TS4.2

This study presents a seismotectonic analysis of the Miocene-aged Bodensee-Hegau Graben, a major tectonic element in the northern foreland of the European Central Alps. The graben is characterized by comparatively low strain rates and low to moderate seismicity. Our study builds on the seismological analysis of earthquakes recorded by a recently densified seismometer network. The derived high-precision absolute and relative hypocenter relocations allow to identify seismogenic structures in the pre-Mesozoic basement, which we relate to bounding faults on either side of the NW-SE striking graben. A cluster of seismicity on the SW side of the graben is associated with the previously mapped Neuhausen Fault. In contrast, the seismogenic, SW-dipping bounding faults on the opposite side of the graben, between the extinct Hegau volcanic field and the Bodanrück peninsula of Lake Constance, cannot be associated with any known fault. A set of 51 focal mechanisms allows for a high-resolution analysis of kinematics and stress regime of the graben. Our results show that the bounding faults of the graben are optimally oriented to be reactivated in transtensional mode in the present-day stress field. Slip rates across the Neuhausen and Randen faults estimated from geodetic data are likely <0.1 mm/yr. In comparison with historic seismicity over the past 600 years and geomorphic field observations, these rates appear overestimated. Nevertheless, historic seismicity over the past 600 years suggests that fault dimensions and slip rates are certainly sufficient to generate M_W 5.0 earthquakes within this slowly deforming transtensive fault zone in the foreland of the Alpine collision zone.

How to cite: Diehl, T., Madritsch, H., Schnellmann, M., Spillmann, T., Brockmann, E., and Wiemer, S.: Seismotectonic evidence for present-day transtensional reactivation of the slowly deforming Bodensee-Hegau Graben in the northern foreland of the Central Alps, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6018, https://doi.org/10.5194/egusphere-egu22-6018, 2022.

The Lower Rhine Graben (LRG) is an area of slow intra-plate extension in north-western Europe. Located in a densely populated area, this rift, with moderate but rather continuous seismic activity, poses significant seismic hazard. The LRG NW-trending fault system is 200-km long and accommodates a total extension of ~0.1 +/- 0.03mm/yr. While the major active faults are well known, the activity of this complex system as a whole remains poorly understood. This is partly due to the fact that the tectonic signal issued from such low strain rates deformation is often overprinted by other natural or anthropogenic processes. Thus, previous fault models do not integrate minor structures associated with limited deformation and remain elusive about precise fault geometry and branching. A high-resolution DEM, created from Lidar-based DEMs recently available in the surrounding countries, allows us to retrieve detailed tectonic information and refine the fault traces and scarp geometry. We thus present, for the entire region, a revised and homogeneous fault map, based on morphological observations of fault scarps and offset alluvial terraces, complemented by external information from paleoseismological surveys and geophysical profiles. The high-resolution topography shows a clear difference in fault morphological expression between the eastern and western sides of the graben, with clear scarps and sharp boundaries along the eastern side and smoother cumulative scarps in the west, suggesting contrasting fault behavior across the graben. Based on this detailed mapping, we propose a new active faults model for the whole LRG, reflecting the uncertainties in fault geometry. This is compiled in a database, including several levels of fault mapping (traces, fault sections, faults, main faults), where the fault traces are ranked according to the certainty of their identification and location.

Another limitation for seismic hazard assessment in the area is the relative scarcity of fault-displacement data compared to the large number of structures. In the southern part of the graben, a well-developed terrace allows us to estimate the activity of most faults over the Quaternary, but such an extended marker is missing in the northern part of the LRG, resulting in only few localized data. To complement these offset observations, we use several 3D-geological models. After a selection of the most representative geological layers, we automatically retrieve their offsets at several locations along each fault, to obtain the spatial slip distribution at different timescales.  We observe that along individual faults, the slip profile evolves laterally and in time, showing some fault linkage, while at the scale of the graben borders the total slip does not show significant lateral variations. Moreover, although the surface-expression differs between the two sides of the graben, the total slip rates are fairly equivalent on both sides, suggesting a symmetrical extension, at least for the northern area.

All offset measurements available for different marker horizons are also included in the new LRG fault database, thus providing an integrated tool which allows the user to choose the most relevant timescale and degree of geometrical complexity for advanced seismic hazard assessment.

How to cite: Lefevre, M., Vanneste, K., Demoulin, A., and Hubert-Ferrari, A.: Quantifying fault activity over different time scales in the Lower Rhine Graben, towards an improved fault database for seismic hazard assessment., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6469, https://doi.org/10.5194/egusphere-egu22-6469, 2022.

The eastern Rhine Graben Boundary fault zone (RGBF) constitutes the oriental margin of the Upper Rhine Graben (URG) which forms part of the European Cenozoic Rift System. The URG with low to moderate intraplate seismicity is one of the most seismically active areas in the plate interiors of Central Europe. Assessing seismic hazard in intraplate Europe is challenging as modest lithospheric deformation (<1 mm/yr) resulting from far-field stresses is accommodated by slow-slip faults. The instrumental and historical earthquake catalog of the URG (dating back to 800 AD) is too short to include the complete earthquake history and, for instance, document the occurrence of large earthquakes, potentially leading to underestimate capable faults.

