SEISMOGENIC LANDSLIDES AND PALEOSEISMICITY AT LAKE TAHOE

Analysis of fourteen piston cores delineates a Holocene record of strong ground shaking and submarine landsliding in Lake Tahoe. The cores show widespread turbidite deposits throughout the central and northern lake which account for >55 percent of sediment accumulation in some areas. As many as 12 post-Tsoyowata ash (7600-8000 ybp) events are identified in the basin, with 8 of these events widely distributed. These episodic turbidites are likely the result of strong earthquake shaking events and landslides within the basin. Stratigraphic correlation, magnetic susceptibility, and preliminary grain-size analysis of turbidites indicate three general areas in which the turbidity currents may originate; North Tahoe, McKinney Bay, and South Tahoe, though the other walls of the basin are not ruled out as a source. The presence of multiple source areas for a single turbidite event indicates strong earthquake shaking within the basin. Also, turbidites that do not originate near local stream or river sources are likely seismically induced. Several generations of moderate to large landslides occur in McKinney Bay, including a megaslide that scattered slide blocks of coherent Pleistocene sedimentary strata throughout the lake, deposited a 70-m-thick chaotic layer, and changed the basin morphology in the Late Pleistocene. These slides in the west Tahoe basin attest to several episodes of faulting and landslide activity. The West Tahoe, North Tahoe, and Incline Village faults likely rupture in large displacement M>7 events, as does the Genoa fault to the east of the basin. Radiocarbon dating of one core, when combined with stratigraphic correlations, leads to a recurrence of basin-wide turbidites every ~1000 yrs., consistent with recent slip-rate estimates and recurrence intervals on the three faults in the basin and the Genoa fault.