Earthquakes & Volcanos - CFrench

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Southern New Hampshire University *

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103

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Geography

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Apr 3, 2024

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Earthquakes Earthquakes are a natural process that occur due to the interactions between cracks in the Earth's crust, known as plates, and where these plates meet are known as fault boundaries. Earthquakes are caused due to pressure between two plates along their boundary, and when that pressure reaches a point where it the stores energy overpowers the standard friction between those two plates, the overflow of power that is the result is what creates the earthquake. These typically happen along a convergent boundary, where two faults are pushing into each other, as the tension build up causing one fault to jump over the other once the tension has released. Earthquakes occur thousands of times a day, but stronger earthquakes, such as a magnitude 7, only happen around five times a year (Lutgens et al., 2021, p. 114) . The stored energy referenced previously is known as seismic waves, which are used to detect when an earthquake is coming, how we measure its magnitude, and where it is happening by using a seismograph. These seismic waves begin at a hypocenter, where the buildup releases in the fault line, and creates the epicenter on the surface directly above it. These seismic waves roll out like water wades and ripples out when you throw a pebble in a pond. Finding the center of these waves is how we find the location of where the earthquake began. Seismic waves are made up of two different types of waves, body, and surface. Body waves are broken down into primary, or p, and secondary, or s, waves. Primary waves are known as the push and pull waves, as they push and pull rocks in the direction that the waves are travelling, and travel through solids, liquids, and gases. The actions are similar to stretching out a slinky and pushing it in a bit before pulling it back out. The motion of one section moving along the length of the slinky is similar to how p waves act. Secondary waves shake objects in the way the waves are moving, similar to shaking one end of a rope when the other side is stationary. The wave that ripples through the rope is act that the s waves take on. When it comes to surface waves, there are also two waves. One causes everything on Earth's surface to move up and down, like an object caught in a wave, and the other makes the Earth's surface move side to side. A seismograph records the first p wave, followed by the first s wave, and then reads the surface waves. The order of the waves read are due to how they travel. P waves can move through any material, so they travel the fastest, and s waves work slightly slower as they are not able to move through gases as easily, and surface waves are read last as they typically can only be read through the Earth's surface (Lutgens et al., 2021, p. 114) . The term for what the seismograph reads is magnitude. It is the measurement of the earthquakes size, which is read by the height of the ripples on a seismograph (Lutgens et al., 2021) . In other words, it is a measurement of the amount of energy that is released during an earthquake. The more energy released, the more dangerous it will be. The intensity of an earthquake, however, is a measurement of the effects of the earthquakes (British Geological Survey 1998 - 2019, n.d.) . This measurement is done after the earthquake has ended, and the intensity does not always directly correlate to the magnitude, as geography and distance, among other factors, play a role in how much damage is done. Both magnitude and intensity are used to measure earthquakes, but what they measure are two different aspects and have two different measuring devices.

Faults, as mentioned previously, are where tectonic plates meet. Faults are measured by their dip, the angle, and their slip, which is the direction. The movement along the dip and slip is categorized between normal or reverse, also known as thrust. Dip-slip faults move along a dip plane in some degree of vertical and slide against one another, while strike-slip faults move in some degree of horizontal and slide past one another. Dip-slip faults are considered normal, whereas strike-slip faults are considered thrust faults. This type of fault is common with convergent boundaries, like the fault line Japan rests on. A normal fault is where the hanging block of rock above an inclined fault moves down relative to the block of rock that lies below. These typically occur on divergent boundaries. A reverse, or thrust, fault, is the opposite of a normal fault. Instead of the hanging block moving down, it moves up, and happen along convergent boundaries. Strike-slip faults are also broken down into two categories, left-lateral and right-lateral. Left-lateral strike-slip faults is where the displacement is on the left far block, and the right-lateral is when this displacement is on the right. The most well-known right0lateral strike-slip fault is the San Andreas Fault. In 1906, and earthquake occurred and displaces objects on the fault line as much as 15 feet. The displacement was movement to the right, which made it right-lateral, and it is a strike-slip because the plates moves horizontally (Lutgens et al., 2021, p. 133); ( What Is a Fault and What Are the Different Types? , n.d.) . Most commonly, natural disasters kill 10,000 to 20,000 people a year, with earthquakes and cyclones bringing that total up to tens of hundreds of thousands (Ritchie, 2024). Deaths and damage from earthquakes don't happen directly from the ground moving, but what that motion causes. The shaking can cause buildings and walls to collapse, objects to fall, and objects like glass to be sent flying. This is where the death and damage come from (Maine Emergency Management Agency, 2020). The extent of this destruction and harm depends on many factors, including magnitude, time of day, intensity, geology, and duration, to name a few. According to the World Health Organization, earthquakes caused nearly 750,000 deaths globally and left 125 million effected by being injured, displaces, or homeless (2019). "Forty-five states and territories in the United States are at a moderate to very high risk of earthquakes" (Maine Emergency Management Agency, 2020). The circum-Pacific seismic belt is where roughly 81% of Earth's highest magnitude earthquakes occur. This belt has earned the nickname of "Ring of Fire" due to the belt traveling along the east coast of the western hemisphere continents and looping around to continue along the western coast of the eastern hemisphere continents ( Where Do Earthquakes Occur? , 2012). This long range makes it the biggest global earthquake risk. When it comes to the United States, Alaska lies on the Alaskan- Aleutian Megathrust. Alaska accounts for 11 percent of the world's earthquakes, 17.5 percent of earthquakes in the U.S., with an average of a magnitude 7.0-8.0 earthquake every year and one greater than 8.0 happening every 13 years ( Which U.S. State Has the Most Earthquakes? , n.d.). When it comes to the conterminous United States, California takes first place for the most earthquakes. The earthquakes are formed from the San Andreas Fault System, which is a transform strike-slip fault. In addition to this, the Alaskan-Aleutian Megathrust and the San Andreas Fault are a part of the Ring of Fire. Both of these faults are where the Pacific plate meets the North American Plate ( Plate Tectonics and the Ring of Fire , n.d.). As the name suggests, Alaskan-Aleutian Megathrust is a right-lateral strike-slip fault, and as previously mentioned, the San Andreas Fault is a strike-slip fault as well. The biggest dangers

