Earthquake focal mechanisms of the induced seismicity in 2006 and 2007 below Basel (Switzerland)

To stimulate the reservoir for a "hot dry rock" geothermal project, that was initiated by a private/public consortium in the city of Basel, approximately 11500 m3 of water were injected between December 2nd and 8th, 2006, at high pressures into a 5km deep well. More than 10500 seismic events were recorded during the injection phase, and minor sporadic seismic activity was still occurring more than two years later. The present article documents the focal mechanisms of the 28 strongest events, with M L between 1.7 and 3.4, that have been obtained by the Swiss Seismological Service (SED) during and after the stimulation. The analysis is based on data that was recorded by a six-station borehole network, operated by the project developers, as well as by several permanent and temporary surface networks. The hypocenters of the events are located inside the stimulated rock volume at depths between 4 and 5km within the crystalline basement. Of the 28 faultplane solutions two are normal faulting mechanisms and one is a strike-slip mechanism with a strong normal component. All others are typical strike-slip mechanisms with mostly NS and EW striking nodal planes. As a consequence, the T-axes are all nearly horizontal and oriented in a NE or SW direction (mean azimuth 46 ± 11 degrees) and the P-axes of the strike-slip events point in a NW or SE direction (mean azimuth 138 ± 13 degrees). Overall, the observed focal mechanisms agree with what would be expected from both the stress observations within the well and the stress field derived from the previously known natural seismicit

Multi-scale imaging of a slow active fault zone: contribution for improved seismic hazard assessment in the Swiss Alpine foreland

Seismic hazard assessment of slow active fault zones is challenging as usually only a few decades of sparse instrumental seismic monitoring is available to characterize seismic activity. Tectonic features linked to the observed seismicity can be mapped by seismic imaging techniques and/or geomorphological and structural evidences. In this study, we investigate a seismic lineament located in the Swiss Alpine foreland, which was discussed in previous work as being related to crustal structures carrying in size the potential of a magnitude M 6 earthquake. New, low-magnitude (−2.0 ≤ ML ≤ 2.5) earthquake data are used to image the spatial and temporal distribution of seismogenic features in the target area. Quantitative and qualitative analyses are applied to the waveform dataset to better constrain earthquakes distribution and source processes. Potential tectonic features responsible for the observed seismicity are modelled based on new reinterpretations of oil industry seismic profiles and recent field data in the study area. The earthquake and tectonic datasets are then integrated in a 3D model. Spatially, the seismicity correlates over 10–15 km with a N–S oriented sub-vertical fault zone imaged in seismic profiles in the Mesozoic cover units above a major decollement on top of the mechanically more rigid basement and seen in outcrops of Tertiary series east of the city of Fribourg. Observed earthquakes cluster at shallow depth (<4 km) in the sedimentary cover. Given the spatial extend of the observed seismicity, we infer the potential of a moderate size earthquake to be generated on the lineament. However, since the existence of along strike structures in the basement cannot be excluded, a maximum M 6 earthquake cannot be ruled out. Thus, the Fribourg Lineament constitutes a non-negligible source of seismic hazard in the Swiss Alpine foreland

Earthquake focal mechanisms and stress orientations in the eastern Swiss Alps

This study presents an updated set of earthquake focal mechanisms in the Helvetic and Penninic/Austroalpine domains of the eastern Swiss Alps. In eight cases, based on high-precision relative hypocentre locations of events within individual earthquake sequences, it was possible to identify the active fault plane. Whereas the focal mechanisms in the Helvetic domain are mostly strike-slip, the Penninic/Austroalpine domain is dominated by normal-faulting mechanisms. Given this systematic difference in faulting style, an inversion for the stress field was performed separately for the two regions. The stress field in the Penninic/Austroalpine domain is characterized by extension oriented obliquely to the E-W strike of the orogen. Hence, the Penninic nappes, which were emplaced as large-scale compressional structures during the Alpine orogenesis, are now deforming in an extensional mode. This contrasts with the more compressional strike-slip regime in the Helvetic domain towards the northern Alpine front. Relative to the regional stress field seen in the northern Alpine foreland with a NNW-SSE compression and an ENE-WSW extension, the orientation of the least compressive stress in the Penninic/Austroalpine domain is rotated counter-clockwise by about 40°. Following earlier studies, the observed rotation of the orientation of the least compressive stress in the Penninic/Austroalpine region can be explained as the superposition of the regional stress field of the northern foreland and a uniaxial extensional stress perpendicular to the local trend of the Alpine mountain bel

Seismic wave attenuation from borehole and surface records in the top 2.5km beneath the city of Basel, Switzerland

