases still poorly constrained.

In this work, we focus on a group of offshore faults located between the northern sector of the Messina Strait and the Gioia Basin (southern Tyrrhenian Sea). We aim at understanding the kinematics and the style of deformation in this area, and to investigate the role played by the main fault networks in the framework of the regional complex geodynamic setting of the Ionian-Tyrrhenian transition zone. This study is based on the interpretation of a multichannel seismic dataset (TIR10 survey), combined with the analysis of morpho-bathymetric and geodetic data, and with numerical modeling. This multidisciplinary and multiscale approach can contribute to unravel the particular role of this region in the context of a stepwise migrating subduction system and provides new constrains for the study of this highly populated area characterized by severe seismic and tsunamigenic hazard.

How to cite: Conti, A., Cuffaro, M., Ficini, E., Petricca, P., Billi, A., Bosman, A., Ferrante, V., Muccini, F., Romano, V., Ligi, M., Doglioni, C., and Bigi, S.: Fault pattern and kinematics at the Tyrrhenian-Ionian transition zone (northern Messina Strait), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13263, https://doi.org/10.5194/egusphere-egu24-13263, 2024.

Hamza Kristou, Frédéric Masson, Néjib Bahrouni, Mustapha Meghraoui, and Patrice Ulrich

Tunisia lies at the centre of the East-West trending convergence zone between the Nubian and Eurasian plates, at the eastern end of the large tectonic structures of the Atlas and Tell mountains and to the west of the Pelagian block and Sicily. As a result, its complex tectonics along the plate boundary show N-S to NW-SE oblique convergence expressed by E-W- to WNW-ESE-trending right-lateral strike-slip faults associated with E-W- to NE-SW-trending thrust faults that affect the Neogene and Quaternary units of the Tell and Sahara Atlas of Tunisia.

Although this region is generally characterized by moderate seismicity, it is known for its historical and instrumental seismic activity that has resulted in human and materiel losses, such as in Utique 408 AD, Kairouan 859 AD, Tozer 1997 and recently in March 2018 an earthquake felt between Tunis and Bizerte and in April 2023 an earthquake felt in Metlaoui, both earthquakes registered (Mw 5).

A partnership between the National Office of Mines ONM-Tunisia and ITES-Strasbourg is being set up to develop spatial geodesy work using GNSS measurements to characterize and quantify the active deformation of Tunisia alongside previous tectonic and seismotectonic works.

A network of already existing 21 GNSS stations spread over the Tunisian territory is managed by OTC (Office of topography and cadaster) so in the framework of this project 6 days/year of records from 2012 to 2019 has been purchased.

To improve the resolution of the acquired data and fill the gaps between the OTC stations, a national network consisting of 24 mobile stations is set up and three campaigns of 3 days of records in 2019, 2021 and 2023 have already been carried out.

Between 2022 and 2023, five more permanent stations have been installed to provide a continuous flow of data.

Two target areas, Gafsa and Kairouan have been chosen to install regional networks consisting of 16 sites each around active faults. Three campaigns in 2021, 2022 and 2023 have been carried out and one more is planned in 2024 to detect the deformation in those areas.

All these data allowed the calculation of a precise velocity field of Tunisia based on GPS trends and the establishment of the strain rate distribution across continental Tunisia. These new data will be analyzed in the light of existing knowledge, in particular the recent seismotectonic and paleoseismological work carried out as part of our project.

How to cite: Kristou, H., Masson, F., Bahrouni, N., Meghraoui, M., and Ulrich, P.: Active deformation in Tunisia from GNSS measurements, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9421, https://doi.org/10.5194/egusphere-egu24-9421, 2024.

Marco Cuffaro, Andrea Billi, Barbara Orecchio, Mimmo Palano, Debora Presti, and Cristina Totaro

In the western Mediterranean, the subduction of the Tethyan ocean has progressively come to an end, following the intervening continent-continent collision. Compressional deformation connected with the ongoing Africa (AF) – Eurasia (EU) convergence has therefore progressively resumed mostly along the southern passive margins of the Mediterranean back-arc basins. The use of geodetic, seismological, and pre-existing tectonic data recorded between the Gulf of Cadiz and the Ionian Sea helps to trace this nascent AF-EU boundary and constrain its kinematics. Based on these data, this plate boundary is detected, kinematically defined, and compared with the previously identified boundaries in the same region. The nascent boundary is articulated and formed by variably oriented inherited structures. It is characterized by a discrepancy between the general motion of Africa with respect to Eurasia and the local contractional/compressive axes deduced from geodetic and seismic data. The oblique convergence along the nascent boundary matches that recorded in other instances of subduction initiation elsewhere, but the average convergence rate equal to 5 mm/yr in the Mediterranean seems currently too small for such a subduction initiation. Based on the assumption of a future northward tectonic vergence (i.e., Eurasian foreland), the Tyrrhenian, Algerian, and Betic salients, the Oran and Fès recesses, and the Ionian, Trans-Alboran, and Gibraltar transfer zones are identified along the nascent boundary. The latter zones connect salients and recesses through strike-slip displacements. The Algerian offshore hosts a long segment of the boundary characterized by locally increased seismic rate and actual northward vergence that would suggest this area being the first nucleus of subduction initiation in the western Mediterranean.

