A model for attenuation and scattering in the Earth's crust |
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| Authors: | M Nafi Toksöz Anton M Dainty Edmund Reiter Ru-Shan Wu |
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| Institution: | (1) Arth Resources Laboratory, Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 02138 Cambridge, MA, USA;(2) Present address: Air Force Geophysics Laboratory/LWH, Hanscom AFB, 01731, MA, USA;(3) Physics and Institute of Tectonics, University of California, 95064 Santa Cruz, CA, USA |
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| Abstract: | The mechanisms contributing to the attenuation of earthquake ground motion in the distance range of 10 to 200 km are studied with the aid of laboratory data, coda wavesRg attenuation, strong motion attenuation measurements in the northeast United States and Canada, and theoretical models. The frequency range 1–10 Hz has been studied. The relative contributions to attenuation of anelasticity of crustal rocks (constantQ), fluid flow and scattering are evaluated. Scattering is found to be strong with an albedoB
0=0.8–0.9 and a scattering extinction length of 17–32 km. The albedo is defined as the ratio of the total extinction length to the scattering extinction length. TheRg results indicate thatQ increases with depth in the upper kilometer or two of the crust, at least in New England. CodaQ appears to be equivalent to intrinsic (anelastic)Q and indicates that thisQ increases with frequency asQ=Q
o
f
n
, wheren is in the range of 0.2–0.9. The intrinsic attenuation in the crust can be explained by a high constantQ (500 Q
o2000) and a frequency dependent mechanism most likely due to fluid effects in rocks and cracks. A fluid-flow attenuation model gives a frequency dependence (Q Q
o
f
0.5) similar to those determined from the analysis of coda waves of regional seismograms.Q is low near the surface and high in the body of the crust. |
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| Keywords: | Attenuation scattering crust strong motion Rg coda |
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