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Attenuation and backscattering from local coda

SCHOOL OF GEOPHYSICAL SCIENCES GEORGIA INSTITUTE OF TECHNOLOGY, ATLANTA, GEORGIA 30332

Abstract

Coda decay and excitation for local events were examined at Mammoth Lakes and Morgan Hill, California, Monticello, South Carolina, and New Brunswick, Canada, in the frequency range of 3 to 50 Hz. The single-scattering theory of attenuation and coda generation was used to interpret the data. Coda decay was parameterized by the total apparent turbidity, which is inversely proportional to the conventional coda Q, and the backscattering turbidity, which is proportional to the ratio of the coda power at the time of the direct S and the energy of the direct S pulse. This is a scattering parameterization, since turbidities are cross-sections per unit volume; the term "apparent" is used for the total apparent turbidity because in actuality this parameter contains the effect of anelastic attenuation as well as scattering. For short times (less than 10 sec), the total (apparent) turbidity determined from coda decay was about 0.1 km^–1 for all regions, or a Q of about 60 (3 Hz) to 1000 (50 Hz). The backscattering turbidity determined from coda excitation at short times is often larger than the total apparent turbidity and thus indicates strong, multiple scattering in the upper crust, especially for frequencies in the 3- to 10-Hz range. This means that coda Q is not equivalent to direct S Q in this frequency range, since it does not include the full effect of scattering. At times longer than 10 to 15 sec for the codas from Monticello and New Brunswick, the coda energy appeared to be channeled into a horizontally propagating mode such as Lg. This change was seen as a change in decay rate and excitation; the total turbidity for this portion of the coda was lower than for short codas, about 0.01 km^–1, indicating less scattering. At Monticello, the conditions for the validity of the single-scattering theory appeared to be satisfied, meaning that coda Q includes the effect of scattering and is a proper measure of the Q of the wave type that makes up the coda—most likely Lg. The backscattering turbidity, which is controlled solely by scattering, and the total apparent turbidity at Monticello both show a minimum around 10 Hz. This correlation between coda excitation and decay indicates that a substantial portion (at least 50 per cent) of coda Q in this case is due to scattering. The evidence for strong scattering observed in other codas indicates that scattering will play an important role in attenuation at those sites also. Codas from California, however, did not show the phenomenon of a change at 10 to 15 sec, indicating either that this mode is not present or that it is more strongly scattered. All of the scattering observed in this study probably occurs in the crust. Reverberation under the station cannot be excluded for most codas, but the existence of two types of coda at Monticello and New Brunswick demonstrates that this cannot be the dominant mechanism at these sites.

Footnotes

^* Present address: Earth Resources Laboratory, Massachusetts Institute of Technology, 42 Carleton Street, Cambridge, Massachusetts 02142.

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