Chapter 13 Slides

Chapter 13 Slides - Geology 1001 PHYSICAL GEOLOGY Chapter...

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Unformatted text preview: Geology 1001 PHYSICAL GEOLOGY Chapter 13 Earthquakes The Great San Francisco Earthquake, 1906 One of the most devastating events in US history First earthquake that was studied in detail Taught us a lot about how earthquakes work Casualties and damage: San Francisco, 1906 Fatalities > 3,000 Homeless - 225,000 of about 400,000 # Buildings Destroyed - 28,000 # Monetary Loss - More than $400 million 1 Length of Rupture Evidence of Rupture What are the odds? 2 Oakland CA: October, 17, 1989 Richter Magnitude: 6.9 Chapter 1: Earth's Plate Tectonic System Earth' 3 Volcanoes (red) & Earthquakes (black) Earthquakes Direct evidence of plate motion Size of the earthquake depends on: Type of plate boundary Rate of plate motion Length of fault slip Largest earthquakes are found along subduction margins and continental transform margins Stress: Local force per unit area. 4 Strain: amount of distortion due to stress. Strength: critical stress at which rocks break. Earthquake: when rocks under stress suddenly break 5 The elastic response Strength Stress Time Variations in strength and rate of stress 6 Time A Time B Time C 7 Rupture 1 Seismic waves Focus: point at which slip begins Rupture 2 Rupture 3 8 Rupture 4 Time D Fault Slip: distance of displacement. Focus: point at which slip begins 9 Epicenter: point on the surface above the focus Epicenter: Wave: disturbance that transfers energy through a medium Seismic wave: vibrations transferred through a medium 10 Seismic waves: types Distinguished by speed, motion, & travel path. Primary (P) Waves: Fastest waves Compressional: travel as a series of contractions and expansions Travel through solid, liquid, & gas P-waves Motion is parallel to wave direction Seismic waves: types Secondary (S) Waves: Slower (1/2 of P) Shear: move up and down Travel only through solids 11 S-Waves Motion is perpendicular to wave motion Seismic waves: types Surface Waves: Slowest waves Move as ripples (like water) Travel on the surface of the Earth Surface Waves Vertical ripples Lateral ripple 12 Studying earthquakes Seismograph: seismic wave recorder. Studying earthquakes Seismograph: seismic wave recorder. 13 Seismic waves: velocities Locating the epicenter Locating the epicenter Time interval between P and S waves depends on travel distance Know time between P&S arrival Know distance traveled 14 Locating the epicenter Locating the epicenter Data from multiple stations used to fix location Locating the epicenter 15 Measuring an earthquake Richter Magnitude: based on ground motion Magnitude: P S Amplitude =23 mm P-S arrival time = 24 seconds Data: wave amplitude and (P-S) time. Data: (P- Measuring an earthquake Interval between S and P waves (s) Distance (km) Richter magnitude Amplitude (mm) 16 Measuring an earthquake Interval between S and P waves (s) Distance (km) Richter magnitude Amplitude (mm) Measuring an earthquake Richter Magnitude: Each unit of magnitude represents 10X greater ground motion. motion. 3 is 10X greater than 2 6 is 100X greater than 4 Richter Magnitude: Each unit of magnitude represents 33X greater energy released. released. 8 is 33 X 33 or 1000X greater than 6 Measuring an earthquake Moment Magnitude: based on fault displacement (Area of scarp) X (Average displacement) 17 Measuring an earthquake What do we feel? Modified Mercalli Intensity: how Intensity: much shaking do humans observe? Measuring an earthquake What do we feel? Patterns of Earthquakes Data: Position and Depth 18 19 35 km 70 km 150 km 300 km Earthquakes in a subduction zone 20 21 Earthquakes in a transform margin 22 Spreading Ridge 23 Quiz 6 Name Signature Date 1. Name the 3 types of seismic waves in order from fastest to slowest. 2. What 2 types of plate boundaries produce the largest earthquakes? Earthquake Hazards Earthquake Hazards Primary Hazards: Faulting and shaking Hazards: 24 Earthquake Hazard: Intensity of shaking Where the Earth moves Earthquake Risk: expected damage Where we live and build Earthquake Hazards Secondary Hazards: Liquefaction Hazards: Unconsolidated, saturated sediments loose internal cohesiveness. 25 Earthquake Hazards Earthquake Hazards Mexico City, 1985: magnitude 8.1 26 Earthquake Hazards Earthquake Hazards Mexico City, 1985 Near the most active subduction zone in the Western Hemisphere. Built on unconsolidated lake sediments. 10,000 people died, 20,000 injured. 5 billion dollars (U.S.) in damage. Reducing Risk: Vibration-absorbing designs Vibration- 27 Earthquake Hazards Secondary Hazards: Tsunamis Hazards: Earthquake Hazards Secondary Hazards: Tsunamis Hazards: 28 Earthquake Hazards Indian Ocean, 2004 Earthquake Hazards Indian Ocean, 2004 Tsunami triggered by a magnitude 9.1 earthquake in the Indian Ocean. Second largest earthquake ever recorded. More than 250,000 people killed. Maximum wave height = 15 m (~50 ft) 29 Reducing Risk: Tsunami walls Earthquake Hazards Secondary Hazards: Hazards: Landslides Earthquake Hazards Secondary Hazards: Fires Hazards: San Francisco, 1906 30 Reducing Risk: Predicting Disaster Recurrence interval: average time between interval: large earthquakes. Based on annual slip rates Not very accurate: error could be in decades or centuries. Reducing Risk: Predicting Disaster Short-Term Predictions: warnings of ShortPredictions: immediate danger. Based on detecting foreshocks Inconsistent results: not all small quakes precede large quakes. Reducing Risk: Be prepared HAVE A PLAN !! 31 ...
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This note was uploaded on 05/02/2009 for the course GEOL 1001 taught by Professor Baksi during the Spring '07 term at LSU.

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