Washington State is one of the most seismically active regions in the United States, facing a complex and continuous earthquake threat. An earthquake is defined as the sudden slip on a fault, a fracture in the Earth’s crust, resulting in the release of stored energy in the form of seismic waves. Thousands of minor events are recorded annually, though most go unnoticed. The nature of this risk is unique because the state is subject to three fundamentally different types of earthquakes, all stemming from its unique geologic position.
The Driving Forces: Washington’s Tectonic Setting
The underlying cause of Washington’s seismic activity is the slow, relentless collision of two massive tectonic plates off the coast. The smaller, denser Juan de Fuca Plate is moving eastward and sliding beneath the larger, lighter North American Plate in a process known as subduction. This boundary forms the Cascadia Subduction Zone, a 700-mile-long fault that stretches from northern California to British Columbia.
The Juan de Fuca Plate is subducting beneath North America at a rate of approximately 3 to 4 centimeters per year. In the upper portion of the subduction zone, the plates are currently “locked,” meaning they are stuck together due to immense friction. This locking causes the continental North American Plate to slowly compress and deform, storing vast amounts of elastic strain energy.
This stored energy will eventually be released, causing ground motion that will shake the entire region. GPS monitoring confirms this ongoing deformation as the continental margin slowly bulges upward and shortens. This plate interaction results in three distinct earthquake sources that threaten the Puget Sound region and the Washington coast.
Three Classes of Washington Earthquakes
Washington State experiences three distinct classes of earthquakes, each with a different depth and destructive potential. The first type is the shallow crustal earthquake, which occurs on faults located entirely within the North American Plate at depths between 0 and 30 kilometers. These faults, like the Seattle Fault Zone, are near the surface, meaning the resulting shaking is intense but quickly diminishes with distance.
The second type is the deep slab, or intraslab, earthquake, which occurs within the subducting Juan de Fuca Plate itself. These events initiate at greater depths, typically between 30 and 70 kilometers, as the descending plate bends and fractures. Because the rupture source is far beneath the surface, the shaking is less intense than a shallow quake but is felt over a much wider geographic area.
The third class is the megathrust earthquake, which occurs along the interface where the Juan de Fuca Plate meets the North American Plate. These are the largest earthquakes possible globally, with potential magnitudes exceeding 9.0. A megathrust rupture along the Cascadia Subduction Zone would cause prolonged and violent shaking, and it is the only regional earthquake type that can generate a widespread tsunami.
Key Seismic Zones and Historical Events
The Puget Sound region, including Seattle and Tacoma, is the area of highest seismic risk due to its population density and concentration of active faults. The area is particularly susceptible to deep slab earthquakes, which have historically been the most common source of damaging seismic events. These deep quakes occur roughly every few decades.
Washington’s recorded history includes several significant deep slab earthquakes that shaped the state’s seismic awareness and building codes. A magnitude 6.7 deep earthquake struck near Olympia in 1949, causing widespread damage, particularly to unreinforced masonry structures. This was followed by a magnitude 6.5 event near Seattle in 1965, resulting in similar destruction.
The most recent large event was the 2001 magnitude 6.8 Nisqually earthquake, which occurred at a depth of 57 kilometers beneath the southern Puget Sound. This intraslab earthquake caused an estimated $1–4 billion in damage, demonstrating the vulnerability of infrastructure and older buildings. While Western Washington faces the most complex threat, Eastern Washington is also seismically active, experiencing the largest historical event, the estimated magnitude 7.3 Lake Chelan earthquake in 1872.
Preparing for Seismic Events
Individual preparedness is the most immediate and effective defense against earthquake hazards. The fundamental action during shaking is to “Drop, Cover, and Hold On.” This involves dropping to the ground, taking cover under a sturdy table or desk, and holding onto it until the shaking stops, which protects against falling objects that cause most injuries.
Proactive Preparedness Measures
Securing the home environment is an important proactive measure. Residents should take several steps to mitigate risk and ensure safety:
- Heavy furniture, such as bookcases, dressers, and water heaters, should be anchored to wall studs to prevent them from toppling.
- Prepare an emergency kit with enough supplies, including water and non-perishable food, to be self-sufficient for a minimum of three days following a major event.
- Establish a communication plan, including an out-of-area contact person, since local phone lines may be overwhelmed or inoperable.
- Know how to safely shut off utilities like gas and water to prevent secondary hazards, such as post-earthquake fires and water damage.
- Practice the “Drop, Cover, and Hold On” protocol through drills like the annual Great Washington ShakeOut to ensure a swift and correct response.