Abstract

The seismic waves from subduction zone earthquakes are significantly affected by the presence of 3D variation in crust and upper-mantle structure around the source area. These heterogeneous structures also profoundly modify the character of seismic waves as they propagate from the source area to regional distances. This is illustrated by studying shallow, interplate earthquakes along the Mexican subduction zone, and deeper, inslab, normal-faulting earthquakes in the subducted Cocos plate beneath Mexican mainland. The strong-motion recordings of these earthquakes are used to evaluate the character of wave propagation along the path between the source region and Mexico City. We compare the wavefield from two large earthquakes of different source type. During the shallow (H = 17 km), interplate, 1995 Guerrero earthquake (Mw 7.3), the Lg phase is the most prominent feature at regional distances of about 150 to a few hundred kilometers from the source. The presence of a lateral velocity gradient in the crust, caused by the subduction of the Cocos plate, enhances the Lg-wave amplitude, which is then amplified further in the Mexican volcanic belt by amplification in the low-velocity volcanic rocks. Both effects lead to very large ground motions along the path from the coast to the Mexican inland, in the frequency band from 0.2 to 4 Hz. However, for the deeper (H = 40 km), inslab, normal-faulting, 1999 Oaxaca earthquake (Mw 7.5), the amplitude of the Lg phase is too small to produce the abnormal wave propagation, and the direct S wave and its multiple SmS reflections between the free-surface and Moho show a simple attenuation with increasing distance. We compare these observations with numerical simulations of seismic-wave propagation using the Fourier spectral method. The results provide a key to the understanding of seismic-wave field generated by shallow interplate and deeper inslab earthquakes in a realistic 3D heterogeneous structure.

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