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 SCS, 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, tgcs are 5.0 f 1.1 s for a double passage through the mantle, whereas for the shield areas, they are around 2.8 f 0.4 s. For the tectonic regions of North America, our estimates of t& are 4.8 f 1.6 s. A mean t& of 6.3 f 0.9 s has been obtained for paths across the E Pacific while the N W and SE Pacific are characterized by moderate attenuation with average t& values of 4.4 f 0.7 and 4.2 f 0.8 s respectively. For central South America, the estimated tics is 2.7 f 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.

Upper mantle Q structure beneath the East Pacific Rise

“…This probably correlates with the high-velocity lid which is seen beneath older oceanic crust [Yu and Mitchell, 1979]. This is mildly surprising in that the region we have studied appears to have lower than average shear velocity to depths as great as 400 km [Wielandt and Knopoff, 1982] [Lay and Wallace, 1983] to over 300 in central South America [Chan and Der, 1988]. Mitchell [1976] and Canas and Mitchell [1978] used fundamental mode data in the period range 20-110 s to derive an attenuation model for the shallow mantle beneath the Pacific as well.…”

Global Imaging of the Earth's Deep Interior: Seismic Constraints on (An)isotropy, Density and Attenuation

“…large Q, is often found within stable continents (e.g. Chan & Der, 1988;Sipkin & Revenaugh, 1994;Tian et al, 2009;Kennett & Abdullah, 2011). This is in contrast to the typically high attenuation in the vicinity of hotspots (e.g.…”

Attenuation Tomography of the Earth's Mantle: A Review of Current Status

“…The existence of large lateral variations of Q in the crust (see MITCHELL, 1995 for a review) and the upper mantle, is well documented from various regional studies, either using surface waves (e.g., MITCHELL, 1975;MITCHELL, 1978, 1981;NAKANISHI, 1979a;BUSSY et al, 1993) or body waves, such as multiple ScS phases (NAKANISHI, 1979b;SIPKIN and JORDAN, 1980;LAY and WALLACE, 1983;CHAN and DER, 1988;SIPKIN and REVENAUGH, 1994) or multiple S and depth phases (SHEEHAN and SOLOMON, 1992;WIENS, 1990, 1994;DING and GRAND, 1993). These lateral variations can be an order of magnitude larger than observed lateral variations in velocity, commonly exceeding 50-100%.…”