New insights into theP- andS-wave velocity structure of the D″ discontinuity beneath the Cocos plate

Abstract

S U M M A R YBroad-band P-and S-waves from earthquakes in South America recorded at Californian network stations are analysed to image lateral variations of the D -discontinuity beneath the Cocos plate. We apply two array processing methods to the data set: a simplified migration method to the P-wave data set and a double-array method to both the P-and S-wave data sets, allowing us to compare results from the two methods. The double-array method images a dipping reflector at a depth range from 2650 to 2700 km in the southern part of the study area. We observe a step-like topography of 100 km to a shallower reflector at about 2600 km depth to the north, as well as evidence for a second (deeper) reflector at a depth range from 2700 to 2750 km in the north. Results from the simplified migration agree well with those from the double-array method, similarly locating a large step in reflector depth in a similar location (about 2650 km depth in the south and about 2550 km in the north) as well as the additional deeper reflector at the depth of about 2750 km in the north. Waveform modelling of the reflected waves from both methods suggests a positive velocity contrast for S waves, but a negative velocity contrast for P waves for the upper reflector in agreement with predictions from mineral physical calculations for a post-perovskite phase transition. The data also show some evidence for the existence of another deeper reflector that could indicate a double intersection of the geotherm with the post-perovskite stability field, that is, the back-transformation of post-perovskite to perovskite close to the core-mantle boundary.

Seismic Detection of Post-perovskite Inside the Earth

“…Note the enhanced visibility of the PdP phase when the change in Vp is the opposite sign to the change in Vs, and the reversal of the PdP polarity for P wave speed decreases compared to P wave speed increases seen in conjunction with negative (up to −3 %) or small positive (<1 %) changes in P wave speed (e.g. Lay and Helmberger 1983;Kendall and Nangini 1996;Ding and Helmberger 1997;Thomas et al 2004a;Kito et al 2007;Hutko et al 2008). The example given in Fig.…”

“…Lay and Helmberger 1983;Weber and Kornig 1990;Weber and Davis 1990;Davis and Weber 1990;Young and Lay 1990;Nataf 1992, 1993;Weber 1993;Weber and Wicks 1996;Kendall and Nangini 1996;Thomas and Weber 1997;Reasoner and Revenaugh 1999;Russell et al 2001;Thomas et al 2002Thomas et al , 2004aWallace and Thomas 2005;Lay et al 2006;Avants et al 2006;van der Hilst et al 2007;Kito et al 2007;Hutko et al 2008Hutko et al , 2009Chaloner et al 2009). This discontinuity, usually referred to as "the D″ discontinuity", appears to have a complex structure, with both small and long wavelength lateral variations in its strength and depth.…”

“…In such cases, reflectivity modelling (e.g. Weber 1993; Kito et al 2007;Chaloner et al 2009) demonstrates that the polarity of the PdP waveform will be the same as that of the P and PcP ("positive polarity") for a velocity increase at the D″ discontinuity, whereas the polarity of the PdP will be reversed relative to P and PcP for a velocity increase ("reverse polarity"; likewise for S wave phases)- Fig. 13.8.…”

“…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.…”

“…To date, most estimates of velocity jumps at D″ have been based upon full-waveform reflectivity modelling in 1-D (e.g. Weber 1993; Kito et al 2007;Chaloner et al 2009) or ray tracing in 2-D (Thomas and Weber 1997), which cannot accurately incorporate the effect of 3-D topography. Full-waveform modelling of the D″ discontinuity in 3-D at the high frequencies at which it is observed is currently beyond available computational capacity.…”