Comparison of Theory and Experimental Results on Seismic Wave Attenuation Ian Jackson a , Ulrich Faul a , John Fitz Gerald a , Stephen Morris b , Yoshitaka Aizawa a,c & Auke Barnhoorn a a Australian National University b University of California, Berkeley c Okayama University, Misasa
25
Embed
Comparison of Theory and Experimental Results on Seismic Wave Attenuation Ian Jackson a, Ulrich Faul a, John Fitz Gerald a, Stephen Morris b, Yoshitaka.
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Comparison of Theory and Experimental Results on Seismic Wave Attenuation
Ian Jacksona, Ulrich Faula, John Fitz Geralda, Stephen Morrisb,
Yoshitaka Aizawaa,c & Auke Barnhoorna
a Australian National University b University of California, Berkeleyc Okayama University, Misasa
Relaxation time e = bd/GURelaxation strength = 0.57(1-) = 0.42 (for = 0.26 & truncation after 100 terms!)Relaxed modulus and height of Debye dissipation peak
GR/GU = 1/(1+) Q-1D = (/2)/(1+)1/2
HOWEVER, since hj ~ j-2 , the infinite sum j3hj2 fails to
converge implying zero U,
Jackson et al., Mat. Sci. Eng. A, in press
Grain-edge morphology& relaxation strength
Jackson et al., Mat. Sci. Eng A, in press
Transient diffusional creep (Raj)
Duration of diffusional transient (following elastically accommodated sliding)
d = (1-)kTd3/[403GUDb].
Transient creep rate is enhanced relative to steady-state diffusional creep rate by factor (t/d)-1/2
which integrates to a creep function of Andrade form t1/2 (Gribb and Cooper) yielding a wide absorption band with Q-1 ~ To
1/2d-3/2
Viscoelastic behaviour of olivine ± melt: observations vs theory
location: To ~ d exp (EP/RT)*, EP ~ 720 kJ/mol Peak inconsistent with melt squirt between tubules
* c. f. theory
Melt-related viscoelastic relaxation
Faul et al., JGR, 2004
constantrelaxation time: (/)
Squirt of basaltic melt ( 1-100Pa s @ 1300-1200 C) << 1 s; also ~ 0.01 for squirtbetween tubules
Attribution of 1/Q peak to elastically accommodated grain-boundary sliding g. b. viscosity ~ 104-109 Pa s @ 1300-1000 C -intermediate between melt& specimen viscosities
p = 2
p = 3
p = 1
after Schmeling (1985)
Dislocation relaxation
Dislocation motionby formation
& migration of kinksin response to shear
stress yz
Estimated relaxation times(Karato, PAGEOPH, 1998;
Jackson, Treatise on Geophysics,submitted)
Conclusions
Mildly frequency- and grain-size-sensitive background-only behaviour of fine-grained melt-free olivine (c. f. silicon nitride, carbide and alumina) suggests diffusionally accommodated grain-boundary sliding (gbs)
Prior elastically accommodated gbs inhibited by tight grain-edge intersections in melt-free materials?
1/Q peak + background for melt-bearing materials with grain edges rounded at triple-junction melt tubules suggests gbs with mix of elastic & diffusional accommodation
Need improved micromechanical model incorporating compliant grain-edge tubules & allowing possibility of sliding with concurrent elastic & diffusional accommodation
Dislocation relaxation & role of water remain to be systematically addressed
Contrasting microstructures of ‘dry’ &
‘wet’ Anita Bay dunite
‘dry’: patchy distributionof silicate melt & some
fluid-filled pores
‘wet’: homogeneously
distributed water-rich fluid phase
Influence of water II
widely dispersed water-rich fluid phase
Anita Bay dunite, d ~ 100m0.3 wt% H2O, welded Pt capsule