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S-wave earthquake mechanism equations

1967; Seismological Society of America; Volume: 57; Issue: 1 Linguagem: Inglês

10.1785/bssa0570010099

1943-3573

Anne E. Stevens,

Earthquake Detection and Analysis

abstract The orientation of force axes at the focus of an earthquake can be calculated using S polarization angles in a set of S -wave mechanism equations (Stevens, 1964). These equations are rederived in this paper in a more direct manner and each equation is shown to represent the plane of polarization of S at a point on the focal sphere. Two special cases of the general mechanism equations, the single-couple and the double-dipole focal mechanism, have been used to study a number of earthquakes. Mechanism solutions for seven Kamchatka-Kuril earthquakes and one Alaskan earthquake discussed previously (Stevens, 1964) are reconsidered. The former single couple solutions for the earthquakes of 1949 Nov 3, 1951 Nov 6, 1952 Nov 5 and 1957 Dec 17 are rejected. No meaningful solution of either type is considered possible with the limited data available. The single couple solution for the earthquake of 1955 April 17 is changed to a double dipole solution; the single couple solution for the earthquake of 1958 July 10 and the double dipole solution for 1953 Nov 10 are altered slightly; the single couple solution of 1960 July 25 is changed significantly. Mechanism solutions are presented for five Aleutian earthquakes using S polarization angles published by Stauder and Udias (1963). All five earthquakes are the double dipole type. The principal compressive stress axes are nearly horizontal and perpendicular to the trend of the Aleutian arc, the intermediate stress axes are horizontal and trend along the arc and the principal tensile stress axes dip steeply under the continent.