HR: 0800h
AN: T41C-1234    [Abstracts]
TI: Oblique Subduction as a Controlling Factor for Stresses and Earthquakes in Subducting Slabs at the Cascadia and Nankai Subduction Zones
AU: * Wada, I
EM: ikukow@uvic.ca
AF: School of Earth and Ocean Sciences, University of Victoria, Victoria, BC V8W 3P6, Canada, Victoria, BC V8W 3P6 Canada
AU: Wang, K
EM: kwang@nrcan.gc.ca
AF: Pacific Geoscience Centre, Geological Survey of Canada, Sidney, BC, V8L 4B2, Canada, Sidney, BC V8L 4B2 Canada
AB: Obliquely subducting slabs experience viscous mantle resistance in both updip and margin-parallel directions. The updip component is locally balanced by downdip slab-pull, but there is no body force to balance the margin-parallel component locally. Margin-parallel slab resistance thus has a first-order impact on in-slab stresses. In-slab stresses and thermally controlled dehydration embrittlement are jointly responsible for intermediate-depth earthquakes. At both the Cascadia and Nankai subduction zones, young and warm slabs subduct at 4 - 5 cm/yr with a right-lateral obliquity, but oblique subduction affects in-slab stresses differently in the two places because of differences in age distribution and kinematics of the subducting plates. We determine stresses in these slabs by inverting earthquake fault mechanisms and investigate the different effects of oblique subduction. Along the E-W striking Nankai subduction zone, margin-parallel slab resistance pulls the slab eastward. The oldest part of the slab is subducting at the Kyushu margin to the west, and the slab pull force stretches the slab westward. The balance of these two forces results in a margin-parallel tension in the slab, consistent with the state of stress determined from focal mechanisms. Along the N-S striking Cascadia subduction zone, the slab resistance is in the southerly direction and, together with other boundary forces, gives rise to a torque that tends to rotate the slab clockwise. Shear force along the Nootka shear zone to the north generates a torque that tends to rotate the slab in the opposite direction. The torque balance results in an E-W stretch in the slab that decreases from north to south. The observed E-W tension in the northernmost part of the slab and a southward decrease in slab seismicity are consistent with the torque balance mechanism.
DE: 8123 Dynamics, seismotectonics
DE: 8150 Plate boundary--general (3040)
DE: 8164 Stresses--crust and lithosphere
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting