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Earthquakes and Earth’s Interior

3. Earthquakes and Earth’s Interior. Questions : How many have felt an earthquake? What is an earthquake (EQ)?. Earthquake Statistics 30,000 EQ’s strong enough to feel annually 75 significant EQ’s each year 1960 Southern Chile, M 9.5 1964 Alaska, M 9.2

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Earthquakes and Earth’s Interior

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  1. 3 Earthquakes and Earth’s Interior

  2. Questions: • How many have felt an earthquake? • What is an earthquake (EQ)? • Earthquake Statistics • 30,000 EQ’s strong enough to feel annually • 75 significant EQ’s each year • 1960 Southern Chile, M 9.5 • 1964 Alaska, M 9.2 • 2004, Sumatra-Andaman, M 9.1 5

  3. (M 9.5), So. Chile-1960 (M 9.2), Alaska-1964 subduction zone earthquakes (M 9.1), Sumatra-Andaman-2004 6

  4. How are earthquake vibrations similar to a rock entering water? Earthquake – vibrations within the earth that produce rapid release of energy in the form of seismic waves 7

  5. Is there evidence for earthquakes? San Francisco, 1906 8

  6. July 21, 1952 Bakersfield EQ 19th st and Chester 23 mi south of Bakersfield 19th street, Bakersfield

  7. Earthquake Terminology Focus actual rupture takes place Epicenter rupture point directly above the focus Seismic Waves vibrations released from an actual rupture 10

  8. I a good earthquake. Discuss with a friend: • How would you define an earthquake? • 2. About how many significant EQ’s • take place per year? • 3. Define the following: • epicenter seismic waves • focus I will get an A on my exams and quizzes. 11

  9. What is taking place? 12

  10. How do EQ’s occur? • The Elastic Rebound Theory: • Stresses begin to build and strain the crust. • Continued stress stores energy and begins to • reach the “elastic” limits of the rock. • The rock SNAPS, releasing stored energy • in the form of seismic waves. • Rock begins to reposition back into place, • producing aftershocks. 13

  11. “Good Vibrations” – released seismic waves • Three major seismic waves are released: • The P-wave • The S-wave • The L-wave (surface wave) 14 body waves • What measures the incoming seismic waves? • The Seismograph The Seismograph The inertia of the suspended mass keeps the seismograph motionless while the seismic waves vibrate the recording drum anchored to the bedrock – produces a seismogram.

  12. The P-wave • The P-wave (the primary wave) • compressional wave • compresses and expands rock material • fastest of the 3 seismic waves • typically 6 km/s in the upper crust • travels through both solids and liquids 15

  13. The S-wave 16 • The S-wave (secondary wave) • the shearing wave • moves particles up and down (right angles to • each other) • travels 2/3 the speed of the P-wave • typically moves about 3.5 km/s in the • upper crust • travels through solids ONLY

  14. The L-wave and Rayleigh wave (Surface Waves) 17 • The L-wave (Love Wave) • exhibits horizontal motion (like an S-wave) • travels the slowest • travels along the surface – producing horizontal • motion • The Rayleigh wave • travels similar to an ocean wave • the slowest wave • travels along the surface – producing a • rolling motion

  15. SAF Delano - Bako Surface Waves 18

  16. 19

  17. Seismogram 20

  18. I a good earthquake. Discuss with a friend: • 1. Explain the elastic rebound theory. • 2. Describe the motions of P, S, and surface • waves. • 3.Give three characteristics of each wave. I will get an A on my exams and quizzes. 21

  19. Did you feel the earthquake? – Measuring an EQ What’s the difference between an 8 magnitude and a 6.5 magnitude? Three ways to measure EQ’s: MercalliIntensity Scale Richter Scale Moment Magnitude Scale 22

  20. Modified Mercalli Intensity Scale • measures the intensity of the EQ – how much • damage occurs • closer to the epicenter-- more damage • farther from the epicenter -- less damage • people report the damage • destruction is assigned a Roman Numeral (I – XII) • I – least intensity • XII – greatest intensity Less intense More intense 23

  21. Richter Scale (Charles Richter – 1935) • measures the intensity of an EQ using a seismogram • maximum intensity = largest amplitude recorded • EQ intensity is based on the logarithmic scale. • each step (M1…M2…M3..) increases the amplitude X10 • M5 has an amplitude 10 times larger than an M4 • M5 has an amplitude 100 times larger than an M3 • What about the energy released? • Each increase in Richter amplitude (by 10) increases energy 32 times so, • An M6 releases 32 more times energy than an M5. • An M6 releases _____ more times energy than an M4. • An M6 releases _____ more times energy than an M3. • An M6 releases ________ more times energy than an M2. largest amplitude 1,024 32,768 1,048,576 What observations can you make about the increasing energy release from one magnitude to the next? 24

  22. Richter Magnitudes Per/year • < 2.0 generally not felt, but recorded 600,000 • 2.0 -2.9 potentially perceptible 300,000 • 3.0 -3.9 felt by some 49,000 • 4.0-4.9 felt by most 6,200 • 5.0 – 5.9 damaging shocks 800 • 6.0 – 6.9 destructive in populous regions 266 • 7.0 – 7.9 major EQ – inflicts serious damage 18 • 8.0 great EQ – destroys communities near epicenter 1.4 25

  23. M 6.6 M 4.2 M 5.6 M 7.6 M 8.0 M 7.8 26

  24. The Moment Magnitude Scale (MMS or Mw) • based on “work” done (Mw) • measures the energy released • based on the moment of the earthquake Rigidity of the earth Average amt. of slip and size of the area that slipped X Moment = (rock properties) • developed in the 1970’s – replaces the • Richter Scale • uses magnitude values defined by the • Richter Scale • used by the USGS folks 27

