What are Earthquakes?
• The shaking or trembling caused by the sudden
release of energy
• Most commonly found at or near to plate margins
• Usually associated with faulting or breaking of rocks
• Continuing adjustment of position results in
aftershocks
What is an Earthquake?
•
Rock does not move
smoothly against rock.
– Pressure at plate
boundaries builds up
– Eventually the
pressure is too great
and the rock moves
– Energy is released in
waves that radiate
outward from the
fault
The Focus and Epicenter of an Earthquake
•
•
The point within Earth
where faulting begins is
the focus, or
hypocenter
The point directly above
the focus on the surface
is the epicenter
FAULT LINE
EPICENTRE
SHOCK WAVE
FOCUS
Zhang Heng invented the seismoscope, or Di Dong Yi (Earth Motion Instrument)
for measuring earthquakes. A similar device did not reach Europe until sixteen
hundred years later, when a seismoscope was "invented" again in
France. Heng's device was made of bronze in the shape of an urn eight feet in
diameter. Although Heng's original seismoscope was destroyed, Wang Zhenduo
reconstructed it in 1951. Inside the seismoscope is a copper pendulum shaft
connected to eight copper arms. Outside the device are eight dragon heads
around the top, each with a ball in its mouth. Around the bottom, eight frogs squat
with open mouths, each directly under a dragon head. When an earthquake
occurred, a ball fell out of a the dragon's mouth into a frog's mouth, making a
noise that was supposedly loud enough to wake the Emperor's household,
alerting them of the earthquake.
Seismographs record
earthquake events
At convergent boundaries,
focal depth increases
along a dipping seismic
zone called a Benioff
zone
The Slinky and The Rope
Earthquakes generate several kinds of seismic waves including P,
for "Primary" and S, for "Secondary" waves.
P Waves
The P waves move in a compressional motion similar to the motion of a slinky,
while the S waves move in a shear motion perpendicular to the direction the wave
is travelling.
S Waves
Body Waves: P and S waves
•
Body waves
– P or primary waves
• fastest waves
• travel through solids,
liquids, or gases
• compressional wave,
material movement is
in the same direction
as wave movement
– S or secondary waves
• slower than P waves
• travel through solids
only
• shear waves - move
material
perpendicular to
wave movement
Surface Waves: R and L waves
•
Surface Waves
– Travel just below or along the ground’s surface
– Slower than body waves; rolling and side-to-side
movement
– Especially damaging to buildings
How is an Earthquake’s Epicenter Located?
Seismic wave behavior
– P waves arrive first, then S waves, then L and R
– Average speeds for all these waves is known
– After an earthquake, the difference in arrival times at a
seismograph station can be used to calculate the distance
from the seismograph to the epicenter.
How is an Earthquake’s Epicenter Located?
Time-distance graph
showing the average
travel times for P- and Swaves. The farther away a
seismograph is from the
focus of an earthquake,
the longer the interval
between the arrivals of
the P- and S- waves
How is an Earthquake’s
Epicenter Located?
•
•
•
Three seismograph stations
are needed to locate the
epicenter of an earthquake
A circle where the radius
equals the distance to the
epicenter is drawn
The intersection of the
circles locates the
epicenter
How are the Size and Strength of an Earthquake Measured?
•
•
Intensity
– subjective measure
of the kind of
damage done and
people’s reactions
to it
– isoseismal lines
identify areas of
equal intensity
Modified Mercalli Intensity Map
– 1994 Northridge, CA earthquake,
magnitude 6.7
How are the Size and Strength of an Earthquake Measured?
•
Magnitude
– Richter scale
measures total amount
of energy released by
an earthquake;
independent of
intensity
– Amplitude of the
largest wave produced
by an event is
corrected for distance
and assigned a value
on an open-ended
logarithmic scale
Excellent Guardian Earthquake Site
• http://www.guardian.co.uk/flash/0,5860,112
1610,00.html
Where Do Earthquakes Occur and How Often?
~80% of all earthquakes occur in the circum-Pacific belt
– most of these result from convergent margin activity
– ~15% occur in the Mediterranean-Asiatic belt
– remaining 5% occur in the interiors of plates and on
spreading ridge centers
– more than 150,000 quakes strong enough to be felt are
recorded each year
Three Types of Faults
Strike-Slip
Thrust
Normal
What type of
fault?
THRUST !!
Strike-slip Fault Example
Strike-slip Fault Example
1906 San Francisco Earthquake
Normal Fault Example
Dixie Valley-Fairview Peaks, Nevada earthquake
December 16, 1954
Thrust Fault Example
Thrust Fault Example
The Economics and Societal Impacts of EQs
•
•
•
•
Damage in Oakland, CA, 1989
Building collapse
Fire
Tsunami
Ground failure
This event is
still WAITING
to happen.
What are the Destructive Effects of Earthquakes?
•
Ground Shaking
– amplitude, duration, and damage increases in poorly
consolidated rocks
Can Earthquakes be Predicted?
Earthquake Prediction Programs
– include laboratory and field studies of rocks before, during,
and after earthquakes
– monitor activity along major faults
– produce risk assessments
What are the Primary and Secondary
Effects of Earthquakes.
• The primary effects of an earthquake are those
things which happen as the ground is shaking.
• The secondary effects are those things which
happen after the ground has stopped shaking as a
result of the quake. These may be minutes to
years after the event.
• Which do you think create the most problems,
Primary or Secondary effects.
Primary Effects - Ground Shaking
Northridge, CA 1994
Primary Effects Ground Shaking
Northridge, CA 1994
Primary Effects - Ground Shaking
Loma Prieta, CA 1989
KGO-TV News ABC-7
Primary Effects - Ground Shaking
Kobe, Japan 1995
Primary Effects - Ground Shaking
Kobe, Japan 1995
Primary Effects - Surface Faulting
Landers, CA 1992
Primary Effects - Liquefaction
Source: National Geophysical Data Center
Niigata, Japan 1964
Primary Effects - Landslides
Source: National Geophysical Data Center
Turnnagin Heights,Alaska,1964
Secondary Effects - Fires
Loma Prieta, CA 1989
KGO-TV News ABC-7
Secondary Effects - Tsunamis
1957 Aleutian Tsunami
Photograph Credit: Henry Helbush. Source: National Geophysical Data Center
Create a table listing the positive and
negative effects of Earthquakes
Brainstorm factors which are positive and negative
about earthquakes. Fill them in on a table.
Positive Affects
Weak Buildings Fall
Negative Affects
Homes/ Buildings destroyed
Positive Effects
Negative Effects
Weak Buildings Fall, strong buildings
survive
Homes/ Buildings destroyed or
damaged
Brings communities together
People die or are injured
Improves emergency services
Bridges/roads damaged/blocked
Research and prediction improves
Looting, crime, lawlessness
Aid from overseas leads to short term
improved quality of life
Fires from gas leaks
Improves building standards
Electricity outages, gas and telephone
services disrupted
Work in Construction
Water and Sewage pipes break
Controls Population-survivors benefit.
Disease
Insurance rates increase
Industries damaged