and Earth s Interior Earth Science, 13e Chapter 8 Stanley C. Hatfield Southwestern Illinois College General features Vibration of Earth produced by the rapid release of energy Associated with movements along faults Explained by the plate tectonics theory Mechanism for earthquakes was first explained by H. Reid Rocks spring back a phenomena called elastic rebound Vibrations (earthquakes) occur as rock elastically returns to its original shape Elastic rebound Video clip 5.1a elastic rebound (in folder) Take your own notes of video! General features are often preceded by foreshocks and followed by aftershocks Earthquake waves Study of earthquake waves is called seismology Earthquake recording instrument (seismograph) Records movement of Earth Record is called a seismogram 1
Seismograph Video 5.1b seisomgraph Take own notes on video! A seismogram records wave amplitude vs. time Types of earthquake waves Surface waves Complex motion Slowest velocity of all waves Can be horizontal and vertical Surface waves Earthquake waves Types of earthquake waves Body waves 1. Primary (P) waves Push-pull (compressional) motion Travel through solids, liquids, and gases Greatest velocity of all earthquake waves 2
Earthquake waves Types of earthquake waves Body waves 2. Secondary (S) waves Shake motion Travel only through solids Slower velocity than P waves Video clip 5.1c seismic wave motions Take your own notes on video! Activity 4.1 In a group of 2 or 3, come up with a way to visually demonstrate horizontal and vertical surface waves. You got 5 minutes before you present it to the class! Activity 4.2 Giant slinky demo of P and S Body waves Activity 4.2 Giant slinky demo of P and S Body waves Now take 5 minutes and draw a picture of the difference of P and S waves, or use your own words to describe them! Locating an earthquake Focus the place within Earth where earthquake waves originate Epicenter Point on the surface, directly above the focus Located using the difference in the arrival times between P and S wave recordings, which are related to distance 3
Earthquake focus and epicenter A time-travel graph What do you notice about the difference between S and P waves on this timetravel graph? The epicenter is located using three or more seismic stations around the world!! Locating an earthquake Earthquake zones are closely correlated with plate boundaries Circum-Pacific belt Oceanic ridge system Alpine-Himalayan belt Magnitude 5 or greater earthquakes over 10 years Earthquake intensity and magnitude Intensity A measure of the degree of earthquake shaking at a given locale based on the amount of damage Most often measured by the Modified Mercalli Intensity Scale Magnitude Concept introduced by Charles Richter in 1935 4
Earthquake intensity and magnitude Magnitude Often measured using the Richter scale Based on the amplitude of the largest seismic wave Each unit of Richter magnitude equates to roughly a 32- fold energy increase Does not estimate adequately the size of very large earthquakes Video clip 5.1d Wave Properties Make own notes on Wave height (crest and trough), Wave length, and amplitude Earthquake intensity and magnitude Magnitude Moment magnitude scale Measures very large earthquakes Derived from the amount of displacement (or movement) that occurs along a fault zone Earthquake destruction Factors that determine structural damage Intensity of the earthquake Duration of the vibrations Nature of the material upon which the structure rests The design of the structure Damage caused by the 1964 earthquake in Alaska Earthquake destruction Destruction results from Ground shaking Liquefaction of the ground Saturated material turns fluid Underground objects may float to surface Tsunami, or seismic sea waves Landslides and ground subsidence Fires 5
Damage from the 1964 Anchorage, Alaska earthquake Formation of a tsunami Tsunami travel times to Honolulu Video clip 5.1e liquefaction Make own notes on: What liquefaction is Difference between dry compaction and liquefaction Earthquake prediction Short-range no reliable method yet devised for short-range prediction Long-range forecasts Premise is that earthquakes are repetitive Region is given a probability of a quake Earthquake Lab assignment Make a earthquake GeoBlox model and questions 6
Earthquake lab assignment Read the lab entirely first then begin! Questions due for HW Have fun! 4.2 notes: Earth s layered structure Most of our knowledge of Earth s interior comes from the study of P and S earthquake waves Travel times of P and S waves through Earth vary depending on the properties of the materials S waves travel only through solids Earth s Interior Shallow Components of Earth! Draw a quick sketch of this on the provided paper! And this 39 Mineralogy of Earth s Layers Crust Thin, rocky outer layer Varies in thickness Roughly 7 km (5 miles) in oceanic regions Continental crust averages 35 40 km (22 25 miles) Exceeds 70 km (40 miles) in some mountainous regions http://pubs.usgs.gov/gip/interior/ 7
