IN SEARCH OF THE SURFACE TRACE OF THE DOG VALLEY FAULT NEAR TRUCKEE, CALIFORNIA

Magnitude 6 earthquakes on 29 December 1948 and 12 September 1966 near Truckee, California, were attributed to the Dog Valley Fault (DVF); however, the actual trace of the DVF has not been located on the ground or in two exploratory trenches (Hawkins et al., 1986). The inferred location of the DVF is marked by a ~5 km-wide zone of NE-trending geomorphic lineaments that are roughly parallel to fault-plane solutions for the 1966 event and to the trend of associated aftershocks and ground effects. Noteworthy are three lineaments extending from Highway 89 NE into the Snoke Valley: a trend SE of Billy Hill bearing ~37° that marks the inferred DVF trace in the Quaternary Fault and Fold Database of the US, a trend NW of Billy Hill bearing ~40°, and a subtle lineament bearing ~32° that extends from the NW end of Prosser Creek Reservoir towards the NE abutment of Stampede Dam. We investigated this third trend.

Several small-displacement faults are exposed in the roadcut on County Highway 261 (Dog Valley Road) and on the drought-exposed upper shoreface at the NE abutment of Stampede Dam, and have strikes that average 28°±10° with dip angles of between 36-56°NW and 74-87°SE. No shear striae were recognized along these faults, probably due to the coarse granular character of the sheared materials.

Using recent refinements in the Seismo-Lineament Analysis Method (Cronin et al., 2008; bearspace.baylor.edu/Vince_Cronin/www/SLAM/), we defined seismo-lineaments for all relevant earthquakes for which published focal mechanisms are available, including a M4 earthquake from 3 July 1983 and a M3.2 earthquake from 30 August 1992. The 1983 and 1992 events have epicenters just SW of the 1966 M6 event, and all three have left-lateral NE-striking fault-plane solutions. We interpret all three earthquakes to have occurred on the DVF, as did Reed (2014). The average orientation of their mean fault-plane solutions strikes 31°±5° dipping 78°±20°. Greensfelder (1968; Kachadoorian et al., 1967) wrote that ground effects of the 1966 M6 event were in a "narrow zone about 10 miles long trending N30°E." The more subtle lineament that we studied, and that largely coincides with the zone of ground effects and aftershocks of the 1966 event, parallels the mean fault-plane solution for the 1966, 1983 and 1992 events and is within the seismo-lineaments of all three earthquakes.