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

PROBING THE DEEP EARTH

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

SEISMIC WAVES

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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

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RECORDING SEISMIC WAVES

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 --->

WAVES: FUNCTIONS OF SPACE AND TIME

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

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

TWO TYPES OF SEISMIC WAVES

Davidson 5.5

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 BUTSHEAR (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

SEISMIC WAVE SPEED (VELOCITY)

AS SEISMIC WAVES TRAVEL FROM MATERIAL OF ONE SPEED AND DENSITY TO ANOTHER

INCIDENT (INCOMING) WAVES ARE REFLECTED AND TRANSMITTED (REFRACTED)

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)

FAST

SLOW

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

FROM FASTER TO SLOWER MATERIAL- REFRACTED WAVES BENDS TOWARDS VERTICAL

FROM SLOWER TO FASTER MATERIAL- REFRACTED WAVES BENDS TOWARDS HORIZONAL

FAST

SLOW

SNELL’S LAW

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

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

CRITICAL ANGLE

REFLECTION (R) AND TRANSMISSION (T) COEFFICIENTS

RATIO OF AMPLITUDES (SIZE) AS WAVES TRAVEL FROM MATERIAL OF ONE SPEED (V) AND

DENSITY () TO ANOTHER

TRANSMISSION COEFFICIENT:

Transmitted/Incident

REFLECTION COEFFICIENT:

Reflected/Incident

VL, LVR, R

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 PLOTSSHOW 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

Davidson 5.7

SLOW

FAST

STRAIGHT 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

Davidson 5.12

SLOW

FAST

SLOPE OF TRAVEL TIME CURVE = TIME/DISTANCE= 1/SPEED

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

DIRECT

SURFACE REFLECTED

SURFACE CONVERTED S -> P

LIQUID: NO S WAVES

SOLID

GIVE DEPTH TO CORE & PROPERTIES THERE

PATHS REFLECTING FROM CORE-MANTLE BOUNDARY (CMB)

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

4 MAJOR REGIONS:

CRUST, MANTLE, OUTER CORE, INNER CORE

Davidson 5.8

Near melting at base of plates

Rapid velocity increase

Smooth velocity increase

Rapid velocity increase

Velocity decrease

SOLID LIQUID SOLID

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

TEMPERATURES IN THE EARTH - GEOTHERM

BELOW MELTING CURVE (SOLIDUS) IN MANTLE AND SOLID INNER CORE

ABOVE MELTING CURVE IN LIQUID OUTER CORE

LIQUID

SOLID

SOLID

SOLID INNER CORE FREEZES FROM OUTER CORE

Fractional crystalization as earth cools

SolidSolid

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

Heat released by freezing and gravitational energy released as the denser solid sinks drive convection in the outer core and thus generate magnetic field

Phase diagram

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

Diamond cell - reach core conditions8% denser

410 km: olivine to

spinel phase

change

Piston Cylinder - reach upper mantle conditions

Davidson 5.2

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

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

SEISMIC WAVES TRAVEL THROUGH THE EARTH ON MANY DIFFERENT PATHS

COMBINE PATHS FOR A DETAILED PICTURE OF VELOCITY VARIATIONS INSIDE

SEISMIC TOMOGRAPHY

Davidson 5.14

Davidson 5.16

East African Rift: Africa splits into Nubia (West Africa) and Somalia (East Africa)

1-2 mm/yr

Hot rock indicated by slow wave speed rises to fill gap as plates separate

Forsyth et al, 1998

SEISMIC TOMOGRAPHY OF MIDOCEAN RIDGEHOT UPWELLING MAGMA HAS LOW VELOCITY

TOMOGRAPHY SHOWS THERMAL STRUCTURE OF SUBDUCTING PLATE (SLAB)

COLD (HIGH SEISMIC VELOCITY) OCEANIC PLATE SUBDUCTS INTOWARMER (LOWER SEISMIC VELOCITY) MANTLE

WARM REGION OF BACK ARC MELTING CAUSES VOLCANOES

SLAB HEATS UP SLOWLY (MILLIONS OF YEARS)

DATA

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 GAS- TRAPPING STRUCTURES IN SEDIMENTARY BASINS

ALSO USED FOR DETAILED STUDIES OF THE DEEP CRUST

SEISMIC REFLECTION ON LANDUSES "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

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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) TIME ON SEISMOGRAM

DEPTH = 2 x SPEED x TIME TO REFLECTOR

OIL?

VANCOUVER ISLAND

IMAGE OF JUAN DE FUCA PLATE SUBDUCTING FENEATH NORTH AMERICA FROM SEISMIC REFLECTION DATA