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(NAS Colloquium) Earthquake Prediction The Scientific Challenge (1996) / Chapter Skim
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Scale dependence in earthquake phenomena and its relevance to earthquake prediction
Pages 3740-3747

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From page 3740...
... Scale dependence in earthquake phenomena and its relevance to earthquake prediction KEITTI AKI Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740 ABSTRACT The recent discovery of a low-velocity, low-Q zone with a width of 50-200 m reaching to the top of the ductile part of the crust, by observations on seismic guided waves trapped in the fault zone of the Landers earthquake of 1992, and its identification with the shear zone inferred from the distribution of tension cracks observed on the surface support the existence of a characteristic scale length of the order of 100 m affecting various earthquake phenomena in southern California, as evidenced earlier by the kink in the magnitudefrequency relation at about M3, the constant corner frequency for earthquakes with M below about 3, and the sourcecontrolledfmax of 5-10 Hz for major earthquakes. The temporal correlation between coda Q-~ and the fractional rate of occurrence of earthquakes in the magnitude range 3-3.5, the geographical similarity of coda Q-~ and seismic velocity at a depth of 20 km, and the simultaneous change of coda Q-~ and conductivity at the lower crust support the hypotheses that coda Q-~ may represent the activity of creep fracture in the ductile part of the lithosphere occurring over cracks with a characteristic size of the order of 100 m.
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From page 3741...
... Since we cannot hypothesize any room-like structure in the earth, we attribute seismic coda waves to backscattering from numerous heterogeneities in the earth. We may consider seismic coda as waves trapped in a random medium.
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From page 3742...
... , seismic energy per unit volume at x and t; I3, velocity of wave propagation; a, total attenuation coefficient: 71 = Us + Hi (energy decays with distance X as expt-71~x~; hi, intrinsic absorption coefficient; Us, scattering attenuation coefficient; Qs ~ = 71s,B/w, scattering Q-~; Qua ~ = 71i,~/w, absorption Q-~; B = ~s/r1' albedo; LO = 1/~, extinction distance; L = 1/71S' mean free path; 13Eo~t) , rate of energy radiated from a point source at xO at t.
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From page 3743...
... If we define the distance at which the correlation first comes close to 0 as the coherence distance, the average coherence distance is ~135 km for the time window 30-60 see, 90 km for the window 20-45 see, and 45 km for the window 15-30 sec. The above observation offers strong support to the assumption that coda waves are composed of S to S back-scattering waves, because the distances traveled by S waves with a typical crustal S wave velocity of 3.5 km/see in half the lapse time 60, 45, and 30 see are, respectively, 105, 79, and 53 km, which are close to the corresponding coherence distance-namely, 135, 90, and 45 km.
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From page 3744...
... 3. Spatial autocorrelation function of coda Q- ~ at various frequencies measured from the time window 30-60 see in southern California obtained by Peng (44~.
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From page 3745...
... lying within a 0.2° x 0.2° region for frequency 12 Hz and time window 20-30 sec. Larger circles correspond to greater coda Q-1 as indicated on the right (in 1O-3~.
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From page 3746...
... Discussion The characteristic magnitude Mc attributed to the characteristic scale length of creep fracture in southern California is 3.0, which corresponds to the fault length of a few hundred meters. The closeness of this length to the fault zone width estimated from trapped modes suggests a generic relation between them.
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From page 3747...
... (1990) San Andreas Fault System, California (U.S.
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Key Terms

  • time window
  • coda waves
  • scale length
  • trapped modes
  • characteristic scale

  • corner frequency
  • seismic coda
  • landers earthquake
  • unique scale
  • shear zone

  • temporal change
  • zone found
  • creep fracture
  • discrete hierarchy
  • earthquake processes

  • waves trapped
  • zone width
  • constant corner
  • lapse time
  • various frequencies

  • san andreas
  • earthquake phenomena
  • seismic network
  • mean free
  • autocorrelation function

  • lower crust
  • scale lengths
  • earthquake prediction
  • creep fractures
  • spatial autocorrelation


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