Swiss Seismological Service Research Articles

A New Empirical Magnitude Scaling Relation for Switzerland

Abstract We estimate moment magnitudes M w for earthquakes in Switzerland recorded between 1998 and 2009 using three different spectral methods. The M w estimation in Switzerland is extended to lower magnitudes (local magnitude M L 0.1), and scaling relations between M L and M w are investigated. Above M L 4, the obtained M w estimates are consistent with the previously obtained scaling relation of M w = M L -0.3 at the Swiss Seismological Service (SED). Below M L 4, all three methods indicate that a 1:1-type relationship is inappropriate. Therefore, we propose a new piecewise empirical scaling relation for earthquakes in Switzerland. The scaling is linear below M L 2 and above M L 4. To obtain a smooth transition between the two linear scales we fit a quadratic relation in between (2≤ M L ≤4). This scaling relation is also consistent with M w estimates from moment-tensor (MT) solutions based on broadband waveform fitting of local earthquakes with M L >3.0. We have tested all three methods carefully to ensure that the observed break in scale at around M 3 cannot be attributed to bias in the M w determination. However, we cannot determine with certainty from the dataset at hand whether the break in scaling is due to bias in the routine determination of M L or to physical properties of the source.

Earthquakes in Switzerland and surrounding regions during 2010

This report of the Swiss Seismological Service summarizes the seismic activity in Switzerland and surrounding regions during 2010. During this period, 407 earthquakes and 85 quarry blasts were detected and located in the region under consideration. With a total of only 19 events with ML ≥ 2.5, the seismic activity in the year 2010 was below the average over the previous 35 years. The two most noteworthy earthquakes were the ML 3.4 Barrhorn event near Sankt Niklaus (VS) and the ML 3.0 event of Feldkirch, both of which produced shaking of intensity IV.

Magnitude problems in historical earthquake catalogues and their impact on seismic hazard assessment

SUMMARY Areliablehistoricalearthquakecatalogueisahighlycriticalcomponentinanyregionalseismic hazard analysis. Although many of the various catalogues used for constructing national or regional seismic hazard maps in Europe cover much of the same territory and use standardized parameters,somediscrepancieshavebeenrevealedbetweendifferentcatalogues.Switzerland’s ECOS catalogue, compiled by the Swiss Seismological Service (SED), and Italy’s CPTI04 catalogue, compiled by the CPTI Working Group, spatially overlap each other to a large extent and employ a uniform moment magnitude scale (Mw). However, a careful review of these two catalogues has revealed significant inconsistencies in the moment magnitude (Mw): (1) The moment magnitudes used for the same earthquakes were found to differ, which could potentially have a substantial impact on any regional seismic hazard assessment that is based on these catalogues. This type of inconsistency often arises when different regression relationships are used to convert other magnitude scales or event intensity into Mw, which can cause many of the event magnitudes in one catalogue to be systematically biased higher or lower with respect to those in another catalogue. In this case, the magnitudes of small to medium historical earthquakes in Italy’s CPTI04 catalogue are systematically higher than those in Switzerland’s ECOS catalogue. (2) An abnormally high frequency of large magnitude events was observed for some of the time periods, particularly if most of the available data was on intensity. This has been observed in both the CPTI04 and ECOS catalogues for the time period prior to 1975 and is possibly due to a bias during the intensity-to-magnitude conversion. (3) A systematic bias in magnitude resulted in biased estimations for the a and b values of the Gutenberg–Richter magnitude–frequency relationships. All of these issues can lead to skewed seismic hazard results, or inconsistencies in the seismic hazard from different studies. By constructing seismic hazard models based on the ECOS and CPTI04 catalogues, we can evaluate the impact of these inconsistencies on the regional seismic hazard. The results show that the seismic hazard from the two models can disagree by as much as a factor of three at some locations, which demonstrates the importance of developing consistent and unbiased earthquake catalogues across neighbouring countries.

