PIT EXCAVATION ON A MOUNTAIN SLOPE: A POSSIBLE PALEOSEISMIC INVESTIGATION FOR SHORT AND LOW SLIP-RATE FAULTS WITHOUT SUITABLE SITES FOR TRENCHING

Paleoseismology has developed mostly based on studies of major active faults with high slip rates, but low slip-rate faults are also potential sources of catastrophic earthquakes. Especially in regions under tectonically undeveloped regime where many short and low slip-rate faults accommodate regional strain, such faults should be much more important for seismic hazard assessment. We believe that further understanding of low slip-rate faults and tectonically undeveloped regions is one of the important directions of paleoseismology in the 21st century.

One big challenge about low slip-rate faults would be difficulty in obtaining paleoseismic data, because there are fewer suitable sites for trenching along them. In particular, most of low slip-rate strike-slip faults occur within mountainous regions and therefore traditional trenching is much less effective for these faults. Here, we propose pit excavation on a mountain slope as a possible paleoseismic investigation for such faults. By excavating a pit across a small uphill-facing fault scarp on a mountain slope and analyzing the sediment trapped by the scarp, we possibly estimate timing of recent paleoseismic events on the fault.

We carried out two test excavations (pits D1 and D2) on a fault in intraplate Japan for the purpose of evaluating application and problems of this method. The tested fault is the Daguchi fault, one of the short strike-slip faults within the Nosaka mountains north of Lake Biwa, southwest Japan, which is typical of intra-montane low slip-rate faults lacking available trench sites. Pit D1 on the northern segment of the fault successfully reveals evidence of latest two surface-rupturing events. Coupled with the extremely small catchment area upstream of the pit site that precludes recycling of detrital woods, identification of species and portion for wood fragments from the trapped sediment allows accurate estimation of the timing of the paleoseismic events. Pit D2 on the southern segment also reveals evidence of multiple paleoseismic events, but lack of wood fragments in the sediment prevents accurate age control. Pending results of pollen analysis, tephra analysis, and radiocarbon dating should provide constraints on the approximate timing of paleoseismic events and the cause for lack of woods.