Abstract
The Median Tectonic Line active fault zone (MTL), with slip rates as high as 5-10 mm/yr, is one of the most active inland faults in Japan. However, the seismic hazard evaluation on the MTL is hampered by insufficient paleoseismological data, mainly due to sparse distribution of trenching sites along the fault zone. We have conducted a first major paleoseismological study of the Kawakami fault, a segment of the MTL in northwestern Shikoku. The study includes aerial photograph interpretation and paleoseismic trenching. Two trenches were excavated across the Kawakami fault northeast of downtown Komatsu, Ehime Prefecture in 1997. The sediments exposed on the walls in the eastern trench are middle to late Holocene fluvial deposits ranging in texture from silt to gravel. These sediments are offset along an almost vertical,3-m-wide fault zone. The sense of apparent displacement across the fault zone is down to the north, consistent with Holocene scarps around the trench site.
The sedimen t s in the trench contain evidence for three episodes of surface-rupturing earthquake in the past 4000 years B. P. The most recent surface-rupturing earthquake, event A, on the Kawakami fault occurred sometime during the 7th and 19th century AD. Historical documents report extensive damage around the trench site during an earthquake in 1596, and this earthquake may be correlated to the latest faulting on the Kawakami fault. The penultimate event, event B, occurred sometime during the 1st and 10th century AD. Another earthquake, event C, occurred after the deposition of layer VII, which was radiometrically dated at about 4000years B. P.
