Your textbook estimates there are approximately 30,000
earthquakes occur every year. According the USGS, several million
earthquakes occur each year on average. On average only about 75
per year are very large. Most of these earthquakes occur in remote
locations where very little harm is done. Occasionally, these large
earthquakes occur in areas with a large population of people and
they are one of the most destructive forces on earth.
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EARTHQUAKE DAMAGE Turkey JapanTaiwan Chile Haiti
California
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WHAT CAUSES AN EARTHQUAKE? The surface of the earths crust is
made of plates. Below these plates is hot molten rock called magma.
The heat from this magma causes the plates to move. There are
boundaries between the different plates. When the plates move it
creates pressure and friction between these plate boundaries. When
the pressure that builds up is released, an earthquake occurs.
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WHERE AN EARTHQUAKE OCCURS When an earthquake occurs, there is
a specific place usually along a plate boundary where the pressure
is released and a large seismic wave is produced. The actual
specific place where the pressure is released is called the focus.
Most earthquake foci occur at great depths. The epicenter is a
location on the surface of the earth located directly above the
focus.
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EPICENTER AND FOCUS
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EPICENTERS AND FAULT LINES When geologists are trying to
describe to the general public the location of an earthquake, it is
not necessary to explain how deep the earthquake was. Most people
generally would like to know the location of the quake in relation
to a familiar place. That is why epicenters are most often used. A
fault line is the fracture or boundary between two plates. It is
most commonly where earthquakes occur.
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WHAT IS THE ELASTIC REBOUND HYPOTHESIS? The elastic rebound
hypothesis is an explanation for how seismic waves are produced and
generated. Basically, if you think of the earths crust as a stick,
when the stick is bent, it stores elastic potential energy.
Eventually the stick (earths crust) will build up enough energy to
overcome the bonds that hold the stick together. When this occurs
the stick will break. When the stick breaks, it releases all its
stored energy, producing a wave (seismic wave).
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ELASTIC REBOUND HYPOTHESIS
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WHAT ARE AFTERSHOCKS AND FORESHOCKS If you think of the analogy
of the stick from the elastic rebound hypothesis, it is easy to
understand where aftershocks and foreshocks come from. If you were
to bend the stick, you might hear cracks occurring before the stick
snapped in half. These cracks are the equivalent to a foreshock in
the earths crust. Geologists can sometimes detect foreshocks
occurring before a large quake.
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FORESHOCKS AND AFTERSHOCKS Foreshocks are small earthquakes
that often occur before a large earthquake. Aftershocks are small
earthquakes that occur after a large earthquake occurs. These were
recorded from the Japan earthquake that occurred on March 11,
2011.
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SEISMOLOGY AND THEIR INSTRUMENTS Seismology is the study of
earthquakes. Waves produced by earthquakes are called seismic
waves. Instruments that record seismic waves are called
seismographs. Seismographs produce a written or digital record of
seismic waves produced from earthquakes. The recorded seismic waves
are called seismograms.
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SEISMOGRAPHS AND SEISMOGRAMS
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SEISMIC WAVES Seismologists can tell a lot about earthquakes
and the earths interior from seismic waves. There are basically
three main types of seismic waves. Surface Waves these waves only
travel through the earths outer layer. These waves move similarly
to the way an ocean wave moves. These waves move both up and down,
as well as side to side. Surface waves are the most destructive
seismic waves.
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SEISMIC WAVES The other two types of waves are called body
waves because they can travel through the earth. The first body
waves are called P waves or primary waves. P waves are the fastest
seismic waves. Sometimes they are also called push-pull waves or
compression waves because they compress and expand the rocks as
they travel through the earth. They move similarly to a spring
being compressed.
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SEISMIC WAVES The second body wave is called an S wave or
secondary wave. S waves shake the ground at right angles to the
direction the wave is traveling.
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SEISMIC WAVES
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SEISMOGRAMS AND SEISMIC WAVES Seismic waves are recorded on
seismograms. P waves are the fastest waves and are about 1.7 times
faster than S waves. Surface waves are the slowest and are about 90
percent of the speed of the S waves. Knowing the speed of the waves
can tell seismologists the distance to an earthquakes epicenter and
if at least three distances are known, the exact location can be
determined by triangulation.
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READING A SEISMOGRAM
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READING A DISTANCE TRAVEL TIME GRAPH
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FINDING THE LOCATION OF AN EPICENTER
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WHERE DO EARTHQUAKES OCCUR? About 95% of most earthquakes occur
along active plate boundaries. Most of these occur along the
Circum-Pacific belt, also known as the Pacific Ring of Fire.
Another belt where many earthquakes occur is called the
Mediterranean-Asian belt.
