Transcript Stress and Strain (Energy Transfer)

Today’s Schedule:
HAZARD UPDATE!
Review Lecture 3
XXXX
Earthquakes (cont.)
Stress and Strain (energy transfer)
Elastic (bounces back) vs. Plastic (stays deformed)
Elastic Rebound Theory
Today’s Material
Earthquakes (cont.)
Stress and Strain (energy transfer)
Elastic (bounces back) vs. Plastic (stays deformed)
Elastic Rebound Theory
Class Review
Preview Next Class
http://earthobservatory.nasa.gov/IOTD/view.php?id=84282&src=fb
http://earthobservatory.nasa.gov/IOTD/view.php?id=84282&src=fb
314 years ago (January 26, 1700), an
estimated M9 earthquake unzipped
the Cascadia Subduction Zone fault
from northern California, USA to
southern British Columbia, Canada,
much like recent events in 2011 in
Japan and 2010 in Chile.
The newly released “Cascadia
Subduction Zone Earthquakes: A
Magnitude 9.0 Earthquake Scenario”
examines how the Pacific Northwest
may fare after the next great
‘megathrust’ earthquake and tsunami.
http://www.crew.org/sites/default/file
s/cascadia_subduction_scenario_2013
.pdf
Turbidite Event History—Methods and
Implications for Holocene
Paleoseismicity of the Cascadia
Subduction Zone
http://pubs.usgs.gov/pp/pp1661f/
Earthquake
http://www.youtube.com/watch?v=HQFTDAzhbZM&feature=player_embedded
Animation shows the buildup of stress along the margin of two
stuck plates that are trying to slide past one another. The rock is
deformed as it builds up strain in the plates; stress increases
along the contact.
http://www.iris.edu/hq/programs/education_and_outreach/animations/4
Stress and Strain (Energy Transfer): What is stress
and what is strain? How do these relate to
earthquakes?
What is stress?
http://www.geology.sdsu.edu/visualstructure/vss/htm_hlp/stres_d.htm
Stress is defined as a force (F) acting on some area (A).
Additional resources:
http://en.wikipedia.org/wiki/Stress_%28mechanics%29
http://serc.carleton.edu/quantskills/methods/quantlit/stressandstrain.html
Stress: Tension vs. Compression vs. Shear
Tensional stress is the stress that tends to pull
something apart. It is the stress component
perpendicular to a given surface, such as a
fault plane, that results from forces applied
perpendicular to the surface or from remote
forces transmitted through the surrounding
rock.
Compressional stress is stress that squeezes
something. It is the stress component
perpendicular to a given surface, such as a
fault plane, that results from forces applied
perpendicular to the surface or from remote
forces transmitted through the surrounding
rock.
Shear stress is the stress component parallel
to a given surface, such as a fault plane, that
results from forces applied parallel to the
surface or from remote forces transmitted
through the surrounding rock.
Image courtesy of Michael Kimberly, North Carolina State Univ.
http://earthquake.usgs.gov/learn/glossary/
What is strain?
http://physics.bgsu.edu/~stoner/p201/shm/sld002.htm
Strain is the relative change in shape or size of an object due to
externally-applied forces (e.g. stress).
Hooke’s Law: Stress is
directly proportional to
strain. >>>>>>>>
http://en.wikipedia.org/wiki/Hooke%27s_law
What does “elastic” mean?
http://en.wikipedia.org/wiki/Elastic_and_plastic_strain
Elastic refers to internal strain in a material. This internal strain is
observed as a distortion of the material. Elastic deformation
returns to it's original shape after a strain is applied.
What does “plastic” mean?
Plastic refers to internal strain in a material. This internal strain is
observed as a distortion of the material. Plastic deformation
stays in the deformed shape after a strain is applied.
http://www.iris.edu/hq/programs/education_and_outreach/animations/1
Earthquake Machine
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.
Activity 1
http://en.wikipedia.org/wiki/Elastic_rebound_theory
What is the Elastic Rebound Theory?
The elastic rebound theory is an explanation for how energy is
spread during earthquakes. As rocks on opposite sides of a fault
are subjected to force and shift, they accumulate energy and
slowly deform until their internal strength is exceeded. At that
time, a sudden movement occurs along the fault, releasing the
accumulated energy, and the rocks snap back to their original
undeformed shape.
•
•
The rocks accumulate energy and slowly deform until their
internal strength is exceeded.
At that time, a sudden movement occurs along the fault,
releasing the accumulated energy, and the rocks snap back
to their original undeformed shape.
Reid's Elastic Rebound Theory
http://earthquake.usgs.gov/regional/nca/1906/18april/reid.php
• From an examination of the
displacement of the ground surface
which accompanied the 1906
earthquake, Henry Fielding Reid,
Professor of Geology at Johns Hopkins
University, concluded that the
earthquake must have involved an
"elastic rebound" of previously stored
elastic stress.
• If a stretched rubber band is broken or
cut, elastic energy stored in the rubber
band during the stretching will
suddenly be released. Similarly, the
crust of the earth can gradually store
elastic stress that is released suddenly
during an earthquake.
• This gradual accumulation and release
of stress and strain is now referred to
as the "elastic rebound theory" of
earthquakes. Most earthquakes are the
result of the sudden elastic rebound of
previously stored energy.
The following diagram illustrates the
process. Start at the bottom.
1.
2.
3.
A straight fence is built across the San
Andreas fault.
As the Pacific plate moves northwest,
it gradually distorts the fence. Just
before an earthquake, the fence has
an "S" shape.
When the earthquake occurs the
distortion is released and the two
parts of the fence are again straight;
but now there is an offset.
This diagram greatly exaggerates the
distortion. Actually, the distortion is
spread over many miles and can only be
seen with precise instrumentation (e.g.
GPS).
Review Lecture 4
• Earthquakes: What are earthquakes? What
causes them? What is the results of an
earthquake?
• Stress and Strain (Energy Transfer): What is
stress and what is strain? How do these relate
to earthquakes? What are the 3 different kinds
of strain?
• Elastic Rebound Theory: What is this and how
does this relate to earthquakes?
One Minute Paper, that lasts three minutes.
I want to read about what you do understand. What is
the most exciting thing you learned today?
I want to read about what you do not understand.
What may have been confusing?
What is one question that you have about today’s
lecture?
We will cover these issues during our review of
today’s class at the beginning of our next class.
Next Class:
• Origin of the Earth
• Earth Structure
• Plate Tectonics