Slip Rate and Recurrence Interval of the WKLFZ

An estimate of the long-term slip rate on the WKLFZ near Crater Lake can be obtained from geologic mapping of offset lava flows that have been dated by the K–Ar method. For example, a dacite flow dated at 50±6 ka appears to be offset ~15 m vertically, down to the east, along the Annie Spring fault ~750 m west of Rim Village. This result implies an average rate of vertical displacement of 0.3 mm/yr. This rate is corroborated by vertical offsets of older lava flows along the same fault up to 10 km south of The Watchman (table 4). In addition, a minimum displacement on the related Red Cone Spring fault northwest of the caldera (fig. 1 and plate 1) suggests a similar rate (“A” in table 4). Should the fault dip 60° and have a purely normal sense of motion, the east–west tectonic extension rate would be 0.17 mm/yr and the slip rate in the plane of the fault would be 0.35 mm/yr. This result is similar to long-term average slip rates of 0.1--0.6 mm/yr determined by Pezzopane and Weldon (1993) for faults at the west edge of their Central Oregon fault zone ~100 km east of Crater Lake.

Estimates of slip rates on faults of the WKLFZ south of Crater Lake are consistent with our data for the Annie Spring and Red Cone Spring faults. Hawkins and others (1989, table 3) measured offsets in ~130--150 ka and ~10-30 ka glacial moraines and in early Holocene deposits at the mouths of the canyons of Dry, Sevenmile, Threemile, and Cherry Creeks where they enter the Klamath graben and reported an average slip rate of 0.17 mm/yr for the last ~130,000 years. They noted that at least one 1–2 m surface displacement event has occurred in the last 10,000 years.

Recurrence intervals are unknown for earthquakes that cause surface displacement on the WKLFZ. If all of the displacement on these faults occurred in events with, say, 1–3 m of vertical offset, significant earthquakes would be expected to recur at an average rate of one event in ~3,300–10,000 years. This inference is consistent with paleoseismic data for the Ana River fault ~100 km east-southeast of Crater Lake (Pezzopane and others, 1996). We cannot give a more rigorous estimate of the probability of a large earthquake on the WKLFZ without knowledge of the time and amount of co-seismic displacement of the last event (Working Group on California Earthquake Probabilities, 1990), such as might be obtained by trenching across the fault trace.