Soo-nah-mee

by Ruth Ann Angus

One thing cannot be denied—life in California is definitely shaky. Sooner or later, if you live here long enough, you are going to experience an earthquake. Sometimes these occur far out in the depths of the sea where the earth moves up or down, causing a bulge or depression in the water surface. The result can be the birth of a tsunami.

Tsunamis are not isolated incidents. As recently as last month a tidal wave reached the shores of the West Coast following the 8.9 earthquake in Japan. And while the waves were nothing like what happened in that country, the tidal surge was something to see, damage did occur in several harbors, and one person lost their life.

In March of 1964, following an earthquake in Alaska, a wall of water slammed into Crescent City, California, knocking buildings from their foundations and sweeping cars and structures out to sea. The waves were so strong that a 50,000 pound concrete tetrapod in the harbor was knocked from its base. The inundation swept inland 800 to 2000 feet in the commercial area and a mile inland along a low stream. Water depths reached eight to 13 feet. More than $7.4 million in damages occurred and 12 people lost their lives.

Tsunami is a Japanese word meaning "harbor" (tsu) "wave" (nami) and is the scientific term used for a seismic sea wave. Undersea earthquakes, landslides, volcanic eruptions, and meteorite impacts can all cause a tsunami.

Most tsunamis occur along the Ring of Fire, a zone of seismic activity and volcanoes that encircles the Pacific Ocean. These waves can have lengths (widths) of 60 to 120 miles. They may travel hundreds of miles across the deep ocean, visually undetectable but reaching 450 to 500 miles per hour. The wave may be only a foot or two high out to sea but suddenly grows rapidly upon entering shallow waters. Reaching shore the wave may be 50 feet high or more. Tsunamis affect a great volume of water and possess a tremendous energy. They are capable of obliterating coastal settlements.

Much of the Atlantic and Gulf Coasts lie less than 10 feet above mean sea level. Storm surges generated by hurricanes have historically been responsible for more loss of life than any other wave related incident and characteristically affect these areas.

Storm surge is water that is pushed toward the shore by the force of swirling winds. This surge combines with the normal tides to create a mean water level of 15 feet or more. In addition, wind waves are superimposed on the storm tide. Severe flooding occurs when these surges coincide with normal high tides. Communities near a shallow slope of the continental shelf suffer more from storm surges, although large breaking waves also affect steeper slope areas.

Many problems occur from storm surge, including severe beach erosion, the undermining of building foundations, damage to harbors, ships, marinas and pleasure boats, and salt-water intrusion into fresh water areas.

Evacuation of these coastal areas is common in the event of a hurricane and expected storm surge and notice to evacuate should always be taken seriously.  A storm surge killed an estimated 500,000 people in Bangladesh in 1970.

People enjoy watching waves crash against rocky shores and jetties. During winter storms fierce waves of 20 feet or more regularly blast away at the breakwaters in Morro Bay. At these times the parking lot at Morro Rock is filled with eager spectators who clamber over the boulders to get a closer look. Often they are doused by splashing seawater and risk being swept away. People have died this way.

The tidal surge with our recent tsunami appeared to be the most significant thing about the event. Video of the water receding from the Morro Bay harbor area showed a rapidly receding tide, much swifter than any regular tidal flow. The waves north of Morro Rock that surfers are so fond of appeared to advance in rapid sets and surge all the way up the beach.

You cannot surf a tsunami. They are not the curling or v-shaped waves surfers long for. They can occur any time of day or night, in all weather conditions and seasons. The most vulnerable places are beaches open to the ocean, bay mouths or tidal flats, and shores of large coastal rivers. While the East and Gulf coasts must contend with hurricane generated waves, the tsunami is a West Coast phenomenon and, while rare, we now know they have struck in the past and will do so again.

What To Do in the Event of a Tsunami

-If you feel an earthquake in a coastal area, duck, cover, hold, and watch for falling objects. Then move to high ground. The earthquake may be your only warning that a tsunami is coming.

-Never go to the coast to watch for a tsunami if you hear that a warning has been issued. Tsunamis move faster than a person can run. Any incoming traffic in the coastal area hampers safe and timely evacuation.

-If you see the ocean recede, do not go out to save stranded fish or sea life. The incoming wave will move faster than you can run.  A large withdrawal of water from the shore is a clue that a large wall of water may follow.

-If you hear an unusual roar or rumble—like a freight train coming from the ocean—move to higher ground.  It may be a tsunami.

-Evacuate if told to do so by authorities.

-All tsunamis are potentially dangerous, though they may not damage every coastline they strike.

For further information:
Tsunamis
International Tsunami Information Center
Box 50027
Honolulu, Hawaii 96850
(808) 532-6423

Earthquakes
Office of Emergency Services
Coastal Region Earthquake Program
1300 Clay St. Suite 400
Oakland, CA 94612
(510) 286-0873