We investigate attenuation (Q−1) of sediments of 2.5-3.5km thickness underneath the city of Basel, Switzerland. We use recordings of 195 induced events that were obtained during and after the stimulation of a reservoir for a Deep Heat Mining Project in 2006 and 2007. The data set is ideally suited to estimate Q as all events are confined to a small source volume and were recorded by a dense surface network as well as six borehole sensors at various depths. The deepest borehole sensor is positioned at a depth of 2.7km inside the crystalline basement at a mean hypocentral distance of 1.8km. This allows us to measure Q for frequencies between 10 and 130 Hz. We apply two different methods to estimate Q. First, we use a standard spectral ratio technique to obtain Q, and as a second measure we estimate Q in the time domain, by convolving signals recorded by the deepest sensor with a Q operator and then comparing the convolved signals to recordings at the shallower stations. Both methods deliver comparable values for Q. We also observe similar attenuation for P- and S- waves (QP∼QS). As expected, Q increases with depth, but with values around 30-50, it is low even for the consolidated Permian and Mesozoic sediments between 500 and 2700

Rupture complexity of a moderate intraplate earthquake in the Alps: the 1996M5 Epagny-Annecy earthquake

The magnitude 5 Epagny-Annecy earthquake of 1996 July 15 is the largest seismic event to have occurred in the Alps since the introduction of modern digital instrumentation. This strike-slip event was located on the Vuache Fault, near the town of Annecy, in the northern French Alps. The aim of our work was to retrieve the main parameters of the rupture process of this earthquake from seismograms recorded at local and regional distances (20-300 km). To eliminate path and site effects from the seismograms, we compared the main shock recordings at each station with those of the largest aftershocks nearby. We used a combination of techniques, including pulse-width measurements and cross-correlation of velocity traces, comparison of P-wave displacement pulses, and empirical Green's function deconvolution, to retrieve the apparent duration of the rupture process as seen at each station. Our results demonstrate that, in the absence of on-scale data, P-wave pulse-width measurements on clipped signals can be misleading if the rupture process is complex. In the case of the Annecy earthquake, comparisons of on-scale P-wave displacement seismograms and the empirical Green's function deconvolutions show that the rupture process consisted of at least two subevents separated by 0.2-0.3 s, and with a total duration of about 0.5 s. The systematic azimuthal dependence of both the shape and duration of the apparent source-time function is consistent with a nearly unilateral propagation of the main rupture phase in a southeast direction along the fault plane and parallel to the direction of slip. An isochron analysis reveals that the first subevent occurred slightly to the northwest of the nucleation point but that the second subevent was located further to the southeast, thus confirming the overall rupture directivity towards the southeast. An interpretation of our results in light of the previously documented aftershock distribution and of observations of ground cracks in the epicentral area suggests that the main shock occurred on the Vuache Fault, and that rupture in a northwest direction was inhibited by a right-lateral stepover in the fault. Accordingly, the vast majority of the subsequent aftershocks, which include several magnitude 3-4 events, occurred on a fault segment that is slightly offset from the inferred surface trace of the Vuache Fault and that was activated by the main shoc

Moment tensor inversions of icequakes on Gornergletscher, Switzerland

We have determined seismic source mechanisms for shallow and intermediate-depth icequake clusters recorded on the glacier Gornergletscher, Switzerland, during the summers of 2004 and 2006. The selected seismic events are part of a large data set of over 80,000 seismic events acquired with a dense seismic network deployed in order to study the yearly rapid drainage of Gornersee lake, a nearby ice-marginal lake. Using simple frequency and distance scaling and Green’s functions for a homogeneous half-space, we calculated moment tensor solutions for icequakes with M_w-1.5 using a full-waveform inversion method usually applied to moderate seismic events (M_w>4) recorded at local to regional distances (≈50–700 km). Inversions from typical shallow events are shown to represent tensile crack openings. This explains well the dominating Rayleigh waves and compressive first motions observed at all recording seismograms. As these characteristics can be observed in most icequake signals, we believe that the vast majority of icequakes recorded in the 2 yr is due to tensile faulting, most likely caused by surface crevasse openings. We also identified a shallow cluster with somewhat atypical waveforms in that they show less dominant Rayleigh waves and quadrantal radiation patterns of first motions. Their moment tensors are dominated by a large double-couple component, which is strong evidence for shear faulting. Although less than a dozen such icequakes have been identified, this is a substantial result as it shows that shear faulting in glacier ice is generally possible even in the absence of extreme flow changes such as during glacier surges. A third source of icequakes was located at 100 m depth. These sources can be represented by tensile crack openings. Because of the high-hydrostatic pressure within the ice at these depths, these events are most likely related to the presence of water lenses that reduce the effective stress to allow for tensile faulting

