1. GRAVITY STUDIES MEASURE VARIATIONS IN DENSITY
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

2. LIKE SOUND WAVES, LIGHT (ELECTROMAGNETIC) WAVES, OR WATER WAVES
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

3. RECORDING SEISMIC WAVES
2ND CENTURY CHINESE SEISMOGRAPH SEISMOMETERS-
BALLS COME FROM DRAGON’S MOUTH USE SPRING-MASS SYSTEM TO
WHEN WAVES ARRIVE RECORD GROUND MOTION WITH TIME
- 3 components (north-south; east-west, up-down)
- Precise timing (GPS satellites)
- Amplitude calibrated
- Digital recording and data available on www
Davidson 8.14

4. 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 --->

5. COMPRESSION - VOLUME CHANGES SHEAR- DISTORTION: SHAPE CHANGES
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

6. 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

7. 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

8. INCIDENT (INCOMING) WAVES ARE REFLECTED AND
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)

9. 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
RELATED BY SNELL’S LAW
(SIN i)/VELOCITY = CONSTANT
FAST
SLOW
FROM FASTER TO SLOWER MATERIAL- REFRACTED WAVES BENDS TOWARDS VERTICAL
FROM SLOWER TO FASTER MATERIAL- REFRACTED WAVES BENDS TOWARDS HORIZONAL

10. 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

11. 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

12. REFLECTION (R) AND TRANSMISSION (T) COEFFICIENTS
RATIO OF AMPLITUDES (SIZE) AS WAVES TRAVEL FROM MATERIAL OF ONE SPEED (V) AND DENSITY () TO ANOTHER
VR, R
VL, L
TRANSMISSION COEFFICIENT:
Transmitted/Incident
REFLECTION COEFFICIENT:
Reflected/Incident

13. TSUNAMI wave speed = (water depth x acceleration of gravity)1/2
NY Times

14. SHOW SEISMIC WAVE SPEED (DISTANCE/TIME)
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

15. PATHS TRAVELED BY DIFFERENT SEISMIC WAVES
DIRECTION CHANGES AS WAVE TRAVELS THROUGH DIFFERENT VELOCITY REGIONS
STRAIGHT
BENT
SLOW
FAST
Davidson 5.7

16. 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

17. 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

18. PATHS REFLECTING FROM CORE-MANTLE BOUNDARY (CMB)
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

19. RAY PATHS THROUGH THE OUTER CORE AND INNER CORE
LIQUID: NO S WAVES
GIVE DEPTH TO CORE & PROPERTIES THERE

20. 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

21. TRAVEL TIME CURVES COMPILED FROM MANY EARTHQUAKES & SEISMOMETERS USED TO FIND VELOCITY AT DEPTH

22. CRUST, MANTLE, OUTER CORE, INNER CORE
ANALYSES OF TRAVEL TIME CURVES GIVE VELOCITIES, COMPOSITIONS, AND CHANGES WITH DEPTH
4 MAJOR REGIONS:
CRUST, MANTLE, OUTER CORE, INNER CORE
Near melting at base of plates
Rapid velocity increase
Rapid velocity increase
Smooth velocity increase
Velocity decrease
SOLID LIQUID SOLID
Davidson 5.8

23. 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 THAN AT SURFACE

24. 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

25. 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
Remaining liquid core enriched in S
Composition of inner & outer cores differ
Solid
Solid
Heat released by freezing and gravitational energy released as the denser solid sinks drive convection in the outer core and thus generate magnetic field

26. 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 ( km depth) AND LOWER ( 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

27. 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

28. 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

29. 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 X-RAYS (ELECTROMAGNETIC RADIATION) TO “SEE” VARIATIONS IN ABSORPTION DUE TO BONES, TISSUE ETC

30. SEISMIC WAVES TRAVEL THROUGH THE EARTH ON MANY DIFFERENT PATHS
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

31. 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

32. SEISMIC TOMOGRAPHY OF MIDOCEAN RIDGE
HOT UPWELLING MAGMA HAS LOW VELOCITY
Forsyth et al, 1998

33. TOMOGRAPHY SHOWS THERMAL STRUCTURE OF SUBDUCTING PLATE (SLAB)
COLD (HIGH SEISMIC VELOCITY) OCEANIC PLATE SUBDUCTS INTO
WARMER (LOWER SEISMIC VELOCITY) MANTLE
WARM REGION OF BACK ARC MELTING CAUSES VOLCANOES
SLAB HEATS UP SLOWLY (MILLIONS OF YEARS)
DATA
TEMPERATURE MODEL

34. ALSO USED FOR DETAILED STUDIES OF THE DEEP CRUST
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 GAS- TRAPPING STRUCTURES IN SEDIMENTARY BASINS
ALSO USED FOR DETAILED STUDIES OF THE DEEP CRUST

35. SEISMIC REFLECTION ON LAND
USES "VIBROSEIS" TRUCKS THAT SHAKE THE GROUND, OR EXPLOSIONS
Vibroseis used to map rocks in constructing Chicago Deep Tunnel Project that stores storm water overflow in the combined sewer system

36. 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

37. VANCOUVER ISLAND
IMAGE OF JUAN DE FUCA PLATE SUBDUCTING FENEATH NORTH AMERICA FROM SEISMIC REFLECTION DATA