HR: 1320h
AN: S43A-0981    [Abstracts]
TI: Site Amplification, Scattering and Intrinsic Attenuation in the Mississippi Embayment
AU: * Jemberie, A L
EM: ajemberi@memphis.edu
AU: Langston, C A
EM: clangstn@memphis.edu
AB: Site amplification, intrinsic Q (Qi) and scattering Q (Qs) values are measured from coda of broad-band data recorded by the Cooperative New Madrid Sesimic Network stations. For the frequency range between about 0.5and 1.0 Hz, site amplification factors for the transverse component recorded by stations in the Mississippi embayment are between 1.7 and eighteen, compared to stations outside the embayment. The amplification factor becomes smaller at center frequencies lower than 0.5Hz and higher than 1.0Hz. The radial component shows similar amplification, however, the vertical component shows less amplification compared to the other components. Knowledge of site effect is a useful parameter for seismic hazard assessment of a given region. There is a general belief that the impedance contrast between the basement rocks and the overlying sediments of sedimentary basins is responsible for the amplification of coda amplitudes recorded by stations there. However, we found it very difficult to explain this using a simple theory of plane wave propagation through a layered structure of New Madrid Seismic Zone. The Physical mechanism responsible for the amplification of seismic amplitudes under stations in the Mississippi embayment is not known. For each seismogram used in this study, we computed the average pre-event noise level, which is part of the seismogram between the zero recording time and the onset of the P-wave. As is in the case of the SH coda, the pre-event noise recorded by stations within the embayment are larger than those recorded by stations outside the embayment. Qi and Qs are measured from coda using an energy flux model at 0.5 and 1.0Hz for lapse time bands: between the direct SH-wave arrival time (td) and twice this time (2td), and between 2td and a time where the noise level begins. For band 1, most stations within the embayment show higher Qi, but lower Qs, values than stations outside the embayment. Energy from reverberations beneath stations in the embayment may cause the apparent high Qi values there. For band 2, stations outside the embayment have higher Qi and Qs values than stations in the embayment. Energy from reverberations under stations is assumed to be absent for band 2. Both Qi and Qs values are lower at 0.5Hz than at 1.0Hz, showing a frequency dependence. Partially fluid/gas saturated sediments or fluid flow in pore spaces or cracks in the upper crust may be the possible mechanisms for low Qi (high intrinsic attenuation) found in the embayment. Presence of large lateral velocity heterogeneities in the embayment may cause Qs values to be low there.
DE: 7223 Seismic hazard assessment and prediction
DE: 7260 Theory and modeling
DE: 7212 Earthquake ground motions and engineering
SC: Seismology [S]
MN: 2004 AGU Fall Meeting