1IGPP 0225, U.C. San Diego, La Jolla, CA, 92093
2U.S. Geological Survey, MS 966, Box 25046 DFC, Denver,
CO 80225
Email:
pshearer@ucsd.edu
poster/oral: poster
| At high frequencies (~1 Hz), most of the seismic energy arriving at teleseismic distances is not found in the main phases (e.g., P, PP, S, etc.) but is contained in the extended coda that follows these arrivals. This coda results from scattering off small-scale velocity and density perturbations within the crust and mantle and contains valuable information regarding the depth dependence and strength of this heterogeneity as well as the relative importance of intrinsic vs. scattering attenuation. Most analyses of seismic coda to date have concentrated on S-wave coda generated from lithospheric scattering for events recorded at local and regional distances. Here we examine the globally averaged teleseismic ( > 20 degree range) vertical-component, 1-Hz wavefield for earthquakes recorded in the IRIS FARM archive from 1989 to 1999. We apply an envelope-function stacking technique to image the average time-distance behavior of the wavefield. Unlike regional records, our images are dominated by P and P-coda owing to the large effect of attenuation on PP and S at high frequencies. Modeling our results is complicated by the need to include a variety of ray paths, the likely contributions of multiple scattering, and the possible importance of P-to-S and S-to-P scattering. We adopt a stochastic, particle-based approach in which millions of seismic "photons" are randomly sprayed from the source and tracked through the Earth. Each photon represents an energy packet that travels along the appropriate ray path until it is affected by a discontinuity or a scatterer. Discontinuities are modeled by treating the energy normalized reflection and transmission coefficients as probabilities. For example, if the reflection coefficient is 10% then there is a 10% chance that an individual photon will be reflected by the interface. Scattering probabilities and scattering angles are computed in a similar fashion, assuming random velocity and density perturbations characterized by an exponential autocorrelation function. Intrinsic attenuation is included by reducing the energy contained in each packet as an appropriate function of travel time. Preliminary results suggest that most scattering occurs in the lithosphere and upper mantle, as previous results have indicated, but that some lower mantle scattering may also be required. Models found to date that successfully predict P-coda amplitudes severely underestimate observed PP amplitudes at 70 to 95 degree range. The origin of this discrepancy is not clear. |