that lie with this is fault is the collapse of bridges and buildings. There are also minor changes, such as railroad tracks bending and shifting, but left unchecked can cause train derailment, adding onto the hazardous dangers that already exist. Strike-slip faults have some of the most dangerous aftershocks of any faults. The reason being that they are unpredictable and can have a large magnitude, causing structures weakened by the main shock to collapse (Lay et al., 2013) . The biggest earthquake to ever hit the United States was in Alaska in 1964. A 9.2 magnitude earthquake hit the lower region of Alaska, causing not only a terrible earthquake, but a tsunami as well. Combined, they took 131 lives, 112 from the tsunami and nine from the earthquake. It also caused 311 million dollars’ worth of damage, 1.1 billion in today’s money. Furthermore, Anchorage, some 120 kilometers away, sustained heavy landfalls and 30 blocks of property being destroyed, including one that overtook 130 acres ( Largest Earthquake in Alaska , n.d.) . Farmington, Missouri has a low risk of earthquakes, with only 19 earthquakes since 1931 and a 4.06% chance of a 5.0 earthquake happening within 50 km in the next five years. Also, the highest magnitude earthquake to occur was a 3.5 in 1976 that was 24 km away (Homefacts.com, n.d.) . Volcanoes Volcanoes are vents that are found In Earth's crust. The cone shaped volcanoes that we picture when we think of them, is formed when lava cools at the surface and hardens on top of the magma that has already solidified at the surface building up the shape of the volcano (Lutgens et al., 2021) . These vents and magma formations are formed from divergent boundaries. These two plates pulling away from each other is what allows these vents to form, and the magma flows up. There are currently 1,350 active volcanoes worldwide ( How Many Active Volcanoes Are There on Earth? | U.S. Geological Survey , 2019) . "Shield volcanoes are produced by the accumulation of fluid basaltic lavas and exhibit the shape of a broad, slightly domed structure that resembles a warrior's shield" (Lutgens et al., 2021). These volcanoes typically form as seamounts, and some will grow large enough to form volcanic islands, such as the Hawaiian volcanic system. These can also be characterized by slow mounds, as the lava flows very "fast and far". Cinder volcanoes build from "ejected basaltic lava fragments that begin to harden in flight, producing the vesicular rock scoria" (Lutgens et al., 2021). They are mostly composited of these scoria’s; they also can make extensive lava fields. And are known for their steeper slopes and their craters, which are topically formed from one high eruption (Lutgens et al., 2021). Composite volcanoes are also known as stratovolcanoes, the ones that we picture in our minds when we think of the word. They are very large and nearly symmetrical structures that have been built up by many layers of magma from eruptions. These shapes are from slower forming magma, the opposite of shield volcanoes (Lutgens et al., 2021). According to plate tectonics, the location and frequency of volcanism are formed from the plate division and the creation of the deep plumes found around Earth's crust. Volcanoes are most frequently found in the same place Earthquakes occur, the Ring of Fire. The name actually comes from the varying lava pools that are there that have related these volcanoes. Divergent plate boundaries, allowing for these lava pools to come forward. Convergent plate boundaries

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