Effects of subduction zone structure on explosion-generated Rayleigh waves: 3-D numerical simulations

Abstract

SUMMARY Three-dimensional linear elastic finite difference calculations were used to investigate effects of near-source scattering on long-period surface waves radiated by shallow explosions located in the arc of a subduction zone. These simulations were motivated by the teleseismic Rayleigh wave amplitude patterns of explosions detonated at the Amchitka test site. Amplitude patterns of these explosion-generated 20–40s surface waves show evidence of both tectonic release and source-receiver path effects. A 3-D velocity model was constructed based on the Aleutian subduction zone structure of Boyd & Craeger (1991) and crustal refraction results reported by Lambert et al. (1970). Two 3-D finite difference calculations were performed to assess the sensitivity of the results to the source location and fine details of the velocity model. Each finite difference grid consisted of 2 million nodes and the calculations were designed to model surface waves in the 20–40s period range at distances up to 400 km from the source. Displacements on the free surface were analysed to show the effects of the velocity structure upon the 3-D propagation of Rayleigh waves from an explosion source. Azimuthally dependent Rayleigh wave amplitudes are clearly seen as well as Rayleigh-to-Love wave conversion along the strike of the subduction zone structures. Teleseismic Rayleigh wave amplitude anomalies of up to a factor of two are clearly evident from the near-source scattering. Love waves 1/3 to 1/4 the amplitude of the Rayleigh waves are radiated in narrow azimuthal ranges. A method for the continued propagation of surface waves is presented based on a 2-D Fresnel-Kirchhoff integral for surface waves. The Fresnel-Kirchoff integral accounts for diffraction effects assuming uniform propagation outside the finite difference grid. This hybrid procedure using both the finite difference simulation and the Fresnel-Kirchoff integral predicts 50 per cent Rayleigh wave amplitude anomalies at teleseismic distances due to structure within 400 km of the Amchitka test site.