Earthquakes

1
Earthquakes

• Prepared by Betsy Conklin for
• Dr. Isiorho

2
Introduction

• earthquake a trembling or shaking of the ground
  caused by the sudden release of energy stored in
  the rocks beneath the earths surface
• seismic waves the waves of energy produced by an
  earthquake
• elastic rebound theory the sudden release of
  progressively stored strain in rocks that results
  in movement along a fault

3
Causes of Earthquakes

• movement along faults
• explosive volcanic eruptions
• mineral transformations within the downgoing rock
  as pressure collapses one mineral into a denser
  form

4
Seismic Waves

• focus the point within the earth where seismic
  waves originate
• epicenter the point on the earths surface
  directly above the focus

5
Body Waves

• body waves seismic waves that travel through the
  earths interior, spreading outward from the
  focus in all directions
• P (primary) wave a compressional (or
  longitudinal) wave in which rock vibrates back
  and forth parallel to the direction of wave
  propagation can travel through rock, gas, or
  liquid

6
Body Waves (cont.)

• S (secondary) waves a slower, transverse wave
  that travels through near surface rocks at 2 to 5
  kilometers per second the rock vibrates
  perpendicular to the direction of wave
  propagation can travel through rock but not gas
  or liquid

7
Surface Waves

• surface waves seismic waves that travel on the
  earths surface away from the epicenter
• love waves waves that have no vertical
  displacement they move side to side in a
  horizontal plane that is perpendicular to the
  direction the wave is traveling or propagating
  do not travel through liquids because of
  horizontal movement the waves tend to knock
  buildings off their foundation

8
Surface Waves (cont.)

• Rayleigh waves behave like rolling ocean waves
  cause the ground to move in an elliptical path as
  the wave passes tend to be incredibly
  destructive because they produce more ground
  movement and take longer to pass

9
Measuring Earthquakes

• seismograph a recording device that produces a
  permanent record of earth motion detected by a
  seismometer, usually in the form of a wiggly line
  drawn on a moving strip of paper

A seismograph for horizontal motion. Modern
seismographs record earth motion on moving strips
of paper. The mass is suspended by a wire from
the column and swings like a pendulum when the
ground moves horizontally. A pen attached to the
mass records the motion on a moving strip of
paper.
10
Measuring Earthquakes
A simple seismograph for detecting vertical rock
motion. The pen records the ground motion on the
seismogram as the spring stretches and
compresses with up and down movement of the
spring. Frame and recording drum move with the
ground. Inertia of the weight keeps it and the
needle relatively motionless.
11
Measuring Earthquakes (cont.)

• seismogram the paper record of each vibration

12
Determining the Location of an Earthquake

• travel-time curve a plot of seismic-wave arrival
  times against distance

A travel time curve is used to determine the
distance to an earthquake. Note that the time
interval between the first arrival of P and S
waves increases with distance from the
epicenter.Seismogram X has a 3-minute interval
between P and S waves corresponding to a distance
of 2,000 km from the epicenter, Y has an
interval of 8 minutes, so the earthquake
occurred 5,300 km away, and Z an interval of 12
minutes, and is a distance of 9,000 km from the
epicenter.
13
Determining the Location of an Earthquake

• depth of focus the distance between focus and
  epicenter

Locating an earthquake. The distance from each of
three stations (Denver, St. Johns, and Lima) is
determined from seismograms and the travel-time
curves shown in figure 16.9. Each distance is
used for the radius of a circle about the
station. The location of the earthquake is just
offshore of Vancouver, British Columbia, where
the three circles intersect.
14
Measuring the Size of an Earthquake

• intensity a measure of an earthquakes effect on
  people and buildings
• modified Mercalli scale scale expressing
  intensities of earthquakes (judged on amount of
  damage done) in Roman numerals ranging from I to
  XII
• magnitude a measure of the energy released
  during the earthquake
• Richter scale a numerical scale of magnitudes
• moment magnitude an earthquake magnitude
  calculated from the strength of the rock, surface
  area of the fault rupture, and the amount of rock
  displacement along the fault

15
Effects of Earthquakes

• ground motion
• fire
• landslides
• liquefaction (a special type of ground failure)
• permanent displacement of the land surface
• aftershocks small earthquakes that follow the
  main shock

Liquefaction of soil by a 1964 earthquake in
Niigata, Japan, caused earthquake-resistant
apartment buildings to topple over intact.
An example of permanent displacement of the land
surface - fence compressed by ground movement,
Gallatin County, Montana, 1959.
16
Tsunamis

• tsunamis (seismic sea waves) huge ocean wave
  produced by displacement of the sea floor

17
World Distribution of Earthquakes

• Circum-Pacific belt the most important
  concentration of earthquakes which encircles the
  rim of the Pacific Ocean. Approximately 80 of
  the worlds shallow-focus quakes, 90 of the
  intermediate-focus quakes, and nearly 100 of the
  deep-focus quakes occur there.
• Mediterranean-Himalayan belt a major
  concentration of earthquakes which runs through
  the Mediterranean Sea, crosses the Middle East
  and the Himalayas, and passes through the East
  Indies to meet the circum-Pacific belt north of
  Australia

18
World Distribution of Earthquakes
19
World Distribution of Earthquakes

• Benioff zones distinct earthquake zone that
  begins at an oceanic trench and slopes landward
  and downward into the earth at an angle of about
  30o to 60o
• island arc Benioff zones that slope under a
  continent or a curved line of islands

20
First-Motion Studies of Earthquakes

• By studying seismograms of an earthquake on a
  distinct fault, geologists can tell which way
  rocks moved along that fault. Rock motion is
  determined by examining seismograms from many
  locations surrounding a quake. Each seismogram
  station can tell whether the first rock motion
  recorded there was a push or a pull. If the rock
  moved toward the station (a push), then the pen
  drawing the seismogram is deflected up. If the
  first motion is away from the station (a pull),
  then the pen is deflected downward.

Map view of two possible solutions for the same
pattern of first motion. Each solution has a
different fault orientation. If the fault
orientation is known, the correct solution can
be chosen. The star marks the epicenter, and rock
motion is shown by arrows.
21
Earthquakes at Divergent Boundaries

• divergent boundaries where plates move away from
  each other
• the rock motion that is deduced from first-motion
  studies shows that the faults here are normal
  faults, parallel to the rift valley

22
Earthquakes at Transform Boundaries

• transform boundaries where plates move
  horizontally past each other
• first-motion studies indicate strike-slip motion
  on faults parallel to the boundary

Narrow band of shallow-focus earthquakes shown as
stars along single fault
Broad band of earthquakes along a system of
parallel faults
23
Earthquakes at Convergent Boundaries

• convergent boundaries where plates move toward
  each other
• two general types - one marked by the collision
  of two continents, the other marked by subduction
  of the ocean floor under a continent

24
Pictures

• All pictures used in this power point
  presentation were taken from the following
• Carlson, Diane H., David McGeary and Charles C.
  Plummer. Physical Geology Updated Eighth
  Edition. New York City, McGraw-Hill Higher
  Education, 2001.