Slow slip event in the focal area of the 1975 Kurile tsunami earthquake inferred from unusual long-term seismic quiescence

In subduction zones, slow slip events (SSEs) have been observed in the portion deeper than the downdip edge of seismogenic zone. However, since it is far offshore from geodetic networks on land, shallow SSEs near the trench axis are hardly observed. Despite of less quantitative than seafloor geodetic observations, a method for inferring the shallow SSEs based on seismic quiescence was presented in this study. Unusual decrease in occurrence rate of M >= 5.0 earthquakes was found in the southwestern Kurile Islands. The occurrence rate was similar to 1.3 events/year between 1977 and around 2004 and no earthquake was observed during 16 years after 2004. The spatial pattern of the seismic quiescence can be explained qualitatively by the Coulomb failure stress change due to shallow SSE and its fault plane is on the upper boundary of the subducting Pacific plate in the focal area of the 1975 tsunami earthquake.

Automated summary

This study presents a method for inferring shallow slow slip events (SSEs) based on seismic quiescence. Shallow SSEs near the trench axis are difficult to observe because they are located far offshore from geodetic networks on land. The study found an unusual decrease in the occurrence rate of M >= 5.0 earthquakes in the southwestern Kurile Islands, with no earthquakes observed for 16 years after 2004, compared to an average rate of approximately 1.3 events per year between 1977 and 2004. The spatial pattern of seismic quiescence can be explained qualitatively by the Coulomb failure stress change due to shallow SSE, and its fault plane is on the upper boundary of the subducting Pacific plate in the focal area of the 1975 tsunami earthquake.