HR: 0800h
AN: T11E-1322    [Abstracts]
TI: ScS-S Differential Attenuation from Spectral Ratios and Instantaneous Frequencies via the Hilbert and Continuous Wavelet Transform
AU: * Ford, S R
EM: sean.ford@asu.edu
AF: Arizona State University, Department of Geological Sciences, Tempe, AZ 85287-1404 United States
AU: Garnero, E J
EM: garnero@asu.edu
AF: Arizona State University, Department of Geological Sciences, Tempe, AZ 85287-1404 United States
AB: Measurement of differential t* between ScS and S phases leads to a better understanding of attenuation in the D'' layer. Here, we obtain differential t* between ScS and S from instantaneous frequencies via the standard Hilbert transform and the complex continuous wavelet transform. The waveform is attenuated until the instantaneous frequency at the S envelope peak matches that of the unattenuated ScS envelope peak. Differential t* between ScS and S is then the t* operator required to match the instantaneous frequencies. The instantaneous frequency at the peak of a seismic phase is equal to the average Fourier spectral frequency of the phase weighted by its amplitude. We also compute differential t* between ScS and S from spectral ratios by windowing the S and ScS phases and calculating the ratio of the log magnitude of their spectra. The slope of this ratio is then the differential t* between ScS and S. All methods produce consistent results for high signal-to-noise ratio (SNR) synthetic waveforms with S and ScS phases well separated in time. The instantaneous frequency method gives more stable results than the spectral ratio method for low SNR waveforms or with diminished differential time between ScS and S arrivals. The discrepancy between the methods at low SNR can be mitigated somewhat by careful choice of the bandwidth used in the spectral ratio calculation. The instantaneous frequency matching method is applied to broadband seismic data sampling the deep mantle beneath Central America. Differential t* between ScS and S is calculated from the data permitting the calculation of a regional model of shear wave attenuation in the lower mantle.
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 5144 Wave attenuation
DE: 7207 Core and mantle
DE: 7215 Earthquake parameters
DE: 0935 Seismic methods (3025)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting