HR: 14:05h
AN: T13E-02 INVITED     [Abstracts]
TI: Seismic imaging of North China: insight into intraplate volcanism and seismotectonics
AU: * Zhao, D
EM: zhao@sci.ehime-u.ac.jp
AF: Ehime University, Geodynamics Research Center, Matsuyama, 790-8577 Japan
AB: We used seismic tomography to study the detailed three-dimensional (3-D) seismic velocity structure of the crust and mantle beneath North China for understanding the intraplate volcanism and seismotectonics of the Asian continent. Two active volcanoes, Changbai and Wudalianchi, exist in Northeast China and they have erupted several times in the last 1000 years. The origin of the active intraplate volcanoes is still unclear. Global tomography shows that the subducting Pacific slab becomes stagnant under NE Asia and strong low-velocity (low-V) anomalies exist in the upper mantle under the two volcanoes (Zhao, 2004). Recently we determined a 3-D P-wave velocity structure under the Changbai volcano using teleseismic data recorded by 19 portable seismic stations in NE China (Zhao et al., 2004). Our result shows a columnar low-V anomaly extending to 400 km depth and high-velocity anomalies in the mantle transition zone with deep-focus earthquakes of about 600 km depth. These results indicatie that the Changbai and Wudalianchi volcanoes are not hotspot like Hawaii but a kind of back-arc volcano related to the deep subduction and stagnancy of the Pacific slab under NE Asia. A detailed 3-D P-wave tomography of the crust and uppermost mantle under the Beijing region is determined by using local earthquake arrival times recorded by the newly installed Chinese Capital Seismic Network with 101 short-period seismic stations coving the region densely and uniformly (Huang and Zhao, 2004). The results show that large crustal earthquakes, such as the 1679 Sanhe earthquake (M 8.0) and the 1976 Tangshan earthquake (M 7.8), generally occurred in high-velocity areas in the upper to middle crust. In the lower crust to the uppermost mantle under the source zones of the large earthquakes, however, low-velocity and high-conductivity anomalies exist, which are considered to be associated with fluids. The fluids in the lower crust may cause the weakening of the seismogenic layer in the upper and middle crust and thus contribute to the initiation of the large crustal earthquakes. Similar features are also found in the source areas of the 1995 Kobe earthquake (M 7.2) in Japan (Zhao et al., 1996) and the 2001 Bhuj earthquake (M 7.8) in India (Mishra and Zhao, 2003). Zhao, D. (2004) Global tomographic images of mantle plumes and subducting slabs: insight into deep Earth dynamics. Phys. Earth Planet. Inter. 146, 3-34. Zhao, D., J. Lei, R. Tang (2004) Origin of the intraplate Changbai volcano in Northeast China: Evidence from seismic tomography. Chinese Science Bulletin 49(13), 1401-1408. Huang, J., D. Zhao (2004) Crustal heterogeneity and seismotectonics of the region around Beijing, China. Tectonophysics 385, 159-180.
UR: http://www.ehime-u.ac.jp/~grc/
DE: 8180 Tomography
DE: 8110 Continental tectonics--general (0905)
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8123 Dynamics, seismotectonics
DE: 7209 Earthquake dynamics and mechanics
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting