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Earthquakes
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Earthquakes
Most earthquake are not very strong or powerful,
and little, if any, damage occurs A few powerful
earthquakes occur each year and can cause death
and destruction A few tremendously powerful
earthquakes occur each century which cause
staggering devastation and uncountable death
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Earthquakes
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Worst Earthquake Ever
July 5, 1201 AD Easter Mediterranean Several
other earthquakes and tsunamis also occurred
around 1201 Egypt and Syria suffered the worst
damage 1,100,000 dead Mud brick buildings caved-in
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Where Do Earthquakes Occur?
The majority of earthquakes occur along tectonic
plate boundaries These boundaries are faults or
fault lines Faults are breaks or fractures in
rocks along which there has been movement of one
side of the fault relative to the other side
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Where Do Earthquakes Occur?
Plates do not smoothly move into each other or
pass by one another Instead they move in a series
of lurches Each lurch is an earthquake The bigger
the lurch, the stronger the earthquake
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Types of Earthquake Faults

• The main types of faults that cause earthquakes
  are
• Normal
• Thrust (or Reverse)
• Strike-slip

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Types of Earthquake Faults
The fault movement can be caused by three main
type of forces push (compression) pull
(tension) shove (shear)
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Types of Earthquake Faults
Normal faults generally occur in where the
lithosphere is being stretched apart by tension
forces For example, in the African Rift Valley
or the mid-oceanic ridges
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Types of Earthquake Faults
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Types of Earthquake Faults
Thrust faults generally occur in where one
tectonic plate is being subducted under another
plate or two continental plates are colliding
These collisions create compression forces
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Types of Earthquake Faults
These compression forces create the strongest and
most damaging earthquakes For example, in
northern India in the Himalaya Mountains or along
the west coast of South America
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Types of Earthquake Faults
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Types of Earthquake Faults
Strike-slip faults generally occur in where one
tectonic plate sliding past another plate The
San Andreas fault in California is the most
famous example
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Types of Earthquake Faults
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Intraplate Earthquakes

• Not all earthquakes occur along plate boundaries
• Intraplate earthquakes occur in plate interiors
• The three New Madrid earthquakes in 1811-12 and
  Charleston earthquake in 1886 are prime examples
  (we will talk about both)
• It is believed that intraplate earthquakes occur
  on old faults that were once parts of ancient
  plate boundaries, which remain zones of crustal
  weakness

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Where Do Earthquakes Occur?
A plot of all earthquakes from 1975 to 1995
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Recent USA Earthquakes
A map showing all of the detectable earthquakes
that occurred in the USA over a 7 day period 95
western US 4 eastern US 74 Alaska 11
Hawaii 16 Puerto Rico
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Future USA Earthquakes
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Recent Tennessee Earthquakes
A map showing all of the detectable earthquakes
that occurred over an 11 year period Note that
most occurred near Dyersburg
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Where Earthquakes Occur

• In summery
• Majority of earthquakes occur around tectonic
  plate boundaries and margins
• Worlds strongest and most deadly earthquakes
  occur at convergent plate boundaries
• Deepest earthquakes also occur at convergent
  plate boundaries

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Where Earthquakes Occur

• More summery
• Normal faults in rift valleys and in seafloor
  spreading
• Transform fault boundaries, such the San Andreas
  fault in California
• Intraplate earthquakes occur along old plate
  boundaries

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Earthquakes
The elastic rebound theory of earthquakes
explains how repeated cycles of stress
accumulation and release along a fault line
create earthquake of all magnitudes
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Earthquakes
The two sides of the fault are locked together
Stress begins Elastic deformation begins
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Earthquakes
Stress continues to build up in the rocks over
decades or centuries or longer Elastic
deformation continues to get worse
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Earthquakes
The stress exceeds the strength of the rock The
fault slips and the earthquake occurs The rocks
rebound (snap back) elastically
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Creep Versus Earthquake
When movement along a fault is smooth and gradual
it is called creep or aseismic slip Cannot be
felt and rarely causes damage
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Focus
The focus, or hypocenter, is the point on a fault
at which the first movement or break occurs
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Epicenter
The point of the earths surface directly above
the focus is called the epicenter
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Focus and Epicenter

• Typically given as a location and depth
• Seismologists report that the epicenter of last
  nights destructive earthquake was located 56
  kilometers southeast of Los Angeles at a depth of
  10 kilometers.

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Earthquake
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Earthquake
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Earthquake
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Earthquake
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Elastic Rebound
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Elastic Rebound
Note the straight fence line after the elastic
rebound!!
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Foreshocks
One or more smaller earthquakes that precede a
larger earthquake Occurs in the same vicinity as
the larger earthquake, perhaps on the same fault
or adjacent faults Many earthquakes are NOT
proceeded by foreshocks
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Aftershocks

• Smaller earthquakes that occur afterwards as the
  consequence of a larger earthquake
• Usually along the same fault, but can be on
  adjacent faults
• Can occur for years after a very large earthquake
  
• Some aftershocks can cause serious damage and
  death

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Foreshocks and Aftershocks
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Seismic Waves

• When an earthquake occurs, it releases the
  stored-up energy as seismic waves
• There are 3 kinds of seismic waves generated by
  earthquakes and they are characterized by
  distinct kinds of motion
• P (primary) waves
• S (secondary) waves
• Surface waves

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Seismic P Waves
P waves travel as a series of contractions and
expansions, pushing and pulling on the rock in
the direction of their path of travel
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Seismic P Waves
P waves travel through rock at the rate of 6-8
kilometers per second
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Seismic S Waves
S waves are shear waves that push material at
right angles (side to side) along their path of
travel in the rock
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Seismic S Waves
S waves travel 4-5 kilometers per second through
rock
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Seismic Surface Waves
Surface waves move across the Earths surface
Surface waves travel slightly slower than S
waves There are two types of surface waves
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Seismic Surface Waves
In one type the ground surface moves vertically
in a rolling, elliptical motion that dies down
with depth beneath the surface
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Seismic Surface Waves
In the second type, the ground shakes sideways,
with no vertical motion
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Seismograph
A seismograph instrument is used to detect and
record ground motion caused by seismic waves
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Seismic Waves
Seismic waves generated at an earthquakes focus
travel thru the Earth interior and on the surface
and arrive at a seismograph far from the
earthquake
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Speed of Seismic Waves
P waves, S waves and surface waves travel at
different speeds and arrive at the seismograph at
different times P first, S second and surface last
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Locating the Epicenter

• The difference between the arrival times of the P
  and S waves at a recording station is a function
  of the distance from the epicenter
• Therefore, if you have at least three seismic
  stations, you can determine the location of an
  epicenter by triangulation

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Locating the Epicenter
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Locating the Epicenter
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Type of Earthquake Fault
The first motion of the P wave is used to
determine the orientation of the fault plane
direction of slip
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Magnitude and Intensity

• 2004 Southeast Asia earthquake caused the entire
  Earth to vibrate 1/2 inch and lasted 10 minutes
• 1964 Alaska earthquake displaced 70,000 square
  miles of the seafloor by up to 50 feet
• 1906 San Francisco earthquake moved the city 8.5
  feet
• 2007 East Tennessee earthquake was not felt by
  most people even though it occurred at 605 pm
• Hundreds of thousands of silent earthquakes are
  so mild that they can only be detected by
  seismographs

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Magnitude and Intensity

• Magnitude and intensity are the two major ways to
  judge the power of an earthquake
• Richter Magnitude Scale is based on amount of
  energy released by the earthquake
• Modified Mercalli Intensity Scale is based on
  damage and human perception

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Richter Scale

• The Richter Magnitude Scale was developed in 1935
  by Charles Richter to rate the power of
  California earthquakes
• It measures the amount of energy received 100 km
  from epicenter
• It is a logarithm scale based upon magnitude
• For every unit increase in Richter magnitude,
  ground displacement increases by a factor of 10,
  while the energy released increases by a factor
  of 30

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Richter Scale
Based upon a relationship of the amplitude of the
largest wave and the time interval between the
arrival of the P and S waves
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Moment Magnitude Scale

• Geologists tried to improve upon the Richter
  scale
• The Moment Magnitude Scale is more directly
  related to the force of the earthquake than the
  Richter Scale
• Proportional to the logarithm of the area of the
  break in the fault
• Also proportional to the logarithm of the seismic
  energy released
• Can be determined more accurately
• However, the news media persistently reports the
  Richter magnitude

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Moment Magnitude Scale
Relationship between Moment Magnitude and the
energy released by the earthquake
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Shaking Intensity

• The Richter Scale and Moment Magnitude measure
  force
• An Italian geologist, Giuseppe Mercalli, devised
  an intensity scale that described what the
  shaking felt like to a human observer and the
  damage that was done
• Many similar scales have been developed and are
  in use world-wide

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Modified Mercalli Intensity Scale
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Modified Mercalli Intensity Scale
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Modified Mercalli Intensity Scale
Intensities Associated with the 1811 earthquake
at New Madrid, Missouri Richter scale of 8.4 and
Mercalli of XI East Tennessee was a VI
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Modified Mercalli Intensity Scale
Intensities Associated with the 1886 earthquake
at Charleston, South Carolina Richter scale of
6.9 and Mercalli of XI Again East Tennessee was
a VI
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Damage Due to Earthquakes
Ground Movement Earthquakes do not kill people,
buildings kill people.
16 died in this building in California
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Damage Due to Earthquakes
Ground Movement Freeways kill people too This
elevated freeway in Kobe, Japan was overturned
during a 1995 earthquake (bad day to commute!)
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Damage Due to Earthquakes
Fire The fire caused by the 1906 San Francisco
earthquake did considerable damage
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Damage Due to Earthquakes
Landslides
Two towns were buried by a massive landslide in
Peru during a magnitude 8 earthquake in 1970
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Damage Due to Earthquakes
Flood Dams giving way during or after earthquakes
can cause tremendous flooding
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Damage Due to Earthquakes
Tsunami
Movement of seafloor during an earthquake
produces a surge of water that becomes a long sea
wave
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Damage Due to Earthquakes
Tsunami
A tsunami only a few centimeters in the open
ocean can produce waves that are many meters high
on shore
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Damage Due to Earthquakes
Tsunami
A tsunami only a meter high in the open ocean can
produce waves that are many tens of meters high
on shore
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Damage Due to Earthquakes
Tsunami
A tsunami wave can travel thousands of miles and
still cause staggering damage
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Southeast Asia Tsunami 2004
758 am on December 26, 2004 Magnitude 9.2
(second strongest earthquake ever recorded on a
seismograph) Lasted 10 minutes (longest lasting
earthquake in history)
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Southeast Asia Tsunami 2004
229,866 confirmed dead (which includes 42,883
missing) More than 7 billion dollars damage
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Southeast Asia Tsunami 2004
This earthquake was unusually large in
geographical extent An estimated 1,200 km
(750 mi) of fault line slipped about 15 m (50 ft)
along the subduction zone where the
Indo-Australian plate dives under the Eurasian
plate
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Southeast Asia Tsunami 2004
Four tsunami waves came ashore over a 90 minute
time period The resulting tsunami devastated the
shores of Indonesia, Sri Lanka, India, Thailand
and other countries with waves up to 30 meters
(100 feet) It caused serious damage and deaths as
far as the east coast of Africa, with the
farthest recorded death due to the tsunami
occurring in South Africa, 8,000 kilometers
(5,000 miles) away from the epicentre
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My First Big Earthquake
Kern County, California 1952 Living in Oxnard,
California Occurred at 455 am on July 21,
1952 Magnitude 7.8 on Richter scale Epicenter was
a couple hundred miles away in the Mojave
Desert Dozens of aftershocks
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My First Big Earthquake
11 were killed in Tehachapi Overall, there was
very little property damage considering the
magnitude of the quake The quake was strongest in
areas of low population Oxnard High school was
destroyed (cracked concrete) So, for 6 months, I
only had an abbreviated afternoon school session
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My Second Big Earthquake
San Fernando, California 1971
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1971 San Fernando Earthquake
The earthquake occurred at 601 AM, on February
9, 1971 The epicenter was located about 20 miles
north of downtown Los Angeles in the San Fernando
Valley area
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1971 San Fernando Earthquake
This is the actual seismic profile measured on a
seismograph in Los Angeles, which was only 20
miles from the quake From start to finish, the
quake lasted 28 seconds Note that the P, S and
surface waves all arrived at about the same times
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1971 San Fernando Earthquake
A Mercalli intensity scale map
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1971 San Fernando Earthquake
This intersection of interstate I-5 and C-14 was
under construction The overpasses dropped 40 to
60 feet onto the freeway below At 6am the traffic
was very light
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1971 San Fernando Earthquake
The upper picture is another view of the 1971
damage
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1971 San Fernando Earthquake
The exact same overpasses were destroyed again in
the 1994 Northridge earthquake After the 1971
quake, they had not bother to strengthen the
overpasses for enhanced earthquake resistance
(duh!)
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1971 San Fernando Earthquake
47 patients died in this VA hospital when it
caved in It had been built in 1925 and did meet
earthquake code
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1971 San Fernando Earthquake
The Lower San Fernando Dam was seriously damaged
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1971 San Fernando Earthquake
Liquefaction in the upstream side led to a major
land slide along nearly half the length of the
dam 80,000 people were evacuated downstream of
the dam until the lake was drained
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1971 San Fernando Earthquake
A map showing the areas with the greatest
monetary damage
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1971 San Fernando Earthquake
The earthquake was also followed by many
aftershocks, including four of magnitude 5.0 or
greater 65 people were killed Property damaged
total 511 million dollars And this was only a
6.7 magnitude quake...