Understanding the Physical Process of Unusually Long-duration Slow Slip Events: Insights from Stress Interaction and Environmental Influences in the Nankai Trough

JAMSTEC have been monitoring changes in underground fluid pressure, or "pore pressure," from boreholes near the site of the 1944 Tonankai earthquake in southwestern Japan. These changes are linked to Slow Slip Events (SSEs), which occur on the boundary between the Eurasian plate and the subducting Philippine Sea plate beneath the Nankai Trough. By connecting their borehole observatory (LTBMS) to a seafloor monitoring network (DONET), they now collect real-time pore pressure data, allowing them to update their SSE catalog.

This updated catalog revealed something unusual: the SSE in February 2012 lasted significantly longer than similar events. Researchers studied pore pressure and seafloor pressure data to understand why. We found that the February SSE moved more slowly and lasted longer because of two key factors: internal and external forces.

Internally, the SSE occurred in a region where little stress had built up on the fault, causing it to slip more slowly, consistent with frictional behavior on faults. Externally, we found that changes in seafloor pressure, driven by shifts in the Kuroshio Current (a major ocean current), coincided with the end of the February SSE. This suggests that the Kuroshio Current's meander may influence the duration of SSEs.

Our study highlights that SSEs are not only shaped by fault interactions but also by environmental factors like ocean currents and atmospheric pressure. Understanding these influences is key to better predicting such events. These findings are based on a paper accepted by Tectonophysics (https://doi.org/10.1016/j.tecto.2024.230439), and we plans to share additional insights and recent practical analysis in our presentation.