Earthquakes
Vibration of
earth produced
by rapid release
of energy
(seismic waves)
with radiate in
all directions
from the source
(focus) Elastic
Rebound Theory
Explains how
this works.
Like ripples from dropping
a stone in a pond, energy
dissipates with distance
Earthquakes don't occur
randomly.
Occur on faults or
fractures
within the earth
Explained by plate
tectonics.
Most occur on plate
boundaries.
Sometimes in plate
interiors
if enough stress is built
up
Earthquakes
Most destructive forces on Earth. But it is
buildings
and other human structures that cause injury
and
death, not the earthquake itself
1988 - Soviet Armenia: magnitude 6.9, 25,000 people
died
1985 - Mexico City: magnitude 8.1, 9500 people
1989 - Loma Prieta, CA: magnitude 7.1, 40 people
died
1995 - Kobe, Japan: magnitude 7, ~6000 people died
Soviet Armenia: 1988
Mexico City - 1985
Kobe, Japan 1995
Alaska 1964
30,000
earthquakes
occur
worldwide
annually
that are
strong
enough to
be felt, but
typically
only 75 of
them are
considered
to be
significant
Mechanism
Elastic rebound theory
Forces bend rock on either side of fault, rock
strains ever so slowly, then weakest point
breaks. Break sends out shock waves, which
migrate outwards from the original break,
causing shaking. Stress is released
Aftershocks are adjustments to that change in
stress. They are always less strong than the
main shock, but they may cause more damage
to weakened structures.
Duration of Shaking Varies
1960 San Francisco: 40 seconds
1989 Loma Prieta: 15 seconds
1962 Alaska: 4 minutes!!!!
Earth material can either
fold or fault when put
under stress
If the Earth folds, it is said
to be ductile.
If the Earth faults, it is said
to be brittle.
Normal Reverse
The following animations
are types of faults in
motion: Normal,
Reverse/thrust, and Strike
Slip
There are 2 types of
Earthquake Waves: Body
Waves and Surface
Waves
Body Waves: Because
they travel through the
body of the earth.
Body Waves
P waves:
Pressure or compressional waves. Vibrate
parallel to direction of wave travel like a
slinky.
Fast travel: 4-7 km/sec (15,000 mph)
P is primary, or first wave to arrive at
recording station
Body Waves
S waves:
Shear waves. Vibrates perpendicular to
direction of wave travel. Like snapping a rope
Slower than P wave: 2-5 km/sec (11,000
mph)
So S is secondary, or second wave to arrive
at recording station
Body Waves
S waves:
Shear waves. Vibrates
perpendicular to
direction of wave
travel. Like snapping
a rope
Slower than P wave:
2-5 km/sec (11,000
mph)
So S is secondary, or
second wave to arrive
at recording station
Earthquake waves in review
Examples of Body Waves
on a Seismogram
Surface Waves: Because
they travel along the
surface of the Earth.
These waves travel
slower than Body Waves
and are very destructive.
Love Waves: cause
particles to move from
side to side, perpendicular
to direction of travel,
similar to S wave.
Rayleigh Waves: travel
more slowly than Love
Waves and cause
particles to move in
elliptical patterns
Types of Waves
Earthquake waves = seismic waves.
Recorded on seismometers on
seismographs.
Types of Waves
Surface waves - travel on Earth's
surface, away from epicenter.
• Very slow waves. Cause a lot of damage,
rolling feeling at end of earthquake
Body Waves - travel through Earth's
interior, spread outward from focus
Earthquake waves in review
Seismogram Tracings
Locating the Epicenter of an
Earthquake
P, S and surface waves all start out at same
time.
The further you are away from the quake, the
longer the time span between arrival of P and
S wave.
The distance of the seismometer to the
earthquake can be determined by the time
between the arrival of P wave and arrival of S
waves.
Can tell the distance, but not the direction.
Therefore, at least 3 sites must be used to
find epicenter.
Studying Earth’s Interior
Travel of P and S Waves
Through the Body of the
Earth
How did geophysicists
discover what was inside of
our earth?
Travel of P and S Waves
Through the Body of the
Earth
They discovered that P
waves will travel through
solid and liquid, whereas S
Waves will only travel
through a solid.