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					                                                                          EARTHQUAKES
         ESSC 100 – Intro to Earth Science
                                                                       Ancient cultures offered a variety of
                                                                       explanations for earthquakes activity
                                                                       (seismicity), most of which involved
                                                                       the action or mood of a giant animal
                                                                       or god.
    EARTHQUAKES AND                                                    Today we know that an earthquake is
                                                                       the motion or trembling of the ground

     EARTH’S INTERIOR                                                  produced by several factors,
                                                                       including:
                                                                       • sudden displacement of rock in the
                                                                        Earth's crust;
                                                                       • volcanic eruptions;
                                                                       • giant landslides;
                                                                       • a meteorite impact, or
                                                                       • underground nuclear-bomb tests.




   STRESS AND STRAIN, PLASTIC AND                    ELASTIC REBOUND
       BRITTLE DEFORMATION                                                     • Along plate boundaries,
                                                                                 rocks are under stress.
•Stress is the push, pull or shear                                             • Rocks initially deform
 that a material feels when it is                                                plastically, but when the
 subjected to a force.                                                           stress exceeds the
•Strain is the change in shape of                                                strength of rocks, they
 a material in response to the                                                   break along a fault.
 application of a stress.                                                      • The accumulated strain is
•Brittle deformation: permanent                                                  suddenly released as
 deformation in which the rock                                                   seismic waves.
 fractures or crack, instead of                                                • Rocks are somewhat
 flowing or bending.                                                             elastic, so they try to snap
•Plastic (ductile) deformation:                                                  back after they break.
 permanent deformation in which                                                • The fault remains as a
 a rock may change its shape by                                                  weakness in the rock.
 flowing or bending.
                                             See textbook figure 7.4




               SAN FRANCISCO, 1906
                                                       EARTHQUAKES
                                             General features
                                             • Vibration of Earth produced by a 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




                                                                                                                1
                        TERMINOLOGY                                                            EARTHQUAKE WAVES
• When an earthquake occurs, the sudden movement of the rock causes
 seismic waves to radiate out from the area where the movement occurred                    Earthquake waves
 (the earthquake hypocenter or focus) at a speed of several kilometers
 per second.                                                                                • Types of earthquake waves
                                                                                                 • Body waves
                                      • EPICENTER: directly above the
                                                                                                    • generated at the focus when an earthquake occurs.
                                        hypocenter, is the location of earthquakes
                                        projected at the surface (in latitude and                   • travel in the interior of the Earth (3-7 km/sec)
                                        longitude.                                                  • P waves - Primary
                                                                                                    • S waves - Secondary
                                      • Main Shock: largest and generally First
                                                                                                 • Surface waves
                                        earthquake in a sequence.
                                                                                                    • Produced when body waves “hit” the surface
                                      • Aftershock: smaller earthquakes after                       • Complex motion
                                        first main shock. Can last as much as a                     • Slowest velocity of all waves
                                        month afterward. Can be almost as large
                                        as main shock, generally smaller.
                                                                                            • Study of earthquake waves is called seismology
                                        Decrease in magnitude with time.                    • Earthquake recording instrument (seismograph)
                                                                                                 • Records movement of Earth
                                      • Foreshock: an earthquake that occurs
                                                                                                 • Record is called a seismogram
                                        prior to a large one.




             PRIMARY (P) WAVES                                                               SECONDARY (S) WAVES
• Push-pull (compressional)                                                          • "Shake" motion: they propagate
  motion: particles are displaced                                                      by laterally displacing the
  parallel to the direction of wave                                                    medium through which they
  propagation.                                                                         move
• Travel through solids, liquids,                                                    • Travel only through solids
  and gases                                                                          • Slower velocity than P waves
• Greatest velocity of all                                                             (about 3.5 km per second.)
  earthquake waves (up to 7                                                          • However, S waves, because of
  km/sec)                                                                              their shearing motion, are far
• Because P waves are like sound                                                       more damaging to structures
  waves, when they reach the                                                           than P waves.
  surface they can create sound
  waves in the air that are audible
  to humans and animals.




                                                                                            RECORDING EARTHQUAKES:
                 SURFACE WAVES                                                                   SEISMOGRAPHS
 Surface waves are slower than body waves. Because their motion is                                                  What we want to do to record an earthquake is to
 restricted to the surface of the earth, they generally have a longer distance to                                   measure the shaking of the earth.
 travel to reach a particular point than do body waves.
                                                                                                                    •However, everything attached to the earth,
 Rayleigh (R) waves - make the surface of the ground to go up and down,                                              including our measuring instruments, will move
 like ripples on the surface of a pond.                                                                              with the earth.
 Love (L)waves - are a horizontal displacement at the surface, that is, they                                        •We need a stationary frame of reference from
 cause the surface of the ground to shear sideways. The horizontal shaking of                                        which we can measure the shaking without being
 Love waves is particularly damaging to building foundations.                                                        a part of it.
                                                                                                                    •Although we cannot easily detach our
                                                                                                                     instruments from the earth, we can take
                                                                                                                     advantage of inertia to isolate them from earth
                                                                                                                     movements.
                                                                                                                    •Inertia is the tendency of an object at rest to
                                                                                                                     remain at rest.
 Rayleigh                                                                                                           • The key to a seismograph is the presence of
                                           Love (L)
 (R) waves                                                                                                           a weight that stays fixed in space while
                                            waves
                                                                                                                     everything else moves around it.




