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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. - PowerPoint PPT Presentation
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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
Different kinds of waves exist within solid materials
Body waves – propagate throughout a solid medium
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Vs =μ
ρ
Vp =κ +
4
3μ
ρ
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)
= Hypocenter
Difference between P-wave and S-wave arrival can be used to locatethe location of an earthquake more effectively…
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.
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
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)
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
Beneath subduction zones
Note the occurrence of deep earthquakes co-located with the down-going slab
“Helicorder” record of the Sumatra Earthquake and aftershocks recorded in the Czech Republic
(December 26, 2004)
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
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…
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
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
Different ways of measuring Earthquakes – Part 2. Richter Scale
Two pieces of information used to calculate size of Earthquake:a)Deflection of seismometer, b)b) distance from source (based on P & S wave arrivals)
Equivalency between magnitude and energy
Different ways of measuring Earthquakes – Part 2. Richter Scale
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M w =2
3log10
E s
1(N ⋅m = Joule)− 2.9
⎛
⎝ ⎜
⎞
⎠ ⎟
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…
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
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
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
• 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…