ElijahMiniukBlock 7WhartonThe Effect of Earthquakes and Why They HappenThe Ring of Fire is a major area inthe basin of the Pacific Ocean where a high percentage of earthquakes andvolcanic eruptions occur. Within a 40,000-kmhorseshoe shape that is casted upon the perimeter of the Pacific plate, it isassociated with a nearly continuous series of oceanic trenches, volcanic arcs,volcanic belts, and plate movements (Wikipedia, 2017).
Four-hundred-fifty-twovolcanoes are in this mountain chain that rests atop the basin of the Pacific (What is the “Ring of Fire?”, 2017), and the reason whya large majority of volcanoes are a part of this volcanic mountain chain is dueto the constant subduction of the tectonic plates. In addition, many earthquakes are the resultof the constant subduction. Subductionis 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 ofthe crust and upper mantle) is forced under another plate (What is the “Ring of Fire?”, 2017).
Three types of earthquakes are produced dependingupon the interaction of the tectonic plates, divergent plate boundary,convergent plate boundary, and the transform plate boundary. A divergent boundary occurswhen two tectonic plates move away from each other. Along these boundaries, lava spews from longfissures creating new crust. As themagma solidifies it transforms into a dark, dense rock called Basalt (NOAA, 2013). Convergent plate boundaries happen when twoplates collide together. When the platesstrike, one plate rises upward while the other down.
Typically, when this happens a mountain ridgeis formed parallel to the boundary. Also, a typical volcano can be formed thisway. Powerful earthquakes shake a widearea on both sides of the boundary (NOAA, 2013). The phenomenon when two plates slide againstone another is known as a transform plate boundary. As the plates alternately press in apropagational direction against each other, earthquakes are produced through awide boundary zone. In contrast toconvergent and divergent boundaries, no magma is formed. Thus, crust is crackedand broken at transform margins, but is not created or destroyed (NOAA, 2013). When one of the three platescreate tension, the focal point of an earthquake is determined, also known asthe epicenter.
The focal point refers tothe 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 itcommonly is classified as mid-focus or intermediate-depth earthquakes. But in subduction zones, where older andcolder oceanic crust descends beneath another tectonic plate, deep-focus earthquakes may occur at much greater depths in the mantle, ranging from 300-km upto 700-km.
Generally, the shallower the earthquake the more damaging theyare (William Spence, 1989).Duringthe formation of an earthquake’s hypocenter or focus point, fault lines formfrom 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 horizontalmovement of rocks but have little to no vertical movement. Normal Faults are likewise to divergent plateboundaries, because the earth’s crust is pulled away manipulating the landscape. Reverse Faults, also called thrust faults, slide one block of crust on topof another. These faults are commonly found in collisions zones, where tectonicplates push up mountain ranges (Oskin, 2017).Whenan earthquake ensues, there aretwo main categories of seismic waves, body waves and surface waves.
Body waves are of two types: compressional or primarywaves and shear or secondary waves. P-waves and S- waves are called “bodywaves” because they can travel through the interior of a body such as theEarth’s inner layers, from the focus of an earthquake todistant points on the surface (BRAILE, 2007). P-waves are the fastest and strongest seismicwave being able to travel through solid rock and liquid. They travel by vibrating in a fixed directionof propagation called a longitudinal wave (BRAILE, 2007). S-waves are the second waves recorded by theseismograph. They travel in a series ofundulated motions called transverse waves. S-waves also possess a weaker frequency withit only capable of traveling through solid rock rather than a liquid medium andare the aftershock of the P-waves (BRAILE, 2007).
Asurface wave is a seismic wave that is trapped near the surface ofthe Earth. Though it is the slowest of the waves, it causes greatdestruction when it breaches the surface. Whenthese waves are recorded they are categorized into different levels ofmagnitude. Themagnitude of an earthquake is used to describe the intensity and relative sizeof an earthquake with each level being ten times stronger than the previouslevel. Earthquakes with a magnitude of2.5 or less on the Richter Scale is considered minor and cannot be felt on thesurface. When the magnitude reaches 2.5to 5.
4 minor shakes can be felt. As anearthquake’s magnitude increases from then on, from 5.5 to 6.
0 minor damage isinflicted 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 themovement 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 thatalters the stresses and strains on the Earth’s crust. Most induced seismicity is of a low magnitude. Examples of inducedearthquakes are waste water disposal wells. It is a common misconception that fracking is the major influence ofinduced earthquakes, but wastewater disposal wells typically operate forlonger durations and inject much more fluid than hydraulic fracturing, makingthem more likely to induce earthquakes (USGS, n.d.
). For example, in Oklahoma, which has the mostinduced earthquakes in United States, only 1-2% of the earthquakes can belinked to hydraulic fracturing operations. The remaining earthquakes areinduced by wastewater disposal.
Butmost injection wells are not associated with felt earthquakes. A combination ofmany factors is necessary for injection to induce felt earthquakes. Theseinclude: the injection rate and total volume injected; the presence of faultsthat are large enough to produce earthquakes that are felt; stresses that arelarge enough to produce earthquakes; and the presence of pathways for the fluidpressure to travel from the injection point to faults (USGS, n.
d.). Other examples that result in inducedearthquakes are artificial lakes, mining, and geothermal energy. Another way in which an earthquake can beproduced is through volcanic activity. Earthquakesrelated to volcanic activity may produce hazards which include ground cracks,ground deformation, and damage to manmade structures. There are two generalcategories of earthquakes that can occur at a volcano: volcano-tectonicearthquakes and long period earthquakes.Earthquakes produced by stress changes insolid rock due to the injection or withdrawal of magma are called volcano-tectonicearthquakes (Chouet, 1993).
These earthquakes can cause land tosubside and can produce large ground cracks. These earthquakes can occur asrock is moving to fill in spaces where magma is no longer present.Volcano-tectonic earthquakes don’t indicate that the volcano will be eruptingbut can occur at any time.The second category of volcanic earthquakes are long period earthquakes whichare produced by the injection of magma into surrounding rock.
These earthquakesare a result of pressure changes during the unsteady transport of the magma.When magma injection is sustained a lot of earthquakes are produced (Chouet,1993). This type of activity indicates that a volcano is about to erupt.Scientists use seismographs to record the signal from these earthquakes. Thissignal is known as volcanictremor.