Download What is a wave?

ESS 202 - Earthquakes
What is a wave?
! A wave is a disturbance that travels far
through a medium while particles of the
medium move a small amount back and
forth and do not experience a net
translation.
! Examples: ripples on a pond, the ”wave” at
sports events
Profs. Vidale & Creager
Bolt, 5-17
Seismic wave radiation
Doppler shift
1
! Radiation - waves that travel outward
and carry energy
2
3
" Examples
1
# Light energy from space heater
2
3
• Travels too fast to see go, 300,000 km/sec
# Water waves from a splash, few m/sec
# Sound waves from a speaker, 300 m/sec
! Seismic waves (motions) are just
vibrations of the ground, like sound
waves are vibrations of the air
zebu.uoregon.edu/~js/space/ lectures/lec05.html
Moving wave sources
Lower
pitch
Higher
pitch
For example, approaching siren has
a higher pitch than a receding siren
Breaking
sound barrier
Doppler shift
Sonic boom
http://www.fly.faa.gov/carf/
Shuttle sonic boom
The “Thrust SST”, 1997
Breaking sound barrier
on land! 763 mph, 1997
X-15 world records
! 4520 miles per hour - Mach 6.7
" 2 km/sec, no longer champ
! 354,000 ft off the ground
" 100 km high
Old speed record - Mach 6.7
4520 miles per hour
! Flew for just 2 minutes at a time
! Intended to prototype moonships
" NASA vs Air Force turf war, Air Force lost
X-43A - new record in 2004
! Mach 9.6 (7000 mph), off California coast
! Unpiloted, 12-foot long vehicle
! Ramjet powered (air breathing)
" Burned for just 10 seconds
! $250M research project
Types of Seismic Waves
! P waves
! S waves
Amplitude
Walt on
Disneyland
train
body waves
! Surface wave train
" Love
" Rayleigh
Wavelength
Period
Frequency
Types of Seismic Waves
Earthquake
Station
! Body waves
*
Frequency:
How many waves pass a point in a given
amount of time
! Surface waves
Earthquake
Station
*
For sound: frequency=pitch
P waves
! Longitudinal - material moves back and
forth (vibrates) in same direction that
wave travels, produces
compression/dilatation cycle
! Fastest type of wave, so arrives first
Raypaths
and
wavefronts
P waves
" termed Primary wave
! Typical velocities in crust: 5 - 7 km/sec
! Travels through solids or fluids
Another view
of P wave motion
Different “First Motions” in
different directions
Strike-slip P-wave radiation pattern
This is left lateral
strike-slip faulting
in map view
N
L
Strong in some directions:
Lobes
Weak in other directions:
Nodes
First direction
of motion:
beach ball
diagrams
N
L
N
N
L
Up
Down
Up
Vertical ground
motion
Down
Up
L
Down
Up
Raypaths bend
as seismic waves
travel
Down
First Motions and Rupture modes
3-D view
Map views
Three primary focal mechanisms
Deformation
Map view of first motions
Strike-slip
Normal
Thrust or reverse
S waves
S waves
! Shearing - material moves back and forth
perpendicular to the direction the wave travels
in a twisting motion.
! Slower than P wave, arrives second
" termed Secondary wave
! Typical velocities in crust: 3-5 km/sec
" P waves travel 5-7 km/s
! Travels through solids, but not fluids
" because there is no restoring force for the
perpendicular motions
S wave in a solid
Surface Waves
! Travel on surface of Earth
! Two types
" Love waves
" Rayleigh waves
! Travel a bit slower than S waves
! Are the largest amplitude waves
" so the P wave can serve as a warning to take
cover or shut down critical facilities
" warning ranges from a few to 100 seconds
# Can get 1 s of warning for each 10 km in distance
More surface waves
! Need a surface to travel along,
which is the rock-air interface at
the Earth’s surface.
" Motion is strongest near the
surface
" Most strongly generated by
earthquakes near the surface
Rayleigh waves
Love waves
Where is energy?
