Download Earthquakes

Earthquakes
What is an earthquake?
• Defined as movements of the ground that
are caused by a sudden release of energy
when rocks along a fault move.
– Sudden slip on a fault
– Resulting ground shaking
– Radiated seismic energy caused by the slip
• Caused by volcanic or magmatic activity,
• Caused by other sudden stress changes in
the earth.
What causes earthquakes?
• Tectonic plates move past each other
causing stress. Rocks near tectonic plate
boundaries experience a great amount of
stress.
– Elastic deformation – rock stretches then
reaches a breaking point, releasing energy.
Plate motion simulation
Elastic Rebound – sudden return of
elastically deformed rock to its
undeformed shape
http://www.uwgb.edu/dutchs/EarthSC-102VisualsIndex.HTM
Focus – point inside the Earth where the
first motion of an earthquake
begins
Epicenter – point on Earth’s surface above
focus
Seismic Waves
• Body Waves – waves that travel through
the body of a medium
– P Waves
– S Waves
• Surface Waves – waves that travel along
the surface of a body rather than through
the middle
– Love Waves
– Rayleigh Waves
Primary Waves (P Waves)
• A type of seismic wave that compresses and expands
the ground.
• The first wave to arrive at an earthquake.
• Particles of rock move in a back-and-forth direction
that is parallel to the direction in which the waves are
traveling.
• Can move through solids, liquids and gases.
http://daphne.meccahosting.com/~a0000e89/insideearth2.htm
Secondary Waves (S Waves)
• Second fastest wave.
• A type of seismic wave that moves the ground up and
down or side to side.
• Can only travel through solid material.
http://daphne.meccahosting.com/~a0000e89/insideearth2.htm
Comparing Seismic Waves
Surface Waves
The surface waves are the slowest of the
three earthquake wave types.
Two basic types of surface waves
• Love Waves
• Rayleigh Waves
1. L-waves or love waves.Complex
motion. Up-and-down and side-toside. Slowest. Causes damage to
structures during an earthquake.
2. Rayleigh waves involve rolling
motions. A surface particle moves in a
circle or ellipse in the direction of
propagation.
Using Seismic Waves to
Study Earth's Interior
Seismic Waves travel through the
entire Earth
Both S and P waves travel throughout
the body of the earth, and can be
picked up by seismograph - machines
that record earthquakes (ground
vibrations) - anywhere in the world.
Seismic waves as “x-rays” to
look inside the earth
• P-Waves travel
through solid and
liquid
Seismic waves as “x-rays” to
look inside the earth
• However, it turns
out that S waves
cannot travel
through the core,
and only P waves
are recorded in
some places:
S-Waves travel only through solids
Using Seismic Waves to Study
Earth's Interior
• Seismic waves travel
faster through denser
material.
• Because of this, the
path traveled by a
seismic wave is bent
towards the surface.
• Shadow zones are
areas where there are
no direct seismic
waves from a
particular earthquake.
Earthquakes and Tectonic
Environments
Fault zones form at plate boundaries because intense stress occurs there when
the plates separate, collide, subduct, or slide past each other.
• Convergent Oceanic Environments
–
–
–
–
Plates move toward each other and collide
Denser plate subducts
Top plate scrapes across subducting plate
Occurs between two oceanic plates or one oceanic plate and one
continental plate
Earthquakes and Tectonic
Environments
• Divergent Oceanic Environments
– Plates move away from each other
– Oceanic lithosphere is pulling away from both sides of each
ridge
– Spreading motion causes earthquakes
Earthquakes and Tectonic
Environments
• Continental Environments
– Continental plates converge, diverge, or move horizontally in
opposite directions
– Rock experiences stress – mountains form or earthquakes occur
Seismograph
• A seismograph is an
instrument that records
ground vibrations from
earthquakes. Mechanical
versions work by way of a
large mass, freely
suspended.
• In the example on the left, a
rotating drum records a red
line on a sheet of paper. If
the earth moves (in this
case from left to right) the
whole machine will vibrate
too.
