Microseismic characterization of Fribourg area (Switzerland) by nanoseismic monitoring

This thesis aims to investigate the present level of microseismicity generated within the Fribourg area (Switzerland). In particular, it focuses on a 20- 30 km N-S trending seismic lineament located east and north of the city Fribourg in the Western Swiss Molasse Basin. The so-called Fribourg Lineament has recently received a lot of attention, following the detection by the Swiss Seismological Service (SED) of three earthquake sequences in 1987, 1995 and 1999. On the basis of seismic survey interpretation, the three sequences have been interpreted to be related to a 20- 30 km long active tectonic fault reaching the Basement [Kastrup et al., 2007]. The size of the inferred fault would carry the potential of a magnitude 6 earthquake and thus constitute significant seismic hazard source for the Fribourg region. The implications of the recent seismic activity within the Fribourg zone have raised questions about the vulnerability of critical infrastructures such as the Mühleberg Nuclear Power Plant (KKM), located NE of Fribourg and NW of Bern. In that context, the Earth Sciences Institute of Fribourg University was mandated by the RESUN AG1 and subsequently the BKW to investigate the spatiotemporal characteristics of the microseismicity generated within the Fribourg area. The objective is to assess the link between located source processes and potential subsurface tectonic features in order to estimate an upper magnitude bound for an earthquake to be generated within Fribourg region. Based on a new systematic and consistent interpretation of seismic survey imaging by [Interoil, 2010], a 3-D tectonic model of the Fribourg area has been built in ArcScene software and integrated in a comprehensive GIS tectonic database. The main outcome of this newly interpreted dataset is the apparent decoupling of interpreted fault zones within the Molasse Basin from the pre-Mesozoic Basement and thus the significant reduction of potential earthquake source size. The GIS tectonic database was used to verify the level of coincidence betweendifferent independent datasets: surface data (structural, geomorphologic, hydrologic…), subsurface data (seismic surveys, geopotential data) with the GIS based earthquake catalog established in the present study. Microseismic investigation in Fribourg area was carried out by nanoseismic monitoring techniques [Wust-Bloch and Joswig, 2006; Joswig, 2008; Sick et al., 2012]. Nanoseismic monitoring takes advantages of mini-arrays and advanced signal processing tools to optimize seismic event detection and location down to the noise threshold. Signal to noise (SNR) conditions are maximized by deploying the seismic arrays as close as possible from the target seismogenetic source. Events are detected by waveform screening under the form of sonograms. Sonograms are logarithmically scaled spectrograms with an automatic noise-adapted filter that enhance the display of weak signal energy down the background seismic noise level. Further event identification is carried out by HypoLine software. This innovative interactive software uses a jackknife approach which displays points of highest probability in space-time for potential event evaluation and location. Between 2010 and 2013, two seismic mini-arrays were deployed semi-permanently north and south of the Fribourg Lineament. In addition, available seismic records from three local stations of the Swiss network were processed by sonogram analysis for event detection. During the investigated period, the SED detected 45 events within the Fribourg target zone. The application of nanoseismic monitoring techniques in Fribourg region enabled the detection of 282 additional events. Due to low SNR conditions, a reliable location, with horizontal uncertainty below 1-2 km was possible only for the stronger events. Absolute location of earthquakes outline three area of apparent enhanced seismicity: the Fribourg Lineament, the St-Sylvester Structure (further south) and the Fribourg Cluster (located under the city Fribourg). All events seem to be generated within the sedimentary cover. Based on a waveform similarity analysis, it can be shown that most events are in fact related to a few main “earthquake families”. Events related to these families display striking waveform similarities, with correlation coefficient above 80-90% within a 2-10 Hz frequency band. In consequence, we interpret these families as the expression of repeated ruptures generated within the same fault zone under same rupture mechanisms. Focal mechanism solutions computed by SED for the stronger events derive almost pure strike-slip with leftlateral N-S motion for events related to the Fribourg Lineament. Structural observation in Fribourg area reveals an intensely deformed zone of left-lateral N-S oriented tear faults along-strike of the Fribourg Lineament. Further south, the Alpine orogenic front (Subalpine Molasse) shows geomorphological evidence of leftlateral N-S strike-slip offset. Important strike-slip fault zones are known to cross-cut the Jura fold-andthrust belt further north and west and to extend in the Molasse Basin (e.g. [Sommaruga, 1997]). Therefore, we propose that the Fribourg Lineament represent such a fault zone, which starts within the Subalpine Molasse and runs into the Molasse Basin as a result of ongoing Alpine compressive stresses. On the basis of located source processes, we infer a 5-10 km active N-S trending fault zone located east of the city Fribourg. This fault zone presently triggers low-magnitude (-2 ≤ ML ≤ 2) short recurrence-time seismic activity. Such a pattern of energy release fits well field structural observation of small scale (< 100 m) en-échelon faults. However, individual en-échelon faults can interact and connect towards an anastomosing larger fault zone. Such a phenomenon would generate an enlarged rupture area and therefore a higher magnitude event. Therefore, on the basis of the rupture zone delineated by our earthquake catalog, we infer a potential of a magnitude 5 earthquake for the tectonic feature related to the Fribourg Lineament and do not exclude the possibility of an even higher magnitude event. Thus, the Fribourg Lineament constitutes significant hazard for the region. In 1996, an event with ML 5.3, located near Annecy (France) was generated within the sedimentary cover (1-3 km depth) and reached MSK intensities of VII-VIII, provoking great damages in the epicentral area. This earthquake was originated by the Vuache fault, a sinistral strikeslip fault zone which links the Subalpine Chains to the Jura Mountains south of Geneva at the southwestern tip of the Molasse Basin, in a tectonic framework that is comparable to the Fribourg Zone.