The Mw 6.0–6.8 Quasi‐Repeating Earthquakes Off Miyagi, Japan, With Variable Moment Release Patterns Due To a Hidden Adjacent Slip Patch

Abstract After the 2011 M9 Tohoku earthquake, several interplate earthquakes of Mw 6 occurred in close proximity in the area off the northern Miyagi prefecture in Japan, including the 2015 Mw 6.8 earthquake. To advance our understanding of the similarity and diversity in earthquake ruptures at the same asperity, this study compares the source processes of six Mw 6.0–6.8 earthquakes in this region. The results of hypocenter relocation and source process inversion show that these Mw 6.0–6.8 events repeatedly ruptured the same patch west of their hypocenters; the rupture always began in this patch and tended to propagate westward. Moment releases for this patch are similar, approximately Nm (Mw 6.3) before and after the Tohoku earthquake. However, the Mw 6.0 event that occurred 20 days after the Tohoku earthquake had only half this moment release, likely reflecting less than usual healing of frictional strength. Moreover, the fourth earthquake after the Tohoku earthquake, which is the 2015 Mw 6.8 earthquake, released a much larger moment of Nm (Mw 6.5) in the same patch and caused a subsequent large slip in a second patch to the east. The presence of the eastern hidden patch likely affected the slip amount of the western patch that typically causes Mw 6.3 events. An examination of M JMA 2.0–5.3 repeating earthquake activity suggests that this eastern patch also affected the preparation period for the 2015 event. The present observation suggests that even locations with well‐predetermined rupture areas and slips can produce much larger slips than usual under the presence of adjacent hidden slip patches.