Tsunamis

A tsunami is a wave, or series of waves, generated by an earthquake, landslide, volcanic eruption, or even large meteor hitting the ocean (The Japanese word tsu means “harbor”; nami means “wave”). What typically happens is a large, submarine earthquake (magnitude 8 or higher) creates a significant upward movement of the sea floor resulting in a rise or mounding of water at the ocean surface. This mound of water moves away from this center in all directions as a tsunami. A tsunami can travel across the open ocean at about 500-miles per hour, the speed of a jet airliner. As the wave approaches land and as the ocean shallows, the wave slows down to about 30 miles per hour and grows significantly in height (amplitude).

Although most people think a tsunami looks like a tall breaking wave, it actually resembles a flood or surge.

Tsunamis generally affect coastal communities and low-lying (low-elevation) river valleys in the vicinity of the coast. Buildings closest to the ocean and near sea level are most at jeopardy. Visit our Tsunami Inundation Maps to identify areas that are vulnerable to tsunami inundation.

In order to determine whether a tsunami has been generated following a large earthquake, scientists from the National Tsunami Warning Center monitor an array of buoys and tide gauges that measure vertical changes to the ocean surface (more info on ocean tides and currents). If a potentially damaging tsunami is headed towards California, a warning will be broadcast through the Emergency Alert System and the National Oceanic and Atmospheric Administration (NOAA), National Weather Service (NWS) Weather Radio (http://www.nws.noaa.gov/nwr/). Check with your local city or county to see what ways they will notify you of a tsunami.

In situations where tsunami travel times are short (due to nearby earthquakes or landslides), it is difficult for government agencies to identify and warn the public. Individuals should know what the natural warning signs of a tsunami are and have a plan to evacuate if necessary. One noticeable, but not universal, sign is the rapid receding of ocean water from the beach before the first tsunami wave hits. In many accounts (including the 2004 Indian Ocean tsunami), this effect has caused greater loss of life because it became a curiosity that attracted people to the oceanfront.

Very strong ground shaking along the coast is an indication of an earthquake that could cause seafloor displacements and/or a submarine landslide large enough to generate a tsunami. Though many large earthquakes have occurred along the coast without causing a tsunami, you should still be aware of the potential and plan accordingly. In the event you are at the coast and feel strong shaking, it would be prudent to move to higher ground.

Education and preparation are the best ways to avoid injury and increase your chances for survival. Whether you live, work, or visit in California, it is important to be aware that the coastal region​ is vulnerable to tsunamis. Tsunamis can be generated locally or come across the ocean great distances.

California’s large active offshore faults and unstable submarine slopes can cause tsunami activity along the coast. Strong ground shaking from an earthquake is the natural warning that a tsunami might be coming. If you are on the beach or in a harbor and feel an earthquake, immediately move inland or go to high ground. If strong shaking lasts for 20 seconds or more, everyone within the tsunami evacuation area should evacuate as soon as it is safe to do so.

Other natural warnings include seeing the ocean quickly recede to expose the ocean bottom, or hearing an unusually loud roar coming from the ocean. A tsunami can arrive within minutes and may last for several hours.

For tsunamis coming from across the ocean, local communities use a variety of communication methods to broadcast emergency information. Be informed on what signals, sirens, and public services will be employed for your area. Practice tsunami evacuation drills.

Know whether you are in a potential tsunami zone by observing street signs or looking online to see if you are in a zone. Know the evacuation routes for your area. Contact your local city and/or county government to see what the evacuation plan is for your area and where you will be expected to evacuate to. Have a “Go bag” ready, in the event you have to evacuate. Do not return to the evacuated zone until officials tell you it is safe to do so. The first tsunami is not always the largest, and tsunami waves, flooding and strong currents can last for several hours.

For more information about tsunami preparedness go to the TsunamiZone web site.

Tsunami Preparedness Brochure for Recreational and Commercial Boaters

The California Office of Emergency Services and California Geological Survey have produced a tsunami preparedness brochure for recreational and commercial boaters. The March 11, 2011 tsunami caused nearly $100M in damage to the California maritime community. The state tsunami program is working with the state’s maritime communities to provide more detailed maps and guidance that will help them improve their tsunami planning. This brochure will help educate individual boaters about how they should prepare BEFORE the next tsunami arrives on our coast.

