2D analysis of earthquake ground motion in Haifa Bay, Israel

We study the earthquake response of the Zevulun Valley basin, underlying northern Israel‘s largest urban area with two-dimensional viscoelastic seismic modeling of a detailed geological section. We found that amplification of the horizontal vibrations, the ratio of basin to no-basin response spectra, correlates with basin depth. In the deepest portion of the basin (Qishon graben) long periods (2-5 sec) are amplified by 400% and in the shallowest portion of the basin (Afeq horst) shorter periods (~0.5 sec) are amplified by 300-400%. These resonances in the vertical direction through the basin are strong enough that their amplitude overwhelms the amplitude of a previously recognized basin-edge effect. H/V Fourier spectral ratios based on 124 ambient noise measurements do not fully coincide with the basin-to-no-basin Fourier spectral ratios of the simulation, but the resonance frequencies found in both methods are alike. Moreover, the relation between the resonance frequency and the depth of the corresponding seismic reflector in the simulation is almost identical to the empirical frequency-depth relations obtained from measurements. This indicates that the average shear-wave velocity of the sedimentary column in the model is consistent with measurements. To evaluate the necessity of 2D analysis we performed additional 1D simulation at two locations along the section. For the Qishon graben 1D analysis underestimates the amplification factor relative to 2D by 25%, whereas for the Afeq horst 1D and 2D simulations are similar. For a hard layer within the soft Qishon graben fill, we found that when the hard layer is thinner than ~50 m its influence on ground motion is small.