HR: 1340h
AN: S13B-1047    [Abstracts]
TI: Scattering and Attenuation of Seismic Waves in the Northeastern United States
AU: * Cicerone, R D
EM: rcicerone@bridgew.edu
AF: Bridgewater State College, Department of Earth Sciences and Geography, Bridgewater, MA 02325 United States
AU: Doll, C G
EM: doll@ll.mit.edu
AF: Earth Resources Laboratory, MIT, 42 Carleton Street, Cambridge, MA 02142 United States
AU: Doll, C G
EM: doll@ll.mit.edu
AF: now at Lincoln Laboratory, MIT, 244 Wood Street, Lexington, MA 02420 United States
AU: Toks\"oz, M N
EM: nafi@mit.edu
AF: Earth Resources Laboratory, MIT, 42 Carleton Street, Cambridge, MA 02142 United States
AB: 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.
DE: 7260 Theory and modeling
DE: 7294 Instruments and techniques
DE: 7203 Body wave propagation
DE: 7205 Continental crust (1242)
SC: Seismology [S]
MN: 2004 AGU Fall Meeting