HR: 0800h
AN: S51B-0160    [Abstracts]
TI: Structure of the Ryukyu Subduction Zone at its Western end: Slab Buckling, Double Seismic Layer, and the Effect of Dehydration
AU: * Chou, H
EM: d92224001@ntu.edu.tw
AF: Institute of Geosciences, National Taiwan University, P.O. Box 13-318, No.1, Sec. 4, Roosevelt Rd., Taipei, 106 Taiwan
AU: Kuo, B
EM: byk@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, No.128, Sec. 2, Academia Rd., Nankang, Taipei, 115 Taiwan
AU: Hung, S
EM: shung@ntu.edu.tw
AF: Institute of Geosciences, National Taiwan University, P.O. Box 13-318, No.1, Sec. 4, Roosevelt Rd., Taipei, 106 Taiwan
AU: Chiao, L
EM: chiao@ntu.edu.tw
AF: Institute of Oceanography, National Taiwan University, No.1, Sec. 4, Roosevelt Rd, Taipei, 106 Taiwan
AU: Wu, Y
EM: drymwu@ntu.edu.tw
AF: Institute of Geosciences, National Taiwan University, P.O. Box 13-318, No.1, Sec. 4, Roosevelt Rd., Taipei, 106 Taiwan
AU: Zhao, D
EM: zhao@sci.ehime-u.ac.jp
AF: Geodynamic Research Center, Ehime University, 2-5, Bunkyo-cho, Matsuyama, 790-8577 Japan
AB: Network data of Taiwan and Japan were integrated to illuminate the collision-oblique subduction complex in a region within 100 km of the island's coast, which has been poorly resolved by either single network. We relocated 4814 events applying a series of 1-D velocity model inversion and double difference method. These processes reduce the variance of traveltime residuals by about 70% with respect to each network value. The relocated seismicity delineates better the curving of the trench-forarc system toward the island and the 15--20 km apart double seismogenic layer (DSL) within the slab of the subducted Philippine Sea Plate in the depth range of 30-70 km. Not revealed before is the seismicity distribution that clearly defines bulging of the slab near its western end continuing from 50 km depth to 90 km. The overall geometry and the focal mechanisms suggest that the slab buckles against the Eurasian plate under lateral compression while subducting obliquely toward the continent. Tomographic inversion of the data set further reduces variance by 61%. The images reveal that much of the DSL is punctuated by low velocity anomalies (LVA) on the upper layer. We propose two hypotheses to explain the buckling-DSL-LVA combination, based on a previous notion that the DSL results from lateral compression. It could be that diabase and olivine present different creep rheology for crust and mantle causing the double layer, and that dehydration of hydrous minerals triggers melting that lowers the seismic velocity. Lateral compression could have thickened the typical oceanic crust to accommodate the at least 15 km gap for DSL. Or, a non-basaltic section of crust, thick and low in velocity, is subducted, which causes the 15--20 km separated double layer and the low anomalies in tomography. Subduction of a buoyant segment of crust has various implications for the dynamics of this subduction-collision junction.
DE: 8180 Tomography
DE: 7220 Oceanic crust
DE: 7230 Seismicity and seismotectonics
DE: 8159 Rheology--crust and lithosphere
DE: 7218 Lithosphere and upper mantle
SC: Seismology [S]
MN: 2004 AGU Fall Meeting