Amplitude of the core–mantle boundary topography estimated by stochastic analysis of core phases

Abstract

The core–mantle boundary (CMB) topography is an important parameter for constraining the models of mantle dynamics and core–mantle coupling. However, the various large wavelength seismological models of the CMB topography, which have been obtained up to now, are poorly correlated. Moreover, their maximum amplitudes vary considerably from one model to another, with values ranging from ±4 to ±12 km. These large discrepancies may be due to the difficulty to separate, in the travel time anomalies, the contribution of the CMB topography from that of the highly heterogeneous D″ region at the base of the mantle. In order to better constrain the amplitude of the CMB topography, we perform a stochastic analysis of the core phases which sample the CMB as transmitted waves and/or as reflected waves. In particular, we analyse underside reflected PKKP phases, which help to discriminate between CMB topography and D″ structure, and have in addition a great sensitivity to CMB topography. The other phases used are the upperside reflected waves PcP, and the transmitted waves PKP. The analysis is performed on the travel time residual file obtained by Engdahl et al. [Engdahl, E.R., van der Hilst, R., Buland, R.P., 1998. Global teleseismic earthquake relocation with improved travel times and procedures for depth determination. Bull. Seismol. Soc. Am. 88, 722–743.] after earthquake relocation and phase re-identification. After careful travel time data selection, the stochastic analysis allows us to separate coherent signal from random signal in the data at different length scales ranging from 300 to 1500 km. Then the CMB topography variance is obtained at the different length scales from a joint analysis of the coherent signal in the different core phases, in taking into account their different sensitivities to CMB topography. The estimated CMB topography variance has a significant signal in the wavelength range that we have investigated, showing that 95% of CMB topography amplitude is in the range ±∼4.0 km for wavelengths larger than 300 km. This value decreases to ±∼1.5 km for wavelengths larger than 1200 km, indicating that the long wavelength CMB topography is significantly lower than previously proposed. However, it has not been possible to invert the data for deriving a map of the CMB topography. A checkerboard pattern analysis reveals that, despite the introduction of the PKKP phase, the relative contributions of D″ heterogenieties and CMB topography cannot be separated with the presently available data.