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 .

T he writer recognizes deposits of seven glacial advances, in three groups, in the San Joaquin drainage system on the western slope of the Sierra Nevada and in Rock Creek on the eastern slope. Glacial advances are distinguished for the most part on geomorphic criteria involving abrupt changes in aspect of subsequent erosion, deposition, and weathering. Quantitative data include ratios of fresh-to-weathered granite boulders and boulder frequency counts. The fresh-to-weathered ratio counts confirm correlations between canyons and across the crest of the Sierra Nevada. All the oldest (Group I) glacial deposits recognized are outside areas of younger ice advances. They have been extensively weathered but show less marked weathering than deposits in areas not recognized as having been glaciated. Few morainal forms remain, and locally granitic erratics rest on lava. The writer believes that the deposits are Sherwin. Group II includes deposits representative of three advances: the Tahoe, the Tioga, and a previously unrecognized advance, termed Tenaya. The Tenaya deposits are recognized in the San Joaquin drainage system in the Yosemite Valley, and in several canyons on the east side of the Sierra Nevada. Group-II deposits are abundant at middle elevations. The Hilgard deposits, the oldest of Group III, extend several miles from the cirques and are characterized by a sharp reversal in ratio of fresh-to-weathered granitic boulders. The writer believes that they postdate the Thermal Maximum. The succeeding Recess Peak deposits are within or near the cirques, and although the deposits are very fresh, the slopes are stabilized. Most Matthes deposits consist of cliff glacierets and rock glaciers that are still unstable. The writer interprets the Group-III deposits as Neoglacial.