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…
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