There’s a reason California is highlighting tsunami preparedness in March. The two most devastating tsunamis in state history occurred this month. Last Friday marked 11 years since the Great East Japan earthquake and in two weeks we mark 58 years since the 1964 Alaska earthquake and tsunami.
Mother Nature gave us a little tsunami boost this year with the first tsunami warning issued for California’s shores in over a decade. The volcanic explosion in the Tonga Islands on January 15 prompted a tsunami warning that caused strong currents, minor flooding and required several Coast Guard rescues in the state.
How common are tsunamis along our coasts? Fifteen years ago, I did an exercise with Paul Whitmore, the director of the National Tsunami Warning Center (NTWC) in Palmer, Alaska. We looked at all earthquakes that have occurred in the Pacific basin since 1900 and, using current protocols, estimated what type of tsunami warning would have been issued. For much of this period, there were no tsunami warning centers; The NTWC was only created in 1967 and alert definitions have changed several times since then.
We found that tsunami warnings would not be as rare as one might think. The NTWC issues three types of warning bulletins: Tsunami Warning (significant land flooding possible), Tsunami Warning (risk limited to beaches, bays and ports) and Tsunami Watch (a possible tsunami threat is being assessed ). Over the past 122 years, we estimate that 12 warnings, 11 advisories and 15 watches would have been issued.
Five of the warnings were triggered by large earthquakes far away from us and would have put all or most of California’s coastline on warning. The sources of 1946 (M8.6), 1952 (M 9.0), 1960 (M9.5), 1964 (9.2), 2011 (M9.1) were 4.5 to 16 hours away from the tsunami. All of these tsunamis caused damage in California and three (1946, 1964, 2011) caused deaths. No one in California would have felt them and we would need the NTWC alert for our county emergency managers to make evacuations.
The other seven warnings would have been issued for earthquakes of magnitude 7 to 8, located near the west coast of the United States. The “San Francisco” earthquake of 1906 would have triggered an alert from Monterey to Humboldt County. The 1927 “Lompoc” earthquake would have caused a warning for Southern California and the rest would have been limited to our North Coast region.
These “near-field” earthquakes are much more difficult to manage from a warning center perspective. Unlike large earthquakes far away from us, the travel time between the earthquake and the arrival of the first tsunami waves in these near events is half an hour or less. The NTWC would not have the luxury of hours to assess whether a tsunami was actually generated and to run models to predict likely tsunami wave heights. When time is of the essence, it is important to err on the side of caution.
For nearby earthquakes, people will feel the quake minutes before NTWC issues a bulletin. These large earthquakes produce many seconds of shaking, i.e. a tsunami could be on its way. We always recommend reacting quickly to the shake alert; you may never receive an official alert due to infrastructure damage. If you are on the beach or in a tsunami risk area, evacuate to higher ground as soon as the shaking subsides enough to allow you to move around safely. Large coastal and offshore earthquakes in the past have produced small tsunamis, but fortunately none have caused any damage.
Our study projected 12 advisory bulletins since 1900. Advisories, such as the Tonga alert of January 15, have a more limited potential impact than warnings. There is no need for extensive evacuations from the tsunami hazard area as the threat is limited to beaches and port areas. Of these, eight were from large earthquakes elsewhere in the Pacific and three from north coast earthquakes located further offshore than those warranting a warning. January 15 was the eccentric – the first warning ever issued for a volcanic eruption. Two of these advisory-level events (1957 and 2006) caused damage to Crescent City.
Thirteen earthquakes of the past century would have reached the monitoring level. All were caused by large earthquakes at least four hours away from California and would have been canceled before any part of the state was placed on an advisory or warning. Some, however, would have resulted in warnings for other parts of the United States and it can be confusing to know if your area is on alert. NOAA’s website https://www.tsunami.gov/ provides quick information on warning areas and county emergency notification systems will let you know if you need to be concerned.
Next week, the Humboldt County Board of Supervisors will proclaim March 21-25 Tsunami Preparedness Week. The Redwood Coast Tsunami Task Force has planned a number of events/activities. The Manila community on the Samoa Peninsula will hold a tsunami evacuation drill on March 19 and a tsunami communications test will be held on March 23.
Evacuation drills are one of the best ways to prepare for tsunamis. By driving down the road to get to a safe area, you develop a muscle memory of what to do to protect yourself and your family in a real earthquake/tsunami. Beginning at 9:30 a.m. on March 19, Humboldt County Sheriff’s Office deputies will patrol the streets of the Manila neighborhood sounding the Hi-Lo evacuation siren. It is the signal for residents and visitors to head for the heights. Volunteers from Humboldt County CERT Teams and the RCTWG will help direct you to the nearest evacuation site. I will be one of them and we will all be happy to answer all your questions.
Note: See the new Tsunami Preparedness page https://rctwg.humboldt.edu/ on the Redwood Coast Tsunami Task Force website. It includes more information about the Manila exercise, the March 23 Tsunami Communications Test, and general information about tsunamis and local tsunami hazards.
Lori Dengler is professor emeritus of geology at Cal Poly Humboldt, specializing in tsunami and earthquake risk. Do you have questions or comments about this topic, or would you like a free copy of the Living on Shaky Ground readiness magazine? Leave a message at 707-826-6019 or email [email protected]