THE S WAVE PROJECT FOR FOCAL MECHANISM STUDIES.
Final rept., 1 Aug 62-31 Dec 65,
SAINT LOUIS UNIV MO DEPT OF GEOPHYSICS AND GEOPHYSICAL ENGINEERING
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The general purpose of the investigation was the routine investigation of the focal mechanism of the larger earthquakes of each year. The focal mechanism of thirty-six earthquakes of 1962, of thirty-five earthquakes of 1963, and to date of forty-five earthquakes of 1964, including twenty-four earthquakes of the Alaska earthquake sequence of that year, have been determined. The project is continuing, in order to complete all earthquakes of 1964, under other sponsorship. The Alaska earthquake sequence of 1964 is reported in some detail. The mechanisms were found to fall into two groups, one with epicenters in the Kodiak Island region, the other with epicenters in the Prince William Sound region. All have a single steeply dipping well-determined P nodal plane. Orientation of this plane follows, in general, the trend of the tectonic features as they curve from the Alaska panhandle around the Prince William Sound area to the Kenai peninsula. The second nodal plane, identified by S wave data, is near horizontal. The mechanism is open to interpretation either as a reverse fault or as an overthrust. Reasons of strike of the P nodal planes, the spatial distribution of the earthquake foci, and peculiarities in mechanism along the line of greatest flexure of the trend of the tectonic features favor the overthrust hypothesis. General conclusions of the Project are summarized. These include especially the establishment of S wave as an independent means of source mechanism investigation or as an adjunct to P wave first motion, resolution of ambiguity of P wave data in certain types of radiation patterns, and ability to exclude centers of compression in some cases as the source of seismic waves. Author