Earthquake ground motion amplification in the coastal plain and Zevulun Valley
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 2D 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%; 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. The horizontal/vertical (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 simulations 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.
Published paper: Gvirtzman Z, Louie JN (2010) 2D analysis of earthquake ground motion in Haifa Bay, Israel. Bull Seismol Soc Am 100:733–750. https://doi.org/10.1785/0120090019
3D Effects of Sedimentary Wedges and Subsurface Canyons: Ground-Motion Amplification in the Israeli Coastal Plain
We study the propagation of seismic waves, the resulting ground motions, and their amplification atop sedimentary structures underlying continental passive margins. We employ a set of generic models with increasing complexity within a framework of a 3D numerical scheme. The basic geological structure and velocity model were derived from the subsurface of the Israeli coastal plain where soft sediments form a wedge over the stiffer bedrock and fill subsurface canyons that incise deep into the bedrock. Ground motions were modeled for both seaside and landside seismic sources. We show that for a landside source, peak ground velocities (PGVs) atop a sedimentary wedge are amplified by a maximum factor of 2.6 and on average by a factor of 1.6, relative to a reference model. This amplification is mainly due to the ellipticity of Rayleigh waves in the soft sediment layer. Spatial distribution of amplification factors shows that sedimentary wedges do not exhibit a prominent edge effect. Atop sediment-filled canyons and landside source, PGVare amplified by a maximum factor of 3.3, relative to a reference model, along the exposed part of the canyon. The PGVamplification factor in the canyon relative to adjacent hard-rock site is up to 2.4. PGV amplification atop the sediment-filled canyons is mainly due to the geometrical focusing of SH waves. Based on our findings, we present a simplified ground-motion amplification map for the Israeli coastal plain.
Ground motion amplification atop the complex sedimentary basin of Haifa Bay (Israel)
The Zevulun Valley (ZV) is a sedimentary basin underlying the heavily populated and industrialized petrochemical hub of Haifa Bay, Israel. With active tectonic faults at close range and a mixture of large population and vulnerable facilities, the seismic risk in the ZV is high. However, until now the national seismic network in Israel only included rock stations with no measurements supporting the expected difference between the ZV and its surroundings. Moreover, a detailed analysis of ground motions atop sedimentary basins using earthquakes data was never conducted in Israel for any basin. In this paper, we present a dataset collected during a 16 months monitoring campaign with a transportable network deployed in the ZV. For the first time in Israel we simultaneously recorded earthquake (3.1 < Mw < 5.5) ground motions at basin- and reference-sites. Spectral ratios reveal amplification factors tangibly higher than those previously reported by horizontal-to-vertical-spectral-ratio (HVSR) techniques and 2-D modeling. In particular, the deeper parts of the valley exhibit ground motion amplification up to a factor of 8 at frequencies lower than 1 Hz. Comparison of the measured spectral ratios with the results of 1-D linear-elastic analysis shows partial correlation reflecting the complexity of the sub-surface structure.
