Transcript Module 5 - Earthquakes - IST Akprind Yogyakarta

Module 5 - Earthquakes
Introduction
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E a r t h q u a k e s are probably the most frightening
naturally occurring hazard encountered. Why?
Earthquakes typically occur with little warning. There
is no escape from an earthquake! Earthquakes have
devastating effects, resulting in hundreds to
thousands of deaths and injuries, and millions to
billions of dollars worth of property damage.
The earthquake's location, magnitude of the
earthquake, surface geology, and population density
are major factors contributing to earthquake damage.
Although earthquakes can occur anywhere on earth,
most earthquakes (>90%) occur where tectonic
plates move against one another. The boundaries
along each plate are referred to as margins. Different
types of stresses are associated with each type of
margin. Convergent-plate margins have
compressional stresses (come together Þ Ü ,
therefore result in crustal shortening); divergent-plate
margins have tensional stresses (move apart Ü Þ ,
resulting in crustal extension); and transform-plate
margins have shear stresses (the plates slide past
each other ). Each type of margin has a
corresponding fault type associated with it.
Types of faults
• Earthquakes result from movement along a fault. Faults
and earthquakes are cause and effect. The sense of
motion on faults describes how the block move relative
to each other. Faults may move along preexisting
fracture or may form a new one. There are 3 basic types
of faults: normal, reverse, and strike-slip. Normal and
reverse faulting result in vertical slip, while strike-slip
faulting results in horizontal slip. In nature, motion is
seldom absolutely along one direction. There can be a
combination of vertical and horizontal slip, which would
make the movement along the fault oblique.
Normal faults
• Normal faults are associated with
extension. A good example of normal
faulting is the Basin and Range
topography of the western United
States. The western part of the North
American plate has been pulled apart
into a series of "blocks". Most Basin
and Range structures result from the
tilting of these blocks. A major Basin
and Range fault zone is the Wasatch
Fault zone, which is 220 miles long
(360 kilometers) and extends from Utah
into Idaho.
Reverse faults
• Reverse faults are
associated with
compressional forces- 2
plates or fault blocks
pushing towards each
other. One side ends up
on top! Thrust faults are
reverse faults that move
up a shallower angle than
ordinary reverse faults.
Reverse faults
• Strike-slip faults are associated with shear stresses. One
side of the fault "slides" past the other. "Sometimes" it is
fairly easy to recognize where movement on a strike-slip
fault has occurred. The photo below shows a creek
located along the San Andreas Fault. The zigzag effect
(offset) of the creek channel is the result of movement
along the fault.
• Compare the photo of the San Andreas Fault with the
strike-slip fault diagram. The San Andreas Fault is a
right-lateral strike-slip fault.
Earthquake processes
• Rupturing rocks release huge amounts of
energy. The sudden release of energy is
what is felt in an earthquake. Earthquake
energy is in the form of seismic waves.
The seismic waves radiate out from a
central point, called the focus or
hypocenter, like ripples moving outward
from a pebble tossed into a lake. The
location directly above the hypocenter, on
the earth's surface, is called the epicenter.
Seismic waves
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Four types of seismic waves are generated when faulting
triggers an earthquake. All the seismic waves are
generated at the same time, but travel at different speeds
and in different ways. Body waves penetrate the earth and
travel through it, while surface waves travel along the
surface of the ground.
Primary and secondary waves are body waves. Primary
waves (P-waves) travel the fastest and can move through
solids and liquids. The P-wave energy causes the ground
to move in a compressional motion in the same direction
that the wave is traveling. Secondary waves (S-waves) are
slower and travel only through solids. The S-wave energy
causes the ground to move in a shearing motion
perpendicular to the direction of wave movement.
Rayleigh and Love waves are the two types of surface
waves. Rayleigh wave energy causes a complex heaving
or rolling motion, while Love wave energy causes a
sideways movement. The combination of Rayleigh and
Love waves results in ground heave and swaying buildings.
Surface waves cause the most devastating damage to
buildings, bridges, and highways.
Detecting, locating, and measuring
earthquakes
• Several thousand stations monitor earthquakes all
over the world. Each station contains an instrument,
called a seismograph, used to detect arrival times and
record seismic waves. The seismograph consists of a
seismometer (the detector) and a recording device.
The seismometer electronically amplifies wave motion.
