Download How are seismic waves generated-Elastic rebound theory Describe

Nepal Earthquake, Spring 2015
EARTHQUAKES
Tommy Her , Ben Lee , James Edwards , Chandler Collier, Brent Dorn,
and April Bartholomew (photographer)
Introduction to Physical Science, EMPACTS Project
C. Dianne Phillips, Instructor, NWACC, Bentonville, AR 72712
How are Seismic waves generated?
Seismic waves are tremors or vibrations in the Earth's crust that are caused by
the build up or accumulation of pressure (more correctly termed stress).
This accumulation of stress causes the rocks that make up the crust to deform
elastically. This is very similar to what happens when you squash or stretch a
spring and causes a form of energy to be stored in the rocks of the crust technically described as elastic potential energy.
When this stress gets too large, it exceeds the strength of the rocks in the crust
and causes a brittle failure. Brittle failures are failures where fractures form
through the material.
https://www.youtube.com/watch?v=yOGoKCK17a4
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.
https://www.youtube.com/watch?v=zGcnUUaYXv4
Different types of Seismic Waves
 P waves- P-waves, also known as primary waves or pressure waves, travel at
the greatest velocity through the Earth. When they travel through air, they
take the form of sound waves.
 S waves- S-waves, also known as secondary waves, shear waves or shaking
waves, are transverse waves that travel slower than P-waves. In this
case, particle motion is perpendicular to the direction of wave
propagation. Again, imagine a slinky partially stretched, except this time, lift
a section and then release it, a transverse wave will travel along the length
of the slinky. S-waves cannot travel through air or water but are more
destructive than P-waves because of their larger amplitudes.
Seismogram
A seismogram is a graph output by a seismograph. It is a record of the ground
motion Seismograms typically record motions in three cartesian axes (x, y, and
z), with the z axis perpendicular to the Earth's surface and the x- and y- axes
parallel to the surface.
https://www.youtube.com/watch?v=Gbd1FcuLJLQ
Surface waves are similar in nature to water waves and travel just under the
Earth’s surface. They are typically generated when the source of the
earthquake is close to the Earth’s surface. Although surface waves travel more
slowly than S-waves, they can be much larger in amplitude and can be the
most destructive type of seismic wave. There are two basic kinds of surface
waves:
 Rayleigh waves- these waves travel as ripples like water.
 Love waves- they cause horizontal sheering of the ground. They are a bit faster
than Rayleigh waves.
What can seismic waves tell us?
Studies of the different types of seismic waves can tell us much about the
nature of the Earth’s structure.
For example, seismologists can use the direction and the difference in the
arrival times between P-waves and S-waves to determine the distance to the
source of an earthquake. If the seismographs are too far away from the event
to record S-waves, several recordings of P-waves can be crunched in a
computer program to give an approximate location of the source.
Different kinds of Plate Boundaries
all generate earthquakes
They could occur on a Convergent Boundary, Divergent Boundary or a
Transform Fault.
Convergent boundary: one plate is forced over another plate during movement creating a
thrust fault
Divergent boundary: plates are force apart each other, using forming a Rift Zone. This kind is
coming in ocean floors where new floors are created
Transform fault: The plates slip by each other this is also called Strike-Slip
Convergent Plate Boundaries are the
location of earthquakes at variable depths.
The zone of earthquakes
along a subduction zone
is also known as a
Benioff Zone.
A Benioff Zone is a planar
seismic zone dipping at
45 degrees. Earthquake
foci fall along these
planes and exist at
shallow, medium and
deep depths.
http://earthquake.usgs.gov/learn/glos
sary/?term=Benioff%20zone
Faulting?
Faulting is a break in the rocks that
make up the Earth’s crust, along which
rocks on either side have moved past
each other.
Faulting of rocks can also generate
seismic waves.
Difference between a fault and
a tectonic boundary.
Fault
 A fault is a break in the rock,
where each side of the rock
moves in opposite directions
Tectonic boundary
 A tectonic plate boundary is
simply where 2 plates meet
Links
 http://earthquakestoday.info/
 http://earthquake.usgs.gov/
 http://www.world-earthquakes.com/
 http://environment.nationalgeographic.com/environment/naturaldisasters/earthquake-profile/
 http://ds.iris.edu/seismon/
 http://quakes.globalincidentmap.com/
 http://www.emsc-csem.org/#2
Seismograph
Tommy Herr, Benjamin Lee, Brent Dorn, Chandler Collier,
James Edwards & Jamie Schneider
Physical Science
Dianne Phillips
Introduction
Our team focused on Earthquakes and so we decided to make our own seismograph.
A seismograph is an
instrument that records
the shaking of the earth's
surface caused by
seismic waves.
https://youtu.be/83GOKn7kWXM
How do seismographs record seismic
energy?
A seismograph is an instrument
that records the shaking of the
earth's surface caused by
seismic waves.
A seismogram is the paper trace
generated by the motion of the
seismograph from seismic waves
transmitting energy.
Click on image for video
http://www.iris.edu/hq/programs/education_and_out
reach/animations/8
Materials used
(2) 2x4
(1) 1x1
(1) 1x6
String
1 inch screws
Dremel
Pencil
Plumber Tape
How we did it?
 The first thing we did was use goggles, because SAFETY
FIRST.
 After that we did a blueprint layout of how the
equipment will work.
 We used the 1x6 as our foundation. After that we
screwed one of the 2x4 perpendicular to the
foundation using plumber tape. We used the 1x1 to
support our 2x4.
 We used the last 2x4 running it parallel to the
foundation and screwed it to the other 2x4 that is
standing perpendicular to the foundation.
 Next, we tied the string to the 2x4 running parallel from
the foundation.
 We drilled a hole at the end of the 2x4 and plugged
the pencil at the end of it.
How it is used?
 Our homemade seismograph sits on a table and picks us
the earth’s movement from the seismic waves.
 The seismic waves carry energy and the free swinging arm
of the seismograph will sway back and forth as the earth
moves.
 A pencil is the exact distance from the paper used to
record the movement as it hangs on the underside of the
freely moving arm of the seismograph machine.
 The pencil will mark the paper and leave a graphic record
of the earth’s movement on the paper under the
suspended pencil.
 The graphic record is a seismogram.
Acknowledgements
 EMPACTS Lab – Lee Stidham
 Professor Phillips
 Our classmates
 James for the materials he donated