Identifying and characterizing active faults is essential towards a comprehensive seismic hazard assessment in the URG. Several research efforts have been made towards this direction, focusing on the western RGBF and the southern end of the eastern RGBF. However, neotectonic studies integrating the whole eastern RGBF are lacking. As a first step, we here present a study of the neotectonic imprint in the morphology of the eastern margin of the URG based on the 12 m resolution TanDEM-X DEM and the 1 m resolution DEM of Baden-Württemberg derived from LIDAR data together with data from regional geological maps. We performed geomorphological mapping of Quaternary deposits, paleoseismic features, and faults. Besides, we calculated several morphometric parameters, including mountain front sinuosity, basin asymmetry, knickpoints, and hypsometric curve analysis to depict long-term deformation. The eastern RGBF consists of several NNE-SSW parallel fault strands marked by topographic steps that constitute the boundary between the Rhine River plain and the eastern uplifted URG shoulder. We have identified along the fault landforms that appear typical of active tectonic landscapes: a) topographical scarps, b) well-defined triangular facets developed on the hillslope associated with the main fault trace, c) displaced alluvial fans, d) left-lateral channel deflections and beheaded channels, and e) hanging valleys; that allows us to prove the kinematics of the fault as transtensional left-lateral strike-slip which is consistent with the regional stress (S_{H max}). The occurrence of these neotectonic features varies along the 300 km long eastern RGBF fault, which, together with the results from the morphometric analysis, allow us to differentiate areas with differential tectonic activity suggesting fault segmentation. These results point out the seismic potential of the eastern RGBF, are critical to find suitable sites for paleoseismological trenching and are key to later propose plausible rupture scenarios for further PSHA studies.

How to cite: Pena-Castellnou, S., Baize, S., Hürtgen, J., and Reicherter, K.: Morphotectonics of the eastern Rhine Graben Boundary Fault (Germany): an active fault within the plate interiors of Central Europe, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8806, https://doi.org/10.5194/egusphere-egu22-8806, 2022.

Strong and rare or infrequent intraplate quakes in densely populated areas pose a significant risk to humans, infrastructure, and the environment. The Lower Rhine Graben is tectonically one of the most active zones in central Europe, and it is part of the European Cenozoic rift system. The destructive 1756 CE Düren Earthquake (Mw 6.4±0.3; located in western Germany), or the  1992 CE Roermond Earthquake (Mw 5.9; located in eastern Netherlands), both caused by normal faults of the Lower Rhine Graben, inflicted tremendous damage and demonstrate the need of hazard assessments and prevention in this highly industrialized area. Therefore, mapping and detecting of the traces, historical activity and kinematics of faults and related fault systems, is of high importance for hazard assessment of critical infrastructure   (i.e. pipelines, highways, lifelines) and cities in the Dutch, Belgian and German border region.

The 1756 CE Düren earthquake was one of the most destructive ones in the area, and in entire Central Europe, the observed damage (landslides, sackungen) and magnitude suggest a surface rupturing event. The causative fault is still under debate, also epicentral area and hypocentral depth remain enigmatic although different studies investaged several faults in the area, e.g. the normal Rurrand Fault or the Schafberg Fault. The Rurrand Fault does not exhibit seismic surface rupturing events younger than 2.3 ka, whereas the Schafberg Fault is much too short to produce a M > 6 event and trenching failed.

Trenching at the Birkesdorfer Sprung (or Fault), a NW-SE trending normal fault with a minimum length of c. 9 km, revealed a set of SW dipping normal faults associated with colluvial wedges and unconformities. Geophysical ground survey methods (GPR and ERT) as well as GIS-based morphotectonic analyses identified a long fault trace. Radiocarbon (¹⁴C) charcoal dating of displaced colluvial deposits revealed very young ages of c. 240 y BP and evidence for a second event older than c. 3.6 ky BP, the latter has been already described. However, this older event can be bracketed here much better in between 9.1 ± 1.5 ky BP and 3.6 ± 0.03 ky BP. Hence, in the Holocene, the recurrence period of surface rupturing earthquakes is lower than thought before and other seismic sources, such as the Birkesdorfer Fault must be considered.

How to cite: Steinritz, V., Hürtgen, J., and Reicherter, K.: A possible surface rupture of the 1756 Düren earthquake (Lower Rhine Graben), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10132, https://doi.org/10.5194/egusphere-egu22-10132, 2022.

The island of Cyprus sits at the boundary between the Anatolian and African plates, at a transition between oceanic subduction and incipient continental collision. Seismicity has been recorded here for millenia, with at least 12 town-destroying earthquakes recorded over the last 2,000 years. However, the instrumental coverage on the island has remained poor until relatively recently, and there is no bespoke velocity model or local magnitude scale, meaning that local seismicity is relatively poorly understood. Larger earthquakes, mainly to the south and west of the island, have revealed a mix of strike-slip and reverse faulting mechanisms. More enigmatic is the onshore seismicity, and questions remain over deformation within the Cyprus slab and uplift mechanisms of the Troodos ophiolite. We investigate seismicity in and around the island, in order to better understand these processes and their associated seismic hazard. We combine records of a temporary deployment of five broadband seismometers with the 13 permanent broadband seismometers on the island, as well as two accelerometers, to create a two-year local earthquake catalogue. We locate earthquakes both within the overriding Cyprus crust and the underthrusting African plate, and identify previously unrecognised seismically active regions on the island, especially around the Troodos ophiolite. We use this earthquake catalogue to constrain a new 1-D velocity model and local magnitude scale for the region. We also constrain new focal mechanisms and interpret these in the context of the regional tectonics.

How to cite: Merry, T., Bastow, I., Green, D., Ugo, F., Bell, R., Pilidou, S., and Dimitriadis, I.: The seismicity of Cyprus, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5732, https://doi.org/10.5194/egusphere-egu22-5732, 2022.

