Global seismology
and wave propagation
Reading: Fowler p100-111
EPS 122: Lecture 9 – Global seismology and wave propagation
The importance of
Seismology
Seismology is the most powerful technique
for sampling and constraining the physical
structure of the Earth’s interior
• By studying the propagation of
elastic waves through the Earth
we can learn about the physical
properties of the Earths interior
• Seismic waves sample narrow
swaths of the Earth’s interior
• We can produce a CAT-scan of
the Earth
• Seismology was central to the
discovery of plate tectonics
…and then there is
earthquakes
Upwelling
beneath
Iceland
Subduction
beneath the
Pacific
Northwest
Not Johnny Cash
http://www.youtube.com/watch?v=Ls2De3yF4Ps
EPS 122: Lecture 9 – Global seismology and wave propagation
1
Seismological techniques
Global seismology
• Earthquake sources
• Global ray paths
• Imaging 3D structure
of the Earth’s interior
Refraction & Reflection seismology
• Controlled sources
• Crustal and uppermost mantle ray paths
• Crustal structure
EPS 122: Lecture 9 – Global seismology and wave propagation
Body waves and surface waves
Point source seismic disturbance:
• Wavefront expands out from the
point
• Body waves: sphere
• Surface waves: circle
• Rays: perpendicular to wavefront
How will the amplitudes
vary with distance?
EPS 122: Lecture 9 – Global seismology and wave propagation
2
Body waves
• Energy traveling
through the interior
of the earth
• Rays bend and
reflect due to
variations in
physical properties
of the Earth’s
interior
Snell’s Law
(just like optics)
What do these curved
ray paths tell us about
the variation in velocity
with depth?
EPS 122: Lecture 9 – Global seismology and wave propagation
Energy sources
and receivers
Seismic receivers
• Globally distributed
• Primarily on the
continents
Earthquake sources
• Globally distributed
• Primarily along tectonic
plate boundaries
EPS 122: Lecture 9 – Global seismology and wave propagation
3
P-waves
• P for “primary” or “push-pull”
• Compression and rarefaction, no rotation
• Causes volume change as the wave propagates
• Similar to sound waves traveling through air
EPS 122: Lecture 9 – Global seismology and wave propagation
S-waves
• S for “secondary” or “shear” and “shake”
• Shearing and rotation
• No volume change as the wave propagates
EPS 122: Lecture 9 – Global seismology and wave propagation
4
Seismic waves
EPS 122: Lecture 9 – Global seismology and wave propagation
Seismic waves
EPS 122: Lecture 9 – Global seismology and wave propagation
5
Waves – a reminder
v = f
Terms:
• Velocity, v
• Wavelength, • Frequency, f
• Period, T = 1/f
EPS 122: Lecture 9 – Global seismology and wave propagation
Seismic waveform
(or radial)
epicentral
distance: ~90°
minutes
EPS 122: Lecture 9 – Global seismology and wave propagation
6
Seismometer
Use inertia of a mass
to measure ground
motion
Measure
three
components
EPS 122: Lecture 9 – Global seismology and wave propagation
EPS 122: Lecture 9 – Global seismology and wave propagation
7
P and S-velocities
P-velocity
S-velocity
change of shape and volume
change of shape only
Density, = mass / volume
Bulk modulus, • Ratio of increase in pressure to
associated volume change
• Always positive
Shear modulus, μ
• Force per unit area to change the shape of
the material
• μ of a liquid = 0, therefore = 0 in fluid
Is Vp or Vs greater?