The True Case of the 1276 Fake Earthquake

In the pre-1900 time-window of earthquake catalogs, it is not unusual to come face-to-face with “fake” earthquakes erroneously listed among true ones. Such mistakes have been disclosed for many events, and their nature revealed and discussed in quite a number of papers in the last thirty years. Some discoveries of fake earthquakes originated from the revision of national catalogs ( e.g. , for France, Vogt 1979; for Italy, Guidoboni and Ferrari 1986 and Bellettati et al. 1993; for Germany, see a summary in Grunthal 2004), and others come from projects dealing with European-wide catalogs ( e.g. , Stucchi and Camassi 1997). As a consequence of these investigations, in some well-studied European regions recent catalogs have been purged of most of these mistakes. A list of fake earthquakes is now available in some regional databases (France: BRGM-EDF-IRSN/SisFrance 2010; Switzerland: Swiss Seismological Service 2002; Northern Europe: Grunthal et al. 2009), and a section devoted to fakes can be found in the European Archive of Historical Earthquake Data (AHEAD 2010). Most fakes are in fact duplications deriving from historical records that supply different locations and dates of an individual event ( e.g. , Alexandre 1991), in many cases because dates are expressed according to different calendars in use in the same period in neighboring countries. Also, fake earthquake catalog records might be the result of a mistaken interpretation of rather obscure records on other natural phenomena, such as storms or landslides ( e.g. , Alexandre 1990; Albini et al. 1990; Albini and Vogt 1992; Alexandre 1993). Among the most elusive catalog records to be unraveled as fakes are the ones originating from wrong locations of true earthquakes. Such misinterpretations of historical earthquake records might be found in 19th to early 20th century seismological literature, and from there they …

Earthquakes in Switzerland and surrounding regions during 2009

This report of the Swiss Seismological Service summarizes the seismic activity in Switzerland and surrounding regions during 2009. During this period, 450 earthquakes and 68 quarry blasts were detected and located in the region under consideration. The three strongest events occurred about 15 km NW of Basel in southern Germany (ML 4.2), near Wildhaus in the Toggenburg (ML 4.0) and near Bivio in Graubunden (ML 3.5). Although felt by the population, they were not reported to have caused any damage. With a total of 24 events with ML ≥ 2.5, the seismic activity in the year 2009 was close to the average over the previous 34 years.

Automatic computation of moment magnitudes for small earthquakes and the scaling of local to moment magnitude

SUMMARY Moment magnitudes (M W) are computed for small and moderate earthquakes using a spectral fitting method. 40 of the resulting values are compared with those from broadband moment tensor solutions and found to match with negligible offset and scatter for available M W values of between 2.8 and 5.0. Using the presented method, M W are computed for 679 earthquakes in Switzerland with a minimum M L = 1.3. A combined bootstrap and orthogonal L1 minimization is then used to produce a scaling relation between M L and M W. The scaling relation has a polynomial form and is shown to reduce the dependence of the predicted M W residual on magnitude relative to an existing linear scaling relation. The computation of M W using the presented spectral technique is fully automated at the Swiss Seismological Service, providing real-time solutions within 10 minutes of an event through a web-based XML database. The scaling between M L and M W is explored using synthetic data computed with a stochastic simulation method. It is shown that the scaling relation can be explained by the interaction of attenuation, the stress-drop and the Wood–Anderson filter. For instance, it is shown that the stress-drop controls the saturation of the M L scale, with low-stress drops (e.g. 0.1–1.0 MPa) leading to saturation at magnitudes as low as M L = 4.

Earthquakes in Switzerland and surrounding regions during 2008

This report of the Swiss Seismological Service summarizes the seismic activity in Switzerland and surrounding regions during 2008. During this period, 451 earthquakes and 75 quarry blasts were detected and located in the region under consideration. The three strongest events occurred in the Valais, near Lac des Toules (M L 3.6), and in Graubunden, near Ilanz (M L 3.7) and Paspels (M L 4.0). Although felt by the population, they were not reported to have caused any damage. However, with a total of only 15 events with M L ≥ 2.5, the seismic activity in the year 2008 was far below the average over the previous 33 years.

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 5 km 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 5 km 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 seismicity.

Methodology and main results of seismic source characterization for the PEGASOS Project, Switzerland

