Oct. 8, 2005 Pakistan earthquake, as recorded at Idaho Springs, Colorado

Aftershock

Each line represents15 minutes of ground motions

M 6.61882

300 s

Indonesia - Alaska M 8.22/17/96P S

surface

A seismogram (record of ground motion) has several waves...

Aspen - KS M 4.6

9/13/94

60 s

P Ssurface

Earthquake Waves2 classes:

Body Waves

Surface Waves

P wave

P wave

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are needed to see this picture.

S wave

S wave

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Surface wave

x

t1 t2 t3 t4

• shape stays same with time • peaks, troughs migrate with time

What’s a Wave?

Waves travel in both space and time

Basic Wave Properties

time

amplitude period(T)

frequency (f) = 1/T

Distance

amplitude wavelength(λ)

Wave Velocity (V) = λ/T = f λ

Basic Wave Relationλ = VTorV = f λ

relates frequency (or period) to wavelength

long wavelength = long period (or low frequency)short wavelength = short period (or high frequency)

Low-frequency signal

High-frequency signal

T = 5 sec V = 8 km/s

λ= VT = (8)*(5) = 40 km

Density, g/cc

Velocity km/s

1 2 3 4 50

2

4

6

8

10

Speed of wave: depends on rock

P

S

P waves always faster than S

Layer V(P)

Crust 6.4 km/s

Mantle 8.0 km/s

V(P)/V(S) ~ 1.8

Example:

60 s

Distance = 500 km

in crust, P wave travels for (500 km)/(6.4 km/s) = 78 sec

S wave travels(500 km)/(3.6 km/s) = 139 sec

These numbers can vary several percent

Seismic wave propagation: What’s recorded?

PS

PS

PS

surface

PSsurface

Ssurface

P S

P S

• P always faster than S• S - P time delay increases with distance

Seismograms sorted by Distance

P

Ssurface wavesTi

me,

s

Distance20° 70°200

2000

Use these signals to locate Earthquakes

How do we measure? Seismograph networks record Quakes, Explosions, ....

Measuring Earthquakes(Seismometers/Seismographs)

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Western U.S. Temporary Seismic Stations

EARTHSCOPE: USArray

Namche STS2

Namche tele

Travel Time Curve

Locating earthquakes

Earthquake location - equations• Time = Distance / velocity• P wave arrival time = origin time +

Distance/(P velocity)• S wave arrival time = origin time +

Distance/(S velocity)• S wave arrival time - P wave arrival time =

Distance/(S velocity) - Distance/(P velocity)• From above equation calculate distance• Get distance to 3 stations and triangulate

End of Lecture 1, 3/9/06

Seismic Travel-time Curve -a graphical way to get distance

Locating the Epicenter

Locating earthquakes.

Magnitude 3.8, BP 1-5 Hz2 s

Magnitude 4.25 s

Earthquake motions.

Strike - angle between fault plane and northDip - angle between fault plane and horizontal

Cockscomb Ridge, S. Utah

Dip

Strike

P.L. Kresan

Dipping Sedimentary Beds

Chris Pellant

Right-lateral or Left-lateral?

Gudmundar E. Sigvaldason

Strike-slip Fault

Schematics of the collision between India and Asia.

Thrust

Normal

Strike-slip

Sample focal mechanisms

Blue waveforms = Observed dataRed waveforms = synthetics

Moment Tensor Inversion with grid search over depth and bandpassRed = 10-20 sec Green = 12-30 secBlack = 5-15 sec Blue = 20-50 sec

De la Torre et al., 2005

Full waveform moment tensor inversion of selected earthquakes

Focal Mechanisms

Blue = previously published solutionsBlack = moment tensor inversion from this study De la Torre et al., 2005

Cross section of focal mechanisms

De la Torre et al., 2005Circle - extension (T) axes, Solid - compression (P) axes

Himalaya