Transcript Document

PROBING THE DEEP EARTH
SEISMOLOGY USES TRAVEL
TIMES AND AMPLITUDES OF
SEISMIC WAVES TO STUDY
VARIATIONS IN SEISMIC WAVE
VELOCITY & DENSITY
+
GRAVITY STUDIES MEASURE
VARIATIONS IN DENSITY
+
MAGNETIC STUDIES GIVE
INSIGHT INTO CORE'S MAGNETIC
FIELD
+
LABORATORY STUDIES OF ROCK
PROPERTIES
----------->
MODELS OF EARTH'S
COMPOSITION, TEMPERATURE, &
INTERNAL PROCESSES
SEISMIC WAVES
DISTURBANCES AS
FUNCTION OF SPACE
& TIME THAT TRAVEL
THROUGH SOLID
EARTH
LIKE SOUND WAVES,
LIGHT
(ELECTROMAGNETIC)
WAVES, OR WATER
WAVES
USE SEISMIC WAVES
TO “SEE” INSIDE
EARTH
LIKE LIGHT (VISION,
XRAYS) OR SOUND
(ULTRASOUND,
SONAR, DOLPHINS,
BATS)
Davidson 8.11
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
RECORDING SEISMIC WAVES
2ND CENTURY CHINESE SEISMOGRAPH
SEISMOMETERSBALLS COME FROM DRAGON’S MOUTH
USE SPRING-MASS SYSTEM TO
WHEN WAVES ARRIVE
RECORD GROUND MOTION WITH TIME
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
Davidson 8.14
- 3 components (north-south; east-west, up-down)
- Precise timing (GPS satellites)
- Amplitude calibrated
- Digital recording and data available on www
WAVES: FUNCTIONS OF SPACE AND TIME
LIKE SWIMMER'S VIEW
FROM FIXED LOCATION:
WAVE AT BEACH AS
FUNCTION OF TIME
LIKE LIFEGUARD'S VIEW
AT FIXED TIME: WAVES
ALL OVER BEACH AS
FUNCTION OF POSITION
LIKE SEISMOGRAM --->
SEISMIC WAVES OCCUR
BECAUSE AN APPLIED FORCE
MAKES ROCK DEFORM
ELASTICALLY & THEN
RETURNS TO ORIGINAL SHAPE
COMPRESSION - VOLUME
CHANGES
SHEAR- DISTORTION: SHAPE
CHANGES
Davidson 5.3
TWO TYPES OF SEISMIC WAVES
P OR COMPRESSIONAL
WAVES - VOLUME
CHANGES MATERIAL
COMPRESSED OR
EXPANDED IN DIRECTION
WAVE PROPAGATES
S OR SHEAR WAVES DISTORTION WITHOUT
VOLUME CHANGES MATERIAL SHEARED IN
DIRECTION
PERPENDICULAR TO
WAVE PROPAGATES
P WAVES TRAVEL FASTER
(ABOUT 1.7x) THAN S WAVES
Davidson 5.5
SEISMIC WAVE SPEED (VELOCITY)
DEPENDS ON ELASTIC
CONSTANTS (MODULI) & DENSITY
OF MATERIAL
K - BULK MODULUS - LARGER K IS
HARDER TO COMPRESS
 - SHEAR MODULUS - LARGER 
IS HARDER TO SHEAR
 - DENSITY
COMPRESSIONAL (P) WAVE SPEED VP DEPENDS ON BOTH MODULI BUT
SHEAR (S) WAVE SPEED VS DEPENDS ON ONLY SHEAR MODULUS
S WAVES CANNOT TRAVEL THROUGH LIQUID ( = 0) LIKE OUTER CORE
IN CRUST, VP ABOUT 6.5 km/s - 14,650 miles/hr
AS SEISMIC WAVES TRAVEL
FROM MATERIAL OF ONE
SPEED AND DENSITY TO
ANOTHER
INCIDENT (INCOMING)
WAVES ARE REFLECTED
AND
TRANSMITTED (REFRACTED)
FAST
SLOW
BOTH THE ANGLES AND SIZE (AMPLITUDE) OF WAVES CHANGE,
DEPENDING ON VELOCITY AND DENSITY CHANGE
THIS EFFECT LETS US STUDY EARTH STRUCTURE
(AND IS USED IN LENSES, EYEGLASSES, ETC. FOR LIGHT)
SNELL’S LAW
DESCRIBES BENDING AS
SEISMIC WAVES TRAVEL
FROM MATERIAL OF ONE
SPEED TO ANOTHER
ANGLES OF THE INCIDENT,
REFLECTED, AND
REFRACTED (TRANSMITTED)
RAYS
FAST
SLOW
RELATED BY SNELL’S LAW
(SIN i)/VELOCITY = CONSTANT
FROM FASTER TO SLOWER MATERIAL- REFRACTED WAVES BENDS
TOWARDS VERTICAL
FROM SLOWER TO FASTER MATERIAL- REFRACTED WAVES BENDS
TOWARDS HORIZONAL
SNELL’S LAW DESCRIBES OTHER FAMILIAR
EFFECTS
DIFFERENT COLORS (WAVELENGTHS) OF
LIGHT TRAVEL AT DIFFERENT SPEEDS IN
WATER OR GLASS, SO REFRACTED AT
DIFFERENT ANGLES
RAINDROPS CAUSE THE RAINBOW.
