SEISMIC STRUCTURE OF THE DEEP KURILE SUBDUCTION ZONE

Three-dimensional modeling of P wave travel time residuals was perform ed to investigate the deep slab-related velocity structure within the Kurile subduction zone. Travel times of P waves from earthquakes at al l depths within the Kurile subduction zone to World-Wide Standard Seis mograph Network and Canadian stations were measured. Instead of analyz ing the travel time residuals of a single event, relative travel time residuals of P waves to common stations have been analyzed and used in this study to isolate near source contributions from the total travel lime residuals. The effect of uncertainties in long wavelength hetero geneity far from the source in the lower mantle and the effect of unce rtainties in the reference Earth model have been significantly reduced by analyzing relative travel time residuals. The effect of uncertaint ies in station corrections are eliminated by relative travel time resi duals. Our measured P wave travel time data clearly indicate that high -velocity anomalies exist in the lower mantle beneath the Kurile subdu ction zone but they do not appear to be caused by simple slab continua tion into the lower mantle. The travel time modeling was accomplished using a fully three-dimensional finite difference technique for wave p ropagation within the source region. The modeling results show that hi gh-velocity anomalies extend several hundred kilometers into the lower mantle but they appear to be very complicated. A large number of mode ls consisting of simple slab continuation into the lower mantle were t ested and the results show that none of these models was able to produ ce synthetics which match the observed P wave travel time data. The sl ab-related velocity anomalies in the upper mantle of the Kurile subduc tion zone are generally consistent with the seismicity. The high-veloc ity anomalies in the lower mantle below the Kurile subduction zone are broad and geometrically change from the southern to the northern Kuri les. Below the southern Kuriles, the high-velocity anomalies appear to be subhorizontal. However, beneath the central Kuriles, there are bro ad, about 600-km-wide, high-velocity anomalies on the continental side of the slab which extend several hundred kilometers into the lower ma ntle. The high-velocity anomalies found below the northern Kuriles are broad and on both the continental and the ocean side of the slab, and extend several hundred kilometers into the lower mantle.