Passive seismic multiscale subsurface imaging and characterization by utilizing natural quakes

Abstract

This thesis investigates the potential of passive seismic methods that make use of body waves, and especially the passive reflection method, as cost-effective applications for multiscale subsurface imaging and characterization. For this purpose, we develop several seismic techniques for different scales: basin, crustal, and lithospheric. For the basin scale, we developed horizontal- and vertical-components spectral ratio of global earthquake phases to estimate the basin depth. We also used the Sp-wave method and analysis of the frequency-dependent quality factor to characterize the basin’s heterogeneities. The results show good agreement with active-seismic profiles. At the crustal scale, we investigated the application of seismic interferometry (SI). Comparison among different SI methodologies suggests that multidimensional deconvolution based on the truncated singular-value decomposition gives better structural imaging than do the conventional crosscorrelation or crosscoherence approaches, but also better than multidimensional deconvolution based on the damped least-squares scheme. This crustal-scale SI could be useful, for example, as a prescreening-exploration tool for deep geothermal reservoirs whose targets can be as deep as 10 km. At the lithospheric scale we studied not only the Earth, but also the Moon. For the Earth, we applied SI with global phases to obtain detailed images of aseismic parts of a subduction slab. Although the interpretation of the imaging results of the aseismic parts is not sufficiently decisive, the results suggest that the applied method is helpful for imaging aseismic parts of slabs. Furthermore, the radiation efficiency of intermediate-depth earthquakes is estimated to understand the source mechanism as a function of focal depth. The results indicate that there is a larger amount of non-radiated energy for intermediate-depth earthquakes. This implies one of the mechanisms for the slabs to be aseismic at certain depths. For the Moon, we applied SI to deep moonquakes to obtain reflection imaging of the lunar subsurface. With this application, the lunar Moho is interpreted to be around 50 km depth, indicating the potential usefulness of SI for other celestial bodies. Following the results obtained in this thesis, we conclude that the passive seismic methods with natural quakes have excellent potential usage in both the resource industry and academia.