STUDY ON THE TYPES OF EARTHQUAKE FAULTING OF THE HAIFENG EARTHQUAKE SWARM

The major Haifeng-Meilong fault trending NE and a series of small faults parallel to it are distributed in the Haifeng earthquake region. Besides, there also exist disconnected conjugate faults trending NW. The nodal planes of the focal mechanisms of the largest shocks of each of the three sub-swarms are quite similar and one of the nodal plane is trending NE and that of the other NW. According to the space distribution of the aftershock sequences and the direction of the longer axis of the maximum intensity region, the faults causing the earthquake ML 3.3 on February 26 and ML 4.2 on April 9 should strike NE while that associated with the event ML 3.4 on March 14 strikes NW. In addition, by examining the data of signs of first motion of 137 small earthquakes recorded by four seismic stations, we found that there are eight types of combinations from which composite focal mechanisms can b eobtained. Therefore the fracturing process associated with the main-shock as well as theⅠ, Ⅵ type small earthquakes are stick-slip of rock masses along conjugate faults trending NE and NW. The small earthquakes of the Ⅱ, Ⅲ, Ⅳ, Ⅴ, Ⅶ, Ⅷ types are concerned with the. shear fractures caused by a transformed stress field resulted from compression on adjacent rockmasses at the front and tension on the rear and parallel to the direction of slip during the stick-slip of rockmasses.