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Unformatted text preview: Geology 1001
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.
- Spring '07