Influence scattering and Q in the lithosphere

This project examined the contribution of scattering to the attenuation of short pulses within the crust. Coda decay and excitation for local events were examined at Mammoth Lakes and Morgan Hill, Calif., Monticello, SC; and New Brunswick, Canada, in the frequency range 3-50 Hz. For short times (less than 10 seconds), the total turbidity determined from coda decay was about 0.1/km for all regions, applying a magnitude bias of 0.2 in m sub b if 10 km of such material is traversed. Since the total turbidity is independent of frequency, implying geometrical scattering, this would not be detectable by spectral radio methods. The backscattering turbidity determined from coda excitation at short times indicates strong scattering in the upper crust, especially for frequencies in the 3-10 Hz range. At times longer than 10-15 seconds for the codas from the eastern North American regions, Monticello and New Brunswick, the coda energy appeared to be channeled into a horizontally propagating mode such as Lg. The total turbidity for this portion of the coda was lower than for the short codas, about 0.01/km, indicating less scattering, a result born out by the backscattering turbidity. Codas from California, however, did not show this phenomenon, indicating either that this mode is not present or that it is more strongly scattered. This result indicates that attenuation for Lg can be estimated from the coda after 10 seconds, as proposed by other workers.