How to cite: Cuffaro, M., Billi, A., Orecchio, B., Palano, M., Presti, D., and Totaro, C.: Retracing the Africa-Eurasia convergent boundary in the western Mediterranean based on seismic, geodetic and tectonic data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13305, https://doi.org/10.5194/egusphere-egu24-13305, 2024.

Francisco J Nunez-Cornu and Diana Nunez

Oaxaca is the most seismically active region in Mexico and one of the most studied using different methodologies. This seismic activity is due to the subduction of the Cocos Plate beneath the North American Plate, which is considered an anomalous subduction zone since it is a truncated continental margin. Seventy-four earthquakes (M> 7.0) have been identified in the last 510 years, which is an average of one earthquake every 6.8 years. The seismic sequence occurred between 1928 and 1937 is the key to understand the regional seismotectonics. The locations of these nine events (M>7.0) reported by different authors differ by more than 100 km for the same earthquake. We relocated the aftershocks of these earthquakes using the seismograms from TAC (Tacubaya) and VCM (Veracruz) stations available at the Seismological Seismic Network (Mexico) archive reading pre-phases S-P.  To calibrate these readings, we relocate the seismicity in the region between 1950 and 1982 with the JHD Method using 1978, 1982, 1965 and 1968 earthquakes as Master Events. We look from this catalog the earthquakes registered in TAC and VCM in the period 1950 - 1982 and whose seismograms were in the archive were selected. The S-P prephases in TAC and VCM were read with the same criteria used previously. With these data we fitting a time-distance curve for each station. These curves were used to obtain more reliable aftershock area for each of the coastal earthquakes occurred during the 1928 – 1937 sequence.

How to cite: Nunez-Cornu, F. J. and Nunez, D.: The 1928 – 1937 Oaxaca Earthquake Sequence (Mexico), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-179, https://doi.org/10.5194/egusphere-egu24-179, 2024.

Zaur Bayramov, Renier Viltres, Cecile Doubre, Alessia Maggi, Frederic Masson, Behzad Zamani, and Marie-Pierre Doin

The Caucasus and Northern Iran lie within the central part of the Alpine-Himalayan belt, where the Arabian and Eurasian plates started colliding over 100 My ago and caused the building of mountain chains associated with complex tectonics, including transform faulting systems. The region contains many tectonic features including the EW-trending Greater Caucasus and the NW-trending Lesser Caucasus thrust belt separated by the Kura basin. In the southern part of the region, the tectonics are complicated by the Anatolia-Eurasia-Arabia triple junction and the northern end of the Talysh and Alborz thrust belts. There have been several destructive earthquakes in the region, including the Shamakhi earthquake sequences in 1667(8) and 1902 at the junction of the controversial and mostly a-seismic West-Caspian Fault and the Eastern Greater Caucasus and the 1721 and 1780 earthquakes on the North Tabriz fault in NW Iran. 

Investigations of the few publicly available seismic catalogs of the region have been insufficient to understand the seismo-tectonic behavior of the regional structures due to sparse existing seismic networks. To better characterize the active structures in the Caucasus and  Northern Iran we produced  regional-scale mean line-of-sight velocity maps and time-series of the surface displacement from the north-eastern Caucasus to northern Iran. To obtain this dataset we performed Synthetic Aperture Radar interferometry using the NSBAS processing (Doin et al., 2011) of Sentinel-1 images along both ascending and descending tracks for 9-years (2015 to 2023). Main processing steps (such as atmospheric correction, multilooking and filtering) were applied to counter biases and loss of coherence due to the snow and vegetation coverage in the Greater Caucasus mountains. We produced two regional-scale interseismic velocity maps that highlight crustal motions of the large-scale tectonic structures. Moreover, we have identified coseismic deformation due to the 5.2 ml Shamakhi earthquake in the SE Caucasus mountains (Feb. 2019), the 5.9 Mw Torkamanchay earthquake in the Bozgush mountains of NW Iran (Nov. 2019), and possible aseismic strike-slip along the West Caspian fault after the large seismic events in Türkiye in February, 2023. Our results can also be used to study the local deformation of mud volcanoes in the Eastern part of Azerbaijan.

How to cite: Bayramov, Z., Viltres, R., Doubre, C., Maggi, A., Masson, F., Zamani, B., and Doin, M.-P.: Present-day crustal deformation of the Caucasus and Northern Iran constrained by InSAR time series  , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13308, https://doi.org/10.5194/egusphere-egu24-13308, 2024.

Seismicity in Subduction Zones
Lin Shen, Michael Steckler, Eric Lindsey, Bar Oryan, and Jeng Hann Chong

The Indo-Burma subduction zone (IBSZ) is an entirely subaerial plate boundary, where the Indian plate obliquely converges with the Burma microplate. Because the incoming plate includes the 16-20 km thick sediment of the Ganges-Brahmaputra Delta, the accretionary prism is over 250 km wide with numerous active splay thrust faults and strike slip faults. Accurately assessing the long- and short-term dynamics of this complex region is critical for determining its earthquake hazard.