Crust 2 types: 1. Continental crust Upper crust composed of granitic rocks Lower crust is mostly basalt Average density is about 2.7 g/cm 3 Up to 4 billion years old Crust 2. Oceanic Crust Basaltic composition Density about 3.0 g/cm 3 Younger (180 million years or less) than the continental crust Lithosphere Crust and uppermost mantle (about 100 km thick) Cool, rigid, solid Asthenosphere Beneath the lithosphere Upper mantle To a depth of about 660 kilometers Soft, weak layer that is easily deformed Mantle Below crust (and lithosphere and asthenosphere) to a depth of 2900 kilometers (1800 miles) Composition of the uppermost mantle is the igneous rock peridotite (coarse grain made of minerals - changes at greater depths) Igneous rocks are one of the three main types of rocks: Igneous formed from cooling and solidifying of magma or lava Metamorphic transformed existing rock through temp and pressure changes Sedimentary sediment deposited by water then solidifies over time Mesosphere (or lower mantle) 660 2900 km More rigid layer Rocks are very hot and capable of gradual flow Outer core Liquid layer 2270 km (1410 miles) thick Convective flow of metallic iron within generates Earth s magnetic field Video 5.1f Earth magnetic field (in folder, only 1 st half) 8
Video 5.1f Earth magnetic field (in folder, only 1 st half) Ponder this: Based on this knowledge, do you predict the other planets to have a magnetic field Why does Earth have one? http://www.spaceanswers.com/solar -system/why-do-planets-havemagnetic-fields/ Nov. 2012 Video 5.1f Earth magnetic field (in folder, only 1 st half) Ponder this: Based on this knowledge, do you predict the other planets to have a magnetic field Why does Earth have one? Mercury has a magnetic field.... Venus does not have a magnetic field.... Earth has a magnetic field.... Mars does not have a magnetic field.... Jupiter has a magnetic field.... Saturn has a magnetic field.... Uranus has a magnetic field.... Neptune has a magnetic field So far the layers: Crust (oceanic and continental) Lithosphere Asthenosphere Mantle now Outer core then Inner core Outer Core Below mantle A sphere having a radius of 3486 km (2161 miles) Composed of an iron-nickel alloy Average density of nearly 11 g/cm 3 Inner Core Sphere with a radius of 1216 km (754 miles) Behaves like a solid 9
Views of Earth s layered structure How did Seismology help us discover earth s inside? Firstly, remember that earthquake waves spread out in all directions and can therefore reach all parts of the globe P-waves travel through liquid and solid S-waves only through solid Using the concept of wavespeed (higher density = higher velocity), and refraction of waves through different medium, the following was discovered: The Moho Seismic-wave velocities are faster in the upper mantle Crust / Mantle boundary sudden large increase in wave velocity, which indicates a change in medium (more dense) Velocity increases w depth, waves bend back to surface. Waves that travel via mantle arrive sooner at far destinations Think of it this way! When diving in water, in which medium do you travel faster air or water? What about sound waves air, water or solid? Which state are molecules most compact Quick 1 min break Take a quick stretch and Think of a question with a partner I may ask you on a quiz relating to speeds of seismic waves in the earth s layers? Let s here them 10
Wave Velocities Wave Velocities Crust slow Mohorovičić discontinuity Upper Mantle Fast Crust slow Mohorovičić discontinuity Upper Mantle Fast Asthenosphere Slow Asthenosphere Slow Lower Mantle Fast Lower Mantle Fast The P-Wave Shadow Zone The S-Wave Shadow Zone P-waves through the liquid outer core bend, leaving a low intensity shadow zone 103 to 143 degrees away from the source, here shown as the north pole HOWEVER, P-waves traveling straight through the center continue, and because speeds in the solid inner core are faster, they arrive sooner than expected if the core was all liquid. Since Shear (S) waves cannot travel through liquids, the liquid outer core casts a larger shadow for S waves covering everything past 103 degrees away from the source. Behavior of waves through center reveal Earth s Interior Black arrows P waves White arrows S waves What do you notice? Continuous curve through mantle...why? Change in direction through outer core and then again through inner core why? What would be noticed by seismic stations less than 103 away from epicenter? What would be noticed by seismic stations between 103 &142? What would be noticed on the other side of the earth? Take 5 minutes to answer these?s with a partner Possible seismic paths through the Earth 11
Earth s layered structure Let s make sure we really get this! Discovering Earth s major layers Discovered using changes in seismic wave velocity Moho discontinuity Velocity of seismic waves increases abruptly below 50 km of depth Separates crust from underlying mantle Earth s layered structure Discovering Earth s major layers Shadow zone Absence of P waves from about 105 degrees to 140 degrees around the globe from an earthquake Explained if Earth contained a core composed of materials unlike the overlying mantle Not proven but explains seismic wave properties we observe End of Chapter 8 HW: p. 252 #1,3,4,9,10, AND summarize how the information you learned about earthquakes will help you during the Big One 12