Topography effect on seismic waveform tomography: a quantitative study

Abstract

SUMMARY In seismic tomography practices the Earth's surface is sometimes assumed to be either spherical or flat for convenience in forward modelling calculations. The effect of irregular surface topography on seismic wave propagation is thus ignored, resulting in biases in the phases and amplitudes of synthetic seismograms, which contribute to the residuals that are mapped into velocity structures in tomography inversions. In this study, we conduct a series of inversion experiments based on the adjoint waveform tomography method to quantitatively assess the topography effect on waveform-based inversion results. We first employ models with simplified topography to better highlight and quantify the topography effect. Results show that when topography effect is ignored in the forward modelling, it is mapped into velocity perturbations, leading to spurious velocity anomalies in tomography models. The strength of the spurious velocity anomaly is quasi-linearly related to locally averaged topography gradient. Our inversion experiments demonstrate that in places of strong topography variation, such as the Longmenshan Fault Zone region where the 2008 Mw 7.9 Wenchuan earthquake occurred, topography effect can lead to spurious relative velocity anomalies of up to 10 per cent, which cannot be ignored in waveform-based tomography inversions.