Ground motion and macroseismic intensities of a seismic event related to geothermal reservoir stimulation below the city of Basel—observations and modelling

To stimulate a geothermal reservoir below the city of Basel, Switzerland, fluid was injected in December 2006 at high pressure into a 5 km deep borehole. This stimulation led to an increase in local seismicity with the largest seismic event (ML = 3.4) occurring on 2006 December 8. The event was widely felt by the local population, slight non-structural damage to buildings was reported and ultimately this event was the cause to put the geothermal project on hold. In this study, we present recorded ground motion and determinations of macroseismic intensity of the ML 3.4 event as well as simulations of seismic wave propagation and a model to predict macroseismic intensities. As the two models are based on different simplifying assumptions and different underlying physical processes, the predicted intensity distributions exhibit differences in their details. However, the first-order characteristics of the observed macroseismic intensity distribution are well matched. Based on this result, we compute intensities also for hypothetical scenarios of earthquakes with larger magnitude

The false Basel earthquake of May 12, 1021

The Basel (CH) area is a place with an increased seismic hazard. Consequently, it is essential to scrutinize a famous statement by Stumpf (Gemeiner loblicher Eydgnoschafft Stetten, Landen und Völckeren Chronikwirdiger thaaten beschreybung. Durch Johann Stumpffen beschriben, 1548) that allegedly a large earthquake took place in Basel in 1021. This can be disproved unambiguously by applying historical and philosophical method

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 Switzerlan

Seismic monitoring and analysis of deep geothermal projects in St Gallen and Basel, Switzerland

Monitoring and understanding induced seismicity is critical in order to estimate and mitigate seismic risk related to numerous existing and emerging techniques for natural resource exploitation in the shallow-crust. State of the art approaches for guiding decision making, such as traffic light systems, rely heavily on data such as earthquake location and magnitude that are provided to them. In this context we document the monitoring of a deep geothermal energy project in St Gallen, Switzerland. We focus on the issues of earthquake magnitude, ground motion and macroseismic intensity which are important components of the seismic hazard associated to the project. We highlight the problems with attenuation corrections for magnitude estimation and site amplification that were observed when trying to apply practices used for monitoring regional seismicity to a small-scale monitoring network. Relying on the almost constant source-station distance for events in the geothermal ‘seismic cloud' we developed a simple procedure, calibrated using several ML > 1.3 events, which allowed the unbiased calculation of ML using only stations of the local monitoring network. The approach determines station specific ML correction terms that account for both the bias of the attenuation correction in the near field and amplification at the site. Since the smallest events (ML < −1) were only observed on a single borehole instrument, a simple relation between the amplitude at the central borehole station of the monitoring network and ML was found. When compared against magnitudes computed over the whole network this single station approach was shown to provide robust estimates (±0.17 units) for the events down to ML = −1. The relation could then be used to estimate the magnitude of even smaller events (ML < −1) only recorded on the central borehole station. Using data from almost 2700 events in Switzerland, we then recalibrated the attenuation correction, extending its range of validity from a minimum source-station distance of 20 km down to 1 km. Based on this we could determine the component of the previously derived station specific ML corrections due to local amplification. We analysed ground-motion and detailed macroseismic reports resulting from the 2013 July 20 St Gallen ML = 3.5 ± 0.1 (Mw = 3.3-3.5 ± 0.1) ‘main shock' and compared it to a similar ML = 3.4 ± 0.1 event (Mw = 3.2 ± 0.1) that occurred in 2006 at another deep geothermal project in Basel, Switzerland. Differences in ground motion amplitudes between the Basel and St Gallen events and to an extent, the associated macroseismic observations, were investigated in terms of the different source terms: Mw for long-period motions and the source-corner frequency (related to the source rupture velocity and stress-drop) for short period

New predictive equations and site amplification estimates for the next-generation Swiss ShakeMaps

We present a comprehensive scientific and technical update of the Swiss customization of United States Geological Survey ShakeMap, in use at the Swiss Seismological Service since 2007. The new Swiss ShakeMaps are based on predictive equations for peak ground-motions and response spectra derived from stochastic simulations tailored to Swiss seismicity. Using synthetics allows overcoming the difficulties posed by: (i) the paucity of strong-motion data recordings in Switzerland; (ii) the regional dependence of shear wave energy attenuation and focal depth distribution in the Swiss Alps and foreland; (iii) the depth dependence of stress parameters suggested by macroseismic and instrumental observations. In the new Swiss ShakeMaps, VS,30 is no longer used as proxy for site amplification at regional scale, and is replaced by macroseismic intensity increments for different soil classes, based on the recently revised earthquake catalogue of Switzerland (ECOS-09). The new implementation converts ground-motion levels into macroseismic intensity by means of ground-motion to intensity conversion equations based on the Italian strong-motion and intensity databanks and is therefore well constrained for intensities larger than VII. The new Swiss ShakeMaps show a satisfactory agreement with the macroseimic fields of both large historical events and recent well-recorded earthquakes of moderate magnitude. The new implementation is now fully consistent with the state-of-the-art in engineering seismology in Switzerlan

