Download Defining an Earthquake

DEFINING AN EARTHQUAKE
WHAT IS AN EARTHQUAKE?
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An earthquake is the vibration of Earth
produced by the rapid release of energy
ÐEnergy
released radiates in all directions
from its source, the focus
ÐEnergy is in the form of waves
ÐSensitive instruments around the world
record the event
EARTHQUAKE FOCUS/EPICENTER
WHAT IS AN EARTHQUAKE?
Ò
Elastic rebound
ÐMechanism
for earthquakes was first
explained by H.F. Reid
× Rocks
on both sides of an existing fault are
deformed by tectonic forces
× Rocks bend and store elastic energy
× Frictional resistance holding the rocks together
is overcome
FAULTS AND EARTHQUAKES
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Faults: fractures in crust along which rocks on one side
move past rocks on the other side
Measured according to amount of displacement: over
several million years, west side of San Andreas fault has
moved about 450 km to north
Some faults produce earthquakes when they shift,
others produce almost none
Most faults along plate boundaries, few faults in stable
interior regions of continents
Some faults first detected by earthquake or marked by
fault scarp
Fault scarp near West Yellowstone, Montana, formed
during the 1959 Hebgen Lake, Montana, earthquake.
Figure 3-1a p33
Figure 3-1b p33
TYPES OF FAULTS
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Normal faults
É Crustal
extension
É Rocks above steeply-inclined fault surface slip down
and over rocks beneath fault surface
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Reverse faults
É Crustal
compression
É Rocks above steeply-inclined fault surface slip up and
over rocks beneath fault surface
É Thrust faults are same as reverse faults, but have
more gently-inclined surface
Ð Blind
thrusts don’t reach the Earth’s surface
TYPES OF FAULTS
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Strike-slip faults
É Vertical
surface
É Rocks on one side of fault slip laterally past rocks on
other side of fault
É Where rocks on far side of fault slip to right: rightlateral strike-slip fault
É Where rocks on far side of fault slip to left: left-lateral
strike-slip fault
Figure 3-2c p34
Figure 3-2a p34
Figure 3-2b p34
CAUSES OF EARTHQUAKES
Commission appointed after great San Francisco
earthquake of 1906 to find cause of earthquakes,
headed by Andrew Lawson
Ò Lawson and students had mapped San Andreas
fault (SAF), but had no idea it caused earthquakes
Ò After 1906 earthquake, they found that west side
of SAF had shifted north by as much as 7 m
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CAUSES OF EARTHQUAKES
Commission theorized that the two sides of SAF
had been stuck for years, accumulating energy,
while surrounding areas shifted
Ò Stuck segment of fault finally slipped, released
energy in form of earthquake
Ò Elastic rebound theory has since been confirmed
by rigorous testing
Ò
Figure 3-4a p35
Figure 3-4b p35
Figure 3-4c p35
Figure 3-4d p35
CAUSES OF EARTHQUAKES
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Stress: forces imposed on rock
Strain: change in shape of rock, in response to stress
Rocks deform elastically under low stress: revert to
former shape when stress is removed
Rocks deform plastically under high stress: permanently
change shape or flow
Fault ruptures under brittle failure
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Smaller slips as fault begins to fail are foreshocks
Smaller slips as fault continues to adjust after mainshock are
aftershocks
Fig. 3-5, p. 35
CAUSES OF EARTHQUAKES
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Size of earthquake is related to amount of
movement on fault, in terms of offset and
rupture length
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Offset is distance of movement across fault
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Surface rupture length is total length of break
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Largest earthquake expected for particular fault
depends on total fault length
Fig. 3-6, p. 36
CAUSES OF EARTHQUAKES
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Some faults move continuously, rather than
suddenly snapping
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Strain is released by creep
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Central SAF undergoes creep
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Sections of faults that creep might be unable to
build up significant stress and therefore unlikely
to generate large earthquakes
This curb in Hayward, California, has been offset by creep along the Hayward Fault.
Fig. 3-7, p. 37
WHAT IS AN EARTHQUAKE?
Ò
Foreshocks and aftershocks
ÐAdjustments
that follow a major earthquake
often generate smaller earthquakes called
aftershocks
ÐSmall
earthquakes, called foreshocks, often
precede a major earthquake by days or, in some
cases, by as much as several years
THE EARTHQUAKE MACHINE
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Activities
2
1.
1. Measure Slip of the block by noting the
position of the block before and after an event
occurs. As described in Activity 1, in this model
slip is proportional to the magnitude of the
event.
2. Measure Time by watching how much tape
is pulled through the eyelet in the block of
wood. In the model we assume that the
measuring tape or plate, is moving at a
constant rate of speed; thus distance can be
converted into time. For simplicity 1cm/year is
a good rate to use.
