Download Earthquake Waves

Constructive Forces
Part III: Earthquakes
Shake, Rattle and Roll
Earthquakes
A. Stress
Earthquake - shaking of
Earth’s crust and rigid
mantle caused by a
release of energy
1. Causes
- volcanic eruptions
- Collapse of a large
cavern
- Meteorite.asteroid
impact
- Large, underwater
landslides
Earthquakes (contd.)
2. Elastic Rebound Theory
- all materials can be
stretched to a certain
point without breaking elastic limit
- States that earthquakes
occur when stress
(pressure) overcomes
the force of friction
between two sections of
crust
- A sudden release of
energy and movement
occurs
Earthquakes (contd.)
- after the movement,
the pieces of crust
snap back, or
rebound close to
their original shape
Earthquakes (contd.)
3. Depth of an Earthquake
Focus - point underground
where an earthquake
originates
Epicenter - point on
Earth’s surface directly
above the focus
- earthquake (seismic)
waves travel in all
directions from the
focus, like ripples in a
pond
Earthquakes (contd.)
B. Earthquake Waves
- 2 main types
1. Body Waves - seismic
waves that travel
through Earth’s crust
a. P-wave (Primary Wave)
- compression (back
and forth wave)
- 1st wave to arrive at a
seismic station
- - can travel through
solids, liquids, and
gases
Earthquakes (contd.)
b. S-waves (Secondary
Waves)
- transverse (up and
down) waves
- Arrives 2nd at a seismic
station
- Cannot travel through
liquids
- Used as evidence of
Earth’s liquid outer core
Earthquakes (contd.)
2. Surface Waves created when p- and
s-waves reach the
surface
a. L-waves (Love Waves)
- waves move side to
side
- - movement of the
crust is perpendicular
to the direction the
wave travels in
Earthquakes (contd.)
b. Rayleigh waves
- waves travel in an
elliptical a pattern,
like the ripples in a
calm bod y of water
created when you
toss a stone in the
water
Earthquakes (contd.)
C. Locating and
Measuring
Earthquakes
1. Seismographs - an
instrument that detects
and measures seismic
waves
2. Seismograms - physical
record of seismic
waves taken from a
seismograph
- the first “jump” on a
seismogram indicates
the arrival of a p-wave
Earthquakes (contd.)
3. Determining the
Distance to an
Epicenter
- use travel time graph
(ESRT p. 10)
- Shows relationship
between the distance
seismic waves travel
and how fast they travel
Earthquakes (contd.)
- As the distance between
a seismic station and
the epicenter increase,
the difference in arrival
times between the two
waves increases as well
- if you know the
difference in arrival
times between the pand s-waves, you can
determine the distance
to the epicenter by
using the graph
Earthquakes (contd.)
- the travel time graph
can also be used
along with a
seismogram to
determine the origin
time of the
earthquake (when
the earthquake
occurred at local
time)
Earthquakes (contd.)
C. Locating Epicenters
- need three
seismograph
stations triangulation
Earthquakes (contd)
D. Earthquake Magnitude
1. Richter scale - scale of
earthquake magnitude
(strength)
- scale of 1 to 10
- Each value on the
Richter scale is 32
times more powerful
than the previous value
- Established by Charles
Richter in 1935
Earthquakes (contd.)
2. Moment Magnitude more acccurately
desribes the amount of
energy released by an
earthquake
- more difficult to measure
than Richter magnitude,
but provides a more
accurate description of
the energy released
Earthquakes (contd.)
3. Modified Mercalli
Scale
- describes the level
of damage caused
by an earthquake
- - based upon
anecdotal (personal)
accounts and
historical records
Earthquakes (contd.)
E. Earthquake Waves Inside
Earth
1. P- and S-wave Velocities
sharp change in
velocities at 2 900 km
depth
P-waves slow, s-waves
stop
Evidence of the liquid
portion of Earth’s interior
- partial recovery of pwaves at 5 200 km
depth
Earthquakes (cpntd.)
2. Moho
- named for Andrija
Mohorivicic
- - noticed difference in
wave speeds
- Reasoned difference
was caused by waves
traveling through more
dense material under
the crust
- 32 km under continents
- 8 km under oceans
Earthquakes (contd.)
3. Shadow Zone - wide
belt around Earth on the
side opposite the focus
of the earthquake
- caused by Earth’s outer
core
- P-waves are double
refracted (bent) by outer
core
- S-waves can’t travel
through outer core