How fault segments grow and connect in regions with moderate to high seismic activity is key to assess associated hazards. Earthquakes may affect populated areas and can trigger tsunamis that threaten coastal areas and affect marine infrastructures. Regions accommodating relatively slow tectonic deformation may still enclose active fault systems capable of generating moderate to large magnitude earthquakes, albeit at long recurrence intervals (10³ to 10⁴ years). Although the Alboran Sea is currently characterised by slow tectonic deformation and by earthquakes of low to moderate magnitude, large historical and instrumental events have also occurred (i.e., the Almeria 1522 _IEMS98 VIII-IX or the Al-Idrissi 2016 M_w 6.4 earthquakes). This Neogene basin located in the westernmost Mediterranean Sea absorbs most of the convergence between the Eurasian and Nubian plates (3 - 5 mm/year) by means of four tectonic-scale fault systems: the Carboneras and Al-Idrissi left-lateral strike-slip faults, the Yusuf right-lateral strike-slip fault and the Alboran Ridge thrust.

Our study characterises the North-South fault system on the northern Alboran Sea to better understand the kinematics of the region on a larger scale. This system is proposed as the northern termination of the Al-Idrissi fault, and it may be presently evolving due to the transtensional stress field that affects the area. The first step to characterise the fault system has been to elaborate a detailed geomorphological map of the area to describe the identified scarps, their distribution, and structural relations. To achieve this, we have used very high-resolution bathymetric data (1x1 m pixel resolution) acquired with an autonomous underwater vehicle. The bathymetry shows several fault scarps striking N-S, resulting in horst and graben systems. The second step has involved the interpretation of high-resolution multichannel airgun and sparker seismic profiles running across the N-S faults. The integration of this dataset allows us to relate the morphological scarps with different normal faults interpreted in the seismic profiles. These faults cut the post-Messinian seismostratigraphic units (last 5.3 Ma) up to the seafloor, which supports that the fault system is currently active. Finally, the high segmentation of the North-South fault system and its small accumulated fault displacements supports it is in its initial stage of evolution.

How to cite: Canari Bordoy, A., Perea, H., Martínez - Loriente, S., Gràcia, E., Fernández - Blanco, D., and Llopart, J.: Seafloor expression of the deep structure during initiation of transtensional fault systems, as seen in the North-South fault system of the Alboran Sea, SE Iberia., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-514, https://doi.org/10.5194/egusphere-egu22-514, 2022.

Paleoseismology is a fundamental method to characterize the activity of faults in low to moderate strain regions such as SE Spain. Among the different parameters to characterize such activity, the slip rate is one of the most crucial for fault-based probabilistic seismic hazard assessments (PSHA) as it controls the rates of earthquake occurrence and ultimately the hazard levels likely to be exceeded in a given time period.

The Alhama de Murcia Fault (AMF) is the most active structure within the Eastern Betics Shear Zone (EBSZ), a transpressive fault system that accommodates the largest part of the Africa-Eurasia convergence in SE Iberia. The AMF has caused some of the most important earthquakes in the EBSZ since historical times, including the damaging 2011 Mw 5.2 Lorca event. In this setting, paleoseismic studies in the EBSZ have paid special attention to this fault, and particularly to its central segment (Lorca-Totana) as this is one of the most geomorphologically prominent.  Despite this, the segment comprises a wide deformation zone where the fault splays into five subparallel slip-partitioned branches, four of these still unstudied to date. We present a comprehensive paleoseismic study that integrates paleoseismic data from four out of the five branches that compose the segment. Our aim is to improve the representativeness of the geological slip rates by accounting for a nearly complete transect of the fault zone: we excavated eight new trenches across the four branches including seven fault-perpendicular and one parallel trench to measure vertical and lateral displacements, respectively. Fault slip analysis combined with OSL and radiocarbon dating allowed the calculation of slip rates for each branch and for the whole transect, as well as their variability over time.

A total net slip rate of 1.60 +0.16/-0.11 mm/yr for the past 18-15 ka is obtained, which is almost twice the previous estimations from a single fault branch (0.9±0.1 mm/yr). This points out the relevance of accounting for all structures of a fault zone for a more reliable characterization. The slip rate variability analysis depicts cyclic patterns of short slip rate accelerations followed by longer quiescence periods, some of which are interestingly similar to those identified in the neighboring Carrascoy Fault in previous studies. This may, for the first time, suggest potentially synchronous activity among faults in Iberia. The present study is therefore an important step to improve the representativeness of the slip rate estimations in the AMF, and ultimately for subsequent PSHA studies in the area. Despite this, two main challenges still need to be assessed; first, the intermittent deposition of alluvium in the area makes it difficult to have correlative time periods between sites to integrate slip rates. Second, the lack of data in one of the five fault branches and the lack of detailed 3D trenching in most branches suggests that the obtained slip rate values could be a minimum. In this sense, integrating data from new paleoseismic sites and refining the existing data would likely allow to refine the current estimations and potentially fill the present knowledge gaps.

How to cite: Gómez-Novell, O., Ortuño, M., García-Mayordomo, J., Insua-Arévalo, J. M., Rockwell, T. K., Baize, S., Martínez-Díaz, J. J., Pallàs, R., Ollé, M., and Masana, E.: Quaternary slip rates from multi-site paleoseismic analysis of a complex deformation zone in the Alhama de Murcia Fault (SE Spain): improvements and challenges, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3768, https://doi.org/10.5194/egusphere-egu22-3768, 2022.

The Italian Apennines is a slowly deforming area, despite not properly being an intraplate region. This is particularly true for the Northern Apennines, where<= 2mm/yr of extension is accommodated by low-slip rate normal faults, often organized in parallel systems partitioning the regional deformation. As a result, large earthquakes on individual faults are separated by long (>~1ka) recurrence intervals. This makes earthquake geology a fundamental tool for characterizing the seismic hazard.

