The Origin of Volcano-Tectonic Earthquake - Term Paper Example

Earthquakes The earth is made up of four layers, from the outermost, the crust, mantle, outer core and inner core. Earthquakes occur as a result of energy being suddenly released in the crust of the earth, following blocks of the earth slipping past each other and creating seismic waves. The crust consists of tectonic plates, which slowly slide past and bump into each other. Faults are first seen at the boundaries of the tectonic plates, and that is where most of the earthquakes occur. The boundaries occur in three key forms, namely transform, convergent or spreading depending on, respectively, whether they move laterally, toward or away from one another. According to the plate tectonic theory, earthquakes only take place in the brittle plates, which have relatively low temperatures and account for the 100 outermost kilometers of the earth. Temperature variations causes convection of rocks found deeper inside and induce stress, resulting in disturbance of the plates overlaying them. The stress deforms the overlaying plates and store tremendous amounts of energy in them. When the accumulated stress surpasses the rocks’ strength, they suddenly break and release the elastic energy stored in them, which results in an earthquake. Earthquakes Introduction The two common causes if earthquakes are the collision of tectonic plates and volcanic eruptions (Roman & Cashman, 2006). The shock waves emanating from testing nuclear weapons and some man-made explosions may also account for insignificant events of earthquakes, but they are not really considered as earthquakes because the shock waves do not have natural origins. Other artificial causes include the breaking of rocks to build tunnels for subways, railroads and roads as well as mines, although the resultant seismic waves are not considerably strong. Through the layers of earth, seismic waves which are energy waves, travel through the layers, and those of relatively low amplitude are known as ambient vibrations. Periods of changing activity often precede major earthquakes and are known as foreshocks. There can also be periods in which less frequent shocks are experienced as the masses of rock are temporarily ‘stuck’ and locked together (Tanimoto & Lay, 2000). After the main shock, further movements known as aftershocks are experienced, which are caused by the masses of rock settling into the new positions they have assumed. Aftershocks occur in the same regions as the main shock but are characteristically of lower magnitude. However, there have been incidences of aftershocks being of larger magnitude than the main shock, which leads to the designating of the main shock as the aftershock and vice versa. Aftershocks are a particular challenge to rescue efforts because of their potential to bring down structures that were initially weakened during the main quake. Tectonic earthquakes are more common than volcanic earthquakes and up to 90% of them in the world take place along the boundaries of the tectonic plates (Tanimoto & Lay, 2000). The rocks at these areas are usually weaker than those within the plate and are prone to yield to stress more readily. The other 10% takes place further away from the boundaries, for example, the 1811 and 1812 New Madrid and Missouri earthquakes that were felt beyond 3 million square kilometers and still continue to show seismic activity (Roman & Cashman, 2006). The actual ground shaking and vibrating during an earthquake is caused by the releasing of stress that has slowly built up from a few to several hundred years within seconds. A tectonic earthquake can take place anywhere on earth so long as there is sufficient elastic strain energy stored to cause fracture propagation along the plane of a fault. Discussion In the ideal situation, the sides of faults can smoothly, or aseismically, slide past each other if no asperities, or irregularities, exist between them, which means they encounter less frictional resistance (Coen, 2012). However, the actual situation is that the asperities do occur on most of the fault surfaces, which leads to stick-and-slip sort of behavior. Continuous relative motion occurring between the plates after locking of the fault increases stress and, as a result, strain energy is stored in the rock mass around the surface of the fault. This phenomenon will go on until a point where the stress will have sufficiently risen to break through the roughness, permitting the abrupt slipping over the segment of the fault that is locked, and the stored energy is released. The released energy combines seismic waves in the form of radiated elastic strain, cracking rocks and frictionally heated fault surface, culminating into an earthquake. The process involving the gradual swelling of stress and strain interposed by occasional and sudden failure of earthquake is termed the theory of elastic rebound. Earth scientists have established that the larger portion of an earthquake’s energy is either transformed into high temperature produced by friction or goes to powering the growth of the earthquake fracture (Coen, 2012). The types of faults capable of causing earthquakes occur in three main forms, which are strike-slip, reverse (or thrust) and normal. Reverse and normal faulting are dip-slip models in which the dislodgment along the fault takes the direction of the dip. On the other hand, the movement on them is on a vertical constituent. Normal faults mainly take place in places where the crust undergoes extension, which could be around the divergent boundary. A divergent boundary is defined by plate tectonics as a linear feature occurring between two plates slipping away from one another. For clarity, when divergent boundaries occur within continents, they produce initial rifts that, in turn, culminate into rift valleys. Divergent boundaries sometimes form volcanic islands that result from the plates moving apart to form gaps that molten lava will rise to fill. On their part, reverse faults mainly occur during and at the shortening of the crust at convergent boundaries. Convergent boundaries can be understood to be the in opposition of divergent boundaries, as the plates slip toward each other. Strike-slip faults are in the form of vertical structures in which the two faces of a fault slide past each other horizontally. A particular type of this fault is the transform boundaries. Many earthquake occurrences are due to the movement on faults having elements of both strike-slip and dip-slip, termed by earth scientists as oblique