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Geometrical focusing as a mechanism for significant amplification of ground motion in sedimentary basins: analytical and numerical study |
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| Authors: | Shahar Shani-Kadmiel Michael Tsesarsky John N Louie Zohar Gvirtzman |
| Institution: | 1. Department of Geological and Environmental Sciences, Ben Gurion University of the Negev, Beersheba, Israel 2. Department of Structural Engineering, Ben Gurion University of the Negev, Beersheba, Israel 3. Nevada Seismological Laboratory, University of Nevada, Reno, USA 4. Geological Survey of Israel, Jerusalem, Israel |
| Abstract: | 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 $\pm $ 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. |
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