Download You can simulate convergent plate motion by placing your hands

What are earthquakes ? Why do they
occur ? Why can't we predict them ?
Although we still can't predict when an
earthquake will happen, we have learned
much about earthquakes as well as the
Earth itself from studying them. We have
learned how to pinpoint the locations of
earthquakes, how to accurately measure
their sizes, and how to build flexible
structures that can withstand the strong
shaking produced by earthquakes and
protect our loved ones.
An earthquake is a sudden
shaking of the ground. They
generate seismic waves which
can be recorded on a sensitive
instrument called a
seismograph. The record of
ground shaking recorded by the
seismograph is called a
seismogram.
Earthquakes generate seismic
waves which can be detected with
a sensitive instrument called a
seismograph.
Advances in seismograph
technology have increased our
understanding of both
earthquakes and the Earth itself.
Perhaps the earliest seismograph
was invented in China A.D. 136
by a man named Choko.
This early eastern seismoscope
consisted of a copper vessel with eight
dragon heads attached to it, positioned
above eight frogs.
Each dragon head held a ball in its
mouth, which, when dropped due to the
strong shaking of an earthquake, would
fall into the open mouth of the frog
directly below it.
By noting which frogs contained balls
after a strong earthquake, it was
possible to determine how the Earth had
moved in response to the earthquake.
Floating Plates
The Earth's outermost surface is broken into 12
rigid plates which are 60-200 km thick and float on
top of a more fluid zone, much in the way that
icebergs float on top of the ocean.
Shifting and Sliding
Along the boundary separating any two plates, the relative motion
between the plates can be classified into one of 3 categories:
Divergent
Convergent
Transformatinal
Divergent Motion: Movin' Apart
This is the most common kind of motion along the
mid-ocean ridges. This is a system of undersea
mountain ranges that extends beneath the world's
oceans and connects together like the seams on a
baseball.
You can simulate divergent plate motion by placing your palms down
with your index fingers touch. Now pull your hands apart.
Convergent Motion: Comin' Together
This is the most common kind of motion at subduction
zones. This motion happens where dense oceanic plates
collide and slide beneath continental plates.
You can simulate convergent plate motion by placing your hands
together in front of you with your palms down. Now gently slide your
bottom hand downward and your top hand upward, while keeping
them in contact.
Transformational Motion:
A good example of this type of motion is the San Andreas
Fault which runs through California.
You can simulate transformational plate motion by placing your
hands together in front of you with your palms down. Now slide
your left hand away from you and your right hand toward you.
The movement between plates and along faults is not
smooth. They move in jerks, giving rise to earthquakes.
The locations of earthquakes throughout the world
delineate the major tectonic boundaries.
Faults are narrow zones in the Earth, usually
extending no more than about 10 miles deep,
which separate rigid crustal blocks.
A well known fault is the San Andreas Fault
which separates the Pacific plate from the North
American plate. The Pacific plate has San
Francisco and Los Angeles on it, while the North
American plate contains the rest of California and
the U.S.
The Pacific plate is moving to the northwest at a
rate of about 4 inches per year.
Earthquakes occur on faults. Because faults
have friction, they resist the forces trying to
move the pieces apart. As the forces build,
the fault remains locked and the blocks get
deformed because of the increasing stress.
Eventually the stresses get so high that the
fault breaks.
This releases the built up stress and allows the sides of the
fault to slide past one another. This is what we call an
earthquake. The blocks return to their locked state until the
stresses build up enough to cause another earthquake.
How to Release Stress
You can simulate the earthquake by
placing your hands together with
your thumbs up. By pushing your
hands together you create pressure
between your hands.
As you try to slide your hands apart,
you create friction which stops you
from sliding your hands easily.
Stress builds up in your hands and
arm.
Spreading the Motion
Tectonic plates are somewhat flexible. The motion
between them is not confined entirely to their own
boundaries. The motion extends into their interiors
and is spread out among a system of faults all around
the plate's boundary.
Earthquakes create seismic waves which shake
the ground as they pass. They sometimes cause
buildings to topple. Earthquakes create waves just
like waves of water moving across the ocean and
waves of air moving across a field of wheat.
Consider what happens when a drop of rain hits a pond of water. The
drop disturbs the flat surface of the water and creates waves that
travel outward in all directions from the disturbance. These waves
travel on the surface of the pond, along the interface between the
water and the air.
Earthquakes generate seismic waves which
travel all around the world and can be
detected with a sensitive instrument called a
seismograph.
.
This was first discovered in 1889 by E. von Rebleur Paschwitz
who noted that waves recorded on a horizontal pendulum in
Potsdam, Germany were generated by an earthquake far away in
Tokyo, Japan.
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
A Shaking Record
A seismogram is a record of the ground shaking recorded by a
seismograph.
The P waves travel fastest through the Earth so they arrive at a
seismograph first, followed by the S waves and lastly by the
surface waves.
The figure above is the seismogram of the 1989 Loma Prieta
earthquake recorded at a seismograph in Kongsberg, Norway,
8400 km (about 5,200 miles) away.
Locatin' the Shakin'
Seismologists locate earthquakes by measuring
the time between the P and S waves in a
seismogram.After a seismogram "feels" an
earthquake, scientists compare the time difference
of these waves to figure out how far away the
earthquake is. It takes at least three seismograms
to locate exactly where the earthquake is.
One seismograph can only tell how far away it is from that
seismograph. The earthquake could be located anywhere on a circle of
radius equal to this distance and centered on the observation station.
By measuring the S-P times at 3 or more stations these circles can be
drawn around each station and where they meet indicates the
earthquake location
The Bigger The Better
For very large earthquakes (Magnitude greater than 7), the Richter
scale saturates, so that the S waves from an M 7 earthquake are not
appreciably different from those of an M 7.8 earthquake.
For these larger events, seismologists use a slightly different
measure of the earthquake size called the moment magnitude. The
moment magnitude is directly related to the size of the earthquake
rupture area or fault plane, and does not saturate for large events.
For smaller events the Richter and moment magnitudes are similar.
The Great San Francisco earthquake
occurred on Apr. 18, 1906 at 5:12 a.m.
and had a magnitude of 8.25 on the
Richter scale. It is the largest
earthquake to occur along the San
Andreas fault in the last 200 years.
700 deaths were attributed to the
earthquake, though current estimates
place the death toll 3-4 times higher.
Though the ground shaking only lasted some 45-60 seconds, the
earthquake triggered fires throughout the city that lasted over 12 hours
and destroyed much of the downtown.
The Loma Prieta earthquake of 1989 was
centered in the southern Santa Cruz mountains.
It was our nation's most costly natural disaster,
causing over $6 billion in property damage, 62
deaths and 3,757 injuries. 41 of the deaths
occurred when the double-decked section of the
880 freeway in Oakland collapsed
The convergent motion generated by
the fault raised the Santa Cruz
mountains by approximately 3 feet,
leading scientists to propose that the
Santa Cruz mountains have grown
from repeated fault ruptures over
millions of years.
Photos by Deanne Fitzmaurice.