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