| Summary: | 碩士 === 國立臺灣大學 === 地質科學研究所 === 99 === The southern Ryukyu trench off northeast Taiwan marks the plate boundary where the Philippine Sea plate obliquely subducts beneath the Eurasian plate. The Ryukyu trench is oriented in a NE-SW direction to the east of 124oE and bends more E-W oriented to the west. To illuminate the nature of the mantle wedge flow around the edge of the Ryukyu subducting slab, we map the strength and geometry of upper mantle seismic anisotropy constrained by shear wave splitting analysis of local S waves and teleseismic SKS and S waves recorded at two stations IGK and YNG closest to Taiwan operated by the Japan F-net seismic network.
Our results indicate that the fast directions of both local and teleseismic shear waves observed at the two stations are approximately parallel to the local strike of the Ryukyu trench. Fast directions observed at station PCYB located in the Okinawa trough to the north of Taiwan show more trench-perpendicular. The fast directions observed at stations TWBB and TIPB in the northeast corner of Taiwan exhibit the transition from east-west to north-south directions. The split time delays of the teleseismic SKS and S waves are about 1.5-2 s, while those of the local S waves increase with the propagation path lengths ranging from 0.2 to 1 s, suggesting that seismic anisotropy at least extend down to 300 km depth.
A 2D corner flow induced by the drag of a downgoing slab represents the classical view of the mantle flow at SZs. The lattice preferential orientation (LPO) of the fast a-axis of olivine is known to be parallel to the flow direction, that is, trench normal for the corner flow geometry, which is contradictory to the observed trench-parallel fast directions. Recent laboratory experiments on olivine aggregates suggest that under the condition of higher stress and higher water content that may exist in the fore arc region, the B-type fabric becomes dominant with the faster b-axis aligned parallel to the trench.
To test whether the B-type texture in the fore-arc region can be sampled by finite-frequency local and teleseismic shear waves resulting in the observed trench-parallel anisotropy, we conduct full wave propagation modeling through the upper mantle structure in the framework of a 2D corner flow field using the numerical pseudospectral method. The adopted seismic velocity structure is based on LPO of olivine and pyroxene aggregates and the corresponding anisotropic elastic structure calculated from the plastic deformation model. The predicted fast directions from synthetic split SKS waves recorded above both the mantle wedge and subslab mantle are parallel to the flow direction, implying that the trench-normal anisotropy induced by the A-type fabric in the back arc and subslab mantle predominantly contribute to the observed SKS splitting. We also test the model with serpentinization in hydrated mantle wedge and subducted slab. The resulting pattern of shear wave splitting is similar to that from B-type fabric. Therefore, the alternative model like complex 3D geometry of the mantle wedge flow around the slab edge may need to explain the observed seismic anisotropy.
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