THIS IS WHY YOU ONLY SEE A RAINBOW
WHEN THE SUN IS BEHIND YOU
CRITICAL ANGLE
WAVES GOING INTO A HIGHER
VELOCITY MATERIAL (V2>V1) CAN
HAVE TOTAL INTERNAL
REFLECTION WHERE
REFRACTED (TRANSMITTED)
WAVE IS ALONG INTERFACE
IN EARTH, REFRACTED WAVES
ALONG CRUST/MANTLE
BOUNDARY (MOHOROVICIC
DISCONTINUITY)
OTHER APPLICATIONS- FIBER
OPTICS- LIGHT “TRAPPED” IN A
LOW VELOCITY FIBER
REFLECTION (R) AND TRANSMISSION (T) COEFFICIENTS
RATIO OF
AMPLITUDES (SIZE)
AS WAVES TRAVEL
FROM MATERIAL OF
ONE SPEED (V) AND
DENSITY () TO
ANOTHER
VL, L
VR, R
TRANSMISSION COEFFICIENT:
Transmitted/Incident
REFLECTION COEFFICIENT:
Reflected/Incident
http://www.earth.northwestern.edu/people/seth/demos/STRING/string.html
TSUNAMI wave speed =
(water depth x acceleration of gravity)1/2
NY Times
TRAVEL TIME PLOTS
SHOW WHEN SEISMIC WAVES
ARRIVE AT DIFFERENT DISTANCES
FROM AN EARTHQUAKE OR
SEISMIC SOURCE
SHOW SEISMIC WAVE SPEED
(DISTANCE/TIME)
TRAVEL TIME CURVE USED TO FIND
VELOCITY AT DEPTH, WHICH
CONTROLS PATH OF WAVE
LEARN ABOUT STRUCTURE AT
DEPTH
Davidson 5.8
PATHS TRAVELED BY DIFFERENT SEISMIC WAVES
DIRECTION CHANGES AS WAVE TRAVELS THROUGH DIFFERENT
VELOCITY REGIONS
STRAIGHT
SLOW
FAST
Davidson 5.7
BENT
3 IMPORTANT PATHWAYS FOR
SEISMIC WAVES IN CRUST
WAVES MAY TRAVEL FROM
SOURCE TO RECEIVER BY
DIFFERENT PATHS AND
DIFFERENT VELOCITIES
USE TRAVEL TIME CURVE TO
FIND SPEED OF WAVES IN
MATERIAL AT DEPTH
SLOPE OF TRAVEL
TIME CURVE =
TIME/DISTANCE=
1/SPEED
SLOW
FAST
Davidson 5.12
SEISMIC WAVES TAKE
DIFFERENT PATHS
THROUGH
SPHERICAL EARTH
RAY PATHS HAVE
DIFFERENT NAMES, &
THEIR TRAVEL TIME
CURVES USED TO
FIND VELOCITY AT
DIFFERENT DEPTHS
PcP TRAVELS AS A P
WAVE THROUGH THE
MANTLE (P),
REFLECTS OFF THE
MANTLE/OUTER
CORE BOUNDARY (c)
AND THEN RETURNS
TO THE SURFACE AS
A P WAVE (P)
Davidson 5.9
RAY PATHS WITHIN MANTLE
PATHS REFLECTING FROM CORE-MANTLE
BOUNDARY (CMB)
SOLID
DIRECT
SURFACE
REFLECTED
LIQUID: NO S WAVES
SURFACE
CONVERTED
S -> P
GIVE DEPTH TO CORE &
PROPERTIES THERE
RAY PATHS THROUGH THE OUTER CORE
AND INNER CORE
LIQUID: NO S WAVES
GIVE DEPTH TO CORE & PROPERTIES THERE
CORE SHADOW ZONE
WHERE P ARRIVALS ARE
WEAK & NO ARRIVALS OF
DIRECT S WAVES
LIQUID IRON OUTER CORE
HAS LOWER P-WAVE
SPEED THAN ROCKY
MANTLE, SO P-WAVES
ENTERING IT (called PKP)
ARE BENT DOWNWARD
NO S-WAVES CAN
