West Shore Lake Oroville Lineament Geologic Investigation, Northern California, Part 2 of 2

Date: 4/24/2019

Time: 06:00 PM

Room: Grand Ballroom

The ML5.7 1975 Oroville earthquake occurred seven years after construction of Oroville Dam, the tallest earth-fill embankment dam in the United States. The Oroville earthquake and aftershocks resulted in recognition of Quaternary activity along the Cleveland Hill fault (CHF), Swain Ravine fault zone (SRF), located west of the Foothills fault system, where the seismic hazard is relatively low based on moderate earthquake activity and small co-seismic displacements. For relicensing, lidar data was acquired to evaluate the possible northward extension of the CHF. An approximately 10 km-long, northsouth oriented zone of topographic lineaments were identified in the lidar along the West Shore of Lake Oroville (WSLO) north of Oroville Dam. The lineaments are coincident with the northern projection of the CHF and are expressed as scarps, benches, and saddles along the steep slopes of WSLO. In order to assess the origin of the lineaments, and determine whether or not the prominent lineament is related to recent fault activity that could pose a co-seismic surface rupture potential at Oroville Dam, our study integrated geomorphic mapping, field reconnaissance, and four trenches across the lineament. Detailed mapping documented a roughly north-south oriented bedrock fabric throughout the region associated with a series of parallel lineaments. Field reconnaissance and trench exposures revealed a robust correlation between strength of the Jurassic metavolcanic bedrock and localized erosion and slope failures. These surface processes exploit weaker zones within the bedrock, resulting in differential erosion and stepped topography. The stepped topography is accentuated by side-hill benches formed by colluvium that infills areas between resistant bedrock zones. The result is a youthful zone of topographic lineaments. Furthermore, a clay-rich saprolitic unit (~175 ka) was mapped in trench T3 that crosses the lineament in Bear Meadow. No faulting or deformation was observed in the saprolite; therefore, precluding an active fault consideration.

Presenting Author: Ozgur Kozaci

Authors