Seismic imaging constraints on megathrust fault zone properties
Monday, 15 December 2014
Several lines of evidence suggest that subduction zone thrusts lie within overpressured channels. Seismic reflection data often shows a relatively thin, high-reflectivity surface with occasional bright spots, indicative of rapidly varying impedance contrasts over length scales of tens of meters. Scattered coda of teleseismic P waves, such as in receiver functions, often show a thin low-velocity layer corresponding to the top of the subducting plate. The latter have been best documented in Cascadia, where a 2-4 km thick very low velocity channel is seen above a moderately slow subducting crust, and in Alaska where similar structure has been seen. High-reflectivity bright spots occur in the same region, although perhaps over more limited areas. The low velocity zones are characterized by elevated Vp/Vs ratios (>2.0), and extend both throughout the locked, seismogenic fault zone and downdip into the region where episodic tremor and slip occur. Commonly, this combination of low velocities and high Vp/Vs is taken to indicate high pore pressures, and hence a fault zone that can withstand only very low shear stresses. However, models of the low wavespeeds suggest static porosities of 2-5% throughout a 2-4 km thick layer, extending to depths of 40 km, a situation that seems difficult to sustain. At both the Alaska and Cascadia margins, low Vp, high Poisson’s ratios, and high anisotropies should result in part from the subduction of sediments well into and beyond the seismogenic zone. The presence of a significant thickness of subducted and underplated sediment is consistent with observations of preserved subduction “channels” in exhumed examples from tens of km depth. Although some elevation of pore pressure may be still needed to explain observations, if the subduction of 2-4 km of sediment is a significant factor in generating the seismic signatures, then the geophysical observations could reflect a much stronger thrust zone than one sustained by high pore pressure alone.