Castle Mountain

The Castle Mountain fault is one of several major east-northeast-striking faults in southern Alaska, and it is the only fault with historic seismicity and Holocene surface faulting (Lahr and others, 1986; Detterman and others, 1974). The Castle Mountain fault is approximately 200 km long, and is one of the longest structures in the Cook Inlet basin. Martin and Katz (1912) first noted the fault, but it was delineated on a regional scale by Detterman and others (1974, 1976). They mapped and divided it into two physiographic segments: the western Susitna Lowland and eastern Talkeetna Mountains segments (Fig. 1). Haeussler (1994, 1998) mapped and examined the 30-km-long region between the two Detterman and others (1974, 1976) maps.

This report is a compilation of the three USGS maps that cover the location of the Castle Mountain fault in some detail (Detterman and others, 1974, 1976; Haeussler, 1998), with the purpose of providing land managers with an authoritative source for the location of the fault in the Talkeetna Mountains and Susitna Lowland. There are other maps that also cover parts of the Castle Mountain fault (Reger and others, 1995a,b,c; Clardy, 1974; Fuchs, 1980), but these do not alter the location of the fault. Thus far, there are no land use or building regulations associated with proximity to the Castle Mountain fault.

The surface trace of the Castle Mountain fault is not the only earthquake hazard associated with the fault. The two historic earthquakes on the Castle Mountain fault were located on the part of the fault where there is no surface expression (Lahr and others, 1984), and thus even the part of the fault with no scarp should probably be considered active. In addition, Haeussler and others (2000) showed there is a 3-4 km wide fault-cored anticline on the north side of the fault near Houston. The faults in the core of the anticline do not crop out at the surface, but certainly also represent a seismic hazard. Saltus and others (2001) use aeromagnetic data to show that this anticline continues for the length of the Castle Mountain fault in the Susitna Lowland.