Slow-and-fast phenomena in main slip region of the 2025 Mw 6.6 Sanriku-Oki, Japan, earthquake

The 2025 M w 6.6 Sanriku-Oki earthquake occurred in a region where slow fault-slip phenomena coexist with ordinary earthquakes. We estimated its source process and investigated relationships of the slip model with seismic and aseismic activity. Joint inversion of S-net seismic and pressure-gauge records revealed a main slip region northeast of the epicenter, with a maximum slip of 1.2 m. Within this region, the rate of increase in cumulative tremor counts gradually decreased, while intervals between tremor bursts increased, possibly reflecting the long-term evolution of postseismic deformation following the 2011 Tohoku earthquake. The average stress drop within the main slip region was 1.4 MPa, which may reflect low effective normal stress associated with low confining pressure and elevated pore-fluid pressure. The spatial coexistence of slow and fast fault-slip phenomena may reflect heterogeneous frictional properties under such a low effective normal stress.