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.

@en

The Eastern Mediterranean contains two active tectonic borders: the Dead Sea Transform (DST) and the Cyprus Arc. Along the DST system of faults several deep sedimentary basins are found. Due to moderate seismicity and sparse spatial coverage of the Israel Seismic Network, ground motion analysis atop the basins is limited. In this paper, we present the results of two different approaches for ground motion amplification analysis. For the Amiaz Basin (Dead Sea region) we performed 3-D, high resolution numerical model using different source types and locations. We use the abundance of seismically induced clastic dikes to constrain the results of the numerical model and show the complexity of structure-source interaction. Specifically, we show the importance of seismic shielding by high velocity structures. For the Zevulun Valley, which underlays the densely populated and industrialized area of Haifa Bay, we present the results of seismic monitoring campaign using a transportable array. We calculate spectral amplification factors using out of basin reference stations and show the limitations of standard 1-D analysis.@en

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, PGV are amplified by a maximum factor of 3.3, relative to a reference model, along the exposed part of the canyon. The PGV amplification 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.

@en

We study the geometrical and material conditions which lead to focusing of seismic waves traveling across a concave velocity interface representing the boundary of a sedimentary basin within a denser rock. We approximate, using geometrical analysis for plane-waves, the combination of interface eccentricities and velocity ratios for which the seismic rays converge to a near surface region of the basin. 2-D finite difference modeling is used to compute Peak Ground Velocity (PGV) and spectral amplification across the basin. We show that effective geometrical focusing occurs for a narrow set of eccentricities and velocity ratios, where seismic energy is converged to a region of ± 0.5 km from surface. This mechanism leads to significant amplification of PGV at the center of the basin, up to a factor of 3; frequencies of the modeled spectrum are amplified up to the corner frequency of the source. Finally, we suggest a practical method for evaluating the potential for effective geometrical focusing in sedimentary basins.

@en

The Dead Sea Transform (DST) is the source for some of the largest earthquakes in the eastern Mediterranean. The seismic hazard presented by the DST threatens the Israeli, Palestinian, and Jordanian populations alike. Several deep and structurally complex sedimentary basins are associated with the DST. These basins are up to 10 km deep and typically bounded by active fault zones. The low seismicity of the DST, the sparse seismic network, and limited coverage of sedimentary basins result in a critical knowledge gap. Therefore, it is necessary to complement the limited instrumental data with synthetic data based on computational modeling, in order to study the effects of earthquake ground motion in these sedimentary basins. In this research we performed a 2D ground-motion analysis in the Dead Sea Basin (DSB) using a finite-difference code. Cross sections transecting the DSB were compiled for wave propagation simulations. Results indicate a complex pattern of groundmotion amplification affected by the geometric features in the basin. To distinguish between the individual contributions of each geometrical feature in the basin, we developed a semiquantitative decomposition approach. This approach enabled us to interpret the DSB results as follows: (1) Ground-motion amplification as a result of resonance occurs basin-wide due to a high impedance contrast at the base of the uppermost layer; (2) Steep faults generate a strong edge-effect that further amplifies ground motions; (3) Sub-basins cause geometrical focusing that may significantly amplify ground motions; and (4) Salt diapirs diverge seismic energy and cause a decrease in ground-motion amplitude.

@en