Although the Northwest is thought of as a geologically stable region, many are surprised to learn just how wrong that idea is. More than 1,000 earthquakes occur in the region annually and at least 20 damaging earthquakes have occurred during the past 125 years. What's going on here that could cause this activity?
Washington is situated at a convergent boundary between two tectonic plates. The Cascadia subduction zone, which is the convergent boundary between the North America plate and the Juan de Fuca plate, lies offshore from northernmost California to southernmost British Columbia. The two plates are converging at a rate of about 3-4 centimeters per year (about 2 inches per year); in addition, the northward-moving Pacific plate is pushing the Juan de Fuca plate north, causing complex seismic strain to accumulate. Earthquakes are caused by the abrupt release of this slowly accumulated strain
Shallow Crustal Earthquakes
Shallow crustal earthquakes occur about 30 km of the surface. Recent examples in Washington occurred near Bremerton in 1997, near Duvall in 1996, and off Maury Island in 1995, All these earthquakes had a magnitude of about M5-5.5.(M,for magnitude,characterizes the relative size of an earthquake. In simple terms,this means that at the same distance from an earthquake's epicenter,the shaking of an M5 earthquake will be 10 times more severe than an M4 earthquake). The largest historic earthquake in Washington (estimated at M7.4),the North Cascades earthquake of 1872, is also thought to have been shallow. Because of its remote location and the relatively small population in the region, though, damage was light.
Recent paleoseismology studies are demonstrating previously unrecognized fault hazards. New evidence for a fault system that runs east-west through south Seattle (the Seattle fault) suggests that a major earthquake, M7 or greater, affected the area about 1,000 years ago. Similar large faults occur elsewhere in the Puget Sound but little is known about them.
Subduction Zone (Interplate) Earthquakes
Subduction Zone Earthquakes occur along the interface between tectonic plates. Compelling evidence for great-magnitude earthquakes along the Cascadia subduction zone has recently been discovered. These earthquakes were evidently enormous (M8–9+) and recurred on average every 550 years. The recurrence interval, has apparently been irregular, as short as about 100 years and as long as about 1,100 years. The last of these great earthquakes struck Washington about 300 years ago.
Intraplate or Benioff Zone Earthquakes
Intraplate or Benioff zone earthquakes occur in the subducting Juan de Fuca plate at depths of 25-100 km. The largest of these recorded were the magnitude (M) 7.1 Olympia earthquake in 1949, the M6.5 Seattle-Tacoma earthquake in 1965, the M5.1 Satsop earthquake in 1999, and the M6.8 Nisqually earthquake of 2001. Since 1870, there have been six earthquakes in the Puget Sound basin with measured or estimated magnitudes of 6.0 or larger.
For the February 28, 2001, Nisqually earthquake, the hypocenter, or point beneath the surface at which the rupture starts, was at 52 kilometers (32 miles). The epicenter was just off the Nisqually delta in Puget Sound. The quake was felt as far north as Vancouver, British Columbia, as far south as Salem, Oregon, as far east as Spokane, Wash., and as far southeast as Salt Lake City, Utah. Most of the damage was sustained in the Olympia and Seattle areas.
The present visitor center was one of the buildings that sustained significant damage during the Nisqually earthquake.
What are the principal ways that earthquakes cause damage?