Granularity of the Mantle Inferred From PKP Precursors Amplitude and Duration
Abstract
In this work, we apply Radiative Transfer theory to invert for the small-scale structure of the mantle, by fitting mean amplitude envelopes of precursors to PKP in the time domain. The data set previously used by Hedlin and Shearer (2000) has been augmented with new deep events from the 1997-2000 period, thereby doubling the number of high-quality records (205) selected for this study. Regional and global average amplitudes are measured in 6 bins covering the 124-142o epicentral distance range. The inversion focuses on the global stacks and aims at determining the depth extent and power spectrum of mantle heterogeneities. We find that the restriction of scatterers to the D'' layer is incompatible with observations. Instead, whole-mantle scattering models reproduce with reasonable accuracy the slow onset and steady increase of precursors with time, in agreement with previous findings by Hedlin et al. (1997). We show that exponential correlations predict a spatial rate of decay of the precursor amplitude which is much faster than observed, independent of the correlation length. Thus, we are lead to propose a new model of mantle heterogeneity, richer in small-scale than exponential media, that fits uniformly the data at all epicentral distances. An analytic form of the correlation function and power spectrum of our new model is given explicitely. It is shown that the precise length scale of heterogeneities is fundamentally unresolvable because of the limited range of observations. The perturbations in P wave velocities required to fit the data are of order 0.1 % which implies a discrepancy of more than one order of magnitude with previous studies.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2002
- Bibcode:
- 2002AGUFM.S21D..09M
- Keywords:
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- 7203 Body wave propagation;
- 7207 Core and mantle;
- 7260 Theory and modeling