COMBINING GEOPHYSICS AND GEOLOGY TO SEARCH FOR SEISMOGENIC FAULTS, LAKE TAHOE-TRUCKEE AREA, CALIFORNIA AND NEVADA

We are working to correlate earthquakes with faults that reach the ground surface in the northern part of the Lake Tahoe Basin and Truckee area, within the transition zone between the northern Walker Lane and the Sierra Nevada block. Existing fault mapping and focal mechanism solutions demonstrate that this area has earthquakes on N-striking E-dipping normal faults, NW-striking dextral faults and their conjugates: NE-striking sinistral faults. Fault reconnaissance mapping in this area is challenging due to moderately high relief, forest cover, young glacial sediments, mobile slope deposits and private property limitations. Geophysical data from aerial lidar and InSAR, along with earthquake and geodetic GPS data, augment fieldwork that would otherwise be largely futile in finding seismogenic faults.

Preliminary work using focal-mechanism data from 29 M≥3 earthquakes recorded since 1966 suggests spatial correlation between recent earthquakes and the Dog Valley fault zone (fz), Stateline-North Tahoe fz, West Tahoe-Dollar Point fz, Incline Village fault, Polaris fault and possibly the hypothetical Agate Bay fault. Current work includes additional earthquake events, group relocation of earthquake foci, re-evaluation of focal mechanism solutions, and re-definition of seismo-lineaments using an improved version of the Seismo-Lineament Analysis Method (SLAM; Cronin and others, 2008, Env & Eng Geol 14[3], 199-219) code to attempt spatial correlation between nodal planes and the surface traces of faults. Structural geomorphology along seismo-lineaments, studied using standard 10 m or lidar-based digital elevation models, indicates at least two geomorphic lineament trends that might be previously unrecognized seismogenic faults in the Martis Creek drainage, and a third trend crossing the Truckee River and Prosser Creek reservoir. GPS data from the EarthScope Plate Boundary Observatory is used to define the state of horizontal infinitesimal strain in the area. InSAR data accessed through the Western North America InSAR (WInSAR) Consortium will provide additional insights about local crustal deformation, particularly in the vertical direction. Our results will help paleoseismologists identify fruitful targets for future trench studies.