WHAT ARE EARTHQUAKES?
An
earthquake is the shaking of the ground caused by an abrupt shift of rock along a fracture in the Earth, called a
fault (Figure 1). Within seconds, an earthquake releases stress that has slowly accumulated within the rock, sometimes over hundreds of years.
FIGURE 1
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Block diagrams of fault types. (a) An earthquake is caused by the sudden fracturing of rock along part of a fault surface, shown here as a plane. If the fault reaches the surface, a visible ground fracture is created. The focus or hypocenter is the point on the fault plane where fracturing begins. The epicenter is the point on the ground surface directly over the focus. If the fault plane is inclined, the position of the epicenter will not coincide with the ground fracture. Simple fault motions are shown in (b), (c), and (d); directions of compressive stress are indicated. In a normal fault, (b), adjacent blocks of rock behave as if they were being pulled apart; the upper block slides downward along the fault relative to the other. In a thrust fault, (c), the blocks behave as if they were being pushed together; the upper block rides up the fault plane. In a strike-slip fault (d), one block moves horizontally past the other. Oblique motion of the blocks (not illustrated) combines thrust or normal fault motion with strike-slip motion. From an analysis of the seismic waves generated by an earthquake, called a fault-plane solution, scientists can determine the type of fault motion that occurred.
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It is also possible for the accumulated stress to be released more gradually, by continuous slippage along a fault; this movement may amount to only a few millimeters a year. Such faults are said to undergo aseismic fault creep because the stress release occurs without earthquakes.
Faults are a record of past earth movements, just as fossils are a record of plants and animals that once inhabited the Earth. However, like volcanoes, faults may be extinct or active. Some faults are continuously active, while others may have occasional earthquakes and long periods of quiescence. Thousands of "extinct faults" have been mapped in Washington. A few active faults have also been mapped; these active faults are said to be active because they have experienced surface movement in the last 10,000 years. However, in the last 100 years earthquakes in Washington have not been associated with known active faults.
The earthquake process can be compared to the bending of a stick until it snaps. Stress accumulated during bending is suddenly released when the stick breaks. Vibrations are produced as the stick springs back to its pre-stressed position. In the Earth, seismic waves
(Figure 2) are the vibrations caused by the sudden release of stress built up in rocks on either side of a fault. The rupturing of a fault may release all or only some of the stress. Any residual stress is often released by later minor readjustments along the fault causing smaller earthquakes called aftershocks.
FIGURE 2
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Diagrams of near-surface ground motions produced by seismic waves. The P and S waves, (a) and (b) respectively, travel through the earth in all directions from the focus of the earthquake; the first wave to reach an observer during an earthquake is the P wave. Two types of surface waves shown in (c) and (d), travel along the ground surface, somewhat like water waves, and arrive after the S waves. The direction the wave travels is indicated by the arrow below each diagram; the direction of ground movement caused by each wave is indicated by the solid arrows on the diagrams. P and S waves cause the ground to vibrate in mutually perpendicular directions.
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Earthquakes generate several kinds of seismic waves that vibrate the ground
(Figure 2). These seismic waves travel through the Earth at speeds of several kilometers per second, and they cause ground motions that can be detected by seismographs (or by accelerographs) far from the epicenter of the earthquake. In 1987, the University of Washington was operating more than 100 seismograph stations in Washington and northern Oregon
(Figure 3). Several thousand seismographs are operated throughout the world by other groups of seismologists.
FIGURE 3
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Active seismograph stations in the Pacific Northwest in 1987. Stations operated by the University of Washington are shown as triangles, Canadian stations as squares. Seismic signals from the University's stations are received in Seattle. Signals from stations shown as solid triangles are also transmitted to the National Earthquake Information Service in Golden, Colorado. Station LON, at Mount Rainier, is part of an international recording system known as the World Wide Standard Seismograph Station Network (WWSSN). At LON six seismometers measure the various kinds of seismic waves shown in Figure 2.
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FIGURE 4
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Components and dimensions of a typical remote seismograph station, similar to stations located on Fig. 3. The seismometer (a) converts small ground motions into an electric signal that has varying voltage. An amplifier and a voltage-controlled oscillator amplify this signal and convert it to a frequency-modulated (FM) tone. The radio transmitter (c) and the antenna (d) transmit the tone signal to the recording site.
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Typical components of a modern seismograph station are shown in
Figure 4 The signals produced by the seismographs in response to ground vibrations from an earthquake are commonly recorded on paper and magnetic tape. The display of ground motion versus time on paper record is called a seismogram (
Figure 5).Seismographs can detect ground motions caused by sources other than earthquakes, such as explosions, volcanic eruptions, sonic booms, helicopters, and cars. Each of these sources can generally be identified from their characteristic signals recorded on seismograms.
FIGURE 5
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Seismograms (a) through (e) were recorded by stations in Washington and Oregon and illustrate the range of ground motion frequencies commonly recorded. The seismometers that recorded these motions are similar, and all had natural periods of 1.0 second. The seismograms for (a), (b), and (c) are expanded as (A), (B), and (C) in the lower part of the figure. P and S waves are marked on all seismograms. (a) Seismogram of a small (magnitude 1.2) volcanic earthquake at Mount St. Helens on November 23, 1987. The focus was less than I km below the surface, and the epicenter was less than I km from the station. (b) and (c) Seismograms of a magnitude 0.9 earthquake in the Cascade Range on November 18, 1987. The focus was at a depth of 17 km, and the epicenter was 13 km from the station that recorded (b) and 47 km from the station that recorded (c). (d) and (e) Seismograms from a magnitude 6.3 earthquake in the Imperial Valley of California on November 24, 1987 (d) shows the P wave as recorded at a station in northern Oregon, 1427 km from the epicenter. (e) shows the surface waves, which have lower frequencies, recorded at the same station. The surface waves arrived about 5-1/2 minutes after the P waves.
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