A tsunami is a series of powerful ocean waves caused by the displacement of water, usually resulting from a major undersea earthquake. California is vulnerable to these destructive waves due to its expansive coastline along the Pacific Ocean basin. The state sits on the Pacific Ring of Fire, which is the source of many of the world’s most powerful seismic events. This position means California must prepare for tsunamis generated both thousands of miles away and directly offshore.
The Historical Reality of California Tsunamis
California has a history of tsunamis, with over 80 recorded along the coast. Four events have caused fatalities, and several others resulted in major damage to coastal infrastructure. These incidents prove that even waves generated far away can be disastrous upon arrival.
The most severe impact occurred in 1964, following a magnitude 9.2 earthquake in Alaska. The resulting tsunami killed 13 people in California, 10 of whom died in Crescent City. Waves there reached estimated heights of up to 16 feet, destroying 30 city blocks and causing damage exceeding $160 million.
More recently, the 2011 magnitude 9.0 earthquake off Japan generated a tsunami that caused significant economic damage and one fatality in California. Harbors and marinas were heavily impacted. Even non-seismic events, such as the 2022 underwater volcanic eruption near Tonga, have caused surges that led to millions of dollars in dock and harbor damage, underscoring the coastline’s susceptibility.
Understanding the Source Threat
The tsunami threat is divided into two categories based on the source location. Distant sources, or tele-tsunamis, originate from major subduction zone earthquakes across the Pacific Rim (e.g., Alaska, Japan, or Chile). Since these waves travel across the ocean, they take four to 15 hours to reach the California coast.
This travel time allows for official warnings, enabling coastal communities to evacuate safely. The majority of California’s damaging tsunamis have been generated by these distant sources. The long warning time makes these events less catastrophic for loss of life, though property damage potential remains high.
The more immediate threat comes from local sources, specifically the Cascadia Subduction Zone (CSZ), which runs from Northern California up to Canada. The CSZ is capable of producing a magnitude 9.0 earthquake. A wave generated by a major CSZ rupture would reach the northern California coastline in a matter of minutes, allowing no time for official alerts.
Local tsunamis can also be generated by offshore faults or submarine landslides. These near-shore mechanisms pose a threat because the wave energy has little distance to dissipate, resulting in high run-up heights. The last major CSZ event occurred in 1700, indicating California is within the window for the next catastrophe.
Official Warning Systems and Personal Safety
California relies on a multi-layered system for public alerts, coordinated by NOAA’s Tsunami Warning Centers. These centers issue official alerts, including Advisories, Watches, and Warnings, broadcast through various channels. An Advisory indicates a threat of dangerous currents, while a Warning means widespread inundation is expected or occurring.
For a local-source event like the Cascadia scenario, natural warning signs are the primary indicator of danger. Residents must recognize that a strong earthquake, lasting 20 seconds or more, is a natural tsunami warning. A sudden receding of the ocean water, known as “drawdown,” or a loud, unusual ocean roar also signals imminent danger.
If these natural signs are observed, evacuate immediately without waiting for an official announcement. Evacuation should be to high ground, ideally 100 feet above sea level or two miles inland, following marked routes. Since tsunamis often arrive as a series of waves over many hours, people must remain in the safe zone until local emergency officials issue an all-clear notification.