NH4.5

Strain rates are an important factor to find areas that are under stress. Higher strain rates are usually observed along plate boundaries, while lower strain rates are found in intraplate regions. The increased availability of velocity solutions from Global Navigation Satellite System (GNSS) for entire Europe allows a 2D strain rate to be estimated at high resolution. Thus, regions of high and low strain become clearly visible. Here, we will present a new strain rate model, which is based on a recent velocity field solution by the EUREF Permanent Network Densification (EPND2100). This velocity field is obtained by the combination of weekly position SINEX solutions generated by 28 EPND Analysis Centres. More details on EPND can be found in the www.epnd.sgo-penc.hu website. The homogenized and quality checked velocity field is then interpolated via a least-square collocation using a fixed scale length of 135 km. In addition, the effect of known plate boundaries is considered during the interpolation to avoid a smoothing of nearby velocities on different tectonic plates. We also apply a moving variance approach to avoid effects of non-stationarity, which arise due to the variable station densities. The interpolated velocity model is then used to estimate a 2D strain rate covering most of Europe. We will highlight the situation in intraplate areas with very low strain rates but dense GNSS networks.

How to cite: Steffen, H., Steffen, R., Kenyeres, A., Caporali, A., Zurutuza, J., and Lidberg, M.: A strain rate model for Europe from a dense network of GNSS, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11703, https://doi.org/10.5194/egusphere-egu21-11703, 2021.

Probabilistic Seismic Hazard Assessment (PSHA) is the most common tool used to decide on the acceptable seismic risk and corresponding mitigation measures. One key component of these studies is the earthquake generation model comprising the definition of source zones and recurrence relationships. Slow deforming regions are particularly challenging for PSHA since the inferred return period for large earthquakes is longer than the instrumental and historical seismicity records, and the relationship between known or probable active faults and seismicity is uncertain. Therefore, in these areas PSHA results show a large variability that impairs its acceptance by the political decision-makers and the public in general. We propose two consistency tests to address the variability of earthquake generation models found in PSHA studies: i) one rule-of-thumb test where the seismic moment release from the model is converted to an average slip on a typical fault and compared with known plate kinematics or GNSS deformation field; ii) using a neotectonic model, the computed deformation is converted into seismic moment release and to a synthetic earthquake catalogue. We apply these tests to the W and SW Iberia slow deforming region, where two earthquake source areas are investigated: 1) the Lower Tagus Valley, one of the largest seismic risk zones of Portugal; and 2) the offshore SW Iberia area, considered to be the source for the 1^st November 1755 event (M~8.7). Our results show that some of the earthquake source models should be regarded as suspicious, given their high/low moment release when compared to the expected values from GNSS observations or neotectonic modelling. In conclusion, PSHA studies in slow deforming regions should include a similar sanity check on their models’ evaluation, downgrading the weight of poorly compliant models.

How to cite: Ramalho, M., Matias, L., Neres, M., M. C. Carafa, M., Carvalho, A., and Teves-Costa, P.: A sanity check for earthquake recurrence models used in PSHA of slow deforming regions: the case of SW Iberia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3580, https://doi.org/10.5194/egusphere-egu21-3580, 2021.

Continuous and increasingly dense geodetic monitoring in the last couple of decades has enabled resolving deformation heterogeneities in intraplate environments, where seismic hazard assessment is inhibited by low historical seismicity rates, but damaging earthquakes do occur infrequently. It has also revealed the degree of uncertainty with which we have been able to constrain how elastic strain accumulates in mid-continental faults. The St. Lawrence Valley (SLV) in east North America is the most seismically active region along a paleo-rift system in eastern Canada, and is also located around the general post-glacial rebound hinge-line. Earthquakes along the SLV are mainly located in three active seismic zones, from south to north, the Western Quebec, Charlevoix, and Lower St Lawrence Seismic Zones, but the mechanism for the spatial clustering is not clear. Along the SLV, the crustal deformation or strain rate has been calculated to date as part of global estimations or discrete regional measurements, at a resolution that does not enable detection of small-wavelength features. The aim of this work is to create a high-resolution strain rate map that can detect local changes of the deformation style to quantify possible correlation with intraplate seismicity, taking into account the slow tectonic loading rate and the interaction between ancient basement geological structures and glacial isostatic adjustment. We calculate a preliminary strain rate map with high spatial resolution using publicly available continuous GPS data from Nevada Geodetic Laboratory (NGL), with time series covering up to 20 years. We use a 2D velocity interpolation method: gpsgridder, a module from Generic Mapping Tools (GMT) that grids discrete vectors using a model based on 2D elasticity. This approach includes velocity uncertainties and performs better than biharmonic interpolations for sparse vectors because it considers coupling between the velocity components. We test spatial resolution of the method and station configuration using an approach similar to checkerboard tests applied in seismic tomographic inversions. In addition, the resolution analysis gives a spatial quantification of the reliability of the obtained continuous strain rate distribution, which is key to identify zones that can be improved in terms of GPS coverage including campaign data. We will show that for our 2-D velocity field and using a mesh grid of 0.25° X 0.25°, the method begins to resolve checkerboard lengths of ~50 km in regions where the average spacing between stations is ~40 km. Finally, we will present the length resolution of the station configuration in the SLV, along with the interpolated strain rate map.