The Anghiari fault is a 11 km-long segmented NE-dipping normal fault bounding the western side of the Upper Tiber Valley (Northern Apennines, Italy), and belonging to the well-known Altotiberina low-angle normal fault system. Here, we provide unprecedented evidence of the Holocene activity of the Anghiari fault through geological, geophysical and palaeoseismological investigations.

The fault is composed of at least two nearly parallel splays. One splay runs at the base of the Pleistocene Anghiari ridge, downfaulting the late Quaternary alluvial deposits of the Tiber Valley against Middle Pleistocene continental deposits. The other splay is located within the Middle Pleistocene units of the Anghiari ridge. We focus on the latter.
Detailed geomorphological analysis, geological mapping and near-surface geophysics, enabled us to select two sites for palaeoseismological trenching. Radiocarbon dating of faulted sediments provides constraints for late Holocene and historical surface faulting events significantly contributing to the estimation of the seismic hazard in the region.

How to cite: testa, A., Boncio, P., Baize, S., Mirabella, F., Pucci, S., Pauselli, C., Ercoli, M., Riesner, M., Pace, B., and Benedetti, L.: Palaeoseismological constraints on the Anghiari normal fault (Northern Apennines, Italy): first results., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10362, https://doi.org/10.5194/egusphere-egu22-10362, 2022.

   The Apennine mountain belt is a seismically active region showing coupled extensional- and compressional tectonic regimes. The bulk of the seismic energy is released along the normal-fault systems paralleling the topographic divide where earthquakes with 6.0<M_W<7.0 have occurred both in historical- and recent times. Moderately-energetic compressive/transpressive earthquakes (4.0<M_W<6.0), which occurred in the last 50 years, are associated instead with recent activity along the outer front of the fold-and-thrust belt.

   The relatively-low slip rates (1-3 mm/y), peculiar geological settings, fault systems’ immaturity hamper the assessment of Quaternary fault activity, challenging estimation of the seismic hazard.

   We present the results of multiscale-multidisciplinary approaches carried out in the Sibillini Mts and peri-Adriatic piedmont of Abruzzo and Molise regions, located in the Apennine extensional- and compressional domain, respectively. In detail:

• we investigated the area beyond the northern tip of the Mt Vettore-Mt Bove Fault (VBF), where a remarkable seismicity rate was observed after the 2016 (Mw 6.5) Norcia earthquake. We interpreted primary topographic attributes to direct geological field surveys. We compared (on-surface) evidence of distributed deformation with results coming from 3D assessment of fault slip tendency with computation of Coulomb failure function across the potential fault surfaces. We pointed out the seismogenic character of the ∼13 km-long Pievebovigliana master normal Fault (PBF), which strikes N155°E, dips SW and is in right-lateral en echelon setting with respect to the VBF. The reconstructed geometry of the immature PBF is compatible with the occurrence of Mw≥6.0 earthquakes;
• we addressed the hypothesis of late Quaternary activity along the Apennines Outer Front (SAOF), in central-southern Italy, where compressional tectonics is well documented until the Lower-Middle Pleistocene and the front is buried under Plio-Pleistocene foredeep deposits. By integrating topographic- and fluvial network analyses along with morphotectonic investigation of fluvial terraces we found, in the Abruzzo region, variable evidence of rock uplift along segments of the SAOF and inward structures, on its hanging wall. The observed pattern of anomalies is difficult to explain with long-wavelength regional uplift alone and agrees with the regional seismotectonic framework. Despite the low deformation-rate context challenging the interpretation of the topographic and geomorphic signals, the study suggests a reconsideration of late Quaternary active thrusting in central-southern Italy.

   Despite the different tectonic contexts, the study areas belong to, and the diversity in scale and resolution of the input data, the integration of different methods of investigation turned out successful while dealing with active tectonics in low-deforming-rate regions. Our results along the Apennines confirm how multidisciplinarity boosts the chance to decipher clues of active tectonics and unveil potentially seismogenic sources.

This work has received funding from DiSPuTer - University ‘G. d’Annunzio’ of Chieti-Pescara and from the European Union’s Horizon 2020 research and innovation programme, under Grant Agreement #795396.

How to cite: Ferrarini, F., Arrowsmith, J. R., de Nardis, R., Brozzetti, F., Cirillo, D., Whipple, K. X., and Lavecchia, G.: Boosting detection of active tectonics with multi-source data and integrated methods: recent outcomes from the Apennines, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3646, https://doi.org/10.5194/egusphere-egu22-3646, 2022.

Central Asia’s arid landscape provides a key natural laboratory to study the effects of slow deformation in continental interiors. Far-field stresses of the India-Eurasia collision have created major transpressive fault systems across the continent since the Cenozoic. In the 20^th century the northward progression of this deformation resulted in four major earthquakes in Mongolia, among which was the 1957 M_w 8.1 Gobi Altai earthquake in southern Mongolia. Palaeoseismic research following this event has allowed for quantification of deformation rates since the Late Pleistocene. Yet, the application of classic palaeoseismological methods disregards the possibility of more dispersed deformation, as was suggested in other continental interiors.

The 1957 earthquake ruptured ~350 km of the Bogd fault in southern Mongolia, along the mountain front of a series of Gobi Altai restraining bends just south of the Valley of (Gobi) Lakes basin. The high restraining bends are bound by small, steep alluvial fans that reflect a ~100 kyr climate cyclicity, whereas the low relief Valley of Gobi Lakes is characterized by endorheic lakes and sparsely dated large, gentle fans. To determine whether deformation during the 1957 earthquake was representative of regional deformation, we expanded the active tectonic record by increasing the spatial and temporal scales of our studies. Along the highest restraining bend, Ikh Bogd Mountain (~4,000 m asl), we confirmed vertical slip rates of <0.3 mm/yr along single fault strands. We also observed cumulative deformation and increased steepness of older alluvial fan levels, which could suggest progressive tilting by reverse faults along the mountain front. If this tilting is merely tectonically induced, uplift rates of Ikh Bogd could reach 0.9-1 mm/yr. Morphometric analyses indicate that faults in the restraining bend’s interior still affect river steepness. This could imply that multiple sub-parallel faults are active simultaneously, accumulating to the higher uplift rate suggested by fan tilting.

