Microseismic tomography analysis of the larger Fribourg area (western Swiss molasse basin)

This thesis presents an investigation on the subsurface structure of the larger Fribourg area in the western Swiss Molasse Basin using microseismic tomography analysis. Since the Swiss Seismological Service (SED) has installed the Swiss Digital Seismic Network (SDSNet) and the Swiss Strong Motion Network (SSMNet) in the early 1970 and 1980, the seismic monitoring shows an increased level of seismic activity in this area over the past decade [Kastrup et al., 2007]. It shows seismicity with magnitudes of Mw _ 4:0 distributed with an orientation of NS known as the Fribourg Lineament. This seismic activity draws the attention for the vulnerability of the population and its infrastructure, such as the Muhleberg Nuclear Power Plant (KKM), following the detection of three earthquakes series, each in the year 1987, 1995, and 1999 by the SED, in which the last event at Marly (Fribourg, Switzerland) in 1999 has a magnitude of ML = 4:3 Since 2010 until 2013, two portable sparse mini-arrays (seismic navigating systems/SNS) were deployed to enhance the seismic monitoring capability in the larger Fribourg area, especially for the Fribourg Lineament. The seismograms recorded by these arrays and by the seismic networks of the SED were analyzed using nanoseismic monitoring method [Joswig, 2008; Sick et al., 2012] in order to detect very weak (ML _ 0.5) events, for which signal to noise ratio is minimal. Detected events were located using a non-linear probabilistic earthquake location software (NonLinLoc) [Lomax et al., 2000] using consistent P-phase arrival time picking including uncertainties. Due to this signi_cant lowered detection threshold, 206 events were detected and located in the larger Fribourg area in addition to the 115 earthquakes reported by the SED. A comprehensive local catalog of microseismicity (TomoCat) was build based on these detected and located events. The approach of having a better understanding of the subsurface structure of the larger Fribourg area is to obtain a subsurface imaging using microseismic tomography analysis represented by a three-dimensional (3D) P-wave velocity (Vp) model. The tomography analysis was carried out using SIMULPS2000 [Thurber and Eberhart-Phillips, 1999] on an initial velocity model with an extent (CH1903-LV03) of 500 000 m to 680 000 m (W-E) and 110 000 m to 260 000 m (S-N) with the elevation from -250 00 m to 5 000 m built on the basis of existing interpretation by Interoil [2010] of seismic lines within the study area, velocity values on boreholes within the western Swiss Molasse Basin [Sommaruga et al., 2012], and the 3D P-wave velocity model of Switzerland [Husen et al., 2003]. The TomoCat catalog has 2745 raypaths of P-wave for tomography analysis resulting in a 3D velocity model with a minimum node spacing of 8063 m horizontally and 705 m vertically. The microseismic tomography inversion using the events in TomoCat catalog allows the reconstruction of 90.26% _ 7.5% of a synthetic velocity model in this area. Investigation on the principal stress axes in the larger Fribourg area is obtained through focal mechanism solutions based on the observation of the _rst motion polarity of the P-phases and the 3D velocity model resulting from the microseismic tomography analysis. The focal mechanism solutions were calculated using FPFIT [Reasenberg and Oppenheimer, 1985] with quality assignment according to the World Stress Map Project Guidelines [Barth et al., 2008]. The analysis shows an orientation of maximum horizontal compression (SH) regional of 147.4_ with mostly strike-slip faulting tectonic regime which it correlates with the regional compression of the Prealps from SE to NW towards the Jura Mountains.