                                                                                                                                                                       2
                  HOW A SEISMOGRAPH WORKS                                                                               Horizontal (N-E)
   • Let’s consider a mechanical vertical-motion seismograph consisting of a
     heavy weight (like a pendulum) suspended from a spring.
   • When an earthquake wave arrives and causes the ground surface to move up
     and down, it makes the seismograph frame also move up and down.
   • The weight, however, remains fixed in space. As a consequence, the revolving
     paper roll moves up and down under the pen, which traces out the waveforms
     representing the up-and-down movement.
   • On a real seismograph record, one revolution of the paper cylinder
     corresponds to an hour; a single paper roll holds the record for a whole day.
Figure 7.10




                                                                                                                                 Vertical (Z)




                                   SEISMOGRAMS                                           Seismograms: the written record
              The seismic trace recorded on a seismograph contains a variety of
              information useful for analyzing the intensity, distance from the
                                                                                         of an earthquake
              epicenter, and location of the earthquake.
                                                                                         Seismic waves reach a recording
              One can recognize the different kinds of earthquake waves on the
                                                                                         station at different times producing
              seismic trace.
                                                                                         identifiable sets of waves (pattern).
              The first pulse of waves are the fast moving primary waves (P) waves.
              As they begin to fade, the second large pulse records the arrival of the
              slower secondary (S) waves. Finally, a mishmash of surface waves and       P→ emerge at steep angle producing
              reflected p and s waves arrive.                                            mainly vertical ground motion
                                                                                         S→ last somewhat longer than P
                                                                                         trains (E-N components of motion)
                                                                                         Earthquake coda→ the dying end of                      Wave Dispersion
                                                                                         an earthquake composed of a
                                                                                         mixture of surface waves (L-R) and
   See textbook                                                                          scattered P and S late arrivals
   figure 7.12                                                                           (deeper structures)




                  WHAT CAN WE LEARN FROM                                                                FINDING THE EPICENTER
                                                                                          By accumulating a tremendous amount of data, seismologists have
                      SEISMOGRAMS ?                                                       determined the average times of S and P waves for any specific distance.
              1. Distance: we know how fast the fasted P and S waves travel, so we        These travel times are published as time-distance graphs (or travel-
                 can use the difference in their arrival times (time lag) at the          time curves), illustrating the difference between the arrival times of P
                 seismograph to determine how far away the earthquake was.                and S waves.
                                                                                                                               The farther away a seismograph
              2. Origin Time: once we know how far away an earthquake was, we can                                              station is from the focus of an
                 determine the exact time that it happened.                                                                    earthquake, the longer the interval
                                                                                                                               between the arrivals of the P and S
              3. Location of Epicenter: If we have the distances to the earthquake
                                                                                                                               waves, and hence the distance
                 epicenter calculated from three or more seismic stations, then we can
                                                                                                                               between the two curves on the graph.
                 use triangulation to find the exact location of the epicenter.
              4. Magnitude: the strength of the earthquake is indicated by the
                 amplitude or size of the spikes on the seismograph. However, the
                 farther away the seismic station is, the more attenuated the waves
                 become and the smaller are the spikes produced by a given
                 earthquake. Fortunately, since we know how far away the earthquake
                 was, we can compensate for distance to determine how large the
                 Earthquake was where it happened.




                                                                                                                                                                      3
                      TRIANGULATION
The epicenter of any earthquake can be determined by using a time-
distance graph and knowing the arrival times of the P and S waves at
three seismograph locations.
                                                                                     EARTHQUAKES
1-Determine the distance of the epicenter from each of the
seismographs.                                                              Earthquake intensity and magnitude
2-For each seismograph, draw
on a map a circle whose radius                                              • Intensity
equals the distance from the                                                   • A measure of the degree of earthquake shaking
epicenter.
                                                                                 at a given locality based on the amount of
3-The intersection of the three                                                  damage
circles is the location of the
earthquake’s epicenter.                                                        • Most often measured by the Modified Mercalli
A minimum of THREE locations                                                     Intensity Scale
is necessary because two                                                    • Magnitude
locations will provide two
possible epicenters, and one                                                   • Concept introduced by Charles Richter in 1935
location will provide an infinite
number of epicenters.




