Download CHAPTER 12 EARTHQUAKES

CHAPTER 12
EARTHQUAKES
MOVEMENTS OF THE EARTH THAT ARE
CAUSED BY A SUDDEN RELEASE OF
ENERGY WHEN ROCKS MOVE ALONG A
FAULT.
San Fransisco, CA (1906)
Anatomy of an Earthquake
• Focus - the location within Earth where the first
motion occurs along a fault.
– 90% of earthquakes have a shallow focus. (< 70 km deep)
– Any given shallow focus earthquake will cause less
damage than most deep focus earthquakes
– Shallow focus quakes cause the most destruction overall
• Epicenter- the point on the earth’s surface directly
above the focus.
Seismic Waves
• As rocks move, the rocks release energy in the
form of vibrations called seismic waves.
• These waves travel outward in all directions
from the focus through the surrounding rock.
Two Main Categories of Waves
BODY WAVES
•
•
Body waves travel through the
body of a medium.
P Waves or primary waves or
compression waves
– Fastest; fastest waves (1st detected)
– Cause rock to move back and forth
parallel to the direction the waves
are traveling
– Can move through solids, liquids, and
gases.
– Travel faster through more rigid
materials.
•
S Waves or secondary waves or
shear waves
– Second fastest waves
– Arrive after P waves
– Cause rock to move side to side
perpendicular to the direction in
which the waves are traveling.
– Only move through solids.
SURFACE WAVES
• Surface waves travel along
the surface of a body rather
than through the middle.
• Form from motion along a
shallow fault or from the
conversion of energy when
P waves and S waves reach
Earth’s surface.
• Slowest moving
• Cause the greatest damage
Seismic Waves and Earth’s Interior
• Seismic waves can be used to understand
earth’s interior.
• The composition of the material through
which P and S waves travel affect the speed
and direction of the waves.
• By studying the speed and direction of seismic
waves, scientists can learn more about the
makeup and structure of Earth’s interior.
• Andrija Mohorovicic, 1909…discovered that the
speed of seismic waves increases abruptly at 30
km beneath the surface of continents.
• Because the mantle is denser than the crust.
• Therefore, this marks the boundary between the
mantle and crust.
• The depth of this boundary varies from 10 km
under the oceans to 30 km under the continents.
• Earth is composed of 3 composition layers:
– crust, mantle, core
• Earth is composed of 5 mechanical layers:
– Lithosphere, asthenosphere, mesosphere, outer
core, inner core.
Shadow Zones
• Shadow zones are locations on earth’s surface where
no body waves (P and S) can be detected.
• Shadow zones exist because the materials that make
up the earth’s interior are not uniform in rigidity
(some are liquid; some are solid; furthermore, some
are rigid and some are “plastic”).
• A large S wave shadow zone would be found
covering the side of earth opposite the earthquake.
• The S waves cannot pass through the liquid outer
core.
Earthquakes and Plate Tectonics
• Earthquakes are a result of stresses in Earth’s
lithosphere.
• Most earthquakes occur at
– Convergent oceanic environments
• Convergent plate boundaries - colliding
– Divergent oceanic environments
• Divergent plate boundaries – spreading
– Continental environments
• Convergent, divergent, and transform boundaries
Most Earthquakes occur at Plate
Boundaries
Earthquakes away from Plate
Boundaries
• Not all earthquakes occur at active known
plate boundaries; sometimes ancient fault
zones are buried and result in earthquakes.
• 1811/1812…New Madrid, Missouri
– Felt as far away as S. Carolina
Studying Earthquakes
• Seismology
• Earthquakes are the best tool for investigating
Earth’s internal structure and dynamics.
• Seismometer/Seismograph – equipment that
can detect and record the vibrations of an
earthquake.
• Seismogram – the tracing of vibration intensity
recorded by a seismograph.
– P waves are recorded first, then S waves.
Locating an Earthquake
• Determining the epicenter of an earthquake
• Triangulation
– 3 seismic stations are used
– P Waves travel faster than S Waves
– The arrival of P Waves and S Waves are recorded
– The longer the lag time between the P and S
Waves, the farther away the earthquake.
Earthquake Measurement
• Measure the amount of energy released
• Measure the amount of damage caused
• Magnitude = the measure of the strength of the
earthquake.
• Richter Scale measures magnitude, 20th century
• Moment Magnitude Scale, 21st century
– More accurate for large earthquakes
– Measures the strength based on the size of the area of the
fault that moves, the average distance that the fault blocks
move, and the rigidity of the rocks in the fault zone.
– The larger the number, the stronger the earthquake.
– Largest recorded…9.5
– Magnitudes of less than 2.5 are usually not felt by people.
Intensity: Modified Mercalli Scale
• Before the development of magnitude scale, the
size of an earthquake was determined based on
the earthquake’s effects = intensity.
• Expresses intensity in roman numerals. From I to
XII and provides a description of the effects
caused by the earthquakes.
• XII – highest intensity and total destruction.
• Depends on the earthquakes magnitude, the
distance between the epicenter and the affected
area, the local geology, and the earthquake’s
duration.
Modified Mercalli Intensity Scale
Expected Damages
• Most damage is caused by the collapse of
buildings or falling objects and flying debris.
• Other damage is caused by landslides, fires,
explosions (broken electric and gas lines), and
floodwater (collapsing dams).
• CAN YOU THINK OF ANY OTHER DAMAGE?
Tsunamis
• Caused by an earthquake that occurs on the
ocean floor.
• The shifting ocean floor creates a Tsunami
(giant ocean wave) as energy is transferred.
Earthquake Warnings and Forecasts
• Scientists are trying to predict where
earthquakes might occur.
• They think that areas along faults that have
NOT shown movement (seismic gaps) might
be likely areas for future quakes since the
stress may be building in these areas.
Prediction of Earthquakes
• Foreshocks – little earthquakes that may
precede the main event.
• May be a few seconds up to a few weeks prior
to main event.
• Sensors may also detect slight tilting of the
ground and may be able to detect the stress
and strain building along fault zones.
• Predicting earthquakes is mostly unreliable.