We quantitatively characterize the regional variations in the strength of heterogeneity in the lithosphere of the globe by analysing the observed seismogram envelopes of teleseismic P waves in the frequency band of 0.5-4 Hz. We apply a theoretical scattering model based on the Markov approximation for a plane P wave propagating through the random medium characterized by a Gaussian autocorrelation function. Since this model presumes the vertical incidence of an impulsive plane wavelet, we first analyse teleseismic P waves from deep earthquakes occurring along the western Pacific regions. We measure the ratios of peak intensity of transverse components to that of the sum of the three components, and determine the quantity of randomness ε2z/a, where ε, a and z are fractional fluctuation, correlation distance and thickness of a heterogeneous structure, respectively. Although source time functions of shallow earthquakes are too complex to directly apply the scattering model, a good correlation between the ratios of peak amplitude and the normalized transverse amplitude, which is the square root of the energy partition of the P-coda waves into the transverse component, enables us to use the shallow earthquakes that occur widely around the world. As a result, the quantity ε2z/a extends from 1.15 × 10-4 to 6.34 × 10-2 at 0.5-1 Hz, 2.02 × 10-3 to 1.89 × 10-1 at 1-2 Hz and 1.49 × 10-4 to 1.89 × 10-1 at 2-4 Hz, which are in agreement with the results of previous studies using different methods. The spatial distribution of randomness almost agrees with various tectonic settings and roughly correlates with lateral variations of Lg coda Q and shear wave velocity perturbations at 80 km depth, suggesting that lateral heterogeneity extends from the shallow crust to uppermost mantle.
- Peak amplitude
- Teleseismic P waves