• However, the large mass
tends to stay still, so the
drum shakes beneath the
pen, recording a squiggle!
Seismograph Video
How do scientists calculate how far a location
is from the epicenter of an earthquake?
• Scientists calculate the difference between
arrival times of the P waves and S waves
• The further away an earthquake is, the
greater the time between the arrival of the
P waves and the S waves
Typical Seismogram
tracing of earthquake motion
http://isu.indstate.edu/jspeer/Earth&Sky/EarthCh11.ppt
Earthquake Strength
The intensity or strength of an earthquake is
measured by seismologist in two main
ways:
1.The Richter Scale
• measures the amount of energy that an
earthquake releases as ground motion
• Each number of magnitude is 10x stronger
than the number below it.
How are Earthquakes Measured?
Richter Scale
The Moment Magnitude Scale
Geologists use this
scale today
It’s a rating system that estimates
the total energy released by an
earthquake based on the size of
the fault area that moves, how far
fault blocks move, and the rigidity
of rocks.
Can be used for any kind of
earthquakes, near or far
Some news reports may mention
the Richter scale, but the
magnitude number they quote is
almost always the moment
magnitude for that earthquake
Modified Mercalli Scale
• Measures the intensity of an earthquake
• Ranges from I to XII
• Based on common earthquake
occurrences such as "noticeable by
people" "damage to buildings" chimneys
collapse" "fissures open in the ground”.
The Modified Mercalli Scale
Developed in the twentieth
century to rate earthquakes
according to their intensity
The intensity of an earthquake
is the strength of ground
motion in a given place
Is not a precise measurement
But, the 12 steps explain the
damage given to people, land
surface, and buildings
The same earthquake could
have different Mercalli ratings
because of the different
amount of damage in different
spots
XII is total destruction
•The Mercalli scale uses Roman numerals to
rank earthquakes by how much damage they
cause
How are Earthquakes Measured?
Mercalli Intensity Scale
Click Link for Interactive Demo
http://elearning.niu.edu/simulations/images/S_portfolio/Mercalli/Mercalli_Scale.swf
How Earthquakes Cause Damage
The severe shaking provided
by seismic waves can damage
(building may sway or
collapse) or destroy buildings
and bridges, topple utility
poles, and damage gas and
water mains
With their side to side, up and
down movement, S waves can
damage or destroy buildings,
bridges, and fracture gas
mains.
Tsunami – giant ocean wave
http://www.uwgb.edu/dutchs/EarthSC-102VisualsIndex.HTM
Formation of a tsunami
waves get bigger as they move toward
shore
http://isu.indstate.edu/jspeer/Earth&Sky/EarthCh11.ppt
Tsunami Warning System
http://isu.indstate.edu/jspeer/Earth&Sky/EarthCh11.ppt
Tsumani’s Attack
Causes:
•Undersea earthquakes
•Landslides
•Volcanic eruptions
Earthquake Safety
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•
•
•
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Be prepared before an earthquake occurs
Know evacuation procedure
Stay calm
Protect yourself
Check for fire and other hazards
Earthquake damage in Anchorage on March 27, 1964
Earthquake Warning and Forecasts
• Scientists study past earthquakes to
predict future earthquakes
• Still no reliable way to predict exactly
when or where an earthquake will occur
• Seismic Gaps – an area along a fault
where relatively few earthquakes have
recently but where strong earthquakes
occurred in the past
San Francisco are built on sandy soil or fill. Many
homes built on this type of soil were badly damaged
during the 1989 Loma Prieta earthquake.
Earthquake Warning and Forecasts
• Seismic Gaps – an area along a fault where
relatively few earthquakes have occurred
recently but where strong earthquakes occurred
in the past
• Foreshocks – small earthquakes that can
potentially precede an earthquake by a few
seconds or a few weeks
• Changes in Rocks
– cracks filled with water
– natural gas seepage