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Seismic tomography constrained by bouguer gravity anomalies: Applications in western Washington

Research paper by J. M. Lees, J. C. VanDecar

Indexed on: 01 Jan '91Published on: 01 Jan '91Published in: Pure and Applied Geophysics



Abstract

Tomographic inversions for velocity variations in western Washington indicate a high correlation with surface geology and geophysical measurements, including gravity observations. By assuming a simple linear relationship between density and velocity (Birch's law) it is possible to calculate the gravity field predicted from the velocity perturbations obtained by local tomographic inversion. While the predicted gravity matches observations in parts of the model, the overall correlation is not satisfactory. In this paper we suggest a method of constraining the tomographic inversion to fit the gravity observations simultaneously with the seismic travel time data. The method is shown to work well with synthetic data in 3 dimensions where the assumption of Birch's law holds strictly. If the sources of the gravity anomalies are assumed to be spatially localized, integration can be carried out over a relatively small volume below the observation points and sparse matrix techniques can be applied. We have applied the constrained inversion method to western Washington using 4,387 shallow earthquakes, to depths of 40.0 km, (36,865 raypaths) convering a 150×250 km region and found that the gravitational constraints may be satisfied with minor effect on the degree of misfit to the seismic data.