Probing Earth's small-scale structure using array seismology Dr. Sebastian Rost Department of Geological Sciences Max-Planck SNWG-Symposium, January 26, 2005

Outline Introduction – Array Seismology – Deep Earth studies Ultra-Low velocity zones – CMB structure Ultra-Low velocity zones – CMB structure Conclusions and Outlook Conclusions and Outlook

Earthquakes, Plate boundaries, Volcanoes

There is no shortage of those ! Hypotheses and Interpretations

Things we can measure: Seismic waves Gravity Gravity Magnetic field Magnetic field Surface heat flux Surface heat flux Earth’s shape/rotation Earth’s shape/rotation Basalts Basalts Plate motions Plate motions Cosmic abundances Cosmic abundances Additionaltools: High pressure mineralogy (mineral physics) - laboratory - computational Convection studies (geodynamics) - numerical - experimental Multidisciplinary Approach

PREM Preliminary Reference Earth Model Globally averaged properties with depth Dziewonski and Anderson [1981, PEPI] Earth's important boundary layers

Z=50 km Z=1500 km Z=2880 km [after Ritsema and van Heijst, 2001] HETEROGENEITY Weakest Strong again Strongest Heterogeneity

Lithospheric studies Transition zone structure D” and CMB studies Inner core boundary Rost and Weber, 2001 Rost and Williams, 2003 Rost and Weber, 2002 Rost and Revenaugh, 2001, 2003, 2004 Rost et al., 2004 Rost and Garnero, 2004 Studying boundary layer structure

Array Seismology Very Large Array, New Mexico, USA Image courtesy of NRAO/AUI Seismic Array: term used to describe network of seismometers that allow time-series stacking to increase signal-to-noise ratio of coherent energy Similar to: Chains of geophones Antennae Antennae Radio Telescopes Radio Telescopes

Array Seismology Rost and Thomas [2001]

Core-mantle boundary studies Studies of CMB east of Australia Very hot region of lower mantle Very hot region of lower mantle Edge of Pacific superplume structure Edge of Pacific superplume structure

Current view of CMB Image courtesy of E. Garnero

Ultra Low Velocity Zones Thorne and Garnero, 2004 < 50% of CMB are probed < 10% (CMB area) show ULVZ 0.5 to 10’s km thick 0.5 to 10’s km thick 10 to 30 % velocity decrease 10 to 30 % velocity decrease Density ? Density ?

Probing ULVZ Rost and Revenaugh [2003]

Topography from NOAA 2” dataset Using two small-aperture arrays

Study Area WRA Tonga-Fiji

Tonga-Fiji seismicity Tonga-Fiji seismicity Deep earthquakes Deep earthquakes WRA dataset 97 earthquakes 51 earthquakes

ASAR dataset - 51 earthquakes - deep seismicity - Seismicity from: 11/1996 – 12/2000

WRA beam trace profile N S Rost et al. [2005]

WRA beam trace profile N S

WRA double beam All precursor events + summation trace Precursor summation trace Non-precursor summation

ASAR beam traces N S

ScP/P waveform comparison WRA : 0.5Hz – 1.4Hz ASAR: 1Hz – 3 Hz Higher ASAR resolution gives evidence for SdP and perhaps SPcP

ScP CMB sampling Tomo from Ritsema and van Heijst, 2002 Red: slow seismic velocity Blue: fast seismic velocity

ScP ULVZ evidence ~50 by 50 km ~50 by 50 km Small scale structure Small scale structure sharp boundaries sharp boundaries CRZ evidence from Rost & Revenaugh, Science, 2001

Waveform forward modeling 1D Gaussian Beam Synthetics 1D Gaussian Beam Synthetics constant layer velocity constant layer velocity ScP, ScsP, SdP, SPcP ScP, ScsP, SdP, SPcP PREM background PREM background sharp upper boundary sharp upper boundary 4 parameter grid-search 4 parameter grid-search

Waveform forward modeling Rost et al., 2004

Waveform forward modeling

Best fit grid search Best-fit model properties: Thickness: 8.5 (±1) km Thickness: 8.5 (±1) km  V P : -10 (±2.5) %  V P : -10 (±2.5) %  V S : -25 (±4) %  V S : -25 (±4) %   : +10 (±5) %   : +10 (±5) %

Experiment probes very slow mantle  Experiment probes very slow mantle (Ritsema and van Heijst, 2002)  Region of strong lateral gradient  chemical heterogeneity (Thorne et al., 2004) (Thorne et al., 2004)  Probably dense material at CMB (McNamara and Zhong, 2004) (McNamara and Zhong, 2004) red: lowest velocities for S20RTS green: strongest V S gradients Data and modeling results

5 to 30 vol.% melt no spreading along CMB no spreading along CMB trapped intercumulus liquid trapped intercumulus liquid incompatible-element enriched liquid incompatible-element enriched liquid requires large overlying thermal anomaly downward percolation of melt downward percolation of melt correlation to dynamic instabilities/upwellings correlation to dynamic instabilities/upwellings probably a fixed base for mantle upwellings probably a fixed base for mantle upwellings Preferred physical model Rost et al., 2004

(from Jellinek and Manga, RoG, 2004) D” aspect ratio of tank experiment Geodynamics tank experiment

Why should we care? Ultra-low velocity zones Mantle plumes Heat flow Continent break-up Polar path (magnetic reversal) Core entrainment Ocean Islands

Thermo-chemical piles

Geodynamics and Tomography Iso-surface marking the density interface Single ridge beneath Africa Several ridges beneath the Pacific McNamara and Zhong [in review] Image courtesy of GEON

Geodynamics cross section temperature field: hottest mantle is within dense material McNamara and Zhong [in review] Image courtesy of GEON High-resolution ULVZ study

Better seismic wavefield characterization - more array analysis methods - shorter wavelengths - frequency dependence Important next steps

USArray 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 steps

Important next steps – II Thorne, Garnero, Igel, Treml, Jahnke, Schmerr [in prep] Better wave-propagation tools More realistic 2D and 3D wave propagation Codes will generate “synthetic” data

Summary Array Seismology Allows sharper snapshots of the Earth's interior Use of subtle waveform variations Use of subtle waveform variations New installations  new study areas New installations  new study areas New probes  new study areas New probes  new study areas Ocean bottom installations Ocean bottom installations Planetary Seismology ? Planetary Seismology ?

The trembling rock brings knowledge from afar -: read the signs! Thank you