Quantification of focusing effects of the semi-spherical and semi-cylindrical synclinal basement topography on ground motion characteristics

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.

Research on basin effect based on 2-D geophysical prospecting profile

“…It can be seen that the spectral ratios at the center of this small basin have little to do with the burial depth of the basin's basement, and the amplification of the ground motion is mainly related to the lateral velocity structure of the basin or the geometric characteristics of the basin's basement. For this basin, the anomalous amplification of the ground motion by the center of the basin may be due to the focusing effect of this bowl-like structure on the seismic waves, which is consistent with the results of previous studies (Aki and Larner, 1970;Gao et al, 1996;Alex and Olsen, 1998;Davis et al, 2000;Baher and Davis, 2003;Stephenson et al, 2006;Sahar and Narayan, 2016). This may be the reason for the anomaly in seismic intensity in the Luoquanli Basin.…”

“…There may be two reasons for this. First, this special bowl-shaped structure tends to produce a focusing effect on the seismic waves, and this focusing effect amplifies the high-frequency ground motion more significantly (Davis et al, 2000;Sahar and Narayan, 2016). Second, when the vertical upward propagating seismic wave period is greater than the threshold value, the long-period seismic wave is suppressed because the wavelength is larger than the thickness of the sedimentary layer in the basin.…”

“…The earthquake damages to the corresponding basins caused by the Mexico earthquake, the Northridge earthquake, and the Hyogoken Nambu earthquake are well known to everyone, and as a special site effect, the basin effects are receiving more and more attention from seismologists (e.g., Kawase, 1996;Graves et al, 1998;Olsen, 2000;Komatitsch et al, 2004;Lee et al, 2008a;Lee et al, 2008b, Lee et al, 2009Iyisan and Khanbabazadeh, 2013;Chaljub et al, 2015;Maufroy et al, 2017;Zhang et al, 2019;Zhang et al, 2020). The results of existing studies have shown that the factors affecting the basin effects are mainly the structure of the sedimentary layer, the geometry of the basin's basement strata, and the seismic wave input orientation or source location, among which, the sediment structure (Field, 2000;Frankel et al, 2001Frankel et al, , 2002Fletcher and Wen, 2005;Roten et al, 2006;Hartzell et al, 2016) and geometry of the basin's basement strata (Gao et al, 1996;Alex and Olsen, 1998;Davis et al, 2000;Baher and Davis, 2003;Stephenson et al, 2006;Shani-Kadmiel et al, 2012;Riga et al, 2016;Sahar and Narayan, 2016;Zhu, 2018) have the most significant effects on the ground motion.…”