• The graph on which seismic waves are recorded is
called a seismogram. The amplitude of the recorded
seismic wave is the vertical distance between the crest
and trough of the waveform, therefore, the larger the
earthquake, the greater the amplitude of the
earthquake. The key to locating an earthquake's
epicenter is the difference in arrival time, called lag
time, of P- and S-waves.
Seismogram
Magnitude and intensity
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Earthquakes are categorized in two ways- magnitude and intensity.
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Magnitude indicates the severity of an earthquake using the Richter Scale,
a logarithmic, instrumentally determined measurement. Magnitude rates an
earthquake as a whole. The severity of an earthquake is a rating based on
the amplitude of the seismic waves.
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Larger amplitude waves equals higher magnitude earthquake equals
greater severity.
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Amplitude is the vertical distance between the trough and crest of a
waveform (sound familiar?).
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The Mercalli Scale defines intensity. Intensity is rated by how much damage
was caused by an earthquake and how it affected people.
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Go to the University of Nevada-Reno Richter scale page for an excellent
explanation of the Richter Scale and other earthquake quantifying tools.
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The UNR Seismological Laboratory Page is full of interesting information for
earthquake enthusiasists.
Magnitude
Earthquake damages (secondary
effects)
• Effects of an earthquake can be classified as primary or
secondary.
• Primary effects are permanent features produced by the
earthquake. Examples include fault scarps, surface
ruptures, and offsets of natural or human-constructed
objects.
• An example you have already seen is the creek offset
produced by movement along the San Andreas Fault.
• Secondary effects result when ground movement causes
other types of damage. Examples include landslides,
tsunami, liquefaction and fire.
• The amount of damage caused by an earthquake varies
with magnitude. The greater the magnitude, the greater
the damage potential.
Landslides
• Seismic vibration is a common triggering
mechanism for landslides. In hilly or
mountainous regions, landslides can have
particularly devastating effects. Damages
can range from debris-covered roadways
to extensive property damage and
numerous casualties.
Tsunami
• A tsunami is a sea wave triggered by a violent
displacement of the ocean floor, such as vertical
displacement of the seafloor along a fault. Underwater
earthquakes, submarine volcanic eruptions or landslides
can cause tsunami. Tsunami waves have very long
wavelengths (crest-to-crest) and can be enormous (as
large as 60 miles/100 kilometers). The height of a
tsunami in the open ocean is very low (generally less
than 1.5 feet/0.5 meters), while the speed of the tsunami
is very high. As it approaches a shallow coastline, its
speed is reduced, but the height of the tsunami
increases drastically, causing devastation on land.
Liquefaction
• How much can surface and subsurface material contribute to
earthquake damages? Like many other physical phenomena, the
answer is, "It depends." Thick sequences of unconsolidated
sediments, such as sand, mud, and artificial fill, greatly magnify
ground shaking during an earthquake. Ground shaking transmits
forces to building that most buildings are not designed and
constructed to endure. Ground shaking results in extensive property
damage. Bedrock is less likely to be affected by ground shaking
than is unconsolidated material. Buildings constructed on bedrock
sustain far less damage than those built on unconsolidated material.
Other dangers also come from the ground during an earthquake.
Buildings constructed on sandy soil prone to water saturation have
the greatest potential for complete destruction, because watersaturated sandy soil is subject to a phenomena called liquefaction.
During liquefaction, water-saturated soil behaves as a fluid rather
than as a solid. It becomes incapable of supporting much weight.
(Remember the soil module and the section of soil strength?)
Fires
• Earthquakes cause fires. Even moderate
ground shaking can break gas and
electrical lines, sever fuel lines, and
overturn stoves. Water pipes rupture,
making it impossible to fight the
earthquake-caused fires. The famous San
Francisco earthquake in 1906 ruptured the
city's main water pipes. Extensive fire
damage was the result!
Terms to Look Up and Know:
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* Epicenter
* Focus
* Hypocenter
* Faults:
. . . . normal
. . . . reverse
. . . . strike-slip
* Debris flow
* Ground shaking
• * Liquefaction
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• * Mercalli scale
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• * Modified Mercalli •
scale
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• * Primary effects
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• * Secondary effects
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• * Richter scale
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• * Seismograph
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• * Seismogram
• * Seismometer
* Seismologist
* Seismicity
* Seismic waves:
. . . . P-waves
. . . . S-waves
. . . . Rayleigh waves
. . . . Love waves
* Tsunami