THE IMPLICATION OF CONJUGATE FAULTING IN THE EARTHQUAKE BREWING AND ORIGINATING PROCESS
L. M. Jones(MIT, U.S.A) Deng Qidong and Jiangpy(Institute of Geology, State Seismological Bureau)
The earthquake sequence, precursory and geologo-structural background of Haicheng, Tangshan, Songpan-Pingwu earthquakes are discussed in this, article. As we know that all these earthquakes occurred in a seismic zone controlled by the main boundary faults of a intraplate fault block. But the fault plane of a main earthquake is not the same faults rather a related secondary fault. They formed altogether a conjugate shearing rupture zone under the action of regional tectonic stress field. As to the earthquake sequence the foreshocks and aftershocks might be occurred on the conjugate fault planes within an epicentral region rather than limited in the fault plane of a main earthquake, such as the distribution of foreshocks and aftershocks of Haicheng earthquake. The characteristics of the long-, medium- and imminent term earthquake precursory anomalies of the three mentioned earthquakes, especially the character of well-studies anomaly phenomena in the electrical resistivity, radon emission, groundwater and animal behavior, have been investigated. The studies of these earthquake precursors show that they were distributed in an area rather extensive than the epicentral region. Some fault zones in the conjugate fault network appeared usually as distributed belts or concentrated zones of earthquake precursory anomalies, they can be traced in the medium-long term precursory field, but seem more distinct in the short-imminent term precursory anomalous field. These characteristics can be explained by the rupture and sliding originated along the conjugate shear network and the concentration of stress in regional stress field. Moreover, it is worthy to notice the migration of these precursory anomalies along the conjugate rupture zone in the earthquake-brewing process. The macroscopic precursory anomalies of the Songpan-Pingwu and Haicheng earthquakes did not appear first in the epicentral region, but emerged in a set of fault zone conjugated with the fault plane of main earthquake, or in a conjugate fault zone and their intersection region. These phenomena may be originated from the migration of stress-concentrated region and continuous concentration of stress in the earthquake source area. It is also possible that they were progressively initiated at a definite direction along the rupture zone and sliding caused by conjugate faulting.It can be supposed that the effect of conjugate fault on the earthquake brewing and originating process is significant in theoretical studies of seismology and in predicting practice, it should be drawn public attention.