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
Slide 2
A random tomographic image (Ferris et al., 2006 GJI) Crustal
tomography: Woodlark Rift, Papua New Guinea - Transition from
continental to oceanic crust
Slide 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]
Slide 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%
Slide 5
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
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
Slide 8
Two Approaches (1) Direct measurement of rock velocities
Slide 9
V vs. composition Arc lower crust Behn & Kelemen 2006
Crustal rock variations Brocher, 2005
Slide 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
Slide 11
Whole-rock vs. calculated velocities (Oceanic gabbros, from
Carlson et al., Gcubed 2009)
Slide 12
Measured vs predicted Vp Oceanic gabbros (data) Thick line:
predictions What is going on? Behn & Kelemen, 2003 Gcubed
Slide 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
Slide 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) Grneisen parameter T = 2 nd
(adiabatic) Grneisen parameter (K(T))
Slide 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
Slide 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
Slide 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
Slide 18
Usage notes: rocks mins modes Compositions from Hacker et al.
2003 Metagabbros Metaperidotites Petrology for people who dont know
the secret codes
Slide 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
Slide 20
The mineral database how good? Dry, major mantle minerals: OK
Hydrous, and/or highly anisotropic..??? Shear Modulus (&
derivatives)???
Slide 21
Inside the macro Yellow: extrapolated, calculated from related
parameters, or otherwise indirect V KTKT KGG th Big problems w/
shear modulus
Slide 22
A couple of Apps Hydrated metabasalts (after Hacker, 2008;
Hacker and Abers, 2004) use Perple_X to calculate phases, HAMacro
to calculate velocities
Slide 23
Predict T(P) from model (Abers et al., 2006, EPSL) & Facies
from petrology (Hacker et al., 2003)
Slide 24
H2O Vs 2D model predictions Predictions from thermal/petrologic
model
Slide 25
Serpentinization effect on Vp [Hyndman and Peacock, 2003] Are
downgoing plates serpentinized? (Nicaragua forearc)
Slide 26
Result: low Vp/Vs in deeper wedge Where slab is deep: Vp/Vs =
1.64-1.69 (consistent w/ tomography)
Slide 27
The Andes [Wagner et al., 2004, JGR] 31.1S Flat Slab 32.6S
Vp/Vs < 1.68-1.72
Slide 28
Vp/Vs and composition: need quartz Andes AK wedge AK wedge
Slide 29
Slide 30
What is seismic attenuation? Q = E/E - loss of energy per cycle
EE Amplitude ~ exp(- ftT/Q) T 1/f
Slide 31
What Causes Attenuation? Upper Crust: cracks, pores Normal
Mantle: thermally activated dissipation Cold Slabs: ?? (scattering
may dominate if 1/Q intrinsic is low)
Slide 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
Slide 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.)
Slide 34
Attenuating Signals 2 s DH1 = 0.92 RCK = 0.91 wedge RCKDH1
updip P waves depth 126 km (Stachnik et al., 2004, JGR)
Slide 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):
Slide 36
Path-averaged Qs assumes Q(f) from laboratory predictions
Invert these tomographically
Slide 37
Test of Q theory: Ratio of Bulk / Shear attenuation high 1/Qs
high 1/Qk Alaska cross-section (Stachnik et al., 2004)
Slide 38
Test of HTB: Frequency Dependence Q = Q 0 f Lab: Faul &
Jackson 2005 Observations from Alaska
Slide 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)
Slide 40
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)
Slide 41
Attenuation in Central America (TUCAN) (Rychert et al., 2008
G-Cubed)
Slide 42
Anisotropy
Slide 43
EXTRAS
Slide 44
Attenuation vs Velocity: Physical Dispersion No attenuation
Attenuation + Causality = Delay in high-frequency energy
Attenuation without causality
Slide 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
Slide 46
Physical Dispersion: Faul/Jackson approx. K G anharmonic
anharmonic + anelastic
Net effect: interpreting T from Vs Faul & Jackson, 2005
EPSL
Slide 49
Slide 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!)
Slide 51
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.
Slide 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
Slide 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?