Tsunami About

A tsunami is a series of large ocean waves generated by either large, subduction zone earthquakes which deform the ocean floor or by landslides within or falling into the ocean. When the waves enter shallow depths near a coastline, they may rise to several feet or, in rare cases, tens of feet. If you are on a beach or in low coastal areas it is imperative you are aware that a tsunami could arrive within minutes after a severe earthquake. A tsunami’s danger period can continue for many hours after a major earthquake. Tsunamis can occur during any season of the year and at any time, day or night.

The California Governor’s Office of Emergency Services, Earthquake, Tsunami and Volcano Program is continuously researching, learning, and collaborating with science, industry, and academic experts to develop and confirm the latest, best available knowledge base to help make California’s residents and visitors safer in the event of tsunamic activity. By mapping potential inundation and evacuation areas, providing assistance in response and evacuation planning, implementing outreach, education and warning signage at the coast, as well as determining ways to improve preparedness and resilience of California’s ports and harbors, our staff strives to ensure everyone on the coast remains safe before, during and after the next tsunami.

Causes of Tsunamis

A tsunami is a series of large ocean waves generated by either large earthquakes which deform the ocean floor, or landslides within or falling into the ocean. Any disturbance in the ocean that causes the displacement of large amounts of water could result in a tsunami. Not all earthquakes generate a tsunami. To generate tsunamis, earthquakes must occur underneath or near the ocean, be of a large magnitude, and create vertical movement of the sea floor. Generally, earthquakes on strike-slip faults, such as the San Andreas, do not by themselves generate tsunamis. In some instances, large strike-slip fault earthquakes may trigger landslides which could cause a local tsunami.

Most tsunamis are caused by earthquakes generated on a subduction zone, an area where one tectonic plate is forced under another plate. In subduction zones one plate is forced down and an adjacent plate is forced up causing an earthquake. The movement of the plates displaces water on the ocean floor vertically, resulting in a wave which then propagates horizontally through and across the entire ocean.
How a tsunami wave forms
Eventually, water rushes landward and may flood the shoreline resulting in inundation of dry land. While tsunamis can occur in any oceanic region in the world, more large earthquakes take place on the Pacific Ocean basin than anywhere else.

Near and Distant Source Tsunamis

After an event on the ocean floor (earthquake, landslide) displaces water, a wave is formed which travels out from where the event occurred. Some of the water travels across the ocean basin. Scientists refer to this event as a “distant source tsunami”. Distant source tsunamis usually provide more time than near source tsunamis for warning and preparation.

During distance source tsunamis, you may receive official instructions from your local government officials through the media.

Near source tsunamis are caused when a subduction zone (such as Cascadia) is immediately offshore from your location. They can be more dangerous as they are closer to shore and can reach your shoreline within minutes of the originating earthquake. They may also be larger than a wave coming from across the ocean when they do strike, as their energy has not had time nor distance to dissipate. Near source tsunamis provide little to no time for warning, evacuation, first responder preparation and dangerous circumstances for response.

For near source tsunamis, do not wait for official warnings to evacuate. Strong shaking and other natural warning signs are your indicators that a tsunami could be on the way.

Real Time Tsunami Information

Unlike for earthquakes, natural warnings may occur prior to a tsunami. In such cases, however, the warning indication may occur only a few minutes prior to tsunami impact. Knowing what to watch for, how to react and where to go in the event of a tsunami is the most effective way to know how to protect yourself and your family. The bottom line is…

1. When at the coast know where safe, high ground is located.
2. When you are near the shore and feel a strong ground shaking that lasts a long time, drop, cover and hold on until the shaking stops, then move to higher ground.
3. If you see the water recede out to sea, abnormally far from shore, move to higher ground.
4. If you hear a load ocean roar, move to higher ground.
5. If you observe any of these natural warning signs, a tsunami may arrive in minutes and last for eight hours or longer.
6. Never return to the shore until you are given the “all clear” from a reliable source.

Knowing this information will allow you to react more quickly and safely when a tsunami is expected. Please visit our partner websites for more information.

Understanding Tsunami Signage

Most emergency management agencies responsible for areas along the California coast went through a vigorous process to identify tsunami hazard zones and then placed signs in specific areas to delineate the perimeter of the inundation zone, evacuation routes and appropriate action to be taken by individuals when an earthquake occurs.

In California, the signs shown below were approved by the California Department of Transportation for use in tsunami inundation areas.

If you see a Tsunami Hazard Zone sign, you are in an area that could, potentially, be inundated by a tsunami. Upon seeing this sign, identify possible locations to which you will travel when the water starts moving.

Tsunami Evacuation Zone Sign

The Tsunami Evacuation Route sign is used with arrows to direct individuals toward a safe area. When evacuating after an earthquake, follow the arrows until reaching an evacuation site. These signs can be found on state highways as well as local streets and roads.

Tsunami Hazard Zone Sign The signs indicating you are in a tsunami hazard zone and should go to high ground or inland in the event of an earthquake are generally located near the immediate coastline, in parking lots, at parks and beaches. When seeing this sign, be prepared by taking note of the location to which you will go after an earthquake. In most cases, safe, high ground is reachable by foot.

Entering Tsunami Hazard Zone Sign The Entering/Leaving Tsunami Hazard Zone sign is used to delineate the boundary of defined inundation areas on State highways, local street and roads. You are considered safe, and out of the tsunami hazard zone, if moving inland away from the coast, from these signs.

Tsunami evacuation area sign Signs designating an Evacuation Site may be used to direct road users (including pedestrians and bicyclists) to safe areas. Such a sign is further confirmation that you’ve reached safe, high ground.

Did you Know?

Most tsunamis are not one big breaking wave that overruns the shore. They are usually a series of waves behaving similar to a rapidly rising high tide that can continue repeatedly over many hours, and even days.
In most cases, the first in the series of waves is not the biggest. A tsunami may start small, relatively speaking, and develop over time. People have been killed most often by subsequent waves, coming hours after the “start of tsunami.”
Tsunamis travel faster over the open ocean than they do toward land. In the open ocean the travel a speeds of up to 600 mph, about the speed of a jet
Not all tsunamis are recognizable by the fast recession of water from the shore. Only about 40% of tsunamis produce this natural warning sign.