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Seismic velocity structure around the Hyuganada region, Southwest Japan, derived from seismic tomography using land and OBS data and its implications for interplate coupling and vertical crustal uplift |
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| Authors: | M Tahara K Uehira H Shimizu M Nakada T Yamada K Mochizuki M Shinohara M Nishino R Hino H Yakiwara H Miyamachi K Umakoshi M Goda N Matsuwo T Kanazawa |
| Institution: | aDepartment of Earth and Planetary Sciences, Faculty of Sciences, Kyushu University, Fukuoka 812-8581, Japan bInstitute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, Shimabara 855-0843, Japan cEarthquake Research Institute, University of Tokyo, Tokyo 113-0032, Japan dResearch Center for Prediction of Earthquakes and Volcanic Eruptions, Faculty of Science, Tohoku University, Sendai 980-8578, Japan eNansei-Toko Observatory for Earthquakes and Volcanoes, Faculty of Science, Kagoshima University, Kagoshima 892-0871, Japan fFaculty of Science, Kagoshima University, Kagoshima 890-0065, Japan gFaculty of Environmental Studies, Nagasaki University, Nagasaki 852-8521, Japan hFaculty of Fisheries, Nagasaki University, Nagasaki 852-8521, Japan |
| Abstract: | The Hyuganada region, a forearc region of Southwest Japan, is characterized by several interesting geological and geophysical features, i.e., significant aseismic crustal uplift of  120 m during the past 120 thousand years at the Miyazaki Plain, negative free-air gravity anomalies with the maximum magnitude of −130 mgal, and relatively less cohesive interplate coupling compared with that for off the Shikoku and Kii Peninsula. In order to examine the causes of these observations, we determined a detailed three-dimensional seismic velocity structure based on the seismic data observed by ocean bottom seismometers (OBS) and land stations. P- and S-wave tomographic velocity structures clearly indicate the subducting slab and also the zones of high Poisson's ratio at 25–35 km depth along the coastline of the northeastern part of the Hyuganada. The region with high Poisson's ratio may correspond to the serpentinized mantle wedge as suggested for other mantle wedges, and appears to be coincident with the zone for observed aseismic slips such as the slow-slip and after-slip events. Also, the detection may be related to a relatively weak interplate coupling in the Hyuganada region. The tomographic structures also indicate low velocity zones with a horizontal scale comparable to the Kyushu-Palau Ridge in and around the subducting slab. If we assume that the low velocity zones correspond to the subducted Kyushu-Palau Ridge, then the predicted gravity anomaly due to the density contrast between the low velocity zones and the surrounding region can explain about 60% of the gravity anomaly in the Hyuganada region. The buoyancy is probably an important factor for the crustal uplift observed in the Miyazaki Plain, the steep bending of the subducting slab and the normal fault-type earthquakes around the Hyuganada region. |
| Keywords: | Seismic tomography Interplate coupling Gravity anomaly Philippine Sea plate |
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