A tsunami is a series of waves generated by the rapid displacement of a large volume of water, most commonly from a powerful underwater earthquake. California’s extensive coastline, which stretches for over a thousand miles, makes it particularly susceptible to this natural hazard. Over 80 tsunamis have been recorded along the state’s coast in the last 150 years, some causing significant damage and loss of life. Understanding the mechanisms that cause these events and the actions required for survival is necessary.
Sources and Probability of a California Tsunami
California faces a threat from two distinct types of tsunamis: distant-source and local-source events. Distant tsunamis originate from major earthquakes thousands of miles away across the Pacific Ocean, such as those off the coasts of Alaska, Japan, or South America. These waves take many hours to reach the California coast, allowing for official warnings and a window for evacuation. Although smaller than at their source, these tsunamis can still cause severe damage to harbors and coastal infrastructure, as seen during the 2011 Japan earthquake.
The greater concern comes from local-source tsunamis, which are generated much closer to the shoreline and provide little to no warning time. These can be triggered by underwater landslides or by earthquakes from nearby offshore faults. The most significant local threat is the Cascadia Subduction Zone, a 700-mile fault extending from Northern California up to British Columbia. A megathrust earthquake along the Cascadia zone, similar to the estimated magnitude 9.0 event in 1700, could send destructive waves to the northern California coast within minutes. The energy from a near-source tsunami has little distance to dissipate, making it potentially larger and more devastating upon arrival.
Recognizing Natural Warning Signs and Official Alerts
The public receives notification of a tsunami threat through official alerts and observable natural phenomena. Official alerts are issued by the National Tsunami Warning Center, which disseminates information through the Emergency Alert System (EAS), NOAA Weather Radio, and methods like sirens and reverse 911 calls. For distant tsunamis, these alerts provide crucial hours for preparation and evacuation.
For a local-source tsunami, the first warning may be a natural sign. The most immediate sign is strong ground shaking that lasts for a minute or more, indicating a powerful earthquake occurred nearby. If you are near the coast and the shaking makes it difficult to stand, you must immediately seek higher ground without waiting for an official message.
Another natural sign is a sudden recession of the ocean water, exposing large areas of the seafloor that are normally submerged. This drawback of water is the trough of the wave series, indicating the dangerous crest is rapidly approaching. A loud roar from the ocean can also precede the arrival of the first wave. Recognizing these signs means immediate evacuation is the only viable safety procedure.
Immediate Evacuation and Safety Procedures
The immediate response to a tsunami warning or natural sign is to move out of the inundation zone as quickly as possible. The primary objective is to reach high ground or move inland, with the recommendation being at least 100 feet above sea level or 2 miles inland. Evacuation routes are marked with signage directing people away from the hazard zone.
The first wave is often not the largest, and a tsunami is a series of powerful surges that can continue for many hours. The time between successive waves can be anywhere from a few minutes to over an hour, and the danger persists long after the initial impact. People must not return to the coast to sightsee or assess damage after the first wave.
If moving inland or to higher ground is not possible, seek refuge on the upper floors of a sturdy, reinforced concrete building. Remaining in a low-lying coastal area is hazardous due to the immense force of the water and the debris it carries. Remain in the safe location until local authorities issue an official “all clear” message, which may not happen for hours.
Geographical Vulnerability and Expected Impact
The physical consequences of a tsunami are highly dependent on the local coastal geography, with low-lying areas, harbors, and estuaries being the most vulnerable. Infrastructure in California’s major port areas, such as Los Angeles/Long Beach and San Diego, faces significant risk. The funneling effect of bays and river mouths can amplify the height and destructive power of the incoming waves, pushing the inundation further inland.
The concept of an inundation zone defines the area that would be flooded by a worst-case scenario tsunami. Even relatively small waves can cause widespread destruction due to powerful, fast-moving currents and the battering impact of debris. These currents can overwhelm infrastructure, damage buildings, and pose a severe threat to life. Open coastlines backed by steep cliffs may experience less extensive inundation than protected harbors, but the sheer force of the water remains dangerous. Over a quarter of a million Californians reside in identified potential tsunami inundation areas, and the economic impact on the state’s maritime and trade assets would be substantial.