The Cause of Earthquakes

When people hear the word earthquake, most of them would probably say it is ‘the shaking of the ground’ or ‘buildings collapse during earthquakes,’ while some would probably describe it in one word: chaos, catastrophe, etc. You get the picture. People have such a negative connotation on earthquakes because, indeed, it has only brought damage to property and surroundings. Unfortunately, earthquakes are a natural phenomenon that man is unable to control, thwart or prevent. Thus, at the present, the only thing he could probably do is to try to minimize its damage by trying to know more about earthquakes, and then acting according to that knowledge.

Definition of an earthquake.

To begin this research paper, one has to define what an earthquake is as scientists have understood it. Thus, earthquakes are a form of energy that is released suddenly by the shifting or colliding of crustal plates on the surface of the earth. On the other hand, it could also be the energy released by the fracture of a stressed rock formation located in the crust (Earthquake, 2007) or the result of a volcanic eruption. The latter reason, however, is not always the case.

Also, earthquakes are measured by magnitude and may be described via the Richter magnitude scale or the Modified Mercalli Intensity Scale, which will be further discussed later on. With the magnitude, one may know how serious an earthquake can be. Thus, when reporters talk about an earthquake, they would often include that it has a magnitude of 6.3 or 7.0, for example, with the larger magnitude being the more serious one.

Major recorded earthquakes

Earthquakes probably existed ever since Earth came to being. Geologists studying prehistoric soils have discovered different layers of soil that are usually observed on the aftermath of an earthquake (BC Geological Survey, 2006). In the last century until 2005, 22 major earthquakes have been recorded as listed by the United States Geological Survey (2005). In that list, they’ve also included the location of the earthquakes, the date it occurred, its magnitude and the corresponding number of deaths each earthquake caused. The largest earthquake recorded during that time occurred at Sumatra, Indonesia on December 26, 2004 and had a magnitude of 9.0. This earthquake also caused around 283, 106 deaths, including those caused by the tsunami triggered by the earthquake. However, the earthquake that caused the largest number of deaths, an estimate of 655, 000, happened at Tangshan China in 1976. This earthquake had a magnitude of 7.5. Another article by BBC News 24 (2007) lists three more earthquakes that happened last year. One was in western Iran (magnitude 6.0) and killed 70 people and injured 1,200, and two in Java, Indonesia (6.2 and 7.7) that killed nearly 5,000 and 650 people, respectively. Then just last August, an undersea earthquake with a magnitude of 7.9 caused at least 436 deaths in Ica, Peru.

Measurement of an earthquake

As mentioned earlier, earthquakes are measured by magnitude which increases logarithmically as the size of the earthquake increases. Now, this magnitude is calculated by using specific seismic waves or the duration of such waves (Earthquake, 2007). Then, the Richter and Mercalli scales are used to describe the magnitude and intensity of an earthquake, respectively.

The Richter Magnitude scale measures the magnitude or the strength of a particular earthquake and is the most commonly used. “The magnitude value is proportional to the logarithm of the amplitude of the strongest wave during an earthquake.” (MATTER Project, 1999) Table 1 shows the values of the Richter scale and its corresponding effects.

Table 1. Richter Magnitude scale (MATTER Project, 1999).

On the other hand, the Modified Mercalli Intensity (MMI) scale measures the intensity of the damage caused by an earthquake at a certain location as felt and observed by people near it. Although there have been many scales developed to describe the intensity of earthquakes, the MMI scale is the one currently used in the U.S. The upper half of the intensity scale (I-VI) describes how the people on the earthquake site felt it while the lower half (VII-XII) pertains to the damage caused in the area as observed by the people (US Geological Survey, 2006). Table 2 shows the MII scale and its corresponding values.

Table 2. Modified Mercalli Intensity scale (US Geological Survey, 2006).

Locating an earthquake

When one wants to find the location of an earthquake’s origin, one is basically looking for its epicenter. An earthquake’s epicenter is the location on the Earth’s surface that is directly above its focus, which is the area within Earth where the shifting/colliding of crustal plates take place (Brunious & Warner, 1996). To locate the epicenter of an earthquake, one can use the process called Triangulation. This process uses the distance information of the earthquake collected by three seismic stations. The distance information is calculated from the P-waves and S-waves that were recorded in a seismogram by each seismic station. Then, three circles with each station as the center are drawn on a map. The radius of each circle is the estimated distance of the station to the earthquake. The point of intersection of the three circles on the map is the estimate location of the epicenter of the earthquake (Brunious & Warner, 1996).

