Earthquakes

On Earth, earthquakes are concentrated on long narrow bands, often at the edges of continents. Until the theory of plate tectonics was developed in the 1960s, no one knew why this was, but now the movement of plates relative to each other is accepted as the cause of most earthquakes. Earthquakes are caused by a sudden release of stress from within a body of rock or between two bodies of rock moving relative to each other. The slow rate of movement at the Earth's surface generally allows the interface between the rocks to remain stationary for some time, while stress builds up, and then the rocks move suddenly to relieve the stress.This movement creates the earthquake.

Earthquakes can occur in the interior of plates due to plate flexure, for example, when a plate is rebounding from the weight of a recently removed ice cap. Intraplate earthquakes are relatively uncommon and not well understood. The great majority of earthquakes occur along the boundaries of tectonic plates. Plates moving parallel to each other form long faults that stick and rupture at intervals, creating seismic zones such as the San Andreas fault. Subduction zones, plate boundaries at which an oceanic plate moves under a continental plate, are another fertile ground for earthquakes. In subduction zones earthquakes can occur to depths of about 400 miles (700 km), below which the rock is too plastic from heat to allow stress to build up or to allow the rock to fail in a brittle manner. The deep earthquakes along linear trends were noticed in the early 20th century and named Wadati-Benioff zones, but it was not until the advent of plate tectonic theory that their cause was understood. Earthquakes occur at the interface between the subducting plate and the mantle it is moving through, as well as in the middle of the flexing, subducting plate. Mapping these earthquakes actually makes a picture of the location of the slab in the mantle.

About 3,000 seismic stations around the world are constantly monitoring for earthquakes. Seismic stations contain instruments that measure movement in the two horizontal directions and in the vertical direction. As waves of energy radiate from the site of the earthquake, the surface and subsurface of the Earth literally wave up and down or back and forth. The frequency of these waves varies from 0.001 to about 10 Hertz, depending on the size and distance of the earthquake and the material the energy is moving through. Though many scientists are working on earthquake prediction, it has proven impossible to date, even in the most reliable and well-studied simple fault zones.

Dr. Charles Richter, a professor at the California Institute of Technology, realized during the 1930s that the waves given off by an earthquake are proportional to the earthquake's energy, and thus can be used to compare the sizes of earthquakes around the world. By measuring the heights and frequencies of the waves as recorded by a seismic station, scientists can assign earthquakes a magnitude. The most commonly known scale for earthquakes is Dr. Richter's original work, the Richter scale, though there are a number of other earthquake magnitude scales.The formula for the Richter magnitude of an earthquake is to add the logarithm of the height of the earthquake's wave in millimeters to a factor taking into consideration the distance to the earthquake. Each increment on the Richter scale indicates a thirtyfold increase in earthquake energy. An earthquake designated 3 on the Richter scale is 900 times as strong as an earthquake designated 1. Comparisons of events with Richter scale magnitudes are given in the table below. Each year several hundred earthquakes with a magnitude of 3 occur, but only one or two with magnitude 7 or 8.A magnitude 9 event is thought to happen about every hundred years.

Earthquakes can cause immense damage simply by earth movement breaking roads, electrical, gas, and water lines, and knocking down buildings. Faults sometimes break all the way to the surface of the Earth and cause offsets in height or in lateral distance, leaving a scarp across the landscape or offsetting the line of a road, a fence, or even a river. Earthquakes near coasts can transfer their energy to the water, where waves form tsunamis that sometimes travel across entire ocean basins.Tsunamis can travel as fast as 650 feet per second (200 m/sec) across deep water.When they arrive at coastlines their energy is confined to shallower water and causes the wave to slow down but to increase in height. Tsunamis can be tens of feet tall as they hit a coast, and can run up over the land as far as a couple of miles (a few kilometers).

On Monday, December 27, 2004, the world's most powerful earthquake in 40 years (since the 1964 Good Friday earthquake in Alaska) occurred in the oceanic crust off the Indonesian island of Sumatra. The quake gave off waves that caused the Earth's surface to

THE RICHTER SCALE

Richter magnitude

Corresponding event

1

blast at a construction site

3

earthquakes of 3 and below are not generally felt by people

4

detonation of a small nuclear weapon

6

earthquakes in this range can cause significant damage within about

60 miles (100 km) of the event

8

1906 San Francisco earthquake

9.0

2004 Sumatran earthquake

9.2

1964 Good Friday earthquake in Alaska

9.7

the largest recorded earthquake, Chile, May 1960

12

thought to be the strongest earthquake sustainable by the Earth

move vertically by at least a half-inch (1 cm) as they passed by even at the greatest distances from the quake, and by at least four inches (10 cm) along the coastline of the Indian Ocean.The huge quake immediately triggered alarms in seismic monitoring stations around the world, notably in Hawaii, where a central monitoring station runs a warning system for countries around the Pacific Ocean. This monitoring station uses tide gauges set in deep water to confirm when an oceanic quake is creating a tidal wave. In the Indian Ocean, where the December 27 quake occurred, there is no such monitoring system, and there are no tide gauges. Immediately after the quake, its magnitude was estimated at less than the 9.0 magnitude that later, more careful analysis revealed it to be. This lower magnitude, combined with the lack of tide gauges, led scientists to consider the possibility of a tsunami but to have no certainty that one would result.

The size of an earthquake is not in itself a definitive indication of a tsunami.Water has to be significantly displaced to create a wave, so a landslide or large crustal movement is required. Deeper earthquakes, for example, do not usually cause significant crustal movement. Catastrophically for the residents of the Indian Ocean basin, the Sumatran earthquake occurred shallowly and created crustal movements at the site of up to 65 feet (20 m), initiating an immense tsunami (see the upper color insert on page C-6).The tsunami traveled as fast as 500 miles per hour (800 km/hr) in the open ocean, and then built to heights of 40 to 50 feet (~14 m) as they reached shallower water. Without a warning system or tide gauges, no advance word reached any waterfront. Over 250,000 people were killed by the tsunami throughout Southeast Asia, and a few were killed as far away as the coast of Somalia, 3,000 miles (4,800 km) from the quake.

As with any wave disruption in a water basin, the waves come in sets and not as a single event. Shown in the lower color insert on page C-7, residents who avoided the first wave were sometimes killed by the later sudden rises in sea level, which rose and fell at least four times with great enough magnitude to endanger people. The quake also disrupted the Earth into free oscillations, shown in the figure on page 44. The longest-period free oscillation distorts the Earth into a more football-shaped figure, and this quake created an oscillation in this mode with an average height of about 0.1 mm.A second free oscillation, in which the Earth moves radially in and out as if breathing, is slow to die away and was still detectable four months later.

In the immediate wake of this disaster, scientists and governments began working together to create warning systems for oceans other than the Pacific. Earthquakes of this magnitude occur on the order of several times per hundred years, so fast action to prevent another such disaster was necessary. Given just a half hour warning, almost every life could have been saved simply by moving to higher ground.

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