 Crushed car in Mackay, Idaho after the Borah Peak earthquake of 1983 |
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Earthquake Processes
Rupturing rocks release huge amounts of energy. The sudden release of energy is what is felt in an earthquake. Earthquake energy is in the form of seismic waves. The seismic waves radiate out from a central point, called the focus or hypocenter, like ripples moving outward from a pebble tossed into a lake. The location directly above the hypocenter, on the earth's surface, is called the epicenter.
Seismic Waves
Four types of seismic waves are generated when faulting triggers an earthquake. All the seismic waves are generated at the same time, but travel at different speeds and in different ways. Body waves penetrate the earth and travel through it, while surface waves travel along the surface of the ground.
Primary and secondary waves are body waves . Primary waves (P-waves) travel the fastest and can move through solids and liquids. The P-wave energy causes the ground to move in a compressional motion in the same direction that the wave is traveling. Secondary waves (S-waves) are slower and travel only through solids. The S-wave energy causes the ground to move in a shearing motion perpendicular to the direction of wave movement.
Rayleigh and Love waves are the two types of surface waves. Rayleigh wave energy causes a complex heaving or rolling motion, while Love wave energy causes a sideways movement. The combination of Rayleigh and Love waves results in ground heave and swaying buildings. Surface waves cause the most devastating damage to buildings, bridges, and highways.
Earthquake Locations
Although earthquakes can occur anywhere on earth, most earthquakes (>90%) occur where tectonic plates move against one another. The boundaries along each plate are referred to as margins. Different types of stresses are associated with each type of margin:
* convergent-plate margins have compressional stresses (come together, therefore result in crustal shortening)
* divergent-plate margins have tensional stresses (move apart, resulting in crustal extension)
* transform-plate margins have shear stresses (the plates slide past each other).
Each type of margin has a corresponding fault type associated with it.
Types of Faults
Earthquakes result from movement along a fault. Faults and earthquakes are cause and effect. The sense of motion on faults describes how the block move relative to each other. Faults may move along preexisting fracture or may form a new one. There are 3 basic types of faults, normal, reverse and strike-slip. Normal and reverse faulting result in vertical slip, while strike-slip faulting results in horizontal slip. In nature, motion is seldom absolutely along one direction. There can be a combination of vertical and horizontal slip, which would make the movement along the fault oblique.
Normal faults are associated with extension. A good example of normal faulting is the Basin and Range topography of the western United States. The western part of the North American plate has been pulled apart into a series of "blocks". Most Basin and Range structures result from the tilting of these blocks. A major Basin and Range fault zone is the Wasatch Fault zone, which is 220 miles long (360 kilometers) and extends from Utah into Idaho.
Reverse faults are associated with compressional forces- 2 plates or fault blocks pushing towards each other. One side ends up on top! Thrust faults are reverse faults that move up a shallower angle than ordinary reverse faults.
Strike-slip faults are associated with shear stresses. One side of the fault "slides" past the other. The San Andreas Fault in California is a well-known example of a right-lateral strike-slip fault.
Sometimes it is fairly easy to recognize where movement on a strike-slip fault has occurred. A different view of the San Andreas shows a creek located along the Fault. The zigzag effect (offset) of the creek channel is the result of movement along the fault.
Compare the photo of the San Andreas Fault creek with the strike-slip fault diagram. Notice the offset!
Detecting, Locating and Measuring Earthquakes
Several thousand stations monitor earthquakes all over the world. Each station contains an instrument, called a seismograph, used to detect arrival times and record seismic waves. The seismograph consists of a seismometer (the detector) and a recording device. The seismometer electronically amplifies wave motion. The graph on which seismic waves are recorded is called a seismogram. The amplitude of the recorded seismic wave is the vertical distance between the crest and trough of the waveform, therefore, the larger the earthquake, the greater the amplitude of the earthquake. The key to locating an earthquake's epicenter is the difference in arrival time (called lag time) of the P- and S-waves.
Magnitude and Intensity
Earthquakes are categorized in two ways - magnitude and intensity. Magnitude indicates the severity of an earthquake using the Richter Scale, a logarithmic, instrumentally determined measurement. Magnitude rates an earthquake as a whole. The severity of an earthquake is a rating based on the amplitude of the seismic waves. Larger amplitude waves equals higher magnitude earthquake equals greater severity. Amplitude is the vertical distance between the trough and crest of a waveform.
The Mercalli Scale defines intensity. Intensity is rated by how much damage was caused by an earthquake and how it affected people.
GO TO the University of Nevada - Reno Richter scale page for an excellent explanation of the Richter Scale and other earthquake quantifying tools.
ALSO VISIT the UNR Seismological Laboratory Page which is full of interesting information for earthquake enthusiasts.
Earthquake Damages
Effects of an earthquake can be classified as primary or secondary. Primary effects are permanent features produced by the earthquake. Examples include:
* fault scarps
* surface ruptures
* offsets of natural or human-constructed objects
An example you have already seen is the creek offset produced by movement along the San Andreas Fault. Secondary effects result when ground movement causes other types of damage. Examples include landslides, tsunami, liquefaction and fire. The amount of damage caused by an earthquake varies with magnitude. The greater the magnitude, the greater the damage potential.
For more information visit Environmental Geology, an Idaho Virtual Campus course.