Just to be clear, P-waves and S-waves are types of shockwaves. According to The Geography Site (2006), a P-wave, or primary wave, is characteristically similar to a sound wave. It is a longitudinal wave that has a high frequency, thus a short wavelength. P-waves can travel through solids and liquids. It is also responsible for the small displacements of the ground since it can move the ground forward, if compressed, or backward, if decompressed. In addition, such waves can be reflected, refracted and sometimes change into S-waves. On the other hand, S-waves are transverse waves that are also high in frequency and have a short wavelength. They are typically slower than P-waves and thus arrive right after it. S-waves also move away from its source but in all directions, its velocity depending on the densities of the rocks it is moving through. Unlike the P-waves, though, they cannot travel through liquids. S-waves cause sideways movement, thus it is the reason why a previously straight fence is bended into an s-shape after an earthquake.

There is, however, another shockwave called an L-wave besides the two previously discussed. L-waves, or surface waves, are also transverse waves but these are low in frequency thus have long wavelengths. These waves are observed near an earthquake’s epicenter and can only move through the crust’s outer layers. Also, since L-waves move in a circular motion, the ground in turn rises and falls according to it. Thus, they are the cause of most of the damages in buildings and when further combined with an S-wave, can cause fires, landslides, and the worst, tsunamis (The Geography Site, 2006).

Causes of Earthquakes

Geologists explain that there are two main causes of earthquakes: tectonic activity and volcanic activity. The first one, tectonic activity, was derived mainly from the theory of plate tectonics. Thus, to understand what tectonic activity is and how it causes an earthquake, one must understand first the theory behind it.

Tectonic activity

According to Kious & Tilling (2001), a tectonic plate, also called a lithospheric plate, is an irregularly shaped, but really huge slab of solid rock made up of continental and oceanic lithosphere. If one would remember, the Earth is made up of the crust, mantle and the core. The lithosphere, however, consists of the crust and the first layer of the mantle, while directly below it is the asthenosphere. This other layer of the mantle is believed to be a mobile zone made up of hot, semi-solid materials that could soften and flow when subjected to extreme conditions. It is also believed that this is the force that propels the lithosphere, which is broken into tectonic plates, to move around. Before these facts were discovered, the theory of continental drift and theory of plate tectonics were mostly rejected even if the theories seemed to answer a lot of questions during that time.

The theory of continental drift says that the continents of today were actually part of one bigger continent, Pangaea, while the theory of plate tectonics, says that the outermost layer of Earth is divided into a number of small and large plates that move in a relative pace with one another on top of a hotter and more movable substance. Thus, with the acceptance of the two theories by the scientific community, it was now used to explain occurrences like the specific areas in which earthquakes and volcanic eruptions only occurred.

Now with the theory of plate tectonics, scientists have named the plates that consist the crust of the Earth, namely: Australian, Pacific, Antarctic, Nazca, South American, Scotia, African, Indian, Arabian, Caribbean, Cocos, Juan de Fuca, Philippine, Eurasian, and North American plates, with the last six plates located north of the equator and the rest located south of the equator. But the shapes and sizes of these plates change over time. Because of a plate’s composition, the plates “…can either drift towards each other, away from each other or slide past each other.” (The Geography Site, 2006) For example, the Juan de Fuca plate, a part of the larger Farallon oceanic, is believed to someday vanish as it sinks or submerges under the North American plate (Kious & Tilling, 2001).

Most earthquakes are then believed to occur in the areas wherein plate boundary activities occur, namely, in divergent, convergent and transform boundaries. In divergent plate boundaries, two plates move away from each other causing an oceanic ridge, like the Mid-Atlantic Ridge. Earthquakes produced in these boundaries have relatively low foci, about 20 km and lesser. When two plates slide horizontally past each other, then they are called transform boundaries (Nelson, n.d.). An example of such causing earthquakes is at the San Andreas Fault, California. In this area, the Pacific and North American plate are both moving in the northwest direction with one of it moving faster. The plates grinding each other are the reason for the many small earthquakes that the San Francisco area experiences each year. Then, when of the plates produce a bigger movement, a more violent earthquake, like the one that happened in 1989, could occur (The Geography Site, 2006). Then, there are the convergent plate boundaries wherein two plates collide into each other.

