Tsunamigenic potential of the shallow subduction plate boundary inferred from slow seismic slip

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 30-100 s, anomalously long when compared with the 1-2 s duration of ordinary earthquakes with comparable magnitudes of M-w 3.8-4.9 (ref. 13). 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.