Mitigating the effect of errors in source parameters on seismic (waveform) tomography

Abstract

SUMMARYWe investigate the effect of errors in earthquake source parameters on the tomographic inverse problem and propose mitigation strategies for avoiding artefacts caused by such errors. In global catalogues, mislocation errors can amount to tens of kilometres both horizontally and vertically, while fault plane uncertainties can be of the order of tens of degrees. We conduct a perturbation study investigating both errors in the source location and in the moment tensor. Spatial perturbations of 5 km and fault plane perturbations of 5° result in measured time-shifts of the order of 0.5 to several seconds, which in five iterations lead to artefacts with amplitudes of the order of 0.5–1 per cent spanning up to several hundreds of kilometres. Larger perturbations (e.g. 20 km) lead to artefacts similar in amplitude (∼5 per cent) to the features judged to be interpretable in tomographic models. This can be further exacerbated by the cumulative effect of systematic errors. Mitigation strategies consist of removing a region around the source from the gradient and discarding traces where amplitudes are comparatively small. The most problematic type of error is a horizontal mislocation, because its imprint is not removed by such measures—discarding a ‘suspicious’ event may be the only option if no trustworthy ground truth is available. Although this study focuses on (adjoint) waveform tomography, a large part of the results are equally valid for any other type of imaging method that is based on time- and/or phase-shift measurements. The concerns and mitigation strategies presented here therefore have broader applicability.