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Migrating shallow slow slip on the Nankai trough megathrust, captured by borehole observatories

Williams, Charles1; Saffer, Demian2; Edgington, Joshua2

Research facility: The University of Texas at Austin
Published Apr 17, 2025 on Dryad. https://doi.org/10.5061/dryad.wh70rxwxn

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Abstract

Patterns of strain accumulation and release offshore in subduction zones are directly linked to the potential for shallow coseismic slip and tsunamigenesis, but remain elusive. Here, we analyze formation pore pressure records from three offshore borehole observatories at the Nankai subduction zone, Honshu, Japan to capture detailed slip-time histories of two slow slip events (SSEs) along the outermost reaches of the plate boundary. Slip initiates ~30 km landward of the trench, migrates seaward at 1-2 km d-1 to within a few km of, and possibly breaching, the trench and coincides with the onset and migration of tremor and/or very-low frequency earthquakes (VLFE). The SSE source region lies in a zone of high pore fluid pressure and low stress, providing clear observational evidence linking these factors to shallow slow earthquakes.