TS3.1

EDI
Seismic imaging and characterization of crustal faults

Imaging both fluid-filled fault networks and surrounding heterogeneous crust with geophysical methods is especially challenging. In these settings, fluids interact with deformation-induced seismic sources, influencing both nucleation and development of seismic sequences.

Imaging and characterizing both seismogenic structures and elastic and anelastic properties of the surrounding medium is key to understanding wider tectonic and small-scale deformation processes. Understanding the geometry and kinematics of crustal-scale faults from field observations is also critical for many green-energy applications (e.g., geothermal energy, CO2 storage, mining for minerals important for battery production). This session aims to provide an overview of techniques and applications aimed at characterizing both active and ancient seismogenic fault networks at local and regional scales.
In this session we aim to bring together passive and active-source seismologists to discuss new studies that image and characterize seismically active and ancient faults and fault networks. We welcome contributions from velocity tomography, attenuation tomography (coda, t* method, direct wave attenuation), source imaging and characterization (absolute and relative location techniques, focal mechanism and stress drop analysis, …), active-source seismic techniques (reflection, refraction, integrated drilling data, …), along with multidisciplinary studies. We particularly welcome contributions from early-career researchers and those using novel techniques (e.g., data mining and machine learning).

Co-organized by NH4/SM5
Convener: Simona GabrielliECSECS | Co-conveners: Ferdinando Napolitano, Rebecca Bell, Panayiota SketsiouECSECS, Richard Davy
Presentations
| Wed, 25 May, 15:10–16:30 (CEST)
 
Room K2

Presentations: Wed, 25 May | Room K2

Chairpersons: Richard Davy, Simona Gabrielli, Ferdinando Napolitano
15:10–15:20
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EGU22-1403
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solicited
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Virtual presentation
Ryuta Arai, Shuichi Kodaira, Stuart Henrys, Nathan Bangs, Koichiro Obana, Gou Fujie, Seiichi Miura, Daniel Barker, Dan Bassett, Rebecca Bell, Kimihiro Mochizuki, Richard Kellett, Valerie Stucker, and Bill Fry

The NZ3D OBS experiment performed in 2017-2018 in the northern Hikurangi margin off the east coast of North Island, New Zealand, provided the highest-resolution seismic refraction/wide‐angle reflection data with multi-azimuth ray coverage in subduction zones to date (Arai et al., 2020). The study area extending 60 km in the trench-normal direction and 14 km in the trench-parallel direction covers source regions of a variety of slow earthquake phenomena, such as shallow slow slip events and tectonic tremor (e.g., Wallace, 2020), and thus offers an ideal location to link our understanding of structural and hydrogeologic properties at subduction faults to slip behavior. We applied an anisotropic traveltime tomography analysis to this active-source dataset from 97 ocean bottom seismographs deployed with an average spacing of 2 km on four parallel lines and dense air gun shooting with a 25 m interval, and succeeded in quantitatively constraining the P-wave velocities (Vp) of the upper plate forearc and the subducting slab and their azimuthal anisotropy in three dimensions. The velocity models revealed some locations with significant Vp azimuthal anisotropy over 5 % near the splay faults in the low-velocity accretionary wedge and the deformation front. This finding suggests that the anisotropy is not ubiquitous and homogeneous within the upper plate, but more localized in the vicinity of active thrust faults. While the fast axes of Vp are mostly oriented in the trench-normal direction in the accretionary wedge, which is interpreted as results of preferentially oriented cracks in a compressional stress regime associated with the plate convergence, they are rotated to the trench-parallel direction on the seaward side of the trench and in the landward backstop. This regional variation is consistent with the results of shear-wave splitting analysis (Zal et al., 2020) and the directions of maximum horizontal stress inferred from the borehole breakouts at two IODP drilling sites (Wallace et al., 2019). The significant magnitudes of anisotropy may indicate that in addition to the crack orientation, clay-rich sedimentary sequences that stack and form coherent strata along the accretionary wedge also contribute to seismic anisotropy in the subduction margin.

