Study Of The Application Of The Multi-level Double Difference Tomography

This study uses the wavelet-based double-difference tomography method with sparsity regularization to explore its application effect in real data.For this purpose,we selected two regions which represent different scales,one is the Lushan aftershock fault zone which is at the local scale,and the other is the subduction zone area of northern Chile at the regional scale.Both the spatial domain and the wavelet domain double-difference method are carried out to perform seismic tomography inversion for comparison and analysis.In Lushan region,the wavelet domain method has obtained better inversion results.Compared with the spatial domain imaging,it can reveal the velocity contrast between the two sides of the fault more sharply.The wavelet domain result also reveals the similar feature where different characteristics of seismic distribution and velocity structure above and below 8km at depth.Simultaneously,the wavelet domain imaging method would become more stable with the increase in the number of decompositions and grid density.It can be inferred from the velocity result that the shallow faults are more developed,in concert with previous research,preventing the rupture from spreading to the surface.In the northern region of Chile,we first extract the cross-correlation to time difference data with the BCSEIS program to improve the data quality and then perform seismic imaging in the space domain and wavelet domain.The inversion results show that the wavelet-based double-difference tomography still has specific issues unresolved in large-scale areas.The complex velocity model of the subduction zone may not be the most applicable area of this method.The amount of wavelet transform calculations brought by performing wavelet transformation with a large dataset also poses a great challenge to applying this method at the regional scale.Still,the velocity results obtained by the two methods both show clear double seismic zone and corresponding characteristics of different velocity and Vp/Vs ratio,which might be related to the dehydration of different water-bearing minerals under their corresponding temperature and pressure conditions.