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THE CIRCUM-PACIFIC BELT (PACIFIC RING OF FIRE)
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HOW TO DETERMINE THE MAGNITUDE OF AN EARTHQUAKE The old
familiar scale that was used for determining an earthquakes
magnitude is called the Richter Scale. The Richter Scale is based
on the amplitude of the largest seismic wave recorded on a
seismogram. It is a logarithmic scale which means that if the
amplitude of a 5.0 seismic wave 10 times greater in amplitude than
a 4.0 seismic wave. So the amount of shaking or intensity (energy
released) of a 5.0 earthquake is 33 times greater than a 4.0.
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UNDERSTANDING THE RICHTER SCALE
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LIMITATIONS OF THE RICHTER SCALE AND THE USE OF THE MOMENT
MAGNITUDE SCALE The problem with the Richter Scale is that the
intensity of waves decrease with the distance from the epicenter.
Because of this, the Richter Scale is only accurate out to about
500 km (300 miles). Seismologists use another method called the
Moment Magnitude Scale, which is believed to be more accurate. The
Moment Magnitude Scale is based on three things; the average
displacement along the fault, the surface area of the fault, and
the rigidity (strength) of the rock.
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DESTRUCTION FROM EARTHQUAKES At 5:36 PM, March 27 (Good
Friday), 1964, an enormous earthquake occurred about 75 miles east
of Anchorage. It was the largest recorded earthquake ever recorded
in North America. It was the second largest earthquake ever
recorded. (1960 Chile earthquake was the largest) It had a moment
magnitude of 9.2 and lasted about 3 to 4 minutes. (1960 Chile
earthquake was a 9.5) Luckily, very few people lived in Alaska at
the time, and it occurred on Good Friday (a holiday), so only 131
people died. Which is a relatively small number considering the
enormous magnitude of the earthquake.
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1964 ALASKAN EARTHQUAKE
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WHAT LESSONS CAN WE LEARN FROM THE 1964 ALASKA EARTHQUAKE?
Scientists and engineers have learned a great deal from the 1964
Alaska earthquake. Approximately $300 million dollars worth of
damage. Many building and homes were destroyed by the intensity and
duration of the shaking. Steel framed and wood framed buildings and
home withstood better than unreinforced stone or brick homes and
buildings.
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WHAT TYPES OF BUILDING DESIGNS WORK?
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EARTHQUAKE PROOF BUILDINGS
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EARTHQUAKE PROOF FOUNDATIONS
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SHEAR STRENGTH ON WALLS
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BUILDING DAMAGE FROM THE EARTHQUAKE IN HAITI (JANUARY 12,
2010)
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EARTHQUAKE IN HAITI (2010)
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WHAT WAS DIFFERENT BETWEEN THE EARTHQUAKES IN ALASKA (1964) AND
HAITI (2010)? The earthquake in Haiti was only a magnitude 7.0
compared with Alaskas 9.2 (1964). That is nearly 1000 times less
intensity. Alaska only had 131 fatalities. Haiti had over 300,000
fatalities The earthquake in Haiti made 1,000,000 people homeless,
and 250,000 homes were destroyed. The primary difference was the
building designs. In addition to building design, population size
was also a factor.
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WHAT ARE SOME OTHER FACTORS THAT CAUSE DAMAGE DURING AND AFTER
AN EARTHQUAKE? Normally, under ordinary conditions soil that has a
small amount of moisture in it is very stable. The cohesive
properties of water actually make the soil more stable and help
hold the soil together. But, seismic waves can cause the bonds of
the soil and water to break and cause the stable damp soils to
become like an unstable soup of mud. This is known as liquefaction
of soils.
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LIQUEFACTION OF SOILS
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THE RESULTS OF LIQUEFACTION OF SOILS
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LIQUEFACTION OF SOILS DURING THE 1964 ALASKAN EARTHQUAKE
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WHAT ARE SOME OTHER FACTORS THAT CAUSE DAMAGE FROM AN
EARTHQUAKE? The largest loss of life during the 1964 Alaskan
earthquake as well as many other earthquakes like the one that
occurred in Japan on March 11, 2011 are seismic sea waves called
tsunamis. Tsunamis are usually caused when an earthquake occurs on
the floor of the ocean. The underwater quake quickly displaces a
large volume of water. Tsunamis are hard to detect in the ocean,
and most are only about a foot tall, and travel at speeds up to 500
MPH.
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TSUNAMIS As a tsunami approaches shore, it slows down, builds
up height, sometimes approaching 100 feet or more. Before the
tsunami reaches shore, witnesses have often reported a vacuum
effect occurs and all the water rushes out to sea, exposing the sea
floor. When this happens, usually within five minutes or less the
main wave comes rushing in at enormous speed. After the main wave,
other waves may follow.
