2008 Volume 2008 Issue 29 Pages 35-57
We excavated paleoseismic trenches across the Yamagata-bonchi fault zone that is a 60-km-long westdipping active reverse fault located in northeast Honshu, Japan. We then recovered the evidence for six to seven surface-rupturing earthquakes in the past ten thousand years from the highly deformed strata and faults exposed on the trench walls in the town of Oishida that located on the northern edge of the fault zone. The most recent paleoseismic event in the town is inferred to have occurred sometime between 1,500 and 2,800 cal. y. B. P. In contrast, trench walls in the town of Yamanobe across the southern part of the fault zone exposed evidence for only one surface-rupturing earthquake in the past ten thousand years. The event age is constrained to have occurred sometime between 4,200 and 5,000 y. B. P. Together with the major fault discontinuity at the center of the fault zone, we conclude that the Yamagata-bonchi fault zone is divided into two 30-km-long behavioral segments, both of which might have produced and will generate M-7.3 earthquakes. An average recurrence interval of 1,400-1,500 years and the long elapsed time since the most recent event on the northern segment allow us to calculate 3-14% of 30-yr conditional earthquake probabilities. Since time range of the event on the southern segment overlaps one of the events recovered from the northern segment, we cannot rule out the near future likelihood of the infrequent worst scenario of 60-km simultaneous multiple ruptures that might produce M-7.5 earthquake.
