Modern seismometer

If you speeded up any earthquake signal and listened to it with a hi fi, it would sound like thunder.

east-west

north-south

up-down

Three components of motion can be measured

Station 1

Station 2

Station 3

Station 4

Station 5

Different kinds of waves exist within solid materials

Body waves – propagate throughout a solid medium

Compressional Waves

in one- and two-dimensions

Shear waves

in one- and two- dimensions

€

Vs =μ

ρ

Vp =κ +

4

3μ

ρ

Shear velocity

Compressional velocity

= shear modulus = shear stress / shear strain (restoring force to shear)k = bulk modulus = 1/compressibility (restoring force to compression)

Different types of waves have different speeds

P-waves travel faster than S-waves (and both travel faster than surface waves)

(just like waves on a string)

(a bit like a slinky)

P-waves get there first…

Rayleigh

Love

As well as body waves, there are surface waves that propagate along a surface

Different kinds of damage….

P-wave

S-wave

Sfc-wave

All

P-wavearrival

S-wavearrival

Surface wavesarrival

= Hypocenter

Difference between P-wave and S-wave arrival can be used to locatethe location of an earthquake more effectively…

Difference between p- and s-waves can be used to track location

Need 3 stations to isolate location (and the more the better)

The “first-motion” of the earthquake signal has information about the motion on the fault that generated it.

east-west

north-south

up-down

The sense of motion can be used to infer the motion that caused it.

The orientation of faults can be determined from seismic networks

Go to board for Snell’s law

FAST

FAST

SLOW

SLOW

Back to Snell’s LawAny change in wave speed due to composition change with heightwill cause refraction of rays….

This one applies to the crust

Do this on the board

Seismology can be used to infer the structure of the interior of the Earth

First, recall that wave paths are curved within the Earth due to refraction.

If the Earth werehomogenous in composition…

aesthenosphere

crust

coremesosphere

But seismic velocities show great variety of structure

moho

S waves cannot propagate through the core, leading to a huge shadow zone

S waves cannot propagate in a fluid (fluids cannot support shear stresses)

Shadow zones for P-waves existbut less b/c propagation throughthe core

Animation of P wave rays

Animation of P wave fronts

The pathways from any given source are constrained…

Seismic “phases” are named according to their paths

P – P wave only in the mantle

PP – P wave reflected off earths surface so there are two P wave segments in the mantle

pP – P wave that travels upward from a deep earthquake, reflects off the surface and then has a single segment in the mantle

PKP – P wave that has two segments in the mantle separated by a segment in the core

Ray path examples…

Can be identified from individual seismograms (just about)

What do we know about the interior composition of the Earth?

How does seismology help?

Velocity beneathHawaii…

Beneath subduction zones

Note the occurrence of deep earthquakes co-located with the down-going slab

Beneath subduction zones

Earthquake number by Richter Scale – variations over time?

Earthquakes are bad for you….

Earthquakes are dangerous

Bam, Iran, 2003


Chi-chi Taiwan, 1999


Seattle, 1956 Seattle, 2003


Sichuan, China, 2008

“Helicorder” record of the Sumatra Earthquake and aftershocks recorded in the Czech Republic

(December 26, 2004)


El Salvador, 2001


Kasmir, 2006

Where, when, and how?

U.S. Earthquakes, 1973-2002

Source, USGS. 28,332 events. Purple dots are earthquakes below 50 km, the green dot is below 100 km.

Earthquakes in California – different frequency in different sections of the fault

creeping

1906 break

1857 break

USGS shake maps – 2% likelihood of seeing peak ground acceleration equal to given color in the next 50 years

Units of “g”

USGS shake maps – 2% likelihood of seeing peak ground acceleration equal to given color in the next 50 years

Close to home…

USGS shake maps – 10% likelihood of seeing this level of acceleration inThe next 50 years

USGS shake maps – Shaking depends on what you’re sitting on.

Different ways of measuring Earthquakes – Part 1. By damage


1966 ParkfieldEarthquake

Notorious for busted forecastof earthquake frequency.

I-80 Freeway collapse (65 deaths)


Loma-PrietaEarthquake 1989

Northridge Earthquake, 1994



1906 San Francisco vs. 1811 New Madrid


Extent of damage varies widely

Charleston, MOEarthquake

• quantifies the amount of seismic energy released by an earthquake.

• base-10 logarithmic based on the largest displacement, A, from zero on a Wood–Anderson torsion seismometer output.

ML = log10A − log10A0(L)

A0 is an empirical function depending only on the distance of the station from the epicenter, L.

• So an earthquake that measures 5.0 on the Richter scale has a shaking amplitude 10 times larger than one that measures 4.0.

• The effective limit of measurement for local magnitude is about ML = 6.8 (before seismometer breaks).

Different ways of measuring Earthquakes – Part 2. Richter Scale


Two pieces of information used to calculate size of Earthquake:a)Deflection of seismometer, b)b) distance from source (based on P & S wave arrivals)

Equivalency between magnitude and energy


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M w =2

3log10

E s

1(N ⋅m = Joule)− 2.9

⎛

⎝ ⎜

⎞

⎠ ⎟

Eseismic = M010 -4.8 = 1.6 M0 · 10-5

‘Moment Magnitude’

AdM 0

= force/unit area · displacement · fault area

= shear modulus · displacement · fault area

= total elastic energy released

Earthquake “moment”

a. Total energy released in an earthquake

b. Only a small fraction released as seismic waves

c. Create logarithmic scale…

Different ways of measuring Earthquakes – Part 3. By energy released

Equivalence of seismic moment and rupture length

a)Depends on earthquake sizeb)Depends on fault type


Distribution of slipFor various Earthquakes


If you speeded up any earthquake signal and listened to it with a hi fi, it would sound like thunder.

This is the sound of the 2004 Parkfield 6.0 Earthquake

More information can come from analyzing Earthquake

Am

plitu

de

Frequency

Narrow band filters

A spectrum what you get when you listen to a signal through a series of narrow band filters

Amplitude vs. time for different frequency bands

Lower frequencies have larger amplitudes

Theoretical shapes for earthquakes

And the resulting velocity spectrum

Log10 frequency (hz)

Log

10 M

omen

t (dy

ne-c

m)

1/f (for a box car)

1/f2

(in reality)

But real earthquakes don’t do this

Instead there is a ramp-up time…

The time series of displacement looks very similar

• The theoretical spectrum for a “box car” velocity function decreases as 1/f.

• Observations show a 1/f2 behavior.

• This can be explained as ramping (i.e acceleration) of the velocity at the start and end.

Which fits much better with the velocity spectrum

1/source duration

Scaled moment

1/ramp time

Get lots of useful information from a velocity spectrum…

Log10 frequency (hz)

Log

10 M

omen

t (dy

ne-c

m)

1/f2

To~ 30 seconds

The maximum amplitude gives information about the moment magnitude of the Earthquake

Modern seismometer

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