Crater Lake caldera is at the north end of the Klamath graben, where this N10°W-trending major Basin and Range structure impinges upon the north-south–trending High Cascades volcanic arc. East-facing normal faults, typically 10–15 km long, form the West Klamath Lake fault zone, which bounds the graben on its west side. The fault zone terminates on the south near the epicentral area of the September 1993 Klamath Falls earthquakes. It continues north past Crater Lake as the Annie Spring fault, which is within ∼1 km of the west caldera rim, and Red Cone Spring fault. We have determined a long-term vertical slip rate of 0.3 mm/yr for these two faults using high-precision K-Ar and 40Ar/39Ar age measurements on offset lava flows ranging in age from ca. 35 to 300 ka. Holocene offset reported by Hawkins et al. and epicenters of eight MW 2 earthquakes in 1994 and 1995 indicate that the West Klamath Lake fault zone is active. Empirical relations between earthquake magnitudes and scarp heights or fault lengths suggest that the fault zone is capable of producing earthquakes as large as MW 7¼. Earthquakes on these or other faults of the zone could trigger landslides and rockfalls from the walls of the caldera, possibly resulting in large waves on Crater Lake.