However, due in part to insufficient geodetic observations in the region to constrain the 3D shape of the megathrust and upper plate deformation, the kinematics of this plate boundary zone remain controversial. Ongoing debates focus on how strain is partitioned between the megathrust and strike-slip and oblique faults, whether the subduction zone is locked, and whether the multiple anticlines of the accretionary prism foldbelt are locked or actively deforming aseismically.

In this study, we present the first large-scale Interferometric Synthetic Aperture Radar (InSAR) velocity field over the IBSZ. Considering the operational nature and radar characteristics of different satellites, we processed datasets of multiple satellites spanning from late 2014 to 2023, including Sentinel-1, ALOS-2, and the newly launched L-Band differential InSAR satellite of China, LuTan-1. This approach allows us to more accurately constrain deformation across such a heavily vegetated and topographically-varied region. We incorporated updated horizontal and vertical GNSS velocities from 60 sites obtained from 2003 to 2023 to derive a three-dimensional decomposed velocity field, and then we investigated faults activities by estimating interseismic strain rates across the IBSZ. Our preliminary results reveal how strain is distributed in the region, shedding light on seismic hazard across this densely populated area.

How to cite: Shen, L., Steckler, M., Lindsey, E., Oryan, B., and Chong, J. H.: Large-scale geodetic deformation measurements of the Indo-Burma Subduction Zone from multi-sensor InSAR and GNSS: implications for strain partitioning and earthquake hazard, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11641, https://doi.org/10.5194/egusphere-egu24-11641, 2024.

Silvia Brizzi, Arnauld Heuret, Claudia Piromallo, Fabio Corbi, Francesca Funiciello, and Serge Lallemand

Subduction zones host the world’s largest earthquakes. Recent studies suggest complex interactions between megathrust, upper plate and intraslab seismicity. Understanding the spatio-temporal relationships of seismicity within subduction zones is challenging, yet essential for accurate seismic hazard assessment. A prerequisite for conducting these analyses involves the availability of a reliable and readily updatable dataset that classifies subduction seismicity into the three categories above.

In this work, we compile a comprehensive global database of subduction-related earthquakes from 1976 to 2023, using the ISC-GEM catalog (Storchak et al., 2013; 2015; Di Giacomo et al., 2018) for events with magnitude Mw ≥ 5.5. Building on Heuret et al. (2011), we define 505 trench-normal transects across all active subduction zones, spaced at 1-degree intervals along the trench, partially overlapping and extending 120 km on both sides of a vertical plane. The seismicity in each transect is initially categorized into shallow (≤ 70 km) and deep (> 70 km) events. We then focus on the megathrust region, identifying earthquakes exhibiting a thrust focal mechanism with the  azimuth and dip of the focal planes aligning with the strike and dip of the megathrust along the transect. Subsequent steps involve categorizing the remaining earthquakes in the transect as either upper or subducting plate events. The classification uses the Slab 2.0 model (Hayes et al. 2018) when available, determining whether each earthquake occurs above or below the slab top surface.  In regions lacking Slab 2.0 data, geometric criteria are applied, considering the distance of the hypocenter to the trench. For each transect, this workflow yields three distinct seismicity classes: megathrust, upper plate, and intraslab earthquakes. Subsequently, seismicity from individual transects is merged into 62 broader segments (Heuret et al., 2011), ensuring the uniqueness of earthquakes in each segment.

This automated workflow ensures the application of objective classification criteria and facilitates efficient analyses with each update of the ISC-GEM catalog. We compare key seismic parameters (e.g., maximum magnitude, number of events, cumulated seismic moment, recurrence time) across the different categories and segments. Additionally, we evaluate the correlation with a wide range of geological, geophysical, and geodynamical factors. This process not only provides an overview of the global behavior of subduction-related seismicity but also allows for the statistical identification of the combination of factors influencing subduction seismicity.

How to cite: Brizzi, S., Heuret, A., Piromallo, C., Corbi, F., Funiciello, F., and Lallemand, S.: Unveiling subduction-related seismicity: towards a new global database, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12240, https://doi.org/10.5194/egusphere-egu24-12240, 2024.

Han Chen, Gauhua Zhu, Hongfeng Yang, Shaoping Lu, Chuanxu Chen, and Jian Lin

Intermediate-depth earthquakes (IDEs), i.e., earthquakes at depths of 70 to 300 km, have been observed in subduction zones globally and extensively investigated. However, the seismogenic mechanism of IDEs is still controversial, especially in the southern end of the Mariana Trench, where near-field observations are lacking. By using machine-learning-based methods in three sets of near-field Ocean Bottom Seismogram (OBS) network data, we detected and located more than 1,000 intraplate and interplate earthquakes. The seismogenic volumes in different regions of the subducted plate are different, showing the character of double seismogenic zones (DSZ) and single seismicity layer (SSZ). The seismicity features coincide well with the regional landform, development of outer-rise faults, and hydration scenarios, suggesting a dehydration-related mechanism for the generation of IDEs. The subducted slabs experience different degrees of slab hydration, leading to various seismic behaviors.

How to cite: Chen, H., Zhu, G., Yang, H., Lu, S., Chen, C., and Lin, J.: Impact of outer-rise slab hydration on the intermediate-depth seismicity: Evidence from near field OBS observation in the southernmost Mariana subduction zone, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3277, https://doi.org/10.5194/egusphere-egu24-3277, 2024.

