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CAUSES OF EARTHQUAKES
People feel approximately 1 million earthquakes a year. Few are noticed very far from the source. Even fewer are major earthquakes. Earthquakes are usually felt only when they are greater than a magnitude 2.5. The USGS Earthquakes Hazards Program has a real-time map showing the most recent earthquakes. Most earthquakes occur along active plate boundaries. Intraplate earthquakes (not along plate boundaries) are still poorly understood.
Earthquake energy is known as seismic energy, and it travels through the earth in the form of seismic waves. To understand some of the basics of earthquakes and how they are measured, consider some of the fundamental properties of waves. Waves describe a motion that repeats itself in a medium such as rock or unconsolidated sediments. The magnitude refers to the height, called amplitude, of a wave. Wavelength is the distance between two successive peaks of the wave. The number of repetitions of the motion over time, called cycles per time, is the frequency. The inverse of frequency, which is the amount of time for a wave to travel one wavelength, is the period. When multiple waves combine, they can interfere with each other. When the waves are in sync with each other, they will have constructive interference, where the influence of one wave will add to and magnify the other. If the waves are out of sync with each other, they will have destructive interference. If two waves have the same amplitude and frequency and they are ½ wavelength out of sync, the destructive interference between them can eliminate each wave.
The elastic rebound theory explains the release of seismic energy. When rock is strained to the point that it undergoes brittle deformation, built-up elastic energy is released during displacement, which in turn radiates away as seismic waves. When the brittle deformation occurs, it creates an offset between the fault blocks at a starting point called the focus. This offset propagates along the surface of rupture, which is known as the fault plane.
The fault blocks of persistent faults like the Wasatch Fault of Utah are locked together by friction. Over hundreds to thousands of years, stress builds up along the fault. Eventually, stress along the fault overcomes the frictional resistance, and slip initiates as the rocks break. The deformed rocks “snap back” toward their original position in a process called elastic rebound. Bending of the rocks near the fault may reflect this build-up of stress, and in earthquake-prone areas like California, strain gauges that measure this bending are set up in an attempt to understand more about predicting an earthquake. In some locations where the fault is not locked, seismic stress causes continuous movement along the fault called fault creep, where displacement occurs gradually. Fault creep occurs along some parts of the San Andreas Fault.
The release of seismic energy occurs in a series of steps. After a seismic energy release, energy begins to build again during a period of inactivity along the fault. The accumulated elastic strain may produce small earthquakes (on or near the main fault). These are called foreshocks and can occur hours or days before a massive earthquake, but they may not occur at all. The main release of energy occurs during the major earthquake, known as the mainshock. Aftershocks may then occur to adjust strain that built up from the movement of the fault. They generally decrease over time.
Introduction to Physical Geography by R. Adam Dastrup is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.
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