Earth Science, 10^th edition

Chapter 6: Earthquakes and Earth’s Interior

 

 

    I. Earthquakes

       A. General features

           1. Vibration of Earth produced by the rapid release of energy by rocks that have been stressed beyond their elastic limit.

             2. Associated with movements along faults

                      1. Rocks "spring back"

                           a. Phenomena called elastic rebound

                           b. Vibrations (earthquakes) occur as rock elastically returns to its original shape

             3. Often preceded by foreshocks and followed by aftershocks

         

       B. Earthquake waves

             1. Study of earthquake waves is called seismology (recorded on seismograph)

             2. An earthquake releases it’s energy in two wave types: body waves and surface waves

                 a. Surface waves-complex wave motion that does the most damage in an earthquake.  Slowest velocity of all waves

                  b. Body waves-two types primary and secondary

                      1. Primary (P) waves

                           a. Push-pull (compressional) motion

                           b. Travel through

                                1. Solids

                                2. Liquids

                                3. Gases

                           c. Greatest velocity of all earthquake waves

                      2. Secondary (S) waves

                           a. "Shake" motion

                           b. Travel only through solids

                           c. Slower velocity than P waves

       C. Locating an earthquake

             1. Focus – the place within Earth where earthquake waves originate

             2. Epicenter

                  a. Point on the surface, directly above the focus

                  b. Located using the difference in the arrival times between P and S wave recordings,

  which are related to distance        

       D. Earthquake intensity and magnitude

             1. Intensity

                  a. A measure of the degree of earthquake shaking at a given locale based on

       the amount of damage

                  b. Most often measured by the Modified Mercalli Intensity Scale   

             2. Magnitude

                  a. Concept introduced by Charles Richter in 1935

                  b. Often measured using the Richter scale

                      1. Based on the amplitude of the largest seismic wave

       E. Earthquake destruction

             1. Factors that determine structural damage

                  a. Intensity of the earthquake

                  b. Duration of the vibrations

                  c. Nature of the material upon which the structure rests

                  d. The design of the structure

             2. Destruction from

                  a. Ground shaking

                  b. Liquefaction of the ground

                      1. Saturated material turns fluid

                       2. Underground objects may float to surface

                  c. Tsunami, or seismic sea waves

                  d. Landslides and ground subsidence

                  e. Fires

        F. Earthquake prediction

             1. Short-range – no reliable method yet devised for short-range predictions

             2. Long-range forecasts

                  a. Premise is that earthquakes are repetitive

                  b. Region is given a probability of a quake

 

  II. Earth's layered structure

       A. Most of our knowledge of Earth’s interior comes from the study of P and S earthquake waves

             1. Travel times of P and S waves through Earth vary depending on the properties of the materials

             2. S waves travel only through solids

         

       B. Layers defined by composition

             1. Crust

                  a. Thin, rocky outer layer

                  b. Varies in thickness (5-40 miles)

                  c. Two parts

                      1. Continental crust

                           a. Upper crust composed of granitic rocks

                           b. Lower crust is more akin to basalt  

                      2. Oceanic crust

                                a. Basaltic composition

             2. Mantle

                  a. Below crust to a depth of 2900 kilometers (1800 miles)

                  b. Composition of the uppermost mantle is the igneous rock peridotite

                     (changes at greater depths)

             3. Outer core

                  a. Below mantle

                  b. A sphere having a radius of 3486 km (2161 miles)

                  c. Composed of an iron-nickel alloy

    

       C. Layers defined by physical properties

             1. Lithosphere

                  a. Crust and uppermost mantle (about 100 km thick)

                  b. Cool, rigid, solid

             2. Asthenosphere

                  a. Beneath the lithosphere

                  b. Upper mantle

                  c. To a depth of about 660 kilometers

                  d. Soft, weak layer

                  e. Easily deformed

             3. Mesosphere (or lower mantle)

                  a. More rigid layer

                  b. Rocks are very hot and capable of gradual flow

             4. Outer core

                  a. Liquid layer

                  b. Convective flow of metallic iron within generates Earth’s magnetic field

             5. Inner core

                  a. Behaves like a solid

    

       D. Discovering Earth’s major layers

             1. Discovered using changes in seismic wave velocity

             2. Mohorovicic discontinuity

                  a. Velocity of seismic waves increases abruptly below 50 km of depth

                  b. Separates crust from underlying mantle

             3. Shadow zone

                  a. Absence of P waves from about 105 degrees to 140 degrees around the

       globe from an earthquake

                  b. Explained if Earth contained a core composed of materials unlike the overlying mantle

             4. Inner core

                  a. Discovered in 1936 by noting a new region of seismic reflection within the core

                  b. Size was calculated in the 1960s using echos from seismic waves

                     generated during underground nuclear tests

         

   E. Discovering Earth’s composition

             1. Oceanic crust

             2. Mantle

             3. Core