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Modern seismometer

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Modern seismometer

Three components of motion can be measured

east-west

north-south

up-down

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

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

Different types of waves have different speeds

Shear velocity

(just like waves on a string)

Compressional velocity

(a bit like a slinky)

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

P-waves travel faster than S-waves

(and both travel faster than surface waves)

P-waves get there first…

As well as body waves, there are surface waves

that propagate along a surface

Rayleigh

Love

Different kinds of damage….

P-wave

S-wave

Sfc-wave

All

P-wave

arrival

Surface waves

arrival

S-wave

arrival

Difference between P-wave and S-wave arrival can be used to locate

the location of an earthquake more effectively…

= Hypocenter

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

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

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

east-west

north-south

up-down

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

The orientation of faults can be determined from seismic networks

The orientation of faults can be determined from seismic networks

Go to board for Snell’s law

Back to Snell’s Law

Any change in wave speed due to composition change with height

will cause refraction of rays….

SLOW

FAST

FAST

SLOW

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 were

homogenous in

composition…

But seismic velocities show great variety of structure

moho

crust

mesosphere

core

aesthenosphere

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 exist

but less b/c propagation through

the 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…

Ray path examples…

Can be identified from individual seismograms (just about)

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

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

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

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

How does seismology help?

How does seismology help?

How does seismology help?

How does seismology help?

Velocity beneath

Hawaii…

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

Earthquakes are dangerous

Chi-chi Taiwan, 1999

Earthquakes are dangerous

Seattle, 2003

Seattle, 1956

Earthquakes are dangerous

Sichuan, China, 2008

“Helicorder” record of the Sumatra Earthquake and aftershocks recorded in the Czech Republic (December 26, 2004)

Earthquakes are dangerous

El Salvador, 2001

Earthquakes are dangerous

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

1906 break

creeping

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 in

The next 50 years

USGS shake maps –

Shaking depends on what you’re sitting on.

Different ways of measuring Earthquakes – Part 1. By damage

Different ways of measuring Earthquakes – Part 1. By damage

Different ways of measuring Earthquakes – Part 1. By damage

1966 Parkfield

Earthquake

Notorious for

busted forecast

of earthquake

frequency.

Different ways of measuring Earthquakes – Part 1. By damage

I-80 Freeway collapse (65 deaths)

Loma-Prieta

Earthquake 1989

Different ways of measuring Earthquakes – Part 1. By damage

Northridge Earthquake, 1994

Different ways of measuring Earthquakes – Part 1. By damage

1906 San Francisco vs. 1811 New Madrid

Different ways of measuring Earthquakes – Part 1. By damage

Charleston, MO

Earthquake

Extent of damage varies widely

Different ways of measuring Earthquakes – Part 2. Richter Scale

- 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(DL)
- A0 is an empirical function depending only on the
- distance of the station from the epicenter, DL.
- 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:
- Deflection of seismometer,
- b) distance from source (based on P & S wave arrivals)

Different ways of measuring Earthquakes – Part 2. Richter Scale

Equivalency between magnitude and energy

Different ways of measuring Earthquakes – Part 2. Richter Scale

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

a. Total energy released in an earthquake

Earthquake “moment”

= force/unit area · displacement · fault area

= shear modulus · displacement · fault area

= total elastic energy released

b. Only a small fraction released as seismic waves

Eseismic = M010 -4.8 = 1.6 M0· 10-5

c. Create logarithmic scale…

‘Moment Magnitude’

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

Different ways of measuring

Earthquakes

– Part 3. By energy released

- Equivalence of seismic moment
- and rupture length
- Depends on earthquake size
- Depends on fault type

Different ways of measuring

Earthquakes

– Part 3. By energy released

Distribution of slip

For various Earthquakes

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

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

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

More information can come from analyzing Earthquake

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

Narrow band filters

Amplitude

Frequency

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

But real earthquakes don’t do this

Log 10 Moment (dyne-cm)

Log10 frequency (hz)

1/f (for a box car)

1/f2

(in reality)

Instead there is a ramp-up time…

The time series of displacement looks very similar

Which fits much better with the velocity spectrum

- 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.

Get lots of useful information from a velocity spectrum…

Scaled moment

1/source duration

1/ramp time

Log 10 Moment (dyne-cm)

Log10 frequency (hz)

The maximum amplitude gives information about the

moment magnitude of the Earthquake

1/f2

To~ 30 seconds