learning exercise 5

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East Carolina University *

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1500

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Geology

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Apr 3, 2024

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Dynamic Earth GEOL-1500 Spring 2024 Earthquakes and Seismic Waves Learning exercise Name: Haley Young Date: Feb 27 Learning Goals: • What is fault? • Fault types • Normal, reverse, strike-slip • Theory of elastic rebound • Why earthquakes occur • Different types of seismic waves • P, S, Love, Rayleigh • How to use them to locate an earthquake • Travel time curves and three station method Part 1 Faults and earthquakes: 1. What is a fault? A fracture or zone of fractures between two blocks of rock 2. On Earth, where are faults likely to occur? Along divergent plate boundaries like under the ocean where crust is forming 3. What is the difference between an active and an inactive fault? Active faults are structure along where expected displacements occur, structures that do not produce earthquakes are inactive faults 4. What physical property of rocks along a fault zone allows Earthquakes to occur? Earthquakes occur when energy is stored in elastically strained rocks. 5. What is the overall theory controlling why earthquakes occur repeatedly along plate boundaries? Illustrate this theory by drawing a fence crossing a fault at three successive time steps. The Elastic rebound theory 1

6. A) What kind of fault is this? B) Label the hanging wall and footwall. C) What plate boundary would you expect to find this kind of fault? a. reverse fault c. convergent plate boundries 7. A) What kind of fault is this? B) Label the hanging wall and footwall. C) What plate boundary would you expect to find this kind of fault? a. normal fault c. divergent plate boundries 8. Draw in the fault on the diagram below. What kind of fault is it? Include relative motion arrows. What is the relative motion on this fault (e.g. right or left lateral)? Strike slip fault rightr hanging footwall hanging footwall 2

Part 2 Seismic waves and Earthquakes: 9: List the main body waves. P waves S waves 10: List the main surface waves. Love waves Rayleigh waves 11. Draw a seismogram that includes P-waves, S-waves and surface waves. Label the different waves. P waves s waves surface waves 12. What seismic wave would be expected to arrive first? How about last? P waves arrive first surface laft 13. What type of material would stop the propagation of an S-wave? Fluids or gas 14. Draw a picture that illustrates the difference between a seismic ray path and a wave front. Seismic ray 3

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Wave front 15. Triangulation is a method that uses distance information determined from 3 seismic stations to uniquely locate the earthquake. On a map, circles are drawn around each seismic station. The radius of the circle are scaled to the estimated distance from the station to the earthquake. The 3 circles will share one unique intersection that locates the earthquake. • On each of the attached seismograms determine the time of the P and S arrivals. The name of the station is represented with a three letter code on the seismogram. Record your answers below. Pasadena, California (PAS) P Wave Arrival Time (seconds) ______45______________ S Wave Arrival Time (seconds) _______95_____________ • Calculate S - P Time (subtract P time from S time) ____50____ Dugway, Utah (DUG) P Wave Arrival Time (seconds) _______78_____________ S Wave Arrival Time (seconds) ________135____________ • Calculate S - P Time (subtract P time from S time) ___57_____ Berkley, California (CMB) P Wave Arrival Time (seconds) _______40_____________ S Wave Arrival Time (seconds) _______50_____________ • Calculate S - P Time (subtract P time from S time) ___10_____ • For each station, determine the distance from the station to the event. This is done using the formula D epicenter = (S-P time) * 8 km/s. This approximation is valid for seismic waves traveling in the upper crust with a P-wave velocity of 5.5 km/s and S-wave velocity of 3.26 km/s. • Multiply the S - P times for each station by 8. Record the distance below. (PAS) distance (km) ______400___________ (DUG) distance (km) ______456___________ 4

(CMB) distance (km) ______80___________ Earthquake Location • If you have picked P and S correctly and drawn circles accurately all of the circles will overlap at one point. The point where all of the circles overlap is the approximate epicenter of the earthquake. • Determine the Latitude and Longitude of the earthquake from the map and record it below. Latitude _______117 w_____________ Longitude ________37 n___________ 5

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learning exercise 5

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