 

How to cite: Arai, R., Kodaira, S., Henrys, S., Bangs, N., Obana, K., Fujie, G., Miura, S., Barker, D., Bassett, D., Bell, R., Mochizuki, K., Kellett, R., Stucker, V., and Fry, B.: Seismic anisotropy structure of the northern Hikurangi margin, New Zealand, and its significance for subduction fault systems, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1403, https://doi.org/10.5194/egusphere-egu22-1403, 2022.

15:20–15:25
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EGU22-4047
Thilo Wrona, Indranil Pan, Rebecca Bell, Christopher Jackson, Robert Gawthorpe, Haakon Fossen, and Sascha Brune

Understanding how normal faults grow is critical to an accurate assessment of seismic hazards, for successful exploration of natural (including low-carbon) resources and for safe subsurface carbon storage. Our current knowledge of fault growth is, in large parts, derived from seismic reflection data of continental rifts and margins. These seismic datasets do however suffer from limited data coverage and resolution. In addition, detailed fault mapping in increasingly large seismic reflection data requires a large amount of expertise and time from interpreters. Here we map faults across the entire northern North Sea rift using a combination of supervised deep learning and broadband 3-D seismic reflection data. This approach requires us to interpret <0.1% of the data for training and allows us to extract almost 8000 individual normal faults across a 161 km wide (E-W), 266 km long (N-S) and 20 km deep volume. We find that rift faults form incredibly complex networks revealing a previously-unrecognised variability in terms of fault length, density and strike. For instance, while we observe up to 75.9 km long faults extending from the Stord Basin and Bjørgvin Arch in the south into the Uer and Lomre Terrace to the north, most faults (>90%) are closely spaced (< 5 km) and relatively short (<10 km long). Moreover, these faults show a large range of strikes varying from NW-SE to NE-SW with two dominant fault strikes (NE-SW & NW-SE) almost perpendicular to each other. This observation is difficult to reconcile with previous studies on the extension directions during rifting of the northern North Sea. While previous studies suggest that pre-existing shear zones control faulting in the northern North Sea, we only observe faults aligning with the southern parts of the Lomre shear zone and the eastern parts of the Ninian shear zones, but none of the other eight previously mapped shear zones. Instead we think that these variations in fault strike could occur naturally through the complex evolution of fault networks. As such our innovative approach allows us to map faults across the entire northern North Sea revealing complex networks, which challenge many conventional views of fault growth during continental rifting.

How to cite: Wrona, T., Pan, I., Bell, R., Jackson, C., Gawthorpe, R., Fossen, H., and Brune, S.: Complex fault growth controls 3-D rift geometry: Insights from deep learning of seismic reflection data from the entire northern North Sea rift, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4047, https://doi.org/10.5194/egusphere-egu22-4047, 2022.

15:25–15:30
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EGU22-9287
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ECS
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Virtual presentation
Shashank Verma and Dibakar Ghosal

Ongoing under-thrusting between Indian and Eurasian plate poses a serious concern to millions of human lives residing in the foreland Himalayan. A devastating earthquake in the Central Seismic Gap (CSG) is anticipated in various studies with a possible estimated slip of ~10 m, thereby a thorough analysis of the foreland set up is essential. Present study aims the Kumaon Himalaya of the CSG using the high-resolution seismic sections along profiles PGR4, PGR5 and PGR6. Profiles PGR4 and PGR5 trend N-S direction across the Kaladungi Fault (KF) and Himalayan Frontal Thrust (HFT), whereas the PGR6 trends E-W in the Indo-Gangetic plain. Here we mostly observe Siwalik formation and alluvial deposits of the Dabka and Baur rivers. PGR4 and PGR5 clearly show evidence of south verging faults that displace the Siwalik formation. We observe Upper, Middle and Lower Siwalik rocks at ~0.82 and 0.62 s, ~1.38 and 1.27s, and ~1.88 and 1.85 s TWTT in the footwall and hanging wall sides of KF, respectively. Sedimentary deposits near the KF is highly fractured and host multiple traces of the Fault among which two reach to the surface inferring these are active. We further observe highly folded top sediments with evidence of fault bend folding at North of the KF. In the Indo-Gangetic plain, we observe gentle folding in the Lower Siwalik deposits and trace of a south verging fault that meets the Main Himalayan Thrust (MHT) at ~3 s TWTT displacing the Lower Siwalik deposits by ~ 0.0368 s TWTT. Evidence of such folding and displacements in the formation reveal that foreland Himalaya is conducive to rupture propagation of any major earthquakes developed along MHT over the CSG.