The basin north of Ikh Bogd comprises the endorheic Orog Nuur (lake) which is mostly fed by the Tuyn Gol (river) that drains the Hangay Mountains in central Mongolia. Its large alluvial fans are cross-cut by four tectonic lineaments that can each accommodate M~7 earthquakes and that have a cumulative vertical slip rate that is similar to the Bogd fault. This suggests that they are significant components of the regional structure, yet they were previously overlooked because the recurrence intervals of surface-rupturing events are slower than climatic rates. In the Orog Nuur Basin itself, reflection seismics indicate that Jurassic-Cretaceous extension structures were reactivated by Miocene-Present transpression. The effect these structures have on the Basin’s modern geomorphology indicates that they may still be active, although lacustrine and fluvial sediments do not reflect any tectonic activity since MIS 5 (~120 ka).

By expanding spatial and temporal scales of active tectonic studies in southern Mongolia, we show that variability in the interplay between climate, tectonics, and geomorphology can mask the complexity of a tectonic structure. By adapting methods and incorporating the different processes that affect landscapes, such studies contribute to more complete seismic hazard assessments in slowly deforming continental interiors.

How to cite: van der Wal, J. L. N., Nottebaum, V., Stauch, G., Gailleton, B., Binnie, S., Tully, J., Batkhishig, O., Lehmkuhl, F., and Reicherter, K.: Hidden Tectonics: Finding faults in (seemingly) climate controlled landscapes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1829, https://doi.org/10.5194/egusphere-egu22-1829, 2022.

Most national and international seismic regulations require quantifying seismic hazard based on probabilistic seismic hazard assessment (PSHA) methods. The probabilities of exceeding ground-motion levels at sites of interest over a future time window are determined by combining a source model and a ground-motion model. This research work aims at understanding how the measurement of strain rates by geodesy can provide constraints on the source model.

Earthquake catalogs, merging instrumental and historical data, are usually used to establish earthquake recurrence models. Although these catalogs extend over several centuries, the observation time windows are often short with respect to the recurrence times of moderate-to-large events and in some regions the recurrence models can be weakly constrained.

Here, we compute different realizations of strain rates maps over Europe using a combined velocity field (Piña Valdes et al., JGR submitted). These strain rates are compared to the source model of the new European seismic hazard model (ESHM20, Danciu et al. 2021). More precisely, the moment rates estimated from the earthquake recurrence models are compared to the geodetically-derived moment rates.

We explore the different uncertainties in both models. For geodesy, we integrate uncertainties on the velocities at each station, and the epistemic uncertainties on the different steps of the computation of the geodetic moment rate : filtering of the velocity field (outliers’ removal and smoothing) (Piña-Valdés et al, JGR submitted), parameters used to drive the strain inversion with VISR software (Shen et al. 2015), constants and formula used for the moment computation. The goal is to quantify the impact of each parameter uncertainty or decision on the moment computation.

The first results show that a correlation exists between the seismically and geodetically derived moment rates. In general, the main uncertainty is on the velocities at each stations, followed by the depth taken into account for the moment computation. In areas characterized by high activity, such as Betics or Apennines for example, the moment rates derived by both methods are comparable. In areas of lower activity, such as at the interior of plates, the error associated with geodetic measurements is of the same order of magnitude as the measured strain, and the relation between catalog-based and strain-based moment is not straightforward.

How to cite: Donniol, B., Socquet, A., Beauval, C., Piña-Valdès, J., Danciu, L., and Nandan, S.: Towards integrating information about strain rates in PSHA models in Europe: comparison between seismic moment rates from ESHM20 model and geodetic estimates, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11609, https://doi.org/10.5194/egusphere-egu22-11609, 2022.

The Ar-Hötöl surface rupture along the Khovd fault (Mongolian Altay)

Battogtokh Davaasambuu^1,2, Matthieu Ferry^1,*, Jean-Francois Ritz¹ and Ulziibat Munkhuu²

• Géosciences Montpellier, University of Montpellier, CNRS, France
• Institute of Astronomy and Geophysics, Mongolian Academy of Sciences, Mongolia

Abstract

The Khovd fault is one of the major active faults of the Altay but has been little studied. Detailed mapping based on satellite imagery shows that the Khovd structure exceeds 550 km in length and displays different types of complex rupture segmentation, fresh and mature surface ruptures and a number of co-seismic and cumulative offsets along its entire length.

We present a 1:200,000 scale map of the Ar-Hötöl surface rupture along the Khovd Fault in the Mongolian Altay, presumed to be the surface expression of a Mw ~ 7.8 earthquake that was felt regionally in 1761 CE. The detailed mapping is based on a multi-scale approach combining a range of airborne and terrestrial imaging and topographic techniques: Sentinel-2, Pleiades, TanDEM-X, UAV, and terrestrial laser scanning. This effort led to the detailed quantification of right-lateral and vertical offsets ranging from ~ 1 m to ~ 4 km over a continuous rupture length of 238 km. The distribution of the smaller offset class documents the surface deformation associated with the last surface-rupturing earthquake. Its analysis yields a robust segmentation model comprising 6 segments 18 to 55 km in length, a maximum co-seismic slip value of 4.5 m ± 0.5 m located near the center of the rupture. Our detailed remote sensing and field observations precise the varying kinematics along strike, bring new evidence of repeated faulting and confirm a moment magnitude of 7.8 ± 0.3.

