abstract

Fault parameters of the Ms 7.1 St. Elias, Alaska, earthquake of February 28, 1979, are determined from an analysis of P-wave first motions, fundamental-mode surface waves, and aftershock data. The preferred P-wave nodal plane has a shallow (12°) angle of dip and indicates underthrusting in a northerly (N13°W) direction, which is also close to the azimuth (N8°W) of the deviatoric compression (P) vector. Aftershock activity during the 24-hr interval immediately following the main shock extends over an area of 3200 km2, which is taken to represent the fault area of the main shock. Because aftershock activity outlines a fault area with nonrectangular geometry, fault length (50 to 80 km) and width (50 to 65 km) are not well defined. Estimates of focal depth from aftershock activity fall in the range 10 to 20 km, which surface-wave analysis is unable to restrict further. For the main shock, seismic moment, Mo, is 2.5 × 1027 dyne-cm (0.25 × 1021 N.m) and average dislocation, 2 meters. Average rupture velocity falls in the range 2.5 to 3 km/sec. Apparent stress, η

σ
, is 5 bars (0.5 MPa) and stress drop, Δσ, 35 bars (3.5 MPa). These values are considered typical of large-magnitude interplate earthquakes.

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