EEPS Colloquium: Demian Saffer

Tolman Lecture

Wiring the Megathrust: Monitoring the Creaks and Groans of a Plate Boundary near the Trench in the Nankai Subduction Zone

Summary: The far offshore reaches of subduction megathrusts - nearest the trench - represent a key “blind spot” in our knowledge of plate boundary fault behavior. Yet, understanding strain accumulation and release in this region is central to assessing hazards posed by shallow earthquake rupture and tsunamigenesis. In this talk, I describe a promising pathway to use formation pore pressure, monitored in instrumented IODP boreholes, to detect deformation in this region with both high sensitivity and fidelity. Our 16-year dataset (from 2010-present) captures a family of 13 repeating slow slip events (SSE) offshore of SW Honshu Japan, which occur quasi-periodically on the plate boundary immediately above the currently locked rupture zone of historic great (M8) earthquakes. Of these, most arise spontaneously, and occur at ~1 yr intervals with durations of a 1-3 weeks. The remaining two events are larger, closer to the trench, and are triggered by: (1) the 2011 M9 Tohoku earthquake; and (2) the 2016 M7 Kumamoto earthquake. In some cases, the SSE are accompanied by swarms of low-frequency tremor and low-frequency earthquake activity that migrate trenchward. For most events, the data are well fit by slip of a few cm on a patch of the plate interface with a dimension of 20-40 km. The data are also of sufficiently low noise that we are able to evaluate the time evolution of slip, and capture the migration of deformation with exquisite sensitivity, with detection of as little as a few mm of slip at the trench (~100 km offshore, in 4 km water depth!). 

This increasingly clear picture of fault slip and dynamics provides several key new insights into the dynamics of the plate boundary. First, through comparison with other observations, it enables an increasingly quantitative understanding of faulting processes that underpin the occurrence of locking versus slow slip events, including: (1) in situ pore fluid pressure and stress; and (2) fault friction and restrengthening (healing). Second, we are able to assess the role of slow slip events in the overall plate motion budget, and show that they accommodate ~30-50% of overall plate convergence. Finally, the repeating nature of the events, taken together with apparent triggering by regional earthquakes, indicates that the outermost reaches of the megathrust are highly sensitive to perturbation and are perched near a state of failure. Taken together with emerging similar observations at other subduction margins, these results carry important implications for geodetic models, the mechanics of strain accumulation offshore, and the resulting outlook for hazard associated with shallow coseismic slip.

Host: Phil Skemer

EEPS Colloquia are made possible by the William C. Ferguson Fund