The aim of the present research work is to reveal the main sources of potential destructive earthquakes by identifying the location of past large earthquakes along the fault and estimate their magnitude and recurrence period. Our results would contribute to improve seismic hazard estimation for population of the Altay Mountains.

Keywords: active fault, surface rupture, Altay Range

How to cite: Davaasambuu, B., Ferry, M., Jean-Francois, R., and Munkhuu, U.: The Ar-Hötöl surface rupture along the Khovd fault (Mongolian Altay), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-194, https://doi.org/10.5194/egusphere-egu22-194, 2022.

The western Kunlun (WK) region is characterized by weak to moderate seismicity. However the recent Pishan earthquake (Mw 6.4), which ruptured in 2015 a blind thrust of the Pishan anticline at the front of the WK range, points out the potential for larger earthquakes in this region. Previous studies highlighted the existence of a major thrust sheet, connected at depth to the fault segment that likely ruptured in 2015, spanning ~ 150-180 km, between the western Kunlun front of the chain and another active deformation front further north within the Tarim basin (Mazar Tagh ridge). This active thrust sheet has a probable slip-rate of ~ 0.5-2.5 mm/yr as derived from geological and morphotectonic indicators. Would this structure be fully locked during the interseismic period, it could lead to earthquakes of Mw ~ 8 given the rupture width under consideration.

The present work aims at studying slip partitioning and interseismic loading in this area. GPS data within the Tarim basin lack constraints due to relatively high uncertainties and to a sparse spatial distribution. We present here an InSAR time-series analysis which provides a high space and time resolution to monitor the main active structures. This analysis is however challenging due to sand dunes and vegetation, which alter the coherence of the signal, and to topographic gradients inducing atmospheric phase delays where tectonic deformation is expected. We thus rely for this study on the ForM@Ter LArge-scale multi-Temporal Sentinel-1 InterferoMetry (FLATSIM) service (Thollard et al., 2021) to process the 5 ascending and 5 descending tracks covering our area. We compare parametric signal decompositions and principal/independent components analysis in order to separate tectonic from non-tectonic signals. We finally derive a regional linear velocity map representative of tectonic motions, masking unwrapping errors, atmospheric residuals, and remaining non-tectonic signals.

These first InSAR-based velocity results obtained along the WK-Tarim area allow to discuss the potential locking of the wide thrust sheet, in the context of known moderate ruptures. It also brings new insights into the possible connections between compressive structures and large strike-slip fault systems from the WK front to northwestern Tibet. In the complex junction area of the Western Kunlun, Altyn Tagh and Karakorum faults, several strike-slip and normal faults could be identified as active over the observation period (2015-2021), with slip rates consistent with the ones derived from morphotectonic studies (~ 4-5 mm/yr), and some faults likely showing creep (Longmu-Gozha Co fault system). These results may contribute to better understand the occurrence of normal faulting earthquakes in-between the identified strike-slip segments, such as the 2020 Mw 6.3 Yutian earthquake.

How to cite: Mathey, M., Grandin, R., Lasserre, C., Simoes, M., Doin, M.-P., Durand, P., and Team, T. F.: Active deformation across the western Kunlun range, from NW Tibet to the SW Tarim Basin (China), using Sentinel-1 InSAR data, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3003, https://doi.org/10.5194/egusphere-egu22-3003, 2022.

The Australian continent, being void of plate boundaries, is often perceived as seismically quiescent. However, around 100 magnitude three or larger earthquakes are typically recorded in Australia each year, with a magnitude 6+ occurring every 8-10 years. Such intra-plate activity can pose a significant risk as they are often non-periodic, poorly understood, and sporadically recorded by sparse seismic networks across vast continents. Within Australia the distribution of intra-plate seismicity is non-uniform, but instead tends to concentrate along certain weak zones of increased activity. One such region is the eastern margin of the Gawler Craton in South Australia, one of the oldest building blocks of the continent. Recently several new temporary seismic arrays have been deployed in the region, transforming data coverage across South Australia. So far over 70 new local events have been recorded that would otherwise have gone undetected by the national network. After relocation the pattern of earthquakes becomes more spatially defined and appears to be closely tied to the edge of the Gawler Craton. Supporting evidence suggests that these events may be associated with a trans-crustal scale fault system that adds new constraints on the poorly defined craton boundary.

How to cite: Eakin, C., Agrawal, S., and O'Donnell, J. P.: Intra-plate seismicity of the Lake Eyre Basin and Gawler Craton, Australia, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6846, https://doi.org/10.5194/egusphere-egu22-6846, 2022.

Canada’s earthquake scenario catalogue is a nation-wide collection of possible earthquake rupture scenarios that allows us to understand which populations and assets will be impacted by the rupture of particular faults (or their segments). In the past, scenarios were often generated on an ad hoc basis, when they were needed by practitioners. As new information from geologic, geomorphic, geophysical, and geodetic studies become available, it is possible to model additional earthquake rupture scenarios for inclusion in Canada’s earthquake scenario catalogue, which will be crucial to providing relevant seismic hazard and risk estimates to end users such as community planners and emergency managers. This is especially valuable in the seismically active intraplate regions of eastern Canada, where the seismic risk awareness, perception and, consequently, preparedness, is relatively low. In updating this catalogue, we employed different approaches to modelling earthquake hazard and risk scenarios using the Global Earthquake Model Foundation’s (GEM) OpenQuake Engine. We conducted a ‘known events’ approach, which involved modelling representative events for historical earthquakes and potentially active faults. We also implemented a ‘systematic risk-based’ approach, which involved disaggregating the seismic risk at certain locations into the relative contributions from different seismic source zones, and ranking the seismic risk for each census subdivision (approximately aligned with municipalities) across Canada. The goal of the ‘systematic risk-based’ approach was to mitigate the irregular coverage of the existing catalogue. We compare the nature of the two catalogues for one community, taking into account the ways these kinds of catalogues are used in Canada and elsewhere. Finally, we described the overall spatial variations in seismic risk, focusing on regions where seismic zones are close to densely-populated areas, such as the offshore BC region and Cascadia subduction zone in western Canada; and the Western Quebec, Charlevoix, lower St. Lawrence, and southern Great Lakes seismic zones in eastern Canada. 

