Washington's Largest Reported Earthquakes

Most of the largest earthquakes felt in Washington (Table 2) have occurred in the Puget Sound region between Olympia and the Canadian border, in the Cascade mountains, and along the Washington-Oregon border. Figure 13 shows the locations of epicenters of the largest earthquakes reported in Washington from 1872 to 1987. The earthquakes whose epicenters are shown were felt over an area of at least 50,000 square kilometers or were rated VII or more on the Modified Mercalli Intensity Scale (Table 1). Two earthquakes whose epicenters were on Vancouver Island, British Columbia, are included on Figure 13 because they were widely felt in Washington.

The depths of the largest Washington earthquakes are not precisely known because calculations of depth require a number of seismograms for each earthquake. Before 1949, the number of earthquakes and their locations and sizes were determined almost entirely using newspaper accounts of reported damage. Even as late as 1969 there were only three seismograph stations in Washington and three in westem British Columbia. As a result, information about early earthquakes is incomplete, and the locations, depths and sizes of these earthquakes are less precise than those of earthquakes recorded after 1969 by the University of Washington multistation seismograph network.

The 1949 magnitude 7.1 Olympia earthquake and the 1965 magnitude 6.5 earthquake between Tacoma and Seattle were large enough to be recorded at many seismograph stations around the world. Both of these Puget Sound earthquakes occurred within the subducting Juan de Fuca plate at depths of 54-63 kilometers (Langston and Blum, 1977; Baker and Langston, 1987). Neither earthquake had significant aftershock activity. Like the 1949 and the 1965 earthquakes, almost all large Puget Sound earthquakes have lacked aftershocks. The lack of aftershocks is considered characteristic of deep earthquakes (Algermissen, 1983; Page, 1968), and this has been taken as evidence that the early large earthquakes in the Puget Sound region were mostly deep (Algermissen, 1983). The 1880 earthquake may have been shallow because many aftershocks were reported.

Many aftershocks have been reported following large earthquakes in the Washington Cascade Mountains. The largest such earthquake (perhaps the largest in the state) occurred on Dec. 14, 1872, in the northern Cascade Mountains and was followed by many aftershocks. The estimated location, depth, and size of this earthquake are controversial. The location used in Table 2 and shown on Figure 13 was determined by Malone and Bor (1979) and was estimated using the intensity pattern determined from reports of damage. Other possible locations of the 1872 earthquake, ranging from the Canadian border to Lake Chelan, have also been calculated from the analysis of intensity patterns (Milne, 1956; Bechtel, 1976; Washington Public Power Supply System, 1977; Algermissen, 1983). The numerous aftershocks following this event have been interpreted as evidence that the source for the 1872 earthquake was shallow (Algermissen, 1983). Some argue, however, that a shallow earthquake of this magnitude would have caused a large identifiable surface rupture. Although the Straight Creek fault passes near this area, geological evidence does not support the conclusion that it has had recent movement (Vance and Miller, 1983). The 1872 earthquake is thus a reminder that Puget Sound is not the only site of large, damaging earthquakes in Washington.

Moderate, damaging earthquakes have been reported from eastern Washington. These include the 1893 Umatilla and 1936 Milton-Freewater earthquakes whose epicenters were located along the southeastern Washington-northeastern Oregon border and the 1959 Lake Chelan earthquake along the eastern border of the Cascades. Strong aftershocks followed each of these earthquakes.

Earthquakes Recorded by the University of Washington Seismograph Network

The geographic distribution of recent earthquakes located by the University of Washington seismograph network ( Figure 14 ) coincides broadly with the distribution of the largest Washington earthquakes shown on Figure 13 . Most of Washington's earthquakes occur within the Puget Sound region and along the western side of the Cascade mountains. Eastern Washington is an area of generally low seismicity-except for the westem side of the Columbia River Basin and the Oregon-Washington border (Malone and others, 1975). Numerous earthquakes occur in the Georgia Strait-northern Puget Sound areas of Canada. However, since the installation of regional seismograph networks in Washington and Canada, no earthquakes have been located near the epicenters of the large Vancouver Island earthquakes of 1918 and 1946 shown on Figure 13 (Rogers, 1983).

The east-west cross section shown in Figure 11 provides a subsurface view of Washington's earthquakes. Numerous shallow earthquakes occur in the crust of both eastern and western Washington within 30 kilometers of the Earth's surface. A thin zone of earthquake hypocenters that deepens toward the east from 30 kilometers under the coast to 100 kilometers below the Cascades underlies the zone of shallow earthquakes in westem Washington and western Oregon. A few deep earthquakes have also been reported under the northern California Cascade Range (Walter, 1986). Although many small shallow earthquakes are recorded in the Puget Sound area today, no large shallow earthquakes have been recorded by the multistation seismograph network since 1969. The two largest Puget Sound earthquakes since 1969 were a magnitude 5.1 earthquake on May 16, 1976, located in the northern Puget Sound basin at a depth of 62 kilometers and a magnitude 4.5 earthquake on Sept. 8, 1976, located in the southern Puget Sound basin at a depth of 50 kilometers. Both these deep earthquakes were unaccompanied by aftershocks. In this respect, they were similar to the large, deep 1949 and 1965 earthquakes.

A magnitude 4.6 earthquake on March 11, 1978, near Bremerton, Washington, occurred at a depth of 24 kilometers, in the zone of shallow earthquakes above the subducting Juan de Fuca plate. The mainshock was followed by 44 aftershocks; this was the first well-documented mainshock-aftershock earthquake sequence in the Puget Sound basin (Yelin and Crosson, 1982). Considering the depth of this earthquake and the presence of thick overlying glacial deposits, it is not surprising that the fault producing this earthquake has not been identified at the surface.

Earthquakes in the Cascades are generally shallow except for a few small deep earthquakes in the subducted Juan de Fuca plate. The two largest shallow earthquakes in Washington since 1969 occurred near Elk Lake and Goat Rocks in the southern Cascades. The magnitude 5.5 Elk Lake earthquake occurred on February 13, 1981, at a depth of only 7 kilometers and was felt over 102,000 square kilometers. It had more than 1,000 aftershocks (Grant and others, 1984) distributed on a vertical fault zone, called the St. Helens Seismic Zone by Weaver and Smith (1983), which did not break the Earth's surface ( Figure 15). Weaver and Smith (1983) suggest that the seismic zone, roughly centered on Mount St. Helens, may extend about 90 kilometers in a northwest-southeast direction. Scientists sometimes use the length of a fault to calculate the magnitude of the largest earthquake likely to occur on it. Weaver and Smith (1983) estimated that the St. Helens Seismic Zone could be capable of generating a magnitude 7.0 earthquake.

All earthquakes recorded in eastern Washington have been shallow, and most are at depths less than 6 kilometers. The largest earthquake in eastern Washington since 1969 was a shallow, magnitude 4.4 event northwest of Othello on December 20, 1973. Some of the most active earthquake areas in eastern Washington are near Entiat, south of Lake Chelan, and in the Saddle Mountains, south of Vantage. Many of the earthquakes in eastern Washington occur in clusters near the Saddle Mountains in folded volcanic rocks, which were extruded in southeastern Washington from 16.5 to 6 million years ago (Rothe, 1978; Malone and Bor, 1979).