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A Numerical Study of Effects of Valley-Weathering and Valley-Shape-Ratio on the Ground Motion Characteristics

Narayan J. P., Arafat M. Y., Kamal

Acta Geophysica

2015

Vol. 63, no. 1

154--175

A study of combined effects of valley-weathering and valley-shaperatio on the ground motion characteristics and associated differential ground motion (DGM) is documented in this paper. In order to properly quantify the weathering effects, a forth-order-accurate staggered-grid viscoelastic time-domain finite-difference program has been used for the simulation of SH-wave responses. Simulated results revealed that the defocusing caused by valley is frequency-independent in contrast to the ridge-focusing. A decrease of average spectral amplification (ASA) with an increase of shape-ratio of the non-weathered triangular and elliptical valleys was obtained. Overall, the amplification and de-amplification pattern was larger in case of triangular valleys as compared to the elliptical valleys. It can be concluded that the dwelling within or near the topcorners of weathered valleys may suffer more damage as compared to their surroundings. A weathered triangular valley with large shape-ratio may cause unexpected damage very near its top-corners since both the ASA and DGM are largest.

Study of combined effects of sediment rheology and basement focusing in an unbounded viscoelastic medium using P-SV-wave finite-difference modelling

Narayan J.P., Kumar V.

Acta Geophysica

2014

Vol. 62, no. 6

1214--1245

This paper consists of two parts. First, a fourth-order-accurate staggeredgrid finite-difference (FD) program for simulation of P-SV-wave in viscoelastic medium is presented. The incorporated realistic damping is based on GMB-EK-model. The accuracy of program is validated by comparing computed phase-velocity and quality-factors with same based on GMB-EK-model and Futterman’s relations. The second part of paper presents the combined effects of sediment damping and synclinal basement focusing (SBT) on ground motion. The results reveal SBT focusing, mode conversion and diffraction of incident waves. The response of elastic SBT model reveals an increase of spectral amplification with increasing frequency. The viscoelastic response of SBT model reveals that a particular frequency may get largest amplification for a particular set of values for damping, focal-length and distance from tip of the SBT. This frequency-dependent amplification may explain mysterious damage reported in some past earthquakes if predominantly amplified frequency matches natural frequency of damaged structures.