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

Abstract

SUMMARY The vertical profile of eVective turbidity under Kamchatka is reconstructed from observations of distance-dependent broadening of the inchoherent pulse of highfrequency body waves from small earthquakes, by means of a new approach and data processing scheme developed in Paper I. The key ‘eVective turbidity’ parameter, g e ,u sed is an immediate generalization of the common isotropic turbidity/scattering coeYcient g. Measurements of 200‐600 onset-to-peak delays for P and S waves for five Kamchatka stations are used for interpretation. The estimates based on these data correspond to the 2‐4 Hz frequency band. The inversion of data is performed in terms of the parameters of two generic vertical eVective 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 eVective turbidity decay from the surface down: within the depth interval h=0‐50 km, the decay is gradual; at greater depths it is much steeper, roughly following the inverse cube law. The estimates of average eVective mean free path l e =1/g e are very close for P and S waves: 50‐60 km (±20 per cent) for the 0‐20 km layer; 250‐300 km (±30 per cent) for the 20‐80 km layer; and at h>60‐80 km, l e #100(h/40)’2‐4 for both P and S waves. The value of both the P- and the S-wave optical thickness (total scattering loss) of the upper 200 km is about 0.75 (±25 per cent), and the lithosphericscattering contribution to t* P is estimated as 0.2 s at 1 Hz. The expected S-wave scattering loss agrees reasonably with the standard regional amplitude attenuation curve, probably reflecting the secondary role of intrinsic loss at 3 Hz. The S-wave scattering Q in the lithosphere of Kamchatka is estimated for f =1 Hz as 125, 205 and 255 for hypocentral distances of shallow events of 30, 100 and 300 km, respectively.