How to cite: Verma, S. and Ghosal, D.: Shallow crustal architecture of the foreland Kumaon Himalaya analysing high-resolution seismic data, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9287, https://doi.org/10.5194/egusphere-egu22-9287, 2022.

15:30–15:35
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EGU22-6589
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ECS
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On-site presentation
Richard Davy, Laura Frahm, Rebecca Bell, Joanna Morgan, Ryuta Arai, Nathan Bangs, Stuart Henrys, and Daniel Barker

The northern Hikurangi subduction margin hosts shallow slow-slip events (SSEs) and multiple historic tsunami earthquakes. The physical mechanisms and properties of the subduction interface which enable these dual modes of fault rupture remain largely enigmatic. In 2017-2018, the NZ3D seismic experiment was conducted offshore of Gisborne to image the structure of the overriding plate and subduction interface and infer the physical properties within the region of shallow SSEs. This experiment included a 3D seismic volume collected with four 6 km long streamers, ocean-bottom seismometers, and land stations. Early results from this project have demonstrated the successful application of 2D full-waveform inversion (FWI) to high frequencies along selected seismic inlines.

Here, we present the initial results of acoustic 3D FWI on the collected streamer data. Compared with 2D FWI, 3D FWI benefits from greater azimuthal coverage and the ability to relocate out-of-plane arrivals accurately but is restricted by increased calculation times and file sizes. Velocity models reveal a complex system of thrust faulting, horst and graben structures and bottom-simulating reflectors within the accretionary prism, as well as the decollement below the accretionary prism. Velocity inversions across the imaged thrust faults in the accretionary prism indicate the presence of fluids, potentially supporting the hypothesis that the subduction interface has elevated pore-fluid pressures, which are drained along some thrust faults. Velocity inversions are also observed across bottom-simulating reflectors, which indicate the presence of gas hydrates and free gas. Imaging the shallow decollement reveals an acoustically transparent region of low velocity contrasts in the inferred location of a subducted seamount.

How to cite: Davy, R., Frahm, L., Bell, R., Morgan, J., Arai, R., Bangs, N., Henrys, S., and Barker, D.: Early results from 3D full-waveform inversion imaging of the slow slip region at the shallow Hikurangi Subduction Margin, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6589, https://doi.org/10.5194/egusphere-egu22-6589, 2022.

15:35–15:40
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EGU22-991
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ECS
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On-site presentation
Samuel Zappalá, Alireza Malehmir, Tae-Kyung Hong, Junhyung Lee, Bojan Brodic, Dongchan Chung, Christopher Juhlin, Byeongwoo Kim, Myrto Papadopoulou, Seongjun Park, Jeongin Lee, and Dongwoo Kil

The Korean peninsula is considered a stable intraplate setting with few large magnitude earthquakes and in terms of seismic risk, a low-risk region. However, the peninsula is crosscut by crustal-scale fault systems, some of which may be active or have a potential of reactivation. After the Tohoku-Oki earthquake (Mw 9.0, 2011) offshore Japan, subsequent larger magnitude seismic events were registered along some of the fault systems in the South Korean portion of the peninsula. Following these, seismic risk in the area has been given more attention with several initiatives to study the current state of the seismicity in the region and to better understand the geometry and role of these crustal-scale fault systems.

To provide information on the geometry of subsurface structures causing the seismic events, a number of locations were identified for high-resolution reflection seismic imaging. In November 2020, the first active-source seismic profiles in the region (P1 and P2) were acquired with a total length of approximately 14 km with the aim to better understand the correlation between one of the main faults, the Chugaryeong fault system, and the seismicity in the area. A novel data acquisition survey consisting of a 120-unit micro-electromechanical sensors (MEMS-based) seismic landstreamer and 290 wireless recorders was employed to allow both near-surface and deep imaging of structures. Profile P1, 5 km long, was acquired on the outskirt of Seoul and P2, 9 km long, was acquired in the central part of the city. Acquiring P2 was a significant challenge given that the Seoul metropolitan area is densely populated. Difficulty to obtain good geophone-ground coupling and anthropogenic noise severely degraded the data quality. Nonetheless, final seismic sections from both profiles show encouraging results, particularly along P1 where much deeper imaging was possible (up to 9 km depth). An integrated processing work flow was required to take advantage of both the landstreamer and wireless data and this proved to be instrumental for improved imaging and subsequent interpretation.

