MODELING S WAVE AMPLITUDE PATTERNS FOR EVENTS IN THE KURILE SLAB USING THREE-DIMENSIONAL GAUSSIAN BEAMS

Abstract

The shear velocity structure of the subducted Kurile slab is modeled using shear wave amplitude residual spheres. We measure long-period SH and ScSH phase amplitudes for 10 intermediate and deep focus earthquakes recorded at World-Wide Standardized Seismograph Network and Canadian Seismic Network stations from 1967 to 1986. After correction for source radiation pattern, geometric spreading, and empirical receiver/upper mantle effects, the data for intermediate depth Kurile slab events show smoothly varying amplitude residual sphere patterns with low amplitudes concentrated in the direction down the slab dip. The data are modeled using three-dimensional dynamic ray tracing and a three-dimensional Gaussian beam method. Starting models are taken from previous P and S wave arrival time residual sphere studies. Three-dimensional ray tracing indicates that ray patterns and resulting amplitudes are very sensitive to the depth extent of the slab and to the position of the event relative to the across-slab velocity gradient. This sensitivity allows us to place some constraints on the Kurile slab geometry, position of each event within the slab, and penetration depth of coherent slab structure. Our final slab model is defined by an asymmetric error function that peaks at 4% higher velocity than Preliminary Reference Earthquake Model 25 km from the top of the slab. The total width of the thermal anomaly averages 140 km. The slab is tabular and extends to 670 km in the northern half of the Kurile subduction zone. In the southern half of the subduction zone the slab flattens along the 670 km discontinuity and deflects laterally, persisting to 750 km depth.