The analysis of passive source seismic data recorded by quasi-linear deployments of broadband stations at teleseismic distances has proven to be an effective means of probing the subsurface of the Earth. However, current methodologies are far from being able to exploit all the interpretable signal in these data sets. In this thesis, I describe a 2.5D, frequency domain, visco-elastic waveform tomography algorithm for imaging with this type of data. To compute synthetic seismograms (the forward problem), the general equations of motion are discretized with p-adaptive finite elements. This approach allows for geometric flexibility and accurate solutions as a function of wavelength. Artificial force distributions manifesting Huygen's principle for the teleseismic events are introduced locally through a Bielak layer. Because of the relatively low frequency content of teleseismic data, regional scale tectonic settings can be parameterized with a modest number of variables and perturbations can be determined directly from a regularized Gauss-Newton system of equations. Waveforms generated by the forward problem compare well with analytic solutions for simple 1D media and with those generated in heterogeneous structures by a finite difference technique. It is demonstrated through examples that the regularized approximate Hessian is particularly effective at focusing backpropagated residuals to their true location. It is observed that full waveform inversion can provide significantly better vertical resolution than arrival time tomography and significantly better lateral resolution than standard surface wave tomography. Used in tandem in a multi-scale approach, surface wave tomography followed by joint surface wave/body wave tomography is shown to be an effective strategy for image reconstruction from a simple starting model. This inversion strategy is then applied to body and surface wave teleseismic waves recorded in the Tien Shan. The work of previous investigators is both corroborated and better illuminated by starting from a 1D model and inverting with this joint multi-scale approach. The main results for this application are (1) that the Tarim basin appears to be underthrusting the Tien Shan from the south, (2) a thin crust overlaying a piece of suspected relict lithosphere exists below the Naryn valley, and (3) there is a south dipping high velocity zone on the north side of the Tien Shan suggesting consumption of the Chu depression and the Kazak shield. The north dipping high velocity feature in the south and south dipping high velocity feature in the north are consistent with the hypothesis of dual subduction beneath the Tien Shan.
|Advisor:||Roecker, Steven W.|
|Commitee:||Abers, Geoffrey A., Fox, Peter, McLaughlin, Joyce R., Spear, Frank S.|
|School:||Rensselaer Polytechnic Institute|
|School Location:||United States -- New York|
|Source:||DAI-B 75/06(E), Dissertation Abstracts International|
|Keywords:||Computational seismology, Full waveform inversion, Seismic tomography, Wave scattering|
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