Atmosphere and Ocean Research Institute, University of Tokyo
|When:||Friday, February 14, 2014, 10:30 a.m. to 11:30 a.m.
Join us for coffee beginning at 10:00 a.m.
|Where:||Seminar Conference Room, 10100 Burnet Road, Bldg 196-ROC, Austin, Texas 78758|
|Host:||Nathan Bangs, UTIG|
Click for a Live Broadcast.
Rupture of slow tsunami earthquakes at subduction zones propagates along a shallow plate-boundary fault (i.e., décollement) all the way or nearly all the way to the trench, exciting large tsunamis. Our seismic reflection profiles reveal the shallow décollements have variable reflection polarity and reflection coefficient in the Nankai subduction zone, allowing us to divide the subduction zone into 'wet' (fluid-rich) and 'dry' (fluid-poor) décollement regions. The wet décollement regions are characterized by (1) reverse-polarity reflections with negative reflection coefficients, (2) weak shear strength along the fault, and (3) close proximity to the source area of the 1605 Keicho tsunami earthquake, and therefore, may play a role as conditionally-stable patches with elevated fluid pressures. In contrast, the dry décollement regions are characterized by (1) normal-polarity reflections and positive reflection coefficients, (2) strong shear strength along the fault, (3) interseismic fault locking, (4) frictional heating, and (5) very-low-frequency earthquakes rich in high-frequency components, and therefore, could be seismic patches with no unusual fluid pressures. We propose that when megathrust earthquakes nucleate at shallow depth, failure of the small seismic patches, acting as a nucleation portion, may accelerate adjacent large, conditionally stable patches, generating large tsunamis. As a result, along-strike contrast of fault properties can involve large tsunami earthquakes along the Nankai shallow megathrust fault.