A quantitative study of the site effects observed at the GERESS array

Site effects were studied for the individual sites of the 25 elements of the German Experimental Seismic System (GERESS) array based on the traditional spectral ratio (TSR) technique and horizontal-to-vertical ratios (HVRs) for earthquakes and microtremors, commonly referred to as Nakamura's technique. An understanding of these site effects is required for various applications, in particular for the discrimination of seismic events. The spectral ratio technique identifies three stations (A0, A3, B3) as showing prominent peaks in the frequency band from 1 to 17 Hz up to a factor of 30, with A0 and B3 having the largest site effect at a frequency of 7 Hz. These stations are situated in the immediate vicinity of the array center. A number of other stations, such as A1, B1, B4, B5, C1, C3-C5, D2, D5, and D7-D9, display a broad frequency range preferably above 10 Hz, where the spectral ratios indicate up to 10 times higher spectral amplitudes compared to the reference site, C2. The remaining stations, A2, B2, C6, C7, D1, D3, D4, and D6, show no noticeable site effects. Further study considers the appropriateness for the selection of C2 as reference site for the TSR technique and the comparison of TSR and HVR techniques for the three-component elements of the array. Although typically site response refers to the site amplification of horizontal ground motions, the assessed site response of vertical ground motions seems to yield comparable results. The spectral variance, a parameter based on the smoothness of stacked array spectra from the best 17 array elements of the GERESS array, has been used successfully to distinguish between local earthquakes and mining explosions. It is shown that the spectral variance estimate is considerably improved using all 25 array elements and applying a site response correction.