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Probing Earth's small-scale structure Probing Earth's small-scale structure using using
array seismologyarray seismology
Dr. Sebastian RostDr. Sebastian RostDepartment of Geological SciencesDepartment of Geological Sciences
Max-Planck SNWG-Symposium, January 26, 2005
OutlineOutline
Introduction – Array Seismology – Deep Earth studiesIntroduction – Array Seismology – Deep Earth studies
Ultra-Low velocity zones – CMB structureUltra-Low velocity zones – CMB structure
Conclusions and OutlookConclusions and Outlook
Earthquakes, Plate boundaries, Volcanoes Earthquakes, Plate boundaries, Volcanoes
There is no shortage of those !There is no shortage of those !
Hypotheses and InterpretationsHypotheses and Interpretations
Things we canThings we canmeasure:measure:
Seismic wavesSeismic waves GravityGravity Magnetic fieldMagnetic field Surface heat fluxSurface heat flux Earth’s shape/rotationEarth’s shape/rotation Basalts Basalts Plate motionsPlate motions Cosmic abundancesCosmic abundances
AdditionalAdditionaltools:tools:
High pressure mineralogy High pressure mineralogy (mineral physics)(mineral physics)
- laboratory- laboratory- computational- computational
Convection studies Convection studies
(geodynamics)(geodynamics)- numerical- numerical- experimental- experimental
Multidisciplinary ApproachMultidisciplinary Approach
PREMPREM
PPreliminary reliminary RReferenceeferenceEEarth arth MModelodel
Globally averagedGlobally averagedproperties with properties with depthdepth
Dziewonski and Anderson Dziewonski and Anderson [1981, PEPI][1981, PEPI]
Earth's important boundary layersEarth's important boundary layers
Lithospheric studiesLithospheric studies
Transition zone structureTransition zone structure
D” and CMB studiesD” and CMB studies
Inner core boundaryInner core boundary
Rost and Weber, 2001Rost and Weber, 2001Rost and Williams, 2003Rost and Williams, 2003
Rost and Weber, 2002Rost and Weber, 2002
Rost and Revenaugh, 2001, 2003, 2004Rost and Revenaugh, 2001, 2003, 2004Rost et al., 2004Rost et al., 2004
Rost and Garnero, 2004Rost and Garnero, 2004
Studying boundary layer structureStudying boundary layer structure
Array SeismologyArray Seismology
Very Large Array, New Mexico, USAVery Large Array, New Mexico, USA
Image courtesy of NRAO/AUI
Seismic Array:Seismic Array: term used to describe network of seismometers that term used to describe network of seismometers thatallow time-series stacking to increase signal-to-noiseallow time-series stacking to increase signal-to-noiseratio of coherent energy ratio of coherent energy
Similar to:Similar to: •• Chains of geophonesChains of geophones
• • AntennaeAntennae
• • Radio TelescopesRadio Telescopes
Array SeismologyArray Seismology
Rost and Thomas [2001]Rost and Thomas [2001]
Core-mantle boundary studies Core-mantle boundary studies
Studies of CMB east of AustraliaStudies of CMB east of Australia Very hot region of lower mantleVery hot region of lower mantle Edge of Pacific superplume structureEdge of Pacific superplume structure
Current view of CMB Current view of CMB
Image courtesy of E. Garnero
Ultra Low Velocity Zones Ultra Low Velocity Zones
Thorne and Garnero, 2004Thorne and Garnero, 2004
< 50% of CMB are probed< 50% of CMB are probed
< 10% (CMB area) show ULVZ< 10% (CMB area) show ULVZ
0.5 to 10’s km thick0.5 to 10’s km thick
10 to 30 % velocity decrease10 to 30 % velocity decrease
Density ?Density ?
