1
EPS 122: Lecture 9 – Global seismology and wave propagation
Global seismology and wave propagation
Reading: Fowler p100-111
EPS 122: Lecture 9 – Global seismology and wave propagation
Seismology The importance of
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
Upwelling beneath Iceland
Subduction beneath the Pacific
Northwest …and then there is
earthquakes
Not Johnny Cash http://www.youtube.com/watch?v=Ls2De3yF4Ps
2
EPS 122: Lecture 9 – Global seismology and wave propagation
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?
3
EPS 122: Lecture 9 – Global seismology and wave propagation
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
Earthquake sources
• Globally distributed
• Primarily along tectonic plate boundaries
Seismic receivers
• Globally distributed
• Primarily on the continents
4
EPS 122: Lecture 9 – Global seismology and wave propagation
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
5
EPS 122: Lecture 9 – Global seismology and wave propagation
Seismic waves
EPS 122: Lecture 9 – Global seismology and wave propagation
Seismic waves
6
EPS 122: Lecture 9 – Global seismology and wave propagation
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
minutes
epicentral distance: ~90°
(or radial)
7
EPS 122: Lecture 9 – Global seismology and wave propagation
Seismometer
Use inertia of a mass to measure ground motion
Measure three components
EPS 122: Lecture 9 – Global seismology and wave propagation
8
EPS 122: Lecture 9 – Global seismology and wave propagation
P and S-velocities P-velocity S-velocity
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
change of shape and volume change of shape only
Density, = mass / volume
Is Vp or Vs greater?
EPS 122: Lecture 9 – Global seismology and wave propagation
Birch’s Law Velocity and density
A linear relationship between velocity and density
v = a + b
Crust and mantle rock
observations
Three pressures
9
EPS 122: Lecture 9 – Global seismology and wave propagation
Nafe-Drake curve
Velocity and density
sediments and sedimentary rocks
igneous and metamorphic rocks
VP
VS
EPS 122: Lecture 9 – Global seismology and wave propagation
Northern CA Brocher, 2005
10
EPS 122: Lecture 9 – Global seismology and wave propagation
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
11
EPS 122: Lecture 9 – Global seismology and wave propagation
EPS 122: Lecture 9 – Global seismology and wave propagation
LA earthquake M 5.4, July 29, 2008
12
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
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?
13
EPS 122: Lecture 9 – Global seismology and wave propagation
Seismic waveform
minutes
epicentral distance: ~90°
(or radial)
EPS 122: Lecture 9 – Global seismology and wave propagation
hours
Seismic waveform
14
EPS 122: Lecture 9 – Global seismology and wave propagation
EPS 122: Lecture 9 – Global seismology and wave propagation
Largest earthquakes
149.6 E 44.9 N 8.5 1963 10 13 Kuril Islands 11.
131.62 E 5.05 S 8.5 1938 02 01 Banda Sea, Indonesia 10.
161.0 E 54.0 N 8.5 1923 02 03 Kamchatka 9.
96.5 E 28.5 N 8.6 1950 08 15 Assam - Tibet 8.
178.50 E 51.21 N 8.7 1965 02 04 Rat Islands, Alaska 7.
81.5 W 1.0 N 8.8 1906 01 31 Off the Coast of Ecuador 6.
95.78 E 3.30 N 9.0 2004 12 26 Off the West Coast of Northern Sumatra
5.
160.06 E 52.76 N 9.0 1952 11 04 Kamchatka 4.
175.39 W 51.56 N 9.1 1957 03 09 Andreanof Islands, Alaska 3.
147.65 W 61.02 N 9.2 1964 03 28 Prince William Sound, Alaska 2.
73.05 W 38.24 S 9.5 1960 05 22 Chile 1.
Coordinates Magnitude Date UTC Location
9.3
15
EPS 122: Lecture 9 – Global seismology and wave propagation
Sumatra earthquake December 26, 2004
India moving north at 56
mm/yr
Swaying skyscrapers in Singapore
EPS 122: Lecture 9 – Global seismology and wave propagation
Sumatra aftershock
March 28, 2005
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
16
EPS 122: Lecture 9 – Global seismology and wave propagation
Sumatra earthquake December 26, 2004
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
oce
an
ic l
ith
osp
he
re
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.
(This fig is for water-waves)
Rule of thumb: Peak sensitivity at a depth of 1/3 of their wavelength
17
EPS 122: Lecture 9 – Global seismology and wave propagation
Group and phase velocity Group velocity: velocity of energy
Phase velocity: velocity of phase, i.e. a peak or trough
Both are a function of frequency
Arrival A
• freq decreases with distance
• gradient of the dashed line is the phase velocity
as the freq decreases, the phase velocity increases
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
18
EPS 122: Lecture 9 – Global seismology and wave propagation
Surface wave propagation
time continues
What phases can we see?
EPS 122: Lecture 9 – Global seismology and wave propagation
Dispersion curves and seismic velocity
Use an inversion technique to determine velocity
models that satisfy observed dispersion curves
Love wave group velocity is greater than Rayleigh
Love waves arrive first
Both travel faster in the oceans than on continents
19
EPS 122: Lecture 9 – Global seismology and wave propagation
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
nSl and nTl l – harmonic degree – number of nodes in latitude n – overtone – number of nodes with depth
20
EPS 122: Lecture 9 – Global seismology and wave propagation
Largest earthquakes
149.6 E 44.9 N 8.5 1963 10 13 Kuril Islands 11.
131.62 E 5.05 S 8.5 1938 02 01 Banda Sea, Indonesia 10.
161.0 E 54.0 N 8.5 1923 02 03 Kamchatka 9.
96.5 E 28.5 N 8.6 1950 08 15 Assam - Tibet 8.
178.50 E 51.21 N 8.7 1965 02 04 Rat Islands, Alaska 7.
81.5 W 1.0 N 8.8 1906 01 31 Off the Coast of Ecuador 6.
95.78 E 3.30 N 9.0 2004 12 26 Off the West Coast of Northern Sumatra
5.
160.06 E 52.76 N 9.0 1952 11 04 Kamchatka 4.
175.39 W 51.56 N 9.1 1957 03 09 Andreanof Islands, Alaska 3.
147.65 W 61.02 N 9.2 1964 03 28 Prince William Sound, Alaska 2.
73.05 W 38.24 S 9.5 1960 05 22 Chile 1.
Coordinates Magnitude Date UTC Location
9.3
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
EPS 122: Lecture 9 – Global seismology and wave propagation
Largest earthquakes
21
EPS 122: Lecture 9 – Global seismology and wave propagation
Free oscillations excited by the Sumatra earthquake
EPS 122: Lecture 9 – Global seismology and wave propagation
22
EPS 122: Lecture 9 – Global seismology and wave propagation
EPS 122: Lecture 9 – Global seismology and wave propagation
23
EPS 122: Lecture 9 – Global seismology and wave propagation
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
24
EPS 122: Lecture 9 – Global seismology and wave propagation
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
25
EPS 122: Lecture 9 – Global seismology and wave propagation
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