What is a tsunami?

A series of waves or surges that is most often caused by earthquake fault movement beneath the sea floor.

Tsunamis can cause great loss of life and property damage in coastal areas. Very large tsunamis can cause damage to coastal regions thousands of miles away from the earthquake that caused them.

Thirty-eight tsunamis have been recorded on the Northern California coast since 1933. Most were very small, but five caused damage. The most damage was caused by a tsunami generated by the M 9.2 1964 Alaska earthquake. It flooded 29 blocks of Crescent City’s waterfront, damaged harbors and port facilities as far south as Santa Cruz, and caused 12 deaths in California. 

Tsunamis are Tricky

Don ’t rely on your past experience

Tsunamis are very different from normal ocean wave

  • Surges mayattack from unexpected directions.
  • A tsunami maylooksmall from a distance, but bythe time you realize how big it is,you might not be able to reach high ground.
  • The water moves much faster than it appears. You can’t out run it.
  • Tsunamis penetrate far onto the shore and can travel miles up coastal rivers.
  • You can’t surf tsunamis. There is no curling wave or face to surf.
  • Even strong swimmers can’t survive a tsunami.
  • The next tsunami will be different from the last.

Just when you think it is all over, a bigger surge may arrive

  • First surges are often small; the largest surges mayarrive many hours after the first.
  • The danger can last a day or longer.
  • The water is dangerous even after the tsunami is over – debris and hazardous materials maylurk in the depths.

What areas are at risk?

Beaches, harbors, bays, and river mouths are at the greatest risk. If you are in the YELLOW areas of our Hazard Zone Maps, you should leave after feeling an earthquake that lasts a long time. If you are in the white area, stay where you are.

Damaging tsunamis are not as rare as you might think. In the last century:

  • More than fortytsunamis have been observed on the North Coast.
  • Five caused damage.

Tsunami Facts

  • Tsunamis most commonly are caused by earthquakes, but also may be triggered by landslides, submarine volcanic eruptions and, very rarely, meteor impacts.
  • No two tsunamis are alike. Sometimes they look like sloping mountains of water and other times they rush ashore like a river in a flood, and are usually choked with debris.
  • Large tsunamis may reach heights of 20 to 50 feet along the coast and, in isolated areas, even higher. The first tsunami surge is not the highest. In Northern California, the largest surge may occur hours after the initial wave.
  • It is not unusual for tsunami surges to last 12 hours, and in some cases much longer. It is not safe to approach the coast until officials permit you to return.
  • The time between wave surges may range from minutes to over an hour. It is not possible to predict how many surges or how much time will elapse between waves.
  • The areas at greatest risk are on the beach and low-lying coastal areas. Only in large coastal rivers is a tsunami likely to penetrate farther than two miles inland.
  • Sometimes the first sign of a tsunami is an unusual lowering of ocean water, exposing the sea floor. This “drawdown” always means that the water will surge back strongly. Not all tsunamis are preceded by water lowering—so if you feel ground shaking or hear that a tsunami warning has been issued, evacuate the coast immediately and do not wait to see the water pull back.

How do I know if an earthquake is big enough to cause a tsunami?

  • If you are on the beach and feel an earthquake, no matter how small, move inland or to high ground immediately.
  • If you are in a tsunami hazard zone and feel an earthquake that lasts a long time, evacuate as soon as it is safe to move. Not sure if the earthquake is long enough? When in doubt, DRILL IT OUT. Every earthquake is an opportunity to practice evacuating.
  • GO ON FOOT. Roads and bridges may be damaged by strong ground shaking. Avoid downed power lines. If evacuation is impossible, go to the upper floor of a sturdy building or climb a tree — but only as a last resort.

To read more about tsunamis: 

Visit our Frequently Asked Questions

Marigram (water level recording) from Crescent City during the 2011 Japan tsunami.

Water level recording from Crescent City during the 2011 Japan tsunami.

Each horizontal dashed-line is 12 hours. Each vertical dashed-line is about 1.5 feet. The tidal fluctuation (blue line) affects the total water height. For example, a tsunami arriving on a high tide will cause more innundation and damage than on a low tide." This graphic probably needs a little more explanation which can be done at a later date.

  1. The first tsunami surge arrived in Crescent Cityat 7:30 am on March 11.
  2. During the strongest surge around 10 am, the water level fluctuated from 5 feet below mean low tide, to over 8 feet above it. Fortunately, these strongest surgesarrived at low tide.
  3. When the high tide arrived around 4 pm, the tsunami was 5 feet high.
  4. The absolute highest water occurred at2am on March 12, just before the highest tide arrived. The tsunami was onlyabout 4 feet, but it was on top of a 6.5 foot high tide. The tsunami surges continued for over 6 days.

 


How Tsunamis are Formed

  1. Gravity pulls the offshore Gorda and Juan de Fuca plates beneath the North American Plate. Most of the time the two plates are firmly stuck together along the boundary (red zone).
    Gorda plate, North American plate, stuck with arrows pointing towards eachother

  2. Over time, the North American plate is squeezed and bulges up as the Gorda plate slowly moves beneath it.
    Slow distortion - plates moving to show how the wave is formed

  3. Eventually the stuck area can no longer resist the squeezing and breaks along the boundary, causing a large earthquake. Like a spring, the overriding North American Plate jumps upward and seaward, lifting the water above it. A tsunami is born.
    Earthquak starts tsunami
    Stuck area ruptures, releasing energy in an earthquake - arrows pointing in three directions away from the source of the rupture.

  4. The water bulge divides and sends waves both east towards the coast and west into the Pacific. The first waves reach nearby shores only minutes after the earthquake. The other set of waves may still be large enough to damage distant coastal areas many hours later.
    Tsunami waves spread
    Two arrows one on each plate with rippling waves