RESEARCH ON EARTHQUAKES INDUCED BY WATER INJECTION IN CHINA

In China, the earthquakes induced by water injection have occurred in four oil fields including the Renqiu oil field, and in two mines. Prod uction of oil from the Renqiu oil field began in 1975 and the injectio n of water into the oil field commenced in July 1976. The induced eart hquakes have been occurring in the area for the past 17 years, since D ecember 1976. The controlled experiments of water injection showed the cause and effect relation between water injection and earthquakes. So urce parameters such as source dimension, seismic moment and stress dr op of a large number of the induced earthquakes, and Q factor for the area have been determined. The results indicate that the stress drop v aries from 0.2 to 3.0 bar and the Q factor has an average value of 75. 0. The low-stress drop and low Q factor values imply that the earthqua kes are caused by the brittle fracture of weak rocks under low ambient stresses, due to a decrease in their strength because of the injectio n of water. The induced earthquakes are unevenly distributed in the oi l field. The northern part of the oil field, where the reservoir rocks are characterized by low porosity and low permeability, exhibits high seismic activity with the largest earthquake registering a magnitude of 4.5 and about 68% of the total number of induced earthquakes in thi s part. Whereas, the southern part of the oil field with higher porosi ty and higher permeability is characterized by low seismic activity wi th the largest earthquake registering a magnitude of 2.5 and only 4% o f the total number of earthquakes which occurred in this part. These f eatures of the focal region suggest that larger earthquakes may not oc cur in the Renqiu oil field area.