(435 miles). The focuses of most earthquakes are concentrated in the crust and upper mantle. The depth to the center of the Earth’s core is about 6,370 kilometers (3,960 miles), so even the deepest earthquakes originate in relatively shallow parts of the Earth’s interior.

The epicenter of an earthquake is the point on the Earth’s surface directly above the focus. The location of an earthquake is commonly described by the geographic position of its epicenter and by its focal depth.

Earthquakes beneath the ocean floor sometimes generate immense sea waves or tsunamis (Japan’s dread “huge wave”). These waves travel across the ocean at speeds as great as 960 kilometers per hour (597 miles per hour) and may be 15 meters (49 feet) high or higher by the time they reach the shore. During the 1964 Alaska earthquake, tsunamis engulfing coastal areas caused most of the destruction at Kodiak, Cordova, and Seward and caused severe damage along the west coast of North America, particularly at Crescent City, Calif. Some waves raced across the ocean to the coasts of Japan.

Tsunami destruction on Kamehameha Avenue on Hilo’s waterfront, 1946. (Photograph provided by the U.S. Army Corps of Engineers.)

Liquefaction, which happens when loosely packed, water-logged sediments lose their strength in response to strong shaking, causes major damage during earthquakes. During the 1989 Loma Prieta earthquake, liquefaction of the soils and debris used to fill in a lagoon caused major subsidence, fracturing, and horizontal sliding of the ground surface in the Marina district in San Francisco.

Liquefaction of sands and debris caused major damage throughout the Marina district in San Francisco during the Loma Prieta earthquake.

Landslides triggered by earthquakes often cause more destruction than the earthquakes themselves. During the 1964 Alaska quake, shock-induced landslides devastated the Turnagain Heights residential development and many downtown areas in Anchorage. An observer gave a vivid report of the breakup of the unstable earth materials in the Turnagain Heights region: I got out of my car, ran northward toward my driveway, and then saw that the bluff had broken back approximately 300 feet southward from its original edge. Additional slumping of the bluff caused me to return to my car and back southward approximately 180 feet to the corner of McCollie and Turnagain Parkway. The bluff slowly broke until the corner of Turnagain Parkway and McCollie had slumped northward.

Many homes were damaged by landslides triggered by the 1964 Alaska earthquake (above) and the 1989 Loma Prieta shock (below).

Measuring Earthquakes

The vibrations produced by earthquakes are detected,