Learning log
Describe what you think is happening during an
earthquake.
Essential Question:
What causes earthquakes, how do we know
where they are, and how are they described?
Forces Within Earth
• Earthquakes are natural •
vibrations of the ground
caused by plate
•
movement in Earth’s
crust or by volcanic
eruptions
In some instances a single
earthquake has
killed more than 100,000 people
and destroyed entire cities.
Anyone living in an area prone to
earthquakes should be aware of
the potential danger and how to
minimize the damage that they
cause.
Stress and Strain
• Most earthquakes occur when
rocks fracture, or break deep within
Earth
• Fractures form when
stress (forces acting on
a material) exceed the
strength of the rock
• 3 kinds of stress:
– Compression – decreases
the volume of a material
– Tension – pulls a material
apart
– Shear – causes a material
to twist
Strain is the deformation of
materials in response to stress.
Rocks fail when stress is applied
too quickly, or when stress is too
great.
What is a fault?
• the fracture or system of
fractures along which
movement occurs.
• The surface along which the
movement takes places is
called the fault plane.
Reverse Fault
Normal Fault
• 3 basic types of faults:
– Reverse fault – form as a result
of horizontal compression
– Normal fault – caused by
horizontal tension
– Strike-slip fault – caused by
horizontal shear
Clip 0-13 on Engineering Earthquakes
Strike-slip Fault
Earthquake Waves
• The vibration of the ground
during an earthquake are
called seismic waves.
• Every earthquake generates 3
types of seismic waves:
–Primary waves (P-waves) –
causes rock particles to
move back and forth as it
passes
Earthquake Waves
Secondary waves (S-waves)
causes rock particles to
move at right angles to the
direction of the wave
Earthquake Waves
• Surface waves – causes
rock particles to move both
up and down and side to
side.
• Surface waves travel along
Earth’s surface and produce
the most serious damage.
• P-waves and S-waves (body
waves) pass through Earth’s
interior
Earthquake Waves
• The first waves generated by an earthquake spread out
from the point of failure of rocks
• Focus – Point where an earthquake originates,
usually several km below the surface
• Epicenter – Point on Earth’s surface directly above the
focus is the earthquake’s.
25-26:10 on video
• Liquifaction – when earthquake vibrations
make the loose soil behave like a liquid.
Measuring Energy Released and
Locating Earthquakes
• More than one million
earthquakes occur each
year!
• More than 90% are not
felt and cause little to no
damage
• The ones that make the
news are major seismic
events that cause much
damage.
Locating the Epicenter of an
Earthquake
The Triangulation
Method
Locating the Epicenter
In order to determine the location of an
earthquake, the earthquake needs to be
recorded on three different seismographs
that are at significantly different locations.
The other piece of information needed is the
time it takes for P-waves and S-waves to
travel through the Earth and arrive at a
seismographic station.
The Triangulation Method
Triangulation
A mathematical method for locating the
epicenter of an earthquake using three
or more data sets from seismic stations.
Seismograph - Earthquake monitoring
instrument that records the seismic waves
of the earthquake.
A seismograph records earthquake activity by
plotting vibrations on a sheet of paper to
create a seismogram. Above are some sample
seismograms:
Earthquake Magnitude & Intensity
• The amount of energy
released during an
earthquake is measured by
its magnitude.
• Measured using the Richter
Scale – based on the size of
the largest seismic waves
made by the quake
• 10-Fold (logarithmic) Scale:
– meaning seismic waves of a
magnitude-8 earthquake on the
Richter scale are 10 times larger
than a magnitude-7 and 100 times
larger than a magnitude-6.
Earthquake Magnitude & Intensity
• Most seismologists use
the moment magnitude
scale – takes into account
the size of the fault
rupture, the amount of
movement along the fault,
and the rock’s stiffness
– Moment magnitude
values are estimated from
the size of several types of
seismic waves produced
by an earthquake.
Earthquake Magnitude & Intensity
• Another way to assess earthquakes
is to measure the amount of
damage.
• Modified Mercalli Scale measures the amount of
damage done to the structures
involved and is used to
determine the intensity of an
earthquake.
• uses the Roman numerals I to XII to
designate the degree of intensity.
• Specific effects or damage correspond
to specific numerals; the higher the
numeral, the worse the damage.
Depth of Focus
• Another factor that determines a quake’s intensity
is the depth of the quake’s focus
– Can be classified as shallow, intermediate, or deep
– Deep-focus = smaller vibrations at epicenter
– Shallow-focus = larger vibrations at epicenter
• A shallow-focus, magnitude-6 quake will have greater
intensity than a deep-focus, magnitude-8 quake
• Catastrophic quakes with high intensities are almost always
shallow-focus quakes
End of presentation
Seismic Belts
• Seismologists have collected and plotted locations of epicenters.
– The global distribution of these epicenters reveals an interesting
pattern. They are not randomly distributed
Most earthquakes are associated with tectonic plate boundaries
• 80% occur along the Circum-Pacific Belt
• 15% occur along the Mediterranean-Asian Belt
• Most of the remaining occur at ocean ridges
End of PP
Locating an Earthquake
• Initially, the exact location of an
earthquake’s epicenter and time of
quake’s occurrence is not known
• But, it can be easily determined using
seismograms and travel-time curves
Time of an Earthquake
• The exact arrival times
of the P-waves and Swaves at a seismic
station can be read from
the seismogram.
• Using a travel-time
graph, the time of
occurrence can be
determined by
subtracting the travel
time from the known
arrival time of the wave.
Distance to an Earthquake
• To determine the
location of the
epicenter:
– The locations of 3
seismic stations are
plotted on a map
– A circle whose radius
is equal to the
epicentral distance is
plotted around each
station
– The point of
intersection of these
circles is the epicenter