(Teacher Name)
An earthquake is any seismic event that causes seismic waves. These can be caused by human activity or natural causes. For example, fracking often causes earthquakes which are human caused. Earthquakes often shake or displace the ground or surface of the earth near the epicenter and if the displaced earth is under the ocean, it will usually cause a tsunami. Most earthquakes are caused by geological faults but they can also be caused by landslides, mine blasting, and volcanic activity. Earthquakes are measured by overserving seismometers that are set up across the globe in strategic places for scientists to observe the Earth’s movement underground. (United States Geological Survey. January 29, 2009)
Earthquakes along the faults are called tectonic earthquakes. If the sides of a fault are smooth, they can slip past each other with almost no major shaking, however, if the sides are irregular a jolt or series of jolts will occur, causing the wave of energy to move across the land. This energy is much like a stone that is tossed into a still body of water. The larger the stone, the larger the energy wave. The same thing occurs with earthquakes. The harder the jolt, the farther the energy wave is felt. Tectonic earthquakes happen anywhere on earth where there is enough stockpiled elastic strain energy to create fracture spread on a fault plane. The sides of the fault will move slowly past each other until there is an irregular edge. Where there is irregularity, there will be friction. When the friction builds it will lock up and cause stress that stores up the energy around the fault. This will continue until the stress builds up so much that it has no other alternative but to break free, causing seismic activity or an commonly known as an earthquake. The energy in an earthquake is converted into heat cause by the friction. Although it is minimal, the heat does lower the elastic potential energy. (United States Geological Survey. January 29, 2009)
Three main types of faults are called strike-slip, reverse, and normal inter-plate earthquakes. These reverse and normal faults are called dip-slip. The dip-slip is where displacement on the fault line dips and the movement has a vertical movement. A normal fault will take place in areas that have the crust extended. For example, a divergent boundary will have the crust extended. Reverse faults happen where the crust is shortened. An example of this is a convergent boundary. Strike-slip faults are where the crust is slipping past itself along the fault in a horizontal motion. Transform boundaries are one type of this kind of fault. Earthquakes that are both dip and strike-slip are called oblique slip. (United States Geological Survey. January 29, 2009)
Earthquakes that are magnitude 8 and higher are usually caused by reverse faults on a convergent plate boundary. These are the most powerful and seem to do the most damage to life and property. Continental transforms, such as strike-slips, seem to produce large earthquakes up to magnitude 8. Normal faults produce earthquakes that are less than magnitude 7. Each magnitude of energy on the Richter scale will produce about thirty times more energy than the magnitude below it. For example, a magnitude 5 will produce thirty times more energy than a magnitude 4. Earthquake activity depends on the length and width along converging plate margins. For example, if the dip angle is shallow it is usually referred to as a convergent fault. If the dip angle is deep and wide, the earthquake can be massive, like the 2011 earthquake in Japan. (United States Geological Survey. January 29, 2009)
If the fault is vertical like a strike-slip fault, then a width of about 10km and an earthquake of no more than magnitude 8 is the result. Thrust faults are created by a strike slip, and normal faults are created by the lowest energy generated, or the lowest stress levels. These push the rock in a downward motion and the energy is spread out and down instead of just out. The earthquake is typically a low magnitude. (United States Geological Survey. January 29, 2009)
If the plate boundaries happen in the continental lithosphere, the earthquake is spread over a large area. This is what occurs with the Sand Andreas fault. The activity happens away from the fault and strains the crust near the fault itself. All activity occurs as plates interact with each other and may trigger earthquakes in regions that seem unrelated. For instance, Japan and California. (United States Geological Survey. January 29, 2009).
Most of the tectonic earthquakes start at the ring of fire. These earthquakes are at depths at around 100km or less. The earthquakes that are as shallow as 70 km are usually called intermediate or mid-focus. However, those earthquakes that originate between 70 and 300 km are mid-focus. However, in the ocean seismic activity is not measured as accurately, but the area in a subduction zone can have quakes that are 300-700 km in depth. These are called deep-focus earthquakes. (Committee on the Science of Earthquakes, 2003).
Earthquakes in volcanic areas, such as the ring of fire, are typically caused by tectonic and magma. If the earthquake is caused by magma, it usually warns of a volcanic eruption. For example, before the Mt. St. Helens eruption in 1980, swarms of earthquakes occurred. People had plenty of warning and were able to get away before the volcano erupted. The people of Pompeii were not as aware when Mt. Vesuvius erupted and buried people in the streets and their homes. Scientists keep a close eye on earthquake action near volcanoes to help warn people of the possibility of danger. (Committee on the Science of Earthquakes, 2003).
Tectonic earthquakes occur when the first rupture is at a start point on the surface of the fault. This is commonly called a nucleation. The size of the nucleation zone is unknown. However, there is some evidence like the dimension of the smallest earthquakes that give clues to make an educated guess that they are smaller than 100 m, but slow component shows that by low frequency spectra may show that it is larger. It is possible that the nucleation includes some preparations process that includes tiny shockwaves that precede the larger quake. Sometimes these are referred to as cluster earthquakes, especially if they measure 3.0 or below on the Richter scale and are followed by a larger quake. Aftershocks are tiny quakes that occur after a large earthquake and sometimes finish the damage made the original large earthquake. (Committee on the Science of Earthquakes, 2003).
Earthquake swarms are groups of earthquakes that occur in a specific area in a relatively short period. They are different than the earthquakes that are followed by aftershocks because there is no large event that occurs first. The series is the main event, and each rumble is the same magnitude as the first. Different than the swarms, earthquake storms occur when earthquakes hit a fault in clusters. Each shake starts a new shake and can cause a series of earthquakes that cause severe damage. (Horizon. 1 April 2003)
Although the most common earthquakes occur naturally by the movement of the tectonic plates, human activity also causes earthquakes. Much of the human activity caused earthquakes are preventable. For example, when humans inject the earth with liquid for fracking, blast rock for mining, and drilling for oil all cause earthquakes in regions that are not associated with the plates. Building a dam and a large building has also been the cause of several human-caused earthquakes. Earthquakes that are naturally caused are unavoidable and precautions have been set-up to warm people of imminent danger that could be caused by an earthquake. (United States Geological Survey. January 29, 2009)
Sources Cited
"Earthquake Storms". Horizon. 1 April 2003. Retrieved April 4, 2016
National Research Council (U.S.). Committee on the Science of Earthquakes (2003). "5. Earthquake Physics and Fault-System Science". Living on an Active Earth: Perspectives on Earthquake Science. Washington D.C.: National Academies Press. p. 418. ISBN 978-0-309-06562-7. Retrieved April 4, 2016
Hess, Darrel, and Tom L McKnight. Physical Geography, A Landscape Appreciation, Tom L. Mcknight. 11th ed. Upper Saddle River, N.J.: Prentice Hall, 1996. Print.
"Repeating Earthquakes". United States Geological Survey. January 29, 2009. Retrieved April 5, 2016
S Sibson R. H. (1982) "Fault Zone Models, Heat Flow, and the Depth Distribution of Earthquakes in the Continental Crust of the United States", Bulletin of the Seismological Society of America, Vol 72, No. 1, pp. 151–163
Watson, John; Watson, Kathie (January 7, 1998). "Volcanoes and Earthquakes". United States Geological Survey. Retrieved April 3, 2016
