Cascadia Subduction Zone

The Cascadia Subduction Zone is a convergent plate boundary where the Juan de Fuca, Explorer, and Gorda plates are being thrust beneath the North American Plate. The subducting slab descends at a rate of about 40 millimeters per year, and the interface is locked for long periods, accumulating strain that is released in great earthquakes. The zone is characterized by a deep-sea trench, an accretionary wedge, and a volcanic arc that includes the Cascade Range volcanoes such as Mount St. Helens and Mount Rainier.¹

The structure of the subduction zone varies along strike, with differences in sediment thickness, dip angle, and locking behavior. The northern segment, off Vancouver Island, has a thicker sediment cover and a shallower dip, while the southern segment, off northern California, has thinner sediment and a steeper dip. These variations influence the potential rupture extent and the size of tsunamis generated.²

The Cascadia Subduction Zone is capable of producing megathrust earthquakes of magnitude 8 to 9, with an average recurrence interval of about 500 years, though intervals have ranged from 300 to 700 years. The most recent event occurred on January 26, 1700, and is inferred from Japanese tsunami records and Native American oral traditions. That earthquake generated a tsunami that crossed the Pacific and caused damage in Japan, providing key evidence for the timing and magnitude of the event.³

Such earthquakes can cause violent shaking, ground liquefaction, and coastal subsidence, and they generate tsunamis that can inundate coastal communities within minutes. The 1700 event is estimated to have had a magnitude of about 9.0, and similar events are expected in the future. The probability of a magnitude 8 or larger earthquake in the next 50 years is estimated at about 7–15%, and the potential impacts on infrastructure and population are severe.

Evidence for past megathrust earthquakes comes from buried marsh soils, sand layers deposited by tsunamis, and abrupt subsidence of coastal forests. Radiocarbon dating of these features has revealed a sequence of at least 19 great earthquakes in the last 10,000 years, with the most recent being the 1700 event. The discovery of these buried soils in the 1980s and 1990s, particularly by Brian Atwater and others, was pivotal in recognizing the seismic hazard of the region.⁴

Paleoseismic studies also use turbidite deposits in offshore submarine canyons to infer past earthquakes, and they have identified a similar recurrence pattern. The geological record shows that the subduction zone is capable of rupturing along its entire length in a single event, as well as in smaller segments. This variability complicates hazard assessment but is crucial for understanding the range of possible earthquakes.⁵

Beyond the well-known megathrust risk, the Cascadia Subduction Zone also hosts slow slip events and episodic tremor and slip (ETS) that occur every 14–16 months and last for several weeks. These events release strain without causing earthquakes, and they were first discovered in the early 2000s. The relationship between slow slip and megathrust earthquakes is an active area of research, as slow slip may load the fault and potentially trigger a large event.⁶

The subduction zone also influences the region's geothermal activity and the chemistry of springs and rivers. The Cascadia margin is home to methane seeps and hydrates, which could be destabilized by seismic shaking. Additionally, the zone's offshore sediments record a history of submarine landslides, some of which may have been triggered by earthquakes and generated tsunamis independent of the main rupture.

Megathrust earthquake

A very large earthquake that occurs at a subduction zone, where one plate is thrust beneath another.

Slow slip event

A transient, slow movement along a fault that releases stress without generating seismic waves.

Paleoseismology

The study of prehistoric earthquakes using geological evidence such as fault scarps and sediment layers.