A critical review is given of the present status of the record of earthquakes in the western United States. Special field studies for siting of nuclear reactors and other major structures have brought to light major modifications and revisions of some earlier inferences on the intensity and fault location of historical earthquakes. For example, the hypocenters of the Washington State earthquake of December 14, 1872, and the Lompoc, California, earthquake of November 4, 1927, have recently been redetermined. Presentation and retrieval of both modern and historical seismicity records are still not optimum, with various errors and inconsistencies—some introduced by computer processing.

Increased density of seismographic networks is providing sharper resolution in seismicity mapping. In northern California the pattern of widespread minor seismicity has been defined for the first time; earthquake foci in the Humboldt County region are concentrated in two crustal levels, 0 to 10 km and 18 to 20 km. Use of ocean-bottom seismographs is improving knowledge of the offshore seismicity pattern. Seismotectonic properties of northwestern California, Puget Sound (Washington), and the intermountain seismic belt are now emerging.