2003 Volume 112 Issue 4 Pages 531-543
The Median Tectonic Line active fault system (MTL), with an average slip rate as high as 5-10 mm/yr, is one of the most active inland faults in Japan. However, the long-term seismic risk of the MTL has been poorly known, because of insufficient paleoseismological data, especially timing and displacement associated with the most recent surface faulting.
We carried out a trench excavation survey across the Hatano fault in Doi town, Ehime Prefecture, and were able to precisely determine the timing of the latest faulting event. The survey site is located between range-facing fault scarps level 0.8 m high on an alluvial fan. We first excavated two trenches across the fault to precisely locate fault traces. Faults cutting Holocene sediment were exposed on both walls of each trench. The sense of apparent displacement across the fault zone is down to the south, which is consistent with fault scarps around the trench site. Then we excavated two trenches parallel to the fault zone to expose stratigraphic evidence of horizontal displacement associated with past earthquakes.
The sediment exposed on the trench walls contains evidence of two faulting events in the past 3500 years B.P. The most recent surface faulting along the Hatano fault occurred between 1520 cal A.D and 1660 cal A.D. This is the first paleoseismological data that precisely constrain the timing of the most recent faulting event of the MTL. We have estimated 2.5 ± 0.5 m right-lateral displacement and 0.3-0.5 m vertical displacement up to the north during the most recent faulting event based on an offset of paleo-channel deposit.
