Earthquake Odds in the Pacific Northwest: Deep, Megathrust and Crustal Quakes

The U.S. Pacific Northwest lies along the Cascadia Subduction Zone, where the Juan de Fuca Plate is thrust beneath the North American Plate. The region — northern California, Oregon, Washington and southern British Columbia, together called Cascadia — faces three kinds of earthquake, and each has done serious damage before: deep (intraslab), subduction zone (megathrust) and shallow crustal earthquakes.

Figure 1. Earthquakes of magnitude 4 or more, 1980–2024, notable earlier quakes in Washington and Oregon, the possible extent of Cascadia megathrust ruptures, and crustal faults in the USGS's 2023 National Seismic Hazard Model. USGS.

Estimating the odds

Earthquake catalogs and historical accounts are short compared with the hundreds to thousands of years between large quakes, so scientists also use paleoseismology — dating geological evidence of past quakes — and geodetic measurements of strain building on faults.

Probabilities here assume a time-independent ("Poissonian") model: a quake is equally likely any day, whenever the last one was. For a single fault that is probably wrong, since strain builds over time, but it is a simple, reasonable approach where past dates are uncertain or many faults are combined. The chance in t years is P = 1 − e^(−t/τ), with τ the mean recurrence time. The fact sheet uses 50 years — about the life of a typical building. The National Seismic Hazard Model, used in building codes, gives the fuller picture.

Figure 2. The 50-year probability of each type of earthquake. USGS.

Deep (intraslab) earthquakes

The region's most common moderate quakes happen inside the sinking Juan de Fuca Plate, 25–40 miles (40–60 km) down — perhaps from the slab bending, or releasing fluids as it descends. They cluster beneath Puget Sound: the 1949 M6.8 Olympia, 1965 M6.5 Seattle and 2001 M6.8 Nisqually quakes. M6.5+ deep quakes there come every few decades, damaging buildings especially near the epicenter and on soft soils.

Figure 3. Deep Puget Sound earthquakes of magnitude 6.5 or more. USGS.

• The 16- and 36-year gaps give a mean of 26 years — an 85% chance in 50 years.
• Northern and central Oregon have had no deep quakes of M4+ since 1990, which doesn't rule them out; southern Oregon and northern California have had several under M5 in the Gorda Plate.
• The record is short, deep quakes leave little geological trace, and elsewhere they have reached about M8 (the 2017 M8.2 Chiapas, Mexico, quake) — so future ones could be larger.

Megathrust earthquakes

These strike the boundary between the Juan de Fuca and North American plates. Although none has been observed directly, Cascadia has a ~10,000-year geological record of them — coastal land-level changes, tsunami deposits, landslides, liquefaction and turbidites (undersea landslide and sediment deposits) offshore and in lakes — making it the best-documented subduction zone in the world. Indigenous oral histories add evidence.

• The last one: tree-ring dating of trees killed by subsidence and seawater, plus written records of a tsunami in Japan, date it to January 26, 1700, at about M8.7–9.2.
• At least 19 great (M8+) megathrust quakes have ruptured most of the coast in the past ~10,000 years. Small and moderate quakes on the megathrust are rare.

A "ghost forest" at Copalis, Washington: tree rings show these trees died in the winter of 1699–1700, likely drowned by seawater when the coast sank in the last great Cascadia earthquake. Photograph by Brian Atwater, USGS.

Full-margin (M~9) ruptures recur about every 500 years in the hazard model, based on coastal subsidence, tsunami deposits and turbidites from California to Vancouver Island. Some of these may really be closely spaced partial ruptures (M~8) — but any given stretch of coast would still see a great quake about every 500 years.

Partial-margin ruptures (M~8) affect part of the coast. In southern Cascadia they may come every ~800–1,000 years, for a combined rate of about 300–350 years; in the north, their recurrence is hard to pin down.

Figure 4. How full-margin (about every 500 years) and partial-margin (about every 1,000 years) ruptures combine into an estimated 330-year recurrence for M8–9 quakes in southern Cascadia. USGS.

Crustal earthquakes

These occur on faults in the shallow crust, within about 20 miles (30 km) of the surface. Pacific Northwest faults are often hidden under sediment, water, vegetation and cities, and slip slowly, so they are hard to study.

• Puget Sound: the clockwise rotation of the Oregon coast block and the northward push of the Sierra Nevada block squeeze the region north–south on mostly east–west faults. Past quakes include the 923–924 C.E. M7.5–7.8 Seattle Fault event. All the region's faults together give a recurrence of about 265 years for M6.5+ (17% in 50 years — possibly an underestimate, since mapping is incomplete). The 20 quakes of M4.0–5.4 recorded there from 1960 to 2024 point to a similar ~280 years.
• Western Oregon: the 1873 M6.8 Brookings, 1962 M5.2 Portland and 1993 M5.6 Scotts Mills quakes.
• Eastern Oregon and Washington: the 1872 M6.5–7 Entiat and 1936 M5.8 Milton-Freewater quakes; in southeastern Oregon's Basin and Range, the 1993 Klamath Falls sequence (M6.0 and M5.9).

Offshore, quakes are common near the Mendocino Triple Junction, where the Pacific, North American and Gorda plates meet — the 1980 M7.2 and 2010 M6.5 Eureka, 1992 M7.2 Petrolia, 2022 M6.4 Ferndale and 2024 M7.0 Cape Mendocino quakes. The Gorda Plate area has a few M6.5+ quakes a decade within about 100 miles of the coast, though the rate varies widely.

Slow slip and volcanoes

• Episodic tremor and slip — slow slipping of the subduction interface over hours to years, every 8–22 months, too gentle to feel — occurs just below the locked zone expected to rupture in a megathrust quake. Slow slip has preceded megathrust quakes in Japan and Chile, but no link to megathrust hazard is known in Cascadia.
• Volcanoes cause earthquakes too: millions of small ones accompanied Mount St. Helens' eruptions in 1980 and 2004, and the largest recorded in the Cascade Range were M5+ beneath it in the 1980s.

Getting ready

Earthquakes can also trigger tsunamis, landslides, liquefaction and land-level changes. To prepare:

• find hazards and secure movable items at home and at work;
• make an emergency plan and supply kit, following local guidance;
• practice "Drop, Cover, and Hold On";
• learn tsunami evacuation routes where you live, work and travel;
• join drills, such as the Great ShakeOut.

Sources

Based on "Earthquake probabilities and hazards in the U.S. Pacific Northwest," U.S. Geological Survey fact sheet, citing Atwater and others (2015), Black and others (2023), Goldfinger and others (2012), Ludwin and others (2005), Matthews and others (2002) and the USGS's 2023 National Seismic Hazard Model; a work of the United States government in the public domain.