           MEASURING THE MAGNITUDE                                                 THE RICHTER SCALE
•Earthquake strength is measured against
a scale developed by the seismologist
Charles Richter in 1935.                                               •The Richter scale is logarithmic, that is an increase of 1
•The magnitude of an earthquake is a                                    magnitude unit represents a factor of ten times in
measure of the amount of energy                                         amplitude. The seismic waves of a magnitude 6
released. Each earthquake has a unique
magnitude assigned to it.
                                                                        earthquake are 10 times greater in amplitude than those
                                                                        of a magnitude 5 earthquake.
•The magnitude is calculated as the
logarithm of the amplitude of waves                                    •However, in terms of energy release, a magnitude 6
recorded by seismographs.
                                                                        earthquake is about 31 times greater than a magnitude 5.
•The magnitude is determined by
measuring the maximum amplitude of                                     •The total amount of energy released in the largest
the largest seismic wave (usually a
surface wave) and the difference between
                                                                        earthquakes ever recorded is 9.5 (Chile, 1960) (roughly
the arrival times of the P and S waves.                                 equal to the energy of 10,000 Hiroshima sized atom
•Adjustments are included for the variation                             bombs). Probably rocks are not able to store the energy
in the distance between the various                                     necessary to generate earthquakes of higher magnitude.
seismographs and the epicenter of the
earthquakes.




                                                                                                                            1886




                                                                                                                                     4
 WHERE AND WHY DO EARTHQUAKES OCCUR?                                                EARTHQUAKES AT PLATE BOUNDARIES
•The majority of earthquakes (~80%) occur in the circum-pacific belt.
•The second major seismic belt (~15%) is the Mediterranean-Asiatic belt.            • TRANSFORM MARGINS => shallow focus;
•The remaining 5% of earthquakes take place mostly in the interior of plates and    • DIVERGENT MARGINS AND
along oceanic spreading ridges (divergent plate boundaries).                          CONTINENTAL RIFTS => shallow focus;
                                                                                    • INTRAPLATE => shallow focus
                                                                                    • CONVERGENT MARGINS => shallow to
                                                                                      deep.




                                       CLASSIFICATION
                                  Seismologists recognize three categories
                                  of earthquakes:
                                                                                                    EARTHQUAKES
                                  •Shallow-focus-focal depth less than 70 km.
                                  These earthquakes usually generate along
                                  transform or divergent plate boundaries. In            Earthquake prediction
                                  general these are the most destructive.
                                                                                          • Short-range – no reliable method yet
                                  •Intermediate-focus- foci between 70 and                  devised for short-range prediction
                                  300 km.
                                  •Deep-focus-foci deeper than 300 km.                    • Long-range forecasts
                                  •Approximately 90% of all earthquake foci                   • Premise is that earthquakes are repetitive
                                  occur at a depth of less than 100 km.                       • Region is given a probability of a quake
                                  •Intermediate and deep earthquakes occur
                                  along convergent plate boundaries, especially
                                  along the circum-pacific belt.
                                  •Benioff zones => convergent margins.




                                                                                   Denali Fault Earthquake, M7.9, Nov. 3, 2002
       EARTHQUAKES DAMAGE
     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


     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
Aerial view of the Trans-Alaska Pipeline and Richardson
                                                              An aerial photo of the Trans-Alaska Pipeline System (TAPS) line near the Denali
Highway, looking north. Rupture along the fault resulted in
                                                              fault, looking west. This is where the line is supported by rails on which it can move
approximately 2.5 meters (8 feet) displacement of the
                                                              freely in the event of fault offset. Here the line has moved toward the west end of the
highway, with the north side moving east relative to the      rails. Alyeska Pipeline Service Company reported no breaks to the line and therefore
south side. Photo by Patty Craw, DGGS.                        no loss of oil. Note the transverse crack on the Richardson Highway in lower left.




                STRUCTURAL DAMAGE                                                      LIQUEFACTION
                                                              •In addition to the
• Most buildings and bridges are constructed                  collapse of buildings,
  to withstand the downward force of gravity.                 the shaking from
  Construction materials such as brick and                    earthquakes can
  concrete are very strong in compression                     cause ground
  and can support great weight.                               composed of loose soil
• Unfortunately, these same materials are                     and sand to liquefy.
  brittle and incapable of resisting tensional                •The most impressive
  forces introduced by bending. Side to side                  example of liquefaction
  or upward motion introduces bending                         was seen at Turnagain
  forces and these materials fail and                         Heights in Anchorage,
  collapse. Most people killed in earthquakes                 during the Good Friday
  die from trauma caused by building                          earthquake of 1964,
  collapse and object falling from walls.                     where 60 foot high soft
• The principle goal of earthquake                            clay beach cliffs
  engineering is to prevent loss of life from                 collapsed causing the
  building collapse. Even if a building is                    slumping of developed
  damaged beyond repair, if it does not                       land up to 900 feet
  collapse on the occupants then it will not                  inland along more than
  cause loss of life.                                         a mile of coastline.




         TSUNAMI TRAVEL TIMES
                                                                               EARTH'S LAYERED
             TO HONOLULU
                                                                                 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




                                                                                                                                                        6
                              EARTH'S                            SHADOW ZONE
                              LAYERED
                                                    •Absence of P
                             STRUCTURE               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




  Earth's layered structure
Discovering Earth’s major layers
• Discovered using changes in seismic wave
  velocity
• Mohorovicic discontinuity
   • Velocity of seismic waves increases abruptly
     below 50 km of depth
   • Separates crust from underlying mantle




                                                                               7

				
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