The earthquake of 250 a.d. in Augusta Raurica, A real event with a 3D site-effect?

The Roman city Augusta Raurica is located East of Basel, Switzerland. One important topic of the city's history concerns the hypothesis of an earthquake striking the city in the middle of the third century a.d. This idea had been formulated according to archaeological features and findings, but had not been tested so far. A selection of the archaeological features were reviewed and dated in order to test the hypothesis of a single event. However, archaeological investigations do not draw a conclusive picture; it could not be proven that all features of possible destruction date to the same event. Detailed seismological investigations were performed. These included geological and geotechnical mapping of the unconsolidated sediments. Important parameters such as the thickness and composition of the unconsolidated sediments, the terrain topography and the topography of the bedrock surface were mapped. Ambient vibration H/V measurements provided the fundamental frequency of resonance for the unconsolidated sediments. The velocity of shear waves traveling through sediments is the controlling parameter for amplification of seismic waves. This material property is estimated using the relation between the ellipticity of the fundamental mode Rayleigh wave and the H/V curve. From all information we compiled a three-dimensional model of the surface geology. This model is used to simulate earthquake ground motion and amplification effects in the city, and to map the variability of the amplification. In the part of the city where possible earthquake damage was recognized, amplification occurs in the frequency band of building resonance (2-8Hz). In the other part of the city amplification occurs much above the building's resonance. From 1D modelling we estimate a difference in spectral amplification of about a factor of 2.5 to 3 between the two parts of the city. This corresponds approximately to a difference in macroseismic intensity of one unit. 3D modelling showed a large variability of ground motion within very close distance in the part of the city where possible earthquake damage was recognized. The maximum amplification reaches values up to a factor of nine, which is due to 3D effects and the choice of using vertically incident waves. Finally, all paleoseismological findings in the area of Basel were reviewed in order to find indications of a large event in the time-period of interest. Paleoseismological findings provide no hints to a large earthquake in the third century. If we assume that an earthquake caused at least part of the identified damage in Augusta Raurica, we have to assign to this event a magnitude Mw of about 6.0 or even lower, that is much smaller than the value of 6.9 that is actually in the Swiss earthquake catalogue. The earthquake source of this event must then be very close to the site of Augusta Raurica and a strong site-effect occurred in one part of the cit

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 35years. The two most noteworthy earthquakes were the ML3.4 Barrhorn event near Sankt Niklaus (VS) and the ML 3.0 event of Feldkirch, both of which produced shaking of intensity I

Earthquakes in Switzerland and surrounding regions during 2011

This report of the Swiss Seismological Service summarizes the seismic activity in Switzerland and surrounding regions during 2011. During this period, 522 earthquakes and 92 quarry blasts were detected and located in the region under consideration. With a total of only 10 events with M L≥2.5, the seismic activity in the year 2011 was far below the average over the previous 36years. Most noteworthy were the earthquake sequence of Sierre (VS) in January, with two events of M L 3.3 and 3.2, the M L 3.3 earthquake at a depth of 31km below Bregenz, and the M L 3.1 event near Delémont. The two strongest events near Sierre produced shaking of intensity I

Earthquakes in Switzerland and surrounding regions during 2011

This report of the Swiss Seismological Service summarizes the seismic activity in Switzerland and surrounding regions during 2011. During this period, 522 earthquakes and 92 quarry blasts were detected and located in the region under consideration. With a total of only 10 events with M L≥2.5, the seismic activity in the year 2011 was far below the average over the previous 36years. Most noteworthy were the earthquake sequence of Sierre (VS) in January, with two events of M L 3.3 and 3.2, the M L 3.3 earthquake at a depth of 31km below Bregenz, and the M L 3.1 event near Delémont. The two strongest events near Sierre produced shaking of intensity I

Earthquakes in Switzerland and surrounding regions during 2005

Abstract.: 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 seismicit