Cédric De Meyer, Calum J. Chamberlain, and Martha K. Savage

The formation of new subduction zones, termed Subduction Zone Initiation (SZI), has a large influence on global plate tectonics. However, how stresses within the plate boundary region evolve throughout the evolution of nascent subduction zones remains unresolved. The Puysegur Subduction Zone in Fiordland, near the southern tip of New Zealand’s South Island, is a young, steeply dipping subduction zone and a key site for studying such incipient stages of subduction. Despite the global significance of the Puysegur Subduction Zone, it has received relatively little attention, mostly due to its remote location. Few passive seismic studies have been carried out in the region, and the continuous GeoNet network is too sparse to detect and accurately resolve seismicity around the Puysegur Subduction Zone. Because of this, the present-day structure and stress state of the Puysegur Subduction Zone remain poorly resolved.

We aim to study these two unresolved characteristics by using a combination of temporary seismic networks and the permanent GeoNet network to increase station coverage in the region. We have developed a Puysegur-appropriate workflow consisting of automated earthquake detection and association, manual event evaluation and P-wave polarity determination. Highly accurate earthquake locations are obtained using NonLinLoc and a 3D velocity model. Focal mechanism analyses and stress inversion are conducted using Bayesian approaches. Currently, we have obtained a preliminary catalogue for the period between 02/2018 and 10/2018, which shows that the developed methodology is capable of producing more complete earthquake catalogues compared to the national GeoNet catalogue. Preliminary precise hypocentral locations and well- constrained focal mechanisms for moderate-to-large magnitude events are used to constrain the region’s seismogenic structures, such as the subduction interface and major active faults, as well as provide preliminary constraints on the region’s stress state.

How to cite: De Meyer, C., Chamberlain, C. J., and Savage, M. K.: Seismogenic structures and stress state of the Puysegur Subduction Zone (Fiordland, New Zealand) from detailed earthquake observations., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1460, https://doi.org/10.5194/egusphere-egu24-1460, 2024.

Jonas Folesky

The characteristic stress drop of an earthquake is indicative of its slip to fault dimension. Its value is affected by fault strength, fault topography, the presence of fluids, and other properties. By estimating stress drops throughout an entire subduction zone, namely for the seismically highly active northernmost part of Chile, and combining it with mapped b-values and their corresponding magnitude distribution, this work aims to better constrain the conditions under which earthquakes of different provenances may nucleate.
Database is a recent seismicity catalog, containing over 180,000 events and covering 15 years of seismicity, for which more than 50,000 stress drop estimates were computed. Their class wise spatial average segments the subduction zone into different parts. This difference, however, is small compared to the natural scatter of stress drop values. 
By considering stress drop variations, b-value map, magnitude distribution, and thermal modeling, I describe a variety of mechanisms of earthquake nucleation which might explain the observed stress drop variation. This is done for 1) the plate interface in general; 2) local shallow interface features, i.e., asperities and creeping sections; 3) the highly active intermediate depth seismicity region. In all three cases, the combination of stress drop distribution and b-value mapping helps to better understand the differences in earthquake nucleation and to formulate hypotheses on the controlling factors of earthquake nucleation.

How to cite: Folesky, J.: From stress drop mapping to earthquake nucleation conditions in the northern Chilean subduction zone, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1741, https://doi.org/10.5194/egusphere-egu24-1741, 2024.

Siyuan Yang and Hu Yan

The 2007 Mw8.4 southern Sumatra earthquake provides an opportunity to understand the rheological properties in the southern Sumatra, particularly in the Mentawai gap. In this study, we have derived the first 3-year GPS postseismic observations to study deformation processes based on a three-dimensional viscoelastic finite element model. In the model, a 2-km-thick shear zone attached to the fault is used to simulate the time-dependent and stress-driven afterslip. Model results indicate that a model with a heterogenous shear zone better fits the horizontal GPS observations than a model with a uniform shear zone. This heterogenous shear zone is divided into the southern shear zone and northern shear zone (Mentawai gap), which is separated by the southern edge of the Mentawai gap. The southern shear zone is further divided into an upper (depths of ≤ 20 km) and lower shear zone (depths of > 20 km). The viscosities in these three shear zones are determined to be 5 x 1017 Pa s, 1016 Pa s and 1018 Pa s, respectively. Model results indicate that a weakened mantle wedge is required to better explain the observed uplift in vicinities of the rupture area.

How to cite: Yang, S. and Yan, H.: Rheological structure beneath the southern Sumatra constrained from postseismic deformation of the 2007 Mw8.4 Sumatra earthquake, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11015, https://doi.org/10.5194/egusphere-egu24-11015, 2024.

