Detailed mapping of seismic anisotropy with local shear waves in southeastern Kamchatka

Abstract

SUMMARY The Kamchatka Peninsula lies over a vigorous subduction zone where Pacific and North American plates converge at a rate of almost 80 mm yr −1 . Earthquakes associated with the subduction process provide an excellent source of seismic data for the study of anisotropic properties of the upper mantle and crust overlying the downgoing lithospheric slab. We collected a large set of shear waves from events within the slab recorded by a variety of seismic stations in Kamchatka. Data from permanent and temporary networks cover the entire ∼700 km length of the subduction zone, with 50‐200 km spacing between observing sites, resulting in an unprecedented coverage of the supraslab mantle wedge. We estimated shear wave splitting in selected S waves using two techniques and applied quality tests to ensure measurement stability. Fast directions vary from station to station, and they can vary with backazimuth at individual stations and with direction of propagation for individual sources. In over 350 measurements we recovered meaningful splitting delays, up to 1 s, with most delays in the 0.2‐0.6 s range. Additionally, in nearly 200 measurements splitting could not be resolved, yielding ‘NULL’ observations. Anisotropic properties of the Kamchatka supraslab mantle wedge vary greatly along the volcanic arc and forearc of the subduction zone. Observed anisotropic indicators in the arc and forearc correlate spatially with some tectonic features (e.g. volcanoes). Inland of the volcanic arc most splitting values indicate trench-parallel fast polarization. We do not observe depth dependence in local S-wave splitting delays, consistent with a shallow coherence of anisotropic texture. In the vicinity of Petropavlovsk-Kamchatsky observed anisotropic indicators are coherently trench-normal, and thus consistent with 2-D corner flow. However, splitting above the fragmented slab edge near the Klyuchevskoy volcanic centre is variable and trench-oblique. Birefringence between Petropavlovsk and Klyuchevskoy is weak. Overall, our observations are incompatible with a regional slab-driven corner flow regime.