The Effect of Earthquakes and Why They Happen
The Ring of Fire is a major area in the basin of the Pacific Ocean where a high percentage of earthquakes and volcanic eruptions occur. Within a 40,000-km horseshoe shape that is casted upon the perimeter of the Pacific plate, it is associated with a nearly continuous series of oceanic trenches, volcanic arcs, volcanic belts, and plate movements (Wikipedia, 2017). Four-hundred-fifty-two volcanoes are in this mountain chain that rests atop the basin of the Pacific (What is the “Ring of Fire?”, 2017), and the reason why a large majority of volcanoes are a part of this volcanic mountain chain is due to the constant subduction of the tectonic plates. In addition, many earthquakes are the result of the constant subduction.
Subduction is a geological occurrence that happens when one plate of oceanic lithosphere (the term lithosphere is used to describe the rigid outer part of the earth, which consists of the crust and upper mantle) is forced under another plate (What is the “Ring of Fire?”, 2017). Three types of earthquakes are produced depending upon the interaction of the tectonic plates, divergent plate boundary, convergent plate boundary, and the transform plate boundary. A divergent boundary occurs when two tectonic plates move away from each other. Along these boundaries, lava spews from long fissures creating new crust. As the magma solidifies it transforms into a dark, dense rock called Basalt (NOAA, 2013). Convergent plate boundaries happen when two plates collide together. When the plates strike, one plate rises upward while the other down. Typically, when this happens a mountain ridge is formed parallel to the boundary. Also, a typical volcano can be formed this way. Powerful earthquakes shake a wide area on both sides of the boundary (NOAA, 2013). The phenomenon when two plates slide against one another is known as a transform plate boundary. As the plates alternately press in a propagational direction against each other, earthquakes are produced through a wide boundary zone. In contrast to convergent and divergent boundaries, no magma is formed. Thus, crust is cracked and broken at transform margins, but is not created or destroyed (NOAA, 2013).
When one of the three plates create tension, the focal point of an earthquake is determined, also known as the epicenter. The focal point refers to the depth at which an earthquake is initiated. Earthquakes happen at three depths. Shallow earthquakes occur in depths less than 70-km. When the depth is between 70-km and 300-km it commonly is classified as mid-focus or intermediate-depth earthquakes. But in subduction zones, where older and colder oceanic crust descends beneath another tectonic plate, deep-focus earthquakes may occur at much greater depths in the mantle, ranging from 300-km up to 700-km. Generally, the shallower the earthquake the more damaging they are (William Spence, 1989).
During the formation of an earthquake’s hypocenter or focus point, fault lines form from the direct result of sliding plates. There are three variants of fault lines: Strike-Slip Faults, Normal Faults, and Reverse Faults (Oskin, 2017). Strike-Slip Faults indicate the horizontal movement of rocks but have little to no vertical movement. Normal Faults are likewise to divergent plate boundaries, because the earth’s crust is pulled away manipulating the landscape. Reverse Faults, also called thrust faults, slide one block of crust on top of another. These faults are commonly found in collisions zones, where tectonic plates push up mountain ranges (Oskin, 2017).
When an earthquake ensues, there are two main categories of seismic waves, body waves and surface waves. Body waves are of two types: compressional or primary waves and shear or secondary waves. P-waves and S- waves are called “body waves” because they can travel through the interior of a body such as the Earth’s inner layers, from the focus of an earthquake to distant points on the surface (BRAILE, 2007). P-waves are the fastest and strongest seismic wave being able to travel through solid rock and liquid. They travel by vibrating in a fixed direction of propagation called a longitudinal wave (BRAILE, 2007). S-waves are the second waves recorded by the seismograph. They travel in a series of undulated motions called transverse waves. S-waves also possess a weaker frequency with it only capable of traveling through solid rock rather than a liquid medium and are the aftershock of the P-waves (BRAILE, 2007). A surface wave is a seismic wave that is trapped near the surface of the Earth. Though it is the slowest of the waves, it causes great destruction when it breaches the surface. When these waves are recorded they are categorized into different levels of magnitude.
The magnitude of an earthquake is used to describe the intensity and relative size of an earthquake with each level being ten times stronger than the previous level. Earthquakes with a magnitude of 2.5 or less on the Richter Scale is considered minor and cannot be felt on the surface. When the magnitude reaches 2.5 to 5.4 minor shakes can be felt. As an earthquake’s magnitude increases from then on, from 5.5 to 6.0 minor damage is inflicted upon buildings with greater magnitudes from 6.1 to 8.0 or greater, major damage is induced on structural foundations and land (MichiganTech, n.d.).
There are other ways an earthquake can be produced other than from the movement of the tectonic plates. Earthquakes can be of the direct result of human activity. These man-made tremors are called induced seismicity. Induced seismicity refers to typically minor earthquakes and tremors that are caused by human activity that alters the stresses and strains on the Earth’s crust. Most induced seismicity is of a low magnitude. Examples of induced earthquakes are waste water disposal wells. It is a common misconception that fracking is the major influence of induced earthquakes, but wastewater disposal wells typically operate for longer durations and inject much more fluid than hydraulic fracturing, making them more likely to induce earthquakes (USGS, n.d.). For example, in Oklahoma, which has the most induced earthquakes in United States, only 1-2% of the earthquakes can be linked to hydraulic fracturing operations. The remaining earthquakes are induced by wastewater disposal. But most injection wells are not associated with felt earthquakes. A combination of many factors is necessary for injection to induce felt earthquakes. These include: the injection rate and total volume injected; the presence of faults that are large enough to produce earthquakes that are felt; stresses that are large enough to produce earthquakes; and the presence of pathways for the fluid pressure to travel from the injection point to faults (USGS, n.d.). Other examples that result in induced earthquakes are artificial lakes, mining, and geothermal energy. Another way in which an earthquake can be produced is through volcanic activity. Earthquakes related to volcanic activity may produce hazards which include ground cracks, ground deformation, and damage to man-made structures. There are two general categories of earthquakes that can occur from a volcano: volcano-tectonic earthquakes and long period earthquakes. Volcano-tectonic earthquakes are ensued from the slippage on a fault near a volcano (PNSN, n.d.). Volcanoes are often found in areas of crustal weakness and the mass of the volcano itself adds to the regional strain. Long period earthquakes are produced by vibrations generated by the movement of magma or other fluids within the volcano. Pressure within the system increases and the surrounding rock fails, creating small earthquakes (PNSN, n.d.).