SLIP DISTRIBUTION DURING THE LATEST TWO EARTHQUAKES ON THE SEKIDOSAN FAULT ALONG THE SOUTHEASTERN MARGIN OF THE OUCHIGATA PLAIN, CENTRAL JAPAN

We report nearly characteristic behavior during the latest two surface-faulting events on the Sekidosan fault in central Japan. The Sekidosan fault is a NE-trending, 33-km-long active reverse fault along the southeastern margin of the Ouchigata Plain, an elongated NE-trending tectonic lowland whose northwestern and southeastern sides are bounded by reverse faults. A vertical slip rate of 0.8 - 1.0 mm/yr has been estimated based on vertical separation of the top of a peat layer dated at approximately 6,000 years B.P. However, there is no historical record that indicates surface-faulting events along the fault. In order to clarify the activity of the Sekidosan fault during the Holocene, we conducted the following geological and geomorphological investigations.

First, we reexamined the precise location of the fault traces by interpreting aerial photographs on scales of 1/10,000 to 1/40,000. We then carried out 33 topographic profiling across the tectonic scarps on geomorphic surfaces younger than the last glacial maximum. Among the profiles, 15 profiles are interpreted to represent the vertical offset during the most recent surface-faulting event. As a result, we were able to reconstruct the slip distribution associated with the most recent surface-faulting event along the middle and northeastern part of the Sekidosan fault. The amount of vertical offset along the middle part is fairly constant along the trace (2 - 3 m). In contrast, the amount of vertical offset along the northeastern part is 1 - 2 m and tapers off toward the northeastern end. The amount of vertical offset along other 10 profiles nearly equals to twice the amount of offset during the most recent surface-faulting event, indicating that the amount of offset at individual sites along the Sekidosan fault was almost the same during the latest and penultimate earthquakes.

Subsurface stratigraphy and radiocarbon ages revealed by drilling survey across a flexure scarp 4 - 5 m high on geomorphic surfaces of Holocene age suggests that more than two surface-faulting events occurred after approximately 6,000 years B.P.