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

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.

Methods
Data collected from borehole observatories that were made possible through efforts of the Integrated Ocean Drilling Program (IODP), and through the DONET cabled observatory installed and supported by JAMSTEC. Funding: Observatory installation and data acquisition were founded by attributed NSF Awards.

Data presented are subseafloor pore pressure responses for 2 separate slow slip earthquakes, one in 2015, another in 2020.

Oceanopgrahic tides have been removed. Data are linearly detrended.