Seismic imaging of the laterally varying D″ region beneath the Cocos Plate

Abstract

S U M M A R YWe use an axisymmetric, spherical Earth finite difference algorithm to model SH-wave propagation through cross-sections of laterally varying lower mantle models beneath the Cocos Plate derived from recent data analyses. Synthetic seismograms with dominant periods as short as 4 s are computed for several models: (1) a D reflector 264 km above the core-mantle boundary with laterally varying S-wave velocity increases of 0.9-2.6 per cent, based on localized structures from a 1-D double-array stacking method; (2) an undulating D reflector with large topography and uniform velocity increase obtained using a 3-D migration method and (3) cross-sections through the 3-D mantle S-wave velocity tomography model TXBW. We apply double-array stacking to assess model predictions of data. Of the models explored, the S-wave tomography model TXBW displays the best overall agreement with data. The undulating reflector produces a double Scd arrival that may be useful in future studies for distinguishing between D volumetric heterogeneity and D discontinuity topography. Synthetics for the laterally varying models show waveform variability not observed in 1-D model predictions. It is challenging to predict 3-D structure based on localized 1-D models when lateral structural variations are on the order of a few wavelengths of the energy used, particularly for the grazing geometry of our data. Iterative approaches of computing synthetic seismograms and adjusting model characteristics by considering path integral effects are necessary to accurately model fine-scale D structure.

Seismic Detection of Post-perovskite Inside the Earth

“…However, a few studies in 2.5-D have been or are being performed (e.g. Thorne et al 2007); (Hempel et al, article in preparation). Investigations by Hempel et al (pers.…”

“…These include the following: double-array stacking Kito et al 2007;Thorne et al 2007;Lay 2008); vespagram analysis (slant-stacking) (Thomas and Weber 1997;Thomas et al 2002;Chaloner et al 2009); simplified seismic migration (Thomas et al 1999;2004b;Kito et al 2007;Chaloner et al 2009); Kirchhoff migration (Hutko et al 2006); and the generalised radon transform (GRT) (Chambers and Woodhouse 2006;van der Hilst et al 2007). Array-based methods such as vespagrams (Davies et al 1971;Muirhead and Datt 1976) require visual inspection of individual seismograms which is time intensive, restricting studies to small data sets; on the other hand, methods such as the GRT which allow large data sets to be processed quickly are not subjected to the same level of quality control.…”

Deep Earth Structure: Lower Mantle and D″

“…The velocity increases at the top of D 00 are primarily detected by reflections and triplications, which arrive ahead of the corereflected PcP and ScS phases (e.g., Avants et al, 2006b;Chaloner et al, 2009;Ding and Helmberger, 1997;Gaherty and Lay, 1992;Houard and Nataf, 1993;Hutko et al, 2008Hutko et al, , 2009Kendall and Nangini, 1996;Kendall and Shearer, 1994;Kito and Krü ger, 2001;Kito et al, 2004Kito et al, , 2007aKohler et al, 1997;Lay and Helmberger, 1983a;Lay et al, 2004aLay et al, , 2006Reasoner and Revenaugh, 1999;Sun and Helmberger, 2008;Sun et al, 2006Sun et al, , 2009Takeuchi and Obara, 2010;Thomas and Weber, 1997;Thomas et al, , 2004aThorne et al, 2007;Weber, 1993;Weber and Davis, 1990;Wright and Lyons, 1975;Wright et al, 1985;Lay, 1987a, 1990). The clearest observations of deep mantle triplications are from distances of 65 to 95 , where the critical angle interaction with the velocity increase greatly enhances the amplitudes of the reflected signals relative to precritical distance ranges.…”

Global SH-wave propagation in a 2D whole Moon model using the parallel hybrid PSM/FDM method

“…Igel and Gudmundsson (1997) investigated the frequency-dependent effects of S and SS waves with the similar FDM method. Thorne et al (2007) studied lateral heterogeneity in the lower mantle beneath the Cocos Plate with a 2D global SH-wavefield simulation based on FDM. Jahnke et al (2008) performed 2D global SH-wave modeling on a parallel computer.…”