How to cite: Meneses, G., Onwuemeka, J., Harrington, R., and Liu, Y.: Small wavelength features of the strain rate distribution for the St. Lawrence paleo-rift zone, eastern Canada, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10998, https://doi.org/10.5194/egusphere-egu21-10998, 2021.

Methods to determine seismic hazard in any region vary depending on the regional seismicity, but can be roughly grouped into two main groups: one based on probabilistic methods that use data about known seismicity in the region, and another, which is based on data related to the faulting processes and determination of seismically active faults. Both groups of methods are relatively good for seismically active regions. However, in regions of low seismic activity and slow deformations, there is neither enough data for proper probabilistic determination of seismic hazard, nor enough data about deformation that can indicate possibly active faults. Because of that, all sets of data have to be combined in order to gather necessary information needed to determine seismic hazard for a given area.

One of such regions of low seismicity and very slow deformation is the region of Carpatho-Balkan orogen, situated in Eastern Serbia. This orogen represents the western part of the Carpatho-Balkan orogenic chain, extending in the north to the Romanian Southern Carpathians and in its southeastern part to the Balkan massif in Bulgaria. In its central part, in Eastern Serbia, Carpatho-Balkanides are made up of a system of east-vergent nappes, that have been formed in Early Cretaceous and were multiply activated during their geological history. This activity led to the formation of faults that are favorably oriented in respect to the main thrust system. It is suspected that some of these fault systems are also active in recent times.

Relatively complex geological structure and existence of a large number of rock discontinuities, as well as relatively long time during which these geological units have been exposed on the surface, led to intensive karst process and formation of both surface and underground karst forms. Therefore, investigations of faults and deformations on the field surface are very difficult, but investigations of neotectonically active faults inside the karst caves can give a lot more information.

In this abstract, we present evidence about the youngest and recently active faults in the region of interest, based on data from karst caves. Age of activity of faults mapped inside the caves was determined based on indicators of faults cutting speleothems, forming fault breccias that incorporate cave sediments (broken speleothems), and based on speleogenetic considerations. Samples for radiometric dating have been collected, that will help to quantify fault activity rate.

Preliminary results show that the research area is characterized by strike-slip tectonics, most likely resulting from far-field stress generated by the collision of the Adriatic microplate, the Moesian indenter and the tectonic units in-between. Such stress field is shown to be highly heterogeneous even in this relatively small research area, so local areas of transtension and transpression have also been very important in controlling the recent fault kinematics in this part of the Carpatho-Balkanides. These preliminary conclusions are also of high importance for seismic hazard characterization.

How to cite: Mladenović, A. and Ćalić, J.: Determining seismic hazard in slowly deforming region: Can we gather enough information from karst caves?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8723, https://doi.org/10.5194/egusphere-egu21-8723, 2021.

Intact speleothems can be used as indicators for an upper limit of the level of horizontal ground motions of past earthquakes which affected the cave. Candlestick stalagmites have eigenfrequencies well in the frequency-band of regional earthquake ground motions. An earthquake can therefore break such elongated structures if the ground movement is strong enough. In the study of the response of speleothems to earthquakes, eigenfrequencies are a fundamental parameter. A study was carried out at Han-sur-Lesse (Belgium Ardennes) to estimate these frequencies for the so called Minaret stalagmite, an imposing 4.5 m tall structure.

Three-component seismic sensors were used to record the ambient noise during a period of 22 days on the stalagmite, at its base on the cave floor, and at Earth’s surface. This technique allows a precise determination of the first eigenfrequencies of the stalagmite (two firsts mode shapes) and the polarization of the motions associated with the frequencies. The use of three-component seismic sensors is a precondition to identifying these polarizations. Moreover, the horizontal motions recorded on the stalagmite show significant amplification (4 and 14 times depending on the orientation) compared to those recorded at the free surface outside the cave. The long recording period allows the measurement of transient events like earthquakes or quarry blasts.