Probabilistic seismic hazard assessment of Switzerland: best estimates and uncertainties

We present the results of a new genera tion of probabilistic seismic hazard assessment for Switzerland. This study replaces the previous intensity-based generation of national hazard maps of 1978. Based on a revised moment-magnitude earthquake catalog for Switzerland and the surrounding regions, covering the period 1300-2003, sets of recurrence parameters (a and b values, M max ) are estimated. Information on active faulting in Switzerland is too sparse to be used as source model. We develop instead two models of areal sources. The first oriented towards capturing historical and instrumental seismicity, the second guided largely by tectonic principles and express ing the alterative view that seismicity is less stationary and thus future activity may occur in previously quiet regions. To estimate three alterna tive a and b value sets and their relative weighting, we introduce a novel approach based on the modified Akaike information criterion, which allows us to decide when the data in a zone deserves to be fitted with a zone-specific b value. From these input parameters, we simulate synthetic earthquake catalogs of one-million-year duration down to magnitude 4.0, which also reflect the difference in depth distribution between the Alpine Foreland and the Alps. Using a specific predictive spectral ground motion model for Switzerland, we estimate expected ground motions in units of the 5% damped acceleration response spectrum at frequencies of 0.5-10Hz for all of Switzerland, referenced to rock sites with an estimated shear wave velocity of 1,500m/s2 in the upper 30m. The highest hazard is found in the Wallis, in the Basel region, in Graubünden and along the Alpine front, with maximum spectral accelerations at 5Hz frequency reaching 150cm/s2 for a return period of 475years and 720cm/s2 for 10,000year

Understanding Eurasian Convergence: Application Of Kohonen Self-Organizing Maps

Kohonen self-organizing maps (SOMs) are employed to examine economic and social convergence of Eurasian countries based on a set of twenty-eight socio-economic measures. A core of European Union states is identified that provides a benchmark against which convergence of post-socialist transition economies may be judged. The Central European Visegrád countries and Baltics show the greatest economic convergence to Western Europe, while other states form clusters that lag behind. Initial conditions on the social dimension can either facilitate or constrain economic convergence, as discovered in Central Europe vis-à-vis the Central Asian Republics. Disquiet in the convergence literature is resolved by providing an analysis of the Eurasian states over time

Active strike-slip faulting in the Chablais area (NW Alps) from earthquake focal mechanisms and relative locations

Abstract.: The Chablais area is characterized by a complex geological setting, resulting from the transport of nappes of various internal origins (the Prealpine nappes), thrusted in Oligocene times onto the Helvetic cover of the external zones of the Alps. While the structural setting and timing of nappe emplacement are well understood, current tectonics and associated faulting remain unclear. The detailed analysis of the Bonnevaux and Samoëns earthquakes, presented in this study, constitutes a significant contribution to the active tectonics of the Chablais area. The associated seismotectonic regime appears to be constant with depth, both focal mechanisms yielding a strike-slip regime, one in the crystalline basement at around 17km depth and the other probably cross-cutting the cover/basement interface at around 5km depth. Relative location techniques, applied in this study to the seismic sequence associated to the Samoëns earthquake, represents the best way to identify active faults in a region where neotectonic evidence is scarce and controversial. The resulting seismic alignment corresponds to the E-W oriented nodal plane inferred from the Samoëns main shock focal mechanism, thus defining an active near vertical E-W dextral fault. This strike-slip regime, compared to the current regional stress field, corresponds to the one observed in the Jura/Molasse basin area but contrasts with the exclusively dextral and NE-SW-oriented transcurrent regime of the Wildhorn/Martigny regio

Bilinearity in the Gutenberg-Richter Relation Based on M-L for Magnitudes Above and Below 2, From Systematic Magnitude Assessments in Parkfield (California)

Several studies have shown that local magnitude, M L, and moment magnitude, M , scale differently for small earthquakes (M < ~2) than for moderate to large earthquakes. Consequently, frequency‐magnitude relations based on one or the other magnitude type cannot obey a power law with a single exponent over the entire magnitude range. Since this has serious consequences for seismic hazard assessments, it is important to establish for which magnitude type the assumption of a constant exponent is valid and for which it is not. Based on independently determined M , M L and duration magnitude, M d, estimates for 5,304 events near Parkfield, we confirm the theoretically expected difference in scaling between the magnitude types, and we show that the frequency‐magnitude distribution based on M and M d follows a Gutenberg‐Richter relation with a constant slope, whereas for M L it is bilinear. Thus, seismic hazard estimates based on M L of small earthquakes are likely to overestimate the occurrence probability of large earthquakes