Development of a Teleseismic Viscoelastic Waveform Tomography Algorithm with Application to Broadband Seismograms from the Tien Shan

Rensselaer Polytechnic Institute, Troy NY

Investigators

Abstract

This project has two parts: (1) to adapt a suite of waveform tomography algorithms based on a 2D acoustic framework developed over the past 18 years by R. G. Pratt and co-workers and thus far used exclusively in an active source environment to the analysis of recordings of earthquakes at teleseismic distances (what are commonly known as ?receiver functions?) and (2) to apply this algorithm to a collection of broad band seismograms collected in the Tien Shan as part of the recently completed MANAS project. The motivation behind the first part is the demonstrated efficacy of this approach; reasonable models can be constructed without resorting to massively parallel computing. The tasks involved include incorporating a visco-elastic version of this code, which we have already developed, to an adjoint inversion scheme similar to that used in the acoustic case. The researchers will also investigate the feasibility of incorporating anisotropic effects and extending the algorithm to three-dimensions. The second part is motivated partly by our familiarity with the MANAS dataset, but more by results from the application of ?traditional? receiver function migration and arrival time tomography techniques. Specifically, arrival time tomography images indicate the presence of two high wavespeed slabs beneath the Tien Shan that suggest a long history of plate-like lithospheric subduction in this region. The receiver functions show an unusual disruption in the Moho in the areas where these plates appear to be consumed. Both these images have first order implications for the evolution of the Tien Shan, but interpretations suffer from insufficient resolution. Based on active source analogues, the waveform tomography approach will significantly improve the quality of the images of these unusual features in the upper mantle. A better understanding of these features will have a significant impact on our estimation of both the evolution of this region and the root cause of earthquake activity in this part of the world. Arguably the best sources of information we have about the interior of the Earth are derived from seismograph records of earthquakes recorded at long distances (>1000 km) from the source. A particularly fruitful area of research over the past decade has been in modeling those parts of these seismograms that are modified by those parts of the Earth within a few 100 km of the surface beneath a seismic station. In this project the researchers will develop a new technique to extract more information from these types of waves by adapting methodology that has proved to be very useful in subsurface imaging on a much smaller scale using ?active? sources like explosions and vibrators. Despite the difference in scale, the basic physics involved in modeling wave propagation is the samel. The algorithm will be general enough for scientists and engineers to use in any analogous environment to produce subsurface images.