PROPAGATE THROUGH
OUTER CORE
DETERMINED FROM
TRAVEL TIME CURVES
TRAVEL TIME CURVES COMPILED FROM MANY EARTHQUAKES &
SEISMOMETERS USED TO FIND VELOCITY AT DEPTH
ANALYSES OF
TRAVEL TIME
CURVES GIVE
VELOCITIES,
COMPOSITIONS,
AND CHANGES
WITH DEPTH
Near melting at
base of plates
Rapid velocity
increase
4 MAJOR
REGIONS:
CRUST,
MANTLE,
OUTER CORE,
INNER CORE
Smooth velocity
increase
SOLID
Davidson 5.8
LIQUID
SOLID
Velocity
decrease
TRAVEL TIME CURVES GIVE VELOCITY AT DEPTH
TO DETERMINE COMPOSITION, NEED TO KNOW WHAT MATERIALS COULD
EXIST AT THOSE DEPTHS (PRESSURES) AND TEMPERATURES THAT
WOULD HAVE OBSERVED VELOCITIES
PRESSURE = DENSITY x DEPTH x ACCELERATION OF GRAVITY
SO PRESSURE INCREASES WITH DEPTH
3 km depth => 1000 ATMOSPHERES
400 km => 133,000 ATMOSPHERES, ~1500°C
CORE-MANTLE BOUNDARY (2900 km) => 1.3 million ATMOSPHERES,
~3700°C
CENTER OF EARTH (6371 km) => 3.5 million ATMOSPHERES, ~4200°C
MATERIALS BEHAVE VERY DIFFERENTLY AT THESE CONDITIONS
AT SURFACE
THAN
TEMPERATURES IN THE EARTH - GEOTHERM
BELOW MELTING CURVE (SOLIDUS) IN MANTLE AND SOLID INNER
CORE
ABOVE MELTING CURVE IN LIQUID OUTER CORE
SOLID
LIQUID
SOLID
SOLID INNER CORE FREEZES FROM OUTER CORE
Fractional crystalization as earth cools
Phase
diagram
Liquid core
containing iron (Fe)
and Sulphur (S)
cools
Solid iron core
freezes out
Solid
Solid
Remaining liquid
core enriched in S
Composition of
inner & outer cores
differ
Heat released by freezing and gravitational
energy released as the denser solid sinks
drive convection in the outer core and thus
generate magnetic field
INCREASE OF PRESSURE WITH DEPTH HAS TWO EFFECTS
- STEADY COMPRESSION (SQUISHING) OF MATERIAL MAKES IT
STRONGER AND DENSER AND SO INCREASES VELOCITY
GRADUALLY WITH DEPTH IN UPPER (100-410 km depth) AND LOWER
(660-2900 km depth) MANTLE
- AT APPROPRIATE PRESSURE AND TEMPERATURE CONDITIONS
MINERALS TRANSFORM TO DENSER PHASES, CAUSING THE RAPID
VELOCITY INCREASES (DISCONTINUITIES) AT 410 AND 660 km depth
SIMULATING EARTH’S
INTERIOR
Lab experiments
Pressure = Force/Area
Piston Cylinder - reach upper mantle conditions
410 km:
olivine to
spinel
phase
change
8% denser
Diamond cell - reach core conditions
DENSITY INCREASES WITH DEPTH
COMBINED EFFECT OF:
Compression (squishing) slow increase
Phase changes - sharp
increase
Changes in composition
