A Local Seafloor Pressure Anomaly Potentially Triggered by Pore Water Migration during Ocean Current Meander

Owing to the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) and borehole observatories, slow slip events (SSEs) have been detected in the shallow extension of the source region of the 1944 Tonankai earthquake (DONET-1). However, a localized seafloor pressure anomaly—characterized by uplift and subsidence at two DONET-1 stations in 2013—has yet to be reasonably explained.

In this study, we explore possible source models for this pressure anomaly by assuming pore-water migration from compacted reservoirs, either arranged in layered formations or represented as swarms of small spheres, toward a dilated zone beneath the décollement. We also compile observations of seafloor crustal deformation driven by SSEs and oceanographic phenomena under baroclinic conditions to refine the spatio-temporal scaling relationship of seafloor pressure variations.

Our main findings are as follows. (i) The potential compacted pore-water reservoirs spatially overlap with the hypocenters of very low-frequency earthquakes (VLFEs), whereas the dilated zone lies in a region with normal-fault-type VLFE activity. (ii) A Kuroshio meander associated with an abrupt fluctuation in sea surface height (SSH) occurred around DONET-1 during the pressure event. (iii) Taken together, (i) and (ii) suggest that the local seafloor pressure change may be explained by pore-water migration destabilized by the Kuroshio current meander. (iv) As this is the first reported case in which a local seafloor pressure anomaly has been identified from only two observation points, the suggested causal link—namely, that the Kuroshio meander may have promoted pore-water migration—provides a strong scientific motivation for future geological surveys, particularly those monitoring seismic activity and seafloor crustal deformation before and after similar pore-water migration events.