Seismic Source Inversion Of 3D Elastic Finite-frequency Tomography And Its Application To LMSF Zone

Due to its break through high frequency assumption of ray theory, three-dimensional (3D) elastic finite-frequency tomography can provide 3D Earth model with higher resolution, which is now widely used in both seismic exploration and seismology structure tomography. In this study, we developed an accurate and efficient method to determine the small to moderate seismic source parameters of 3D elastic finite-frequency tomography. Topographic effects on seismic source inversion, sensitivity kernels and cross-correlation traveltime of surface wave in 3D elastic finite-frequency tomography are fully considered. Furthermore, we applied the novel framework of 3D elastic finite-frequency tomography by combining this new seismic source inversion method to Longmen Shan fault zone and obtained an initial imaging result.With dense seismic arrays and advanced imaging methods,3D regional earth models have become more accurate. It is now increasingly feasible and advantageous to use a 3D earth model to better locate earthquakes and invert their source mechanisms by fitting synthetics to observed waveforms. In this study, we develop an approach to determine both the earthquake location and source mechanism from waveform information. The observed waveforms are filtered in different frequency bands and separated into windows for the individual phases. Instead of picking the arrival times, the traveltime differences are measured by cross-correlation between synthetic waveforms based on the 3D earth model and observed waveforms. The earthquake location is determined by minimizing the cross-correlation traveltime differences. We then fix the horizontal location of the earthquake and perform a grid-search in depth to determine the source mechanism at each point by fitting the synthetic and observed waveforms. This new method is verified by a synthetic test with noise added to the synthetic waveforms and a realistic station distribution. We apply this method to a series of Mw3.4-5.6 earthquakes in the Longmen Shan fault (LMSF) zone, a region with rugged topography between the eastern margin of the Tibetan plateau and the western part of the Sichuan Basin. The results show that our solutions result in improved waveform fits compared to the source parameters from the catalogs we used and the location can be better constrained than the amplitude-only approach. Furthermore the source solutions with realistic topography provide a better fit to the observed waveforms than those without the topography, indicating the need to take the topography into account in regions with rugged topography.3D elastic finite-frequency tomography attempts to take into account a finite-frequency band effects of seismic waves. The distribution and magnitude of 3D sensitivity kernels reflect how much of the scattering effects which caused by the velocity perturbation of background velocity filed and lead to waveform variation. In addition to velocity perturbation can cause scattering, an event in the proximity of steep topography can generate significant scattering for incident seismic waves especially to surface wave which are propagated in directions parallel to the surface of the Earth. In this study, we investigate the topographic effects on Rayleigh waves by means of synthetic data using a non-uniform-grid in a non-staggered finite-difference code and we calculate the 3D finite-frequency sensitivity kernels with topography.Finally, we applied the novel framework of 3D elastic finite-frequency tomography by using part of our location and moment tensor inversion results of small earthquakes to Longmen Shan fault zone with rugged topography and obtained an initial result.