Adriano Gualandi, Luca Dal Zilio, Davide Faranda, and Gianmarco Mengaldo

Earthquakes are understood as frictional instabilities taking place in weak zones of the Earth crust called faults. On the one hand, the lengthy recurrence time of earthquakes makes numerical simulations an invaluable tool to study consecutive ruptures of a given fault. On the other hand, it makes a direct comparison with nature difficult, if not currently impossible. Slow earthquakes, exhibiting lower recurrence times, serve as a viable alternative for validating models against real-world observations. We investigate similarities and differences between natural and simulated slow earthquakes using nonlinear dynamical system tools. We focus on slow earthquakes derived from Global Navigation Satellite System (GNSS) position time series in Cascadia and numerical simulations intended to reproduce their pulse-like nature and scaling laws. We provide metrics to evaluate the accuracy of simulations in mimicking slow earthquake dynamics, and we investigate the influence of spatio-temporal coarsening as well as observational noise. Findings indicate that numerical simulations exhibit average properties akin to natural occurrences. In addition, despite the usage of many degrees of freedom in numerical simulations, we retrieve a low average dimension, like the one obtained for Cascadia slow earthquakes, suggesting that a reduced order model may be a viable representation of slow slip events. Time-dependent, instantaneous properties show strong dependence on the variable considered for the analysis for numerical simulations, but not for natural observations. Our exploration show a possible way to extract dynamic attributes from kinematic information, and enriches the picture that we have of natural-scale friction We propose to use the suggested metrics as an additional tool to narrow the divergence between slow earthquake observations and dynamical simulations.

How to cite: Gualandi, A., Dal Zilio, L., Faranda, D., and Mengaldo, G.: Similarities and differences between natural and simulated slow earthquakes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20926, https://doi.org/10.5194/egusphere-egu24-20926, 2024.

Chia-Han Tseng, Po-Yu Chu, Cheng-Feng Wu, and Ruei-Juin Rau

The Taiwan Island is the product of the orogeny resulting from the collision of the two tectonic plates, the Eurasia Continent Plate and the Philippine Sea Plate. The Philippine Sea Plate has subducted the Eurasia Continent Plate and formed Ryukyu Volcanic Arc in northern and northeastern Taiwan. The Datun Volcano Group (DVG) being located in northern Taiwan is the westernmost member of the Ryukyu Volcanic Arc and has the widest extent and largest eruption amount among the volcanic rock areas. About 1 Ma, compressional stress transformed into extensional stress in northern Taiwan, and magma from the depth erupted to form younger volcanoes (~20) in the same area. During this period, the Taipei Basin and the Jinshan Basin gradually formed as half grabens on a normal fault, namely the Shanjiao Fault.

The DVG and the Shanjiao Fault have been identified to be active for micro-earthquake activities and topographical features, respectively, revealed by dense and high-resolution surficial monitoring systems in the Datun Mountain area. However, owing to rugged landscape and dense vegetations, geological boreholes are few and shallow (10 to 20 meters) so that the underground geological structures in the Datun Mountain area are still unclear. In this study, microtremor cross the presumed fault trace of the Shanjiao Fault are recorded and analyzed by applying the horizontal-to-vertical (H/V) spectral ratio method, and the H/V spectrum is further decomposed into E-W and N-S components.

The H/V spectral ratio reveals different dominant frequency for different volcanic products. The results indicate that the stations on thin loose deposits (pyroclastic debris) underlying by lava flow (andesite) show the higher dominant frequency, and these stations are near crater, while the stations farther from the craters have lower dominant frequency with thick loose deposits. And these results are also consistent with the topography revealed by high-resolution digital terrain model of the Datun Mountain area.

Based on the results, the future work of this study will be describing spatial geometry of the Shanjiao Fault by distinguishing different dominant frequencies corresponding to the footwall and hanging wall.

How to cite: Tseng, C.-H., Chu, P.-Y., Wu, C.-F., and Rau, R.-J.: Shallow geological structure by applying H/V method in volcanic area in northern Taiwan, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14378, https://doi.org/10.5194/egusphere-egu24-14378, 2024.

Andrés Melo, Sumiko Tsukamoto, Margret Fuchs, David Tanner, Christian Brandes, Uwe Kroner, Andrew Nicol, and Richard Gloaguen

The Alpine Fault in New Zealand is one of the world’s major active crustal-scale faults. It builds the boundary between the Pacific and the Australian plate, and branches into strike-slip faults known as the Marlborough fault system. The northeastern region of the southern island of New Zealand has a historical record of large, shallow earthquakes with magnitude (Mw > 6.5) since the Nineteenth Century. The most recent event, the 2016 Kaikōura earthquake, with a magnitude (Mw) of 7.8, is among the strongest. The severe impact on society and landscape explain the importance of a better understanding of the Quaternary activity of these active faults. Recent investigations in other tectonically-active settings worldwide indicate the potential feasibility of applying luminescence dating to unravel the timing and hence, re-occurrence of fault activity as a source for earthquakes.

We aim to test the potential of luminescence dating to determine the relative activity of three active faults in New Zealand. To this end, we collected four dark-gray, fine to very fine grain-size samples classified as cataclasite and gouge from outcrops situated along the fault traces of the Alpine Fault, Hope Fault, and Hundalee Fault. Through sample processing, we obtained polymineral fine grains, ranging from 4 to 11 µm, to conduct post-infrared infrared stimulated luminescence (pIRIR225) dating. The method applied on faults is the signal-resetting event of the fault movement, and if the signal is not saturated in nature, this implies that frictional heating was enough to at least partially reset the system; for feldspar the closure temperature is 40-90 °C.