How to cite: Rimando, J., Hobbs, T., Peace, A., and Goda, K.: A comparative analysis of approaches to expanding Canada’s Earthquake Scenario Catalogue, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1341, https://doi.org/10.5194/egusphere-egu22-1341, 2022.

We use boundary element method modelling to investigate whether subduction zone coupling drives permanent forearc deformation in the northern Cascadia subduction zone. Recent work in this region shows that several active crustal faults accommodate permanent strain north of the Olympic Peninsula in Washington State, USA and British Columbia, Canada. These faults are similar in that they strike west-northwest, have oblique right-lateral slip senses, and have low slip rates (<1 mm/yr). Paleoseismic studies show that despite the region’s low permanent strain rates, these faults have produced large (~M 7) earthquakes. Therefore understanding how and why these structures accommodate permanent deformation is crucial to assessing regional seismic hazard. Previous work has hypothesized this type of permanent forearc deformation may be driven by stress resulting from interseismic subduction zone coupling. To test this hypothesis, we used a 3D boundary element method model to determine whether coupling-driven forearc deformation can account for the observed right-lateral fault slip on one of the recently studied structures, the Leech River--Devils Mountain fault. Our model predicts left-lateral slip on this fault if strain results from subduction zone coupling alone, inconsistent with the observed kinematics. Additionally, if we use our model to mimic strain partitioning, where only strain resulting from the strike-slip component of subduction zone coupling is accommodated in the forearc, the predicted fault slip is also inconsistent with observations of fault kinematics. These simplified models represent a first-order test that contradicts the hypothesis that subduction zone coupling is the primary driver of permanent forearc deformation in northern Cascadia.

How to cite: Harrichhausen, N., Loveless, J. P., Morell, K. D., Regalla, C., and Lynch, E. M.: Using numerical modelling to investigate the driving forces of permanent forearc deformation in northern Cascadia, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3735, https://doi.org/10.5194/egusphere-egu22-3735, 2022.

The evolution of strike-slip restraining bends depends on early fault geometry (e.g., bend angle & stepover distance) and material properties, yet the influence of loading rate on fault system evolution is unknown. Within viscoelastic materials, such as the crust, the relaxation of stresses depends on loading rate. Under faster strain, faults will have shorter recurrence intervals, which reduces the time for stress relaxation during the interseismic period. Because slow strain rates yield greater stress relaxation, the growth of new faults near restraining bends may depend on loading rate. While crustal restraining bends evolve under a range of strain rates, the field expressions of faulting have overprints of early and late deformation, which makes discerning the impact of early strain rate on fault growth difficult. Here, we use scaled physical experiments to directly investigate the impact of strain rate on the evolution of restraining bends. We use wet kaolin as an analog for the crust because it creates sharp faults that remain active even when the loading orientation deviates slightly from the ideal for fault slip. In addition, off-fault stresses within the wet kaolin dissipate over time just as stresses within the crust do via inelastic processes and are tracked with tests on Anton Paar MCR102 rheometer. We directly observe and record the horizontal surface deformation for three experiments with the same initial restraining bend geometry. Computer-controlled stepper motors drive a basal plate at a prescribed velocity to induce faulting within the overlying layer of wet kaolin. The three experimental loading rates of 0.25, 0.5, and 1.0 mm/min scale to crustal loading rates of 2-4, 4-8, and 11-22 mm/yr respectively. We use digital image correlation to calculate incremental displacement and strain field data from overhead photos. Restraining bend experiments with different loading rates produce different deformation histories; slower applied loading produces greater  off-fault deformation and more secondary faults. Furthermore, new oblique-slip faults that grow within the slower loading rate experiments accommodate greater slip than the new faults that grow within the faster loading rate experiments. This suggests that strike-slip fault systems in slow strain rate regions may have slip distributed among several faults whereas slip may localize along a few faults within high strain rate regions. Additionally, the restraining bend geometry becomes more open in the slower loading rate experiments due to greater off-fault deformation. The differences in fault evolution owe to the sensitivity of both the wet kaolin strength and the degree of stress relaxation to strain rate, supported by rheometer tests. The experimental data suggest that loading rate can impact strain partitioning and fault geometry in crustal faults.

How to cite: Elston, H. and Cooke, M.: Velocity influence on deformation partitioning along evolving restraining bends, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8300, https://doi.org/10.5194/egusphere-egu22-8300, 2022.

In the time frame after initial deposition but prior to lithification, sediments are frequently prone to physical, chemical, or biological disturbance. The resultant structures – commonly referred to as soft sediment deformation (SSD) - can be explained by a variety of mechanisms, each defined by a distinct set of parameters. Among the factors responsible for upward-oriented, physically-induced disturbance, two main triggering mechanisms are distinguished: (i) Fluidization of sediment, where SSD occurs as a fluid (typically saline water) passes through a layer of solid particles via areas of available pore space, and (ii) Liquefaction of unconsolidated sands, a process that commonly occurs in response to sudden loading on a bed which forces the sediment to transition from a solid to a liquefied state. Liquefaction can moreover be caused by seismic shocks. When subjected to seismic shocks, unconsolidated sand-size sediments tend to decrease in volume, which in turn produces an increase in pore-water pressure and a decrease in shear strength. A contrasting mechanism responsible for SSD is chemical disturbance which is thought to be the result of desiccation, cementation and crystal growth, thermal expansion, and contraction of partially lithified sediment during a continuous spectrum of diagenetic stages.

