Tomographic imaging of the P-wave velocity structure beneath the Kamchatkapeninsula

A total of 5270 shallow and intermediate-depth earthquakes recorded by the 32 stations of the regional seismic network of the Geophysical Service of R ussia are used to assess the P-wave velocity structure beneath the Kamchatk a peninsula in the Western Pacific. The tomographic inversion is carried ou t in three steps, First, a 1-D tomographic problem is solved in order to ob tain an initial velocity model. Based on the 1-D velocity model, 3-D tomogr aphic inversions with homogeneous and heterogeneous starting models are obt ained. The Conrad (15 km depth) and Moho (35 km depth) discontinuities dete rmined from the 1-D tomographic inversion, and the upper boundary of the su bducting slab are taken into account in the heterogeneous starting model fo r the traveltimes and ray-path determinations. Both velocity structure and hypocentral locations are determined simultaneously in the inversion. The s pacing of the grid nodes is a half-degree in the horizontal direction and 2 0-50 km in the vertical direction. A detailed P-wave tomographic image is d etermined down to a depth of 200 km. The resulting tomographic image has a prominent low-velocity anomaly that shows a maximum decrease in P-wave velo city of approximately 6 per cent at 30 km depth beneath a chain of active v olcanoes. At depth, low-velocity anomalies are also observed in the mantle wedge extending down to a depth of approximately 150 km. These anomalies ar e apparently associated with the volcanic activity, The sedimentary basin o f the Central Kamchatsky graben, to the west of the volcanic front, and the accretionary prism at the trench correlate with shallow low-velocity anoma lies. High-velocity anomalies observed at a depth of 10 km may be associate d with the location of metamorphic basements in the Ganalsky-Valaginskoe up lift and upper crust of Shipunsky cape, The results also suggest that the s ubducted Pacific plate has P-wave velocities approximately 2-7 per cent hig her than those of the surrounding mantle and a thickness of approximately 7 0 km.