Seismic Tomography Methods And Their Applications

Seismic tomography is one of the most powerful tools for imaging the structuralheterogeneities in Earth and can provide important information to improve ourunderstanding of the dynamic evolution of Earth. In this thesis I have discussed themethods and applications of the ray-based traveltime tomography, the finite-frequencytraveltime tomography and the waveform tomography. The ray-based tomography issimple and fast, but it ignores the finite-frequency effects of seismic waves. To take intoaccount the finite-frequency effects, a new finite-frequency traveltime sensitivity kernelwas derived. This finite-frequency kernel can naturally ‘capture’ the influence of theheterogeneous structure off the geometric ray path on the seismic waves propagating incomplex media. Both ray-based and finite-frequency tomography methods rely on theaccuracy of the geometric ray path. But ray tracing techniques sometime get the localminimum traveltime paths in complex media. Solving the full acoustic/elastic waveequation can simulate exactly the propagation of seismic waves, and can provide a largenumber of synthetic data sets. To take these advantages in tomographic research, I haveattempted to develop waveform tomography methods based on solving full acoustic orelastic wave equation. Three different finite-difference methods were proposed toimplement waveform tomography, which all have the properties of high accuracy, highefficiency, low numerical dispersion, low requirement of storage space and producingmore wavefields than the common finite-difference schemes. With respect to theapplication aspects, finite-frequency traveltime and ray-based traveltime tomographymethods were adopted to image the crustal structures of the1995Kobe earthquake (M7.2) source area. Our tomographic results show that the Kobe mainshock and itsaftershocks occurred mainly in low velocity (Vp, Vs), high Poisson’s ratio and low Vp×Vs zones, suggesting the existence of crustal fluids in the Kobe source area. It isthought that the crustal fluids originated from the dehydration of the subductedPhilippine Sea Plate and the penetration of the surface water. In addition, the crustal andupper-mantle structure under the2011Iwaki earthquake (M7.0) and the Fukushimanuclear power plant area was investigated with the same tomography methods. Thetomographic results suggest that the Iwaki earthquake was triggered by the combinedeffects of the ascending fluids from the Pacific slab dehydration and the stress variation caused by the2011Tohoku-oki mainshock (Mw9.0). The similar structures under theFukushima nuclear power plant and the Iwaki source area indicate a possibility of theoccurrence of future large crustal earthquakes under the nuclear power plant site. Thecomparisons of the finite-frequency tomography with the ray tomography show that thetomographic images resulted from the two methods have a high level of similarity. Thewaveform traveltime tomography and the ray-based traveltime tomography methods arealso used to image the2008Iwate-Miyagi (M7.2) earthquake area. The results showthat the Iwate-Miyagi mainshock and its major aftershocks occurred mainly in aboundary zone with strong variations in seismic velocity, which may be associated withthe arc magma and fluids ascending from the upper-mantle wedge.