Abstract The Lake Tahoe basin lies within one of the most active parts of the Sierra Nevada-Great Basin boundary zone. New geologic mapping and new geophysical data show that numerous active faults and landslides occur within the Lake Tahoe basin. Active faults define a northeast-southwest-trending zone, the North Tahoe-Incline Village fault zone, a north-south-trending zone, the West Tahoe-Dollar Point fault zone, and a northwest-trending zone, the Tahoe-Sierra frontal fault zone. These zones include faults on land and on the lake bottom, where scarps are well preserved. Other faults in many other areas around the basin may be active, but data are presently insufficient to characterize most of them. The October 30, 1998, M4.9 earthquake occurred near the North Tahoe-Incline Village fault zone, and other historic earthquakes may have occurred along both fault zones as well . Active faults are spatially associated with landslides along the sides and bottom of the lake, suggesting that at least some of the landslides have been triggered by large earthquakes. Evidence also exists for seiches or tsunamis that may have been generated by past earthquakes and landslides . Currently available data indicate that Lake Tahoe lies within a tectonically active, asymmetric half-graben. Large (M7) earthquakes can be expected along active faults in the basin in the future, and emergency planning scenarios should include provisions for strong ground-shaking, landslides, and seiches. Additional research, both on land and within the lake, is underway to quantify the seismic hazard within the basin .

Abstract Neogene sedimentary rocks, largely lacustrine and fluvial, were deposited in many areas of western Nevada and easternmost California, more or less continuously from ~12 Ma-3 Ma. These sediments were deposited on a surface of moderate primary relief developed on underlying Miocene volcanic rocks, and depositional facies are laterally variable, with sedimentation beginning and ending at different times in different areas. About 3 Ma, the basin system began to break up structurally, with strike-slip, oblique-slip and normal faulting, and local uplift. Modern topography inverts many original highs and lows. Stratigraphy and structure of Neogene strata document the progressive shut-down of the Neogene Cascade arc, the evolution of transtensional tectonics in the Walker Lane, and the encroachment of extensional faulting into the eastern Sierra Nevada .