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TSUNAMI IN JAPAN
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TSUNAMI IN THAILAND DURING THE 2004 INDIAN OCEAN
EARTHQUAKE
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HOW A TSUNAMI OCCURS In 2004, 230,000 people died from a
tsunami that occurred in the Indian Ocean. In 2011, nearly 16,000
people died from a tsunami in Japan. In 1964, 119 out of the 131
people who died from the Alaskan earthquake died from
tsunamis.
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WHAT LESSONS CAN WE LEARN? If at all possible dont live too
close to sea level. If a large earthquake occurs, get to high
ground quickly. The plate itself can sometimes lower in elevation.
The government has created the Pacific Tsunami Warning Center,
located in Hawaii and in coastal communities located around the
Pacific Ocean. If a large earthquake occurs, buoys in the ocean can
usually detect if a tsunami will occur and if it is not already too
late, a warning will be issued.
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WHAT ARE SOME OTHER FACTORS THAT CAUSE DAMAGE FROM AN
EARTHQUAKE? Another major damaging affect of many earthquakes is
large landslides. During the 1964 Alaskan earthquake, many people
lost their homes and land due to the land collapsing beneath their
feet and broken foundations. In 1958, a landslide triggered by an
earthquake caused a massive megatsunami in Lituya Bay, Alaska that
reached a record height of over 1,700 feet.
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LITUYA BAY, ALASKA
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1964 ALASKAN EARTHQUAKE 200 acres of land were swept out to sea
in the Turnagain Heights area of Anchorage, AK
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WHAT ARE SOME OTHER FACTORS THAT CAUSE DAMAGE FROM AN
EARTHQUAKE? Believe it or not, fires can become a major hazard as a
result of an earthquake. During an earthquake, gas and electrical
lines often rupture. When this happens fires can occur. Often
times, broken water lines occur as well, making it very difficult
to put out the fires. In 1906, in San Francisco, 80% of the city
burned down from a fire that was caused by an earthquake.
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THE SAN FRANCISCO FIRE OF 1906
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EARTHQUAKE PREDICTIONS So far, scientists have been unable to
successfully predict earthquakes with any consistent accuracy. By
measuring movements in the ground, pressure and water levels in
aquifers, radon gas emissions from fractures, scientists can gather
clues that can help them assess the risk of an earthquake.
Scientist can also use patterns of when earthquakes have occurred
in the past to locate a seismic gap or an area along a fault which
has not had an earthquake for a long time and therefore is likely
to have one in the future.
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CAN INFORMATION GAINED FROM EARTHQUAKES BE USEFUL? Seismic
waves that travel through the earth give clues to geologists about
the composition of the earth. If the inside of the earth was all
the same material, the seismic waves would travel straight through.
But that is not how they travel. In fact, seismic waves bend or
refract as they travel through the earth. Based on seismic waves,
scientists believe the earth is made up of three zones based on
their composition.
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THE THREE ZONES OF THE EARTH BASED ON COMPOSITION These three
zones based on composition are the crust, the mantle and the
core.
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THREE ZONES OF THE EARTH BASED ON COMPOSITION The Crust A thin,
rocky outer layer made of ocean crust and continental crust. Made
up of mostly rocks like basalt, gabbro (ocean), and granite
(continental). The Mantle A solid rocky material but behaves like a
plastic material that can bend and flow. It is made up of mostly
ultramafic rocks like peridotite. The Core Believed to be an
iron-nickel alloy material.
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FOUR ZONES OF THE EARTH BASED ON PHYSICAL PROPERTIES The earth
can also be divided into four different zones based on physical
properties. The Lithosphere (brittle) The crust and the upper
mantle make up the solid lithosphere. The Asthenosphere (ductile)
The rocks in the asthenosphere are softer than the upper mantle and
lithosphere. These rocks are close to their melting points and can
deform very easily.
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FOUR ZONES OF THE EARTH BASED ON PHYSICAL PROPERTIES The Outer
Core This layer is hot enough to melt rock completely. Therefore
this layer is a liquid layer. The flow of metallic iron in this
region is believed to create the earths magnetic field. The Inner
Core Even though this layer is hot enough to melt the rock, the
immense pressures created by the extreme depths force this material
into a solid state.
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FOUR ZONES OF THE EARTH BASED ON PHYSICAL PROPERTIES
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EARTHS LAYERS AND COMPOSITION Another discovery about the
earths composition is something called the Mohorovicic
discontinuity. A Croatian scientist discovered it and most
scientists just refer to it as the Moho. The Moho is a boundary
layer at about 50 km deep, where the crust is separated from the
mantle. Scientists also determined the core was liquid because S
waves cant travel through a liquid so the core creates a shadow
zone.