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 these patterns remain elusive. In this work, 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 kilometers landward of the trench; migrates seaward at 1 to 2 kilometers per day to within a few kilometers of, and possibly breaching, the trench; and coincides with the onset and migration of tremor and/or very-low-frequency earthquakes. The SSE source region lies in a zone of high pore fluid pressure and low stress, which provides clear observational evidence linking these factors to shallow slow earthquakes. Editor's summary: Slow slip is a quiet but energetic feature of faults that influences earthquakes. Detection along tsunami-prone plate boundaries such as the Nankai Trough offshore of southern Japan helps to assess risk. Using sensors in seafloor boreholes, Edgington et al. linked slow slip to high fluid pressure and to the onset of near-surface tremors and low-frequency earthquakes (see the Perspective by Obara). In the other direction, Ozawa et al. tracked land deformation in the weeks around the 2024 moment magnitude 6.7 Hyuga-nada earthquake to reveal a possible slow-slip trigger of the main shock. Both studies highlight the role of slow slip in how energy is distributed over earthquake cycles. —Angela Hessler [ABSTRACT FROM AUTHOR]