Plastic Instabilities as a Possible Physical Mechanism Causing Intermediate-Depth and Deep-Focus Earthquakes

Thursday, March 21, 2002 - 3:00pm - 3:30pm
Keller 3-180
Michael Riedel (Geo Forschungs Zentrum Potsdam)
Joint work with S. Karato (Department of Geology and Geophysics, Yale University) and D.A. Yuen (Department of Geology and Geophysics, University of Minnesota).

It has been suggested that the occurence of plastic instabilities in the deeper portion of subducting slabs is the responsible mechanism for the generation of deep-focus earthquakes. Heat generated during viscous deformation provides a positive feedback to creep and eventually faulting under high pressure. A similar mechanism could be responsible for the occurence of intermediate-depth earthquakes within portions of the mantle lithosphere, where mechanisms involving dehydration or phase transformations do not apply. Recent detailed receiver function images of the structure of the Japan subduction zone seem to provide support for this notion. First, there is no indication of an existing metastable olivine wedge. Second, the intermediate-depth seismicity seems to be located in the strong and colder portions of the downgoing slab, about 30 km below the oceanic Moho. This suggests that instead of dehydration or phase transformation triggered events, ductile faulting is its predominating cause.

We show that, under certain conditions, a general local criterion for plastic instability can be met for nonlinear power-law creep (dislocation creep) of olivine resp. spinel (below 410 km discontinuity), so that the existence of metastable olivine in the deeper portion of a slab (below 500 km) is not a necessary condition for the generation of deep-focus earthquakes.