Reference point equalization method for determining the source and path effects of surface waves

Abstract

A reference point equalization method is presented for the determination of source and propagation effects of surface waves. The method works on seismic events located in a small source region, allowing the assumption that all events share the same path effects to a given receiver. Two steps in the method are initialization and iteration. Initialization obtains the first reference events in order to compute initial estimates of phase velocity and attenuation coefficient. Iteration simultaneously refines the propagation parameters and determines source parameters of new earthquakes. This method was applied to nine earthquakes in the Pamir Mountains, Central Asia (reference point: 39.58°N, 73.55°E). Source parameters were determined using the linear moment tensor inversion on 26 to 60‐s Rayleigh wave complex spectra. It was necessary to modify the straight least squares inversion method because of its sensitivity to even just a few bad data points in the data set. Residuals obtained from repeated application of the moment tensor inversion over trial focal depths showed two minima: one at depths less than 20 km and the other at depths greater than 70 km, with values of the residuals at these minima close enough to cast doubt on the determination of focal depth. The inversion generally gave three‐couple force systems having significant nonzero intermediate component. These features of source parameter inversions are discussed in light of errors in the data set. Propagation parameters, in particular, the phase velocities, show good qualitative correlation with geologic and geographic features on the Eurasian continent.