In addition, a 3D laser scan of the stalagmite’s shape has been used to construct numerical models. The dynamic behaviour of the models is in good agreement with the measured parameters. The use of the 3D scans clearly increased accordance between model and measurements compared to simply shaped approximations of the stalagmite. The combination of measured and modelled data clearly show that the shape of the stalagmite (elliptical cross-section and shape irregularities) influences the eigenfrequencies and the polarization of the modes while also causing a near-orthogonal split of the natural frequencies.

Knowing that the shape and the height of the stalagmites evolve over time, further steps in this study will be to date the candlestick stalagmites in order to have an approximation of their height (and therefore their eigenfrequencies) during their history and to model their eigenfrequency evolution with time.

How to cite: Martin, A., Lecocq, T., Hinzen, K.-G., Camelbeeck, T., Quinif, Y., and Fagel, N.: Candlestick stalagmite’s eigenfrequency characterisation with ambient seismic noise and 3D scan, a step to support seismic hazard assessment, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1035, https://doi.org/10.5194/egusphere-egu21-1035, 2021.

The 3 April 2017 M_W 6.5, Moiyabana (Botswana) earthquake occurred in the continental interior of the African plate and in a seismogenic region previously considered as stable. We analyse the mainshock and aftershock sequence based on a local seismic network and local seismotectonic characteristics. The earthquake rupture geometry is constrained with more than 1,000 aftershocks recorded over a period of three months and from the InSAR analysis of Sentinel-1 images (ascending orbit). The mainshock (25.134 E, 22.565 S; depth 22 ± 3 km) was followed by more than 500 events of magnitude M ≥ 0.8 recorded in April 2017 including the largest aftershock (M_W 4.6 on the 5 April 2017). Focal mechanism solutions of the mainshock and aftershocks display predominance of NW-SE trending and NE dipping normal faulting. Stress inversion of focal mechanisms obtained from the mainshock and aftershock database are compatible with a NE-SW extension under normal faulting regime. The InSAR study shows fringes with two lobes with 4 to 6 cm coseismic slip on a NW-SE elongated and 30-km-long surface deformation consistent with the mainshock location and normal faulting mechanism. The modelling of surface deformation provides the earthquake rupture dimension at depth with ~ 1 m maximum slip on a fault plane striking 315°, dipping 45°, -80° rake and with M_o 7.12 10¹⁸ Nm Although the seismic strain rate is of low level, the occurrence of the 2017 Moiyabana earthquake, followed by an aftershock sequence in the central Limpopo belt classifies the intraplate region as an active plate interior. 

How to cite: Mulabisana, T., Meghraoui, M., Midzi, V., Saleh, M., Ntibinyane, O., Kwadiba, M. T., Manzunzu, B., Seiphemo, O., Pule, T., and Saunders, I.: Seismotectonic Analysis of the 2017 Moiyabana Earthquake (MW 6.5; Botswana), Insights from field investigations, aftershock and InSAR studies, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5599, https://doi.org/10.5194/egusphere-egu21-5599, 2021.

The 11-11-2019 Le Teil earthquake (Mw4.9), located in the Rhône river valley occurred along the La Rouvière fault (LRF) within the NE termination of the Cévennes faults system (CFS). This very shallow moderate magnitude and reverse-faulting event inverted an Oligocene normal fault which was not assessed to be potentially active, causing surface rupture and strong ground shaking. Its morphology shows no evidence of cumulative reverse faulting during the Quaternary. All of this information raises the question of whether the fault was reactivated for the first time since the Oligocene during the Teil earthquake, or if it had broken the surface before, during the Quaternary period, but could not be detected. In addition, it poses the question of the potential reactivation of other faults of the CFS and other faults in metropolitan France as well.

To tackle those issues, we launched paleoseismic investigations along the LRF to analyze and characterize evidences of paleo-ruptures in Quaternary deposits. Twelve trenches were dug along the section that broke in 2019. The trenches were dug in aeolian deposits and slope colluvium lying against the ancient LRF normal fault mirror carved in the Barremian limestones. Five trenches yielded favorable Quaternary deposits to document deformation suggesting that one paleo-event, maybe more, occurred with kinematic characteristics (sense of movement, amount of displacement) similar to the 2019 event. The radiocarbon dating of the deformed units (“bulks” collected from the colluvium clayey-silty matrix) suggests, in particular, that at least one event occurred in the past 13 Ka (i.e. penultimate event prior to the Teil earthquake) . The fact that these events are not preserved in the morphology is explained by the small amount of displacement and a long return period, consistent with the low strain rate measured by GPS in this region (~10^-9 yrs^-1). Our study shows that it is therefore fundamental to carry out more detailed paleoseismological investigations in metropolitan France, especially along ancient faults favorably oriented with respect to the present stress field. Those are already planned in the next coming months along other segments of the CFS.

