The focal mechanism of the Alaska earthquake of March 28, 1964, and of its aftershock sequence

Focal mechanisms have been determined for one preshock, for the main shock, and for more than 25 aftershocks of the Alaska earthquake of March 28, 1964. For the main shock a single nodal plane with a strike azimuth of 66°, dip 85° southeast, is determinable from the polarity of the P wave. This plane may be taken either as a plane normal to the fault motion (thrust faulting) or as the fault plane (dip-slip motion on a near-vertical plane). A combination of P wave first motion and S wave polarization data make possible the determination of both nodal planes in each shock studied of the aftershock sequence. One of these planes is near vertical and closely resembles the nodal plane of the main shock; the other dips 5° to 15° to the northwest or north. For earthquakes of the Kodiak Island region, the near-vertical plane has the same orientation as that of the main shock. For earthquakes to the east of Prince William Sound, this plane shows a systematic change in orientation corresponding to the change in trend of the tectonic features. Three earthquakes that have foci at increasing depths along the line of greatest flexure of the tectonic features differ from the others. The difference in character of these foci probably provides an important clue to the right interpretation of the motion in the main shock. Although the focal mechanism solutions for the shocks are subject to the same ambiguity of interpretation as that of the main shock, criteria which favor a thrust hypothesis are advanced from the interrelation of the foci. From dislocation theory it is shown that differential slip and/or a dipping thrust plane explain satisfactorily the observed vertical displacements at the surface.