slip (Hyndman & Hyndman, 2009). The energy that is released during an earthquake, known as its magnitude in technical terminology, is proportional to the fault area that raptures and the associated stress drop. This translates into the broader and the longer the faulted area, the larger the resultant magnitude. The most potent earthquakes are associated with reverse faults, especially those occurring along convergent plate boundaries. These are usually of a magnitude of eight and above. Those of around magnitude eight are usually produced by strike-slip faults and, in particular, continental transforms, while those below magnitude seven are a result of normal faults. The outermost brittle part of the crust of the earth and cool tectonic slabs that descend into the warmer mantle are the only portions capable of storing elastic energy to be releases during rupturing of the fault. Rocks that have temperatures higher than 300 degrees Celsius will only respond to the stress by flowing but do not actually rapture during an earthquake. From observations, the maximum length of mapped faults and raptures that could break in a single go are is about 1,000 kilometers (Tanimoto & Lay, 2000). However, scientifically, the key significant parameter that determines the maximum magnitude of an earthquake on a fault is the width available, rather than the length available. This is because the width available has exhibited variances of up to a factor of 20. Typically at approximately 10 degrees, the rapture plane’s dip angle is very shallow along the margins of the converging plates. Consequently, the plane’s width within the earth’s brittle, top crust, can range anywhere between 50 and 100 kilometers, resulting in the most potent earthquakes possible. Earthquakes with magnitudes above eight may not be possible be possible since strike-slip faults have a tendency of being almost vertically oriented, which results in a width of approximately 10 kilometers within the crust’s brittle segments (Schorlemmer, Wiemer & Wyss, 2005). Along most normal faults, it is even more difficult to achieve maximum magnitudes since most of them are situated along the spreading centers in which the brittle layer’s thickness ranges around six kilometers only. Additionally, there is a stress level hierarchy in the three types of faults. The highest intermediate strike slips generate thrust faults, while normal faults are generated by the lowest levels of stress. The perfect way to envision this event is to consider the highest principal stress and its direction, which is the course of the force pushing the mass of rock as the faulting occurs. In terms of normal faults, the mass of rock is vertically forced down, therefore, the highest principal stress, which is the pushing force, is equal to the weight of the mass of the rock itself (Hyndman & Hyndman, 2009). In the perspective of thrusting, the mass of rock tends to escape by taking the upward direction, which is also the course of the minimum principal stress. The effect is that the mass of rock is lifted upwards and the overburden then equals the minimum principal stress Apart from tectonic plate collisions, earthquakes are also known to be related to volcanoes because they sometimes happen, and are also caused, around volcanic regions. Volcanic earthquakes occur in two main forms, namely long period earthquakes and volcano-tectonic earthquakes. Volcano-tectonic earthquakes are produced by changes in stress levels in solid rock because of the injection and extraction of molten rock, known as magma, causing the earth’s surface to subside, producing large cracks on the ground. Such earthquakes may occur as rock moves to fill in the voids left by withdrawal of magma. Although volcano-tectonic earthquakes are not an indication of oncoming volcanic eruptions, they may still occur anytime soon within the event of such earthquakes. Most of the earthquakes that occur directly underneath volcanoes are a result of the moving of magma, which in turn exerts pressure upon rocks until they crack. The magna then squirts its way into the cracks, starting the process of building up pressure again. This is a repetitive process, with an earthquake being cause each time the rock cracks (Schorlemmer, Wiemer & Wyss, 2005). Although such earthquakes may not often be felt because they are too weak, sensitive instruments can still detect and record them. When magma is injected into surrounding rock, it produces long period volcanic earthquakes. The changing pressure as the magma is unsteadily transported is what causes the earthquakes. When the injection of magma is prolonged, it can result in multiple earthquakes. Unlike in the volcano-tectonic type, this activity, known as volcanic tremor, is an indication of an imminent volcanic eruption. Conclusion It can be concluded that earthquakes are a result of the abrupt release of energy in the earth’s crust either after the collision of tectonic plates or volcanic eruptions. Whereas the collision of tectonic plates occurs on the outermost layer of the earth known as the crust, volcanic eruptions also capable of triggering earthquakes around volcanic regions. The sides of the tectonic plates, known as faults, often collide smoothly when they do not encounter irregularities. But when the irregularities occur, them sometimes stick together, store stress energy and later slip apart abruptly when the energy is sufficient, causing an earthquake. The volcanic type further occurs in two forms, which is either long period or volcano-tectonic earthquake. The volcano-tectonic type results from the continuous withdrawal of magma, which cracks the earth’s surface due to its subsiding. On the other hand, long period volcanic earthquakes are a result of injecting magma into the rock surrounding the areas of such activity. When the injection goes on for long periods, they result in continuous quakes and possible volcanic eruptions. References Coen, D. (2012). The earthquake observers: Disaster science from Lisbon to Richter. Chicago: University of Chicago Press. Hyndman, D. & Hyndman, D. (2009). Natural hazards and disasters. New York: Cengage. Roman, D. & Cashman, K. (2006). The origin of volcano-tectonic earthquake swarms. Geology, 34(6), 457-460. Schorlemmer, D., Wiemer, S., & Wyss, M. (2005). Variations in earthquake-size distribution across different stress regimes. Nature, 437(7058), 539–542. Tanimoto, T. & Lay, T. (2000). Mantle dynamics and seismic tomography. Proceedings of the National Academy of Science, U.S.A. 97(23), 12409–10. Read More