Article,
Tsunamigenic potential of the shallow subduction plate boundary inferred from slow seismic slip
Nature Geoscience, 5 (6): 414--418 (Jun 6, 2012)
DOI: 10.1038/ngeo1466
Abstract
At subduction zones, convergence between the two plates at shallow
levels was thought to occur aseismically, accommodated by asiesmic
slip along either the megathrust plate boundary or splay faults that
branch upwards at high angles into a wedge of overlying sediment.
However, some anomalous, enigmatic events are known to occur infrequently
in this region, including tsunami earthquakes that generate tsunamis
disproportionately large for their seismic energy, and very-low-frequency
earthquakes. Here we report close-in observations of very-low-frequency
earthquakes, measured using broadband ocean-bottom seismometers,
occurring at the shallowest parts of the plate boundary at the Nankai
Trough. We find that the very-low-frequency events are generated
by slip on extremely low-angle thrust faults along the plate boundary
beneath the sedimentary wedge. The earthquakes have durations of
30-100 s, anomalously long when compared with the 1-2 s duration
of ordinary earthquakes with comparable magnitudes of Mw 3.8-4.9.
Despite their slowness, the waves are unexpectedly rich in high-frequency
components, a feature consistent with shear failure driven by tectonic
stress and fluid-pressure-controlled tensile fractures. The occurrence
of this slow, yet seismic slip implies that the shallowest part of
the plate boundary could be a source of tsunami earthquakes.
