Estimations of Q from seismic Rayleigh waves

Abstract

The specific attenuation factor, Q(_γ)(^-1), has been estimated from seismic Rayleigh waves in the frequency range 0.015-0.11 Hz. The 95% confidence limits determine a narrow region around all estimates. The observational data consists of digitised Rayleigh wave traces from film chips of the long period vertical component instruments of the WWSSN stations. Events used are nuclear explosions in Movaya Zemlya, the Lop Nor region of China (Southern Sinkiang Province) and the Aleutian Islands. The group velocity and spectral amplitudes are obtained for each seismogram using an improved "multiple filter technique Q(^-1)(_ γ) is estimated by a least squares regression fit to the subsequent amplitude distance plots. Values of Q(^-1)(_ γ) are generally larger when determined from Novaya Zemlya (.004) than for the Lop Nor test site (.003). The largest values of (.009) are found at low frequencies (0.02 Hz), implying a zone of high dissipation in the upper mantle sampled by these frequencies alone.-1The observed values of Q(^-1)*_ Q(^-1)(_ γ) are directly inverted using an extended Monte-Carlo technique - "Hedgehog". This successfully inverted the data from Novaya Zemlya revealing a region of high dissipation coincident with the low velocity zone, although low velocity is not assumed. The inversion model shows Q(^-1)(_a) = .002, Q(^-1)(_β) = .0045 for the uppermost 120 km and Q(^-1)(_a) = .007, Q(^-1)(_β) = .015 (Q(_a) = 140, Q(_β) = 65) in the absorption zone below 120 km.