Seismotectonic map of the Puget Sound Region, Washington

The Puget Sound region is a seismically active area with hundreds of earthquakes occurring each year (Crosson, 1974, 1975, Crosson and Millard, 1975, Crosson and Noson, 1978a, 1978b, 1979). Most of the earthquakes are so small that they can be detected only by sensitive seismographs such as those operated by the University of Washington. Not all of the earthquakes have been small, however, for several damaging ones have occurred in historic time. Little is known about the geologic structures responsible for generating these earthquakes, and knowledge of the causative structures is fundamental to an adequate evaluation of the earthquake risks for the Puget Sound region. The purpose of this map is to summarize the current knowledge about tectonic deformation, seismicity, and the tectonic framework of the Puget Sound region in order to provide a basis for detailed geologic and geophysical studies that will lead to better understanding of the structures and forces responsible for generating earthquakes in the area.

This map is a compilation of all known and inferred faults, including what is known about their age of movement. Also shown are some deformed deposits of Quaternary age and earthquake epicenters. Few of the faults have been studied in sufficient detail to establish whether or not they have been active or inactive during late Tertiary or Quaternary time, but most of the faults are the result of tectonic forces that were active during early and middle Tertiary time. These faults may or may not be related to the tectonic forces responsible for the current seismicity. Approximately 5 percent of the earthquakes recorded in the Puget Sound region originated at depths of 40 Km or more (Crosson, 1972, fig. 11), and these are probably not directly related to exposed or near-surface structures. Most of the earthquakes occur at shallower depths and may be associated with near-surface structures, but no earthquakes have been definitely identified with mapped faults.