Signs of an Approaching Tsunami

On March 11, 2011, a devastating magnitude-9 earthquake struck off the northeastern coast of Japan, triggering a tsunami of catastrophic proportions. As the death toll mounted, people the world over were left wondering how such a horrific thing happened, what they could do to help and whether such a thing could happen to them. Given the disastrous nature of tsunamis, it is important to understand and recognize the signs of an approaching tsunami so you can take action to protect yourself.

Tsunamis Explained

• Tsunamis occur when something causes a large amount of water to be suddenly displaced. Underwater earthquakes most commonly cause tsunamis but landslides, volcanic eruptions or icebergs breaking apart can cause them as well. Described as a series of waves, tsunamis' wavelengths are thousands of times longer than those of regular waves and they travel at speeds of up to 10 times faster than standard waves. There can also be a period of time as long as an hour between tsunami waves.

When to Prepare for a Tsunami

• Despite the suddenly occurring nature of earthquakes, you will find that there is usually a small amount of time to prepare for tsunamis. Tsunami Warning Centers such as The Pacific Tsunami Warning Center (PTWC) and The West Coast/Alaska Tsunami Warning Center (WC/ATWC) may issue tsunami advisories (meaning an earthquake has occurred, and a tsunami may result), tsunami watches (meaning that a tsunami may occur, but is at least two hours away) and tsunami warnings (indicating that a damaging tsunami may occur and residents of the affected area are advised to evacuate). You should heed any warnings issued and evacuate if it is recommended.

Tsunami Warning Signs

• The earliest warning sign of a potential tsunami is an earthquake. An earthquake occurring even thousands of miles away should alert you to the possibility of an impending tsunami if you live in an area where tsunamis frequently occur.

  According to National Geographic, tsunami eyewitnesses often report a sudden change in the water level. If the ocean level drops or recedes suddenly, especially to the point where you are able to see unusual parts of the ocean floor, you should immediately head to higher ground.

  Beware of a white or dark shadow on the horizon, appearing as a wall of clouds sitting on top of the sea. When a tsunami approaches, the wall of water creates a thunderously loud roaring sound, reported to be comparable with a jet engine. If you actually see the tsunami, run for higher ground.

Tips and Warnings

• Remember that a tsunami is not a single wave but a series of waves that can arrive an hour apart. Do not assume that the danger is over until the tsunami warning is officially lifted by authorities. If you see or hear signs of a tsunami, do not wait for an official evacuation order. Head to higher ground immediately. A tsunami that appears mild at one area of the shoreline can be enormous at a different point. Do not conclude that the tsunami is modest at all areas just because that's what you witness.