Paleoseismic Investigation of the Spring Valley Fault, Santa Rosa, California: Evidence for Holocene Activity

The Spring Valley fault is a 5-km-long north-trending fault located on the east side of Santa Rosa, CA, marked by a linear west-facing escarpment, springs, and a linear marsh now occupied by Spring Lake Reservoir. A vertical plane of microseismicity closely aligns with the fault trace. Despite this indirect evidence, the fault is not currently recognized by the State of California as an active fault. Extending between subparallel traces of the Rodgers Creek fault and the Maacama fault, and bounding the eastern margin of the Santa Rosa structural basin, the Spring Valley fault is one of several structures that may play a role in the transfer of slip between these two major faults. Two trenches at Spring Lake Park, located about 1.6 km (1 mi.) apart, were excavated to accurately locate the fault and to assess the timing of activity. The north trench was sited across a faulted Late Pleistocene terrace remnant of Santa Rosa Creek. This trench exposed gently down-warped fluvial terrace deposits on the east, faulted against colluvium on the west. The fault zone is characterized by multiple sub-vertical seams of clay. Apparent displacement is down-to-the-west, with unknown lateral displacement. The lack of matching units on either side of the fault zone precludes measurement of vertical displacement, but fluvial deposits are absent on the downthrown side to the bottom of the trench at 3 m (9 ft.), suggesting the vertical displacement is significant. Detailed logging of faulted and down-dropped colluvial wedges west of the fault permits the interpretation of three to five faulting events. Six samples were collected for radiocarbon analysis from the various colluvial units. The radiocarbon age of sample 2, taken from a depth of 0.79 m in colluvium on the downthrown side of the fault, is 2210 to 2310 CAL yr. BP (2 sigma). The radiocarbon age of sample 5, taken at 1.14 m depth in a colluvial unit underlying sample 2, is 2860 to 3000 CAL yr. BP (2-sigma). Results for four additional samples are pending. These initial radiocarbon results suggest that the Spring Valley fault has been active as recently as the late Holocene.