Displays

In Central Italy more than 393 thousands people live in villages and towns located at less than 5 km distance from a known, mapped, active fault, capable of generating Mw>6 earthquake. Improving seismic risk estimates in such places requires the use of (i) informative databases of active faults and (ii) the implementation of appropriate building-codes. 
The current level of knowledge regarding activity of active faults in Central Italy has been stored in a recently compiled database (160 slip rates estimates for 88 faults). Given the complex nature of fault ruptures, we adopted a multi-fault rupture approach (SHERIFS) that accounts for both individual ruptures and multi-fault complex ruptures, involving more than one seismogenic fault section. Our earthquake rupture forecast model includes 1249 possible combinations of fault ruptures with lengths ranging from 7 to 42 km. Slip rates and associated errors are used to estimate recurrences of the ruptures assuming a  Gutenberg-Richter frequency-magnitudedistribution. The computed distribution is validated against the CPTI15 catalogue.
The multi-fault model approach and a seismogenic area approach are used to estimate damages based on published typological fragility curves for typical building classes derived from 30 years of data in Italy (Rota et al., 2006) assuming earthquake occurrence for the faults follows a Poisson time-independent process. Two fragility curves are considered here: one for reinforced concrete designed according to seismic regulations and one for masonry with irregular layout and without tie rods and tie beams, a typical typology for the region. Expected levels of damage for 150 villages and towns in Central Italy are computed for all damage states considering a 50 years risk target period. 
Results obtained with the fault approach show a much higher variability of the estimated risk depending on the location of the village/town w.r.t. the fault system and the hanging-wall/footwall location. The probability of collapse in 50 years for a typical masonry building ranges between 0.01 and 0.07 in the fault approach and 0.01 and 0.04 for the area approach. For both approaches, the probability of collapse for reinforced concrete buildings is ~90 % less than that for typical masonry structures. Even if this can be considered obvious, it must be underlined that most buildings in Italy were built before 1975 (before the first applicative decree of the seismic Italian law No. 64 of 1974). Thanks to the availability of the detailed database of active faults a strategy to prioritize resources for seismic risk reduction could be adopted.
 
Rota, M., Penna, A. & Strobbia, C. (2006). Typological fragility curves from Italian earthquake damage data. First European Conference on Earthquake Engineering and Seismology Geneva, Switzerland, 3-8 September 2006 Paper Number: 386

How to cite: Scotti, O., Visini, F., Benedetti, L., Boncio, P., Faure Wlaker, J., Pace, B., Peruzza, L., and Roberts, G.: On the importance of fault modelling for seismic risk estimate , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19434, https://doi.org/10.5194/egusphere-egu2020-19434, 2020.

Towards a comprehensive European fault database for induced seismic hazard research

Seismogenic faults and fault systems in tectonically active regions are extensively studied as a source of seismic hazard and especially of high magnitude natural earthquakes. Global research has already resulted in several databases and models presenting location, characteristics and kinematic behavior of such faults (e.g. GEM Global Active Faults Database, SHARE European Database of Seismogenic Faults, USGS Quaternary faults database).

Faults that are inactive under present-day geological conditions are far more abundant, yet less-well documented. Nevertheless, these faults can potentially pose significant hazards under anthropogenic activities, particularly when the stress state of such faults is influenced by adjacent active fault systems (e.g. Northern Italy). Subsurface extraction and injection of fluids can either alter the in-situ stress state to a level exceeding the critical stress threshold (e.g. through pressure-induced compaction) or reduce the fault strength to a point where natural stresses can trigger fault movements (e.g. through the invasion of fluids into the fault zone). Well-known cases are reported among others in Basel – Switzerland (geothermal stimulation), Oklahoma – US (waste water injection) and Groningen – The Netherlands (conventional hydrocarbon extraction).

Here, we present the development of a pan-European fault database by the project GeoERA-HIKE. The database incorporates the locations, geometries, characteristics and scientific references of both active and inactive faults and fault systems and will be complementary to existing databases of seismogenic faults. The database information is derived from national mapping studies and local assessments by the European Geological Survey Organizations and includes, amongst others, surface outcrop observations, geophysical monitoring, boreholes and geological modelling studies.

The primary goal of the database is to support induced hazard studies with better access to harmonized data and knowledge on fault characteristics and behavior. The correlation of fault systems across Europe with a generic semantic concepts framework provides better insight into the genetic links between active and inactive fault systems within the greater structural geological development of Europe. The integration of data from different geoscience disciplines will improve the understanding of in-situ characteristics and behavior. Ultimately, the database is intended to become a collaborative tool for future fault characterization and research by geoscience institutes.

The GeoERA-HIKE project has received funding from the European Union’s Horizon 2020 research and innovation programme under agreement No. 731166

How to cite: Van Gessel, S., Middelburg, H., Hintersberger, E., Larsen, T., Ben Rhouma, S., Diepolder, G., and Di Manna, P.: Towards a comprehensive European fault database for induced seismic hazard research, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21420, https://doi.org/10.5194/egusphere-egu2020-21420, 2020.

Most of the seismic hazard along subduction zones is posed by great tsunamigenic earthquakes associated with the interplate megathrust fault. However, crustal faults are ubiquitous along overriding continental plates, some of which have been triggered during recent megathrust earthquakes. In Chile, the 2010 Maule earthquake (M8.8) triggered a shallow M7 earthquake on the Pichilemu fault, which had not been mapped and was unknown. In fact, M~7 earthquakes have recently occurred along unknown faults in California and New Zealand, emphasizing the need for better and more detailed mapping initiatives. A first step towards a synoptic assessment of seismic hazards posed by continental faults at the national level is mapping at a homogeneous scale to allow for a systematic comparison of faults and fault systems. Here, we present the first map of active and potentially-active faults in Chile at 1:25,000 scale, which includes published studies and newly-identified faults. All the published faults have been re-mapped using LiDAR and TanDEM-X topography, where available. Using different scaling relations, we estimate the seismic potential of all crustal faults in Chile. For specific faults where we have conducted paleoseismic and tectonic geomorphic field studies (e.g., Liquiñe-Ofqui, El Yolki, Mesamavida, and Pichilemu faults) we provide new estimates of slip rate, recurrence interval, and deformation style. We propose a segmentation model of continental faults systems in Chile, which are associated with distinct morphotectonic units and have predominant kinematics and relatively uniform slip rates. Using stress transfer models, we explore the potential feedbacks between upper-plate deformation and the megathrust seismic cycle.

How to cite: Melnick, D., Maldonado, V., Contreras, M., Jara-Muñoz, J., Cortés-Aranda, J., Astudillo, L., Martínez, J. M., Tassara, A., and Strecker, M.: Inferring seismic hazards from a new 1:25,000 scale map of active and potentially-active continental faults in Chile, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10399, https://doi.org/10.5194/egusphere-egu2020-10399, 2020.

Probabilistic fault displacement hazard analysis (PFDHA) is needed for a numerical estimate of the displacement likely to occur at a site near an active fault in case of a surface faulting earthquake. The methodology is based on parameters describing the probability of occurrence, and the spatial distribution of the displacement on and off-fault. The methodology was created for normal faulting setting, and has been later complemented with the parameters for other slip types, especially regarding the principal fault rupturing. Based on empirical fault displacement data in the Worldwide and Unified Database of Surface Ruptures (SURE), we are presenting new regression parameters for distributed faulting for dip-slip earthquakes. The parameters are used in a computational model for assessing the surface rupture hazard near active dip-slip faults. The modelling results the probability distribution of exceeding a chosen level of displacement, and can be used in stcture design and land-use related decision making in areas where surface faulting hazard should be considered.

How to cite: Nurminen, F., Baize, S., Boncio, P., Pace, B., Scotti, O., Valentini, A., and Visini, F.: PFDHA modelling based on new empirical regressions for distributed faulting on dip-slip earthquakes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-422, https://doi.org/10.5194/egusphere-egu2020-422, 2020.

