TIMING AND DISPLACEMENT OF THE MOST RECENT SURFACE FAULTING ALONG THE MEDIAN TECTONIC LINE IN SHIKOKU, SOUTHWEST JAPAN

An active fault system extends for about 190 km along the Median Tectonic Line (MTL) in Shikoku, southwest Japan. The MTL is an arc-parallel, right-lateral strike-slip fault related to the oblique subduction of the Philippine Sea plate beneath the Eurasian plate along the Nankai trough. It is one of the most active inland faults in Japan. However, the long-term seismic risk of the MTL has been poorly understood, because of insufficient paleoseismological data, especially the timing and amount of displacement associated with the most recent surface faulting along the fault zone.

In order to better define seismic potential of the MTL, we carried out a trench excavation survey across the Hatano fault in central Shikoku. We excavated two trenches across the fault trace and two trenches parallel to the fault trace to expose stratigraphic evidence for horizontal displacement associated with the past earthquakes. The sediments exposed on the trench walls contain evidence for two faulting events in the past 3500 years B.P. The most recent surface faulting along the Hatano fault occurred between 1510 cal A.D and 1630 cal A.D. This is the first paleoseismological data, which precisely constrain the timing of the most recent faulting event of the MTL. We have also estimated 2.0-3.0m right-lateral displacement and 0.3-0.5m vertical displacement up on the north during the most recent faulting event based on offset of paleo-channel deposit.

Detailed analysis of large-scale aerial photographs led us to identify numerous geomorphic and artificial features that are offset during the most recent faulting along the MTL. These features include terrace risers, streams, roads, and rice-paddy dikes. We have made detailed geomorphic maps using an EDM and measured the amount of offset at each site. The amount of offset ranges from about 7 meters in eastern Shikoku and 2-3 meters in western Shikoku, which is concordant with geologic slip rates. This implies that the characteristics earthquake model proposed by Schwartz and Coppersmith (1984) is applicable to the MTL active fault system.