The Alaska earthquake of July 10, 1958: Seismic studies

Abstract

The instrumental epicenter of the Alaska earthquake of July 10, 1958, has been located on the hanging-wall side of the Fairweather fault near the surface expression of the fault and at the southeast end of the greatly disturbed area. The fault-plane solution from P waves gives a fault plane which differs by 15° from the strike of the observed surface faulting. The theoretical relative amplitudes computed from the fault-plane solution are interpreted as partly explaining the residuals of P arrivals and inconsistent directions of the first motion of P. The directions of polarization of the S waves are found to conform to the pattern expected for a single couple as the point-source model of the focal mechanism. A method is suggested for using S waves to check fault-plane solutions from P and to select the fault plane from the two nodal planes of P. Observations of S at near stations do not correspond to the pattern expected for a couple. Large transverse motion is observed along the azimuth of the fault. In the western United States, at stations along the same azimuth, the SH motion changes progressively to SV with distance. Large SV components at distances of 25° to 27° indicate that the critical angle for SV may be reached at these distances and that even long S waves are refracted by structure within the crust.