The expected surface displacement in the aftermath of an earthquake is an important issue to consider, among others, for pipeline damage. While estimates of permanent ground deformation after an earthquake event is often performed nowadays through the acquisition of Interferometric Synthetic Aperture Radar (InSAR) scenes, this method is only applicable to onshore regions.

In this work we explore possible methodologies for fault hazard assessment to be applied in offshore regions.

Methods to estimate the surface rupture hazard for faults of known location and geometry are reviewed, such as the Okada equations available in the Coulomb3 software. However since fault data may be lacking or scarce in offshore areas we also explore the availability of methods to estimate a probabilistic surface rupture assessment, to be applied within the same framework of Probabilistic Seismic Hazard Assessment studies. A simple application of both methods is presented in a hypothetic case study where an early warning system for pipeline damage inspection is required.

How to cite: Fiorini, E.: An investigation of methods for Fault Displacement Hazard Assessment for offshore studies, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21876, https://doi.org/10.5194/egusphere-egu2020-21876, 2020.

The prototype version of the DISS was launched and published in July 2000. Twenty years later we present an appraisal of how the database started off, how it evolved, and how it served the seismological and engineering communities.

During the early years of its development we learned that the three fundamental requirements of any SHA-oriented fault database are:

1) the capacity to represent seismogenic sources in 3D, thus providing a standardized quantitative basis for subsequent SHA calculations and stressing the hierarchy relationships among all existing active faults;

2) the completeness, i.e. the ability to portray the vast majority of seismogenic sources existing in the region of relevance and to progressively address the emerging lack of knowledge;

3) the reliability of the geometrical parameters of each seismogenic source and of the relevant slip and strain rates, and the ability to assess the associated uncertainties.

Given these requirements, we found it hard to build a database around existing studies of individual large faults, which are often carried out for non-SHA purposes; as such they do not necessarily involve a 3D delineation and a hierarchization of the master fault. Furthermore, most published studies concern surface-breaking faults occurring onshore; they are most relevant to surface faulting hazard, but in shaking-oriented SHA they are less crucial than deeper, hidden faults.

We initially developed the concept of “Individual Seismogenic Source” (ISS), a simplified but geometrically coherent representation of the presumed causative fault of the largest earthquakes of the investigated region. An ISS is based on original observations, seismological/geophysical evidence, and literature data. Since large portions of the Italian territory are characterized by blind or hidden faulting, we developed strategies based on the analysis of geomorphic evidence for cumulative tectonic strain, on the reappraisal of commercial seismic lines and subsurface data, and on geological and geodetic evidence.

In 2005 we introduced the “Composite Seismogenic Sources” (CSSs): generalized, unsegmented sources designed to increase the database geographic coverage and completeness, based on the same type of information used for the ISSs and on regional-scale synopses of ongoing tectonic strain. Their identification was progressively extended to offshore areas, often scarcely considered in traditional fault mapping. In 2015 we also introduced the 3D definition of the subduction slabs and associated interfaces for the whole Mediterranean region.

The ISSs are routinely used in engineering applications aimed at investigating the shaking scenario associated with known earthquakes or well-identified quiescent fault segments. In contrast, the CSSs are not assumed to be capable of a specific-size earthquake; as such, they can be used in any standard PSHA procedure after estimating their activity rate and frequency magnitude distribution, based on tectonic slip rates integrated with the record of past earthquakes and GPS-determined strains, or derived from regional-scale geodynamic models.

DISS also served as a template for developing EDSF,  the European Database of Seismogenic Faults. Over the years, DISS and EDSF have become the basic geological input for PSHA and PTHA, both at Italian scale (MPS04, MPS19, MPTS19) and European scale (ESHM13, ESHM20, NEAMTHM18).

How to cite: Valensise, G., Basili, R., Burrato, P., Fracassi, U., Kastelic, V., Maesano, F. E., Tarabusi, G., Tiberti, M. M., Vallone, R., and Vannoli, P.: Italy’s Database of Individual Seismogenic Sources (DISS), 20 years on: lessons learned from the construction of a SHA-oriented fault database, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13132, https://doi.org/10.5194/egusphere-egu2020-13132, 2020.

The H2020 Project SERA (WP25-JRA3; http://www.sera-eu.org) is committed to updating and extending the 2013 European Seismic Hazard Model (ESHM13; Woessner et al., 2015, Bull. Earthquake Eng.) to form the basis of the next revision of the European seismic design code (CEN-EC8). Following the probabilistic framework established for ESHM13, the 2020 update (ESHM20) requires a continent-wide seismogenic model based on input from earthquake catalogs, tectonic information, and active faulting. The development of the European Fault-Source Model (EFSM20) fulfills the requirements related to active faulting.

EFSM20 has two main categories of seismogenic faults: crustal faults and subduction systems. Crustal faults are meant to provide the hazard model with seismicity rates in a variety of tectonic contexts, including onshore and offshore active plate margins and plate interiors. Subduction systems are meant to provide the hazard model with both slab interface and intraslab seismicity rates. The model covers an area that encompasses a buffer of 300 km around all target European countries (except for Overseas Countries and Territories, OTCs), and a maximum of 300 km depth for slabs.

The compilation of EFSM20 relies heavily on publicly available datasets and voluntarily contributed datasets spanning large regions, as well as solicited local contributions in specific areas of interest. The current status of the EFSM20 compilation includes 1,256 records of crustal faults for a total length of ~92,906 km and four subduction systems, namely the Gibraltar Arc, Calabrian Arc, Hellenic Arc, and Cyprus Arc.

In this contribution, we present the curation of the main datasets and their associated information, the criteria for the prioritization and harmonization across the region, and the main strategy for transferring the earthquake fault-source input to the hazard modelers.

The final version of EFSM20 will be made available through standard web services published in the EFEHR (http://www.efehr.org) and EPOS (https://www.seismofaults.eu) platforms adopting FAIR data principles.

The SERA project received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No.730900.

How to cite: Basili, R., Danciu, L., Carafa, M. M. C., Kastelic, V., Maesano, F. E., Tiberti, M. M., Vallone, R., Gracia, E., Sesetyan, K., Atanackov, J., Sket-Motnikar, B., Zupančič, P., Vanneste, K., and Vilanova, S.: Insights on the European Fault-Source Model (EFSM20) as input to the 2020 update of the European Seismic Hazard Model (ESHM20), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7008, https://doi.org/10.5194/egusphere-egu2020-7008, 2020.

The Betic Cordillera (S Spain), located in the convergent plate boundary between Eurasia and Nubia, is an area of moderate seismicity. These plates converge at a rate of approximately 4 to 6 mm/yr in the NW-SE direction (see review by Nocquet, 2012). Between 2.7 to 3.9 mm/yr of present-day plate convergence is accommodated in N Africa. Active shortening must occur at rates ranging from 1.6 to 2.7±0.6 mm/year across the Algero-Balearic Basin and the SE Iberian Peninsula (Serpelloni et al., 2007; Pérez-Peña et al., 2010; Echeverría et al., 2013). In the Betic Cordillera, most of the deformation is concentrated in the Betic Internal Zones, while the Betic External zones are considered as a slow-strain area.

In SE of Spain onshore active deformation and seismicity are mainly located along the Eastern Betic Shear Zone (EBSZ), a major strike-slip tectonic corridor belonging to the Betic Internal Zones. Regional and local geodetic studies indicate that the EBSZ is absorbing between 0.2 and 1.3 mm/yr (Serpelloni et al., 2007; Pérez-Peña et al., 2010; Echeverría et al., 2013; Borque et al., 2019), i.e. only a portion of regional deformation. We postulate that part of this deformation not absorbed by the EBSZ is accommodated in the eastern Betic External Zones, located to the north of the EBSZ, where several major historical earthquakes occurred (e.g., the 1748 Estubeny, 1396 Tavernes, and 2017 Caudete earthquakes). These major events have been attributed to the Jumilla Fault, the only major active structure described in this area (Giner-Robles et al. 2014; García-Mayordomo, J. and Jiménez-Díaz, A., 2015).

