Numerical modelling of the effects of an underground ridge on earthquake-induced 0.5-2.5 Hz ground motion

Abstract

SUMMARY This paper presents a numerical simulation of the effects of an underground ridge (UGR) on seismically-induced ground motion characteristics using 2.5-D modelling with variable grid size. The responses of models with and without an UGR, estimated group velocity of later arrivals, horizontal differential ground motion, snapshots and spectral analysis of surface waves have confirmed high frequency surface wave generation near the ridge. Exponential decay of surface wave amplitude with depth was obtained in the vertical component but not in the radial and transverse components. Changes of focal mechanism parameters mainly affected the amplitude but not the spectral shape or bandwidth of UGR-induced surface waves. We predict that anomalous macroseismic intensity due to UGR-induced surface waves might arise in a zone of width of about 2.2 km parallel to a ridge and at a distance of about 0.5 km from the ridge edge, for the model geometry and the soil parameters considered. The UGR-induced surface waves were observed even when a UGR was overlain by a soil deposit. Increased depth or changes in the slope of the flanks of the UGR causes decrease of spectral amplitudes but does not affect the spectral shape, bandwidth or dominant frequency of induced surface waves. The dominant frequency of Rayleigh waves appeared to be twice that the fundamental frequency ( f 0) of first soft soil layer. For Love waves, the dominant frequency was twice that of f 0 for f 0 ≤ 0.63 Hz and was more than twice f 0, when f 0 > 0.63 Hz. The lower cut-off frequency for surface wave generation was approximately f 0. Only spectral amplitudes were affected by a change of S-wave velocity in first soil layer but not the spectral bandwidth, spectral shape or dominant frequency when f 0 was maintained constant. An increase of spectral bandwidth with decrease of f 0 of the first soil layer was found. We obtained a decrease of surface wave amplitude in accordance with the decrease of amplitude of body waves with angle of incidence. The amplitude of surface waves on the side of incidence of body waves was more than the opposite side. Further, it was inferred that dominant frequency and spectral bandwidth were unaffected but spectral shape was highly dependent on the angle of incidence of body waves. The simulations revealed that UGR effects deserve particular attention for earthquake-resistant design and seismic microzonation.