Understanding earthquakes and tsunamis

NOAA's Pacific Tsunami Warning Center provides warnings for Pacific basin teletsunamis -- tsunamis that can quickly spread across the entire ocean and cause widespread damage far away from their source -- to most Pacific rim countries and island states. A few destructive teletsunamis are generated each century by great earthquakes around the Pacific rim.

Preparations for tsunami events start long before the earthquakes occur. After seismometers detect a potentially tsunami-triggering quake, warnings use historical data to help predict when and where the waves could cause damage.

To confirm an actual tsunami, the Pacific Tsunami Warning Center uses a network of pressure sensor buoys anchored to the floor of the ocean that can detect surface wave heights. That data is fed into computer models along with the latest earthquake data from additional satellite and ocean wave measuring sources.

"Tsunamis don't behave like when you throw a pebble in a pond and identical ripples move in every direction," said Diego Arcas, who creates computer models of tsunamis at the Pacific Marine Environmental Laboratory in Seattle, Wash. "They actually like to travel only in specific directions determined by the topology of the seafloor."

What Causes Earthquakes?

Most earthquakes are caused by faulting: a sudden movement of rock along a rupture in the Earth's surface. This surface is in constant slow motion, because deeper in the earth, hot rock continually flows. Plates of the Earth’s crust cover the entire surface of the globe and can rub against each other in certain spots, sliding above or below each other. But if the motion isn't smooth, strain builds up until the "fault" ruptures, slipping to new position to relieve the strain. An earthquake is the resulting shaking that radiates out from the breaking rock.

What Are Aftershocks?

Aftershocks are smaller quakes that occur after the main quake. They happen because the newly moved rock must re-settle in its new formation. Bigger earthquakes have more and larger aftershocks. Because they are so unpredictable, an aftershock can be as damaging as the initial quake, especially if building structures were weakened but not destroyed the first time around.

How Quakes Are Measured

An earthquake's magnitude describes how much the ground moves. The scale is logarithmic, which means that when the magnitude increases by one (say from 3 to 4, or from 4 to 5) the amount of ground motion increases by ten times. That is, a magnitude 3 quake leads to ten times as much ground motion as a magnitude 2 quake, and a magnitude 2 leads to ten times as much motion as a magnitude 1. The magnitude scale also tells us just how much energy an earthquake released. For example, a magnitude 1 earthquake releases the same amount of energy as 30 pounds of TNT exploding. Although a magnitude 2 earthquake makes the ground move ten times as much as a magnitude 1, it releases 32 times as much energy -- or roughly as much as a ton of TNT. A magnitude 5 earthquake packs the punch of a moderate nuclear weapon, and a magnitude 12 quake would be enough to put a crack all the way through the center of the Earth.

USGS World Quake Map

The U.S. Geological Survey has devised the best tool yet for forecasting when and where earthquake aftershocks could occur: an online map, available to the public, that displays the probability of the ground shaking significantly over the next 24 hours.

The online map updates itself hourly. But it doesn't predict a primary earthquake. Instead, the map uses patterns in the initial aftershocks that follow big quakes to forecast when and where more will strike. It won't forecast the first of a series of quakes.

USGS Quake World Map: 1.usa.gov/dUxuow

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The following excerpts were compiled using material from previous reports by Inside Science News Service (ISNS) and Discoveries & Breakthroughs Inside Science (DBIS) and from information in the public domain.

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