Tsunamis

How to Survive a Tsunami in Tutuila and Manu'a Islands
Tsunami Zone (English)
Tsunami Zone (Samoan)

What is a tsunami?
A tsunami is a series of ocean waves most commonly caused by an earthquake beneath the ocean floor.

How are tsunamis generated?
Tsunamis are caused by any large, abrupt disturbance of the sea-surface. These disturbances can be caused by earthquakes, volcanoes, or landslides. If the disturbance is close to the coastline, local tsunamis can demolish the coast within minutes. A very large disturbance can cause local devastation and send tsunami destruction thousands of miles away.

Illustration of a tsunami. — Click on image for tsunami animation video.
NOAA

Where and how frequently do tsunamis occur?
Since science cannot predict when earthquakes will occur, they cannot determine exactly when a tsunami will be generated. There is an average of two destructive tsunamis per year in the Pacific Basin. Pacific wide tsunamis are a rare phenomenon, occurring every 10-12 years on average.

Tsunamis do not have a season and do not occur regularly or frequently. However, they do pose a major threat to the coastal populations of the Pacific and other world oceans. Nothing can be done to prevent them, but the adverse impact on the loss of life and property can be reduced with proper preparedness.

What determines the destructive nature of a tsunami?
When a tsunami finally reaches the shore, it may appear as a rapidly rising or falling tide, or a series of breaking waves. Reefs, bays, entrances to rivers, undersea features and the slope of the beach all help to modify the tsunami as it approaches shore. The first wave may not be the largest in the series of waves. One coastal area may see no damaging wave activity while another may receive large, violent, destructive waves.

lhoknga_iko_2004364 — Before and after a tsunami.
NASA

What are some tsunami warning signs?

Earthquake, especially one of prolonged duration and extreme strength (you have less than 15 minutes to reach safety)
Reef revealed as the ocean seems to be sucked away from the coast (the wave is imminent)

What should I do in case of a tsunami?
In the event that you do find yourself near the coast when an earthquake hits, be aware of the following guidelines:

Move to higher ground immediately. A tsunami may be coming within minutes.
Go on foot.
If there is no high ground, move inland away from the coastline.
Stay away from the coast. Later waves can be higher than the first. Waves may continue to arrive for hours.
Listen to your radio for the "all clear" signal.

What happened in American Samoa?
On the morning of Tuesday, September 29, 2009 at 6:48am American Samoa was struck by a powerful earthquake. Registering a magnitude 8.1 and originating about 150 miles away in the Tongan Trench, it generated the largest tsunami the Samoan Archipelago has seen in recent history. The first wave struck the island only 15 minutes after the earthquake stopped, catching many islanders off-guard.

The National Park of American Samoa headquarters and visitor center were housed in Pago Plaza at the very head of Pago Pago Harbor. The narrowing of the harbor at this point makes it particularly vulnerable to tsunami damage, as water is funneled there. The first floor office included the visitor center with newly installed exhibits, cultural resources storage and the administrative and superintendent's offices. There was a dive locker and two containers in the parking area of Pago Plaza that stored the equipment needed to perform marine, terrestrial and maintenance duties. Most work days begin at 7am, so the earthquake and tsunami caught most employees either at work or on their way.

As employees ran up the mountain across the street from the headquarters, the first wave of water surged into the parking area destroying the fleet and storage facilities located there. The water continued to rise, topping out just below the second floor offices of Pago Plaza. There were four more large waves to follow through the next hour each slightly smaller than the last. Employees were gathered and damage assessed. The first floor offices were completely destroyed.

Villages on every side of the island were affected. Thirty-five people lost their lives. Scores of people lost their homes.