Site Amplification, Scattering, and Intrinsic Attenuation in the Mississippi Embayment from Coda Waves

Abstract

We estimate site amplification, intrinsic Q ( Q I ) and scattering Q ( Q S ) values from three-component coda of broadband data recorded by Cooperative New Madrid Sesimic Network stations for center frequencies 0.2, 0.5, 1.0, 2.0, and 5.0 Hz. Near 0.5 and 1.0 Hz, site amplification factors for the transverse component recorded by stations in the Mississippi embayment are between 1.7 and 18, compared with stations outside the embayment. The amplification factor becomes smaller at frequencies lower than 0.5 Hz and higher at those greater than 1.0 Hz. The radial component shows a similar amplification; however, the vertical component shows relatively less amplification. Q I and Q S are estimated from coda using an energy flux model for two time windows after the direct S wave and at 0.5 and 1.0 Hz center frequencies. The two time windows include coda up to and after twice the S -wave travel time at each station. Coda parameters change with the time window and are attributed to a mixture of incoherent coda with coherent S-wave reverberations directly after the S wave. Q I and Q S inferred from stations outside of the embayment have higher values than those from stations within the embayment. A plane wave model qualitatively explains the lack of coda amplification at high frequencies compared with a low frequency without recourse to anelastic attenuation arguments. However, the plane wave model cannot explain the very large observed coda amplifications at 1 and 0.5 Hz seen in the data, suggesting that other wave-scattering effects must be occurring in the embayment velocity structure. This is consistent with the relatively low Q S values inferred from the energy flux model for stations within the embayment. In addition to coda amplification, we performed a synoptic noise study that revealed that pre-event ambient background noise behaves in a similar manner to the coda for stations within and outside of the embayment. The presence of large lateral velocity heterogeneities in the embayment may cause Q S values to be low there.