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Transcript
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