Tomographic low‐velocity anomalies in the uppermost mantle around the northeastern edge of Okinawa Trough, the backarc of Kyushu

Abstract

Abstract.A simultaneous inversion for threedimensional velocity structure and hypocentral parameters is used to determine the tomographic images of the backarc of Kyushu, southwestern Japan. The data used in the imaging are the first arrival times of 2736 local earthquakes recorded at 305 stations in a wide regicn from western Japan to eastern China. Additional data are provided by including arrival times of 357 teleseismic earthquakes. We got the high resolution of the uppermost mantle images for P waves at least in the depth range to 150 km, the latitude range of 30ø-34øN and the longitude range of 128ø-132øE. Results of inversion reveal a major zone of strong low velocity under west off Kyushu Island in the depth from 40 km to 100 km. This feature may result from partially melting in relation to the upwelling of hot mantle around the northeastern edge of the extending Okinawa trough.

Thermal structure beneath Far Eastern Russia inferred from geothermobarometric analyses of mantle xenoliths: Direct evidence for high geothermal gradient in backarc lithosphere

“…Yamamoto et al (4) described that the shallower part of the upper mantle throughout the backarc region in eastern Asia shows characteristic low P-wave velocity with > 2% compared to Archean craton (Duan et al, 2009;Huang and Zhao, 2009;Huang et al, 2010;Sadeghi et al, 2000;Tian et al, 2009;Zhao et al, 2009Zhao et al, , 2010. Such seismic observations are not consistent with the mantle convection model proposed to explain the high surface heat flow in Cascade because vigorous mantle convection in the backarc mantle eliminates the local distribution of low seismic wave velocity anomaly.…”

Velocity structure in the crust beneath the Kyushu area

“…This gross structural trend is compatible with the observed Bouguer gravity anomaly, which is positive in western Kyushu and negative in eastern Kyushu. Oda and Ushio (2007) suggested that the shallow Moho and the low velocity zone estimated by Sadeghi et al (2000) reflects lower crustal erosion associated with a small-scale mantle upwelling in the Okinawa Trough and that the deeper Moho corresponds to the oceanic Moho of the subducting Kyushu-Palau ridge.…”

“…They detected a low-velocity zone corresponding to the KyushuPalau ridge in and around the subducting PHS plate and suggested that buoyancy due to the subducted ridge plays a role in crustal uplift in the Miyazaki Plain and steep bending of the subducting slab. Sadeghi et al (2000) obtained tomographic images beneath western Kyushu, and documented the existence of a low-velocity zone related to the inferred upwelling mantle near the northeastern edge of the extending Okinawa Trough. Nakamura et al (2003) obtained more detailed structure images of the Okinawa Trough and described a low-velocity zone with high V p /V s extending along the Okinawa Trough at a depth of approxi- mately 50 km.…”

Boron systematics of Hisatsu and Kirishima basaltic rocks from southern Kyushu, Japan

“…This low-velocity zone is consistent with mantle upwelling around the northeastern edge of the currently rifting Okinawa Trough (Sadeghi et al, 2000;Sibuet et al, 1995) and is believed to have activated volcanism in the Kyushu backarc and Ryukyu arc (Kimura et al, 2005).…”