  25. Richter scale vs. Moment magnitude using different starting assumptions: 28 • Richter Scale: • assumes EQ focus is a point • best suited for EQ energy release from a • small area • EQ intensity is based on amplitude • measurements. • Moment Magnitude Scale: • assumes energy is released over a large area • measures the actual energy released and • considers the various rock types that waves • move through

  26. I a good earthquake. Discuss with a friend: • Explain the differences between the • Mercalli Intensity scale, Richter • scale and Moment scale. • 2. What’s wrong with the Mercalli Scale • when compared to the Richter scale? I will get an A on my exams and quizzes 29

  27. How do EQ’s provide scientists a look into the earth’s interior? • Seismic waves propagate through the earth’s • interior encountering different rock types. • Seismic waves behave differently depending on the materials • they pass through. • Seismic discontinuity: • A boundary inside the earth where the velocities and • directions of seismic waves change abruptly. • refracted: • waves are bent as they pass from one material • to another • reflected: • waves are bounced back • absorbed: • waves are blocked or impeded 30

  28. The Mohorovicic discontinuity (the moho) and asthenosphere Rocks are ductile with little strength (upper mantle) – low velocity zone The mantle: Mesosphere: 350km to the core mantle boundary is highly compressed rock (more dense) increasing seismic wave velocities. Seismic waves are refracted, or reflected changing the wave velocity within the material they pass through. The earth’s mantle has several seismic boundaries 32 Seismic discontinuities

  29. How do geologists understand the “layering” of our earth?? An earthquake takes place. P and S waves measured at seismic stations The absence of both P and S waves – NO WAVES The Shadow Zone 40o P-waves ONLY 33

  30. I a good earthquake. Discuss with a friend: • What is a seismic discontinuity? • Explain the differences between refraction, • reflection and absorbsion of seismic • waves. • 3. What seismic evidence suggests a low- • velocity zone within the upper mantle? • 4. What is the shadow zone? I will get an A on my exams and quizzes 34

  31. What is a tsunami? • an enormous ocean wave created from an earthquake • or an undersea volcanic eruption 1.0 2.0 3.0 Sea floor shifts, displacing water and creating tsunami. 4.0 5.0 6.0 7.0 35

  32. I A good earthquake • Explain why the shadow zone exists • 2. Why do P-waves occur on the other side • of the earth and S-waves do not? • 3. How is the earth’s interior layering • understood --- if a person has not been • there?????? Discuss with a friend I will get an A on my exams and quizzes 37

  33. Can earthquakes be predicted??????????? • At this time – EQ’s cannot successfully be • predicted. • In order to predict an EQ – science folks must • predict a small range of uncertainty • as to the location and timing. • Predictions must be able to produce few failures • and no false alarms. 38

  34. Can earthquakes be predicted along the SAF?? Seismologists warn that an EQ larger than 6.5 will occur along the San Andreas Fault. How do seismologists predict this warning? Bakersfield 39

  35. How geologists predict EQ’s Long Range EQ-Prediction – Seismic gap method • Prediction of an EQ is based on probability – What is the probability • of an EQ? • Statistical estimation when EQ’s take place over a given time • span (the frequency of EQ’s) • Assumption that EQ’s are cyclic -- repetitive Seismic gap method - SAF Based on studies of the San Andreas Fault, the probability of an EQ is: • Less than 10% in Northern CA • Parkfield (central CA) = 90% • Southern CA = 30% 40

  36. Use of the seismic gap method along the SAF Cyclic EQ’s in Parkfield,CA 1857 1881 1901 1922 1934 1966 1 EQ about every 22 years Cyclic EQ M6 or greater rupturing in the same area of the fault zone Parkfield Bakersfield 90% The next predicted EQ before 1993 DID NOT HAPPEN! 90% chance of a major EQ 41

  37. EC – Did the EQ on 9/28 reduce the probability of 90%? www.quake.usgs.gov/recenteqs/ September 28, 2004 M6 at Parkfield 42

  38. I a good earthquake Discuss with a friend: • Can EQ’s be predicted? – Why or Why not? • 2. Draw a picture of CA and show where • the San Andreas fault is located. • 3. Explain how the seismic-gap method • works when predicting earthquakes. I will get an A on my exams and quizzes 43

  39. Structural Geology Faults 44

  40. Structural Geology – Earth Deformation Fault Folds 45

  41. How is rock deformed? (stress and strain relationships) • Stress • force (pressure) acting on the rock surface • Strain • a change in the shape (deformation) to the • response of stress Compression Tensional undisturbed cube Shear

  42. Faults Joint – fracture within a rock body Fault – movement along a fracture or joint fault B Joint B A A 47

  43. Displacement

  44. I A good earthquake • Describe the differences between stress • and strain • 2. Describe the stress types compression • tensional, and shear • Draw a fault block and label: • hanging/footwalls • fault plane Discuss with a friend I will get an A on my exams and quizzes 50

  45. Normal Fault • HW down relative to FW • Tensional stress • Extension of crust FW HW FW HW • Reverse Fault • HW up relative to FW HW • Compression stress • Shorting of crust FW 52

  46. Vertical type faults FW HW FW HW Reverse faulting (compression of crust) Normal faulting (extension of crust) FW HW FW HW 53

  47. Strike-slip faulting: two blocks sliding past one another Map View Turned to Right Turned to Left

  48. What is the type of strike-slip fault? Right-Lateral – strike slip

  49. Wallace Creek Right-Lateral Strike slip San Andreas Fault – Wallace Creek, Carrizo Plane 56

  50. What type of fault ? Normal fault 57

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