Multiple-Event Deformation on the West Tahoe Fault from Lidar and Trenching: Reconciling On and Offshore Paleoseismology

Abstract:

The West Tahoe Fault has a mapped length of 45 km, and its vertical slip rate is estimated at 0.6-1.0 mm/yr (Dingler et al., 2009). It is the range bounding, east dipping normal fault along the west margin of the basin, and the highest slip rate section is located along the western base of Lake Tahoe at a water depth of 400-500 m. In the lake, the fault has clearly defined scarps that displace submarine fans, lake-bottom sediments, and the ~50 ka age (Kent et al., 2005; Smith et al, 2013) McKinney Bay slide. Onshore the fault displaces tree-covered glacial and fluvial landforms.

In October 2013 we conducted the first trenching study of the geomorphically well-defined West Tahoe Fault. Using Lidar, we selected a site with fine-grained surface sediments, as opposed to the boulder-rich moraine sediments that cover most of the onshore fault. The site is located ~3.5 km from the southern end of the mapped fault, adjacent to Osgood Swamp. In the excavation, we observed an east-dipping, normal fault juxtaposing glacial deposits against datable peat-bearing and charcoal-rich younger alluvial sediments. The glacial deposits and peat layers can be matched across the fault. Clear evidence for two earthquakes was based on scarp-derived colluvium, fissures, faulted scarp-related alluvium, liquefaction, and upward terminations of faults. Retrodeformation of the sediments results in nearly equal vertical displacements of about 1.7 m/event. Results from multiple C-14 analysis place the most recent event at ~5.3 ka and the penultimate event at ~7.4 ka.

The fault scarp at the trench is 3.5 m high; at the bottom of the lake at a depth of 400 m a postulated same-age fan as the trench site post-glacial deposits, has a vertical scarp of 10-12 m. As expected the slip rate decreases towards the southern end of the fault. However, if the same number of events resulted in the underwater scarp then events must average about 5-6 m vertical displacement per event. This has serious implications for earthquake generated lake tsunamis and permanent shoreline subsidence.

These types of onshore results are needed to validate more established paleoseismic results from Lake Tahoe based on offshore methodologies: CHIRP seismic profiling, lake cores showing earthquake triggered turbidites and ROV dives.