Surface wave tomography method based on data from distant earthquakes

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The standard method of surface wave tomography assumes the use of data on surface wave velocities measured on paths wholly located inside the region being investigated and oriented in different directions. If the data on group velocities obtained in the regional network of stations are used for this purpose, then the earthquake sources also should be situated within the limits of this region. If the region is small, then this requirement restricts the range of periods of surface waves and, correspondingly, the depth of research. The use of data from distant earthquakes enlarges the range of periods. However, it does not enable one to use the usual tomography method because of an inappropriate configuration of the system of paths: outside of the region in which the stations are located, the paths do not intersect, but, at the same time, the paths from a particular earthquake source toward the different stations of networks are very close over a large distance, and the mean correction to the velocity in the path sections outside of the network of stations can be accepted as identical. This assumption forms the basis of the proposed modification of the method of surface wave tomography, in which group velocities measured at stations of the local network from distant earthquakes are used. The lateral variations in the velocity are determined within the limits of the network, and the mean corrections to the velocity are determined on the paths from different earthquake sources in the sections outside of the network. As earlier, the condition of smoothness is imposed on the distribution of velocity variations, and the condition of smallness of the sum of their squares is imposed on the values of the corrections to the velocity. Testing of this method based on model examples and on real data obtained in the network of stations in Tibet showed its advantage over the standard tomography method.