oct. 8, 2005 pakistan earthquake, as recorded at idaho...
TRANSCRIPT
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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are needed to see this picture.
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