Attenuation of multiple ScS in various parts of the world

Abstract

A waveform and amplitude matching scheme is used to study regional variations of QScS at low-frequency band between 0.02 and 0.1 Hz for paths across various parts of the world. The scheme estimates attenuation by convolving the first well-recorded, clear ScSH phase with multiples of trial t* operators until the phases so derived match the amplitude decay-rates and waveform characteristics of the observed later multiples. Thirty-five intermediate-to-deep focus earthquakes recorded at seismographic stations throughout the world were used in this study. The regions sampled are the tectonic and shield areas of Eurasia, tectonic regions of North America, shield regions of South America, and tectonic portions of NW, SW and E Pacific Ocean. In general, the regional differentials in t* between successive multiples of ScS are similar to those reported in earlier studies (Nakanishi 1979; Spikin & Jordan 1980). For the tectonic regions of Eurasia, t*ScS are 5.0 ± 1.1 s for a double passage through the mantle, whereas for the shield areas, they are around 2.8 ± 0.4 s. For the tectonic regions of North America, our estimates of t*ScS are 4.8 ± 1.6 s. A mean t*ScS of 6.3 ± 0.9 s has been obtained for paths across the E Pacific while the NW and SE Pacific are characterized by moderate attenuation with average t*ScS values of 4.4 ± 0.7 and 4.2 ± 0.8 s respectively. For central South America, the estimated t*ScS is 2.7 ± 0.7 s. Assuming that the lateral variations in Q observed using the ScS data are confined to a 200–300 km thick layer in the upper mantle, where the lateral velocity variations are also the most pronounced, these results imply lateral variations in shear wave Q of about an order of magnitude. The lateral variations of mantle Q in the upper mantle derived from these analyses of long-period ScS phases correlate well with mantle Q estimates from short period data. It is therefore unlikely that Q(f) appropriate to various types of upper-mantle structures converge at low frequencies of 0.02 to 0.1 Hz as required by the ‘absorption-band shift’ models.