Signature of remnant slabs in the North Pacific from P-wave tomography

A 3-D ray-tracing technique was used in a global tomographic inversion in order to obtain tomographic images of the North Pacific. The data reported by the Geophysical Survey of Russia (1955-1997) were used together with the catalogues of the International Seismological Center (1964-1991) and the US Geological Survey National Earthquake Information Center (1991-1998), and the recompiled catalogue was reprocessed. The final data set, used for following the inversion, contained 523 430 summary ray paths. The whole of the Earth's mantle was parametrized by cells of 2 degrees x 2 degrees and 19 layers. The large and sparse system of observation equations was solved using an iterative LSQR algorithm. A subhorizontal high-velocity anomaly is revealed just above the 660 km discontinuity beneath the Aleutian subduction zone. This high-velocity feature is observed at latitudes of up to similar to 70 degrees N and is interpreted as a remnant of the subducted Kula plate, which disappeared through ridge subduction at about 48 Ma. A further positive velocity perturbation feature can be identified beneath the Chukotka peninsula and Okhotsk Sea, extending from similar to 300 to similar to 660 km depth and then either extending further down to similar to 800 km (Chukotka) or deflecting along the 660 km discontinuity (Okhotsk Sea). This high-velocity anomaly is interpreted as a remnant slab of the Okhotsk plate accreted to Siberia at similar to 55 Ma.