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Interpreting Seismic Observables Geoff Abers, Greg Hirth Velocities: compositional effects vs P,T Attenuation at high P, T Anisotropy (Hirth) Upload from bSpace -> Seismic_Properties: Hacker&AbersMacro08Dec2010.xls & various papers

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A random tomographic image (Ferris et al., 2006 GJI) Crustal tomography: Woodlark Rift, Papua New Guinea - Transition from continental to oceanic crust

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Arc crust velocities Arc Vp along-strike Aleutians Vs. SiO2 in arc lavas [Shillington et al., 2004] Arc lower crust predictions [Behn & Kelemen, 2006]

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Velocity variations within subducting slab W E Green: relocated, same velocities. yellow: catalog hypocenters CAFE Transect, Washington Cascades (Abers et al., Geology, 2009) km from coast dlnVs = 10-15% dlnVs = 2-4%

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Unusual low Vp/Vs in wedge Vp/Vs = 1.65-1.70 Alaska (Rossi et al. 2006) Andes 31S (Wagner et al. 2004) Normal N Honshu Zhang et al. (Geology 2004) Vp/Vs = 1.8-1.9 Strange: no volcanics * PREM: Vp = 8.04 km/s, Vp/Vs = 1.80

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Velocities & H2O in metabasalts Crust Hydrated at: low P, or low T eclogite blueschist amphibolite gr-sch (Hacker et al., 2003a JGR; Hacker & Abers, 2004 Gcubed) 100 92 87 95 81 84 %Vp/Vp HARZ %Vp ~ 99-103 % (eclogite/peridotite) %Vp ~ 85-95 % (hydrated/peridotite)

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What else affects velocities? (b) temperature (c) fluids = bulk modulus = shear modulus = bulk modulus = shear modulus Takei (2002) poroelastic theory Temperature Pore fluids melts H2OH2O Faul & Jackson (2005) anelasticity + anharmonicity aspect ratio 0.1-0.5

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Two Approaches (1) Direct measurement of rock velocities

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V vs. composition Arc lower crust Behn & Kelemen 2006 Crustal rock variations Brocher, 2005

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Second Approach (2) Measure/calculate mineral properties, and aggregate Disaggregate rock into mineral modal abundances For each mineral, look up K, G, V, at STP & derivatives Extrapolate K(P,T), G(P,T), Aggregate to crystal mixture Calculate Vp, Vs Eclogite: Abalos et al., GSABull 2011 Peridotites: Lee, 2003

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Whole-rock vs. calculated velocities (Oceanic gabbros, from Carlson et al., Gcubed 2009)

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Measured vs predicted Vp Oceanic gabbros (data) Thick line: predictions What is going on? Behn & Kelemen, 2003 Gcubed

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Calculating seismic velocities from mineralogy, P,T (Hacker et al., 2003, JGR; Hacker & Abers 2004, Gcubed) Thermodynamic parameters for 55 end-member minerals - 3rd order finite strain EOS - aggregated by solid mixing thy. Track V, , H 2 O, major elem., T,P minerals elastic parameters

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Compiled Parameters o = (P=0 GPa,T=25 C) = density K T0 = isothermal bulk modulus (STP) G 0 = shear modulus (STP) 0 ; d /dT or similar = coef. Thermal expansion K = dK T /dP = pressure derivative = dlnG/dln = T derivative (G(T)) G = dG/dP = pressure derivative th = 1 st (thermal) Grneisen parameter T = 2 nd (adiabatic) Grneisen parameter (K(T))

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Elastic Moduli vs. P, T Computational Strategy: First increase T thermal expansion Second increase P 3 rd order finite strain EoS Integrate in T Integrate in P STP From Hacker et al. 2003a

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Aggregating & Velocities Mixture theories, simple: Voight-Reuss-Hill average K, 1/K, both Complex Hashin-Shtrikman Mixtures sorted/weighted averages Finally, turn elastic parameters to seismic velocities using the usual

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Usage notes Raw data table: elastic parameters & derivatives Intermediate calculation table Work table: Enter compositions, P,T here Mineralinformation & stored compositions database includes references & notes on source of values

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Usage notes: rocks mins modes Compositions from Hacker et al. 2003 Metagabbros Metaperidotites Petrology for people who dont know the secret codes

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Usage Notes: you manipulate rocks sheet Enter compositions here (adds to 100%) and P,T here (optional: d, f for anelastic correction) then click to run (primary output) More info below

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The mineral database how good? Dry, major mantle minerals: OK Hydrous, and/or highly anisotropic..??? Shear Modulus (& derivatives)???

