Earthquake Sources in the Pacific Northwest
Seismologists categorize the earthquakes of the Pacific Northwest into 4 different types, depending on where they originate. Three types – Cascadia Subduction Zone earthquakes, deep intraplate earthquakes, and crustal fault earthquakes – can produce strong shaking that threatens life and property. The fourth type, volcanic earthquakes, are generally too small and remote to be felt by people directly, but they provide strong clues about potential volcanic eruptions.
Cascadia Subduction Zone Earthquakes In Cascadia, strain arises from the interaction of three tectonic plates. The North American, Juan de Fuca, and Pacific plates all are moving with different trajectories. Offshore of British Columbia, Washington, Oregon, and northern California, a young oceanic plate called the Juan de Fuca plate is spreading away from the Pacific plate and plunging beneath North America. Locked together since the great Cascadia Subduction Zone Earthquake of 1700, strain is building on the interface between these two plates that will be released in future earthquakes. | Deep Intraplate Earthquakes The crust of the Juan de Fuca plate, which consists of basalt and marine sediments, is placed under strain as it is forced beneath North America and down into the Earth's mantle. The slab is bent and pulled by gravity. In a zone that ranges from near the coast to the foothills of the Cascades, but centered on the Puget Lowland, faults in this subducting slab produce deep intraplate earthquakes. Most of the damaging historic earthquakes in Washington State, such as the 2001 Nisqually, 1965 Puget Sound, and 1949 Olympia earthquakes, were from this deep earthquake source. |
Crustal Earthquakes Shallow or crustal earthquakes occur on faults in the North American plate. Some are so shallow that they can break or deform the ground surface. Others can occur up to 35 km deep, and may not be associated with faults observed at the surface. Earthquakes along these crustal faults are driven by the rotation and shearing of the North American plate due to the forces imposed by the subducting Juan de Fuca plate, the Pacific plate grinding north along the North American plate's western edge in California, and extension in the Basin and Range Province further inland. | Volcanic Earthquakes Volcanic earthquakes are triggered by changes in the magmatic systems within and beneath a volcano. When magma is injected into or removed from the system, rocks have to shift to make room for the injected magma or fill in a vacated area. The shifts occur along weak areas in the rocks (i.e., faults) and produce seismic waves. These are called volcanic earthquakes. |