Cartoon showing a map view of the top of a subducting plate modeled on present day subduction at Nankai in southwest Japan
Map view of the top of a subducting plate modeled on present day subduction at Nankai in southwest Japan. The cross section on the right shows the structure of the overriding accretionary prism with schematic temperature distribution. It is assumed that the shale at the top of the downgoing plate changes behavior from velocity-strengthening shallow (labeled as stable zone) to velocity weakening in the locked or seismogenic zone. The inclusion of seamounts, ridges and other topographical features in the shale matrix in the velocity-strengthening zone can lead to slow earthquake phenomena. Credit: Phillips et al. [2020], Figure 10
Source: Geochemistry, Geophysics, Geosystems

While great earthquakes originate in the locked portion of the plate interface of a subduction zone, a transitional zone downdip can produce slow slip events and low frequency earthquakes. These slow earthquake phenomena release strain energy over hours to months rather than seconds to minutes for typical earthquakes.

Phillips et al. [2020] examine rocks exhumed from updip of a locked segment in southwest Japan. These rocks consist of basaltic blocks (e.g. former seamounts) in a shale matrix. They find that this mix strain is localized along the margins of the basaltic blocks in the form of faults, but deformation is distributed in the shale matrix.

The authors model stress and pore fluid pressure in these rocks and show that the mix of velocity-weakening margins of basaltic blocks(for which frictional resistance decreases with sliding velocity) with velocity-strengthening shale can produce a range of slow earthquake phenomena both updip and downdip of the locked portion.

Citation: Phillips, N. J., Motohashi, G., Ujiie, K., & Rowe, C. D. [2020]. Evidence of localized failure along altered basaltic blocks in tectonic mélange at the updip limit of the seismogenic zone: Implications for the shallow slow earthquake source. Geochemistry, Geophysics, Geosystems, 21, e2019GC008839.

—Ulrich Faul, Editor, Geochemistry, Geophysics, Geosystems

Text © 2020. The authors. CC BY-NC-ND 3.0
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