The growth curves of the pIRIR225 signals reveal that the gouge samples extracted from the Hope fault and Hundalee fault approach saturation levels with equivalent doses around 850 Gy and 900 Gy, respectively. In contrast, the equivalent dose of cataclasite samples from the Alpine Fault was clearly below saturation ranging from 220 Gy to 410 Gy. All assessment criteria, including recycling ratio and recuperation rate, meet the rejection criteria for all samples, indicating a reliable signal to dose relationship. These results suggest there were events, which thermally eroded the pIRIR225 signal at the Alpine Fault. The comparison of the equivalent doses from the three faults also indicates that the method is applicable to evaluate the relative fault activity; the Alpine Fault is more active than the Hope and Hundelee faults. However, micro-structural analysis also indicated differences in brittle deformation mechanisms, differences that also potentially influence the variations between the pIRIR225 signals of individual samples. Observed features comprise, for example, grain fracturing, frictional sliding, pressure solution, and twinning. The micro-structural variation suggest differences in deformation, stress and pressure-temperature (P-T) conditions experienced by the studied cataclasite and gouge samples.

This study presents the first findings of pIRIR225 dating on feldspar in active faults in New Zealand and points at the success of luminescence dating. However, we strongly emphasize the importance of combining luminescence analysis with microstructural and mineralogical data obtained through Scanning Electron Microscope (SEM)-Mineral Liberation Analysis (MLA) to better understand the P-T conditions and resulting degree of luminescence signal reset.

How to cite: Melo, A., Tsukamoto, S., Fuchs, M., Tanner, D., Brandes, C., Kroner, U., Nicol, A., and Gloaguen, R.: Direct dating of active faults using luminescence: A case of study in New Zealand, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20437, https://doi.org/10.5194/egusphere-egu24-20437, 2024.

Gerardo Suarez and Sergio Aguilar

The seismicity in the back arc of the Mexican subduction zone is relatively low.  An exception to this is the seismic activity observed in the Isthmus of Tehuantepec, where shallow earthquakes take place mostly along the coast of the Gulf of Mexico.  The largest recorded earthquake occurred on 26 August 1959 (Mw 6.9).  Other moderate earthquakes are recorded also in the southeastern margin of the Gulf of Mexico with magnitudes ranging from 5.3 to 5.9.  Data from the VEOX experiment that registered seismic data continuously on a cross-section across the Isthmus of Tehuantepec were analyzed.  Shallow earthquakes were culled from the continuous records, eliminating events within the subducted slab.  A total of ~40 shallow earthquakes were identified.  The linear geometry of the VEOX array made it difficult to locate the earthquakes.  Thus, additional stations from the Seismological Service of Mexico were used in the analysis.  Hypocentral locations were improved using the double-difference hypocentral algorithm.  The focal mechanisms obtained show consistently reverse faulting, where the axes of maximum compression are oriented NE – SW, like the 1959 earthquake.  This indicates that the lithosphere is deformed by compressive stress oriented in the direction of relative plate motion.  The best-located earthquakes show focal depths ranging from 20 to 50 km.  The depth of the Moho in the Isthmus of Tehuantepec is well controlled by receiver function results.  Therefore, it is possible to identify where the earthquakes occur relative to the depth of the Moho.  Unlike most upper plate deformation in the back arc of the subduction zones, earthquakes in the Isthmus of Tehuantepec occur both in the crust and the upper mantle.    Rheological models suggest that shallow earthquakes occur mostly in the seismogenic part of the upper crust and the upper mantle.  Our observations clearly show that in this region earthquakes reflect lithospheric deformation involving the crust and the upper mantle.  We are currently exploring rheological models that may help explain earthquakes in both the crust and upper mantle. The focal mechanisms suggest that the deformation may be induced by the subduction of the aseismic Tehuantepec Ridge in the Mexican subduction zone.

How to cite: Suarez, G. and Aguilar, S.: Rheological Behaviour of  the Deformation of the Back Arc in the Isthmus of Tehuantepec in South-eastern Mexico, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14489, https://doi.org/10.5194/egusphere-egu24-14489, 2024.

David Tanner, Christian Brandes, Andy Nicol, Jan Igel, Sumiko Tsukamoto, and Julia Rudmann

In outcrops, the hanging-wall and/or footwall structure around a fault are often exposed, while the underlying fault is poorly resolved. In these cases, it is desirable to estimate the location and shape of the fault at depth, especially if it belongs to an active fault system prone to large earthquakes. The Mw 7.8 Kaikōura earthquake occurred two minutes after midnight on 14th November 2016, causing at least 17 faults in the northeast South Island of New Zealand to rupture, including a number of faults that had not been previously mapped. One of these smaller new faults is the Leader Fault, which at the surface displaces Mesozoic interbedded greywacke and argillite. In outcrop, the fault rupture caused an over 3 m high, 20-30 m wide, and over 120 m long hanging-wall fold to appear at the surface.

