以震源精确定位结果分析张北地震序列的破裂特征

RUPTURE CHARACTERISTICS OF THE ZHANGBEI EARTHQUAKE SEQUENCE ANALYZED BY EXACT FOCAL LOCATION

利用远场和近场数字地震观测记录 ,重新测定了张北地震序列ML≥ 3级地震的震源位置 ,结合修订后的震源机制、宏观烈度分布资料 ,给出了张北地震序列的破裂特征。地震序列由走向NWW、倾向NNE、倾角 4 4°、长 11.5km的左旋走向滑动的主破裂面 ,2条NNE走向、高倾角、右旋走滑的次要破裂面组合而成。NWW和NNE走向的破裂面共轭展布 ,2条NNE向的破裂面呈右阶斜列 ,3条破裂面先后依次出现。破裂面埋深 1.4～ 7.6km ,在地壳的浅部。研究表明 ,在没有发现活动断裂的“构造稳定区” ,利用精确可靠的强震序列震源位置、震源机制和宏观烈度分布资料 ,从三维空间分析研究强震序列的震源断层 。

Based upon the far-field and near-field digital seismic records, the hypocenter locations of M L≥3 earthquakes of the Zhangbei earthquake sequence occurred during January, 1998-March, 1999 have been re-determined by using relative locating method. The results show that the process of the earthquake sequence can be divided into three stages. The first stage occurred in January 1988, which is the stage of the occurrence and adjustment of the main shocks. The seismic activity in this stage occurred mostly along NWW-SEE direction. The second stage occurred during February ～August 1998, which is the active period of moderate aftershocks occurred mostly along NNE direction. The third stage was represented by the occurrence of M5.6 earthquake on March 11, 1999, the hypocenter of which was aligned also along NNE direction, but was out of the hypocenter area of the main shock. It seems that the two hypocenters were not connected with each other. The rupture characteristics of the Zhangbei earthquake sequence is given on the basis of the relocation of the hypocenters and the revised focal mechanisms, as well as macroscopic intensity distribution data of this earthquake sequence. It is suggested that the earthquake sequence consists of one main left-lateral strike-slip rupture plane, and 2 right-lateral strike slip secondary rupture planes. The main rupture plane is NWW-trending, NNE-dipping at an angle of 44°, and 11.5km in length. The two secondary rupture planes are NNE-trending with a high dip angle. The main rupture plane is conjugate with the NNE-trending secondary rupture planes while the two NNE-trending rupture planes are aligned in right-step en ecelon. The three rupture planes occurred sequentially, the depth of the which is 1.4～7.6 km at shallow part of the crust. The present study shows that it is a doable approach to research the focal rupture of a strong earthquake sequence from three dimensional space by using exact hypocenter 1ocation, focal mechanism and macro-intensity distribution data in 'tectonically stable area' where no active fault is found.