Scattering and Attenuation of Seismic Waves in the Northeastern United States

The energy-flux model of seismic coda, developed by Frankel and Wennerberg (1987), is used to derive path-averaged estimates of scattering (Q_S^-1) and intrinsic attenuation (Q_I^-1) for the northeastern United States. The model predicts the amplitude of the coda wave vs. time as a function of frequency, Q_S^-1, and Q_I^-1. A non-linear inversion scheme is developed that allows for the estimation of Q_S^-1 and Q_I^-1 as a function of frequency by fitting the model to a narrow-bandpass filtered envelope of the seismic coda for each seismogram at discrete frequency points. The inversion is performed on seismograms from earthquakes recorded by the MIT New England Seismic Network (NESN) over a 15-year period between 1981 and 1995. Preliminary results indicate that scattering is the dominant mechanism of energy dissipation, and that the effects of intrinsic attenuation are secondary. The scattering is strongest at shorter propagation distances and decreases substantially as the propagation distance increases. Conversely, intrinsic attenuation is negligible at shorter propagation distances and increases as the propagation distance increases. These results are interpreted as indicative of a strong scattering region at shallow depth, with the scattering decreasing with increasing depth, and with a subsequent increasing of intrinsic attenuation at greater depth. We propose a second analysis to invert the path-averaged estimates of Q_S^-1 and Q_I^-1 using a constrained linear method with regularization to obtain a one-dimensional model of Q_S^-1 and Q_I^-1 vs. depth in the crust. Possible mechanisms for the scattering include the presence of a weathering layer near the surface, the presence of fractures in the shallow crust, and topography.