In September 2022, we used a differential global navigation satellite system to map the topography of the fold. We collected a total of 1493 points over a map area of 4526 m², i.e. an average point density of ca. 1 point per 3 m². The data were meshed into a three-dimensional triangular surface, which was then sectioned into ten cross-sections, each 10 m apart and perpendicular to the fold axes. We present fault-prediction modelling of two of these sections. In the Movetm software (Petroleum Experts), we used two methods of fault prediction; constant heave and constant slip. Both methods require implicit information about the hanging-wall shape, the position of the fault at the surface and the “regional”, i.e. the position of the hanging wall before deformation. Before the modelling, all this information was known apriori; i.e. we mapped the shape of the ground surface, we knew the fault to outcrop at the break of slope at the front of the leading edge, and the regional is an extension of the undeformed footwall. Both modelling techniques require a seed, i.e., a small portion of fault at the surface with a certain angle of dip. We use a horizontal and a 60° dipping seed.

We can estimate the fault geometry down to a depth of 20-25 m. For both sections, we predict the fault is steep, greater than 60°. Using a flat seed gives a slightly listric fault geometry, but in any case, the fault is steep down to 20 m depth before flattening out slightly. Compared to a small (15 cm) outcrop of the fault plane (dipping 75° WNW) at the surface at the northern end of the outcrop, the best matches are given by modelling with constant slip. The steep fault geometry is governed by the basement rock that has steep bedding that also dips ca. 70° WNW.

How to cite: Tanner, D., Brandes, C., Nicol, A., Igel, J., Tsukamoto, S., and Rudmann, J.: Predicting the fault beneath a newly-created earthquake-related landform: A case study of Leader Fault rupture during the 2016 Kaikōura Earthquake, New Zealand, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7886, https://doi.org/10.5194/egusphere-egu24-7886, 2024.

Intraplate Seismicity
Abigail Clark, Alexander Peace, Carolyn Eyles, and Ethan Davies

Intraplate neotectonism is generally not well documented and understood despite its significance for seismic hazards in areas such as eastern Canada. This study aims to provide an in-depth structural analysis of potential neotectonic pop-up structures in southern Ontario, Canada, leading to a more comprehensive definition of pop-up structures, and ultimately constrain the processes involved and extent to which neotectonism impacts the region. Three locations in Southern Ontario were documented using a combination of ground and drone-based structural analysis: 1) Fletcher Creek Ecological Preserve, 2) Wainfleet Wetlands, and 3) multiple sites on Manitoulin Island. Sites were chosen where previous work had documented neotectonic activity, and/or where initial geomorphic analyses indicated the possibility of pop-up structures. The locations are all located within the Ordovician to the Devonian Niagara Escarpment stratigraphy. Fracture patterns at each site were analyzed using ground-based measurements or drone-based photogrammetry (DJI Phantom 4 V2 and Phantom 4 Pro acquisition followed by analysis in Pix4D), where applicable. Orthomosaics were then analyzed using FracPaq to determine fracture statistics including orientation, intensity, and density. Where access permitted, ground-based structural measurements were also obtained on structures such as fractures and folds, in addition to RTK-DGPS (real time kinematic differential-global positioning system) profiles over potential pop-up structures. The analysis revealed inconsistencies in the definition of a "pop-up", prompting further inquiry into the definition of a pop-up versus stress relief features more generally. To address this ambiguity, a classification system was developed to differentiate between pop-ups and other tectonic stress relief features. It was concluded that pop-up structures exhibit a distinct geomorphic expression, manifesting as a linear elevated ridge. In southern Ontario, regardless of whether a feature is identified as a stress relief feature or a pop-up, it nonetheless demonstrates that the region is tectonically active despite often being characterized as a stable continental interior. This study adds to a growing body of work documenting neotectonic activity in southern Ontario, with the several stress-related structures documented for the first time in this study showing their prevalence over a wide area.

How to cite: Clark, A., Peace, A., Eyles, C., and Davies, E.: Neotectonics in southern Ontario: Pop-up structures and their implications for seismic hazards in intraplate settings, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1542, https://doi.org/10.5194/egusphere-egu24-1542, 2024.

Dibousse Aboubacar, Olivier Averbuch, and Virginie Gaullier

The Dover Strait, at the transition between the Eastern English Channel and the North Sea, lies on a complex faulted bedrock, making it a potential seismic risk area. Resulting from recent periglacial processes, it cross-cuts the geological structures inherited from the major tectonic deformations that affected the West European margin at the Late Jurassic-Early Cretaceous (extension and subsidence due to the propagation of the opening of the North Atlantic Ocean) and during the Cenozoic (compression causing the inversion of basins linked to the African-Eurasian convergence). Cape Gris-Nez is one of the most striking features of the fault system bordering the inverted Weald-Boulonnais basin. The Sirène beach, which has been heavily cleared of sand over the past 20 years, reveals the complexity of the folded and faulted geological structures associated with the development of this deformation zone. Over the last few years, detailed structural surveys have been carried out on land, using GNSS layer-to-layer mapping, and at sea, using very high-resolution SPARKER seismic profiles, providing an overall land-sea map of this fault zone. This first-rate mapping was recently supplemented by photogrammetric surveys by drone at very high spatial resolution (5 cm) making it possible to obtain an ortho-mosaic and a digital terrain model of the foreshore and cliffs of Cap Gris Nez. The interpretation of these very high-resolution images, adopted in a new structural survey campaign, leads to an optimization of the mapping of structures and a better understanding of the geometry and kinematics of deformations at the fault zone preliminary data for a better definition of seismic risk in the sector.