P
Review
of
Waves
! In waves, energy has two forms
S
" Strain or deformation - like the energy
stored by deforming a spring - 1/2 kx2
" Motion or vibration - kinetic energy in
physics - 1/2 mv2
" Vibration is the most damaging, but either
kind of energy can cause damage
Love
Rayleigh
Bolt, 1-9
Bouncing ball on a spring
At rest
stretched
moving
compressed
Water waves are different
! Energy again has two parts
" One part is kinetic energy
" The other part is gravitational
! There is no twisting energy, not much is
strain
Amplitude of seismic waves
! Amplitude is strength of shaking
" Depends on magnitude
" Determines amount of damage
! Amplitude decreases with distance from
the earthquake
" energy spreading out over larger area
! P wave smallest
! S waves larger
! Surface waves largest
Because the waves travel
at different velocities
! As waves radiate outward from the
earthquake, through the Earth, they
separate into a predictable pattern with
" P waves arriving first
" then S waves
" then surface waves
S
P
Time
surface
P waves, then S waves,
then surface waves
Motion has 3 components
Transverse
Radial
S
Love
Rayleigh
At 80°, as drawn
Vertical
10 m
20 m
30-50 m
P
Body waves
Complications for Seismic waves
! Reflection
! Refraction
! Conversion
Reflection on mirror - simple
Refraction - Object in water
(bending of rays)
Refraction (water in fishbowl)
! Refraction of light
by water
" Because speed of
light waves is
slower in water
than in air
! Seismic waves
refract too
! And can also
switch between
P and S
Press, 19-1
Straw magic
Reflection complicated
Includes refraction
through curved glass
www.nashobawinery.com
Light through prism
Glass-air refraction
Conversions of seismic waves
! P waves and S waves can partially
convert to each other when they
encounter a sharp change in seismic
velocity
" usually just a small percent of total energy of
the wave
! The seismic waves quickly get very
complicated
Reflection & refraction of waves
! P & S waves are reflected, refracted
(bent), and converted at sharp changes
(discontinuities) in seismic velocity (as
occur between rock layers)
refracted S refracted P (most energy)
One incident P
or S wave
results in up to
four waves
Slow
Fast
incident P reflected S reflected P
Simulated reflection
Waves bounce
*
Bolt, 1-10
Simulation of the
1992 Landers EQ
1992
Landers EQ
Peyrat et al., 2000
up
Peyrat et al., 2000
North
Simulation of the 1992 Landers EQ
up
Simulation of the 1992 Landers EQ
Peyrat et al., 2000
North
up
South
South
Peyrat et al., 2000
North
up
South
Simulation of the 1992 Landers EQ
Simulation of the 1992 Landers EQ
Peyrat et al., 2000
North
up
South
Simulation of the 1992 Landers EQ
Peyrat et al., 2000
North
up
Simulation of the 1992 Landers EQ
Peyrat et al., 2000
North
up
South
Simulation of the 1992 Landers EQ
Peyrat et al., 2000
North
up
up
South
South
Simulation of the 1992 Landers EQ
Peyrat et al., 2000
North
South
Peyrat et al., 2000
North
up
South
Simulation of the 1992 Landers EQ
Simulation of the 1992 Landers EQ
Peyrat et al., 2000
North
up
South
Simulation of the 1992 Landers EQ
Peyrat et al., 2000
North
up
Simulation of the 1992 Landers EQ
Peyrat et al., 2000
North
up
South
Simulation of the 1992 Landers EQ
Peyrat et al., 2000
North
up
up
South
South
Simulation of the 1992 Landers EQ
Peyrat et al., 2000
North
South
Peyrat et al., 2000
North
up
South
Simulation of the 1992 Landers EQ
Simulation of the 1992 Landers EQ
Peyrat et al., 2000
North
up
South
Simulation of the 1992 Landers EQ
Peyrat et al., 2000
North
up
Simulation of the 1992 Landers EQ
Peyrat et al., 2000
North
up
South
Simulation of the 1992 Landers EQ
Peyrat et al., 2000
North
up
up
South
South
Simulation of the 1992 Landers EQ
Peyrat et al., 2000
North
South
Peyrat et al., 2000
North
up
South
Simulation of the 1992 Landers EQ
Simulation map
Shawn Larsen, LBL
Doug Dreger, UCB
Line source:
Like shock wave
Peyrat et al., 2000
Produced more energy in the shaking to the north
North
up
South
Seismicity map
Accurate
simulation
Hayward-Calaveras Fault
Right-lateral
slip on strand
of San Andreas
fault
Note strong
directivity
First part of fault
breaks
Middle of fault
breaks
After 15 seconds
After 30 seconds
End of fault
Breaks
Fault rupture
over
Waves keep
Going across
basin
Waves keep
Going across
basin
After 45 seconds
After 60 seconds
Fault rupture
over
Fault rupture
over
Reverbs in
basins dying
away
Only middle
of basins
still ringing
a little bit
After 100 seconds
After 75 seconds
1964 Nigata quake
Which waves cause damage?
! S and surface waves are much bigger than P
waves, and thus cause the most damage.
! Most damaged area is close to fault rupture.
" And damaging quakes rupture for > several sec
" P, S, and surface waves have not separated, but
rather arrive almost simultaneously
" So it is hard to isolate each wave in records of
ground motion from damaged areas
! Waves bounce, and convert between P, S,
and surface types, complicating identification.