Along P1 a clear correlation between seismic event clusters and reflection intersections (at two depth intervals of 4.5-5 km and 8-9 km) is observed, suggesting that seismic triggering is coupled to the fault intersections at depth. P2 shows strong westerly-dipping reflections with similar characteristics to the ones seen along P1, but only visible from the near surface to around 1200 m depth. It was not possible to map these faults deeper along P2, probably due to the noise conditions, thus no correlation between fault intersections and seismicity could be made. The encouraging reflection seismic results from both profiles, motivated the acquisition of a much longer profile (P3, 40 km) crossing three major fault systems in 2021. It lies in between P1 and P2 and a similar acquisition strategy was used as before. Preliminary results are ready and these are currently being interpreted together with other seismological and geological information from the area.

How to cite: Zappalá, S., Malehmir, A., Hong, T.-K., Lee, J., Brodic, B., Chung, D., Juhlin, C., Kim, B., Papadopoulou, M., Park, S., Lee, J., and Kil, D.: High-resolution reflection seismic imaging of fault systems in Metropolitan Seoul, South Korea, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-991, https://doi.org/10.5194/egusphere-egu22-991, 2022.

15:40–15:45
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EGU22-11546
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ECS
Implications of Acoustic Full Waveform Inversion on high-resolution Andaman datasets
(withdrawn)
Vikas Vats, Dibakar Ghosal, and Sai Kumar
15:45–15:50
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EGU22-2940
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ECS
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On-site presentation
Heather Kennedy, Amy Gilligan, and Katrin Löer

Faults and fractures are crucial parameters for geothermal systems as they provide secondary permeability allowing fluids to circulate and heat up in the subsurface. In this study, we use an ambient seismic noise technique referred to as the three-component (3C) beamforming to detect and monitor faults and fractures at a geothermal field in Mexico.

Three-component (3C) beamforming extracts the polarizations, azimuths, and phase velocities of coherent waves as a function of frequency, providing a detailed characterisation of the seismic wavefield. In this study, 3C beamforming of ambient seismic noise is used to determine surface wave velocities as a function of depth and propagation direction. Anisotropic velocities are assumed to relate to the presence of faults giving an indication of the maximum depth of permeability, a vital parameter for fluid circulation and heat flow throughout a geothermal field.

We perform 3C beamforming on ambient noise data collected at the Los Humeros Geothermal Field (LHGF) in Mexico. The LHGF is situated in a complicated geological area, being part of a volcanic complex with an active tectonic fault system. Although the LHGF has been exploited for geothermal resources for over three decades, the field has yet to be explored at depths greater than 3 km. Thus, it is currently unknown how deep faults and fractures permeate and the LHGF has yet to be exploited to its full capacity.

3C beamforming was used to determine if the complex surface fracture system permeates deeper than is currently known. Our results show that anisotropy of seismic velocities does not decline significantly with depth, suggesting that faults and fractures, and hence permeability, persist below 3 km. Moreover, estimates of fast and slow directions, with respect to surface wave velocities, indicate the orientation of faults with increasing depth. The North-East and North-West orientation of the fast direction corresponds to the orientation of the Arroyo Grande and Los Humeros faults respectively. Various other orientations of anisotropy align with other major faults within the LHGF at depths permeating to 6 km.

How to cite: Kennedy, H., Gilligan, A., and Löer, K.: Constraints on Fracture Distribution in Geothermal Fields Using Seismic Noise Beamforming, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2940, https://doi.org/10.5194/egusphere-egu22-2940, 2022.

