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Earthquakes 5/23/2017 ©1999 John Culpepper 1 Earthquakes • The movement of tectonic plates creates stress on large areas of rock • Stress within rocks can be relieved by – Bending – Stretching – Breaking 5/23/2017 ©1999 John Culpepper 2 Earthquakes • Occur when stress is relieved by rocks breaking 5/23/2017 ©1999 John Culpepper 3 Earthquakes (cont.) • Usually occur along faults and plate boundaries • Energy is rapidly released in all directions (causing waves) – Like a pebble dropped in a quiet pool of water 5/23/2017 ©1999 John Culpepper 4 Types of Faults • Normal Fault – Hanging wall slides down the fault plane, – Formed by divergent boundary 5/23/2017 ©1999 John Culpepper 5 Reverse Fault (Thrust Fault) Hanging wall slides up the fault plane Formed by convergent boundary 5/23/2017 ©1999 John Culpepper 6 • Strike-slip Fault – Hanging wall and foot wall slide horizontally to the fault plane, – Formed by transform boundary 5/23/2017 ©1999 John Culpepper 7 Important Terms • Fault- a weakness or break in the Earth’s crust • Focus- the place underground where the break occurs (actual location of the earthquake) • Epicenter- the location on the Earth’s surface just above the focus (map location of an earthquake) 5/23/2017 ©1999 John Culpepper 8 Seismic waves • In an earthquake 3 kind of waves are produced – P-waves – S-waves – Surface waves 5/23/2017 ©1999 John Culpepper 9 P-waves (primary waves) • Travel through solids and liquids • Fastest moving seismic waves 5/23/2017 ©1999 John Culpepper 10 S-waves (secondary waves) • Travel only through solid material • Move slower than Pwaves 5/23/2017 ©1999 John Culpepper 11 Surface waves • Do the most damage and move the slowest 5/23/2017 ©1999 John Culpepper 12 5/23/2017 ©1999 John Culpepper 13 Seismic waves are measured by seismometers • Seismic recordings are called seismograms or seismographs • Scientists who study earthquakes are called Seismologists 5/23/2017 ©1999 John Culpepper 14 5/23/2017 ©1999 John Culpepper 15 Earthquakes (cont.) • The exact time of an earthquake is known as its origin time • Earth’s outer core is liquid – Only P-waves can travel through 5/23/2017 ©1999 John Culpepper 16 Earthquakes (cont.) • Moho discontinuity -the boundary between Earth’s crust and mantle – Seismic waves speed up here 5/23/2017 ©1999 John Culpepper 17 Earth’s Interior • Within the Earth density, pressure, and temperature all increase with depth 5/23/2017 ©1999 John Culpepper 18 5/23/2017 ©1999 John Culpepper 19 Faults • Normal fault – Found in areas under tension • Where sections of the landscape are pulling apart Called Fault Block Mountains 5/23/2017 ©1999 John Culpepper 20 Faults (cont.) • Reverse fault – Found in areas under compression • Zones of convergence Like the Himalaya Mountains 5/23/2017 ©1999 John Culpepper 21 Faults (cont.) • Strike-slip faults – More horizontal movement than vertical movement Like the San Andreas fault in California 5/23/2017 ©1999 John Culpepper 22 Shadow zone • A zone on the opposite side of the earth from where an earthquake has occurred where seismic waves will not be felt – Due to the way that seismic waves travel through the different materials within the Earth 5/23/2017 ©1999 John Culpepper 23 5/23/2017 ©1999 John Culpepper 24 5/23/2017 ©1999 John Culpepper 25 Earthquake damage is measured by 2 scales: • Richter Scale 1-10 each increase of 1=10 fold increase in shaking (exponential) • Mercalli Scale 1-7 based on the damage to buildings and observed effects 5/23/2017 ©1999 John Culpepper 26 5/23/2017 ©1999 John Culpepper 27 5/23/2017 ©1999 John Culpepper 28 Deep, Intermediate, & Shallow Earthquakes • Deeper usually = stronger 5/23/2017 ©1999 John Culpepper 29 • Shallow EQ’s occur nearer the trench and deeper occur farther inland • This is due to subducting oceanic crust 5/23/2017 ©1999 John Culpepper 30 EQ 5/23/2017 ©1999 John Culpepper 31 Typical Seismogram 5/23/2017 ©1999 John Culpepper 32 How is an Earthquake’s Epicenter Located? 5/23/2017 ©1999 John Culpepper Seismic wave behavior – P waves arrive first, then S waves, then L and R – Average speeds for all these waves is known – After an earthquake, the difference in arrival times at a seismograph station can be used to calculate the distance from the seismograph to the epicenter. 33 How is an Earthquake’s Epicenter Located? Time-distance graph showing the average travel times for P- and Swaves. The farther away a seismograph is from the focus of an earthquake, the longer the interval between the arrivals of the P- and S- waves 5/23/2017 ©1999 John Culpepper 34 How is an Earthquake’s Epicenter Located? • • • Three seismograph stations are needed to locate the epicenter of an earthquake A circle where the radius equals the distance to the epicenter is drawn The intersection of the circles locates the epicenter 5/23/2017 ©1999 John Culpepper 35 Earthquake Practice: • How many seismic stations do you need to locate the epicenter of an earthquake? • An earthquake occurs 8000 km away, how long will it take P-waves to reach you? S-Waves? • P waves arrived at a seismic station at 12:03, Swaves arrived at 12:07. How far is the epicenter? 5/23/2017 ©1999 John Culpepper 36