Assignment 5-1 PHY103

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Southern New Hampshire University *

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PHY103

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Geology

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Jan 9, 2024

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Sara Rigby Assignment 5-1 PHY103 I. Earthquakes An earthquake is a rapid release of energy cause by the movement of crustal plates that generates waves of vibration (Lutgens, 2021). Along the faults or weak regions of tectonic plates, stress builds up gradually over hundreds of years. This is all released in an instant with a slipping or breaking motion, releasing the shockwaves we know as an earthquake. Earthquakes are happening consistently in the deep parts of the Earth, but they do not often reach the surface (Lutgens, 2021). There are two main categories of seismic waves, body waves, and surface waves (Lutgens, 2021). Body waves can move through the interior of the Earth, while surface waves remain along the upper layers of the crust. Body waves are broken down into further categories, primary or P waves and secondary or S waves. P waves are the first waves to be recorded, followed by secondary and lastly surface waves. Out of all three of these types of waves, the most damaging are the surface waves because they have the greatest amplitude (Lutgens, 2021). Seismographs measure these waves using inertia, the body of the instrument moves with the waves while the sensor above it remains suspended and stationary, the difference between the two is what ends up

being recorded. (Lutgens, 2021). The seismograph uses the nature of the different types of waves to triangulate where the epicenter of the quake occurred. The intensity of an earthquake is the measurement of the amount of ground shaking at a location. The magnitude of an earthquake is an estimate of the how much energy was released (Lutgens, 2021). The Richter scale uses the maximum amplitude of the seismic waves and the distance of the waves to the seismograph to measure the intensity of the quake. The moment magnitude scale was created to help differentiate between larger earthquakes, it uses the strength of the rock, amount of slippage, and area of the fault to measure the magnitude of the energy released (Lutgens, 2021). Faults are weakened points in rock where a fracture has occurred, and movement is occurring on both sides of it. Faults can be placed into four common categories. A normal fault moves in the same direction as the fault itself. The hanging wall block moves down relative to the footwall (Lutgens, 2021). This type of fault is most common in divergent boundaries. In a reverse fault, the motion is the opposite of a normal fault, instead the hanging wall block moves up relative to the footwall (Lutgens, 2021). A thrust fault is the same as a reverse fault, but it occurs at a much lower angle. Convergent boundaries are most often associated with reverse and thrust faults. Strike-slip faults move in the direction of the fault trace (Lutgens, 2021). Transform faults are where strike-slip faults are located. When Earthquakes occur, they often leave a path of death and devastation in their wake. As time has gone on humans have tried to prepare themselves for these natural disasters. In studying earthquakes, scientists have learned what causes the most destruction and have tried to prevent it to the best of their abilities. Intensity and length of shaking is one of the top three

destruction factors (Lutgens, 2021). When an earthquake shakes the earth, that includes buildings as well. Tall buildings don’t have much support to keep them from swaying, they will often collapse, burying the people inside. The structure of the ground beneath the building itself can be a big factor in how it is affected in a quake. If sediment is waterlogged, severe shaking can cause liquefaction, reducing its ability to support buildings. Landslides and tsunamis are also triggered by earthquakes (Lutgens, 2021). Both act like waves, burying anything in its path, the biggest difference is, one is made of the earth itself and the other is made of water. The circum-Pacific belt is the most seismically active area in the world. This includes some of the coastline along the Pacific Coast (Lutgens, 2021). Specifically, the San Andreas Fault near the cost of California has been known to produce destructive earthquakes . The San Andreas fault is a transform fault. The two plates located here are slipping past each other and a release of energy that is long overdue could cause extreme devastation. An earthquake here could easily rupture gas lines leading to fires and cause landslides. This is mostly due to the large population that lives right along this fault. Because of the unpredictability of earthquakes, it is impossible to be completely prepared for this and it could take many people by surprise. I live in Eastern Washington. We have had earthquakes in my lifetime, but only one of them has ever been large enough for me to feel. The epicenter often lies across the Cascade Mountains and is far enough away that we would not see much damage here. However, they are frequent and the Juan de Fuca plate off the coast is a hotspot for seismic activity in the US. We do have several faults that are closer to us, but they are not as violent or frequent as the ones on the coast. There is also earthquakes that emanate from the many volcanoes in the Cascade range.

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