Late Quaternary slip rate and seismic hazards of the West Klamath Lake fault zone near Crater Lake, Oregon Cascades

Crater Lake caldera is at the north end of the Klamath graben, where this N 10 degrees W-trending major Basin and Range structure impinges upon the nor th-south-trending High Cascades volcanic are. East-facing normal faults, ty pically 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 th e epicentral area of the September 1993 Klamath Falls earthquakes. It conti nues north past Crater Lake as the Annie Spring fault, which is within simi lar to 1 km of the west caldera rim, and Red Cone Spring fault. We have det ermined a long-term vertical slip rate of 0.3 mm/yr for these two faults us ing high-precision K-Ar and Ar-40/Ar-39 age measurements on offset lava flo ws ranging in age from ca. 35 to 300 ka. Holocene offset reported by Hawkin s et al. and epicenters of eight M-W 2 earthquakes in 1994 and 1995 indicat e that the West Klamath Lake fault zone is active. Empirical relations betw een earthquake magnitudes and scarp heights or fault lengths suggest that t he fault zone is capable of producing earthquakes as large as M-W 7 1/4. Ea rthquakes 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.