Influence of Scattering on Q in the Lithosphere
Semi-annual technical rept. no. 2, 1 Dec 1983-31 May 1984
GEORGIA INST OF TECH ATLANTA SCHOOL OF GEOPHYSICAL SCIENCES
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Codas of local earthquakes from Monticello Reservoir, South Carolina, and Mammoth Lakes, California, have been examined to determine scattering parameters and attenuation for the crust in these areas. The backscattering turbidity or backscattering cross-section per unit volume and the total coda Q have been determined. In both areas, strong scattering is observed in the near surface crust upper 10 km, sufficiently strong that multiple scattering is occurring. Because multiple scattering is occurring, coda Q at short times will underestimate Q experienced by a short pulse. At long times, the coda at Monticello indicates the presence of a low-scattering crustal channel within which energy probably spreads horizontally this channel is either absent or more strongly scattering at Mammoth Lakes. Results at both sites indicate that both in the near surface region and in the aforementioned crustal channel, scattering is probably an important contributor to Q at least half of the attenuation. The scatterers are probably predominantly forward scattering such as velocity fluctuations, and in the near surface there are many scatterers of size 50 m and greater.