| Tectonics of the Pacific Northwest |
Magnitude 6.8 WASHINGTON
2001 February 28 18:54:32 UTC
Preliminary Earthquake Report
U.S. Geological Survey, National Earthquake Information Center
World Data Center for Seismology, Denver
A magnitude 6.8 earthquake struck roughly 20 kilometers northeast of Olympia, Washington on Wednesday February 28th at 10:54 AM PST. The earthquake resulted from the release of tensional stresses within the Juan de Fuca Plate. Tensional stresses are continually accumulating in the Juan de Fuca Plate as it bends and subducts beneath the North America Plate.
The boundary between the plates, the Cascadia Subduction Zone, lies just off the coast of Oregon and Washington. This subduction zone is the main tectonic feature in the geologically active Pacific Northwest. On shore, the (continental) North American Plate overlays the subducting (oceanic) Juan de Fuca Plate. The Juan de Fuca Plate is smaller than most oceanic plates but its motion is still capable of generating large earthquakes. The interaction of the Juan de Fuca Plate and the North American Plate generates a complex stress field that results in three main types of earthquakes: subduction zone earthquakes, shallow earthquakes (less than about 30 km), and deep earthquakes (between about 30 and 80 km, like the recent Feb-28-2001 Olympia earthquake that struck 59 km below the surface).
Subduction zone earthquakes occur when an area at the interface between the plates ruptures. These earthquakes can be very large but are uncommon. The last large subduction earthquake in the Cascadia region happened about 300 years ago. Shallow earthquakes occur in faults within the North American crust and are far more common. About a thousand crustal earthquakes are recorded annually but most are too small to be felt. However, a few major crustal earthquakes have occurred. The Seattle fault is thought to have generated a shallow magnitude 7+ earthquake about 1,000 years ago and other magnitude 7+ shallow earthquakes in the Pacific Northwest occurred in 1872, 1918 and 1946.
Deep earthquakes, including the recent Olympia earthquake, occur within the subducting Juan de Fuca Plate. Like the shallow events, deep earthquakes are normally small but a few damaging deep events have occurred within the last century. A magnitude in 7.1 earthquake in 1949 caused over $250 million in damage, including damage to the Capitol Building in Olympia, a magnitude 6.5 earthquake in 1965 caused over $85 million in damage, and a magnitude 5.5 in 1999 caused about $300,000 in damage. The damage amounts have been adjusted to todays dollars.
The origin of deep earthquakes in Cascadia and other regions is the topic of ongoing research. Possible origins include mechanical deformation of the subducting plate, and mineral changes as the plate subducts and is exposed to increased temperature and pressure. Thus, determining the precise reason for the Olympia earthquake is difficult. However, we do know the Olympia earthquake was generated by the release of tensional stresses. Tensional stresses are generated within a subducting plate when it is pulled or bent. This can be understood by visualizing bending a stick while holding both ends. The inner side of the curve will be under compression and the outer side of the curve will be under tension. Additionally, if we pull the ends of the stick in opposite directions the entire stick will be under tension. The stresses that caused the February-28-2001 Olympia earthquake were likely generated by the bending or pulling of the Juan de Fuca Plate as it continually subducts beneath the North American Plate.
See also:
Earthquake Hazards in Washington and Oregon
Cascadia Earthquakes PDF File
Deep Quakes In Cascadia
