Spatial distribution of the faulting types of small earthquakes around the 2011 Tohoku-oki earthquake: A comprehensive search using template events

We developed a new method to classify the faulting types of small earthquakes using interevent waveform similarity and applied it to earthquakes in the northeast Japan subduction zone. In the method, we used separate time windows for P and S waves and established a relationship between waveform similarity and differences in focal mechanisms from event pairs whose focal mechanisms we know. Then we applied the relationship to many pairs of such focal-mechanism-known events (5607 from the moment tensor catalogue and 3623 events from the interplate repeating earthquakes catalogue) and focal-mechanism-unknown events for the period from 1984 to 2013. As a result, 8984 earthquakes were newly classified into interplate (N = 5401), noninterplate thrust (N = 631), normal (N = 1070), and strike-slip faulting earthquakes (N = 165). From the new data set, which doubles the number of mechanism types, we confirmed that there have been almost no interplate earthquakes in the area of large coseismic slip of the Tohoku-oki earthquake since that event. We also saw that this trend continued until at least the end of 2013, suggesting a nearly complete stress release and slow interplate stress recovery. The abundant interplate aftershocks also indicate the precise spatial extent of postsesimic slip, which is usually difficult to obtain from land-based geodetic data. The postseismic slip also suggests stress concentration at the asperities of the 1968 Tokachi-oki (M7.9) and 1994 Sanriku-oki (M7.6) earthquakes. The present-day faulting types offshore Tohoku correlated well with the static-stress change from the Tohoku-oki earthquake, suggesting a stress state change during the earthquake cycle of megathrust earthquakes.