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FAQ About Tsunamis

Introduction

What is a tsunami?

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

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

Are there any warning signs of an impending tsunami?

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

Historic Tsunamis in California.  

Introduction

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.

What is a tsunami?

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.

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

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.

The table appended to the bottom of this page contains information on some additional tsunamis in California from 1812 to 2012, 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. Type in an address or city using the CGS Information Warehouse to access Statewide Inundation Maps to see if areas where you live, work, or visit are in tsunami inundation areas.

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 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 now what the natural warning signs of a tsunami are andhave a plan to evacuate if necessary.

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.  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 “to 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 to go www.tsunamizone.org.

Historic Tsunamis in California:

The chart below shows data from some of the tsunamis recorded in central and southern California from 1812 to 2015 (from http://www.ngdc.noaa.gov/nndc/servlet/ShowDatasets ):

year

month

day

travel (hours)

time (minutes)

tsunami location

height (meters)

source location

source event

source magnitude (Ms)

source magnitude (Mw)

1812

12

21

 

 

EL REFUGIO (GAVIOTA), CA

3.4

CA

Purisima

7.7

 

1812

12

21

 

 

SANTA BARBARA, CA

2

CA

Purisima

7.7

 

1812

12

21

 

 

VENTURA, CA

2

CA

Purisima

.7.7

 

1856

9

24

 

 

SAN DIEGO, CA

3.6

Japan

Tokaido

 

 

1859

9

24

 

 

HALF MOON BAY, CA

4.6

N. CA

 

 

 

1862

5

27

 

 

SAN DIEGO, CA

1.2

S. CA

 

5.8

 

1868

10

21

 

 

SAN FRANCISCO BAY, CA

4.5

SF area

 

6.8

 

1868

8

13

 

 

SAN PEDRO, CA

1.8

N. Chile

 

8.5

 

1868

8

13

 

 

WILMINGTON, CA

1.8

N. Chile

 

8.5

 

1877

4

16

 

 

ANAHEIM LANDING, CA

1.8

CA

 

 

 

1877

4

16

 

 

CAYUCOS, CA

3.6

CA

 

 

 

1877

5

10

 

 

GAVIOTA, CA

1.8

N. Chile

 

8.3

 

1877

5

10

 

 

SAN PEDRO, CA

1

N. Chile

 

8.3

 

1877

5

10

 

 

WILMINGTON, CA

1.7

N. Chile

 

8.3

 

1878

11

22

 

 

WILMINGTON, CA

1

S. CA

 

 

 

1896

12

17

 

 

SANTA BARBARA, CA

2.5

S. CA

 

 

 

1896

6

15

 

 

SANTA CRUZ, CA

1.5

Japan

Sanriku

7.6

 

1927

11

4

 

 

SURF, CA

1.8

CA

 

7.3

 

1946

4

1

 

 

ARENA COVE, CA

2.4

Alaska

E. Aleutian Islands

7.3

 

1946

4

1

5

36

AVILA BEACH, CA

1.3

Alaska

E. Aleutian Islands

7.3

 

1946

4

1

 

 

DRAKES BAY, CA

2.6

Alaska

E. Aleutian Islands

7.3

 

1946

4

1

 

 

HALF MOON BAY, CA

2.6

Alaska

E. Aleutian Islands

7.3

 

1946

4

1

 

 

MORRO BAY, CA

1.5

Alaska

E. Aleutian Islands

7.3

 

1946

4

1

5

36

SAN LUIS OBISPO, CA

1.3

Alaska

E. Aleutian Islands

7.3

 

1946

4

1

 

 

SANTA CATALINA ISLAND, CA

1.8

Alaska

E. Aleutian Islands

7.3

 

1946

4

1

 

 

SANTA CRUZ, CA

1.5

Alaska

E. Aleutian Islands

7.3

 

1952

11

4

8

36

AVILA BEACH, CA

1.4

Russia

Kamchatka

8.2

9

1960

5

22

 

 

MONTEREY, CA

1.1

Chile

Central Chile

 

9.5

1960

5

22

 

 

PACIFICA, CA

1.2

Chile

Central Chile

 

9.5

1960

5

22

 

 

PISMO BEACH, CA

1.4

Chile

Central Chile

 

9.5

1960

5

22

14

4

PORT HUENEME, CA

1.3

Chile

Central Chile

 

9.5

1960

5

22

14

12

SANTA MONICA, CA

1.4

Chile

Central Chile

 

9.5

1960

5

22

 

 

STINSON BEACH, CA

1.5

Chile

Central Chile

 

9.5

1960

5

22

13

43

WILSON COVE, CA

1.2

Chile

Central Chile

 

9.5

1964

3

28

 

 

ARENA COVE, CA

1.8

Alaska

Gulf of Alaska

 

9.2

1964

3

28

5

10

AVILA BEACH, CA

1.6

Alaska

Gulf of Alaska

 

9.2

1964

3

28

 

 

CAPITOLA, CA

2.1

Alaska

Gulf of Alaska

 

9.2

1964

3

28

 

 

MARTINS BEACH, CA

3

Alaska

Gulf of Alaska

 

9.2

1964

3

28

 

 

MONTEREY, CA

1.4

Alaska

Gulf of Alaska

 

9.2

1964

3

28

 

 

MOSS LANDING, CA

1.4

Alaska

Gulf of Alaska

 

9.2

1964

3

28

 

 

PACIFICA, CA

1.4

Alaska

Gulf of Alaska

 

9.2

1964

3

28

5

6

SAN FRANCISCO, CA

1.1

Alaska

Gulf of Alaska

 

9.2

1964

3

28

 

 

SAN RAFAEL, CA

1.5

Alaska

Gulf of Alaska

 

9.2

1964

3

28

 

 

SANTA CRUZ, CA

1.5

Alaska

Gulf of Alaska

 

9.2

1964

3

28

5

39

SANTA MONICA, CA

1

Alaska

Gulf of Alaska

 

9.2

1964

3

28

 

 

SAUSALITO, CA

1.2

Alaska

Gulf of Alaska

 

9.2

1964

3

28

 

 

SEA VIEW, CA

3.8

Alaska

Gulf of Alaska

 

9.2

1964

3

28

 

 

TOMALES BAY, CA

1

Alaska

Gulf of Alaska

 

9.2

1975

11

29

 

 

SANTA CATALINA ISLAND, CA

1.4

 

 

7.2

 

1989

10

18

 

 

MOSS LANDING, CA

1

CA

Loma Prieta

7.1

 

2000

11

4

 

 

POINT ARGUELLO, CA

unknown

CA

Pt. Arguello

 

 

                                                                               -END-


 

Web page created by M. Reichle, D. Hoirup, R. Wilson, and E. Mattison, California Geological Survey, 2005.