Some seismological constraints on the processes governing intermediate-depth earthquakes in Japan

Shanna Chu

Stanford University

Date & Time
Online-only seminar via Microsoft Teams
Justin Rubenstein

The mechanisms generating intraslab intermediate-depth earthquakes (~70-200 km depth) remains uncertain because the high temperatures at pressures at those depths ought to prohibit brittle failure. Proposed mechanisms for their nucleation include dehydration-driven processes such as embrittlement and dehydration-induced stressing, and the generation of thermal shear runaways. These processes have hypothetically different observable seismological signatures, such a scaling of radiated efficiency with moment, and reduced aftershock activity in the case of thermal shear runaways. However, variability in the observed character of intermediate-depth earthquakes worldwide makes it difficult to support any particular mechanism. My research analyzes intermediate-depth earthquakes in two tectonic settings in Japan - the younger, warmer Philippine Sea Plate and the older, colder Pacific Plate. We find no significant scaling of radiated efficiency in either slab, indicating that pure thermal shear, as proposed to explain intermediate-depth earthquakes in the Bucaramanga Nest, Colombia, cannot well explain the occurrence of the subduction zone events. We also find a variation in intermediate-depth aftershock activity between the two slabs. Our findings indicate that aftershock activity may correspond to high Vp-Vs ratio and varies along-strike and with distance to the trench. Taken together these results suggest that the generation mechanisms of intermediate-depth earthquakes form a spectrum, where the relative importance of certain mechanisms is modulated by regional fluctuations in slab character.

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