3. Count Number of Events by noting each
time the block moves, even it is only a little bit.
EARTHQUAKE MACHINE VIDEO
http://www.iris.edu/hq/inclass/lesson/defining_an_earthquake
EARTHQUAKE MACHINE
Mechanical Modeling to
Increase Student
Understanding of Complex
Earth Systems v1.1
Photo Credit: Dave Tuttle
Michael Hubenthal - IRIS
Larry Braile - Prudue University
John Lahr - USGS
John Taber - IRIS
Developed with funding from the national science foundation
OBJECTIVES
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Explain earthquakes as a part of the natural Earth System
Describe global trends for Earthquake occurrence and
magnitude
Interpret a Gutenberg Richter plot (Frequency vs.
Magnitude)
Critically analyze an argument
Describe the importance of sharing science results with
peers in the science process
REVIEW…
What is an Earthquake?
USING THEIR MODEL, WHO CAN ILLUSTRATE
THIS DEFINITION?
Bulk of the Plate
Top View
B
Elastic Properties of
Earth Materials
Edge of the Plate
C
Plate has
Constant Velocity
Here of 1cm/year
Exploring with a model…
Explore
Explore
What did you observe?
How would you alter your definition, to
accommodate these observations?
WHAT SORT OF QUANTITATIVE INFORMATION
CAN WE COLLECT FROM OUR MODEL?
Bulk of the Plate
Top View
B
Elastic Properties of
Earth Materials
Edge of the Plate
C
Plate has
Constant Velocity
Here of 1cm/year
Elaborate
Relationship of slip to magnitude
Seismic Moment
Mo = fault length x
fault width x
displacement x
rigidity
Moment Magnitude = Mw = log Mo/1.5 – 10.7
Elaborate
Elaborate
Elaborate
DEVELOPING ARGUMENTS ABOUT
EARTHQUAKE OCCURRENCE
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Guiding Questions:
É How
frequently do earthquakes occur?
É Are all earthquakes large events?
É How frequently do large events occur?
É Can earthquakes be predicted?
É How does the Earthquake Machine model compare
to global data?
É How do scientists strive for objectivity in their
results?
DEVELOPING ARGUMENTS ABOUT
EARTHQUAKE OCCURRENCE
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Content Objectives (Students will be able to):
Explain earthquakes as a part of the natural Earth
System
É Describe the global trends for earthquake occurrence
and size.
É Interpret a Gutenberg Richter plot (Frequency vs
Magnitude)
É Critically analyze data generated by the Earthquake
Machine and use the data to develop a position
É Describe the importance of sharing science results with
peers in the science process.
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GROUP A
Group B
“There are long periods of quiet
between earthquakes”
“Most earthquakes are huge,
deadly and destructive events”
Develop an argument* either for or against this
statement based on your experimentation with
the earthquake machine.
*Note: You must base our argument on minimally 30 events
“There are always long periods between
earthquakes.”
Elaborate
“All earthquakes are huge, deadly, and
destructive events”
Elaborate
Elaborate
EARTHQUAKE MACHINE VIDEOS
Elastic rebound
Ò Earthquake machine – basic operation, single block
model of elastic rebound On the graph, the yellow line
shows the movement of the hand over time, thus a
steady line. The blue line shows the movement of the
block during slip on "earthquakes" thus the jumps in
distance over time
https://www.youtube.com/watch?v=BfZZgSbfYKI
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Earthquake machine – elastic rebound with single block
model – live demo
https://www.youtube.com/watch?v=JYQLIpo4kwk
EARTHQUAKE MACHINE VIDEOS
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Earthquake Machine—Slow Slip, Time vs. Strain –
two block model
The yellow line plots the steady displacement of the
hand. The red line shows the strain on the rubber band
between the hand and the red block. The strain drops
suddenly to a lower level each time the red block slips
(earthquake slip). There is low friction between the red
block and the surface, so "earthquakes" tend to be
more frequent and smaller. The blue line shows the
strain on the rubber band between the blocks. The blue
block has higher friction with the surface, so it tends to
slip in larger "earthquakes.“
https://www.youtube.com/watch?v=sYaXGlwopiI
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EARTHQUAKE MACHINE VIDEOS
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Earthquake Machine—Slow Slip, time versus
distance
This animation shows the change in distance over time.
The red line steps up in small increments, while the
blue line has large sudden movements, similar to the
large movement expected during a catastrophic
earthquake along a subduction zone. Observe that the
strain in the red line constantly builds and is then
released, while in the blue line, the strain builds in
sudden steps until a catastrophic release of energy
occurs.
https://www.youtube.com/watch?v=80oEAwCLyfw
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