The western segment of the Castle Mountain fault poses a significant seismic hazard to the most populated region of south-central Alaska. We identify a previously unrecognized margin of a postglacial outwash channel that is offset right laterally 36 ?? 4 m across the western segment of the Castle Mountain fault. This offset occurred after glaciers withdrew from the lowland 11,300-15,380 cal yr B.P. and after outwash channel margins were cut and stabilized 11,210-13,470 cal yr B.P. Using these ages and the measured separation, we obtain a maximum slip rate of 3.0 ?? 0.6 mm yr-1 and a minimum slip rate of 2.8 ?? 0.7 mm yr-1. These are the first lateral slip rates for the Castle Mountain fault established by a field measurement. Based on timing of the most recent earthquake, 670 ?? 60 yr B.P., the Castle Mountain fault could have accumulated an average single-event slip of about 1.9 m (extremes range from 1.3 to 2.6 m). The fault consists of two segments; a surface-rupturing earthquake likely will be limited to the 62-km-long western segment. Area-magnitude regression calculations suggest that such an earthquake on the western Castle Mountain fault would have a moment magnitude of 6.9 to 7.3.
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Holocene slip rate for the western segment of the Castle Mountain fault, Alaska