Sam Spade's San Francisco

What is a tsunami?

A tsunami is a sea wave generated by an earthquake, landslide, volcanic eruption, or even by a large meteor hitting the ocean. (The Japanese word tsu means harbor; nami means wave.)

The following link illustrates how an earthquake along an ocean-floor subduction zone can create a tsunami: http://www.pbs.org/wnet/savageearth/animations/tsunami/index.html .

Basically, an event like an earthquake creates a large displacement of water resulting in a rise or mounding at the ocean surface that moves away from this center as a sea wave. These sea waves can move more than 800-kilometers (500-miles) per hour. As they approach land and as the ocean shallows, these waves slow down, making them grow in height (amplitude).

What are the sources for and examples of tsunamis that might affect California?

Though damaging tsunamis have occurred infrequently in California, they are a possibility that must be considered in coastal, and even deep-lake shoreline, communities. There are two sources for California tsunamis, based on distance and warning time:

Local sources - Relatively local earthquakes and landslides off the California, Oregon, and Washington coast pose the greatest threat of tsunamis that can reach California’s coastline in less than an hour. An earthquake on the Cascadia subduction zone, off the coast of northern California, could trigger a tsunami that could reach land within minutes. Earthquakes off the rest of the California coast (south of Cape Mendocino) take place mainly on strike-slip faults, and because the movement they generate is mostly lateral, tsunamis from local sources are less likely to occur because the ocean floor and overlying water is not typically thrust upward. For more information regarding tsunamis from the Cascadia subduction zone, go to http://sorrel.humboldt.edu/~geodept/earthquakes/shaky2_tsunami.html.

The more likely source of a landslide-induced tsunami is a large submarine landslide triggered by ground shaking from even a moderate earthquake in the coastal California region. There would be little time for warning about such an event so close to shore. An extreme example of a landslide causing a large tsunami is the rockfall at Lituya Bay, Alaska, in 1958. The water splashed 520 meters (1,700 feet) up the other side of the inlet, and a wave about 30 meters (100 feet) high was created. In California, a magnitude 5.2 earthquake in 1930 off of Redondo Beach is thought to have caused a landslide that generated a six-meter (about 20 ft.) wave. For more information about tsunamis from landslides and rock falls, go to http://www.prh.noaa.gov/itic/library/about_tsu/faqs.html#4 .

Distant sources - A tsunami caused by a very large earthquake elsewhere on the Pacific Rim could reach the California coast many hours after the earthquake. For example, the tsunami caused by the recent magnitude 9.0 earthquake near Sumatra caused a sea level fluctuation in San Diego of about 22 centimeters (8.6 inches) a day later in San Diego. (See the latest West Coast & Alaska Tsunami Warning Center bulletins at http://wcatwc.arh.noaa.gov ) The magnitude 9.5 earthquake in Chile in 1960, the largest earthquake ever recorded, resulted in a 1.6-meter (5.2-foot) wave that reached Santa Monica about 14 hours after the earthquake (http://wcatwc.arh.noaa.gov/web_tsus/19600522/runups.htm).

The most devastating tsunami to affect California in recent history was from the magnitude 9.2 Alaskan earthquake of 1964. Areas of northern California experienced a six-meter (20-foot) tsunami wave that flooded low-lying communities, such as Crescent City, and river valleys, killing 11 people. (The following link shows the travel time of the tsunami wave from the 1964 Alaskan Earthquake: http://wcatwc.arh.noaa.gov/web_tsus/19640328/traveltime.gif).

For more information regarding tsunamis that have affected northern California, see the chart at the following link: http://www.humboldt.edu/~geodept/earthquakes/tsunami!/n_coast_tsunamis.html.

The table appended to the bottom of this page contains information on some additional tsunamis in California from 1812 to 2000, compiled from the following website: http://www.ngdc.noaa.gov/nndc/servlet/ShowDatasets.

How can I determine whether tsunamis are possible where I live, and what kind of warning could I get?

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.

The OES provides generalized maps for projected tsunami inundation to coastal government agencies for emergency planning purposes (http://nthmp-history.pmel.noaa.gov/its2001/Separate_Papers/R-04_Eisner.pdf ). These maps are used as a basic guideline for what areas are prone to tsunami inundation. Efforts are underway by CGS and other organizations to consider the multiple tsunami sources affecting California in order to produce improved inundation maps.

In order to determine whether a tsunami has been generated following a large earthquake, scientists from the West Coast and Alaska Tsunami Warning Center monitor an array of buoys and tide gauges that measure vertical changes to the ocean surface (http://co-ops.nos.noaa.gov/about2.html#ABOUT ). If a potentially damaging tsunami is headed towards California, a warning can 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/ ). Crescent City and the University of California at Santa Barbara have implemented a loudspeaker warning system to alert the public to the possibility of a tsunami. These monitoring and warning systems work ideally for tsunamis that are hours away from California.

In situations where tsunami-warning times are short (caused by nearby earthquakes or landslides), it is difficult for government agencies to identify and warn the public. Individuals should research their personal exposure and have a plan to evacuate if necessary. The public should contact their local city or county governments for help in determining whether they are at risk and what evacuation plans might be in effect.

Are there any warning signs of an impending tsunami?

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 current 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 may be prudent to move to higher ground.

What should I do before, during, and after a tsunami in my area?

Education and preparation are the best ways to avoid injury and increase your chances for survival. Simply put, the best way to avoid a tsunami is to get to higher ground. Contact your local city and/or county government to see if they have an evacuation plan.

Check the following links for vital information regarding what to do before, during, and after a tsunami:

Are You Ready? by the Federal Emergency Management Agency:
English: http://www.fema.gov/areyouready/tsunamis.shtm
Espanola: http://www.fema.gov/spanish/areyouready/tsunamis_spa.shtm

What to Do When They Hit, by the National Atmospheric and Space Administration: http://observe.arc.nasa.gov/nasa/exhibits/tsunami/tsun_hit2.html

Tsunami Hazard Awareness by the National Oceanic and Atmospheric Administration: http://www.pmel.noaa.gov/tsunami-hazard/tsunami_awareness.htm

Tsunami information by the American Red Cross:
http://www.redcross.org/services/disaster/0,1082,0_592_,00.html

Tsunami Safety Rules by the West Coast & Alaska Tsunami Warning Center:
http://wcatwc.gov/safety.htm

Links for additional information about tsunamis:

The following additional links will provide more information on the cause and effects of tsunamis, and help you determine your best plan of action during a tsunami.

General tsunami information:

Tsunami Hazard Awareness by the National Oceanic and Atmospheric Administration:
http://www.pmel.noaa.gov/tsunami-hazard/tsunami_awareness.htm

Tsunami information by the American Red Cross:
http://www.redcross.org/services/disaster/0,1082,0_592_,00.html

Tsunami Safety Rules by the West Coast & Alaska Tsunami Warning Center:
http://wcatwc.arh.noaa.gov/safety.htm

Frequently asked tsunami questions answered by Pacific Marine Environmental Laboratory scientists:
http://www.pmel.noaa.gov/tsunami/Faq/

Tsunami Event Database Search by the National Geophysical Data Center: http://www.ngdc.noaa.gov/seg/hazard/tsevsrch_idb.shtml

University of Southern California Tsunami Research Group:
http://www.usc.edu/dept/tsunamis/linkspage.html

U.S. Geological Survey:
http://walrus.wr.usgs.gov/tsunami/

.