Transcript seismology_2011

Modern seismometer
Works via electromagnetic forces holding a mass in place, and
measuring the current required to do so.
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
Different kinds of waves exist within solid materials
Body waves – propagate throughout a solid medium
Surface waves – propagate at the interface between media
Compressional Waves
in one- and twodimensions
Shear waves
in one- and twodimensions
Different types of waves have different speeds
Shear velocity
m
Vs =
r
(just like waves on a string)
Compressional velocity
4
k+ m
3
Vp =
r
(a bit like a slinky)
= shear modulus = shear stress / shear strain (restoring force to shear)
k = bulk modulus = 1/compressibility (restoring force to compression)
P-waves travel faster than S-waves
(and both travel faster than surface waves)
P-waves get there first…
As well as body waves, there are surface waves
that propagate at the interface (i.e., along a surface)
Rayleigh
Love
Different kinds of damage….
P-wave
S-wave
Sfc-wave
All
A network of seismometers all recording an earthquake
P-wave
arrival
S-wave
arrival
Difference between P-wave and S-wave arrival can be used to locate
the location of an earthquake more effectively…
= Hypocenter
Difference between p- and s-waves can be used to track location
Need 3 stations to isolate location (and the more the better)
The sense of motion can be used to infer the motion that caused it.
east-west
north-south
up-down
The “first-motion” of the earthquake signal has information
about the motion on the fault that generated 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 of
seismometers
Go to board for Snell’s law
Back to Snell’s Law
Any change in wave speed due to composition change with height
will cause refraction of rays….
SLOW
FAST
This one applies to the crust
FAST
SLOW
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 wrongwhy?
If the Earth were
homogenous in
composition…
But seismic velocities show great variety of structure
moho
core
crust
mesosphere
aesthenosphere
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 exist
but less b/c propagation through
the 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)
Theoretical
Arrival times
of different
waves
Actual
arrival times
compiled 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?
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 = Slow
Cold = Blue = Fast
Velocity beneath
Hawaii…
Red = hot = slow
Blue = 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
Earthquakes occur across the US
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
1906 break
creeping
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 in
The 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 Parkfield
Earthquake
Notorious for
busted forecast
of earthquake
frequency.
Different ways of measuring Earthquakes – Part 1. By damage
Loma-Prieta
Earthquake 1989
I-80 Freeway
collapse (65
deaths)
Different ways of measuring Earthquakes – Part 1. By damage
Northridge Earthquake, 1994
-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
Charleston, MO
Earthquake
Extent of damage varies widely
Different ways of measuring Earthquakes – Part 2. Richter Scale
• 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(DL)
A0 is an empirical function depending only on the
distance of the station from the epicenter, DL.
• 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).
Wood Anderson seismometer
Uses inertia of copper ball to record accelerations on
photo-sensitive paper
Wood Anderson seismometer
Milne 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)
Different ways of measuring Earthquakes – Part 2. Richter Scale
Equivalency between magnitude and energy
Different ways of measuring Earthquakes – Part 2. Richter Scale
Different ways of measuring Earthquakes – Part 3. By energy released
a. Total energy released in an earthquake
Earthquake “moment”
M0   d A
= force/unit area · displacement · fault area
= shear modulus · displacement · fault area
= total elastic energy released
b. Only a small fraction released as seismic waves
Eseismic = M010 -4.8 = 1.6 M0 · 10-5
Empirical formula
c. Create logarithmic scale (akin to the others)…
ö
2æ
Es
M w = çlog10
- 2.9÷
‘Moment Magnitude’
3è
1(N × m = Joule)
ø
Different ways of measuring Earthquakes – Part 3. By energy released
Different ways of measuring
Earthquakes
– Part 3. By energy released
Equivalence of seismic moment
and rupture length
a) Depends on earthquake size
b) Depends on fault type
Different ways of measuring
Earthquakes
– Part 3. By energy released
Distribution of slip
for various Earthquakes
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
More information can come from analyzing Earthquake
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
Amplitude
Narrow band filters
A spectrum what you get when you
listen to a signal through a series
of narrow band filters
Frequency
Amplitude vs. time for different frequency bands
Lower frequencies have larger amplitudes
Theoretical shapes for earthquakes
And the resulting velocity spectrum
But real earthquakes don’t do this
Log 10 Moment (dyne-cm)
1/f (for a box car)
1/f2
(in reality)
Log10 frequency (hz)
Instead there is a ramp-up time…
The time series of displacement looks very similar
Which fits much better with the velocity spectrum
• 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.
Get lots of useful information from a velocity spectrum…
Scaled moment
1/source duration
1/ramp time