Probing ULVZ Probing ULVZ
Rost and Revenaugh [2003]
Topography from NOAA 2” datasetTopography from NOAA 2” dataset
Using two small-aperture arrays Using two small-aperture arrays
Tonga-Fiji seismicityTonga-Fiji seismicity Deep earthquakesDeep earthquakes
WRA dataset WRA dataset
97 earthquakes97 earthquakes 51 earthquakes51 earthquakes
WRA beam trace profile WRA beam trace profile
N
S
WRA double beam WRA double beam
All precursor eventsAll precursor events+ summation trace+ summation trace
Precursor summation tracePrecursor summation trace
Non-precursor summationNon-precursor summation
ASAR beam traces ASAR beam traces
N
S
ScP CMB sampling ScP CMB sampling
Tomo from Ritsema and van Heijst, 2002Tomo from Ritsema and van Heijst, 2002
Red: slow seismic velocityRed: slow seismic velocityBlue: fast seismic velocityBlue: fast seismic velocity
ScP ULVZ evidence ScP ULVZ evidence
~50 by 50 km~50 by 50 km
Small scale structureSmall scale structure
sharp boundariessharp boundaries
CRZ evidence from CRZ evidence from Rost & Revenaugh,Rost & Revenaugh,Science, 2001Science, 2001
Waveform forward modeling Waveform forward modeling
Best fit grid searchBest fit grid search
Best-fit model properties: Best-fit model properties:
ThicknessThickness : 8.5 (: 8.5 (±±1) km1) km DDVVPP : -10 (: -10 (±±2.5) %2.5) %
ddVVSS : -25 (: -25 (±±4) %4) % DD : +10 (: +10 (±±5) %5) %
Experiment probes very slow mantleExperiment probes very slow mantle
(Ritsema and van Heijst, 2002)(Ritsema and van Heijst, 2002)
Region of strong lateral gradient Region of strong lateral gradient chemical heterogeneity chemical heterogeneity
(Thorne et al., 2004)(Thorne et al., 2004)
Probably dense material at CMB Probably dense material at CMB (McNamara and Zhong, 2004)(McNamara and Zhong, 2004)
red: lowest velocities for S20RTSred: lowest velocities for S20RTS
green: strongest Vgreen: strongest VSS gradients gradients
Data and modeling resultsData and modeling results
5 to 30 vol.% melt5 to 30 vol.% melt
no spreading along CMBno spreading along CMB
trapped intercumulus liquidtrapped intercumulus liquid
incompatible-element enriched incompatible-element enriched
liquid liquid
requires large overlying thermal anomalyrequires large overlying thermal anomaly
downward percolation of meltdownward percolation of melt
correlation to dynamic instabilities/upwellingscorrelation to dynamic instabilities/upwellings
probably a fixed base for mantle upwellingsprobably a fixed base for mantle upwellings
Preferred physical modelPreferred physical model
Rost et al., 2004Rost et al., 2004
(from Jellinek and Manga, RoG, 2004)(from Jellinek and Manga, RoG, 2004)
D” aspect ratio of tank experiment
Geodynamics tank experimentGeodynamics tank experiment
Why should we care?Why should we care?
Ultra-low velocity zonesUltra-low velocity zones
Mantle plumesMantle plumes
Heat flowHeat flow
Continent break-upContinent break-up
Polar pathPolar path(magnetic reversal)(magnetic reversal)
Core entrainmentCore entrainment
Ocean IslandsOcean Islands
Better seismic wavefield characterization- more array analysis methods- shorter wavelengths- frequency dependence
Important next stepsImportant next steps
Important next stepsImportant next steps
http://www.earthscope.org
USArrayUSArray
Backbone : ~100 broad-band stations (permanent)
Big-Foot : 400 broad band stations
Flexible : 400/2000 bb/hf stations
Better seismic wavefield characterization- more array analysis methods- shorter wavelengths- frequency dependence
• New probes of the interior- many unexploited possibilities exist
Rost and Garnero [in prep]
Important next stepsImportant next steps
Important next steps – IIImportant next steps – II
Thorne, Garnero, Igel, Treml, Jahnke, Schmerr [in prep]
Better wave-propagation tools
More realistic 2D and 3D wavepropagation Codes willgenerate“synthetic”data
SummarySummary
Array SeismologyArray Seismology
Allows sharper snapshots of the Earth's interiorAllows sharper snapshots of the Earth's interior
Use of subtle waveform variationsUse of subtle waveform variations
New installations New installations new study areas new study areas
New probes New probes new study areas new study areas
Ocean bottom installationsOcean bottom installations
Planetary Seismology ?Planetary Seismology ?
The trembling rock brings knowledge from afar -: read the signs!The trembling rock brings knowledge from afar -: read the signs!
Thank youThank you
http://www.erdbebenwarte.de