15:50–15:55
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EGU22-1926
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ECS
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On-site presentation
Simone Masoch, Michele Fondriest, Rodrigo Gomila, Erik Jensen, Giulia Magnarini, Javier Espinosa, Karin Hofer, Tom Mitchell, José Cembrano, Giorgio Pennacchioni, and Giulio Di Toro

Fault zone architecture controls, for instance, the nucleation, propagation and arrest of individual seismic ruptures, the moment magnitude of the mainshocks and the evolution in space and time of foreshock and aftershock seismic sequences. Nevertheless, the architecture of crustal-scale seismogenic sources is still poorly known. Here, we examine the architecture of the >40-km-long, Mesozoic seismogenic Bolfin Fault Zone (BFZ) of the Atacama Fault System (Northern Chile). The exceptionally well-exposed BFZ cuts through plutonic rocks of the Coastal Cordillera and was seismically active at 5-7 km depth and ≤ 300 °C in a fluid-rich environment. The BFZ includes multiple fault core strands consisting of chlorite-rich cataclasites-ultracataclasites and pseudotachylytes, surrounded by chlorite-rich protobreccias to protocataclasites over a zone as wide as 75 m. These cataclastic units are associated with a damage zone, up to 150-m-thick, which comprises strongly altered and brecciated rock volumes, and with clusters of epidote-rich fault-vein networks located at the linkage of the BFZ with other faults. The architecture of the BFZ is the result of fault core widening by cyclic co-seismic frictional melting and post-to-inter-seismic fault healing due to hydrothermal (chlorite + epidote ± K-feldspar) mineral precipitation plus pervasive, possibly associated with mainshocks and aftershocks, damaging of the surrounding rocks. Additionally, we interpret the epidote-rich fault-vein networks as an exhumed seismic source of fluid-driven earthquake swarm-type sequences in agreement with seismological observations of presently active magmatic and hydrothermal regions. 

How to cite: Masoch, S., Fondriest, M., Gomila, R., Jensen, E., Magnarini, G., Espinosa, J., Hofer, K., Mitchell, T., Cembrano, J., Pennacchioni, G., and Di Toro, G.: Geological imaging of a crustal-scale seismogenic source in the continental crust (Bolfin Fault Zone, Atacama Fault System, Chile), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1926, https://doi.org/10.5194/egusphere-egu22-1926, 2022.

15:55–16:00
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EGU22-7059
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ECS
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On-site presentation
Sandro Truttmann, Tobias Diehl, and Marco Herwegh

Despite the generally accepted concept that most earthquakes occur along pre-existing faults, the complex 3D geometries of seismically active fault systems at depth often remain unresolved. However, earthquake nucleation and migration processes are heavily influenced by the geometries and properties of such pre-existing structures, which limits our general understanding of earthquake nucleation and fault interactions.

Under the assumption that faults are reactivated at spatially and temporally different localities, previous studies have attempted to derive fault geometries from hypocenter locations, but were usually limited by the precision of relocation techniques. Enabled by the recent advances in hypocenter relocation techniques, we present a novel Monte Carlo-based method that uses relatively relocated hypocenters and their uncertainties to image geometries, stress states and kinematics of seismically active fault systems. The application of the developed Python toolbox on a natural earthquake sequence along the Rhone-Simplon fault zone in the northern Valais (Swiss Alps) reveals active strike-slip faults with a contractional stepover. Performed stress analyses indicate varying stress states along the fault system, which has direct implications for fault properties such as the reactivation potential or the fluid transmissivity. Overall, we document the migration of an earthquake swarm across a complex strike-slip fault system at an unprecedented spatiotemporal resolution.

Our toolbox can be applied to high-precision hypocenter catalogs of natural earthquake sequences or hydraulic stimulation experiments, which could help to improve our understanding of the role of pre-existing faults on earthquake nucleation and migration processes at various scales.

How to cite: Truttmann, S., Diehl, T., and Herwegh, M.: A Monte Carlo-Based Approach to Image Active 3D Fault Systems from Relocated Hypocenters, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7059, https://doi.org/10.5194/egusphere-egu22-7059, 2022.