• Tsunami Preparedness Brochure for Recreational and Commercial Boaters

Maritime Tsunami Response “Playbooks”

Research indicates that a large tsunami from the Alaska-Aleutian Islands region could cause one third of the boats in California to be damaged or sunk and two thirds of the docks to be damaged or destroyed. Although the impacts from tsunamis cannot be completely eliminated, they can be significantly reduced by improving tsunami planning and response activities. Over the past several years, the California Geological Survey, the California Office of Emergency Services, and the University of Southern California have been working with harbormasters and port captains to develop guidance for maritime response to future tsunamis. In partnership with the Federal Emergency Management Agency, the state has started developing tsunami “playbooks” that will help these harbors and ports respond to tsunamis of various sizes coming from sources around the Pacific Ocean. The following references provide further details about the work these partners are doing to help harbors and ports to prepare:

• Maritime Tsunami Response Playbooks: Background Information and Guidance for Response and Hazard Mitigation Use (CGS Special Report 241, 2016)
• Example DRAFT Tsunami Response Playbook for Ventura Harbor
• Tsunami hazard mitigation activities in California maritime communities

More than eighty tsunamis have been observed or recorded in California in historic times. Fortunately, almost all of these were small and did little or no damage. Though damaging tsunamis have occurred infrequently in California, they are a possibility that must be considered in coastal communities. There are two sources for California tsunamis, based on distance and warning time:

Local sources - Local tsunami sources, like large offshore faults and massive submarine landslides, can put adjacent coastal communities at the greatest risk of a tsunami because the public must respond quickly with little or no official guidance. The Cascadia Subduction Zone is an example of a local tsunami source that could threaten northern California. Stretching from Cape Mendocino, California, to Vancouver Island, British Columbia, this 700-mile long submarine fault system forms the crustal plate boundary where the offshore Gorda and Juan de Fuca plates dive, or subduct, beneath the North American plate. Examples of local tsunamis that have impacted California include:

January 26, 1700 - An earthquake estimated at a magnitude 9 ruptured the entire length of the Cascadia Subduction Zone, likely causing a 50-foot tsunami in parts of northern California. Though there were no local written accounts, scientists have reconstructed the event based on geologic evidence and oral histories from the Native American people in the area, and determined the exact date and time from Japanese documents that describe the effects of a large tsunami that hit the coast of Japan later that same day.

December 21, 1812 – A tsunami struck the Santa Barbara and Ventura coastline shortly after a large earthquake was felt in the area. Though reports of the size of this tsunami have been debated, the event was large enough to inundate lowland areas and cause damage to nearby ships. One theory is that the tsunami was caused by a nearby submarine landslide triggered by the earthquake.

Distant sources - A tsunami caused by a very large earthquake elsewhere on the Pacific Rim could reach the California coast many (4 to 15) hours after the earthquake. The Alaska-Aleutians Subduction Zone is an example of a distant source that has caused destructive tsunamis in California. Notable distant tsunamis that have impacted California include:

April 1, 1946 – A magnitude 8.8 earthquake in the Aleutian Islands generated a tsunami that caused damage along the coast of California, including flooding over 1000-feet inland in Half Moon Bay.

March 28, 1964 – Twelve people were killed in California when a tsunami was generated by a magnitude 9.2 earthquake off the coast of Alaska. A surge approximately 20-feet high flooded 29 city blocks of Crescent City.

March 11, 2011 – A magnitude 9.0 earthquake in the Tohoku region of Japan produced a moderate amplitude tsunami in California. Although it did not generate significant flooding in California, strong tsunami currents caused one death and over $100-million in damages to 27 harbors statewide, with the most significant damage occurring in Crescent City and Santa Cruz. Read more about it at our Tohoku-oki tsunami page.

The table in the next section contains information on some additional tsunamis in California from 1812 to 2012.

The table below shows data from some of the tsunamis recorded in central and southern California from 1812 to 2012 (Source: https://www.ngdc.noaa.gov/nndc/servlet/ShowDatasets).

HINT: To navigate the table left-right on a desktop computer, hold down your Shift key while turning the scroll wheel on your mouse.

The California Geological Survey (CGS) provides geologic and seismic expertise to the public, other State government offices, such as the California Governor’s Office of Emergency Services (CalOES), and local government agencies (cities and counties). For tsunami hazards, CGS works closely with CalOES and the Tsunami Research Center at the University of Southern California to produce statewide tsunami inundation maps and preparedness information for California. CGS is also the Scientific Representative for California on the National Tsunami Hazard Mitigation Program Coordinating Committee, a state and federal cooperative responsible for developing policies and standards for tsunami mitigation efforts in the United States and its territories. Click here for CGS Tsunami Program contact information.

• Tsunami Response Playbooks/FASTER, and CGS Special Reports
• The March 11, 2011 Tohoku Tsunami poster - This poster looks back at the effects of the massive tsunami that originated offshore of Japan and traveled across the Pacific to California.
  
• Evaluation of the 2010 Chile and 2011 Japan tsunamis in California
• Assessment of tsunami-induced current hazards
• Tsunami Education, Outreach Materials and Videos
• CGS Note 55: Tsunamis (PDF file)
• Know Your (Tsunami) Zone flyer (PDF file)
  
• Tsunami Preparedness flyer (MS Word file)
  
• Tsunami Evacuation Drill handout (PDF file)
  
• Site search results matching Geology and Tsunami
  
• Our Related Links page