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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.

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Presentation on theme: "Interpreting Seismic Observables Geoff Abers, Greg Hirth Velocities: compositional effects vs P,T Attenuation at high P, T Anisotropy (Hirth) Upload from."— Presentation transcript:

1 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

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

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

4 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%

5 Unusual low Vp/Vs in wedge Vp/Vs = 1.65-1.70 Alaska (Rossi et al. 2006) Andes 31°S (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

6 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)

7 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

8 Two Approaches (1) Direct measurement of rock velocities

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

10 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

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

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

13 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

14 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) Grüneisen parameter  T = 2 nd (adiabatic) Grüneisen parameter (K(T))

15 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

16 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…

17 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

18 Usage notes: rocks mins modes Compositions from Hacker et al. 2003 Metagabbros Metaperidotites Petrology for people who don’t know the secret codes

19 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

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

21 Inside the macro… Yellow: extrapolated, calculated from related parameters, or otherwise indirect V  KTKT K’GG’  th Big problems w/ shear modulus

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

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

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

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

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

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

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


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

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

32 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

33 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.)

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

35 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):

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

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

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

39 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)

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

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

42 Anisotropy


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

45 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

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

47 Physical Dispersion: Karato approx. Karato, 1993 GRL

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


50 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!)

51 Deep under the hood: 1 st Grüneisen 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.

52 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

53 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?

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