We present new CGPS data analysis that corroborate that the eastern Betic External Zones accommodate a significant part of the present convergence. Furthermore, our preliminary data quantify deformation in this area for the first time, as we obtain a shortening rate in the N-S direction of 1.43±0.06 mm/yr in the western sector of the Jumilla Fault (Murcia sector) and of 1.69±0.07 mm/yr in the eastern sector of the fault (Valencia sector). We propose that this deformation is likely related to the Jumilla Fault. Our study place constraints on the seismic potential of the highly populated eastern Betic External Zones, as the preliminary values that we obtained are significantly higher than those previously stated. Consequently, we propose that a re-assesment of seismic hazard is necessary for this highly populated region. Moreover, we also propose a regional geodynamic model that provide insights into mechanisms controlling earthquakes in the eastern Betic External Zones.

References

Borque et al. (2019). Tectonics, 38, 5, 1824-1839

Echeverria et al. (2013). Tectonophysics, 608, 600-612.

Giner-Robles et al. (2014). Resúmenes de la 2ª Reunión Ibérica sobre Fallas Activas y Paleosismología, Lorca, España, 155-158.

García-Mayordomo, J. and Jiménez-Díaz, A. (2015). In: Quaternary Faults Database of Iberia v.3.0 - November 2015 (García-Mayordomo et al., eds.), IGME, Madrid.

Nocquet, J.M. (2012). Tectonophysics, 579, 220-242.

Pérez-Peña et al. (2010). Geomorphology, 119, 74-87

Serpelloni et al. (2007). Geophysical Journal Internationl, 169(3), 1180-1200.

How to cite: Martin-Rojas, I., Sánchez-Alzola, A., Medina-Cascales, I., Borque, M. J., Alfaro, P., Gil, A. J., Soler-Llorens, J. L., de Lacy, M. C., Andreu, J. M., and Aviles, M.: Surface deformation deduced from CGPS data in the eastern Betic External Zones (SE Spain). Implications on SHA, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17094, https://doi.org/10.5194/egusphere-egu2020-17094, 2020.

The Western Kunlun Range (WKR) is a slowly converging orogen located along the northwestern edge of the Tibetan Plateau, facing the Tarim Basin. The recent Mw 6.4 2015 Pishan earthquake along the mountain front recalls that this region remains seismically active, despite little or moderate historical seismicity. Its low deformation rates can be hardly retrieved from current geodetic data, placing limited constraints on the potential interseismic loading of the region. This is particularly critical as recent structural investigations report the existence of an extremely wide (~150-180 km) frontal thrust sheet, whose dimensions would imply the possibility of major M ≥ 8 earthquakes in the case that it is locked and slips during one single seismic event.

To place further constraints on the seismic hazards of this region, we have conducted morphological and structural analyses of active faults to unravel the geomorphic record of active deformation cumulated other multiple seismic events at specific sites. To do so, field observations, seismic profiles and high-resolution Pléiades images and DEMs were combined together with the dating of fluvial terraces. We find that shortening rates have been of 0.5-2.5 mm/yr, with most probable values of ~2 mm/yr over the last ~300-500 kyr. Our detailed morphological investigations further indicate that this shortening is variably partitioned on one or several blind ramps along the mountain front, and from there is transmitted forward all the way to the deformation front, ~150-180 km further north. As such, this extremely wide single frontal thrust sheet stands most probably as the largest active thrust sheet in the world!

Finally, previously published GPS velocity fields highlight a 2-3 mm/yr gradient in horizontal velocities across the WKR and southern Tarim basin when combined and expressed in a stable Tarim reference. Such gradient, unseen from previous analyses, is consistent with our morphological results on shortening rates. Most importantly, this spatial gradient in velocities may suggest that the frontal thrust sheet is presently partly locked, questioning the possibility of mega-earthquakes in the region.

How to cite: Simoes, M., Guilbaud, C., van der Woerd, J., Barrier, L., Tissandier, R., Li, H., Nocquet, J.-M., Pan, J., Baby, G., Si, J., and Tapponnier, P.: Assessing the seismic hazards associated with one of the largest active thrust sheets: the case of the slowly deforming Western Kunlun mountain range (Xinjiang, China)., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3678, https://doi.org/10.5194/egusphere-egu2020-3678, 2020.

The 2019 M 6.4 Searles Valley and the M 7.1 Ridgecrest earthquakes occurred in the Eastern California Shear Zone (ECSZ) between the southern tip of the Owens Valley fault and the central segment of the Garlock fault. This earthquake sequence, as shown by recent studies based on cumulative (coseismic plus postseismic) Coulomb stress (ΔCFS) modeling, is likely to have been influenced by previous earthquakes in the ECSZ, reinforcing the hypothesis that the spatial and temporal distribution of major earthquakes in this region is controlled by the location and timing of past events. In turn, the 2019 Ridgecrest sequence has likely reshaped the state of stress on neighbouring faults, and as a consequence modified the probability of occurrence of future events in the region.

Here, focusing on the Garlock fault, we calculate the cumulative ΔCFS due to several major (M ≥ 7) earthquakes which occurred in the ECSZ and surrounding areas (e.g. San Andreas fault) following the most recent event on the Garlock fault (A.D. 1450-1640), and up to and including the Ridgecrest sequence. We then use these results to evaluate the influence of stress changes due to past earthquakes on a probabilistic seismic hazard model for the Garlock fault.

In our first probabilistic model, we calculate BPT (Brownian Passage Time) curves of occurrence of a M ≥ 7 event on the central segment of the Garlock fault in the next 30 years, using recurrence time and coefficient of variation values calculated from paeloseismological data. Preliminary results show a probability of occurrence in 30 years of up to 10% when we do not consider the effect of ΔCFS. This increases to about 15% when ΔCFS effects are introduced in the model.

As a next step, we will implement a more complex segmented model for the Garlock fault, where probability calculations take into account multiple possible rupture combinations.

How to cite: Carena, S., Verdecchia, A., Valentini, A., and Pace, B.: The impact of the 2019 Ridgecrest earthquake sequence on time-dependent earthquake probabilities for the Garlock fault, California, USA, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5523, https://doi.org/10.5194/egusphere-egu2020-5523, 2020.

The Wasatch fault zone (WFZ) is one of the most studied normal fault systems in the world and one of the most hazardous in the United States as it has paleoseismic evidence of repeated Holocene surface-faulting earthquakes and occurs within the densely urbanized Wasatch Front region. Here, we develop an earthquake rupture forecast for the WFZ that quantifies the 50-year probability of all potentially damaging earthquakes above M_w 6.2. Our goal is to evaluate the impact that models of fault segmentation (i.e., hard limits on rupture extent) have on seismic hazard. We evaluate the long-term rate of ruptures on the WFZ, adapt standard inverse theory used in the Uniform California Earthquake Rupture Forecast 3, and implement a segmentation constraint where ruptures that cross primary structural complexities are penalized. Penalized ruptures have low rates or are removed from the inversion. We develop and test three segmentation models, including (1) a segmented model in which ruptures are confined to individual segments, (2) a penalized model where some multi-segment ruptures are allowed, and (3) an unsegmented model in which all ruptures are allowed, and none are penalized. Our results show that mean seismic hazard is highest in the segmented model because of more frequent moderate-magnitude (M_w 6.2–6.8) ruptures and lowest in the unsegmented models. We evaluate the change in hazard curves and maps from these segmentation models, test how other parameters such as slip rate and magnitude-scaling relations affect our results and conclude that segmentation exerts a primary control on seismic hazard. Our study demonstrates the need for additional geologic observations of prehistoric rupture extent as well as methods to include this information in hazard assessments.