Under the direction of National Cooperative for the Disposal of Radioactive Waste (NAGRA), a probabilistic seismic hazard analysis was conducted for the Swiss nuclear power plant sites. The study has become known under the name “PEGASOS Project.” This is the first of a group of papers in this volume that describes the seismic source characterization methodology and the main results of the project. A formal expert elicitation process was used, including dissemination of a comprehensive database, multiple workshops for identification and discussion of alternative models and interpretations, elicitation interviews, feedback to provide the experts with the implications of their preliminary assessments, and full documentation of the assessments. A number of innovative approaches to the seismic source characterization methodology were developed by four expert groups and implemented in the study. The identification of epistemic uncertainties and treatment using logic trees were important elements of the assessments. Relative to the assessment of the seismotectonic framework, the four expert teams identified similar main seismotectonic elements: the Rhine Graben, the Jura / Molasse regions, Helvetic and crystalline subdivisions of the Alps, and the southern Germany region. In defining seismic sources, the expert teams used a variety of approaches. These range from large regional source zones having spatially-smoothed seismicity to smaller local zones, to account for spatial variations in observed seismicity. All of the teams discussed the issue of identification of feature-specific seismic sources (i.e. individual mapped faults) as well as the potential reactivation of the boundary faults of the Permo-Carboniferous grabens. Other important seismic source definition elements are the specification of earthquake rupture dimensions and the earthquake depth distribution. Maximum earthquake magnitudes were assessed for each seismic source using approaches that consider the magnitudes of observed earthquakes within analogous tectonic regions. All four expert teams used the PEGASOS earthquake catalogue for estimating earthquake recurrence parameters. This catalogue was developed by the Swiss Seismological Service and provided all historical and instrumental events in a uniform moment magnitude. The teams evaluated alternative declustering approaches and used available historical data to assess catalog completeness as a function of location, magnitude, and time period.

Earthquakes in Switzerland and surrounding regions during 2007

This report of the Swiss Seismological Service summarizes the seismic activity in Switzerland and surrounding regions during 2007. During this period, 531 earthquakes and 92 quarry blasts were detected and located in the region under consideration. Of these earthquakes, 30 are aftershocks of the stimulation of a proposed geothermal reservoir beneath the city of Basel in December of 2006. With 20 events with ML ≥ 2.5, four of which were artificially induced, the seismic activity in the year 2007 was far below the average over the previous 32 years.

Earthquakes in Switzerland and surrounding regions during 2006

This report of the Swiss Seismological Service summarizes the seismic activity in Switzerland and surrounding regions during 2006. During this period, 572 earthquakes and 91 quarry blasts were detected and located in the region under consideration. Of these earthquakes, two occurred in conjunction with the construction of the new Gotthard railway tunnel and 165 were induced artificially by the stimulation of a proposed geothermal reservoir beneath the city of Basel. With 20 events with M L ≥ 2.5, five of which were artificially induced, the seismic activity in the year 2006 was far below the average over the previous 31 years. Nevertheless, six events were felt by the public, most prominently the strongest of the induced Basel events (M L 3.4), which caused some non-structural building damage. Noteworthy are also the two earthquakes near Cortaillod (M L 3.2), on the shore of Lake Neuchâtel, and in Val Mora (M L 3.5), between the Engadin and Val Mustair, as well as the 42 aftershocks of the M L 4.9 Vallorcine earthquake, between Martigny and Chamonix, of September 2005.

Earthquakes in Switzerland and surrounding regions during 2005

This report of the Swiss Seismological Service summarizes the seismic activity in Switzerland and surrounding regions during 2005. During this period, 611 earthquakes, 96 quarry blasts and two landslides were detected and located in the region under consideration. With 19 events with ML ≥ 2.5, the seismic activity in the year 2005 was below the average over the last 30 years. However, with the earthquake of Vallorcine (ML 4.9) located just across the border to France, between Martigny and Chamonix, and the two earthquakes of Rumisberg and Brugg (ML 4.1), located in the lower crust beneath the Jura Mountains of northern Switzerland, the year 2005 saw three events that produced shaking of intensity IV and V (EMS98). Of the 611 recorded earthquakes more than 110 events are aftershocks of the Vallorcine quake. Moreover, 51 events occurred within two days at the end of August during a period of very intense rainfalls. The epicenters of these events were concentrated in several clusters distributed over a wide area of central Switzerland, and their focal depths were shallow, so that they most likely constitute a case of precipitationinduced seismicity.

Earthquakes in Switzerland and surrounding regions during 2004

This report of the Swiss Seismological Service summarizes the seismic activity in Switzerland and surrounding regions during 2004. During this period, 677 earthquakes and 96 quarry blasts were detected and located in the region under consideration. With 22 events with M L ≥2.5, the seismic activity in the year 2004 was close to the average over the last 30 years. As in previous years, most of the activity was concentrated in the Valais and in Graubunden. In addition, several moderate earthquakes occurred in the lower crust below the northern Alpine foreland. Unusual was that five earthquakes were sufficiently strong to cause ground shaking of intensity IV over large portions of the territory. Two were located in Switzerland (Liestal, M L 3.8, and Brugg, M L 4.0). The epicenters of the other three strong events were located outside Switzerland (Besancon in the French Jura, M L 4.8, Waldkirch in southern Germany, M L 5.1, and Lago di Garda in northern Italy, M L 5.3).

