Global seismology and wave propagation Seismology - Berkeley

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EPS 122: Lecture 9 – Global seismology and wave propagation

Global seismology and wave propagation

Reading: Fowler p100-111


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

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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


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?

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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?


Energy sources and receivers

Earthquake sources

• Globally distributed

• Primarily along tectonic plate boundaries

Seismic receivers

• Globally distributed

• Primarily on the continents

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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


S-waves

• S for “secondary” or “shear” and “shake”

• Shearing and rotation

• No volume change as the wave propagates

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Seismic waves


Seismic waves

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Waves – a reminder

v = f

Terms:

• Velocity, v

• Wavelength,

• Frequency, f

• Period, T = 1/f


Seismic waveform

minutes

epicentral distance: ~90°

(or radial)

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Seismometer

Use inertia of a mass to measure ground motion

Measure three components


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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?


Birch’s Law Velocity and density

A linear relationship between velocity and density

v = a + b

Crust and mantle rock

observations

Three pressures

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Nafe-Drake curve

Velocity and density

sediments and sedimentary rocks

igneous and metamorphic rocks

VP

VS


Northern CA Brocher, 2005

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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


Amplitude Larger amplitude than body waves:

conservation of energy, sphere versus cone

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LA earthquake M 5.4, July 29, 2008

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Chatsworth CA earthquake M 4.5, August 9, 2007


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?

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Seismic waveform

minutes

epicentral distance: ~90°

(or radial)


hours

Seismic waveform

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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

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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

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Sumatra earthquake December 26, 2004

India moving north at 56

mm/yr

Swaying skyscrapers in Singapore


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

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Sumatra earthquake December 26, 2004

Ground motion in California

3.5 cm


Velocity sensitivity The amplitude of wave motion decreases with depth

Related to depth/wavelength

Longer wavelengths sample deeper

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ith

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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

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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)


Chatsworth CA earthquake M 4.5, August 9, 2007

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Surface wave propagation

time continues

What phases can we see?


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

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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


Free oscillations

nSl and nTl l – harmonic degree – number of nodes in latitude n – overtone – number of nodes with depth

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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


Largest earthquakes

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Free oscillations excited by the Sumatra earthquake


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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


Remote sensing

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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


Geophysical methods

• Seismology


• Magnetics

• Heat flow

Gravity and bathymetry

• Connected through isostasy

• Tell us about density variations

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Geophysical methods

• Seismology


• Magnetics

• Heat flow

Magnetics

• Changes in the Earth’s magnetic field recoded within surface rocks tell us about earth history


Geophysical methods

• Seismology


• Magnetics

• Heat flow

Surface heat flow

• Related to mantle temperature variations

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Global seismology and wave propagation Seismology - Berkeley