It is at these places that there are compressional stresses. Furthermore, there are two types of convergent plates, subduction boundaries and collision boundaries. Subduction boundaries usually occur along ocean trenches wherein the oceanic lithosphere is forced down towards the mantle while the other plate is above. “Because the subducted lithosphere is cold it remains brittle as it descends and thus can fracture under the compressional stress. When it fractures, it generates earthquakes that define a zone of earthquakes with increasing focal depths beneath the overriding plate.” (Nelson, n.d.) Such earthquakes have focal depths of around 700km. On the other hand, collision boundaries cause mountain belts because of two continental plates or lithosphere colliding into each other. Earthquakes produced in these areas have a ranging from shallow to nearly 200km.

Volcanic activity

The other way earthquakes are produced is via volcanic activity. However, the number of earthquakes produced this way is very small. Only volcanoes that erupt in an explosive manner, like ones that usually produce acidic lava, can trigger an earthquake. Since acidic lava cool and settle very fast, it tends to block the volcanic vent preventing the pressure from escaping. Because of this, pressure builds up until some part of the volcano explodes out of the way. Furthermore, if an extraordinary amount of pressure builds up, the resulting explosion would expectedly be very violent and can then cause an earthquake. An example of this is when Mt. Krakatoa erupted in 1883. Its explosion was so loud that people 5000km away still heard it. In addition, the shockwaves it produced caused tsunamis that were 36m high causing nearly 36,000 deaths (The Geography Site, 2006).

Conclusion

Earthquakes are a natural phenomenon that is a part of Earth’s processes. As such and since it is a constant cause of climactic destruction, claiming lives, homes, and properties, man has studied it for many years trying to understand its processes. However, even with the vast knowledge about it and technology thus developed, man has not been able to fully develop a way to be completely safe from it. Such cannot be a reason to stop studying it though, for with the current information about earthquakes, many people have also been saved. The knowledge of where earthquakes are more likely to occur can help the inhabitants of that area to prepare for it by improving the designs of buildings and bridges and constantly reminding the inhabitants on how to act in the event that one does finally occur.

References

(2007, October 3). Earthquake. City of Kent Washington. Retrieved November 8, 2007 from http://www.ci.kent.wa.us/emergency/KentHazards/EARTHQUAKE.pdf

(2006, February 22). What causes an earthquake? The Geography Site. Retrieved November 8, 2007 from http://www.geography-site.co.uk/pages/physical/earth/cause.html

BBC News 24. (2007, August 16). Deadly history of earthquakes. BBC News. Retrieved November 8, 2007 from http://news.bbc.co.uk/1/hi/world/2059330.stm

BC Geological Survey. (2006, March 8). What is a megathrust earthquake? British Columbia. Retrieved November 8, 2007 from http://www.em.gov.bc.ca/Mining/Geolsurv/Surficial/quake/eq4.htm

Brunious, C. & Warner, A. (1996). Earthquakes and society. University of Michigan. Retrieved November 8, 2007 from http://www.umich.edu/~gs265/society/earthquakes.htm

Kious, W.J & Tilling, R.I. (2001). This Dynamic Earth: the Story of Plate Tectonics (P. Lindeberg, Ed.). Washington: US Government Printing Office.

MATTER Project. (1999). Scales for measuring earthquakes. Matter. Retrieved November 8, 2007 from http://www.matter.org.uk/Schools/Content/Seismology/richterscale.html

Nelson, S.A. (not dated). Earthquakes. Earth Science Australia. Retrieved November 8, 2007 from http://earthsci.org/education/teacher/basicgeol/earthq/earthq.html

Ortiz, A.M. & Bravo, T.K. (2005). Earthquake location: Triangulation with real data [Electronic Version].  Los Alamos, New Mexico: Science Education Solutions.

United States Geological Survey. (2005, October 8). History of major earthquakes. rediff.com India. Retrieved November 8, 2007 from http://in.rediff.com/news/2005/oct/08quake13.htm

United States Geological Survey. (2006, November 17). The Modified Mercalli Intensity Scale. USGS. Retrieved November 8, 2007 from http://earthquake.usgs.gov/learning/topics/mercalli.php