Observation and explanation of spurious seismic signals emerging in teleseismic noise correlations

Pierre Boué,Lei Li,Michel Campillo

doi:10.5194/se-11-173-2020

Abstract

Abstract. Deep body waves have been reconstructed from seismic noise correlations in recent studies. The authors note their great potential for deep-Earth imaging. In addition to the expected physical seismic phases, some spurious arrivals having no correspondence in earthquake seismograms are observed from the noise correlations. The origins of the noise-derived body waves have not been well understood. Traditionally, the reconstruction of seismic phases from inter-receiver noise correlations is attributed to the interference between waves from noise sources in the stationary-phase regions. The interfering waves emanating from a stationary-phase location have a common ray path from the source to the first receiver. The correlation operator cancels the common path and extracts a signal corresponding to the inter-receiver ray path. In this study, with seismic noise records from two networks at teleseismic distance, we show that noise-derived spurious seismic signals without correspondence in real seismograms can arise from the interference between waves without a common ray path or common slowness. These noise-derived signals cannot be explained by traditional stationary-phase arguments. Numerical experiments reproduce the observed spurious signals. These signals still emerge for uniformly distributed noise sources, and thus are not caused by localized sources. We interpret the presence of the spurious signals with a less restrictive condition of quasi-stationary phase: providing the time delays between interfering waves from spatially distributed noise sources are close enough, the stack of correlation functions over the distributed sources can still be constructive as an effect of finite frequencies, and thereby noise-derived signals emerge from the source averaging.

Highlights

The technique of noise correlation is implemented via computation of correlation functions between ambient noise records at receivers
As for the spurious phase observed between FNET and LAPNET, the correlated P and PKPab waves have no slowness or ray path in common, and there is no stationary point on the curve of the P–PKPab time delay (Fig. 9b)
In contrast to previous studies that observed a strong correlation between signal amplitudes and large earthquakes, the spurious signals in this study have no connections to seismicity

Summary

Introduction

The technique of noise correlation is implemented via computation of correlation functions between ambient noise records at receivers. We refer to Campillo and Roux (2015) for a systematic review of recent progress on its theoretical and methodological aspects, and various noise-based applications Both the surface-wave and body-wave parts of Green’s function can be reconstructed from noise correlations. Some authors have demonstrated that deep body-wave signals that propagate through the mantle and core can be extracted from the correlations of continuous seismograms recorded by seismic networks at regional to global scales (Boué et al, 2013b, 2014; Lin et al, 2013; Nishida, 2013). L. Li et al.: Noise-derived spurious seismic signals higher-order correlations to retrieve the ScS arrivals and imaged the lateral variations of the CMB beneath Mexico.

Noise data processing

Observation of the spurious phase

Origin of signals from P–PKPab correlations

Explanation of the quasi-stationary phase

Effect of source distribution

Conclusions