EPS 122: Lecture 9 – Global seismology and wave propagation
Velocity and density
Birch’s Law
A linear relationship between velocity and density
v = a + b
Three pressures
Crust and
mantle rock
observations
EPS 122: Lecture 9 – Global seismology and wave propagation
8
Velocity and density
Nafe-Drake
curve
VP
igneous and
metamorphic rocks
VS
sediments and
sedimentary rocks
EPS 122: Lecture 9 – Global seismology and wave propagation
Northern CA
Brocher, 2005
EPS 122: Lecture 9 – Global seismology and wave propagation
9
Surface waves
• Energy traveling
around the surface
of the earth
• They sample the
physical properties
of the near-surface
rocks
• Deep earthquakes
do not excite
surface waves as
well as shallow
ones
EPS 122: Lecture 9 – Global seismology and wave propagation
Amplitude
Larger amplitude than body waves:
conservation of energy, sphere versus cone
EPS 122: Lecture 9 – Global seismology and wave propagation
10
EPS 122: Lecture 9 – Global seismology and wave propagation
LA earthquake
M 5.4, July 29, 2008
EPS 122: Lecture 9 – Global seismology and wave propagation
11
Chatsworth CA earthquake
M 4.5, August 9, 2007
EPS 122: Lecture 9 – Global seismology and wave propagation
Love and Rayleigh Waves
Ground roll in plane
of propagation
direction
Retrograde rotation
Ground shake in
horizontal direction
Which components of a seismometer
would they be detected on?
EPS 122: Lecture 9 – Global seismology and wave propagation
12
Seismic waveform
(or radial)
epicentral
distance: ~90°
minutes
EPS 122: Lecture 9 – Global seismology and wave propagation
Seismic waveform
hours
EPS 122: Lecture 9 – Global seismology and wave propagation
13
EPS 122: Lecture 9 – Global seismology and wave propagation
Largest earthquakes
Location
Date UTC
1.
Chile
1960 05 22
Magnitude
9.5
Coordinates
38.24 S
73.05 W
2.
Prince William Sound, Alaska
1964 03 28
9.2
61.02 N
147.65 W
3.
Andreanof Islands, Alaska
1957 03 09
9.1
51.56 N
175.39 W
4.
Kamchatka
1952 11 04
9.0
52.76 N
160.06 E
5.
Off the West Coast of
Northern Sumatra
2004 12 26
9.0 9.3
3.30 N
95.78 E
6.
Off the Coast of Ecuador
1906 01 31
8.8
1.0 N
81.5 W
7.
Rat Islands, Alaska
1965 02 04
8.7
51.21 N
178.50 E
8.
Assam - Tibet
1950 08 15
8.6
28.5 N
96.5 E
9.
Kamchatka
1923 02 03
8.5
54.0 N
161.0 E
10.
Banda Sea, Indonesia
1938 02 01
8.5
5.05 S
131.62 E
11.
Kuril Islands
1963 10 13
8.5
44.9 N
149.6 E
EPS 122: Lecture 9 – Global seismology and wave propagation
14
December 26, 2004
Sumatra earthquake
India moving
north at 56
mm/yr
Swaying
skyscrapers in
Singapore
EPS 122: Lecture 9 – Global seismology and wave propagation
March 28, 2005
Sumatra aftershock
Magnitude 8.7
largest earthquake in
2005!
Slip on one segment
makes it more likely the
next segment will fail
e.g. North Anatolian
Fault
EPS 122: Lecture 9 – Global seismology and wave propagation
15
December 26, 2004
Sumatra earthquake
Ground motion in California
3.5 cm
EPS 122: Lecture 9 – Global seismology and wave propagation
Velocity sensitivity
The amplitude of wave motion
decreases with depth
Related to depth/wavelength
Longer wavelengths sample deeper
for oceanic lithosphere
Rule of thumb: Peak sensitivity at a
depth of 1/3 of their wavelength
(This fig is
for water-waves)
Seismic velocity generally increases
with depth.
Surface waves are dispersive, which
means their velocity is dependent on
their wavelength. This is because
longer wavelengths sample deeper
where the velocity is greater.
EPS 122: Lecture 9 – Global seismology and wave propagation
16
Group and phase velocity
Group velocity: velocity of energy
Phase velocity: velocity of phase, i.e. a peak or trough
Arrival A
• freq decreases with distance
• gradient of the dashed line is
the phase velocity
as the freq decreases, the
phase velocity increases
Both are a function
of frequency
Group velocity
For a given freq is a straight
line (can’t see on diagram)
EPS 122: Lecture 9 – Global seismology and wave propagation
Chatsworth CA earthquake
M 4.5, August 9, 2007
EPS 122: Lecture 9 – Global seismology and wave propagation
17
Surface wave propagation
time continues
What phases can we see?
