Crustal deformation in Kyushu derived from GEONET data

Abstract

An investigation was conducted on the horizontal crustal deformation in Kyushu using the coordinate data from GPS Earth Observation Network (GEONET) for the 4 years from 1998 to 2002, for which the annual variations are corrected. We examined the spatial differences in the displacement rate relative to the Amurian plate by subtracting the velocity of the Amurian plate at each of the Global Positioning System (GPS) stations. Models for interplate coupling in Hyuganada and the dilatational source beneath Sakurajima are obtained. The crustal deformation in eastern Kyushu is well explained by the model of interplate coupling in Hyuganada, which is strong in the north and weak in the south. When the effects of the interplate coupling and the dilatational source are removed, a southward extensional field is recognized in and to the south of the Beppu‐Shimabara graben in western Kyushu. In southern Kyushu, to the south of around latitude 32°N, a southeastward movement is conspicuous. It must be noted that the displacement field is extensional, indicating that southern Kyushu is being dragged at the base by a flow in the mantle, not moving like a raft on the flow. It is probable that the flow in the mantle is driven by the rising of hot materials in the Okinawa Trough and a retreat of the trench where the Philippine Sea plate is subducting. A gap in the magnitude of the eastward displacement rate around latitude 32°N implies that the velocity of the flow in the mantle decreases substantially to the north of the line. The Beppu‐Shimabara graben may be a continuation of the Okinawa Trough, but the significant southwestward displacement rate observed in the eastern part suggests that the deformation in the graben, at least to the east of Aso volcano, is affected by the westward movement of the forearc sliver produced by the oblique subduction of the Philippine Sea plate on the Nankai Trough off Shikoku Island. We think that the strong coupling in the Bungo Channel in our model is probably not real, but might be a result of our assumption that all the westward displacements in northeastern Kyushu, a part of which are actually due to the movement of the forearc sliver, are produced by interplate coupling.

Volume change of the magma reservoir relating to the 2011 Kirishima Shinmoe-dake eruption—Charging, discharging and recharging process inferred from GPS measurements

“…Despite the simplicity, this model fits the observation very well, with an average error of approximately 1.0 mm/yr. A similar relation has been presented by Takayama and Yoshida (2007). Our model is an advanced version of their model with modifications of the fixed point and the optimum direction.…”

“…Nishimura et al (2004) proposed that block rotation of the Ryukyu arc is the main factor. Takayama and Yoshida (2007) suggested that temporal variations in the inter-plate coupling affect the deformation rate. Wallace et al (2009) tried to explain the deformation field assuming an east-west striking shear zone in southern Kyushu.…”

Velocity structure in the crust beneath the Kyushu area

“…Kamata and Kodama (1999) suggested that this uplift started synchronously with counter-clockwise rotation of South Kyushu during the last 2 Myr and that these events were caused by a westward shift in the subduction direction of the PHS plate and associated trench retreat along the Ryukyu Trench. The contemporary geodetic velocity eld shows an eastward movement in southern Kyushu with respect to central Kyushu (Takayama and Yoshida, 2007), and the present-day crustal stress eld deduced from microearthquake focal mechanisms also reveals a change in the axis of minimum horizontal compressive stress from north-south to northeast-southwest in central Kyushu (Shimizu et al, 1993). The low-velocity zone beneath the Kyushu Mountains provides important insight into the tectonics of this region.…”

“…Seno (1999) proposed that the stress eld represents viscous drag exerted by lateral mantle ow associated with upwelling in the East China Sea. In a more detailed regional study, Takayama and Yoshida (2007) umented a southward extensional displacement eld in and to the south of the Beppu-Shimabara Graben in western Kyushu and a southeastward extensional eld in southern Kyushu. They suggested, like Seno (1999), that southern Kyushu is being dragged at the base of the crust by ow in the mantle in response to both the uprising of hot material in the Okinawa Trough and retreat of the trench where the Philippine Sea plate subducts.…”

Continuity, segmentation and faulting type of active fault zones of the 2016 Kumamoto earthquake inferred from analyses of a gravity gradient tensor

“…The north-south extensional tectonics is dominant in central Kyushu (Tada 1984;Takayama and Yoshida 2007). The Beppu-Shimabara rift zone (Matsumoto 1979), of which the south end is bounded by the Oita-Kumamoto tectonic line, is a subsidence zone due to the extension and may lead to the Okinawa trough (Tada 1984(Tada , 1985 ( Fig.…”