How to cite: Valentini, A., DuRoss, C., Field, E., Gold, R., Briggs, R., Visini, F., and Pace, B.: Relaxing segmentation on the Wasatch fault zone: impact on seismic hazard, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-104, https://doi.org/10.5194/egusphere-egu2020-104, 2020.

In zones of distributed continental faulting, it is critical to understand how slip is partitioned onto brittle structures over both long-term millennial time scales and shorter-term earthquake cycles. Measuring earthquake slip histories on different timescales is challenging due to earthquake repeat-times being longer or similar to historical earthquake records, and a paucity of data on fault activity covering millennial to Quaternary scales in detail. Cosmogenic isotope analyses from bedrock fault scarps have the potential to bridge the gap, as these datasets track the exposure of fault planes due to earthquakes with millennial resolution. In this presentation, we show new ³⁶Cl data combined with active fault maps to document the spatial and temporal complexity of extensional faulting in western Turkey.

Extensional faulting covers an area ~460 x 460 km in western Turkey. The dynamics controlling extension are debated, but there is an overall pattern of anticlockwise rotation superimposed on N-S directed upper-plate extension related to eastern Mediterranean subduction. This has resulted in several major east-west trending extensional grabens along the western coast of Turkey and NE-SW to NW-SE trending conjugate grabens towards the southern coast and central Anatolia. The active fault map of Turkey is well characterised by the MTA (General Directorate of Mineral Research and Exploration), but recent mid-magnitude earthquakes have occurred on some un-characterised fault zones, suggesting that there is further complexity in the trace and locations of active faults. This complexity may indicate recent reactivation of pre-existing structures.

Most of the major bedrock normal fault scarps are well preserved in carbonate and marble successions distributed across the region. These scarps preserve an excellent record of Late Pleistocene to Holocene earthquake activity, which can be quantified using cosmogenic isotopes that track the exposure of the bedrock fault scarps. ³⁶Cl accumulates in the fault scarps as the footwall is progressively exhumed by earthquakes and the concentration of ³⁶Cl measured up the fault plane reflects the rate and patterns of slip. In this presentation, we utilise Bayesian modelling techniques to estimate slip histories based on new cosmogenic data from several faults across western Turkey. Each sampling site is carefully characterised using field mapping and LiDAR to ensure that fault plane exposure is due to slip during earthquakes and not sediment transport processes. We will compare several neighbouring fault zones with variable slip rates to investigate how they interact over multiple earthquake cycles, and put this temporal complexity into the context of spatial complexity, and the resultant challenges for hazard forecasts in western Turkey.

How to cite: Gregory, L., Goodall, H., Uzel, B., Sümer, Ö., Softa, M., McCaffrey, K., Shanks, R., Houseman, G., and Sözbilir, H.: Active faulting in western Turkey: the challenge of earthquake hazard estimation in a complex extensional regime based on cosmogenic isotope analyses, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18658, https://doi.org/10.5194/egusphere-egu2020-18658, 2020.

It is important to constrain the spatial distribution of strain-rate in deforming continental material because this underpins calculations of continental rheology and seismic hazard. To do so, it is becoming increasingly common to use combinations of GPS and historical and instrumental seismicity data to constrain regional strain-rate fields. However, GPS geodetic sites, whether permanent or campaign stations, tend to be widely-spaced relative to the spacing of active faults with known Holocene offsets. At the same time, the interpretation of seismicity data can be difficult due to lack of historical seismicity in cases where local fault recurrence intervals are longer than the historical record. This causes uncertainty on how regional strain-rates are partitioned in time and space, and hence with uncertainty regarding calculations of continental rheology and seismic hazard. To overcome this issue, we have gained high temporal resolution slip-rate histories for three parallel faults using in situ ³⁶Cl cosmogenic dating of the exposure of three parallel normal fault planes that have been progressively exhumed by earthquakes. We study the region around Athens, central Greece, where there also exists a relatively-dense GPS network and extensive records of instrumental and historical earthquakes. This allows to compare regional, decadal strain-rates measured with GPS geodesy with strain-rates across the faults implied by slip since ~40,000 years BP. We show that faults have all had episodic behaviour during the Holocene, with alternating earthquake clusters and periods of quiescence through time. Despite the fact that all three faults have been active in the Holocene, each fault slips in discrete time intervals lasting a few millennia, so that only one fault accommodates strain at any time. We show that magnitudes of strain-rates during the high slip-rate episodes are comparable with the regional strain-rates measured with GPS (fault strain-rates are 50-100% of the value of GPS regional strain-rate). Thus, if the GPS-derived strain-rate applies over longer time intervals, it appears that single faults dominate the strain-accumulation at any given time, with crustal deformation and seismic hazard localised within a distributed network of faults.

How to cite: Iezzi, F., Roberts, G., Faure Walker, J., Papanikolaou, I., Ganas, A., Deligiannakis, G., and Gheorghiu, D.-M.: Across-strike variations of fault slip-rates constrained using in-situ cosmogenic 36Cl concentrations. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-756, https://doi.org/10.5194/egusphere-egu2020-756, 2020.

A fundamental question in seismic hazard analysis is whether <30º-dipping low-angle normal faults (LANFs) slip seismogenically. In comparison to more steeply dipping (45-60º) normal faults, LANFs have the potential to produce stronger shaking given increased potential rupture area in the seismogenic crust and increased proximity to manmade structures built on the hanging wall. While inactive LANFs have been documented globally, examples of seismogenically active LANFs are limited. The western margin of the Panamint Range in eastern California is defined by an archetype LANF that dips west beneath Panamint Valley and has evidence of Quaternary motion. In addition, high-angle dextral-oblique normal faults displace mid-to-late Quaternary alluvial fans near the range front. To image shallow (<1 km depth), crosscutting relationships between the low- and high-angle faults along the range front, we acquired two high-resolution P-wave seismic reflection profiles. The northern ~4.7-km profile crosses the 2-km-wide Wildrose Graben and the southern ~1.1-km profile extends onto the Panamint Valley playa, ~7.5 km S of Ballarat, CA. The profile across the Wildrose Graben reveals a robust, low-angle reflector that likely represents the LANF separating Plio-Pleistocene alluvial fanglomerate and pre-Cambrian meta-sedimentary deposits. High-angle faults interpreted in the seismic profile correspond to fault scarps on Quaternary alluvial fan surfaces. Interpretation of the reflection data suggests that the high-angle faults vertically displace the LANF up to 70 m within the Wildrose Graben. Similarly, the profile south of Ballarat reveals a low-angle reflector, which appears both rotated and displaced up to 260 m by high-angle faults. These results suggest that near the Panamint range front, the high-angle faults are the dominant late Quaternary structures. We conclude that, at least at shallow (<1 km) depths, the LANF we imaged is not seismogenically active today.

How to cite: Gold, R., Stephenson, W., Briggs, R., DuRoss, C., Kirby, E., Woolery, E., Odum, J., and Delano, J.: Seismic reflection imaging of the low-angle Panamint Valley normal fault system, eastern California, USA, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5625, https://doi.org/10.5194/egusphere-egu2020-5625, 2020.