Homogeneous Moment-Magnitude Calibration in Switzerland

An earthquake catalog containing a uniform size estimate is important for long-term seismic hazard assessment in regions of low-to-moderate seismicity. During the update of the Earthquake Catalog of Switzerland (ecos), we performed regression analyses to convert all earthquake size information in ecos to physically meaningful moment magnitude M w. For 34 events in and near Switzerland, we determined seismic moment (thus M w) by regional waveform inversion. Independent M w estimates for the same events do not exist; however, M w from European-Mediterranean events, obtained in the same way, agree with M w from Harvard CMT solutions. All other size estimates, M L, M D, m b, M S, and intensities, are calibrated relative to these 34 events. Teleseismic M S and m b from international data centers are directly regressed against M w. Most observations in ecos consist of local magnitudes ( M L, M D) and intensities. For local magnitudes, we first calibrated the Swiss Seismological Service’s M L. Then we calibrated magnitudes from observatories in neighboring countries (France, Germany, Italy) using only events in the border region (e.g., France–Switzerland). Modern instrumental records exist only since the mid-1970s. We calibrated the macroseismic dataset, which represents by far the largest period in the catalog, by determining surface wave magnitude M S for stronger twentieth century Swiss earthquakes from analog seismograms. These M S, which were converted to M w, connect intensities and M w. After calibration, all 20,300 events in ecos have a unified M w, including a class-type uncertainty estimate based on the original magnitude scale. ecos covers the period 250–2001, from 44° N to 51° N and 4° E to 13° E. The largest event in ecos is the 1356 M w 6.9 Basle earthquake.

How deep was the Dudley (UK) earthquake of 22 September 2002?

On 22 September 2002, the largest UK earthquake (m b 4.3) of the last 10 years occurred near the town of Dudley in the West Midlands. Here we determine the earthquake focal mechanism and depth using data from stations at regional and teleseismic distances. Short-period teleseismic seismograms are interpreted in terms of P and surface reflections pP and sP. This analysis suggests that the source depth is deeper than the 9.7 km initially determined by the British Geological Survey (BGS). The relative amplitude method is applied to four teleseismic seismograms to support our interpretation of the surface reflections, and constrain the focal mechanism. Our preferred focal mechanism, a near vertical strike-slip with φs = 94°, δ = 88° and λ = −179°, is in reasonable agreement with a moment tensor determined by the Swiss Seismological Service. Synthetic regional surface wave seismograms match the observed seismograms for a model focal depth of 19.5 (±3.0) km and scalar moment, M0, of 3.2 × 1015 N m. Our results emphasize that due to the well-known trade-off between depth and M0 from inversions of long period (0.02–0.1 Hz) surface waves, it is preferable to combine long- and short-period data to constrain reliably the depth and hence estimate M0. Our focal mechanism and depth are further validated by generating short-period synthetic seismograms that match the observations.

Earthquakes in Switzerland and surrounding regions during 2003

This report of the Swiss Seismological Service summarizes the seismic activity in Switzerland and surrounding regions during 2003. During this period, 532 earthquakes and 118 quarry blasts were detected and located in the region under consideration. With 30 events with M L ≥ 2.5, the seismic activity in the year 2003 was slightly above the average over the last 29 years. As in previous years, most of the seismic activity was concentrated in the Valais and in Graubünden. In addition, several earthquakes occurred along the northern front of the Alps. The strongest event, with M L 4.0 and epicentral intensity V, occurred in the area of Urnerboden, on the border between the cantons Uri and Glarus. Two earthquakes with M L 3.9 occurred in the Valais near the town of Salgesch and south of the Sanetsch Pass. In Graubünden, the Sertig valley south of Davos was the site of a sequence of 51 earthquakes that included three events with magnitudes between M L 3.6 and 3.9.

