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 Neuchatel, and in Val Mora (M _L 3.5), between the Engadin and Val Müstair, as well as the 42 aftershocks of the M _L 4.9 Vallorcine earthquake, between Martigny and Chamonix, of September 2005. Editorial handling: Stefan Bucher     

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 Graubünden, 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.     

Microzonation of the city of Basel

During the past centuries, the city of Basel has suffered damage caused by earthquakes. One extraordinary event described in historical documents is the strong earthquake which occurred in 1356. The 1356 event, one of the strongest earthquakes in northwest-Europe, was obviously much stronger than the low-magnitude earthquakes observed in the area during this century. Even though the present seismicity in the Basel area is low, strong earthquakes have to be expected due to the city's geographical location close to the northern boundary of the African-European convergence zone, at the southern end of the Rhinegraben. A crucial step towards preparedness for future events and mitigation of earthquake risk involves a microzonation study of the city. The study is carried out in three steps: (1) a detailed mapping of the geology and geotechnical properties of the area, (2) measurement, interpretation and modelling of ambient noise data, and (3) numerical modelling of expected ground motions during earthquakes. A qualitative microzonation of the centre of Basel is presented, and it is discussed by comparing it to the historically reported damage of the 1356 earthquake.     

Parameterization of historical earthquakes in Switzerland

Macroseismic earthquake parameters of historical events have been reassessed in the framework of the update of the Earthquake Catalogue of Switzerland ECOS-09. The Bakun and Wentworth method (Bakun and Wentworth 1997) has been used to assess location, magnitude, and, when possible, focal depth. We apply a two-step procedure. Intensity attenuation is assessed first by fitting a model with a logarithmic and a linear term, using a set of 111 earthquakes. The magnitude range is 3 and 5.8. Then, intensity to magnitude relation is developed. A subset of the 111 events, all having an instrumental moment magnitude, was used to perform this intensity to magnitude calibration. Five final calibration strategies were developed based on different intensity calibration datasets, regionalized or non-regionalized models, and fixed or variable source depth. The final assessment of the macroseismic earthquake parameters is based on an expert judgment procedure, using the results derived from all five strategies, and taking into consideration the historical knowledge available for the particular earthquake. A bootstrap procedure has been applied to assess the uncertainty of parameters. Indicative lower and upper bounds of uncertainty are derived from distributions of location and magnitude for a number of events, obtained through bootstrap sampling of the intensity field and of the single intensity values. The final uncertainties are given in terms of parameter uncertainty classes already used in previous versions of the earthquake catalogue of Switzerland.     

“Terrae motus factus est”: earthquakes in Switzerland before A.D. 1000. A critical approach

We focus on Swiss earthquakes in antiquity and the early medieval period before A.D. 1000. We have information on less than half a dozen earthquakes within this era, since written records for the first half of the first millennium A.D. are minimal, and there is little hope of finding more written evidence for earthquakes. Furthermore, interpreting the documents at hand is somewhat complex. For the 6th century Gregory of Tours in Historia Francorum gives hints of a rockslide near the castle Tauredunum (Le Grammont) in the Swiss canton Valais, an event that has been considered in the literature as caused by an earthquake. The Carolingian period (ca. 750–950) included the rise of some very important cultural centers in various parts of today’s Switzerland. For instance, the ecclesiastical culture in St. Gallen generated a remarkable number of written records, which survived for our use in a unique manner. From the 9th and 10th centuries, we have evidence for earthquakes in the years 849, 867, 902, and 944. However, information on them remains so scarce that their location and intensity are generally difficult to assess. Nevertheless, the finding of a new document - a memoir written by the abbot of Reichenau - offers some insight into the A.D. 849 event and its reportedly aftershocks.

Extraordinary heat during the 1930s US Dust Bowl and associated large-scale conditions

Unusually hot summer conditions occurred during the 1930s over the central United States and undoubtedly contributed to the severity of the Dust Bowl drought. We investigate local and large-scale conditions in association with the extraordinary heat and drought events, making use of novel datasets of observed climate extremes and climate reanalysis covering the past century. We show that the unprecedented summer heat during the Dust Bowl years was likely exacerbated by land-surface feedbacks associated with springtime precipitation deficits. The reanalysis results indicate that these deficits were associated with the coincidence of anomalously warm North Atlantic and Northeast Pacific surface waters and a shift in atmospheric pressure patterns leading to reduced flow of moist air into the central US. Thus, the combination of springtime ocean temperatures and atmospheric flow anomalies, leading to reduced precipitation, also holds potential for enhanced predictability of summer heat events. The results suggest that hot drought, more severe than experienced during the most recent 2011 and 2012 heat waves, is to be expected when ocean temperature anomalies like those observed in the 1930s occur in a world that has seen significant mean warming.     

Adiabatic deceleration of cosmic rays near their sources

Every plausible source of cosmic rays yields a high flux of cosmic rays near the source. The high flux leads to plasma effects that cause scattering of the cosmic rays, coupling to the interstellar gas and hence to adiabatic deceleration. The cosmic rays are released from the gas only when their pressure has fallen to the cosmic-ray pressure near the Sun multiplied by a factor between 10 and 100. I discuss a model aimed to minimize the deceleration before the cosmic rays are released. The volume which cosmic rays occupy before scattering is maximized by injection into a large but thin disk. Even then, deceleration is reduced only to a factor of two. Such deceleration should cause quasi-supernova remnants somewhat resembling the Cygnus loop but associated with much younger pulsars. Since both the required model and the predicted observations cause difficulties, the problem of adiabatic deceleration remains severe.Work supported by NSF grant GP-34742.     

Response spectra for the deep sediment-filled Rhône Valley in the Swiss Alps

We present numerical modeling of earthquake ground motion for various profiles across the Swiss Rhône Valley and characterize the seismic response in terms of spectral acceleration. First, we evaluate the relative amplification of 2D with respect to 1D response. Then, we show how the selected bedrock spectrum influences the response spectra of the valley sites. Particular attention is paid to how the internal sediment structure and the often weakly constrained Q-factor shape the seismic response. Results obtained for the different profiles are compared with reference spectra (Swiss building code and Eurocode 8) and for one profile with recorded data as well. From this comparison, we infer that the surficial layer strongly influences spectral acceleration values between 0.1 and 1 s. The total thickness of sediments significantly affects the seismic response at longer periods around the fundamental period of the studied valley sections between 1.8 and 3.6 s.