instruments and planets ess 202. stress and strain zstress is force felt ytensional stress - pull...
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Instruments and planetsESS 202
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Stress and strain
Stress is force felt Tensional stress - pull Compression stress - push
Strain is deformation seen Tensional strain - stretch Compressional strain - shrink
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An example
Strain
Strain
Stress
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Stress: Too hard to measure
It is what we want to knowReveals the forces stirring up the EarthUseful answers we would like to know:
How much stress does it take to break rock? It there enough stress in the ground for another
big earthquake?So we measure strain (deformation) instead
Strain can be measured by measuring motion
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MotionConsider an object
A corner of a building A person A fountain pen point
Several ways to record motion Displacement Velocity Acceleration
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Defining an object’s motion
Displacement - how far has object moved?Velocity - how fast is object moving?Acceleration - how is velocity changing?Usually, we choose geographical directions
North, east, up
If we keep track of one quantity, we can calculate the other two
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Displacement,velocity, andacceleration
0 50 100 150 200
Example of motion: Driving
Time
Displacement
Velocity
Acceleration
speeding up
Slowing down
going fastslow slow
starting point
finish
MIT’s solar powered vehicle
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Co-ordinatesMany possible units
Metric (mm, cm, m, km) English system (inches, feet, miles) Other (paces, degrees, furlongs, cubits)
3 numbers required to give complete location, for example X, Y, and Z relative to a reference, or Latitude, longitude, and depth Forward-backward, left-right, up-down
Plus timeUsually, we choose geographical directions
North, East, Up
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Best seismometers measure ground velocity
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Seismometers
Some say it’s really a seismographAn instrument for recording the motions
of the Earth’s surface through time used to record seismic waves
Seismogram - record of ground motionA suspended mass stays in place while
the Earth moves back and forth under it due to inertia
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Simplest horizontal design
Press, 18-1 Ground moves to right
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Simplest vertical designseismometer
Press, 18-2
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Seismometer design
Essentials A heavy weight A way to record the motion of the weight A spring to keep the weight away from the sides A pivot so weight only moves in one direction
Luxuries An airtight box Electronics to extend frequency response A firm anchor for the seismometer
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Zhang Heng
In 132 Zhang invented the first seismograph (really just a seismoscope) for measuring earthquakes.
Earthquakes were significant in China at this time, not only for the destructive power which they unleashed but also because they were seen as punishment from the gods for poor governance of the country.
In his role as chief astrologer he was responsible for detecting signs of bad government that were indicated by earthquakes.
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Seismoscope
132 ADBalls held in
dragon’s mouths were linked to a vertical pendulum
Shaking dislodged balls
Direction back to epicenter indicated by first ball released
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www.kepu.com.cnMuseum of earthquakes
Worked on March 1, 138 A.D. Invented by Zhang Heng Ball dropped from westernmost
dragon’s mouth Days later, report arrived of
earthquake 500 km to the west!
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Seismic waves have a variety of frequencies (periods)
High frequency
Low frequency
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Need three componentsto completely record motion
Bolt, 3-2
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Milne-Shaw seismometer
One of first seismometers Globally distributed in 1890’s
"He always spoke with a quiet Lancastrian accent, which fascinated us lads, as did his nicotine-stained, bushy moustache with a gap burned in it by numerous cigarettes."
JohnMilne
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Brief Milne biographyBy 1895, Milne had been in Japan for 20 years, had
married a Japanese woman, and appeared settled for life.
Then, a fire destroyed his home, his observatory, his library, and many of his instruments.
Disheartened, he returned with his wife to England and settled on the Isle of Wight.
He persuaded the Royal Society to fund 20 earthquake seismographs around the world. The total cost was about $5000.
For 20 years, this obscure bucolic location was the world headquarters for earthquake seismology.
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1931 earthquake in England
Milne-Shaw recordingM6
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US scientists in 1925(J. B. Macelwane Archives, Saint Louis University)
Jesuit priests,paper records,magnifying glass.