The mechanical feasibility of co-seismic displacement along low-angle normal fault systems remains an outstanding problem in tectonics.  In the southwestern Basin and Range of North America, large magnitude extension during Miocene – Pliocene time was accommodated along a regionally extensive system of low-angle detachment faults.  Whether these faults remain active today and, if so, whether they rupture during large earthquakes are questions central to understanding the geodynamics of distributed lithospheric deformation and associated seismic hazard.  Here we evaluate the geometric and kinematic relationships of fault scarps developed in Pleistocene – Holocene alluvial and lacustrine deposits with low-angle detachment faults observed along the western flank of the Panamint Range, in eastern California.  We combine analysis of high-resolution topography generated from airborne LiDAR and photogrammetry with a detailed chronology of alluvial fan surfaces and a calibrated soil chronosequence to characterize the recent activity of the fault system.  The range-front fault system is coincident with a low-angle (15-20°), curviplanar detachment fault that is linked to strike-slip faults at its southern and northern ends.  Fanglomerate deposits in the hanging wall of the detachment are juxtaposed with brecciated bedrock in the footwall across a narrow fault surface marked by clay-rich gouge.  Isochron burial dating of the fanglomerate using the ²⁶Al and ¹⁰Be requires displacement in the past ~800 ka.  The degree of soil development in younger alluvial deposits in direct fault contact with the footwall block suggest displacement along the main detachment in the past as ~80-100 ka.  The geometry of recent fault scarps in Holocene alluvium mimic range-scale variations in strike of the curviplanar detachment fault, suggesting that scarps merge with the detachment at depth.  Moreover, fault kinematics inferred from displaced debris-flow levees and from fault striae on the bedrock range front are consistent with slip on a low-angle detachment system beneath the valley.  Finally, paleoseismic results from a trench at the southern end of the fault system suggest 3-4 surface ruptures during past ~4-5 ka, the most recent of which (MRE) occurred ~330-485 cal yr BP.  Scarps related to the MRE can be traced for at least ~50 km northward along the range front and imply surface displacements of 2-4 meters during this event.  Thus, we conclude that ongoing dextral shear along the margin of the Basin and Range is, in part, accommodated by co-seismic slip along low-angle detachment faults in Panamint Valley.  Our results have important implications for the interaction of fault networks and seismic hazard in the region.

How to cite: Kirby, E., Sethanant, I. (., Gosse, J., McDonald, E., and Walker, J. D.: Geomorphic evidence for active, co-seismic slip along a low-angle normal fault: Panamint Valley, California, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11523, https://doi.org/10.5194/egusphere-egu2020-11523, 2020.

          Usually, an earthquake of magnitude Mw6.0 or greater can produce a rupture zone on the surface of the Earth’s ground. And displacements can be observed along such a rupture zone, called co-seismic displacements. Although these surface displacements are somewhat different from slip on the rupture plane of the causative fault, which is often vertical or sub-vertical, there exists a certain proportional relationship between them. It means that major slip at depth can produce bigger co-seismic displacements on the ground. As assumed above, major fault slip is generated by asperities. Thus it is possible to establish an asperity model in terms of data of ground co-sesimic displacements.

          Asperity models can be used to describe heterogeneities of the rupture plane of the fault as an earthquake source. This work follows such an idea that an asperity is defined as a region in which the slip is larger by a prescribed amount than the average slip over the entire fault. Because co-seismic displacements along a surface rupture zone depend on slip on the subsurface fault, we attempt to construct probability distribution model for a seismic source in terms of such displacements observed on the ground. Using data of 10 historical earthquakes of Ms7.0 or greater in western China, we make a statistical analysis to distributions of co-seismic displacements on surface rupture zones, yielding the probability distribution model based on a series of ratios of maximum displacements to the average ones in intervals on the rupture. Then, upon the lower and upper limit values of these ratios, we infer the asperities along the rupture zones and analyze further the relationships between asperity parameters, rupture geometries, and earthquake magnitudes based on real data of more earthquakes. Finally, we use the data of the 2001 Kunlunshan Mw7.8 event to test this approach for construction of probability model of asperity and discuss its possible application to assessment of seismic hazard. 

Keywords:  Probability model of asperity, fault slip, surface rupture, co-seismic displacement

How to cite: Li, Z. and Zhou, B.: Probability models of asperity constructed in terms of co-seismic displacements along surface rupture zones, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2792, https://doi.org/10.5194/egusphere-egu2020-2792, 2020.

The lack of an unambiguous method for determining the propagation direction of slip events on faults over significant time periods limits our understanding of the long-term stability of fault slip propagation directions. A geological means for determining the propagation direction of slip events during the growth of faults is provided by mutually cross-cutting faults and bed-parallel slip-surfaces in the Ptolemais Basin, northern Greece.

In the Kardia lignite mine, Ptolemais Basin, bed-parallel slip surfaces intermittently offset the Quaternary faults as they grew to form discontinuities on otherwise continuous fault surfaces. Subsequent fault slip increments bypassed these discontinuities to re-establish a continuous fault trace and leave an associated ‘dead’ splay. The geometry and displacement distributions at these fault/bed-parallel slip intersections record the fault displacement at the time of bed-parallel slip and whether the next fault slip increment had an upwards or downwards component to its local propagation vector.

A database (N = 88) of slip propagation directions and fault throws was derived from continuous mapping of mine faces during lignite extraction over an eight year period. The data demonstrate a clear relationship between slip propagation direction and the accumulation of fault displacement on individual faults. During the early stages of fault growth, slip events propagated almost exclusively upwards through the mined sequence, but later stages of growth are marked by slip events showing both upward and downward components of propagation. The data therefore demonstrate that the location of the point of initiation of fault slip events on these Quaternary faults varied over the fault surfaces as the faults grew.

The emergence of systematic results from our analyses suggests that cross-cutting relationships between other synchronously active structures (e.g. conjugate faults) can provide a robust means for determining the propagation directions of slip events on ancient faults at outcrop.

How to cite: Delogkos, E., Childs, C., Manzocchi, T., and Walsh, J.: Changes in the location of the nucleation point of slip events on ancient normal faults, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16378, https://doi.org/10.5194/egusphere-egu2020-16378, 2020.

Crustal-scale active normal faults dominate seismic hazard in some regions and have been intensely studied. However, the lateral tips of these structures have received relatively little attention in the literature so their geometries are poorly known. This is an important omission because locating the tips of normal faults is vital in order to define fault lengths and calculate maximum expected earthquake magnitudes. Identifying tips will be challenging if their geometries, kinematics and rates of deformation are poorly known. Consequently, incorrectly identified tips and hence fault lengths may contribute to uncertainty in Probabilistic Seismic Hazard Assessment.

We investigate the geometry, rates and kinematics of active normal faulting in the western tip zone of the South Alkyonides Fault System (SAFS) (Gulf of Corinth, Greece) by detailed fault mapping and fault offset dating using a combination of new ²³⁴U/²³⁰Th coral ages and in situ ³⁶Cl cosmogenic exposure ages on wave-cut platforms deformed by faults.

Our results reveal that there is no clear singular fault tip and that distributed deformation in the tip zone of the SAFS occurs across as many as eight faults arranged within ~700 m across strike, each of which deforms deposits and landforms associated with the 125 ka marine terrace of Marine Isotope Stage 5e. Summed throw-rates across strike achieve values as high as 1.6 mm/yr, values that approach those close to the centre of the crustal-scale fault of 2-3 mm/yr from Holocene palaeoseismology and 3-4 mm/yr from GPS geodesy. Considering the uncertainty in the location of the western tip induced by distributed faulting, the SAFS fault length is uncertain by up to ± 6%, which equates to a total maximum magnitude uncertainty of Mw 0.1.

The calculated tip displacement gradient summed across parallel faults since 125 ka for the western tip zone of the SAFS is within the upper range compared to data from other normal crustal-scale faults. We discuss stress interaction between the SAFS and a neighbouring along-strike crustal-scale fault as a potential cause of the observed fault complexity and anomalously high throw and investigate this by undertaking Coulomb stress transfer modelling. The results from the study are discussed within the context of fault-based seismic hazard assessment.

 We conclude that identifying the locations of fault tips is challenging. While the results of this study may or may not be typical of other tip zones owing to the interaction, there is a need for further studies that explore the geometry of both non-interacting and interacting fault tip zones.

