Publication Date:
2012-05-06
Description:
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 1 or splay faults that branch upwards at high angles into a wedge of overlying sediment 2. However, some anomalous, enigmatic events are known to occur infrequently in this region, including tsunami earthquakes 3-7 that generate tsunamis disproportionately large for their seismic energy, and very-low-frequency earthquakes 8-12. 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 30-100s, anomalously long when compared with the 1-2s duration of ordinary earthquakes with comparable magnitudes of M w 3.8-4.9 (ref.). 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. © 2012 Macmillan Publishers Limited. All rights reserved.
Print ISSN:
1752-0894
Electronic ISSN:
1752-0908
Topics:
Geosciences
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