16:00–16:05
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EGU22-617
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ECS
Stefania Tarantino, Antonio Emolo, Guido Maria Adinolfi, Gaetano Festa, and Aldo Zollo

We developed a Bayesian technique to infer the double-couple, focal mechanism parameters (strike, dip and slip angles) of an earthquake source. The method uses 3 independent datasets: P-wave peak amplitude and polarity and S-to-P amplitude ratio wherever it is available.

The Bayesian technique works even in absence of one dataset and easily integrates any prior information about the region of study. The parameter space is explored thanks to an octree strategy. The method estimates the Posterior pdf, where the maximum likelihood parameter values (MAP model) both for the principal and auxiliary plane are chosen as the final fault mechanism solution. Furtherly, the uncertainties as the projections of the semi-axis of the 68% confidence ellipsoid centred on the MAP model are provided.

The joint use of the three datasets allows to determine a solution even in the case of a limited number of stations that have recorded the event, which is the case for example for small magnitude earthquakes (M<3).

We applied and tested the methodology to a microearthquake sequence (ML 0.4-3.0) occurred in the Irpinia region, South Italy, using an uninformative prior distribution for the parameters. In this area, the background seismicity occurs in a volume delimited by the faults activated during the 1980 Irpinia M 6.9 earthquake. This faults system is complex and composed of northwest–southeast striking normal faults along the Apennines chain. A network of 3-component accelerometers and velocimeters is currently monitoring the area (Irpinia Seismic NETwork).

We inferred the focal mechanism of the earthquakes of the sequence. Our results show fault mechanism solutions which are consistent with previous studies, well reflecting the regional stress field. The focus on micro-seismicity can reveal characteristics useful to highlight behaviours of larger scale seismicity.

How to cite: Tarantino, S., Emolo, A., Adinolfi, G. M., Festa, G., and Zollo, A.: A Bayesian probabilistic approach to estimate the focal mechanism of micro-earthquakes occurring at the Irpinia fault system, southern Italy., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-617, https://doi.org/10.5194/egusphere-egu22-617, 2022.

16:05–16:10
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EGU22-10564
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On-site presentation
Guido Russo, Grazia De Landro, Ortensia Amoroso, Nicola D'Agostino, Raffaella Esposito, Antonio Emolo, and Aldo Zollo

Repeated tomographic inversions in time (the so called 4D tomography) track physical properties and stress changes in the medium hosting fault systems by measuring changes in P and S seismic velocities. These changes may provide insights on fault system dynamics and earthquake triggering mechanisms. We applied 4D tomography to the volume embedding the Irpinia Fault System (IFS, southern Italy) using more than ten years of continuous seismicity monitoring. The IFS is one of the Italian most hazardous fault systems, being able to generate the 1980 Ms 6.9 earthquake, characterized by a multi-segmented rupture. Seismicity was divided into uneven epochs having almost the same spatial resolution of the volume hosting the IFS.

The resulting images show time-invariant features, clearly related to crustal lithology, and time-changing (up to 20%) velocity anomalies in the central region. Vp, Vs and Vp/Vs anomalies are referred to the tomographic model obtained using all the data set, and occur at depths ranging between 1 and 5 km, and between 8 and 12 km. These anomalies are temporally well-correlated with groundwater recharge/discharge series and geodetic displacements during the same time intervals. This correlation provides evidence for the existence of pulsating pore pressure changes in a fractured crustal volume at depth of 8-12 km, saturated with a predominant gas phase (likely CO2) and correlated with groundwater recharge processes,  

We suggest that tomographic measurements of the Vp-to-Vs spatiotemporal changes are a suitable proxy to track the pore pressure evolution at depth in highly sensitive regions of fault systems.

How to cite: Russo, G., De Landro, G., Amoroso, O., D'Agostino, N., Esposito, R., Emolo, A., and Zollo, A.: Decade-long monitoring of seismic velocity changes at the Irpinia Fault System (southern Italy), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10564, https://doi.org/10.5194/egusphere-egu22-10564, 2022.