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State of theart tool
Rev. James B. Maclewane withthe thirty-inch Dietricheimer globe,the then state-of-the art tool for truelongitudes and geocentric latitudes.
SLU Billikins
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1920’s laboratory
Curve fitter
Calculators
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Recording systems
Smoked paper rotating drumsInk and paper rotating drumsPhotographic film rotating
drumsAnalog tapeDigital tapeHard drive
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Data recovery
Driving to recording site Still often used
Telephone lines Bad during large quakes
Microwave transmissionSatellite transmissionInternet - Frame relay
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Kinds ofsensors
Permanent sites Anchored, wired Some are borehole Some are strong motion
Temporary Wireless, or Local recording Remote sites
Ocean bottomMilitary
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1960’s seismic station
Concretepier
Rotatingdrums
Three seismometersfor 3 components
Underground room
Garland,5-2
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Modern portableseismic station
We bury seismometerand run a wire to a computer with a big hard disk, plus batteries and a big solar panel.
IRIS newsletter
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Less noisein a borehole
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Strong-motion sensor
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Seismic NetworksRegional short-period (1-30 Hz) networks
50-400 instruments, vertical component only emphasis on earthquake detection & location.
Regional broadband (100s - 30 Hz) networks fewer instruments (10-100), 3 components emphasis on understanding bigger quakes
Global networks run by many countries
USA, France, Japan plus stations in regional nets
Ocean bottom seismometers
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UW-USGS-UO-Wash
Co-op
PNSNseismometers
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Uses of seismic networksWatching earthquakes
Mostly long-term research Partly monitoring earthquake hazards
Watching for nuclear weapons tests Are treaties being violated?
Detection of explosionsDiscrimination of explosions from earthquakes
• And mining blasts
Estimating yield of explosions
Sharing data is now a diplomatic issue
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Science & Military conflicts
An example For decades (1960’s? until about 1990) Air
Force operated many small seismic arrays around the world
Finally didn’t need them, and declassified the seismograms that had been recorded
The Generals weren’t particularly helpfulThrew away some invaluable data rather than give
it to scientists who were pestering themScientists like open exchange of all data, military
like to classify it as secret
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OBS’s
Better coverage of Earth’s surfaceCuriosity
Oceanic volcanoes Hot spots Subduction zones Detection of nuclear explosions
Very expensive Hard to emplace Can’t transmit signals back
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Example - LCheapo
Made at UC San DiegoCan buy 100 instruments for
$1,000,000But still need a ship to set upHydrophones
Just record water pressure, not ground motion
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Can hook up OBS’s to a cable
Some trans-oceanic cables are in place Old pre-satellite phone lines Abandoned submarine detectors
Example from Hawaii On Loihi seamount
newest seamount in Hawaii-Emperor chain Has many geophysical instruments
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Various designs
El Cheapo
Expensive
Not so cheap
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1. Drill seafloor2. Drop into hole3. Recover data
OBS onseafloor
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Extensive cabling offshoreSeismometersCosts $400,000,000Should have been operational in 2007
Canadian part is being built US might start in 2009, run from UW
NEPTUNE - initiative under way
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Neptune
Encircling the Juan de Fuca plate with fiber optic cable
Image provided courtesy of the NEPTUNE Project (www.neptune.washington.edu) and CEV
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Planetary seismologyBest way to see layering inside of planetsAlready been some seismometers on
Moon and MarsMany people have proposed to put more
seismometers on MarsUCLA Prof. Paige sent instruments to Mars
To look for water and life, $170M, 1999 But no parachute deployed, landed too fast, oops. One design team used English units (e.g., inches, feet and
pounds) while the other used metric units for a key spacecraft operation. This information was critical to the maneuvers required to place the spacecraft in the proper Mars orbit.
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Mars Polar Lander
Schematic diagram (above), and incorrect guess as to location of wreckage (right).
