Deep Earthquakes
The most common source of damaging shaking in Washington and Oregon is a deep earthquake that ruptures a fault within the subducting Juan de Fuca plate. These deep earthquakes — or "intraplate" earthquakes — mostly occur beneath Puget Sound, typically at depths from 30 km to 70 km (20 to 45 miles). Although subduction zone earthquakes can be much larger and crustal earthquakes can be much closer to our cities, deep earthquakes are the most frequent when it comes to causing damage. Damaging deep earthquakes have historically occurred about every 30 years. The USGS estimates an 85% chance of another deep earthquake of magnitude 6.5 or greater striking the region sometime in the next 50 years.
The crust of the Juan de Fuca plate, which consists of basalt and marine sediments, is placed under strain as it is forced beneath the North American Plate and down into the Earth's mantle. In a zone from the coastline to the foothills of the Cascades, but centered on the Puget Lowland, there are breaks in this subducting slab which produce deep, intraplate earthquakes. They are usually caused by normal faulting, and may also be associated with mineral changes as the plate encounters increased temperature and pressure. While many subduction zones feature deep intraplate earthquakes extending to depths exceeding 600 km, Cascadia's deepest earthquakes are about 100 km deep. Because of their depth, the energy released by these earthquakes spreads out over a larger area, resulting in more widespread shaking felt at the surface rather than a concentrated area of intense shaking.
This schematic diagram above shows the relative positions of the Juan de Fuca and North American Plates. The "deep earthquakes" arrow points to the depth of the 1949, 1965, and 2001 earthquakes. The 1949 earthquake caused over $100 million in damage, including damage to the Capitol Building in Olympia. The 1965 earthquake caused over $50 million in damage. Most recently, in 2001, the M6.8 Nisqually earthquake () near Olympia shook a large portion of Washington and caused an estimated $2 billion worth of damage. Each year we see growth, development, and greater economic activity in the Puget Sound area, increasing the likelihood of costly losses from future earthquakes.