Tomographic imaging of the P-wave velocity structure beneath the Kamchatka peninsula

Abstract

A total of 5270 shallow and intermediate‐depth earthquakes recorded by the 32 stations of the regional seismic network of the Geophysical Service of Russia are used to assess the P‐wave velocity structure beneath the Kamchatka peninsula in the Western Pacific. The tomographic inversion is carried out in three steps. First, a 1‐D tomographic problem is solved in order to obtain an initial velocity model. Based on the 1‐D velocity model, 3‐D tomographic inversions with homogeneous and heterogeneous starting models are obtained. The Conrad (15 km depth) and Moho (35 km depth) discontinuities determined from the 1‐D tomographic inversion, and the upper boundary of the subducting slab are taken into account in the heterogeneous starting model for the traveltimes and ray‐path determinations. Both velocity structure and hypocentral locations are determined simultaneously in the inversion. The spacing of the grid nodes is a half‐degree in the horizontal direction and 20–50 km in the vertical direction. A detailed P‐wave tomographic image is determined 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 velocity of approximately 6 per cent at 30 km depth beneath a chain of active volcanoes. At depth, low‐velocity anomalies are also observed in the mantle wedge extending down to a depth of approximately 150 km. These anomalies are apparently associated with the volcanic activity. The sedimentary basin of the Central Kamchatsky graben, to the west of the volcanic front, and the accretionary prism at the trench correlate with shallow low‐velocity anomalies. High‐velocity anomalies observed at a depth of 10 km may be associated with the location of metamorphic basements in the Ganalsky–Valaginskoe uplift and upper crust of Shipunsky cape. The results also suggest that the subducted Pacific plate has P‐wave velocities approximately 2–7 per cent higher than those of the surrounding mantle and a thickness of approximately 70 km.

Rock seismic anisotropy of the low‐velocity zone beneath the volcanic front in the mantle wedge

“…The profile A of Gorbatov et al [1999] clearly shows a Vp slow (À7 to À3 %) anomaly below the volcanic front at PET from surface down to 90 km depth. Such reduction of Vp % can be for instance explained by the occurrence of 10 to 5 % spherical melt pocket [Mainprice, 1997].…”

“…[2] Previous studies of various subduction zones have imaged an inclined P-and S-wave low-velocity zone with a velocity reduction of 5 -10%, oriented sub-parallel to the down-dip direction of the slab in the mantle wedge (e.g., northeast Japan: Zhao et al [1990], Kamchatka : Gorbatov et al [1999], and Tonga: Conder and Wiens [2007]). The low-velocity zone is commonly attributed to a region of partially melted mantle [Kushiro, 1987], representing a major source of magma for the volcanic chain that forms along the arc (i.e., the volcanic front).…”

Tomography and Dynamics of Western-Pacific Subduction Zones

“…11. The method has been also applied to several other subduction zones, leading to nice tomographic images in those regions, such as Alaska , Taiwan (Ma et al, 1996;Chou et al, 2006Chou et al, , 2009, Tonga (Zhao et al, 1997b), Romania (Fan et al, 1998), Kamchatka (Gorbatov et al, 1999), and South America (Myers et al, 1998;Wagner et al, 2005), in addition to many other applications in various tectonic settings. The boundary-grid approach has been extended to conduct global tomography (Zhao, 2001c(Zhao, , 2004Zhao and Lei, 2004) by considering the lateral depth variations of the Moho, 410-and 670-km discontinuities.…”

“…The 3-D upper mantle structure under the Kamchatka peninsula has been investigated by several researchers to date. Gorbatov et al (1999) applied the tomographic method of Zhao et al (1992b) to determine a 3-D P-wave velocity model down to a depth of 200 km, and their results showed a prominent low-V anomaly beneath the volcanic front and a high-V zone corresponding to the subducted Pacific slab.…”

Average shear-wave velocity structure of the Kamchatka peninsula from the dispersion of surface waves

“…Slavina and Fedotov (1974) have studied P-wave velocities in the upper mantle using the arrival times of the Pn seismic phase. Tomographic inversions of the travel times of P-waves have been provided by Slavina and Pivovarova (1992) and Gorbatov et al (1999). Most of these studies concern P-wave velocities, while the S-wave velocity structure of Kamchatka remains almost unknown.…”