Modern seismometer

Works via electromagnetic forces holding a mass in place, and measuring the current required to do so.

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 4

Station 5

Station 3

Different kinds of waves exist within solid materials

Body waves – propagate throughout a solid mediumSurface waves – propagate at the interface between media

Compressional Waves

in one- and two-dimensions

Shear waves

in one- and two- dimensions

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 at the interface (i.e., along a surface)

Different kinds of damage….

P-wave

S-wave

Sfc-wave

All

P-wavearrival

S-wavearrival

A network of seismometers all recording an earthquake

= 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

The orientation of faults can be determined from seismic networks

Orientation of the fault plane dictates first motions on an array ofseismometers

Plane A

Plan

e B

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

An example with standing waves behind the direct wave(multiple reflections in a slow crust)

Wave ray paths for Earthquake in a slab of rock.

New section: seismology can be used to infer the structure of the interior of the Earth

Wave speed depends on pressure and temperature(increase with pressure, decrease with temperature, pressure term wins typically)

Since velocities tend to increase in the crust, wave paths are curved due to refraction.

This is maybe not wrong- why?(Ken says so)

If the Earth werehomogenous in composition…

aesthenosphere

crust

core

mesosphere

But seismic velocities show great variety of structure

moho

Note, shear waves (s waves) can’t propagate in the liquid core& big drop in p-wave velocity

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…

More ray path examples…

Can be identified from individual seismograms (just about)

TheoreticalArrival timesof differentwaves

Actualarrival timescompiled from global data

Nature works!

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?

Wave speed depends on pressure and temperature(increase with pressure, decrease with temperature, pressure term wins typically)

How does seismology help?

How does seismology help?

How does seismology help?

How does seismology help?

Red = Hot = SlowCold = Blue = Fast

Velocity beneathHawaii…

Red = hot = slowBlue = cold = fast

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

Olympia, 1965 Seattle, 2001

Earthquakes are dangerous

Chi-chi Taiwan, 1999

Earthquakes are dangerous

El Salvador, 2001

Earthquakes are dangerous

Bam, Iran, 2003

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

(December 26, 2004)

Earthquakes are dangerous

Kasmir, 2006

Earthquakes are dangerous

Sichuan, China, 2008

Japan, 2011

Compilation of global earthquakes.

Hmmm…. See any pattern?

360,000 earthquakes

Black = 0 to 70; green = 70-500km; red = 500 to 700km

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 occur across the US

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

Different ways of measuring Earthquakes – Part 1. By damage

Different ways of measuring Earthquakes – Part 1. By damage

1966 ParkfieldEarthquake

Notorious for busted forecastof earthquake frequency.

I-80 Freeway collapse (65 deaths)

Different ways of measuring Earthquakes – Part 1. By damage

Loma-PrietaEarthquake 1989

Northridge Earthquake, 1994

Different ways of measuring Earthquakes – Part 1. By damage

-January 17, 1994 at 4:31 AM

-the ground acceleration was one of the highest ever instrumentally recorded in an urban area in North America.

-72 deaths, 9000 injuries, $20billion

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

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

Wood Anderson seismometer

Uses inertia of copper ball to record accelerations on photo-sensitive paper

Milne seismometerWood Anderson seismometer

Different ways of measuring Earthquakes – Part 2. Richter Scale

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

Equivalency between magnitude and energy

Different ways of measuring Earthquakes – Part 2. Richter Scale

Different ways of measuring Earthquakes – Part 2. Richter Scale

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 (akin to the others)…

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

Empirical formula

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

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

Distribution of slipfor various Earthquakes

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

Axes are distance along fault& depth.

Colors are slip in m

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

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

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…