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The Geology Behind Earth’s
Features
Natural Disasters
Essential Questions
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What is an earthquake?
What is the way to detect an earthquake?
What is a Tsunami?
What are the ways to prepare for a natural
disaster?
What is an earthquake?
Bill Nye - Earthquakes
 http://learning.aliant.net/Player/ALC_Player.
asp?ProgID=DEP_BN027
 Complete the Question sheet provided.
 24mins
 Earthquake:
– Occur at the boundaries between tectonic
plates
– Occur as a result of the forces of stress, strain,
and strength
 Stress
– Is the local force per unit area that causes rocks to
deform
 Strain
– Is the relative amount of deformation, expressed as the
percentage of distortion (eg. compression of a rock by
1% of its length)
 Strength
– Rocks fail that is they lose cohesion and break into two
or more parts when they ae stressed beyond a critical
value
Seismic Waves
 There are three types of seismic waves
– P-waves
– S-waves
– Surface waves
P-waves
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Primary waves, also called P waves or compressional waves
P waves arrive first at any surface location
can travel through solid, liquid and gas
Can travel through rock at 6km/s
are waves that have the same direction of vibration along their direction
of travel, which means that the vibration of the medium (particle) is in
the same direction or opposite direction as the motion of the wave
 As they travel through rock, the waves move tiny rock particles back
and forth -- pushing them apart and then back together
 http://www.geo.mtu.edu/UPSeis/images/P-wave_animation.gif
S-waves
 Secondary waves, also called S waves or shear
waves
 As these waves move, they displace rock particles
outward
 S waves don't move straight through the earth
 only travel through solid material
 the ground is displaced perpendicularly to the moves
alternately to one side and then the other
 http://www.geo.mtu.edu/UPSeis/images/Swave_animation.gif
Surface Waves
 sometimes called long waves, or simply L wave
 are responsible for most of the damage associated
with earthquakes, because they cause the most
intense vibrations
 stem from body waves that reach the surface
 are something like the waves in a body of water -they move the surface of the earth up and down
 L waves are the slowest moving of all waves
 Both P and S waves refract or reflect at points
where layers of differing physical properties meet.
They also reduce speed when moving through
hotter material. These changes in direction and
velocity are the means of locating discontinuities.
 Seismic discontinuities (a surface at which
velocities of seismic waves change abruptly) aid in
distinguishing divisions of the Earth into inner
core, outer core, D", lower mantle, transition
region, upper mantle, and crust (oceanic and
continental).
Earthquakes
 Did you know…
– According to the United States Geological
Survey, more than three million earthquakes
occur every year. That's about 8,000 a day, or
one every 11 seconds!
How does an Earthquake occur?
How do Earthquakes Occur?
 When brittle rocks being stressed suddenly fail
along a geologic fault
 Most large earthquakes are caused by ruptures of
pre-existing faults, where past earthquakes have
already weakened the rocks on the fault surface
 The two blocks of rock on either side of the fault
slip suddenly, releasing energy in the form of
seismic waves
– When the fault slips, the stress is reduced, dropping to a
level below he rock strength
 A fault rupture does not happen all at once
– It begins at the focus and expands outward
along the fault surface ~2-3km/s
– It stops where the stresses become insufficient
to continue breaking the fault or where the
rupture enters ductile material in which it can no
longer propagate as a fracture
 Fault ruptures in the largest earthquakes
can extend for more than 1000km and the
fault slip can be as large as 20m
Earthquake Terminology
 Focus
– The point at which fault slipping begins
 Epicenter
– The geographic point on Earth’s surface directly above
the focus
 Focal depth
– In continental crust is ~2-20km
– Below 20km is rare because under the high
temperatures and pressures the crust behaves as a
ductile material
 However in subduction zones where cold oceanic crust plunges
into the mantle earthquakes can originate at depths as great as
690km
Earthquake Terminology continued
 Foreshock
– A small earthquake that occurs near, but before, a
mainshock
 Aftershock
– Large earthquakes trigger smaller earthquakes
– Follow the mainshock
 Their foci are distributed in and around the rupture plane of the
mainshock
– Happen where that stress exceeds he rock strength
– The number and sizes depend on the magnitude of the
mainshock
– P 348 fig 13.6
The Elastic Rebound Theory
 Proposed by Henry Fielding Reid of John
Hopkins University in 1910
 Explains why earthquakes recur on active
faults
 P345 Fig 13.1
 P 347 fig 13.5
What is the way to detect an
earthquake?
