Research progress on off-great-circle propagation of surface waves
The traditional surface wave tomography is usually based on ray theory and assumes that surface waves propagate along great-circle paths.Due to the lateral heterogeneity of the Earth's medium,surface waves may deviate from the great-circle path,causing arrival angle anomalies of surface waves and multi-path propagation effects.This paper summarizes the research results on the off-great-circle propagation of surface waves,analyzes the main characteristics and causes of off-great-circle propagation of surface waves:the off-great-circle propagation effect of surface waves increases with decreasing period,being particularly pronounced in short periods,and the lateral heterogeneity of the crust-mantle structure is the main factor leading to the deviation of surface waves from great-circle propagation.Then,two types of methods for measuring the arrival angle anomaly of surface waves are introduced:single-station three-component polarization analysis method and array analysis method,with the array analysis method being more reliable and stable compared to single-station three-component polarization analysis.Due to the off-great-circle propagation of surface waves,the surface wave dispersion measured by traditional two-station method will be higher than the true value,while the two station ambient noise dispersion will be lower than the true value.Therefore,the surface wave tomography results based on the great-circle assumption will deviate from the real values.In particular,the anisotropic fast direction of surface waves may be more affected.The paper discusses the impact of off-great-circle propagation of surface waves on surface wave dispersion measurements and surface wave tomography,and emphasizes the necessity of considering the off-great-circle propagation effect of surface waves in high-precision surface wave tomography studies.Finally,the paper looks ahead to future research directions in off-great-circle propagation of surface waves:in event-based surface wave tomography study,the arrival angle anomaly of surface waves can be used to correct the measurement of phase velocity for seismic station pairs,followed by surface wave tomography;using the spectral element method to simulate seismic waves is an effective means to study the off-great-circle propagation of surface waves,which requires considering influence of both the global-scale crust-mantle structure and the fine three-dimensional structures within the study area on the non-great-circle propagation of surface waves.
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