How Much Time Does It Take for a Tsunami to Reach the Shore?

Tsunami Causes

• Tsunamis form when a substantial volume of seawater -- or, rarely, water in a lake -- suddenly becomes displaced. The most common causes of tsunamis are strong earthquakes occurring on a coastline or offshore. Occasionally, however, other natural phenomena can cause tsunamis -- namely, underwater or coastal landslides or volcanic eruptions, or quite infrequently, meteors crashing into water bodies.

Tsunami Propagation

• Unlike tidal waves, tsunamis extend from the surface of the ocean all the way to its floor. In open ocean waters, tsunamis travel faster than any other waves on the planet: about 650 to 950 kilometers per hour -- equivalent to the speed of jet planes. Curiously, tsunamis are barely noticeable to boats sitting in deep water -- their wave height is less than one meter (three feet). But when tsunamis enter the shallow water near a shoreline, water withdraws from the shore and the wave height drastically increases, creating the characteristic towering wall of water. Because earthquakes and the other phenomena that cause tsunamis can̵7;t be forecast accurately, tsunamis also can̵7;t be predicted very far in advance. However, once a potentially tsunami-causing event occurs, scientists ascertain how long waves could take to strike the areas that could be affected. They divide the possible lengths of time into three categories, based on distance.

Local Tsunami

• Local tsunamis are those that impact areas within about 100 miles (60 kilometers) of their point of generation. They strike in an hour or less, sometimes in just minutes. Local tsunamis were produced by the massive earthquake that struck just off the coast of Japan in March 2011; these tsunamis barreled into the nearest coastal towns in less than 35 minutes. Unfortunately, local tsunamis produced by geological events in close proximity to the affected shorelines don̵7;t provide people with enough warning time, making them extremely dangerous. However, people in such areas often feel shaking from the earthquake or other tsunami-generating event as it occurs -- if this is the case, don̵7;t wait for a tsunami warning to be issued; immediately head inland to higher ground.

Regional Tsunami

• Regional tsunamis are formed at greater distances away from coastlines than local tsunamis, and hence take slightly longer to reach those coastlines. They strike regions roughly 100 to 1,000 kilometers (60 to 600 miles) from where they formed. This distance means that their travel time ranges from between one and three hours. For instance, the tsunamis produced by the March 2011 earthquake off the coast of Japan took one to two hours to strike the parts of Japan that were situated further from its epicenter. The increased travel time of regional tsunamis provides people a greater opportunity to flee.

Distant Tsunami

• Tsunamis generated from distant sources take the longest time of all to reach their ultimate destinations. Long-distance tsunamis originate in areas more than 1,000 kilometers (about 600 miles) away from their place of impact. . This equates to a travel time of three hours or more. Tsunamis generated by the March 2011 Japanese earthquake took more than 11 hours to reach the coast of California. Such a considerable amount of time allows authorities an ample chance to issue warnings and assist people in evacuating.