Swiss Seismological Service

About: Swiss Seismological Service is a based out in . It is known for research contribution in the topics: Induced seismicity & Seismic hazard. The organization has 220 authors who have published 619 publications receiving 19848 citations. The organization is also known as: SED.

Variations in earthquake-size distribution across different stress regimes

Abstract: The earthquake size distribution follows, in most instances, a power law1,2, with the slope of this power law, the ‘b value’, commonly used to describe the relative occurrence of large and small events (a high b value indicates a larger proportion of small earthquakes, and vice versa). Statistically significant variations of b values have been measured in laboratory experiments, mines and various tectonic regimes such as subducting slabs, near magma chambers, along fault zones and in aftershock zones3. However, it has remained uncertain whether these differences are due to differing stress regimes, as it was questionable that samples in small volumes (such as in laboratory specimens, mines and the shallow Earth's crust) are representative of earthquakes in general. Given the lack of physical understanding of these differences, the observation that b values approach the constant 1 if large volumes are sampled4 was interpreted to indicate that b = 1 is a universal constant for earthquakes in general5. Here we show that the b value varies systematically for different styles of faulting. We find that normal faulting events have the highest b values, thrust events the lowest and strike-slip events intermediate values. Given that thrust faults tend to be under higher stress than normal faults we infer that the b value acts as a stress meter that depends inversely on differential stress.

A theoretical investigation of average H/V ratios

Abstract: SUMMARY The mode summation method and a finite difference technique are applied to investigate the spectral ratio between the horizontal and vertical components (H/V ratio) of ambient vibrations and to explore the variation of the resonance frequency and the amplitude and shape of polarization as a function of the structure and the source positions. Layered structural models are used by assuming a large number of sources distributed around a receiver, with shallow source depths that are randomly assigned. We identify stable parts of the H/V ratios that are independent of the source distance and are dominated by the ellipticity of the fundamental-mode Rayleigh wave in the frequency band between the fundamental frequency of resonance of the unconsolidated sediments and the first minimum of the average H/V ratio. The ellipticity in this frequency band is determined by the layering of the sediments. The numerical simulations are compared with observations at a site where the thickness and velocity structure of the unconsolidated sediments are known from S-wave and surface wave measurements. Two methods are applied to compute the H/V ratio, the classical method in the frequency domain and a method based on frequency–time analysis that allows us to locate P–SV wavelets in the time-series. The main problem in comparing synthetics with observations is the contribution of SH waves in the observed H/V ratios. We propose a method to minimize these effects and the effects of the superposition of different incoming P–SV waves. An inversion scheme is applied to the stable parts of the observed H/V ratio, based on a genetic algorithm, to retrieve the S-wave velocity structure from a single ambient vibration record.

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

Abstract: 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 …

The November 2017 Mw 5.5 Pohang earthquake: A possible case of induced seismicity in South Korea

Abstract: The moment magnitude (Mw) 5.5 earthquake that struck South Korea in November 2017 was one of the largest and most damaging events in that country over the past century. Its proximity to an enhanced geothermal system site, where high-pressure hydraulic injection had been performed during the previous 2 years, raises the possibility that this earthquake was anthropogenic. We have combined seismological and geodetic analyses to characterize the mainshock and its largest aftershocks, constrain the geometry of this seismic sequence, and shed light on its causal factors. According to our analysis, it seems plausible that the occurrence of this earthquake was influenced by the aforementioned industrial activities. Finally, we found that the earthquake transferred static stress to larger nearby faults, potentially increasing the seismic hazard in the area.

On the Selection of Ground-Motion Prediction Equations for Seismic Hazard Analysis

Abstract: A key element in any seismic hazard analysis is the selection of appropriate ground-motion prediction equations (GMPEs). In an earlier paper, focused on the selection and adjustment of ground-motion models for probabilistic seismic hazard analysis (PSHA) in moderately active regions--with limited data and few, if any, indigenous models--Cotton et al. (2006) proposed seven criteria as the basis for selecting GMPEs. Recent experience in applying these criteria, faced with several new GMPEs developed since the Cotton et al. (2006) paper was published and a significantly larger strong-motion database, has led to consideration of how the criteria could be refined and of other conditions that could be included to meet the original objectives of Cotton et al. (2006). In fact, about a dozen new GMPEs are published each year, and this number appears to be increasing. Additionally, Cotton et al. (2006) concluded that the criteria should not be excessively specific, tied to the state-of-the-art in ground-motion modeling at the time of writing and thus remaining static, but rather should be sufficiently flexible to be adaptable to the continuing growth of the global strong-motion database and the continued evolution of GMPEs