Ancient Earthquakes at Lake Lucerne

Michael Schnellmann is currently pursuing a doc torate in geology at the Swiss Federal Institute of Technology (E ) in Zurich. His thesis advisor, Flavio S. Anselmetti, received his Ph.D. from E in 1994. He then spent four years in postdoctoral training at the University of Miami. There Ansel metti gained considerable experience studying ma rine sediments before he returned to his native Switzerland and joined the Geological Institute at E , where he currently heads the Limnogeology Laboratory. Domenico Giardini received his Ph.D. at the University of Bologna in 1987. Since 1997 he has been a professor of seismology and geodynamics at the Institute of Geophysics at E and the direc tor of the Swiss Seismological Service. Between 1992 and 1999, he served as director of the Global Seismic Hazard Assessment Program for the UN's International Decade for Natural Disaster Reduc tion. Judith A. McKenzie obtained a doctorate in ge ology from E in 1976. During her studies in Zurich and later, while working at the University of Florida in Gainesville, she investigated the chemical and biochemical sedimentation in both modern lakes

Probabilistic earthquake location in complex three‐dimensional velocity models: Application to Switzerland

We present a new approach to determine precise and reliable hypocenter locations in the tectonically complex region of Switzerland. A three‐dimensional (3‐D) P wave velocity model to be used for earthquake relocation is obtained by simultaneously inverting arrival times of local earthquakes for hypocenter locations and 3‐D P wave velocity structure. A 3‐D P wave velocity model derived from controlled source seismology (CSS) is used as an initial reference model. The final 3‐D model thus combines all available information from both CSS and local earthquake data. The probabilistic, nonlinear formulation of the earthquake location problem includes a complete description of location uncertainties and can be used with any kind of velocity model. In particular, the combination of nonlinear, global search algorithms, such as the Oct‐Tree Importance Sampling, with probabilistic earthquake location provides a fast and reliable tool for earthquake location. The comparison of hypocenter locations obtained routinely by the Swiss Seismological Service (SED) to those relocated in the new 3‐D velocity model using a probabilistic approach reveals no systematic shifts but does exhibit large individual shifts in some epicenter locations and focal depths. We can attribute these large shifts in part to large uncertainties in the hypocenter location. Events with a low number of observations (<8) and no observation within the critical focal depth distance typically show large location uncertainties. Improved hypocentral locations, particularly for mine blasts and earthquakes whose routine hypocenter locations had been questionable, confirm that improved velocity model and probabilistic earthquake location yield more precise and reliable hypocenter locations and associated location uncertainties for Switzerland.

Stephan Mueller (1930”1997)

Stephan Mueller, professor emeritus at the Institute of Geophysics at the Swiss Federal Institute of Technology (ETH) in Zurich and highly respected leader of international geoscience, died February 17, 1997. His untimely death, due to pneumonia following intestinal surgery, came just 18 months after his retirement from the ETH Chair of Geophysics and Directorship of the Swiss Seismological Service. He is survived by his wife, Doris, two sons, and six grandchildren.Mueller received a diploma in physics at the University of Stuttgart in 1957 and an M.S. in electrical engineering from Columbia University in New York in 1959. As an undergraduate at Stuttgart, he was influenced by seismologist Wilhelm Hillerand geophysics quickly became his major academic and career objective. After receiving a 1954–1955 German Academic Interchange Scholarship at Columbia, Mueller sought out Maurice Ewing and his group at Lamont Geological Observatory, where Mueller's enthusiasm for geophysics was strongly encouraged. While at Lamont, he participated in the first U.S. deep‐sea geophysical expedition in the Mediterranean Sea during the summer of 1956 aboard the RV Vema.

Seismic monitoring of explosions: a method to extract information on the isotropic component of the seismic source

The design of a monitoring system for detecting explosions is a very topical problem, both for routine data processing at seismological observatories as well as for the monitoring of a Comprehensive Test Ban Treaty. In this framework it is desirable to have the possibility to quantify the presence of the isotropic component in the seismic source. For this purpose a method is presented, which is based on waveform inversion for the full moment tensor retrieval. The method inverts either full waveforms or separate seismic phases and returns the mechanism and time history of a point source. Moreover, it allows to redefine the hypocentral depth of the event and, in a simplistic way, to optimize the structural model as well. In order to model strong laterally heterogeneous structures, different pairs of structural models can be used for each source-receiver path. The source is decomposed into a volumetric part (V), representing an explosive or implosive component, and into a deviatoric part, containing both the double couple (DC) and the compensated linear vector dipole (CLVD) components. The method is applied to an area in central Switzerland and to the network of the Swiss Seismological Service. The events of interest include both earthquakes and explosions. Despite some modelling inadequacies of the source-time function, the explosions can be well identified with the inverted isotropic component in the source, as long as the number of stations used for the inversion is larger than three. The results of the inversion are better for large epicenter-station distances of the order of 40–90 km.