Detection
 Detecting an earthquake is much easier than
predicting one.
– a powerful earthquake can be felt by people in the area,
and the damage it causes can be seen.
 Seismograph
– An instrument that records the seismic waves generated
by earthquakes
 Seismogram
– A record, graphed or digital, of the seismic activities of
an area
Ideal Seismographs
 It would be a device affixed to a stationary
frame not attached to Earth when the
ground shook, the seismograph would
measure the changing distance between the
frame which did not move and the vibrating
ground
Current Seismographs
 Attach a dense mass, such as a piece of steel to Earth so
loosely that the ground can vibrate up and down or side to
side without causing much motion of the mass
 Attachment is usually a spring (for vertical movement) or
hinge (for horizontal movement)
 When seismic waves move the ground, the mass tends to
remain stationary because of its inertia, but the mass and
the ground move relative to each other because the spring
compresses or stretches or the hinge swings left and right
 Record bthe movements automatically
Reading a Seismogram
 When you look at a seismogram, there will be
wiggly lines all across it. These are all the seismic
waves that the seismograph has recorded.
 Most of these waves were so small that nobody
felt them.
Reading a Seismogram
continued
 The P wave will be the first wiggle that is bigger than the
rest of the little ones (the microseisms).
– Because P waves are the fastest seismic waves, they will usually
be the first ones that your seismograph records.
 The next set of seismic waves on your seismogram will be
the S waves.
– These are usually bigger than the P waves.
– If there aren't any S waves marked on your seismogram, it probably
means the earthquake happened on the other side of the planet. S
waves can't travel through the liquid layers of the earth so these
waves never made it to your seismograph.
 The surface waves are the other, often larger, waves
marked on the seismogram.
– They have a lower frequency.
– Surface waves travel a little slower than S waves.
Finding the Epicenter
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Measure the distance between the
first P wave and the first S wave. In
this case, the first P and S waves
are 24 seconds apart.
Find the point for 24 seconds on the
left side of the chart below and
mark that point. According to the
chart, this earthquake's epicenter
was 215 kilometers away.
Measure the amplitude of the
strongest wave. The amplitude is
the height (on paper) of the
strongest wave. On this
seismogram, the amplitude is 23
millimeters. Find 23 millimeters on
the right side of the chart and mark
that point.
Place a ruler (or straight edge) on
the chart between the points you
marked for the distance to the
epicenter and the amplitude. The
point where your ruler crosses the
middle line on the chart marks the
magnitude (strength) of the
earthquake. This earthquake had a
magnitude of 5.0.
Magnitude
 Is related to the total area of the fault rupture
– Most earthquakes are very small and the
rupture never breaks the ground surface
– In large earthquakes surface breaks are
commonb
The Richter Scale
 The Richter Scale is the best known scale for
measuring the magnitude of earthquakes.
 The energy released by an earthquake
increases by a factor of 30 for every unit
increase in the Richter scale.
 An earthquake that measures 4.0 on the Richter
scale is 10 times larger than one that measures
3.0
Richter scale no.
No. of earthquakes per
year
< 3.4
800 000
3.5 - 4.2
30 000
4.3 - 4.8
4 800
Most people notice them,
windows rattle.
1400
Everyone notices them,
dishes may break,
open doors swing.
4.9 - 5.4
Typical effects of this
magnitude
Detected only by
seismometers
Just about noticeable
indoors
500
Slight damage to
buildings, plaster
cracks, bricks fall.
100
Much damage to buildings:
chimneys fall, houses
move on foundations.
7.0 - 7.3
15
Serious damage: bridges
twist, walls fracture,
buildings may collapse.
7.4 - 7.9
4
5.5 - 6.1
6.2 6.9
> 8.0
One every 5 to 10 years
Great damage, most
buildings collapse.
Total damage, surface
waves seen, objects
thrown in the air.
What is a Tsunami?
What are the ways to prepare for a
natural disaster?