Understanding Earthquakes: Causes, Measurements, and Utah's Seismic History, Lecture notes of History

Download Understanding Earthquakes: Causes, Measurements, and Utah's Seismic History and more Lecture notes History in PDF only on Docsity! All About Earthquakes: The Science Behind Earthquakes What is an earthquake? An earthquake is what happens when two blocks of the earth suddenly slip past one another. The surface where they slip is called the fault or fault plane. The location below the earth’s surface where the earthquake starts is called the hypocenter, and the location directly above it on the surface of the earth is called the epicenter. Sometimes an earthquake has foreshocks. These are smaller earthquakes that happen in the same place as the larger earthquake that follows. Scientists can’t tell that an earthquake is a foreshock until the larger earthquake happens. The largest, main earthquake is called the mainshock. Mainshocks always have aftershocks that follow. These are smaller earthquakes that occur afterwards in the same place as the mainshock. Depending on the size of the mainshock, aftershocks can continue for weeks, months, and even years after the mainshock! What causes earthquakes and where do they happen? The earth has four major layers: the inner core, outer core, mantle and crust. (figure 2) The crust and the top of the mantle make up a thin skin on the surface of our planet. But this skin is not all in one piece – it is made up of many pieces like a puzzle covering the surface of the earth. (figure 3) Not only that, but these puzzle pieces keep slowly moving around, sliding past one another and bumping into each other. We call these puzzle pieces tectonic plates, and the edges of the plates are called the plate boundaries. The plate boundaries are made up of many faults, and most of the earthquakes around the world occur on these faults. Since the edges of the plates are rough, they get stuck while the rest of the plate keeps moving. Finally, when the plate has moved far enough, the edges unstick on one of the faults and there is an earthquake. Why does the earth shake when there is an earthquake? While the edges of faults are stuck together, and the rest of the block is moving, the energy that would normally cause the blocks to slide past one another is being stored up. When the force of the moving blocks finally overcomes the friction of the jagged edges of the fault and it unsticks, all that stored up energy is released. The energy radiates outward from the fault in all directions in the form of seismic waves like ripples on a pond. The seismic waves shake the earth as they move through it, and when the waves reach the earth’s surface, they shake the ground and anything on it, like our houses and us! How are earthquakes recorded? Earthquakes are recorded by instruments called seismographs. The recording they make is called a seismogram. The seismograph has a base that sets firmly in the ground, and a heavy weight that hangs free. When an earthquake causes the ground to shake, the base of the seismograph shakes too, but the hanging weight does not. Instead the spring or string that it is hanging from absorbs all the movement. The difference in position between the shaking part of the seismograph and the motionless part is what is recorded. How do scientists measure the size of earthquakes? The size of an earthquake depends on the size of the fault and the amount of slip on the fault, but that’s not something scientists can simply measure with a measuring tape since faults are many kilometers deep beneath the earth’s surface. So how do they measure an earthquake? They use the seismogram recordings made on the seismographs at the surface of the earth to determine how large the earthquake was (figure 5). A short wiggly line that doesn’t wiggle very much means a small earthquake, and a long wiggly line that wiggles a lot means a large earthquake. The length of the wiggle depends on the size of the fault, and the size of the wiggle depends on the amount of slip. The size of the earthquake is called its magnitude. There is one magnitude for each earthquake. Scientists also talk about the intensity of shaking from an earthquake, and this varies depending on where you are during the earthquake. How can scientists