Chapter 8 A Violent Pulse: Earthquakes
Chapter 8 A Violent Pulse: Earthquakes Geology 101
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This 6 page Class Notes was uploaded by Alanna Wight on Friday December 4, 2015. The Class Notes belongs to Geology 101 at Washington State University taught by Wilkie in Fall 2015. Since its upload, it has received 14 views. For similar materials see Geology in Geology at Washington State University.
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Date Created: 12/04/15
Lecture Notes Chapter 8 A Violent Pulse Earthquakes Key Points for today Earthquakes What are they what causes them and where are they likely to occur Be familiar with earthquake vocabulary What is the di erence between an earthquake epicenter and focus How are earthquakes located and what is meant by there magnitude Why do earthquakes a ect areas di erently Earthquake related websites http Vcourseware3 calstatela eduVirtualEarthq uakeVQ uakeExecute html http earthq uakeus gs govrecenteq swwindexhtml Why should you be concerned with earthquakes Does the west coast of WA OR CA have anything to worry about Why would they What are your perceptions of an Earthquake What will you feel How might it affect you What are some of the hazards associated with an earthquake Earthquakes and Plate Tectonics are linked Earthquakes occur at all boundary types modern and ancient and in some cases Within the plates 95 of energy from earthquakes affect narrow zones along plate edges Some earthquakes are quite deep subduction zones What causes earthquakes Earthquakes are vibrations of the ground created by the sudden release of strain energy accumulating in deformed rocks Where does that strain energy come from It is the accumulation of stress energy stored as the rocks are deformed by tectonic pressure As the rocks are deformed strain energy is stored When enough stress is applied so that it overcomes the strength of the rock the rock breaks producing a fault That sudden release of strain energy moves out in all directions from the break as seismic waves of an earthquake Earthquakes are generated by all fault types Normal Reverse Thrust and Strikeslip What is an earthquake Fault crack in Earth Where slip occurs Earthquake slippage along a fault Earthquake focus hypocenter fault slip location Epicente point on the surface directly above the focus Fault Kara fault scarp cliff formed from vertical slip on fault aftershock small earthquakes that follow an initial earthquake in same vicinity foreshocks small earthquakes that sometimes precede a large one by few days Earthquakes occur at all different fault types Normal Reverse Thrust and Strikeslip Seismic Waves The strain energy released by an earthquake is transmitted through the rocks in all directions in the form of waves Seismology is the study of earthquakes and is based on the 2 main types of seismic waves Body Waves Surface Waves Body Waves P Waves Primary Waves waves expand and contact compressional fastest wave 67 kmsec 4 mjsec S Waves Secondary Waves waves move updown side to side slower than P wave 35 kmsec2 misec Surface Waves slowest wave 25 kmsec 15 misec travel along surface of the earth side to side up and down Measuring Earthquakes Seismograph Device that records seismic wave motion Seismogram record of shaking Recipe for locating an earthquake s epicenter Pwaves fastest amp Swave travel at different speeds So you can use this difference to locate earthquakes 1 Measure time between P and S wave on seismogram 2 Use traveltime graph to get distance to epicenter need seismograms from at least 3 locations 3 Draw circle on a map with radius of that distance 4 Three or more circles should intersect at EQ Measuring Earthquakes Modi ed Mercalli scale based on relative destruction and observations by people Delineates 12 intensity levels The Richter scale scale of 110 based on amplitude on seismogram each number represents the shaking power 33X increase in energy released with each number Works best on California rocks The Richter scale uses the maximum amplitude to determine the earthquake s magnitude at a standard distance form the earthquake epicenter 100km Moment Magnitude depth of fault total amount of slip movement on fault strength of rock type The Richter and Moment magnitude produce roughly the same numerical value the moment magnitude scale is preferred by seismologists Average Earthquakes per year Magnitude E ects Averageyr lt25 not felt but 900000 recorded 25 60 minor 31000 61 69 potentially 100 destructive 70 79 major earthquakes 20 gt 80 great earthquakes 1 every 5 years Examples of Recent Earthquakes Nisqually 68 WA 2001 Spokane 40 WA 2001 Northridge 68 CA 1994 Loma Prieta 71 CA 1989 Kobe 72 Japan 1995 Why would earthquakes with nearly the same intensities magnitude produce such different results 1 Different type of earth movement is possible with different faults Earthquakes are associated with all plate tectonic boundaries both active and ancient as well as other areas Normal Divergent shallow earthquakes Strikeslip Transform shallow earthquakes Reverse Convergent shallow to deep earthquakes Hotspots shallow earthquakes 2 The ground can amplify the shaking Building on bedrock is the best Building on land ll is the worst 3 Depth to the focus 4 Duration of earthquake shaking 5 quotConstructive wave interferencequotor quotlensingquot Comparing Northridge and Kobe earthquakes there are many similarities the exact numbers are not important Northridge Kobe Magnitude 68 71 Duration 15 sec 20 sec Depth 11 miles 11 miles Estimated Kobe Earthquake damages totaled 95 billion to 147 billion many times the 15 billion damage in icted by the 1994 Northridge Earthquake the exact numbers are not important The physical properties are not the only factor that a ect how much damage is done to an area by an earthquake Man plays the largest role