Physics of the Earth and Planetary Interiors, 141(3), 183-206, doi:10.1016/j-pepi.2003.11.002, 2004
Where and why do large shallow intraslab earthquakes occur?
Tetsuzo Seno and Masaki Yoshida
Abstract
We try to find how often, and in what regions large earthquakes (M>=7.0)
occur within the shallow portion (20～60 km depth) of a subducting slab.
Searching for events in published individual studies and the Harvard
University centroid moment tensor catalogue, we find twenty such events
in E. Hokkaido, Kyushu-SW, Japan, S. Mariana, Manila, Sumatra, Vanuatu,
N. Chile, C. Peru, El Salvador, Mexico, N. Cascadia and Alaska.
Slab stresses revealed from the mechanism solutions of these large
intraslab events and nearby smaller events are almost always down-dip tensional.
Except for E. Hokkaido, Manila, and Sumatra, the upper plate shows horizontal
stress gradient in the arc-perpendicular direction. We infer that shear
tractions are operating at the base of the upper plate in this direction
to produce the observed gradient and compression in the outer fore-arc,
balancing the down-dip tensional stress of the slab.
This tectonic situation in the subduction zone might be realized as part of
the convection system with some conditions, as shown by previous numerical simulations.