tell where the earthquake happened? Seismograms come in handy for locating earthquakes too, and being able to see the P wave and the S wave is important. You learned how P & S waves each shake the ground in different ways as they travel through it. P waves are also faster than S waves, and this fact is what allows us to tell where an earthquake was. To understand how this works, let’s compare P and S waves to lightning and thunder. Light travels faster than sound, so during a thunderstorm you will first see the lightning and then you will hear the thunder. If you are close to the lightning, the thunder will boom right Liquefaction Maps Liquefaction Potential Map for Cache Valley Cache County, Utah Utah Geological Survey Public Information Series 79 August 2003 Liquefaction Potential || High HEE Moderate - High [0 Moderate | Moderate - Low HE iw | Very Low See reverse ‘side for explanation ‘This map is for general reference only ‘and was modified from Anderson, LR. ‘Keaton, JJR., and Bay, J.A, 1984, Liquefaction potential map for the rnorthem Wasatch Front, Utah: Utah Geological Survey Contract Report 8448, 148 p., scale 148,000, Digitally compiled by Kami Bremser ‘and Deanna Haleeth Utah Geological Survey LIQUEFACTION-POTENTIAL MAP FOR A PART OF DAVIS COUNTY, UTAH UTAH GEOLOGICAL SURVEY Public Information Series 24 August 1994 WEBER COUNTY % LIQUEFACTION POTENTIAL a High § Moderate | Low ES Very Low Landslides GREAT SALT LAKE Pd ao es vt x “4 a »” : «4 7 % v” SCALE 1:200,000 a bie es tsp ms 10 1 °2 3.4 5 6 7 8 _ 9 KILOMETERS Digital compilation by Janine L. Jarva, Utah Geological Survey, facilitated by Automated Geographic Reference Center This map is for general reference only and was modified from Anderson, L.R., Keaton, J.R., Aubry, Kevin, and Ellis, S.J., 1994, Liquefaction potential map for Davis County, Utah: Utah Geological Survey Contract Report 94-2, 50 p., scale 1:48,000. LIQUEFACTION-POTENTIAL MAP FOR A PART OF SALT LAKE COUNTY, UTAH UTAH GEOLOGICAL SURVEY Public Information Series 25 August 1994 LIQUEFACTION POTENTIAL x High ois oe : BB Moderate sf 2a 4 a! ra ial cet ¢ GREAT a v ( Sabi; Salt Lake City. ¢ LAKE & § Tailings Pond $ Canyon s 5400 S Big Cottonwood i) West a Cottonwood cay 2 Sandy @ wa Rivel gd raper y $ P~ a »< 1 as 4 ® - ir + ss “a 2 sf ya Digital compilation by Janine L. Jarva, Soneet-200/00 Utah Geological Survey, facilitated by 5 ° 5 10 mites Automated Geographic Reference Center 6 o 6 10 16 KILOMETERS LIQUEFACTION-POTENTIAL MAP FOR A PART OF WEBER COUNTY, UTAH UTAH GEOLOGICAL SURVEY Public Information Series 27 August 1994 LIQUEFACTION POTENTIAL & High a Moderate i] Low al Very Low SCALE 1:200,000 Landslides BOX ELDER COUNTY DAVIS COUNTY Digital compilation by Janine L. Jarva, Utah Geological Survey, facilitated by Automated Geographic Reference Center This map is for general reference only and was modified trom Anderson, L.R., Keaton, JR., and Bay, JA. 1994, Liquefaction potential map for the northem Wasatch Front, Utah: Utah Geol Survey Gs Contract Report 94-5, 150 p., scale 1:48,000. Copies of this report are available at the Utah Geological Survey All About Earthquakes: Utah Earthquakes 1.) Utah Earthquake History -July 18, 1894: Ogden, VI-VII (Mag 5.0) -August 1, 1900: Santaquin, VI-VII -November 13, 1901: Parowan-Richfield, VIII -November 14, 1901: Parowan-Richfield Aftershocks, VII -November 17, 1902: Pine Valley, St. George, Santa Clara, VII -October-December 1909: Series of 30-60 quakes within Garland and Tremonton, VII -May 22, 1910: Salt Lake City, VII (Mag 5.0) -May 13, 1914: Ogden, VII (Mag 5.5) -September 29, 1921: Elsinore, Monroe, Richfield, 2 strong earthquakes 12 hours apart, VIII -March 12, 1934: Kosmo, shore of Great Salt Lake, VIII (Mag 6.6) -August 30, 1962: Franklin, Logan, Preston, Richmond (Cache County) Mag 5.7 -October 4, 1967: Marysvale, Mag 5.2 -March 28, 1975: Idaho-Utah border, Mag 6.1 **See more info at: http://earthquake.usgs.gov/earthquakes/states/utah/history.php 2.) Largest Earthquake in Utah -March 12, 1934: Hansel Valley, near Kosmo (30 miles north of Great Salt Lake) Mag 6.6 ace as rats The Salt Lake Tribune newspaper University of Utah Libraries CIMT Ce 0 Pe ates a aa me TTL Newspaper Articles: http://www.seis.utah.edu/Iqthreat/nehrp_htm/1934hans/n1934ha1.shtml#efhy