How to cite: Robertson, J., Roberts, G., Iezzi, F., Meschis, M., Gheorghiu, D., Sahy, D., Bristow, C., and Sgambato, C.: Locating fault tips to aid fault length identification: an example from the Gulf of Corinth rift, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-345, https://doi.org/10.5194/egusphere-egu2020-345, 2020.

Multiple measurements of the geometry, kinematics and rates of slip across the well-exposed Auletta fault scarp (Campania, Italy) are presented, and we use these in order to investigate: (1) the spatial resolution of field measurements needed to accurately calculate a representative strain-rate for seismic hazard calculations; (2) what aspects of the geometry and kinematics would introduce uncertainty in calculated strain-rate, if those are not measured in the field. Our results show that the magnitude of the post-glacial maximum (15±3 ka) throw gradually decreases towards the tip of the fault, but variations are observed along strike, across areas of structural complexity such as along-strike bends in the fault plane where the fault dip is greater. We find that if such variations are unnoticed, different values of strain-rate would be produced, and hence different values would result in seismic hazard calculations. To demonstrate this, we calculate the strain-rate across the Auletta fault using all our measurements, and subsequently degrade the dataset removing one measurement at a time and recalculating the implied strain-rate at each step. The results show that excluding measurements can alter strain-rate results beyond 1 σ uncertainty, thus we suggest caution when using only one measurement of slip-rate along a fault for calculating hazard, as a full understanding of the potential implied errors needs consideration. Furthermore, we investigate the effect of approximating the throw profile along the fault using boxcar and triangular slip distributions; we show that this can underestimate or overestimate the strain-rate, with results in the range of 72–237% of our most detailed strain-rate calculation. We suggest that improved understanding of the potential implied errors in strain-rate calculations from field structural data should be implemented in seismic hazard calculations.

How to cite: Sgambato, C., Faure Walker, J. P., and Roberts, G. P.: Uncertainty in strain-rate from field measurements of the geometry, rates and kinematics of active normal faults: Implications for seismic hazard assessment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1409, https://doi.org/10.5194/egusphere-egu2020-1409, 2020.

Earthquakes carry important information about the current state of stress in the subsurface as well as information on the location of weaknesses. Energy exploitation activities and energy storage are inherently connected to changes in pressure in the subsurface and varying pressure rates are applied depending on the level of activity. Especially rapid changes in pressure are known to lead to induced and triggered earthquakes, and in some cases lead to reactivation of otherwise stable and unknown faults. In some cases, increased small magnitude induced seismicity is an indication of possible larger events to follow.

Small earthquakes can be elusive and hard to locate precisely due to low signal-to-noise levels, an insufficient number of seismograph stations as well as over simplified methods and subsurface models. These challenges need to be overcome to be able to more accurately relate microseismicity to anthropogenic activities, and to be able to relate earthquakes to individual faults – both known faults and faults previously unknown. We explore ways to improve the hypocenter locations of earthquakes in exploited areas, principally by studying the effects of improving velocity models from standard 1D models to 3D models. When the geology is relatively uniform over the study area and the number of seismographs is abundant, using a 1D velocity model and a relatively simple method allows for fast and efficient processing yielding useful results. However, a 1D layered velocity model is not the best approximation where anisotropy is high, in areas with localized velocity anomalies, and where substantial velocity jumps in a layered medium control the differential arrival between P- and S-waves resulting in significant depth estimation uncertainties.

We present results using the NonLinLoc software for locating small earthquakes in Denmark, Netherlands and Iceland using 3D velocity models. The location method provides good uncertainty estimates on the hypocenters. To test the quality of the hypocenters calculated with 3D velocity models, results from NonLinLoc is compared to existing hypocenter solutions for the same earthquakes. The results are evaluated with respect to deviations in hypocenters, uncertainty estimates provided by the different methods, efficiency and applicability for different geological settings. The focus of the Danish case study is the oil and gas fields in the North Sea, however due to sparse data and low seismicity the entire Danish region is included in the study. For The Netherlands the focus of the case study is the two decommissioned gas fields, Roswinkel and Castricum, where seismicity occurred after the end of production. The Icelandic case study focusses on the Reykjanes geothermal field located at the southwest point of the Reykjanes Peninsula. Most activity is natural but induced activity has been recorded near the production area of the Reykjanes power plant.

The GeoERA-HIKE project has received funding from the European Union’s Horizon 2020 research and innovation programme under agreement No. 731166

How to cite: Larsen, T. B., Martins, J. E., Kristjánsdóttir, S., Rasmussen, C., Voss, P. H., and Dahl-Jensen, T.: Advanced localization of small seismic events in Europe, case studies in Denmark, Netherlands and Iceland from the GeoERA HIKE project, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8016, https://doi.org/10.5194/egusphere-egu2020-8016, 2020.

Strain caused by earthquakes give rise to many earthquake-related hydrological changes. Mechanisms responsible for them are different from place to place, depending on whether the trigger is the static strain or dynamic strain. Theoretic calculation indicates that the great difference in dependence on epicentral distance is robust enough to discriminate them, however, few studies based on direct strain measurements have tested this hypothesis. The 2016 M6.2 Hutubi Earthquake is a reverse event occurred in the northern Chinese Tien Shan, and the coseismic strain responses have been recorded by nine 4-component RZB borehole strainmeters at the distance from near field to far field. The nearest four stations have recorded resolvable static strain responses, and all stations have perfectly recorded the dynamic strain waves. Our result shows that the difference in the dependence on distance is truly reliable to differentiate static strains from dynamic strains, the static strain is of the same magnitude with the dynamic strain in the near field, and as the distance increase to intermediate and far field, the static strain are a few magnitude smaller. Yet the ratio between them is a complex index relating to the rupturing process itself, the tectonic background, and the seismic wave radiation pattern. Furthermore, the calibrated static strain were also used to relocate the fault plane through a grid-search method, and the result shows that the seismogenic fault is surprisingly a high-angle backthrust fault. The determined fault parameters are 279°/70°/87°, which are also consistent with the aftershock distribution. It indicates that the high-angle backthrust in the Chinese Tien Shan are capable of breaking individually. Considering the high vertical displacement, and their abundance inside the Tien Shan orogenic belt, the high-angle backthrust faults may had also played a significant role in building the modern ultra-high relief in Tien Shan.

How to cite: Gong, Z., Jing, Y., and Li, H.: Static-dynamic strain response to the 2016 M6.2 Hutubi earthquake (eastern Tien Shan, NW China) recorded in a borehole strainmeter network, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1903, https://doi.org/10.5194/egusphere-egu2020-1903, 2020.

GPS velocities indicate active convergence between Adria and the European plate, leading to back-trusting in the Southern Alps, uplift in the Tauern Window and to complex strike-slip tectonics associated to the lateral escape of the Alps East of the Tauern Window. The amount of convergence transferred to the frontal part of the Alpine thrust belt is unknown, due to a lack of seismic and geologic evidence.

The active tectonic regime in the frontal part of the Eastern Alps of Austria has been analyzed trough the integration of different data sources. The orientation of the maximum horizontal stress has been inferred from the analysis of in situ failures in 62 deep boreholes with high resolution image logs (FMI and FMS) between Salzburg and Steyr, covering the frontal of the belt (Flysch and Helvetic Units, Imbricated Molasse) and its foreland (undeformed Molasse, Bohemian Massif and its Mesozoic Cover). S_Hmax is oriented close to N-S, perpendicular to the front of the thrust belt, with very little variations. Furthermore, the orientation of S_Hmax is not affected by the tectonic position. Indications of perturbations of the stress field are observed only in two wells, both located in the eastern part of the study area (close to Steyr) at the northern edge of the thrust belt. In both wells the anomalous rotation of the stress induced failures occurs in an interval close to a large thrust fault located at the base of the Imbricated Molasse. Structural analysis of borehole image logs and seismic data suggests out-of-sequence thrusting as the cause of this anomaly. The observed stress perturbations suggest activity of the out-sequence-thrust fault in recent times.