16:10–16:15
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EGU22-2202
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ECS
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On-site presentation
Ferdinando Napolitano, Ortensia Amoroso, Mario La Rocca, Luca De Siena, Danilo Galluzzo, Vincenzo Convertito, Raffaella De Matteis, Toshiko Terakawa, and Paolo Capuano

The Mt. Pollino area has been affected by a 4-year long seismic sequence, occurred between 2010 and 2014 and characterized by low-to-moderate seismicity and two moderate events (ML 4.3 and ML 5.0). The sequence developed as a combination of swarm-like and aftershocks. The two main earthquakes occurred late in the sequence, with a slow-slip event starting 3-4 months before the largest earthquake and lasting for a year. Despite the lack of historical and instrumental recordings of strong earthquakes (M>6), paleo-seismological investigations confirm the occurrence in the last 10,000 years of at least two M 6.5-7 earthquakes on the Pollino and Castrovillari faults, located in the SE sector of the Mt. Pollino area. Thus, the area has been marked as the widest high seismic hazard gap in Italy.

In this study we present the most recent advancements in the comprehension of the main peculiarities of the last seismic sequence and of its space and time evolution.   

New local 3D P- and S-wave tomographic images offered a detailed picture of the main lithological units involved in the sequence and more reliable earthquake hypocenter locations. The inferred velocity contrasts have been compared with 2D scattering and absorption maps computed for the area, along with total direct wave attenuation. Clusters of events of similar waveforms (cross-correlation higher than 0.8) have been selected and located applying the master-slave relative location technique. New fault mechanisms have been computed. These mechanisms allowed modeling the local stress field and performing a Focal Mechanism Tomography. Its result was an evaluation of the excess of pore fluid pressure in the volume interested by the sequence. A 1D diffusivity analysis suggests a pore fluid pressure diffusion which, in addition to the Coulomb static stress transfer, can explain the delayed triggering of the two larger events.

This work has been supported by the CORE (“sCience and human factor for Resilient sociEty”) project, funded from the European Union’s Horizon 2020 - research and innovation program under grant agreement No 101021746 and by PRIN-MATISSE (20177EPPN2) project funded by Italian Ministry of Education and Research.

How to cite: Napolitano, F., Amoroso, O., La Rocca, M., De Siena, L., Galluzzo, D., Convertito, V., De Matteis, R., Terakawa, T., and Capuano, P.: Unraveling the complexity of the Pollino (Italy) seismic gap fault system., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2202, https://doi.org/10.5194/egusphere-egu22-2202, 2022.

16:15–16:20
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EGU22-4446
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ECS
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Virtual presentation
Sukanta Sarkar, Chandrani Singh, M. Ravi Kumar, Ashwani Kant Tiwari, Arun Kumar Dubey, and Arun Singh

In this study, we have presented the first high-resolution 2-D Sn attenuation tomography image of
Arunachal Himalaya to enlighten the lithospheric structure of the area. The region is one of the
active segments of the Himalaya and least understood because of the inaccessibility and difficult
working conditions. 37 regional earthquakes within the epicentral distance of 250 - 1650 km were
recorded by 29 broadband seismic stations operated in Arunachal Himalaya covering the majority
portions of the eastern Himalaya are used in the present study.
Sn is the uppermost mantle refracted phase travelled with a velocity of 4.3 - 4.7 km/s. It
is highly sensitive to the velocity gradient and attenuation in the uppermost mantle. We have
categorised the propagation efficiencies of Sn as efficient, inefficient and blocked based on a
visual inspection. The inefficient and blocked Sn phases are observed mainly in the western side
of our study region. Further, we have obtained the Sn Q tomography model to examine lateral
variations in attenuation characteristics, employing the Two Station Method (TSM) using 567 station
pairs as input data. The central Arunachal Himalaya exhibits a low Q value (≤ 50) whereas
Tawang and the western part of Arunachal Himalaya show a high value of Q ≤ 300. The obtained
results are well correlated with the tectonic fabric of the area.

How to cite: Sarkar, S., Singh, C., Kumar, M. R., Tiwari, A. K., Dubey, A. K., and Singh, A.: 2-D Sn attenuation tomography of Arunachal Himalaya, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4446, https://doi.org/10.5194/egusphere-egu22-4446, 2022.

