Earthquake ground motion amplification in the Dead Sea Basin
Simulation of Seismic-Wave Propagation through Geometrically Complex Basins:
The Dead Sea Basin
2D ground-motion simulations of the Dead Sea Basin (DSB) indicate a complex pattern of ground motion amplification affected by several geometric features in the basin. To distinguish between the individual contributions of each geometrical feature, we developed a semi-quantitative 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.
Simulation results from six simplified models: (a) reference, (b) horizontal layers, (c) layers plus basin edge faults, (d) layers plus a concave diapir, (e) layers plus a convex sub-basin, and (f) all features combined. 4 and 5 are edge effects related to faults. 6 is de-amplification related to a diapir that distributes seismic waves. 8 is geometrical focusing by concave basin. In each figure lower panel is the geological structure; upper panel is the calculated PGV; and mid panel is amplification ratio relative to reference model. Abbreviations: PGV, peak ground velocity; Amp., amplification.
Published paper: Shani-Kadmiel, S., Tsesarsky, M., Louie, JN., Gvirtzman, Z., 2012.
Simulation of seismic wave propagation through geometrically complex basins - the Dead Sea Basin,
Bulletin of the Seismological Society of America, v. 102, 1729-1739; doi: 10.1785/0120110254;
Geometrical focusing as a mechanism for significant amplification of ground motion in sedimentary basins: analytical and numerical study
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.
(a) Schematics and nomenclature of incident and transmitted rays over a semi- elliptical interface between host-rock (dark gray) and basin-fill (light gray). (b) Schematic cartoon showing pairs of incident rays converge to different locations, z(x), as a function of point of incidence and basin geometry (increasing basin depth). (c) Same as (b) for increasing velocity ratio
Published paper: Shani-Kadmiel, S., Tsesarsky, M., Louie, JN., Gvirtzman, Z., 2014.
Geometrical focusing of seismic waves by buried geological structures – analytical and numerical study,
Bulletin of Earthquake Engineering, DOI 10.1007/s10518-013-9526-4a.
Distributed Slip Model for Forward Modeling Strong Earthquakes
We develop a generic finite-fault source model for simulation of large earthquakes: the distributed slip model (DSM). Six geometric and seven kinematic parameters are used to describe a smooth pseudo-Gaussian slip distribution, such that slip decays from peak slip within an elliptical rupture patch to zero at the borders of the patch. The DSM is implemented to initiate seismic-wave propagation in a finite difference code. Radiation pattern and spectral characteristics of the DSM are compared with those of commonly used finite-fault models, that is, the classical Haskell’s model (HM) and the modified HM with radial rupture propagation (HM-RRP). The DSM accounts for directivity effects in the fault-parallel direction, as well as fault-normal ground motions, and overcomes the unrealistic uniform slip and stress singularities of the Haskell-type models.
We show the potential of the DSM to estimate the ground motions of strong earthquakes. We use this model to initiate seismic-wave propagation during the 1927 ML 6.25 Jericho earthquake and compare calculated macro-seismic intensities to reported intensities at 122 localities. The root mean square of intensity residuals is 0.68, with 56% of the calculated intensities matching the reported intensities and 98% of the calculated intensities within a single unit from the reported intensities. The DSM is an essential step toward robust ground-motion prediction in earthquake-prone regions with a long return period and limited instrumental coverage.
Published paper: Shani-Kadmiel, Tsesarsky. M., and Gvirtzman Z., 2016.
Distributed Slip Model for Forward Modeling Strong Earthquakes, Bulletin of the Seismological Society of America,
Vol. 106, No. 1, pp. 93–103, doi: 10.1785/0120150102
Seismic Energy Release from Intra‐Basin Sources along the Dead Sea Transform and Its Influence on Regional Ground Motions
The Dead Sea Transform (DST) dominates the seismicity of Israel and neighboring countries. Whereas the instrumental catalog of Israel (1986–2017) contains mainly M<5 events, the pre-instrumental catalog lists 14 M 7 or stronger events on the DST, during the past two millennia. Global Positioning System measurements show that the slip deficit in northern Israel today is equivalent to M>7 earthquake. This situation highlights the possibility that a strong earthquake may strike north Israel in the near future, raising the importance of ground‐motion prediction.
Deep and narrow strike‐slip basins accompany the DST. Here, we study ground motions produced by intrabasin seismic sources, to understand the basin effect on regional ground motions. We model seismic‐wave propagation in 3D, focusing on scenarios of Mw 6 earthquakes, rupturing different active branches of the DST. The geological model includes the major structures in northern Israel: the strike‐slip basins along the DST, the sedimentary basins accompanying the Carmel fault zone, and the densely populated and industrialized Zevulun Valley (Haifa Bay area).
We show that regional ground motions are determined by source–path coupling effects in the strike‐slip basins, before waves propagate into the surrounding areas. In particular, ground motions are determined by the location of the rupture nucleation within the basin, the near‐rupture lithology, and the basin’s local structure. When the rupture is located in the crystalline basement or along material bridges connecting opposite sides of the fault, ground motions behave predictably, decaying due to geometrical spreading and locally amplified atop sedimentary basins. By contrast, if rupture nucleates or propagates into shallow sedimentary units of the DST strike‐slip basins, ground motions are amplified within, before propagating outside. Repeated reflections from the basin walls result in a “resonant chamber” effect, leading to stronger regional ground motions with prolonged durations.