EPS 122: Lecture 9 – Global seismology and wave propagation
Dispersion
curves and
seismic
velocity
Love wave group velocity
is greater than Rayleigh
Love waves arrive first
Both travel faster in the
oceans than on continents
Use an inversion
technique to
determine velocity
models that satisfy
observed
dispersion curves
EPS 122: Lecture 9 – Global seismology and wave propagation
18
Free oscillations
The normal modes of the Earth
• standing waves
• periods between 100 sec and 1 hour
Two types:
Spheroidal, S
• radial and tangential
• vertical and horizontal
seismometers
Toroidal, T
• displacement perpendicular
to radial vector
• confined to concentric
spheres
• horizontal seismometers
EPS 122: Lecture 9 – Global seismology and wave propagation
Free oscillations
nS l
and nTl
l – harmonic degree – number of nodes in latitude
n – overtone – number of nodes with depth
EPS 122: Lecture 9 – Global seismology and wave propagation
19
Largest earthquakes
Free oscillations were first observed after the 1960 Chile earthquake
need large earthquakes to excite these modes
with modern seismometers there are ~20 earthquakes a year that generate
detectable free oscillations
Location
Date UTC
Magnitude
Coordinates
1. Chile
1960 05 22
9.5
38.24 S
73.05 W
2. Prince William Sound, Alaska
1964 03 28
9.2
61.02 N
147.65 W
3. Andreanof Islands, Alaska
1957 03 09
9.1
51.56 N
175.39 W
4. Kamchatka
1952 11 04
9.0
52.76 N
160.06 E
Off the West Coast of
5.
Northern Sumatra
2004 12 26
9.0 9.3
3.30 N
95.78 E
6. Off the Coast of Ecuador
1906 01 31
8.8
1.0 N
81.5 W
7. Rat Islands, Alaska
1965 02 04
8.7
51.21 N
178.50 E
8. Assam - Tibet
1950 08 15
8.6
28.5 N
96.5 E
9. Kamchatka
1923 02 03
8.5
54.0 N
161.0 E
10. Banda Sea, Indonesia
1938 02 01
8.5
5.05 S
131.62 E
11. Kuril Islands
1963 10 13
8.5
44.9 N
149.6 E
EPS 122: Lecture 9 – Global seismology and wave propagation
Largest earthquakes
EPS 122: Lecture 9 – Global seismology and wave propagation
20
Free oscillations
excited by the Sumatra earthquake
EPS 122: Lecture 9 – Global seismology and wave propagation
EPS 122: Lecture 9 – Global seismology and wave propagation
21
EPS 122: Lecture 9 – Global seismology and wave propagation
EPS 122: Lecture 9 – Global seismology and wave propagation
22
Summary
Geophysical remote sensing of the Earth’s interior
• Seismology: direct sampling and stress regime
• Gravity and bathymetry: density variations
• Magnetics: rocks act as magnetic tape recording Earth history
• Heat flow: show temperature gradients within the mantle
Seismic wave propagation
• Body waves: P and S; surface waves: Love and Rayleigh
• Propagation paths dependent on the physical properties of rocks
EPS 122: Lecture 9 – Global seismology and wave propagation
Remote sensing
EPS 122: Lecture 9 – Global seismology and wave propagation
23
Geophysical methods
• Seismology
• Gravity and bathymetry
• Magnetics
• Heat flow
Seismology:
• Directly samples the physical properties
of the Earth’s interior
• Earthquakes indicate the stress regime
EPS 122: Lecture 9 – Global seismology and wave propagation
Geophysical methods
• Seismology
• Gravity and bathymetry
• Magnetics
• Heat flow
Gravity and bathymetry
• Connected through isostasy
• Tell us about density variations
EPS 122: Lecture 9 – Global seismology and wave propagation
24
Geophysical methods
• Seismology
• Gravity and bathymetry
• Magnetics
• Heat flow
Magnetics
• Changes in the Earth’s
magnetic field recoded within
surface rocks tell us about
earth history
EPS 122: Lecture 9 – Global seismology and wave propagation
Geophysical methods
• Seismology
• Gravity and bathymetry
• Magnetics
• Heat flow
Surface heat flow
• Related to mantle temperature
variations
EPS 122: Lecture 9 – Global seismology and wave propagation
25