How to cite: Ritz, J.-F., Baize, S., Ferry, M., Hannouz, E., Riesner, M., Bollinger, L., Larroque, C., Audin, L., Manchuel, K., Rizza, M., Jomard, H., Sue, C., Arroucau, P., and Billant, J.: Analyzing the paleoseismic history of the La Rouvière fault, unexpected source of the 11-11-2019, Mw4.9 Le Teil surface rupturing earthquake (Cévennes fault system, France), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13044, https://doi.org/10.5194/egusphere-egu21-13044, 2021.

Due to the low to moderate seismicity of the European Western Alps, few focal mechanisms are available to date in this region, and the corresponding current seismic stress and strain fields remain partly elusive. The development of dense seismic networks in the past decades now provides a substantial amount of seismic records down to low magnitudes. The corresponding data, while challenging to handle due to their amount and relative noise, represent a new opportunity to increase the spatial resolution of seismic deformation fields.

The aim of this study is to assess spatial variations of the tectonic regimes and corresponding stress and strain fields, which will provide new insights into active seismic deformation in this area. The dataset comprises more than 30,000 earthquakes relocalized in a 3D crustal velocity model, and more than 2200 focal mechanisms newly computed in a consistent manner. We inverted this new set of focal mechanisms through several strategies, including a seismotectonic zoning scheme and a Bayesian inversion, which provides a probabilistic 3D reconstruction of the faulting style in the Western Alps.

The global distribution of P and T axes plunges confirms a majority of transcurrent focal mechanisms in the overall alpine realm, combined with pure extension localized in the core of the belt. Extension is found clustered, instead of continuous along the backbone of the belt. Compression is robustly retrieved only in the Po plain, which lays at the limit between the Adriatic and Eurasian plates. High frequency spatial variations of the seismic deformation are consistent with surface horizontal GNSS measurements as well as with deep lithospheric structures, thereby providing new elements to constrain homogeneously deforming zones.

We interpret the ongoing seismotectonic and kinematic regimes as being controlled by the joint effects of far-field forces –imposed by the counterclockwise rotation of Adria with respect to Europe- and of buoyancy forces in the core of the belt, which together explain the high frequency patches of extension and of marginal compression overprinted on an overall transcurrent tectonic regime.

These results shed new lights on seismicity distribution and tectonic regime variations both regionally and at depth. They appear complementary to geodetic constraints on active faults and to existing structural studies, thus allowing us to bring new insights into future seismogenic zoning schemes.

How to cite: Mathey, M., Sue, C., Pagani, C., Baize, S., Walpersdorf, A., Bodin, T., Husson, L., Hannouz, E., and Potin, B.: Seismotectonics of the Western Alps: new insights on seismogenic source characterization, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14963, https://doi.org/10.5194/egusphere-egu21-14963, 2021.

In the last decade, geodetic data has become fundamental in studies of active faults, seismicity and seismic hazard. In particular, GNSS strain rates and velocities are used to constrain fault-slip rates and seismicity parameters, on the premise that these short-term (ca. 10 yr) measurements are representative of long-term (10⁴–10⁶ yr) fault activity. The Western Alps are a good example of such development in a very-low-strain region with a high-density ongoing seismic activity. There, the first-order agreement between GNSS strain rates and earthquake deformation patterns suggest that a large part of the geodetic deformation observed in the area is seismic. This correlation also suggests that geodetic strain rates can provide constraints on seismicity and seismic hazard. With a numerical modeling approach, we point out the similarities between strain rates predicted for Glacial Isostatic Adjustment (GIA) from the Last Glacial Maximum and the geodetic strain rate field, suggesting that a large part of the GNSS signal is related to GIA. However, we show that the apparent compatibility between geodetic strain rates and seismicity hides a strain rate - stress paradox. In fact, stress perturbations due to GIA are not compatible with observed seismicity, and even tend to inhibit fault activity (as observed from focal mechanisms). Thus, the Western Alps present a typical example of a tectonic system where a transient deformation process precludes, or at least strongly complexifies, the use of geodetic strain rates in seismicity and seismic hazard analyses.

How to cite: Grosset, J., Mazzotti, S., and Vernant, P.: Glacial Isostatic Adjustment as a process of deformation but not seismicity in Western Alps: Coupling geodetical strain rate and numerical modeling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10839, https://doi.org/10.5194/egusphere-egu21-10839, 2021.