The origin of SSD remains a disputed topic in clastic sedimentology and a challenging task in outcrop studies. We present the first report of disturbed calcarenites in the “Pietra di Finale”, which crops out along the Ligurian coast, bordering the Ligurian Alps transect of the Western Alps. It represents an Early to Late Miocene mixed carbonate-siliciclastic coastal wedge that unconformably superimposes the Alpine metamorphic units. The "Pietra di Finale" is considered as a low strain region due to the lack of any deformation evidence, including seismic record, suggesting a Miocene tectonic quiescence in the southernmost part of the Alps. The “Pietra di Finale” can be subdivided in two formations: a basal terrigenous sequence resting below calcarenites making up the top of the formation. The calcarenitic formation displays a uniquely well-exposed assemblage of SSD features. These features comprise (i) vertical sediment expulsions recognizable by gross changes in granulometry with respect to that of the hosting sediments, (ii) carbonatic fluid-expulsion veins, (iii) lateral continuity of SSD-prone layers and (iv) sequential vertical and lateral organization of SSDs. The main aim of this study is to unravel the origin of untypically large coarse-clastic injections into the hosting calcarenites, with emphasis put on distinguishing the role of discriminating seismically from diagenetically induced sediment disturbances. Results from a multi-proxy approach: comprising a detailed study of the sedimentological characteristics at the outcrop-scale and photogrammetric investigations of the geometry of the structures and their stratigraphic occurrence; petrographic investigations of both grain and intergranular features (i.e. clasts and cement); as well as compositional and microthermometry analyses of the vein-filling cements, can yield insights into the pivotal role of the fluids as driver of seismicity-induced liquefaction and  uncommon mineralization and intrastratal sediment mobilization.

How to cite: Tamburelli, S., Mueller, P., Amadori, C., Crispini, L., Maino, M., and Menegoni, N.: Soft-sediment deformations in post-collisional calcarenites: a multi-scale descriptive approach, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9228, https://doi.org/10.5194/egusphere-egu22-9228, 2022.

Glacially triggered faulting is the release of stresses induced by the advances and retreats of ice sheets in addition to other stresses that accumulated in the lithosphere. The faulting typically occurred along pre-existing faults or weakness zones before, during or after the last ice melting. This type of faulting is mainly recognized in intraplate regions but is also proposed for some plate boundary areas. Past reactivations were probably accompanied by large-magnitude seismic events triggering hundreds of landslides and seismically induced soft-sediment deformation structures (SSDS) in the region surrounding the glacially induced faults (GIFs).

Classification criteria were developed in the 1980s and 1990s to correctly identify a GIF and distinguish it from the vast number of other faults around the globe. Reliable field evidence for reactivated faults in and (even) around many formerly glaciated areas has considerably increased the number of confirmed and probable GIFs in recent years, which were recently unified in an international database (Munier et al., 2020). It has been generally thought that GIFs, especially the so-called postglacial faults in northern Fennoscandia, were developed during a short period of time towards the end of and shortly after the deglaciation, however, new dating results from Fennoscandia documenting several episodes of fault rupture within the past 14,000 years (Ojala et al., 2018; Olesen et al., 2021) and even connected to the begin of glaciation (Sutinen & Middleton, 2021) challenge this idea. The youngest fault scarp was formed less than 600 years ago (Olesen et al., 2021).

The new findings warrant a discussion of the classification criteria. We introduce revised classification criteria for GIFs, modified from the previous criteria and for easier application expressed as a checklist, see also Steffen et al. (2021).

References

Munier, R., Adams, J., Brandes, C., et al. (2020). International database of Glacially Induced Faults. PANGAEA, https://doi.org/10.1594/PANGAEA.922705.

Ojala, A. E., Markovaara-Koivisto, M., Middleton, M., Ruskeeniemi, T., Mattila, J., Sutinen, R. (2018). Dating of paleolandslides in western Finnish Lapland. Earth Surface Processes and Landforms 43(11), 2449–2462, https://doi.org/10.1002/esp.4408.

Olesen, O., Olsen, L., Gibbons, S., Ruud, B., Høgaas, F., Johansen, T., Kværna, T. (2021). Postglacial faulting in Norway – Large magnitude earthquakes of the Late Holocene Age. In H. Steffen, O. Olesen, R. Sutinen, eds., Glacially-triggered faulting. Cambridge University Press, pp. 198– 217, https://doi.org/10.1017/9781108779906.015.

Steffen, H., Olesen, O., Sutinen, R. (2021). Glacially-triggered faulting – A historical overview and recent developments. In H. Steffen, O. Olesen, R. Sutinen, eds., Glacially-triggered faulting. Cambridge University Press, pp. 3–19, https://doi.org/10.1017/9781108779906.003.

Sutinen, R., Middleton, M. (2021). Porttipahta end moraine in Finnish Lapland is constrained to Early Weichselian (MIS 5d, Herning stadial). Geomorphology 393, 107942, https://doi.org/10.1016/j.geomorph.2021.107942.

How to cite: Steffen, H., Olesen, O., and Sutinen, R.: Revised classification criteria for glacially induced faults, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9137, https://doi.org/10.5194/egusphere-egu22-9137, 2022.