How to cite: Aboubacar, D., Averbuch, O., and Gaullier, V.: Land-sea mapping and deformation kinematics in the Cape Gris-Nez fault zone (Dover Strait, Eastern Channel) , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19788, https://doi.org/10.5194/egusphere-egu24-19788, 2024.

Alastair Sloan, Beth Kahle, Robert Muir, Diego Quiros, Khumo Leseane, Shaakirah Adams, Timothy Jones, Anele Matsebula, Anzani Ramagadane, Guy Salomon, and Benjamin Whitehead

The hazard posed by large intraplate earthquakes is relatively well-known in the Global
North, but Sub-Saharan Africa is poorly represented in compilations of such events, and the
hazard they may pose in this region is not well understood. Much of southern Africa is an
unusual example of an intraplate region undergoing predominantly extensional
deformation, complicating comparisons with otherwise similar regions. Here we present the
locations of moderate magnitude instrumentally-recorded seismicity, as well as eight major
paleoseismic fault scarps across South Africa, Namibia and Botswana. We focus on regions
generally considered to be stable, and compare these data to available aeromagnetic and
seismic tomographic datasets. Major events are primarily focussed on either the boundaries
of the cratonic cores in the region, or in the vicinity of large igneous complexes, suggesting
that variations in large-scale lithosphere rheology provide a first-order control on their
occurrence. Aeromagnetic lineaments, associated with Jurassic-Cretaceous normal faults or
ancient shear zones within mobile belts, are associated with almost all of the major
paleoseismic ruptures, and appear to control fault bends and terminations. Significant
differences in strike over relatively short length-scales suggest the orientation of the faults
are controlled by crustal anisotropy rather than variations in stress orientation. Some of the
scarps are likely to be associated with M7+ events, suggesting that such events can occur in
stable regions experiencing extensional stresses. The association with major crustal
structures likely explains their great length, relative geometric simplicity and unexpectedly
large magnitudes, despite limited recent brittle offset. While intraplate events are relatively
poorly studied in southern Africa the excellent preservation potential of landscape, and the
rarity of extensional events in other comparably stable regions, mean that this region has
excellent potential to increase our understanding of these phenomena.

How to cite: Sloan, A., Kahle, B., Muir, R., Quiros, D., Leseane, K., Adams, S., Jones, T., Matsebula, A., Ramagadane, A., Salomon, G., and Whitehead, B.: Geophysical constraints on intraplate deformation in southern Africa, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21021, https://doi.org/10.5194/egusphere-egu24-21021, 2024.

Karolina Owczarz, Romy Schlögel, Anne Orban, and Hans-Balder Havenith

The Einstein Telescope area was a research site of a project called Ground Deformation from Meteorological, Seismic and Anthropogenic Changes Analyzed by Remote Sensing, Geomatic Experiments and Extended Reality (GERMANE), which aimed to analyze ground deformation hazards induced by meteorological changes and seismotectonic conditions in eastern Belgium, western Germany and the south-eastern Netherlands. Within the project we proposed and applied an approach based on various Synthetic Aperture Radar Interferometry (InSAR) processing methods to detect and measure ground motions in time series. We focused on the Persistent Scatterer Interferometry (PSI), Small Baseline Subset (SBAS)  and Parallel Small BAseline Subset (P-SBAS) methods. An important issue was that the current neotectonic activity in the target area was not well known, but through spatiotemporal analysis of ground deformation we investigated behavior along NW-SE trending normal faults, where karst also develops, as well as along Variscan  NE-SW trending thrust faults. Time series analyzes were performed along Gueule fault and Gulp fault, which cross the Einstein Telescope area in the Pays de Herve (Belgium) and Heerlerheide fault in the Roer Valley Graben (Germany). We calculated the relative double difference (RDD) of Line of Sight (LOS) displacements to estimate relative deformation of one point with respect to the other. Additionally, we detected regression lines with Bayesian information criterion (BIC) that enables to choose the model which represents better the set of data points corresponding to specific InSAR techniques in double difference. In results, annual velocity rates of the benchmarks extracted along the Gueule and Gulp faults were less than -2mm/yr – which are insignificant value. However, comparing the velocity values for the extracted benchmarks along the faults, it can be seen that the Gulp fault is characterized by slightly higher annual velocities than the Gueule fault. Our time series analyses results along the Heerlerheide fault indicated that its eastern face is uplifting with velocity rates of up to 8 mm/yr. The obtained InSAR results are very small and can be described as insignificant, therefore we cannot find increased seismic activity of the analysed faults, especially the Heerlerheide and Gueule ones, as old mining activity may be responsible for the observed deformation. In sum, the faults crossing the Einstein Telescope area do not show significant displacements, which confirms the initial hypothesis of their low seismotectonic activity. Therefore, we consider the possible Einstein Telescope location as being relatively stable.

How to cite: Owczarz, K., Schlögel, R., Orban, A., and Havenith, H.-B.: Local scale of ground deformation along faults in area and vicinity of one possible Einstein Telescope location, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15223, https://doi.org/10.5194/egusphere-egu24-15223, 2024.