Abstract
The 1847 M 7.4 Zenkoji, Japan, earthquake was caused by movement of the west-dipping, western margin reverse-fault zone of the Nagano basin (WFZNB) and produced surface ruptures reported in Japanese historical documents. Despite previous studies to assess fault activity of the WFZNB, our trench excavation at Dannohara, Nagano City, exposed near-surface fault structures associated with the Zenkoji earthquake for the first time and provided geologic evidence for three earlier paleoseismic events. Three core samples 10–15 m long defined a hanging-wall anticline beneath the trench. Although the primary fault was not observed, the revealed subsidiary features enabled us to discuss surface rupture morphology and its repetition patterns. We suggest that: (1) the east-dipping reverse faults exposed on the trench walls and the hanging-wall anticline are developed above a west-dipping blind reverse fault; (2) the east-dipping normal faults represent bending-moment faults at the crest of the hanging-wall anticline; (3) the exposed east-dipping faults slipped during the Zenkoji earthquake to produce elongated linear rise scarps described in historical documents; (4) the hanging-wall anticline grew during the earthquake to generate uplifted lands as reported in historical documents; and (5) faulting similar to that during the Zenkoji earthquake also occurred during the penultimate earthquake. We propose preliminary but better constraints on timing of the third and fourth latest faulting events, and a tentative and maximum recurrence interval of 800–1000 years for the WFZNB. To better understand reverse-fault rupture patterns for appropriate evaluation of seismic hazard, historical surface ruptures should be studied in more detail.
