Earthquakes
Prepared by Betsy Conklin for
Dr. Isiorho
Introduction



earthquake: a trembling or shaking of
the ground caused by the sudden
release of energy stored in the rocks
beneath the earth’s 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
Causes of Earthquakes



movement along faults
explosive volcanic eruptions
mineral transformations within the
downgoing rock as pressure collapses
one mineral into a denser form
Seismic Waves


focus: the point within the earth where
seismic waves originate
epicenter: the point on the earth’s
surface directly above the focus
Body Waves


body waves: seismic waves that travel
through the earth’s 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
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
Surface
Waves
 surface waves: seismic waves that travel on the

earth’s 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
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
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.
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.
Measuring Earthquakes (cont.)

seismogram: the paper record of each
vibration
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.
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.
Measuring the Size of an Earthquake





intensity: a measure of an earthquake’s 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
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.
Tsunamis

tsunamis (seismic sea waves): huge
ocean wave produced by displacement
of the sea floor
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 world;s 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 circumPacific belt north of Australia
World Distribution of Earthquakes


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
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.
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
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
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
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.