It's Raining Fish and Spiders (32 page)

A view of San Francisco after the 1906 earthquake.
U.S. Geological Survey/Department of the Interior

U.S. Geological Survey/Department of the Interior

The size of an earthquake is indicated by a number called its
magnitude.
Magnitude is calculated from a measurement of either the strength or the length of time of recorded
seismic waves,
or what I like to call the “shake, rattle, and roll.” Magnitude is determined from measurements made from seismograms and not on reports of the shaking or interpretations of building damage. (This is different from the Fujita scale described in the chapter about tornadoes, which is based solely on the damage to buildings and other structures.)

Who Moved My Plate?

The rates of plate movement range from about three-quarters of an inch to 4.5 inches per year. That movement can be measured by precise ground-surveying techniques using laser-electronic instruments. Global Positioning System (GPS) has been very useful for studying the shifts in the Earth's crust. Twenty-one satellites are currently in orbit 13,777 miles above the Earth as part of the NAVSTAR system of the U.S. Department of Defense. These satellites continuously transmit radio signals back to Earth. To determine its precise position on Earth (longitude, latitude, elevation), each GPS ground site must simultaneously receive signals from at least four satellites, recording the exact location of each satellite and the time when its signal was received. By repeatedly measuring distances between specific points, geologists can determine if there has been active movement along faults or between plates. The separations between GPS sites are already being measured regularly around the Pacific basin.

“Yo, Plates, We'll Be Watching YOU!”

Dude, Take It from “Tommy Tsunami,” You Do
Not
Want to Catch
That
Wave!

It's very exciting to sit at the beach on a sunny blue-sky day and watch giant waves come crashing ashore. I love watching surfers try to keep those great monster waves from swallowing them. Laird Hamilton is my favorite surfer. He's extremely cool when it comes to riding gigantic waves that are five to eight stories high, shooting the curl on his board at up to 80 mph. However, the waves Laird and his surfer pals ride look nothing like the real killer of a wave called a
tsunami
. Most people who have not experienced a tsunami imagine that it looks like one of those towering, surfable waves with a giant curl. But that's not what a tsunami looks like.

A tsunami is a series of huge waves that occur as the result of a violent underwater disturbance such as an earthquake or a volcanic eruption. The waves travel in all directions from the epicenter of the disturbance, much like the ripples that happen when a rock is thrown into a pond. The waves may travel in the open sea as fast as 450 mph. In the open ocean, tsunami waves are not generally large—hence the difficulty in detecting the approach of a tsunami. In the open ocean, they would not even be felt by ships because their wavelength is hundreds of miles long with a height of only a few feet. But as these powerful waves approach shallow waters along the coast, their velocity is slowed and the waves swell to a great height before smashing into the shore. Wave heights have been known to reach more than 100 feet (30.5 meters)! Even a wave that is 10 to 20 feet high can cause many deaths or injuries.

National Oceanic and Atmospheric Administration/Department of Commerce

The first wave is usually not the largest or most significant wave in the series. Furthermore, one coastal community may experience no damaging waves while another, not that far away, may be pounded by deadly, destructive waves. Depending on a number of factors, some low-lying areas could experience severe inland flooding of water and debris of more than 1,000 feet.

So Who Is “Sue Nomee”?

Tsunami is the Japanese word for “harbor wave.” Tsunamis used to be mistakenly referred to as
tidal waves
, but they have nothing to do with the tides.

The deadliest tsunami in history happened on December 26, 2004, when a 9.0 magnitude earthquake ruptured the floor of the Indian Ocean off the northwest coast of the Indonesian island of Sumatra. The earthquake triggered the deadliest tsunami in world history. More than 225,000 people died from the disaster; thousands were injured, thousands remain missing, and millions were left homeless.

How Will I Know If a Tsunami Is Coming?

The National Oceanic and Atmospheric Administration (NOAA) uses many tools to detect an oncoming tsunami. Tsunami warning centers use earthquake information, tide gauges, and a new tool from NOAA—tsunami detection buoys, developed by NOAA's Pacific Marine Environmental Laboratory. Six of these buoys are now deployed in the North Pacific to help scientists determine whether a tsunami has been generated and is heading for North American coastlines. More buoys are needed, however, especially since the buoys can suffer outages in the harsh North Pacific Ocean.

National Oceanic and Atmospheric Administration/Department of Commerce

Tsunami Watches and Warnings

Tsunami Warning

A high-level threat. Indicates that a tsunami is imminent and that coastal locations in the warned area should prepare for flooding. The initial warning is typically based on seismic information alone. Earthquakes over magnitude 7.0 trigger a warning covering all coastal regions within 2 hours tsunami travel time from the epicenter. When the magnitude is over 7.5, the warned area is increased to 3 hours tsunami travel time. As water level data showing the tsunami is recorded, the warning will be cancelled, restricted, or expanded as conditions indicate.

Tsunami Watch

A lower-level threat than a tsunami warning. The area included in the watch is based on the magnitude of the earthquake and the borders of the warning zone. For earthquakes over magnitude 7.0, the watch area is 1 hour tsunami travel time outside the warning zone. For earthquakes over magnitude 7.5, the watch area is 3 hours tsunami travel time outside the warning zone. Subsequent bulletins will either upgrade the watch to a warning or cancel it, depending on the severity of the tsunami.