MOMENT TENSOR INVERSION OF SOME AFTERSHOCKS OF THE APRIL 18, 1985, LUQUAN, YUNNAN, CHINA, EARTHQUAKE

Based on the three-component accelerograms, recorded at near-field distance by a temporary seismic network consisting of digital cassette tape reacording accelerographs, the focal mechanisms of three aftershocks of the April 18, 1985, Luquan, Yunnan Province, China, earthquake of magnitude Ms=6.1, are calculated using moment tensor inversion technique. The phases of direct P, S and converted SP waves in the displacement seismograms, produced by twice integration of the observed accelerograms, are identified via forward calculation using Green's functions for homogeneous semi-infinite elastic medium, and used in the inversion. The results of inversion show that a better fit of synthetic to the observed seismograms of direct as well as converted phases can be achieved if appropriate weighting functions are used in solving the over-definite linear equations. While these aftershocks are of different magnitudes (ML=4.8, 3.2 and 3.5, respectively) and hypocentral locations, their focal mechanisms are very similar and consistent with that of the main shock. This feature demonstrates the intrinsic correlation between the occurrence of aftershocks and the seismogeneic fault of main shock. Our experimentations show that using the near-field accelerogram obtained from the digital seismic network with appropriate azimuthal coverage on the focal sphere, with the aid of even simple earth model, not only the shear dislocation source, but also the isotropic part and CLVD (compensated linear vector dipole) can be retrieved by the technique of moment tensor inversion.