Identifying 2D Resonance in Microtremor Wave Fields

To predict local site effects during earthquakes, we must know the geometrical and mechanical properties of local sedimentary structures and their resonance behavior. We present a concept to identify resonance modes for 2D structures. This is done for synthetic as well as measured microtremor wave fields. The microtremor data were investigated by using the reference station and horizontal-to-vertical (H/V) methods. Analyzing the synthetic microtremor data in profiles across 2D structures, the reference station method (RSM) turned out to be much more suitable for the detection of 2D resonances than the H/V method. We therefore focused on RSM when analyzing the measurement data from two test sites in Switzerland, the St. Jakob–Tullingen trough near Basel in the north and a site in the Rhone valley near the village Vetroz in the south. The results of these two test sites led to different conclusions. Whereas the spectral peaks in data of the St. Jakob-Tullingen trough were not well developed and could only tentatively be interpreted, those from the Rhone valley clearly exhibited 2D resonance effects: the SH00 and the SV-fundamental mode could be successfully identified at 0.32 (plus-minus) 0.03 and 0.35 (plus-minus) 0.03 Hz, respectively.