We introduce a new method to determine the magnitude of complete- ness Mc and its uncertainty. Our method models the entire magnitude range (EMR method) consisting of the self-similar complete part of the frequency-magnitude dis- tribution and the incomplete portion, thus providing a comprehensive seismicity model. We compare the EMR method with three existing techniques, finding that EMR shows a superior performance when applied to synthetic test cases or real data from regional and global earthquake catalogues. This method, however, is also the most computationally intensive. Accurate knowledge of Mc is essential for many seismicity-based studies, and particularly for mapping out seismicity parameters such as the b-value of the Gutenberg-Richter relationship. By explicitly computing the uncertainties in Mc using a bootstrap approach, we show that uncertainties inb-values are larger than traditionally assumed, especially when considering small sample sizes. As examples, we investigated temporal variations of Mc for the 1992 Landers aftershock sequence and found that it was underestimated on average by 0.2 with former techniques. Mapping Mc on a global scale, Mc reveals considerable spatial variations for the Harvard Centroid Moment Tensor (CMT) (5.3 Mc 6.0) and the International Seismological Centre (ISC) catalogue (4.3 Mc 5.0).

Assessing the Quality of Earthquake Catalogues: Estimating the Magnitude of Completeness and Its Uncertainty

To stimulate the reservoir for a “hot dry rock” geothermal project initiated by a private/public consortium in the city of Basel, Switzerland, approximately 11,500 m3 of water were injected at high pressures between 2 December and 8 December 2006 into a 5-km-deep well below Kleinhuningen (Haring et al. 2008). A six-sensor borehole array, installed by the operators of the project at depths between 317 and 2,740 meters around the well to monitor the induced seismicity recorded more than 10,500 seismic events during the injection phase. Hypocentral locations could be calculated for more than 3,000 of these events. The gradual increase in flow rate and wellhead pressure was accompanied by a steady increase in seismicity, both in terms of event rates and magnitudes. In the early morning hours of 8 December, after water had been injected at maximum rates in excess of 50 l/s and at wellhead pressures of up to 29.6 MPa for about 16 hours (Haring et al. 2008), a magnitude ML 2.6 event occurred within the reservoir. This exceeded the safety threshold for continued stimulation, so that injection was stopped prematurely. In the afternoon and evening of the same day, two additional events of magnitude ML 2.7 and 3.4 occurred within the same source volume. As a consequence, the well was opened and the water allowed to flow back. In the following days about one third of the injected water volume flowed back out of the well (Haring et al. 2008). Though the seismic activity declined rapidly thereafter, even more than two years later sporadic microseismicity was being detected in the stimulated rock volume by the downhole-instruments.

The mainshock was felt distinctly in the urban area of Basel. People reported short, high-frequency shaking lasting 1–3 seconds, often accompanied by a loud bang similar to an …

Earthquakes Induced by the Stimulation of an Enhanced Geothermal System below Basel (Switzerland)

Kinematics of the Iberia–Maghreb plate contact from seismic moment tensors and GPS observations

We study in-plane ruptures on a bimaterial fault governed by a velocity-weakening friction with a regularized normal stress response. Numerical simulations and analytical estimates provide characterization of the ranges of velocity-weakening scales, nucleation lengths and background stresses for which ruptures behave as cracks or pulses, decaying or sustained, bilateral or unilateral. With strongly velocity-weakening friction, ruptures occur under a wide range of conditions as large-scale pulses with a preferred propagation direction, that of slip of the more compliant material. Such ruptures have macroscopic asymmetry manifested by significantly larger seismic potency and propagation distance in the preferred direction, and clearly quantified by the directivity ratio derived from the second order moments of the spatio-temporal distribution of slip rate. The macroscopic rupture asymmetry of the large-scale pulses stems from the difference in the criticality conditions for self-sustained propagation in each rupture direction, induced by the asymmetric normal stress changes operating in bimaterial interfaces. In contrast, crack-like ruptures show macroscopic asymmetry under restrictive conditions. The discussed mechanism is robust with respect to regularization parameters, ranges of stress heterogeneities and a proxy for off-fault yielding and should operate similarly for crustal-scale rupture pulses even in the absence of velocity-weakening. Small-scale pulses, driven by the bimaterial normal stress reduction at the scale of the process zone, can detach from the rupture front of the large-scale pulses that propagate in the preferred direction. However, their occurrence depends on the relaxation scale in the regularization of the normal stress response and their development can be hindered by off-fault yielding.

Cracks, pulses and macroscopic asymmetry of dynamic rupture on a bimaterial interface with velocity-weakening friction

[1] This study is devoted to a systematic analysis of the state of stress of the central European Alps and northern Alpine foreland in Switzerland based on focal mechanisms of 138 earthquakes with magnitudes between 1 and 5. The most robust feature of the results is that the azimuth of the minimum compressive stress, S3, is generally well constrained for all data subsets and always lies in the NE quadrant. However, within this quadrant, the orientation of S3 changes systematically both along the structural strike of the Alpine chain and across it. The variation in stress along the mountain belt from NE to SW involves a progressive, counterclockwise rotation of S3 and is most clear in the foreland, where it amounts to 45°–50°. This pattern of rotation is compatible with the disturbance to the stress field expected from the indentation of the Adriatic Block into the central European Plate, possibly together with buoyancy forces arising from the strongly arcuate structure of the Moho to the immediate west of our study area. Across the Alps, the variation in azimuth of S3 is defined by a progressive, counterclockwise rotation of about 45° from the foreland in the north across the Helvetic domain to the Penninic nappes in the south and is accompanied by a change from a slight predominance of strike-slip mechanisms in the foreland to a strong predominance of normal faulting in the high parts of the Alps. The observed rotation can be explained by the perturbation of the large-scale regional stress by a local uniaxial deviatoric tension with a magnitude similar to that of the regional differential stress and with an orientation perpendicular to the strike of the Alpine belt. The tensile nature and orientation of this stress is consistent with the “spreading” stress expected from lateral density changes due to a crustal root beneath the Alps.

