Bulletin of the Seismological Society of America
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Bulletin of the Seismological Society of America; June 2007; v. 97; no. 3; p. 1019-1024; DOI: 10.1785/0120060109
© 2007 Seismological Society of America
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Short Notes

Holocene Slip Rate for the Western Segment of the Castle Mountain Fault, Alaska

Julie B. Willis1, Peter J. Haeussler1, Ronald L. Bruhn1 and Grant C. Willis1

1 University of Utah
Department of Geology and Geophysics
Salt Lake City, Utah 84112-0111
 (J.B.W., R.L.B.)
2 U.S. Geological Survey
Anchorage, Alaska 99508–4650
 (P.J.H.)
3 Utah Geological Survey
Salt Lake City, Utah 84114–6100
 (G.C.W.)

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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