An accompanying Fault Map of the Puget Sound shows the approximate locations of major faults in the Puget Sound of Western Washington (Gower and others, 1985, "Seismotectonic Map of the Puget Sound Region, Washington", USGS Map I-1613). Many of these faults are inferred indirectly from gravity and magnetic surveys. There is evidence of recent movement on some of these faults. For example, in about 900 AD an earthquake is believed to have occurred on the Seattle Fault which raised Restoration point about 21 feet and Alki point at least 12 feet. These points are located just south of the Seattle Fault on either side of the Puget sound.
The motion on many of these faults is vertical. For example, vertical motion on the Seattle Fault and faults just to the north have caused the block of earth between faults H and I on the map to drop a total of more than 12,000 feet in the last 40 million years. The resulting basin has, of course, been filled with sediments.
Large earthquakes could occur on any of these faults. However, the average time between large earthquakes on any of these faults may be hundreds or even thousands of years. A comparison of the fault map with locations of earthquakes that have occurred in the last few decades indicates that many recent earthquakes are occurring on faults other than the ones shown on the map. Thus, there are probably many other active faults besides the ones shown on the map. Some of these may also be capable of producing major earthquakes.
Note on the Seattle Fault:
The faults labeled "Seattle Fault" and fault "I" on the map may be the same fault. Gravity surveys indicate that the "middle" of a major fault is located along line "I". Other geologic evidence indicates a major fault at the surface passing through Seattle and Bainbridge Island (labeled Seattle Fault). The Seattle Fault is probably not vertical but slants (gets deeper) to the south. Thus, the line labeled "I" is the average position of the deeper portion of the Seattle fault.
The recent magnitude 5.0 earthquake between Seattle and Tacoma on January 28, 1995 may have occurred on the Seattle fault. However, there are other possible interpretations.
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UW Dept. of Earth and Space Sciences ..... PNW EARTHQUAKES