/pdf/stress-field-variations-in-the-swiss-alps-and-the-northern-62xdtmxidt.pdf

Stress field variations in the Swiss Alps and the northern Alpine foreland derived from inversion of fault plane solutions

Earthquake Catalogue of Switzerland (ECOS) and the Related Macroseismic Database

To obtain more reliable information about the focal-depth distribution of icequakes, in April 1997 we operated an array of seven portable digital seismographs on Unteraargletscher, central Swiss Alps. Over 5000 events were detected by at least two instruments during the 9 day recording period. P-wave velocities (3770 m f) were determined from several calibration shots detonated at the glacier surface as well as in a 49 m deep borehole, whereas S-wave velocities (1860 ms–1) were derived from a simultaneous inversion for Vp/Vs6 applied to 169 icequakes. So far, hypocentral locations have been calculated for over 300 icequakes. Besides confirming the occurrence of shallow events associated with the opening of crevasses, our results show that a small but significant fraction of the hypocenters are located at or near the glacier bed. One event was found at an intermediate depth of about 120 m. Three-dimensional particle-motion diagrams of both explosions and icequakes clearly demonstrate that all vertical component seismograms from shallow sources are dominated by the Rayleigh wave. On the other hand, for events occurring at depths greater than about 40 m, the Rayleigh wave disappears almost entirely. Therefore, a qualitative analysis of the signal character provides direct information on the focal depth of an event and was used as an independent check of the locations obtained from traditional arrival-time inversions. Thus, our results demonstrate that deep icequakes do occur and that simple rheological models, according to which brittle deformation is restricted to the uppermost part of a glacier, may need revision.

/pdf/evidence-for-deep-icequakes-in-an-alpine-glacier-2plximtmhk.pdf

Evidence for deep icequakes in an Alpine glacier

SUMMARY We produce fast and automatic moment tensor solutions for all moderate to strong earthquakes in the European-Mediterranean region. The procedure automatically screens near real-time earthquake alerts provided by a large number of agencies. Each event with magnitude M ≥ 4.7 triggers an automatic request for near real-time data at several national and international data centres. Moment tensor inversion is performed using complete regional long-period (50‐100 s) waveforms. Initially the data are inverted for a fixed depth to remove traces with a low signal-tonoise ratio. The remaining data are then inverted for several trial depths to find the best-fitting depth. Solutions are produced within 90 min of an earthquake. We analyse the results for the period 2000 April to 2002 April to evaluate the performance of the procedure. For quality assessment, we compared the results with the independent Swiss regional moment tensor catalogue (SRMT), and divided the 87 moment tensor solutions into three groups: 38 A-quality solutions with well-resolved M w, depth and focal mechanism; 21 B-quality solutions with wellresolved M w; and 28 unreliable C-quality solutions. The non-homogeneous station and event distributions, varying noise level, and inaccurate earthquake locations affected solution quality. For larger events (M w ≥ 5.5) we consistently obtained A-quality solutions. For M w = 4.5‐5.5 we obtained A- and B-quality solutions. Solutions that pass empirical rules mimicking the a posteriori quality for our data set are automatically disseminated.

/pdf/automatic-regional-moment-tensor-inversion-in-the-european-2fb34om3bd.pdf

Automatic regional moment tensor inversion in the European-Mediterranean region

SUMMARY

We present the surface wave dispersion results of the application of the ambient noise method to broad-band data recorded at 114 stations from the Istituto Nazionale di Geofisica e Vulcanologia (INGV) national broad-band network, some stations of the Mediterranean Very Broadband Seismographic Network (MedNet) and of the Austrian Central Institute for Meteorology and Geodynamics (ZAMG). Vertical-component ambient noise data from 2005 October to 2007 March have been cross-correlated for station-pairs to estimate fundamental mode Rayleigh wave Green's functions. Cross-correlations are calculated in 1-hr segments, stacked over periods varying between 3 months and 1.5 yr. Rayleigh wave group dispersion curves at periods from 8 to 44 s were determined using the multiple-filter analysis technique. The study region was divided into a 0.2°× 0.2° grid to invert for group velocity distributions. Checkerboard tests were first carried out, and the lateral resolution was estimated to be about 0.6°. The resulting group velocity maps from 8 to 36 s show the significant difference of the crustal structure and good correlations with known geological and tectonic features in the study region. The Po Plain and the Southern Alps evidence lower group velocities due to soft alluvial deposits, and thick terrigenous sediments. Our results also clearly showed that the Tyrrhenian Sea is characterized with much higher velocities below 8 km than the Italian peninsula and the Adriatic Sea which indicates a thin oceanic crust beneath the Tyrrhenian Sea.

/pdf/surface-wave-dispersion-measurements-from-ambient-seismic-3ks3bvexcr.pdf

Fabrizio Bernardi

Papers

Earthquake Catalogue of Switzerland (ECOS) and the Related Macroseismic Database

Evidence for deep icequakes in an Alpine glacier

Automatic regional moment tensor inversion in the European-Mediterranean region

Surface wave dispersion measurements from ambient seismic noise analysis in Italy

Earthquakes in Switzerland and Surrounding Regions During 2000