Episodic Slow Slipping of Seafloor under Cascadia: What Physical Processes cause Aseismic Deformation Transients?

In several shallow-dipping subduction zones, including Cascadia, the seafloor undergoes episodes of more rapid than usual creep-slippage under the overlying margin, but at rates vastly slower than usual earthquake slip. In some locations, also including Cascadia, non-volcanic seismic tremors occur during the slip episodes. Graduate student Yajing Liu and I have been trying to understand what physical processes underlie these phenomena. We have shown that transients, with features somewhat like the observations, are a natural outcome of modern 'rate and state' formulations of fault zone friction, in a regime for which the ambient fluid pore pressure within the fault zone is very high and close to the compressive normal stress clamping the fault walls together. Evidence for such pressure conditions is provided by independent mechanical and petrological constraints.

James R. Rice, a professor of Engineering Sciences and Geophysics at Harvard, born in 1940, addresses problems in theoretical mechanics as they arise in the geological sciences, particularly in seismology and the science of earthquakes, as well as in civil/environmental engineering and materials physics. His earthquake studies are on the mechanics and physics of fault zone processes.