28 Maximum Earthquake on the WKLFZ

Volcano and Earthquake Hazards in the Crater Lake Region, Oregon

Earthquakes

Maximum Earthquake on the WKLFZ

The maximum earthquake likely to occur on a fault or fault zone can be estimated from fault characteristics such as surface rupture length and downdip rupture area. Critical to this analysis is rupture length, which depends on fault zone segmentation (Schwartz and Coppersmith, 1986). Crater Lake appears to be adjacent to a segment boundary in the greater WKLFZ within which there is no recognized displacement of ~50 ka lava flows (fig. 2, plate 1).

The southern boundary of this segment may be coincident with the end of the WKLFZ where the faults change trend from north-south to north-northwest–southsoutheast in the epicentral area of the 1993 Klamath Falls earthquakes. This suggests a maximum possible rupture length of ~50 km. Consideration of the empirical relation between earthquake magnitude and surface rupture length (SRL) for normal faults given by Wells and Coppersmith (1994, table 1A) and a maximum SRL of 50 km suggests a maximum earthquake of MW = 7.1 ± 0.3 (MW = 7.3 ± 0.3 for SRL = 70 km; MW = magnitude based on seismic moment, the total energy released in an earthquake). This result is similar to the conclusion of Hawkins and others (1989, 1992) who evaluated the potential shaking at dam sites 50–85 km south-southeast of Crater Lake and reported a maximum earthquake for the West Klamath Lake fault zone of MS = 71/4 (MS = surface wave magnitude).

If the hypocentral depths of the 1993 earthquakes are typical of the fault zone, another estimate of the maximum event can be obtained from the empirical relation between downdip rupture area and magnitude (Wells and Coppersmith, 1994, table 1A). This suggests a maximum magnitude of 7.0 ± 0.2 for a maximum depth of 12 km, dip of 60°, and length of 50 km (7.1 for SRL = 70 km). Increasing depth to 15 km raises the calculated magnitude to 7.1 (7.3 for SRL = 70 km). In light of the uncertainties in the above calculations we consider our best estimate of the maximum earthquake likely to occur near Crater Lake to be ML = 71/4 (table 5; ML = local magnitude).

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