This research paper discusses the phenomenon of earthquakes – the geography and distribution of where they occur, and the explanations of the forces and processes involved. The theory explains why they occur more in some places than others and describes related phenomena including volcanic activity.
The accepted theory of the cause of most earthquakes cites the continuous movements of huge sections of the earth’s crust (known as the lithosphere), which comprises tectonic plates (Earthquakes and Plate Tectonics” 1977), which are further classified as either oceanic or continental plates, dependent on their location on the Earth’s surface: beneath the seas or the land masses. The figure below shows the global boundaries of these tectonic plates:
Tectonic Plate Boundaries - Extracted from: “Earthquakes and Plate Tectonics” 1977
Earthquakes do not occur in random locations around the world, but instead are concentrated in zones around the boundaries of these tectonic plates, which is also where mountains and volcanoes are most likely to be found.
Sinvhal, in her book “Understanding Earthquake Disasters” (2010), describes particular earthquake-prone regions in two areas of the planet known as seismic belts. These are known as the Circum Pacific Belt (or ring of fire) and the Alpine-Himalayan Belt respectively, and are areas where major earthquakes are more likely to occur repeatedly, causing considerable losses of life among what she calls “vulnerable populations” (p. 1).
As explained by Rosenberg (n.d.), the so-called “Ring of Fire” is an arc that extends from New Zealand to Asia’s eastern edge, then continues northwards over the Alaskan Aleutian Islands and in a southerly direction to the coasts of both the North and South Americas. That arc is where three quarters of the world’s volcanoes (both active and dormant) are located and where many earthquakes occur. The reason is that it represents the boundaries of major tectonic plates including the Pacific Plate, which is colliding with other tectonic plates and sliding under them, in a process called subduction. The energy generated gives rise to earthquakes and molten magma which finds its way to the Earth’s surface and creates volcanoes.
Tectonic plates move above a lower, mobile level of the Earth’s outer layers called the asthenosphere. The slow but ongoing motion of the tectonic plates creates huge forces and pressures, particularly when friction between adjacent plates causes them to stick, building up the pressure until it reaches a point where sudden movement occurs, resulting in an earthquake (Earthquakes and Plate Tectonics” 1977). In the case of submarine earthquakes, the resultant shockwave can cause a tsunami, flooding the nearest coastal areas.
As far as these oceanic plates are concerned, a particular cause of the earthquakes is that the ridges that occur in mid-ocean are cracks in the sea bed that fill with rising molten material that forces them further apart as it solidifies, putting additional pressure on the plate boundaries with the continental plates (“Earthquakes and Plate Tectonics” 1977).
This action is nicely illustrated in the following diagram:
Plate Tectonics Explained - Extracted from “Plate Tectonics in a Nutshell” 2008
Referring to the numbers in the above diagram:
(1) Indicates the oceanic and continental elements of the lithosphere. Note that the oceanic part is comprised of heavier and therefore denser materials. The plates of the lithosphere float on the semi-solid asthenosphere (2). High pressures and temperatures deep in the asthenosphere cause the rock to become soft and partly molten, which allows slow movement to occur in the form of convection currents (3). Those currents allow hot material to rise from deeper in the mantle (4).
Nearing the surface, those currents diverge exerting a pulling action on the oceanic plate, which splits apart, forming submarine ridges or splits in what is known as a “divergent boundary” (5). Molten rock rising through the splits solidifies, in a continuous process over time (6). The plate is caused to expand as a consequence of this ongoing process (7). Over time, the cooling plate becomes denser and therefore heavier away from the hot ridges, eventually being heavier then the asthenosphere below (8). As it sinks into the asthenosphere, it creates what is called a “subduction zone” (9). The effect is that the sinking leading edge of the oceanic plate drags the rest of the plate after it, probably remaining in its solid form for more than 100 km below the surface of the Earth (10). Although the denser cooling oceanic plate is sinking at this “convergent boundary”, the lighter continental plate floats above the asthenosphere (11). As the sinking oceanic plate gets deeper and deeper, high temperatures and pressures “sweat” fluids out of it, which rise up, tending to cause local melting of the solid mantle, creating liquid pockets of rock or magma (12). These magma pockets rise towards the surface but mostly solidify, forming the cores of mountains such as the Sierra Nevada or the Andes along the subduction zones near the plate boundaries (13). Where that magma remains liquid until it reaches the surface, the pressure is suddenly released as volcanic activity. When this occurs repeatedly over time, volcanic mountain ranges are formed, such as the Cascades and the Columbia River Plateau (14).
McKenzie (1969) states that observations confirm that earthquakes occur predominantly in areas where the mantle temperature is below a certain defined temperature (p. 1) and that the different compositions of oceanic and continental plates indirectly cause the movements at plate boundaries (p. 29).
An interesting phenomenon associated with earthquakes is reported by Fountain (July 2013), whose New York Times article states that not only can earthquakes cause structural damage, landslides and tsunamis, but – according to scientists – can also cause methane gas to be released from the ocean floor. Their findings originate from a 1945 underwater earthquake off the coast of Pakistan, which appears to have caused methane gas to seep up through cracks created. If this is a widespread phenomenon, scientists believe that source of methane gas should be taken into account in climate change predictions; although it could be that much of the gas becomes dissolved in the sea before it can rise to surface level.
As regards the numbers of earthquakes annually, USGS data suggests that the average numbers of earthquakes of magnitude 7.0 or higher remain fairly constant and may in fact have decreased recently (“Are Earthquakes Increasing?” n.d.). The article states that the reason there seem to be more in recent years is that there are more and better seismographic stations nowadays, and the public get to hear about them sooner. The following plot diagram shows – for the thirty-year period 1969-1999 – the numbers of major earthquakes (red) and great earthquakes (blue), where “major” = magnitude 7.0-7.9, and “great” = 8.0 or above.
(Extracted from “Are Earthquakes Increasing?” n.d.)
“Are Earthquakes Increasing?” (n.d.). Discerning the Times. Web. 5 November 2013.
“Earthquakes and Plate Tectonics.” (1977). Earthquake Information Bulletin, vol. 9, no. 6, November - December 1977, by Henry Spall, USGS, Reston, VA. Web. 3 November, 2013.
Fountain, Henry. (July 2013). “Quakes May Help Release Methane.” New York Times. Web. 5 November 2013.
McKenzie, D., P. (1969). “Speculations on the Consequences and Causes of Plate Motions.” Geophys. J. R. astr. SOC. (1969), 18, 1-32. Web. 3 November 2013.
“Plate Tectonics in a Nutshell.” (2008). USGS. Web. 3 November 2013.
Rosenberg, Matt. (n.d.). “Pacific Ring of Fire: Home to Earthquakes and Volcanoes of the Earth.” Geography.About.com. Web. 21 November 2013.
Sinvhal, Amita. Understanding Earthquake Disasters. New Delhi: Tata McGraw Hill Education Private Limited, (2010). Print.