Application of very fast simulated annealing to the determination of the crustal structure beneath Tibet

The simulated annealing method has recently been applied to several multiparameter optimization problems, including those of geophysical inversion. A new variant of simulated annealing, called very fast simulated annealing (VFSA), overcomes some of the drawbacks of a conventional simulated annealing; it has been found to be a practical tool for geophysical inversion (Sen & Stoffa 1995). The method is particularly useful for non-linear problems with multiple parameters where a grid-search method is prohibitively expensive. Here, we have applied VFSA to retrieve the crustal structure beneath seismic stations in Tibet using teleseismic body-waveform data. Our approach is to compare the radial-component records with generalized ray synthetics directly in the time domain. For any given crustal structure, we have formulated a direct relationship between the radialand the vertical-component signal, so that we can generate synthetic radial-component data from recorded vertical-component seismograms (Zhao & Frohlich 1996). We have tested the VFSA inversion algorithm using synthetics and confirmed that it works very well, i.e. it finds solutions very close to the true solution. Using the algorithm, we have determined the crustal structures beneath 11 stations in Tibet. From south to north, the total crustal thickness is quite uniform. Our tectonic interpretation of these crustal models is that they may represent crust from the Eurasian plate injected beneath the crust of the converging Indian plate. Certain features of the models are consistent with the presence of a small convection cell or plume beneath north-central Tibet, as suggested by Molnar (1990).