EARTHQUAKES
Chapter 8
EARTHQUAKES
• According to your textbook, 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.
EARTHQUAKE DAMAGE
Turkey
Japan
Chile
Haiti
Taiwan
California
WHAT CAUSES AN EARTHQUAKE?
• The surface of the earth’s 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.
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.
EPICENTER AND FOCUS
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.
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 earth’s crust as a stick,
when the stick is bent, it stores elastic potential energy.
• Eventually the stick (earth’s 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).
ELASTIC REBOUND HYPOTHESIS
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
earth’s crust. Geologists can sometimes detect
foreshocks occurring before a large quake.
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.
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.
SEISMOGRAPHS AND SEISMOGRAMS
SEISMIC WAVES
• Seismologists can tell a lot about earthquakes and the
earth’s interior from seismic waves.
• There are basically three main types of seismic waves.
• Surface Waves – these waves only travel through the
earth’s 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.
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.
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.
SEISMIC WAVES
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 earthquake’s epicenter and if at least
three distances are known, the exact location can be
determined by triangulation.
READING A SEISMOGRAM
READING A DISTANCE TRAVEL TIME GRAPH
FINDING THE LOCATION OF AN EPICENTER
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.
THE CIRCUM-PACIFIC BELT (PACIFIC RING OF FIRE)
HOW TO DETERMINE THE MAGNITUDE OF AN EARTHQUAKE
• The old familiar scale that was used for determining an
earthquake’s 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.
UNDERSTANDING THE RICHTER SCALE
UNDERSTANDING THE RICHTER SCALE
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.
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.
1964 ALASKAN EARTHQUAKE
1964 ALASKAN EARTHQUAKE
1964 ALASKAN EARTHQUAKE
1964 ALASKAN EARTHQUAKE
1964 ALASKAN EARTHQUAKE
1964 ALASKAN EARTHQUAKE
1964 ALASKAN EARTHQUAKE
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.
WHAT TYPES OF BUILDING DESIGNS WORK?
EARTHQUAKE PROOF BUILDINGS
EARTHQUAKE PROOF FOUNDATIONS
SHEAR STRENGTH ON WALLS
BUILDING DAMAGE FROM THE EARTHQUAKE IN
HAITI (JANUARY 12, 2010)
EARTHQUAKE IN HAITI (2010)
EARTHQUAKE IN HAITI (2010)
EARTHQUAKE IN HAITI (2010)
WHAT WAS DIFFERENT BETWEEN THE
EARTHQUAKES IN ALASKA (1964) AND HAITI (2010)?
• The earthquake in Haiti was only a magnitude 7.0 compared
with Alaska’s 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.
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.
LIQUEFACTION OF SOILS
THE RESULTS OF LIQUEFACTION OF SOILS
THE RESULTS OF LIQUEFACTION OF SOILS
LIQUEFACTION OF SOILS DURING THE 1964
ALASKAN EARTHQUAKE
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.
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.
TSUNAMI IN JAPAN
TSUNAMI IN THAILAND DURING THE 2004 INDIAN
OCEAN EARTHQUAKE
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.
WHAT LESSONS CAN WE LEARN?
• If at all possible don’t 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.
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.
LITUYA BAY, ALASKA
1964 ALASKAN EARTHQUAKE
• 200 acres of land were swept out to sea in the
Turnagain Heights area of Anchorage, AK
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.
THE SAN FRANCISCO FIRE OF 1906
THE SAN FRANCISCO FIRE OF 1906
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.
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.
THE THREE ZONES OF THE EARTH BASED ON
COMPOSITION
• These three zones based on composition
are the crust, the mantle and the core .
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.
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.
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 earth’s 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.
FOUR ZONES OF THE EARTH BASED ON
PHYSICAL PROPERTIES
EARTH’S LAYERS AND COMPOSITION
• Another discovery about the earth’s 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 can’t travel through a liquid so the core
creates a shadow zone.
THE MOHO AND SHADOW ZONES