16:20–16:25
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EGU22-7998
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ECS
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On-site presentation
Simona Gabrielli, Aybige Akinci, Ferdinando Napolitano, Edoardo Del Pezzo, and Luca De Siena

The Amatrice-Visso-Norcia seismic sequence struck the Central Apennine (Italy) in 2016. Previous works brought to light how fluid movements likely triggered the sequence and reduced the stability of the normal fault network following the first earthquake (Amatrice, Mw6.0), and the subsequent events of Visso (Mw5.9) and Norcia (Mw6.5) mainshocks.
Seismic attenuation has the potential to visualize fluids presence and fractures in a seismic sequence and to image the effect of fluid migration in the events nucleation.

This work aims to provide 3D images of scattering and absorption at different frequency bands for two datasets, one before the sequence (March 2013-August 2016) and a second from the Amatrice-Visso-Norcia sequence (August 2016-January 2017). To measure scattering and absorption we used peak delay mapping and coda-attenuation tomography, respectively.
Previous 2D imaging of scattering and absorption showed a difference between the pre-sequence and the singular sequences at different frequency bands. Structural discontinuities and lithology control scattering losses at all frequencies, while a single high-absorption anomaly developed NNW-SSE across the seismogenic zone during the seismic sequence, probably related to the migration of deep-CO2 fluids from a deep source of trapped CO2 near the Amatrice earth.
The 3D preliminary results are in agreement with the 2D mapping, with high-scattering anomalies following the main structural and lithological elements of the Central Apennines (e.g. Monti Sibillini thrust), both during the pre-sequence and the sequence, also in depth. As for the 2D, the high absorption anomaly is widespread in the area before the Amatrice event, while it is mainly focused on the seismogenic zone during the sequence. This spatial expansion can be related to the deep migration of CO2-bearing fluids across the fault network also at seismogenic depths.

How to cite: Gabrielli, S., Akinci, A., Napolitano, F., Del Pezzo, E., and De Siena, L.: 3D Scattering and Absorption model during the 2016-2017 Central Italy Seismic Sequence, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7998, https://doi.org/10.5194/egusphere-egu22-7998, 2022.

16:25–16:30
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EGU22-9588
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On-site presentation
Felix Borleanu, Laura Petrescu, Fabrizio Magrini, Anica Otilia Placinta, Bogdan Grecu, Mircea Radulian, and Luca De Siena

The Carpathian-Pannonian region (CPR) is one of the geotectonically most exciting areas of Europe due to a diversity of tectonic processes activating in close proximity: extensional basin evolution, oceanic subduction, post-collisional volcanism, as well as active crustal deformation associated with the push of the Adria plate or the pull of the actively detaching Vrancea slab. This makes CPR an excellent natural laboratory to study the behavior of the lithosphere-asthenosphere system in a special tectonic setting. To emphasize the lateral heterogeneity and physical properties of the crust in the CPR we investigate noise data recorded by the vertical components of broadband stations that have been operational in 2007, 2009, 2010, 2011 and 2020 in Eastern Europe, kindly provided by the Romanian Seismic Network and EIDA-European Integrated Data Archive. With the advent of this large amount of data and by applying a new processing method of ambient seismic noise field based on the continuous wavelet transform, we computed cross-correlations between various station pairs to transform every available seismic station into a virtual source. The inter-station cross-correlograms were used to determine the coda quality factors (Qc) in three different period ranges (2.5–5 s, 5–10 s and 10–20 s) and invert them using a modified version of the open-access code MURAT2D to construct the highest resolution attenuation tomography of the region. By mapping the attenuation features, within the study region, our results reveal high attenuation features throughout the Bohemian Massif, Alcapa unit, and Vrancea area, as well as a strong difference in attenuation between the Pannonian Basin, and stable platform regions located in front of the Carpathians. In addition, Qc variations are larger at short period in agreement with the strong heterogeneities in the uppermost crust. Finally, our findings demonstrate that noise correlation approaches are more efficient in analyzing Qc at lower frequencies than those previously proposed for earthquake data analyses.

How to cite: Borleanu, F., Petrescu, L., Magrini, F., Placinta, A. O., Grecu, B., Radulian, M., and De Siena, L.: Seismic attenuation tomography in the Carpathian-Pannonian region from ambient seismic noise analysis, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9588, https://doi.org/10.5194/egusphere-egu22-9588, 2022.