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MarsViking I and II in July and September,
1976.So far as I know, the seismometers
only recorded wind noise.Scientists involved mostly want to
forget experiment.Another failure: NetLander 2007
Victim of US-France tension
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1969 - 1972Moon passive studies
Apollo 11, 12, 14, 15, and 16 had seismometers, so there were up to 5 seismometer locations Found moonquakes
Mostly tidally triggeredAbout 1000 km deepMostly less than magnitude 2
Saw about 2000 impacts0.5 to 5000 kg meteoritesFound a lunar core about half the radius of Earth’s core
http://cass.jsc.nasa.gov/pub/expmoon/Apollo16/A16_Experiments_PSE.html
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Seismometer
Lander
MoonPassive experiment
Walter Kieffer’s page
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More lunar results
And did active experiment Placed seismometers in a 90-m long line Thumped the ground along the line Set off nine explosions up to 3.5 km from
landing site0.05 to 2.5 kg of explosives
They found Very low P wave velocities (100-300 m/s) About 1.5 km layer of basalt under surface
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Mortar for active sourcesRadio-controlled explosive
Walter Kieffer’s page
Explosions for seismic experiments
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Thumper
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Lunar laser ranging
Reflector installed on the MoonFinds distance to moon within 3 cmConfirms presence of small (< 350
km) coreMeasures that the moon is receding
from the Earth 3.8 cm/yr
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Walter Kieffer’s page
Laser beam and reflector
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Geodesy - key tool
Measuring how the ground moves over intervals from hours to years
GPS (Global positioning satellite)Simultaneously measures distance
from several satellitesOriginally guided cruise missiles
Guidings cars, watching kids as wellWe can now watch the plates move
with GPS
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Dashboard satellite guidance
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10 maccuracy.
Easy.
Now included in many phones
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>24 satellites,20,000 km up,12 hours orbits,they broadcasta signal backto Earth.
GPS constellation
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GPSsatellite
From any pointon Earth, > 4satellites willbe visible
Side benefit: Solves the timing problem
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GPS launch
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Using GPSQuickTime™ and a
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QuickTime™ and aTIFF (Uncompressed) decompressor
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Onstar car theft-prevention
A 40 year-old Wisconsin man has put in a strong bid for the dumbest criminal of the year after he allegedly stole a GPS tracking device used to monitor criminals on probation.
Ankle bracelet for felons
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CVHS - typical receiverAntenna Electronics
Southern California
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South Pole
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Mt. Everest
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Monserrat Volcano
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QuickTime™ and aTIFF (Uncompressed) decompressor
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Can see global plate motions with GPS
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Western drift of Turkey and Greece
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Cal.-Nevadamotion
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InSAR: Latest and greatest
Actually, US satellite not yet upDedicated satellite
Sends out a signal Then listens for the echo Scans the ground with 100m square pixels
Can repeat surveys every month Hopefully, more often soon
So far mainly 1 component of position Sometimes now getting all three components
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Landersmotion
map
Crippen
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Hector mine movement
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Ice flow inSouth
Patagonian Icefield, Chile
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Glacierspeedometer
Petermann glacier in Greenland
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Subsidence of Las Vegas
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Oil withdrawal
Example of Deformation Mapping in the Belridge Oil Fields(Belridge, California)
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LA rise and fall
http://www.npagroup.co.uk/insar/
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Secret meetings, quake danger?
Rising cityof LA in 2005
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Londonsubwaysubsidence
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Oregon volcano inflation slows1998 - 3 cm/yr, now 1 cm/yr
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New initiative: EarthScope
$400,000,000 project10 km hole to look at San Andreas Fault
Turned out to be 3 km
1000 new seismometers in the US The wave passed us last year
100’s of GPS and strain instruments In and working
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Drill tothe quakes
Hole started June 1, 2003Ran out of money, 2007
(small accident)
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Earthscope graphic