Advanced Seismic Velocity And Attenuation Tomography And Its Applications

Double-difference (DD) seismic tomography method has been widely used to study the structures of fault zones, volcanoes and subduction zones because of its high precision and resolution. Thus, we used the DD tomography to study the detailed Vp, Vs and Vp/Vs ratio structure of the aftershock zone of the Mw6.6 Lushan earthquake. Many studies have been done about the geometry of the fault, the velocity structure and rupture history of the Lushan earthquake. However, due to the relatively low resolution of these studies, the detailed structure of the fault still is not clear. In our study, the relocations of the aftershocks of Lushan earthquake clearly show a "y" like conjugate fault system with the Lushan earthquake occurring on the main thrust fault. From the 3-D velocity model, it shows a sharp contrast at the depth of-8km, with low Vp, Vs and high Vp/Vs ratio at the shallow part and high Vp. Vs and low Vp/Vs ratio at the deeper part associated with the aftershocks. Comparing the velocity model with the slip model of the Lushan earthquake, it clearly shows a high Vp and Vs anomaly associated with high slips, which may be caused by the earthquake asperity. From the rupture history, slips are very small above the depth of-8km. Our Vp/Vs ratio model clearly shows a high anomaly layer at the depth of-8km, which may act as a barrier of the earthquake rupture.Seismic attenuation tomography can complement the images of velocity structure and give us more information about the subsurface. In general, the attenuation structure is inverted using t* values calculated from waveform spectra. However, due to the limitation of signal to noise ratio, we usually do not have enough data to guarantee the stability of tomographic system. Besides, we also have no idea about the quality of t* values, as well as the probability distribution of its errors. Thus, we will get a lot of artifacts in the inverted model. To solve these problems, we suggested a cross-gradient constrained seismic attenuation tomography method to enforce the attenuation model to be structurally similar to the velocity model. We tested this new method on the attenuation tomography of the aftershock zone of the Mw6.6 Lushan earthquake. Compared to the traditional attenuation tomography, the new method gets a more smoothing model without lots of high frequency artifacts. And the new model is more structurally similar to the velocity model compared to the model determined by the traditional attenuation tomography as expected.The standard off-line tomography does not have necessary time resolution, thus it has been a challenge to image velocity changes in real time by seismic travel time tomography. If more seismic events are included in the tomographic system, the inverted velocity models do not have necessary time resolution to resolve velocity changes. But if fewer events are used for real time tomography, the system is less stable and the inverted model may contain some artifacts and thus resolved velocity changes may not be real. To solve these issues, we propose a wavelet-based real time double-difference (DD) tomography method. The new method combines the multiscale property of wavelet representation and the fast converging property of the simultaneous algebraic reconstruction technique (SART) to solve the velocity model at multiple scales and in near real time. We first test the new method using synthetic data constructed using real event and station distribution for Mount Etna volcano in Italy. Then we show its effectiveness to determine velocity changes for the 2001 and 2002 eruptions of Mount Etna volcano. Compared to off-line standard DD tomography that uses seismic events from a longer time period, real time tomography better resolves velocity changes that may be caused by fracture closure and opening before and after volcano eruptions.