What Would Happen If A Tsunami Hit The West Coast

The damage from a West Coast tsunami would reach well beyond the beach. The biggest threat lies just offshore along the Cascadia Subduction Zone, where a major earthquake could send the first wave ashore in Oregon and Washington in under half an hour. That would leave little time to reach high ground, and the danger would not end once the first wave pulled back. Floodwater could travel surprisingly far from the ocean, and some of the damage to the coast could last for years after the water drains away.

Some Towns Could Have As Little As 15 Minutes To Evacuate

At the Cascadia Subduction Zone, oceanic crust descends beneath the North American Plate, and a large rupture can abruptly displace the seafloor and the water above it. Along parts of the Pacific Northwest coast, the resulting tsunami could arrive roughly 15 to 30 minutes after the earthquake begins. Several minutes of strong shaking could consume part of that window. For someone in a tsunami hazard zone, strong or prolonged shaking is itself a warning. National Weather Service guidance is to protect yourself during the earthquake, then follow evacuation routes to high ground or outside the hazard zone as soon as it is safe. Waiting for the ocean to visibly change could use up the remaining escape time.

A tsunami crossing the Pacific would generally arrive with more warning. The waves that reached Crescent City in March 1964 traveled from Alaska in about four hours. Warning centers now use seismic instruments and ocean measurements to assess the threat while a distant tsunami is still crossing the ocean. With that extra time, officials can order coastal evacuations before dangerous water arrives.

Floodwater Could Reach More Than A Mile Inland Along Rivers

A tsunami arriving at the beach might look like a rapidly rising flood. As the waves enter shallower water, they slow down and can grow taller, pushing a large volume of water across low ground. A powerful surge can move buildings off their foundations and sweep people away. Floating wreckage can strike people and buildings, while the returning flow can carry survivors and debris out to sea.

The coastline would not flood evenly. Offshore depth and the shape of the shore influence where water concentrates. Tsunami water can travel inland through river mouths, and a large tsunami can flood low coastal land more than a mile from the ocean. At Seaside, Oregon, the 1964 tsunami surged up the Necanicum River and Neawanna Creek, wrecking houses in the Venice Park neighborhood along the creek and spilling into streets and parking lots. The water also tore the decommissioned Fourth Avenue bridge from its pilings. Some of Seaside's damage occurred well away from the beach, along the waterways running through town.

One In Three Boats In California Marinas Could Be Damaged Or Sunk

At Crescent City in 2011, the largest tsunami surges coincided with low tide, limiting flooding on land. Inside the harbor, dangerous surges and currents lasted for more than 24 hours. Every dock in the small-boat basin was heavily damaged or destroyed, and 16 boats sank. Dredging and harbor repairs continued for well over a year.

Water forced into a confined harbor can accelerate through narrow sections. At Santa Cruz, the long, rectangular shape of the harbor amplified the incoming surges, buckling docks where the harbor narrows. Twenty-three of its 29 docks sustained significant damage.

Commercial shipping would also be exposed. In a 2013 U.S. Geological Survey planning scenario, researchers modeled a plausible magnitude 9.1 earthquake off the Alaska Peninsula and estimated about $100 million in damage to cargo at the ports of Los Angeles and Long Beach. They also projected that about one in three boats in California's coastal marinas could be damaged or sunk, causing navigation and environmental problems.

Coastal Water And Sewer Service Could Take Up To 3 Years To Restore

On the Oregon coast, a locally generated tsunami could reach communities already damaged by the earthquake that caused it. Landslides could block roads before the first wave arrived. Shaking could damage bridges, and liquefaction could cause waterlogged ground to lose strength beneath infrastructure.

Damage of that kind could split coastal populations into isolated pockets. A surviving road within a town would be of limited use for bringing supplies from outside if its connecting bridge had failed. Damage to utilities could also leave people who escaped the flood without working water or sewer service. The Oregon Resilience Plan, presented to state lawmakers in 2013, estimated one to three years to restore drinking water and sewer service on the coast after a Cascadia earthquake and tsunami.

A distant tsunami would arrive without that preceding local earthquake damage, and help could reach an affected waterfront far more quickly over intact inland roads.

Some Coastline Could Sink Up To 6.5 Feet

After a large Cascadia earthquake, some coastal land could sit several feet lower than it had before. This sinking, called subsidence, happens as the crust deforms during the rupture. The tsunami water would eventually drain away, but the lower ground could leave homes and roads more exposed to ordinary coastal flooding.

A 2025 study of Cascadia subsidence modeled coastal sinking of up to roughly 6.5 feet. In the most severe modeled cases, about 14,350 additional residents and 22,500 additional structures would fall within the coastal floodplain, more than doubling flood exposure. That floodplain represents land with a 1 percent annual chance of flooding, separate from the area briefly covered by a tsunami. Ghost forests along the Washington coast, including dead cedar stands on the Copalis River, contain evidence of this kind of change. The trees died after the land sank and saltwater entered during the 1700 earthquake.

Some Washington communities have already started building for a shortage of reachable high ground. Near Westport, Ocosta Elementary School has a refuge on the reinforced roof of its gymnasium, designed to hold 1,000 people. The surrounding land has few elevated places to escape to, so the roof is a nearby place to climb. Engineers designed the refuge to withstand a magnitude 9 Cascadia earthquake and the forces of the tsunami that could follow, including strikes from floating wreckage. Four concrete stair towers at the corners lead from the ground outside up to the roof.