Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
March 1, 2010 Agenda: • Eprep 4/5 completed in boxes • Discuss SAF field trip • Pick up remaining quizzes/exams • begin discussion of EQ’s • Eprep 6/7 due 3-3-10 1 San Andreas Fault Appreciation Day Fun filled day of fault finding Field Trip – March 6, 2010 • Leave BC at 7:00 PROMPLY • Carpooling a must • FT – itinerary online = 3-2-10 • from start (BC) to Parkfield 168.5 miles • 100 points EC/ student 2 3 Earthquakes and Earth’s Interior • Describe an earthquake. • seismic waves released • elastic rebound theory • understand earthquake terms GOALS • Understand the characteristics of the San Andreas Fault (SAF) and earthquake prediction. • Explain how seismic waves predict the “make-up” of the Earth’s interior. • Understand the relationship between “stress” and “strain” relating to basic structural geology. • Describe basic fault structures and their associated stresses. 4 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 (M 9.5), So. Chile-1960 (M 9.2), Alaska-1964 (M 9.1), Sumatra-Andaman-2004 6 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 Is there evidence for earthquakes? San Francisco, 1906 8 More evidence for earthquakes --- Los Angeles 9 Earthquake Terminology Focus actual rupture takes place Epicenter rupture point directly above the focus Seismic Waves vibrations released from an actual rupture 10 I a good earthquake. Discuss with a friend: 1. 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 What is taking place? 12 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 “Good Vibrations” – released seismic waves Three major seismic waves are released: •The P-wave body waves •The S-wave •The L-wave (surface wave) 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. 14 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 The S-wave 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 16 The L-wave and Rayleigh wave (Surface Waves) The L-wave (Love Wave) 17 •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 Surface Waves 18 19 Seismogram 20 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 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: Mercalli Intensity Scale Richter Scale Moment Magnitude Scale 22 Modified Mercalli Intensity Scale • measures the intensity of the EQ – how much damage occurs •closer to the epicenter-- more damage Less intense •farther from the epicenter -- less damage • people report the damage • destruction is assigned a Roman Numeral (I – XII) •I – least intensity •XII – greatest intensity More intense 23 Richter Scale (Charles Richter – 1935) • measures the magnitude of energy released • based on the largest amplitude of P,S,or surface wave • based on the logarithmic scale: • 10-fold increase in the amplitude of the wave signal • 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 step in the Richter Scale = 32-fold energy increase so, •An M6 releases 32 more times energy than an M5. 1,024 more times energy than an M4. •An M6 releases _____ •An M6 releases 32,768 _____ more times energy than an M3. •An M6 releases 1,048,576 ________ more times energy than an M2. What observations can you make about the increasing energy release from one magnitude to the next? 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 M 6.6 M 4.2 M 5.6 M 7.6 M 8.0 M 7.8 26 The Moment Magnitude Scale (MMS or Mw) •based on “work” done (Mw) •measures the energy released •based on the moment of the earthquake Moment Rigidity of the earth Average amt. of slip X = and (rock properties) Size of the area that slipped • developed in the 1970’s – replaces the Richter Scale • uses magnitude values defined by the Richter Scale • used by the USGS folks 27 Richter scale vs. Moment magnitude using different starting assumptions: Richter Scale: • assumes EQ focus 28 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 I a good earthquake. Discuss with a friend: 1. 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 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: 30 waves are blocked or impeded Seismic waves are bent as the wave propagates into another media (due to a density change). Seismic waves are bounced at the interface of two different materials. 31 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 Seismic discontinuities 32 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 I a good earthquake. Discuss with a friend: 1. What is a seismic discontinuity? 2. Explain the differences between refraction, reflection and absorbsion of seismic waves. 3. What seismic evidence suggests a lowvelocity zone within the upper mantle? 4. What is the shadow zone? I will get an A on my exams and quizzes 34 What is a tsunami? an enormous ocean wave created from an earthquake or an undersea volcanic eruption 2.0 1.0 3.0 4.0 5.0 7.0 6.0 Sea floor shifts, displacing water and creating tsunami. Would not want to be these folks! 36 I A good earthquake 1. 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 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. Imagine this situation: Evacuating Los Angeles? • the cost of evacuating millions of people • arranging living accommodations • providing lost wages And then having a false ALARM!!!! 38 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? 39 Bakersfield 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% Use of the seismic gap method along the SAF Cyclic EQ’s in Parkfield,CA Parkfield 90% Bakersfield 41 1857 1 EQ about 1881 every 22 years 1901 1922 Cyclic EQ M6 or greater 1934 rupturing in the same 1966 area of the fault zone The next predicted EQ before 1993 DID NOT HAPPEN! 90% chance of a major EQ EC – Did the EQ on 9/28 reduce the probability of 90%? www.quake.usgs.gov/recenteqs/ September 28, 2004 M6 at Parkfield 42 I a good earthquake Discuss with a friend: 1. 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 Structural Geology Faults 44 Structural Geology – Earth Deformation Fault Folds How does solid rock “bend” or “break?” Stress and Strain relationships: Stresses- an amount of force subjected to rocks and applied in various directions Strain – amount of deformation produced by stresses changes the shape or volume of rocks Three types of stresses ROCKS Compressional stress – rocks are pushed from opposite sides ROCKS Tensional stress – rocks are pulled in opposite directions ROCKS Shear stresses – rocks are pushed from opposing directions 46 Faults Joint – fracture within a rock body Fault – movement along a fracture or joint B A 47 fault Joint A B Fault terminology Fault plane 48 49 I A good earthquake 1. Describe the differences between stress and strain 2. Describe the stress types compression tensional, and shear 3. Draw a fault block and label: 1. hanging/footwalls 2. fault plane Discuss with a friend I will get an A on my exams and quizzes 50 Determining the type of fault using the hanging-wall (HW) and foot-wall (FW) Normal Fault HW down relative to FW HW FW FW HW HW Reverse Fault HW up relative to FW FW 51 Normal Fault • HW down relative to FW • Tensional stress • extension of crust HW FW FW HW HW FW 52 Reverse Fault • HW up relative to FW • Compression stress • shorting of crust Vertical type faults FW HW FW HW Normal faulting FW Reverse faulting HW HW FW 53 Strike-slip faulting: two blocks sliding past one another Map View 54 Turned to Right Turned to Left What is the type of strike-slip fault? Right-Lateral – strike slip 55 Wallace Creek Right-Lateral Strike slip San Andreas Fault – Wallace Creek, Carrizo Plane 56 What type of fault ? Normal fault 57 I a good earthquake Discuss with a friend: 1. Draw a diagram showing the differences between a normal and reverse fault. 2. Explain the differences between the cross-sectional fault views of a normal, reverse, and strike-slip fault 3. Describe the type of stresses that produce both normal and reverse faults. I will get an A on my exams and quizzes 58 SQ-8 59