ACCURATE HYPOCENTER DETERMINATION OF AFTERSHOCKS OF THE 1985 LUQUAN EARTHQUAKE

Based on the cassette tape readings acquired from a temporary seismic network of 4 digiatal DCS-302 seismometers,the detected aftershocks are determined of the M 6.1 Luquan earthquake which occurred on 18 April 1985 in Luquan County of Yunnan Province. An alternative algorithm of least squares solution to the hypocenter parameters without normalizing the observation equations is used in locating the events. The advantage of the proposed algorithm over the damped least squares method and the singular value decomposition method is assessed in terms of processing simplicity and computation precision. The 28 aftershocks determined are located in an area roughly surrounded by the Xi Ma River,the Jiu Long River,and a number of NW and NE trending faults. The space distribution of these events suggests that they are closely related to the faulting activities of the NE direction.