Vertical profile of effective turbidity reconstructed from broadening of incoherent body-wave pulses-II.Application to Kamchatka data

The vertical profile of effective turbidity under Kamchatka is reconstructed from observations of distance-dependent broadening of the inchoherent pulse of high-frequency body waves from small earthquakes, by means of a new approach and data processing scheme developed in Paper I. The key ‘effective turbidity’ parameter,ge , used is an immediate generalization of the common isotropic turbidity/scattering coefficientg. Measurements of 200–600 onset-to-peak delays forP andS waves for five Kamchatka stations are used for interpretation. The estimates based on these data correspond to the 2–4Hz frequency band. The inversion of data is performed in terms of the parameters of two generic vertical effective turbidity structures: a piecewise-constant profile (PCP) and truncated-inverse-power-law profile (TPLP), both used in several variants. The variants of the inversions give consistent results, but also reveal rather limited resolution, not permitting the recovery of detailed profiles or a comparison of results among individual stations. The inversions indicate that the values of effective turbidity decay from the surface down: within the depth intervalh = 0–50km, the decay is gradual; at greater depths it is much steeper, roughly following the inverse cube law. The estimates of average effective mean free pathle= 1/ge are very close forP andS waves: 50–60km (±20 per cent) for the 0–20km layer; 250–300km (±30 per cent) for the 20–80km layer; and ath >60–80km,le ≈ 100(h/40)−2–4 for bothP andS waves. The value of both theP- and theS-wave optical thickness (total scattering loss) of the upper 200km is about 0.75 (±25 per cent), and the lithospheric-scattering contribution tot*P is estimated as 0.2s at 1Hz. The expectedS-wave scattering loss agrees reasonably with the standard regional amplitude attenuation curve, probably reflecting the secondary role of intrinsic loss at 3Hz. TheS-wave scatteringQ in the lithosphere of Kamchatka is estimated forf = 1Hz as 125, 205 and 255 for hypocentral distances of shallow events of 30, 100 and 300km, respectively.