The geomechanical models of 10 wells located in different tectonic/geographic positions along the Alpine Foreland Basin indicate that active thrusting is found mostly in the Flysch and Imbricated Molasse around the area of Steyr, whereas a strike slip regime prevails in the central and western sectors of the study area.

Further evidences of active thrust tectonics in the area of Steyr are provided by the record of quaternary fluvial-glacial terraces and by a morphometric analysis of creeks. The terraces of the Mindel Glaciation (MIS 12) appear to be uplifted up to 20 meters along out-of-sequence thrust faults that emplace the Helvetic Units on top of the Rheno-Danubian Flysch Units. Further active thrusting in the same area is observed in the Northern Calcareous Alps, where the profile of a few creeks displays characteristic morphometric shapes associated with mapped out-of-sequence thrust faults.

The results of this study suggest that active thrusting in the frontal part of the Eastern Alps is confined to the Eastern Sector of the study area around the city of Steyr, located a few tens of kilometers West of the area where the Bohemian Massif spur collided with the thrust belt, resulting in intense out-of-sequence thrusting in the hanging wall. This indicates a potential relationship between the observed active tectonic regimes and the subduction of the Bohemian Massif below the Eastern Alps.

How to cite: Levi, N., Habermueller, M., Exner, U., Wiesmayr, G., and Decker, K.: Present day tectonic regime in the frontal part of the Eastern Alps inferred trough an integrated approach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18369, https://doi.org/10.5194/egusphere-egu2020-18369, 2020.

The formation of pull apart basins and normal faulting at splays along the Vienna Basin strike-slip fault system resulted in the dissection of the Pleistocene river terraces of the Danube. Displacements of terrace segments are visible on the surface as fault scarps or dells what allows mapping the system of active faults. Furthermore displacement rates can be estimated from the elevation of the basis and the thickness of Quaternary fluvial sediments.

With regard to the prospective utilization of geothermal resources in the area of Vienna a research group was built (Geotief Explore 3D, funded by Wien Energie and FFG) with the objective to identify, map, and assess, Quaternary faults, because such rupture zones are not suitable for the reinjection of thermal water in view of the hazard of triggered earthquakes.

Normal splay faults define the eastern and western margins of Pleistocene Danube terraces north of Vienna. The bodies of these terraces are built up of coarse sandy gravel and sand whereas their surfaces are covered with aeolian and alluvial sediments of the last glacial. Tectonic displacements during the Pleistocene left distinct marks in the late glacial landform configuration of the terraces. Therefore many fault scarps and fault related valleys are clearly cognizable in high resolution LiDAR and satellite images.

During the last decade three distinct fault scarps of the Vienna Basin Transform Fault situated at the terrace edges could be investigated by trenching and transect analysis. Actual research has the objective to model the 3D geometry of the base of the Quaternary strata (horizon Base Quaternary) from a compilation of shallow drillings and the construction of a regional isopach map showing the thickness of Quaternary (growth-) strata.

In the course of research it becomes apparent that within the tectonically subsided areas evidence of neotectonics is overprinted by fluvial sediments and alluvium what hinders accurate localization of faults. However, the sinuosity of palaeochannels in the Danube floodplain seems to be related to tectonics and therefore the pattern of former river channels can be used as sign for tectonic activity during the Pleistocene. In places where signs for active faulting are completely overprinted by fluvial sedimentation and cryoturbation the approved methods for the localization and the assessment of active faults are electrical resistivity tomography and near-surface seismics.

How to cite: Weissl, M., Kurt, D., Flores-Orozco, A., and Steiner, M.: Analysing landforms, borehole logs, and geophysics, for localization and assessment of active faults in the central Vienna Basin (Austria), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18834, https://doi.org/10.5194/egusphere-egu2020-18834, 2020.

In the framework of the III level Seismic Microzonation of the Pieve del Grappa municipality (Treviso, NE Italy), three paleoseismological trenches were dug, in order to investigate activity and capacity of the Crespano del Grappa backthrust.

The study area is located in the Veneto foothills, where the Plio-Quaternary external front of the Eastern Southern Alps (Castellarin and Cantelli, 2000) presently propagates with a 2-3 mm/y velocity towards the south (Serpelloni et al., 2016). The external front is composed of a series of arcuated WSW-ENE striking, S verging structures (Galadini et al, 2005). Moreover, the area is characterized by a medium-to-low seismicity with only one M>6 earthquake during the last millennium: the 1695 Asolo event, Mw 6.45 (Rovida et al., 2016).

Regarding the structural framework, the study area is located between the Bassano-Vittorio Veneto Thrust to the north and the Bassano-Cornuda Thrust to the south. The investigated tectonic structure, i.e. the Bassano-Cornuda backthrust, is a N-verging E-W striking reverse structure. Moving from east to the west, it widely crops out near the Castelcucco village, causing a hundred meters displacement in the Miocene Molasse (Braga, 1970). In particular in Crespano village the thrust is responsible of an about 10 m vertical throw in the Quaternary alluvial conglomerates of Lastego river (Parinetto, 1987). Because of the urbanization, the paleoseismological trenches were realized at the eastern (Col Canil) and western (San Vito) borders of the village. In the former case, the trench cut through thick colluvial deposits that probably buried an abandoned valley. Differently, the second and the third trenches affected wide coalescent LGM alluvial fans, which border the southern slope of Mt. Grappa.

The results testify an intense Pleistocene-Holocene deformation of the Crespano del Grappa backthrust. Particularly, active deformation evidence deals with:

• back-tilting of the Holocene colluvial units;
• pronounced polyphasic liquefaction episodes, locally completely altering the sedimentary structures of colluvial units;
• a wide damage zone in the proximity of the morphological scarp and associated with the peak of the induced polarization. This observation testifies that the Crespano del Grappa backthrust reached the surface and displaced topography in the past, probably at the occurrence of one or more events which generated the paleoliquefaction effects;
• the 3-4 m displacement of the LGM alluvial fan deposits.

Concerning the age of the deformation, the dating of the involved units suggests a post LGM activation, probably recent-to-historical.

REFERENCES

Braga GP, 1970. Rendiconti Fisici dell’Accademia dei Lincei, serie 8, 48(4): 451-455.

Castellarin A. and Cantelli L., 2000. Journal of Geodynamics. DOI: 10.1016/S0264-3707(99)00036-8.

Galadini et al., 2005. Geophysical Journal International. DOI: 10.1111/j.1365-246X.2005.02571.x.

Parinetto A., 1987. Aspetti morfotettonici del versante meridionale del Grappa e delle colline antistanti. Unpublished degree thesis. University of Padova, Italy.

Rovida et al., 2016. DOI: http://doi.org/10.6092/INGV.IT-CPTI15.

Serpelloni et al., 2016. Tectonophysics, https://doi.org/10.1016/j.tecto.2016.09.026.

How to cite: Patricelli, G., Poli, M. E., Paiero, G., Monegato, G., Marinoni, F., Olivotto, M., Marchesini, A., Abu Zeid, N., Unterprentinger, E., and Zanferrari, A.: PALEOSEISMOLOGICAL INVESTIGATIONS AT THE FRONT OF THE EASTERN SOUTHERN ALPS (NE Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19681, https://doi.org/10.5194/egusphere-egu2020-19681, 2020.

