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.

Ten years of monitoring microearthquakes with a regional seismic network has revealed the presence of a well-defined, linear zone of seismic activity in eastern Tennessee. This zone produced the second highest release of seismic strain energy in the United States east of the Rocky Mountains during the last decade, when normalized by crustal area. The data indicate that seismicity produced by regional, intraplate stresses is now concentrating near the boundary between relatively strong and weak basement crustal blocks.

Most natural crustal earthquakes fall into one of two categories:(1) those associated with the dynamics of plate tectonics, or (2) those associated with the elements of the hydrologic cycle. This paper presents a comprehensive listing of published examples of hydroseismicity, a hypothesis that attributes most intraplate and near-intraplate earthquakes to the dynamics of the hydrologic cycle, which includes hurricanes and typhoons. Results from 30 worldwide studies of earthquake-rainfall correlations published during the past 22 years are referenced. These investigations were conducted in both intraplate and plate marginal environments on five continents. Collectively, they provide strong support for the hydroseismicity hypothesis as a viable explanation via pore-fluid-pressure diffusion for the occurrence of many earthquakes, regardless of the host tectonic regime. Signatures of pore-fluid-pressure diffusion in the Earth's crust are ubiquitous. Slow earthquakes result from crack development driven by pore-fluid-pressure diffusion. These earthquakes, also called silent earthquakes, take days, weeks, or even months to release energy instead of seconds or minutes as in normal earthquakes. Typhoons can trigger slow earthquakes in some areas. Hurricanes are believed to have triggered earthquakes in the eastern United States. Their explanation is provided for the most part by Biot's theory for wave propagation and pore-fluid-pressure diffusion in poroelastic media.

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