Broadband observations of upper-mantle seismic phases in northern Australia and the attenuation structure in the upper mantle

Abstract

Abstract Sources in the earthquake belt through Indonesia and New Guinea, recorded at a broadband Guralp CMG3 seismometer at the Warramunga array in the Northern Territory of Australia, give good coverage of propagation through the upper part of the mantle. The midpoints of the propagation paths lie along the northern margin of the Australian continent. Beyond 18° there is a significant difference in the frequency content of P and S waves. The P waves remain high frequency, but the S waves returned from the transition zone and below are of intermediate period (0.1–0.5 Hz), which would be difficult to record without a broadband instrument and a quiet site. The later branches associated with the 410 and 660 km discontinuities are clearly seen in individual seismograms. The S waves recorded on the radial (SV) and tangential (SH) components are of comparable quality, because the hard-rock recording site minimizes the influence of coupling to P on the radial component. We interpret the observed difference in frequency content of P and S waves at transition-zone distances as the effect of a layer under the seismically fast lid which possesses a large degree of shear dissipation. We quantify the observation in terms of the decay rate of the spectral ratio of S and P waves. That quantity can be interpreted in terms of path integrals of the difference in inverse Q for S waves and P waves, t s * − t p * . We analyse 22 seismograms from the WRA broadband instrument and a further four from portable broadband instruments deployed near Warramunga. The measured slope of the S/P spectral ratio is consistently small out to 18° epicentral distance, where it increases dramatically. Assuming that shear dissipation dominates over bulk dissipation, these measurements are consistent with an average quality factor for S waves in the lid of the order of Q s = 1400 (Q p = 2800) on top of a highly attenuative asthenosphere of 200 km thickness with Q s = 100 (Q p = 200) , which is underlain by a transition zone with Q s = 600 (Q p = 1200) . The low- Q zone in the asthenosphere can equally well be modelled with a thinner layer with a lower Q, e.g. a layer of 100 km thickness with Q s = 60 . Frequency dependence of Q may render the above estimates of Q about 25% too large. These results compare fairly well with published results from 0.3 Hz body-wave observations under the Eurasian shield. Notable exceptions are that we obtain a somewhat lower value for Q s in the asthenosphere and a much higher value in the lid. The lid under northern Australia is unusually