Abstract
The anomalous damage patterns developed by the focusing of seismic waves due to deep and shallow seated synclinal basement topography were reported during the Northridge earthquake of 1994 and the Nisqually earthquake of 2001, respectively. This paper presents the role of sediment velocity, depth and geometry of the basement topography in basement focusing effects on ground motion characteristics. An increase of amplitude of the mode converted and the diffracted waves with an increase of impedance contrast and curvature of the basement topography was inferred. It is concluded that the basement focusing effect is unaffected by the change of focal length due to the change of either sediment velocity or radius of curvature for a fixed chord length in the absence of sediment damping. Further, under a favourable condition, the focusing of multiples of the transmitted waves may cause much larger amplitude amplification than that caused by the focusing of the transmitted wave itself. Extensive spatial variations in ground motion level were obtained with the change of depth and chord length of the basement topography. A comparative analysis of the responses of semi-spherical basement topography (3D) and semi-cylindrical basement topography (2D) strongly suggests that 1D or 2D response of an area underlain by a 3D depression should not be used in predicting the ground motion.
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The second author is grateful to the Ministry of Earth Sciences, New Delhi, for financial assistance through Grant Number MES-484-EQD.
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Sahar, D., Narayan, J.P. Quantification of focusing effects of the semi-spherical and semi-cylindrical synclinal basement topography on ground motion characteristics. J Seismol 20, 167–183 (2016). https://doi.org/10.1007/s10950-015-9518-8
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DOI: https://doi.org/10.1007/s10950-015-9518-8