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Inside the macro Yellow: extrapolated, calculated from related parameters, or otherwise indirect V KTKT KGG th Big problems w/ shear modulus

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A couple of Apps Hydrated metabasalts (after Hacker, 2008; Hacker and Abers, 2004) use Perple_X to calculate phases, HAMacro to calculate velocities

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Predict T(P) from model (Abers et al., 2006, EPSL) & Facies from petrology (Hacker et al., 2003)

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H2O Vs 2D model predictions Predictions from thermal/petrologic model

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Serpentinization effect on Vp [Hyndman and Peacock, 2003] Are downgoing plates serpentinized? (Nicaragua forearc)

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Result: low Vp/Vs in deeper wedge Where slab is deep: Vp/Vs = 1.64-1.69 (consistent w/ tomography)

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The Andes [Wagner et al., 2004, JGR] 31.1S Flat Slab 32.6S Vp/Vs < 1.68-1.72

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Vp/Vs and composition: need quartz Andes AK wedge AK wedge

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What is seismic attenuation? Q = E/E - loss of energy per cycle EE Amplitude ~ exp(- ftT/Q) T 1/f

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What Causes Attenuation? Upper Crust: cracks, pores Normal Mantle: thermally activated dissipation Cold Slabs: ?? (scattering may dominate if 1/Q intrinsic is low)

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Seismic Attenuation (1/Q) at high T Faul & Jackson (2005), adjusted to 2.5 GPa d=1 mm 10 mm At High T, Q Has: strong T sensitivity some to H 2 O, grain size, melt weak compositional sensitivity shear, not bulk 1/Q

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High-Temperature Background (HTB) Simple model (Jackson et al. 2002) grain size period activation energy temperature = 0.2-0.3 (frequency dep.) m = (grain size dep.)

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Attenuating Signals 2 s DH1 = 0.92 RCK = 0.91 wedge RCKDH1 updip P waves depth 126 km (Stachnik et al., 2004, JGR)

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Q Measurements Fit P, S spectra: T/Q, M 0, f c 0.5 (10-20) Hz Forearc PathWedge Path S waves, slab event, ~ 100 km u(f) = U 0 A source (f) e - fT/Q Q and amplitude u(f):

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Path-averaged Qs assumes Q(f) from laboratory predictions Invert these tomographically

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Test of Q theory: Ratio of Bulk / Shear attenuation high 1/Qs high 1/Qk Alaska cross-section (Stachnik et al., 2004)

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Test of HTB: Frequency Dependence Q = Q 0 f Lab: Faul & Jackson 2005 Observations from Alaska

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Forearcs: cold; subarc mantle: hot Heat flow in northern Cascadia: step 20-30 km from arc (Wada and Wang, 2009; after Wang et al. 2005; Currie et al., 2004)

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Results from Alaska (BEAAR): 1/Q S In wedge core: Q S ~ 100-140 @ 1 Hz 1200-1400C (dry) lo Q hi Q (Stachnik et al., 2004 JGR)

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Attenuation in Central America (TUCAN) (Rychert et al., 2008 G-Cubed)

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Anisotropy

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EXTRAS

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Attenuation vs Velocity: Physical Dispersion No attenuation Attenuation + Causality = Delay in high-frequency energy Attenuation without causality

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Attenuation vs Velocity: Physical Dispersion No attenuation Attenuation + Causality This means: Band-limited measurements of travel time are late Band-limited measures give slower apparent velocities As T increases, both V and Q decrease

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Physical Dispersion: Faul/Jackson approx. K G anharmonic anharmonic + anelastic

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Physical Dispersion: Karato approx. Karato, 1993 GRL

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Net effect: interpreting T from Vs Faul & Jackson, 2005 EPSL

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Deep under the hood: adiabatic vs. isothermal Important distinction between adiabatic (const. S) and isothermal (const. T) processes Useful: Bina & Helffrich, 1992 Ann. Rev.; Hacker and Abers, 2004 GCubed Labs & petrologists usually measure this Seismic waves see this (not the same!)

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Deep under the hood: 1 st Grneisen parameter relates elastic to thermal properties E is the internal energy, related to temperature S is entropy e.g. defines the adiabat A more useful relationship can be obtained with some definitions/algebra = coef. Thermal expansion K T, K S = (isothermal, isentropic) bulk modulus C V, C P = specific heat at const. (volume, P) Useful: Bina & Helffrich, 1992 Ann. Rev.; Anderson et al., 1992 Rev. Geophys.

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The other parameters & scalings - Relates thermal expansion (of volume) to thermal changes of bulk modulus K = K/P is usually around 4.0 see Anderson et al., 1992 T ~ + K In absence of any data - Same for shear modulus

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Related/useful: Adiabatic Gradient Some monkeying around gives Useful: Bina & Helffrich, 1992 Ann. Rev.; Hacker and Abers, 2004 GCubed So that the adiabatic gradient is This is a useful formulism: ~ 0.8 1.3 for most solid-earth materials (1.1 is good average) g ~ 10 m s 2 throughout upper mantle HOMEWORK: what is the geothermal gradient?