Estimation of the Slip Rate Along the Unruptured Fault Segment of the M7.2 1896 Rikuu Earthquake, Northeast Japan

Abstract

The Ou Backbone Range (OBR) is one of the most seismically active intraplate regions in northeast Japan. Over the past 200 years, it has been the locus of five large magnitude earthquakes, including the M7.2 1896 Rikuu earthquake, which produced coseismic surface deformation along the northern segment of the Eastern Margin Fault Zone of Yokote Basin (EFZYB), while the southern segment remained unruptured. Despite the extensive paleoseismic investigations along the ruptured segment, the slip rate estimates, and the recurrence interval remained largely unresolved along the southern unruptured segment. This study reports the long-term slip rate, and recurrence interval along the Higashi Chokai san Fault and the signatures of active faulting along the Kanazawa and Omoriyama Faults. Based on the seismic reflection and borehole survey, we have estimated a vertical displacement of 29.3–36.1 m across the Higashi Chokai san Fault, a long-term slip rate of 1.6–1.9 mm/yr over the past 35 ka, and a recurrence interval of about 3,800–4,600 years. Combining our results with the already published data along the OBR, revealed two seismic gaps, first along the western margin of the OBR, along the partial rupture zone of the 1970 Akita-ken earthquake, and second, along the eastern margin of the OBR, north of the 2008 Iwate-Miyagi Nairiku earthquake (IMEQ) rupture zone. Given most of the slip is released elastically, we propose that these seismic gaps have a potential to produce earthquakes with magnitudes Mw7.1 and Mw7.3, respectively if they were to rupture in a single event.