RESULTS OF TRENCHING THE CANYON RIVER FAULT, SOUTHEAST OLYMPIC MOUNTAINS, WASHINGTON

The Canyon River fault forms a prominent antislope scarp that impounds at least three sag pond in the southeast Olympic Mountains in Washington. In a prior investigation, we dated charcoal in colluvium that washed onto the scarp at about 1880±70 yr BP. In this investigation, we trenched the scarp across one of the sag ponds where a GPR survey showed the depth to bedrock to be less than 5 m. The trench showed basalt of the Eocene Crescent Formation thrust up over Holocene debris flow deposits along a fault dipping about 700 to the south. The net vertical separation between the footwall and highwall is about 3.35 meters, measured both on the top of the Crescent Formation and on the modern ground surface. Abundant slickensides and grooves (some with the intact pebbles that carved the grooves still in place) on the highwall show a left-lateral sense of slip. The rake of one set of these grooves is 25 degrees, and a second set is 65 degrees. It was not clear that one set consistently truncated the other. Assuming and correcting for a rake of 25 degrees implies a total slip of about 7.9 meters; at a rake of 65 degrees, net slip is about 3.7m. The fault plane opens up to a positive flower structure in the upper 2 m, further supporting strike-slip motion.

A prominent colluvial wedge overlies three forest soils dated at 2620±70, 2050±90, and 1790±40 yr BP. respectively on detrital charcoal. These three soils merge into the modern surface at a distance of about 3 m from the fault, so we think they are all the same soil thrust over one another by drag, but they could also represent separate small events. Clearly, though, most of the scarp was built by a single event. Considering an inbuilt age of the charcoal of several hundred years suggests an event a few hundred years after 1800 yr BP Applying the relation of Wells and Coppersmith, 1994 suggests that the Canyon River fault generated an earthquake with a magnitude on the order of 6.7-7.8 shortly after about 1800 yr BP.