HR: 1340h
AN: DI53A-1096    [Abstracts]
TI: Topography of the 660-km discontinuity beneath northeast China: Implications for a retrograde motion of the subducting Pacific slab
AU: * Li, J
EM: juanli@mail.iggcas.ac.cn
AF: Institute of Geology and Geophysics, Chinese Academy of Sciences, No. 19 Bei Tu Cheng Xi Lu, Beijing, 100029,
AU: Chen, Q
EM: chenqf@seis.ac.cn
AF: Institute of Earthquake Science, Chinese Earthquake Administration, No. 63 Fuxing Lu, Beijing, 100036,
AU: Vanacore, E
EM: evanacor@rice.edu
AF: Department of Earth Sciences, Rice University, 6100 Main Street, Houston, TX 77005,
AU: Niu, F
EM: niu@rice.edu
AF: Department of Earth Sciences, Rice University, 6100 Main Street, Houston, TX 77005,
AB: Mapping the topography of the 660-km seismic discontinuity provides complimentary information to tomography images for understanding mantle circulation. Here we use S to P conversion waves from deep focus earthquakes to image fine-scale lateral variations in the depth of the 660 beneath northeast China, one ideal location to study interaction between a subducting slab and the upper and lower mantle boundary. Our method does not require using a reference velocity model of the heterogeneous upper mantle, and subsequently permits accurate estimate of the 660 depth. Multiple array data from eight deep events occurring beneath northeast China and the Japan Sea were processed with a 2^nd root slant stacking technique. Clear S₆₆₀P arrivals can be identified from stacked seismograms with incident angles well consistent with theoretical predictions. We measured the differential travel times between S₆₆₀P and P with a cross correlation method and used them to compute the depths of the 660 with 3D velocity models. We found a rapid change in the depth of the 660 within a narrow longitudinal range of 130.8{°}E-131.4{°}E, coinciding with the place where the lower boundary of the subducting Pacific slab encounters the 660. East to the rapid transition, the depth of the 660 appears to be flat, with no statistically significant variations. Towards the west, the 660 deepens steadily as it approaches the cold core of the subducting slab. The maximum depression occurs at the west end of the studied region with an amplitude of ~20 km. Our estimate of the depth of the 660-km discontinuity is consistent with previous results of SS precursor studies, indicating that the large-scale depression and low-amplitude topography of the 660 in this area is probably a true feature. Our observations also imply a scenario for a relatively soft slab lying over the 660 progressively from west to east due to a continuous trench retreat associated with the retrograde motion of the subducted Pacific plate.
DE: 1213 Earth's interior: dynamics (1507, 7207, 7208, 8115, 8120)
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 7203 Body waves
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7240 Subduction zones (1207, 1219, 1240)
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting