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Recurring and triggered slow slip events near the trench along the Nankai subduction thrust

Slow slip events (SSE), non-volcanic tremor, and very low-frequency earthquakes (VLFE) are well documented down-dip of the seismogenic zone of major faults, yet similar observations for the shallowest reaches of subduction megathrusts are rare. We document a family of repeating strain events in the Nankai subduction zone, updip of rupture zone of great (M8) earthquakes, using data from two borehole observatories which penetrate the hanging wall accretionary prism: IODP Site C0002, located 36 km landward of the trench; and Site C0010, located 25 km landward.

After filtering oceanographic noise using a local hydrostatic reference at each site, the pressure records over a 6 year period from 2010-2016 reveal 8 strain transients that are synchronous at the two holes. Of these, 6 arise spontaneously, and occur at ~1 yr intervals with durations of ~7-21 days. All are positive in sign at C0010, with consistent magnitudes of ~0.3-0.9 kPa; at Site C0002 three are negative in sign and two are positive, with magnitudes of ~0.3-0.7 kPa. The remaining 2 events are larger (1.7-2.7 kPa), negative in sign at both sites, and immediately follow: (1) the 2011 M9 Tohoku earthquake; and (2) the 2016 M7 Kumamoto earthquake. In some cases, the pressure transients are accompanied by swarms of low-frequency tremor and VLFE activity that appear to migrate trenchward. We interpret the pressure signals to reflect volumetric strain in response to SSEs. The data are well fit by slip of ~1-4 cm on a patch at the plate interface that extends 20-40 km in the dip direction, and is centered beneath Site C0002 (spontaneous events) or slightly updip (triggered events). A key implication is that the SSE accommodate ~30-50% of plate convergence across the outer ~40 km of the forearc. This coincides with a region of the shallow-most megathrust characterized previously by: (1) elevated pore fluid pressure; (2) transitional frictional behavior that promotes the nucleation of unstable slip at low sliding velocities; and (3) low stress magnitudes as constrained by analysis of wellbore failures. The repeating nature of the events, taken together with apparent triggering by regional earthquakes and the inference of low in situ stress magnitudes, indicates that the outermost reaches of the megathrust are highly sensitive to perturbation and are perched near a state of failure.

Speaker: Demian Saffer, Penn State Univ.

Tuesday, 01/21/20


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