Empirical estimate of resonance frequency at the Dead Sea basin

Abstract

We carried out assessments of the fundamental frequencies of the southern Dead Sea basin using records of teleseisms and local earthquakes, obtained by the DESERT2000 (13 events) and DESIRE (40 events) projects that aimed in studying the Dead Sea fault in general and in particular the Dead Sea basin. The observed ground motions are from teleseisms with magnitude larger than 6.5 at distances of 1340–14,800 km, and of local earthquakes in the magnitude range 3.8–4.9 at distances of 90–660 km. We determine the fundamental resonance frequencies of the Dead Sea basin using (1) ratio between the spectra of seismograms recorded on site and those recorded at a reference site and (2) horizontal-to-vertical S-wave spectral ratio. Our results allow dividing the study area into three segments of different lengths: first 22 km (stations JB12–IB13), second 5 km (stations IB15–IB17), and third 45 km (stations IB19–ID27) with fundamental frequencies 0.1, 0.2, and 0.1 Hz, respectively. Amplification factors vary from 2 to 5. Thirteen reference stations are located on rock, at distances of 1–10 km away from the basin, which recorded vibrations in the range of 0.1–0.5 Hz, generated in the Dead Sea basin. We did not find locally induced surface waves, causing significant lengthening of the shaking duration and strong low-frequency amplification. It can be explained by the linear dimensions of the Dead Sea basin, its shape, and the relatively high velocity of shear waves that varies with depth from 800 to 3200 m/s. We use stochastic optimization algorithm to calculate the velocity model, yielding 1-D transfer function matching the observed H/V curves and resonance peaks.