Great earthquakes generated along the Himalayan mega-thrust plate boundary have been shown to rupture the surface. The Mw 8.3 1934 Bihar-Nepal earthquake is one of these major seismotectonic events. Previous studies focused on sites located at the western end of the fault trace concluded that the surface rupture associated with this earthquake is still locally preserved. Here we document a new site, along the Khutti Khola rivercut, in the core of the mesoseismal area. The effects of the earthquake in that area were described as cataclysmic, generating massive damages, landslides blocking one of the local rivers at 4 sites. The Khutti river cuts the frontal range, incising a 4 m- high cumulated scarp exposed along a 19 m-long stretch of Siwaliks claystone-sandstone and alluvial deposits. A detailed study of the river cut revealed the presence of faults emplacing Siwaliks over quaternary alluvials. These units are sealed by a colluvial wedge and wash as well as by recent underformed alluvials. The C14 radiocarbon analyses of 10 detrital charcoals collected reveal that the last surface-rupturing event at that site occurred after the 17^th century and prior to the post-bomb deposition of the young alluvials. The only historical earthquake known within that period is the 1934 earthquake, inferring that for this event the rupture reached the surface at that site. The rupture was followed by rapid aggradation and sealed by ~2 meters of sediments. In addition to being another rare example for the preservation of the 1934 earthquake, these observations demonstrate that, despite their magnitude and potential surface rupture, the study of the great Himalayan paleo-earthquakes are still challenging however necessary to constrain their lateral extent.

How to cite: Riesner, M., Bollinger, L., Rizza, M., Klinger, Y., Sapkota, S. N., Guérin, C., Karakaş, Ç., and Tapponnier, P.: Surface rupture and landscape response within the core of the great Mw 8.3 1934 earthquake mesoseismal area: the case of the Khutti Khola, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12360, https://doi.org/10.5194/egusphere-egu2020-12360, 2020.

The Alhama de Murcia Fault (AMF) is one of the most seismically active faults in the Iberian Peninsula, with important associated historical and instrumental seismicity (e.g. the 1674 I_EMS VIII and 2011 Mw 5.1 Lorca earthquakes), and numerous geomorphic and paleoseismic evidence of paleoearthquakes. It is an oblique left-lateral strike slip fault within the Eastern Betics Shear Zone (EBSZ), a nearly 500 km long fault system that absorbs a great part of convergence between the Nubian and Eurasian plates. Previous paleoseismic studies have mainly focused on the southwestern and especially the central segment of the fault and yielded slip rate values ranging from 1.0 up to 1.7 mm/yr. In the central segment (Lorca-Totana), the fault splays into several branches, the two frontal ones forming a pressure ridge. Paleoseismic trenches have exclusively been dug in the northwestern fault of the pressure ridge, where most of the displacement is along strike, while the expected reverse southeastern branch has never been directly observed.

We present the first results of paleoseismic trenching across a complete transect of the pressure ridge in the Lorca-Totana segment of AMF. To do so we excavated an exceptionally large trench (7 m deep) in the NW branch and 5 trenches in the SE branch. We have been able to: a) extend the paleoearthquake catalogue in the NW branch by interpreting a total of 13 paleoearthquakes, 6 of which were not identified in previous studies. A restoration analysis has been performed; b) unveil the existence and recent activity (Holocene) of the thrust that bounds the pressure ridge to the SE. We have interpreted at least 5 surface ruptures, with the last one being younger than 8-9 kyr BP, based on new radiocarbon dates.

The study of these two sites allows for the refinement of the seismic parameters of the fault, formerly inferred from the study of a single branch. In this sense, the more complete paleoearthquake catalogue will allow for reassessment of the recurrence intervals assigned to the fault and new slip rate estimates will be inferred by combining data from the two studied sites. Furthermore, forthcoming OSL dates may allow us to prove or reject the synchronicity of surface ruptures on both sides of the pressure ridge, shedding light on the rupturing style of this fault system during the Late Quaternary. We discuss how these new data on fault-interaction may affect several seismic parameters and their repercussion in source modelling for fault-based probabilistic seismic hazard assessments (PSHA) of the region.

How to cite: Gómez-Novell, O., Ortuño, M., García-Mayordomo, J., Masana, E., Rockwell, T., Baize, S., and Pallàs, R.: Paleo-surface ruptures at both sides of a pressure ridge along Alhama de Murcia Fault (SE Spain), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3117, https://doi.org/10.5194/egusphere-egu2020-3117, 2020.

How to cite: Lunina, O. and Denisenko, I.: Single-event throw distribution along the Delta fault (Baikal rift) from geomorphological and ground-penetrating radar investigations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-92, https://doi.org/10.5194/egusphere-egu2020-92, 2020.

We model the change of Coulomb Failure Stress (δCFS) during the Weichselian glaciation up until today at 12 locations in Latvia, Lithuania and Russia that are characterised by soft-sediment deformation structures (SSDS). If interpreted as seismites, these SSDS may point to glacially-induced fault reactivation. The δCFS suggests a high potential of such reactivation when it reaches the instability zone. We show that δCFS at all 12 locations reached this zone several times in the last 120,000 years. Most notably, all locations exhibit the possibility of reactivation after ca. 15 ka BP until today. Another time of possible activity likely happened after the Saalian glaciation until ca. 96 ka BP. In addition, some models suggest unstable states after 96 ka BP until ca. 28 ka BP at selected locations but with much lower positive δCFS values than during the other two periods. For the Valmiera and Rakuti seismites in Latvia, we can suggest a glacially-induced origin, whereas we cannot exactly match the timing at Rakuti. Given the (preliminary) dating of SSDS at some locations, at Dyburiai and Ryadino our modelling supports the interpretation of glacially-induced fault reactivation, while at Slinkis, Kumečiai and Liciškėnai they likely exclude such a source. Overall, the mutual benefit of geological and modelling investigations is demonstrated. This helps in identifying glacially-induced fault reactivation at the south-eastern edge of the Weichselian glaciation and in improving models of glacial isostatic adjustment.

This work has been published in Steffen et al. (2019).

Reference:

Steffen, H., Steffen R., Tarasov L. 2019. Modelling of glacially-induced stress changes in Latvia, Lithuania and the Kaliningrad District of Russia. Baltica, 32 (1), 78–90.

How to cite: Steffen, H., Steffen, R., and Tarasov, L.: Indication of glacially-induced fault reactivation in Latvia, Lithuania and the Kaliningrad District of Russia from models of glacial isostatic adjustment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3453, https://doi.org/10.5194/egusphere-egu2020-3453, 2020.

During the last years many studies focused on soft-sediment deformation structures (SSDS) to identify past seismic events. However, in regions that were affected by glaciations and periglacial processes like northern and central Europe, the use of SSDS as paleo-earthquake indicator is challenging. Interpretations require great care. Earthquakes are only one possible trigger mechanism of many that can cause liquefaction and/or fluidization of sediments, which leads to the formation of e.g. sand volcanoes, clastic dykes, flame structures, or ball-and-pillow structures.

SSDS triggered by seismic shaking are so-called seismites. Originally the term ‘seismites’ was used by Sailacher (1969) and only referred to sediment beds that were deformed by earthquake-related shaking. Pleistocene seismites are described from former glaciated areas in Germany, Denmark, Sweden, Poland, Latvia, and Lithuania.

However, ice-sheet loading, glaciotectonism as well as freeze and thaw processes in periglacial environments are also potential trigger mechanisms causing the formation of similar looking types of SSDS, which can be easily mistaken for seismites. Therefore, it is important to use a set of clear criteria to recognize seismites in the field.

Extensive studies of Pleistocene sediments in northern Germany have shown that deformation bands are a suitable indicator for paleo-fault activity. Deformation bands that are developed close to the tip line of a fault in combination with e.g. sand volcanoes, clastic dykes, flame structures, or ball-and-pillow structures is the most robust indicator for paleo-earthquakes.

References

 Seilacher, A. (1969). Fault-graded beds interpreted as seismites. Sedimentology, 13, 155-159.

How to cite: Müller, K., Winsemann, J., Lege, T., Spies, T., and Brandes, C.: Limitations of soft-sediment deformation structures as indicator for paleo-earthquakes in formerly periglacial and glaciated areas, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5399, https://doi.org/10.5194/egusphere-egu2020-5399, 2020.