摘要
Through the accurate determination of hypocenter location, the measurement of the main focal mechanism solutions and the analyses of time dependent processes for Sichuan Mianzhu earthquake sequence with M S=5 0 occurred along the middle segment of Longmenshan fault zone on November 30, 1999, the distribution of focal depth section, the stress release and seismicity features are given to reveal the earthquake-generating structure of the earthquake sequence. The obtained results show the activity features for this sequence as follows: ① There is no obvious foreshock, the accumulated strain energy releases in nearly pulse way, fluctuation of the seismic activity level is not quite evident during the duration of the sequence, and the statistical relation between the large events and the small events is less compatible with the traditional G-R estimation; ② The epicenters of the earthquake sequence are not distributed on the main Longmenshan piedmont faults emerging out ground, but near the hidden Longmenshan piedmont faults. The direction of epicenter distribution is not very coincident with the fault strikes. The microscopic epicenters are relatively far from the macroscopic epicenters. The focal depths range from 5km to 16km; ③ The fractures of focal fault plane with the NE strike appear out the strike-slip displacement with a few overthrust components under the major principal compressive stress of NNE direction. From these, we consider it should be paid more attentions to the underground hidden faults near the ground fault zone on a large scale for their generating earthquake risk.
Through the accurate determination of hypocenter location, the measurement of the main focal mechanism solutions and the analyses of time dependent processes for Sichuan Mianzhu earthquake sequence with M S=5 0 occurred along the middle segment of Longmenshan fault zone on November 30, 1999, the distribution of focal depth section, the stress release and seismicity features are given to reveal the earthquake-generating structure of the earthquake sequence. The obtained results show the activity features for this sequence as follows: ① There is no obvious foreshock, the accumulated strain energy releases in nearly pulse way, fluctuation of the seismic activity level is not quite evident during the duration of the sequence, and the statistical relation between the large events and the small events is less compatible with the traditional G-R estimation; ② The epicenters of the earthquake sequence are not distributed on the main Longmenshan piedmont faults emerging out ground, but near the hidden Longmenshan piedmont faults. The direction of epicenter distribution is not very coincident with the fault strikes. The microscopic epicenters are relatively far from the macroscopic epicenters. The focal depths range from 5km to 16km; ③ The fractures of focal fault plane with the NE strike appear out the strike-slip displacement with a few overthrust components under the major principal compressive stress of NNE direction. From these, we consider it should be paid more attentions to the underground hidden faults near the ground fault zone on a large scale for their generating earthquake risk.