Dense
mostly
iron
liquid
Denser
solid iron
Davidson 5.2
TOMOGRAPHY (“SLICE PICTURE”)
BEAMS TRAVEL THROUGH
OBJECT AT MANY DIFFERENT
ANGLES
COMBINE BEAMS FOR DETAILED
PICTURE OF VARIATIONS INSIDE
OBJECT
(CAT= COMPUTED AXIAL
TOMOGRAPHY)
MEDICAL TOMOGRAPHY USES XRAYS (ELECTROMAGNETIC
RADIATION) TO “SEE” VARIATIONS
IN ABSORPTION DUE TO BONES,
TISSUE ETC
SEISMIC
TOMOGRAPHY
Davidson 5.14
SEISMIC WAVES TRAVEL THROUGH THE EARTH ON MANY
DIFFERENT PATHS
COMBINE PATHS FOR A DETAILED PICTURE OF VELOCITY
VARIATIONS INSIDE
East African Rift: Africa splits into Nubia (West Africa) and Somalia (East
Africa)
1-2 mm/yr
Davidson 5.16
Hot rock indicated by slow wave speed rises to fill gap
as plates separate
SEISMIC TOMOGRAPHY OF MIDOCEAN RIDGE
HOT UPWELLING MAGMA HAS LOW VELOCITY
Forsyth et al, 1998
TOMOGRAPHY SHOWS THERMAL
STRUCTURE OF SUBDUCTING
PLATE (SLAB)
COLD (HIGH SEISMIC VELOCITY)
OCEANIC PLATE SUBDUCTS INTO
WARMER (LOWER SEISMIC
VELOCITY) MANTLE
DATA
WARM REGION OF BACK ARC
MELTING CAUSES VOLCANOES
SLAB HEATS UP SLOWLY
(MILLIONS OF YEARS)
TEMPERATURE MODEL
SEISMIC REFLECTION PROFILING
ARTIFICAL SOURCES GENERATE
SEISMIC WAVES THAT BOUNCE
OFF BOUNDARIES BETWEEN
DIFFERENT TYPES OF ROCK.
REFLECTIONS RECORDED
PLOTTED AS LINES ON SEISMIC
SECTION THAT SHOWS FEATURES
LIKE FAULTS, FOLDS AND ROCK
TYPE BOUNDARIES.
TIMES OF REFLECTIONS GIVE
DEPTH OF REFLECTING
STRUCTURES
SEISMIC REFLECTION USED TO EXPLORE FOR OIL- AND NATURAL GASTRAPPING STRUCTURES IN SEDIMENTARY BASINS
ALSO USED FOR DETAILED STUDIES OF THE DEEP CRUST
SEISMIC REFLECTION ON LAND
USES "VIBROSEIS" TRUCKS THAT SHAKE THE GROUND, OR
EXPLOSIONS
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Vibroseis used to map rocks in
constructing Chicago Deep Tunnel
Project that stores storm water overflow
in the combined sewer system
SEISMIC SURVEY RESULTS
SEISMOGRAMS ARE
COLLECTED AND PROCESSED
BY COMPUTERS TO FORM
SEISMIC SECTIONS
THESE SHOW STRUCTURE AT
DEPTH AS AN IMAGE OF THE
REFLECTED SEISMIC WAVES CAN SEE FAULTS, FOLDS,
DOMES, ETC
STRUCTURES SHOW THE
GEOLOGY OF AN AREA AND
MAY BE "TRAPS" FOR OIL OR
NATURAL GAS (WHICH CANNOT
BE SEEN DIRECTLY)
OIL?
TIME ON SEISMOGRAM
DEPTH = 2 x SPEED x
TIME TO REFLECTOR
VANCOUVER ISLAND
IMAGE OF JUAN DE FUCA
PLATE SUBDUCTING
FENEATH NORTH AMERICA
FROM SEISMIC REFLECTION
DATA