The Sanriku-Oki low-seismicity region on the northern margin of the great 2011 Tohoku-Oki earthquake rupture

Abstract

We examine a region of the megathrust fault offshore of northeastern Honshu (38.75°–40.25°N, 141.5°–143.25°E) that we designate as the Sanriku-Oki low-seismicity region (SLSR). The SLSR, located near the northern termination of the 2011 Tohoku-Oki (M_w 9.0) rupture, lacks historical great earthquake ruptures and has relatively low levels of moderate-size (M_j ≥ 5.0) earthquakes, with subregions having many small events (M_j 2.5–5.0) in the Japan Meteorological Agency unified catalog. The SLSR is located downdip along the megathrust from the rupture zone of the great 1896 Sanriku tsunami earthquake and the great 1933 Sanriku outer trench slope normal-faulting event; weak seismic coupling of the SLSR had been deduced based on the occurrences of those unusual events. Relatively low slip deficit on the SLSR megathrust was estimated based on GPS deformations prior to 2011 compared with adjacent areas with strong inferred locking to the south and north. The southern portion of the SLSR appears to have had, at most, modest levels (<5 m) of coseismic slip during the 2011 event. Some thrust-faulting aftershocks did occur in the SLSR, primarily at depths near 40 km where there had previously been small (M_j ~5.0) repeating earthquakes (e.g., the Kamaishi repeater). An M_w ~ 7.4 underthrusting aftershock occurred near the northeastern edge of the SLSR ~22.5 min after the great 2011 event. Postseismic convergence along the megathrust is peaked in the SLSR. The collective observations indicate that the SLSR is primarily undergoing quasi-static aseismic convergence, and the lack of regional strain accumulation likely delimited the northern extent of the great 2011 rupture as well as the downdip extent of the 1896 rupture. The triggering of the M_w 7.4 aftershock and heightened activity in the downdip repeater regions suggest that high postseismic strain rates drove the region to have ephemerally increased seismic failure, but it appears unlikely that a great earthquake will nucleate or rupture through this region. Similar properties may exist on the megathrust near the southern end of the 2011 rupture.

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