The ocean floor of the Pacific Northwest is a highly active geological zone dominated by the Juan de Fuca Ridge and the Cascadia Subduction Zone, where the Juan de Fuca plate dives under the North American plate. Key features include the highly active Axial Seamount, a frequently erupting underwater volcano, and a region experiencing active crustal tearing.Axial Seamount: Located ~300 miles off the Oregon coast, this is the most active volcano in the region, with eruptions predicted for 2026 based on seafloor inflation. It lies on the Juan de Fuca Ridge. Cascadia Subduction Zone (CSZ): A 1,000 km long fault stretching from Vancouver Island to Northern California, capable of producing magnitude 9.0+ earthquakes and tsunamis, with a major event expected every 300-500 years. Active Tearing: Recent studies show the Juan de Fuca plate is tearing apart in stages at its northern end, a process occurring over millions of years. Hydrothermal Vents: The Juan de Fuca Ridge features hydrothermal vents, which are underwater hot springs supporting unique ecosystems, including microbes and marine animals. Seafloor Deformation: The seafloor in this region undergoes constant change, with monitoring showing the ocean floor around the Axial Seamount expanding and contracting .

Volcanic Eruptions: While the Axial Seamount erupts periodically, its eruptions are deep and typically do not pose a tsunami risk to the coast. Seismic Hazards: The CSZ poses a significant risk for massive earthquakes and tsunamis, with potential for significant subsidence (sinking) along the Washington coast, particularly near Neah Bay. Ecosystems: Deep-sea ecosystems in this area rely on chemosynthesis, converting chemicals from the seafloor into energy. The ocean floor of the Pacific Northwest is a highly active geological zone dominated by the Juan de Fuca Ridge and the Cascadia Subduction Zone, where the Juan de Fuca plate dives under the North American plate. Key features include the highly active Axial Seamount, a frequently erupting underwater volcano, and a region experiencing active crustal tearing.Axial Seamount: Located ~300 miles off the Oregon coast, this is the most active volcano in the region, with eruptions predicted for 2026 based on seafloor inflation. It lies on the Juan de Fuca Ridge. Cascadia Subduction Zone (CSZ): A 1,000 km long fault stretching from Vancouver Island to Northern California, capable of producing magnitude 9.0+ earthquakes and tsunamis, with a major event expected every 300-500 years. Active Tearing: Recent studies show the Juan de Fuca plate is tearing apart in stages at its northern end, a process occurring over millions of years. Hydrothermal Vents: The Juan de Fuca Ridge features hydrothermal vents, which are underwater hot springs supporting unique ecosystems, including microbes and marine animals. Seafloor Deformation: The seafloor in this region undergoes constant change, with monitoring showing the ocean floor around the Axial Seamount expanding and contracting . The Cascadia Subduction Zone (CSZ) "megathrust" fault is a 1,000 km long plate boundary that stretches from Northern Vancouver Island, British Columbia to Cape Mendocino, California. It separates the Juan de Fuca and North America plates. New oceanic crust is created offshore along the Juan de Fuca ridge, contributing to both the Juan de Fuca and Pacific Plates. The production of new, warm, buoyant crust moves cooler and denser older crust away from spreading ridges. The Juan de Fuca plate moves towards the North America Plate at about 4 cm per year, causing it to slowly subduct beneath North America.

Earthquake sources in the Pacific Northwest, highlighting the Cascadia subduction zone. Image from Washington State Department of Natural Resources. At depths shallower than around 30 km, the two plates of the CSZ are locked together by friction. Strain (deformation) slowly builds as the subduction forces continue to act upon the locked plates. Once the fault's frictional strength is exceeded, the rocks slip past each other along the fault in a "megathrust" earthquake. The fault's frictional properties change with depth. Immediately below the locked part is an area that slides in slow slip events ( "slow slip events" ) that slip a few centimeters every dozen months or so. This relieves the plate boundary stresses there, but adds to the stress on the locked part of the fault. Below this transition zone, geodetic (GPS) evidence suggests that the fault slides continuously and silently at long term plate slip rate. From its surface trace offshore to a depth of possibly 5 km, all remote from land, observations are minimal. It remains unknown whether the fault is stuck or slipping silently.Graphic from IRIS showing how different frictional "zones" vary with depth in the CSZ. Great megathrust earthquakes occur in the “Locked" Zone", slow slip events occur in the “Episodic Tremor and Slip” zone, and the plates move continuously past each other in the “Continuous Slip” zone. Subduction zone megathrust faults are the only faults on Earth that can produce earthquakes greater than M8.5. The Cascadia Subduction Zone has produced magnitude 9.0 or greater earthquakes in the past, and undoubtedly will in the future. The last known megathrust earthquake in the Pacific Northwest was on January 26th, 1700, just over 300 years ago, with an estimated magnitude between 8.7 and 9.2 ( 1700 Cascadia Subduction Zone Earthquake ). Geological evidence shows at least 19 great earthquakes (M8+) occurring over the past ~10,000 years in the Pacific Northwest, with an average recurrence interval of ~500 years. There is evidence for both full-margin ruptures (~M9), where the entire coastline from Canada to California experiences an earthquake, and partial-margin ruptures (~M8), where only part of the coastline experiences an earthquake. The USGS estimates a 10-15% chance of a full-margin ~M9 earthquake occurring on the Cascadia Subduction Zone in the next 50 years. To learn more about the history of the Cascadia Subduction Zone and the science that led to the discovery of it, see the sections below, which describe land level changes and turbidites created by Cascadia Subduction Zone earthquakes.