Abstract: The observed values of radon concentration in gas from Babaoshan fault zone, western Beijing through May 1992 to December 1996 are given in the paper, and the factors influencing the radon anomalies and the correlation between the anomalies and earthquakes are discussed. The measurements of radon concentration at 2 measurement positions (“a” and “b” ) vary greatly, and the discreteness of radon concentration at “a” point is relatively larger than that at“b” point. The mean value of radon concentration at “a” point is 6. 5 Bq/L with a square root-error of 2. 3, while them eanvalue and the square root-error at “ b” point are 4. 4 and 1. 6 Bq/L, respectively. Both the annual episodic changes of the radon concentration and the annual periodic fluctuations of the atmospheric tempera ture and pressure and earth tem perature at the depth of 3.2 m exist. The temperature and humidity of soil are considered to be the main factor dominating the annual episodic changes of radon concentration. The radon concentration change is relatively small when no earthquake occurs, but it rapid increases before and after earthquakes. Thirteen earthquakes ( M_L≥4.0) occurred in Beijing and its neighbouring area from 1992 to 1996 and 10 of them occurred in the period of November 1992 to December 1996. Radon concentration anomaly occurred before and after 7 earthquakes, but two radon anomalies did not associate with any earthquake, and 3 earthquakes did not show radon anomaly. The distance between the measurement position and the epicenter of earthquake associated with radon concentration anomaly is relatively larger in the NE-SW direction, since them easurement positions are situated in thew esternedge of the continentalrift. Data show that escaped radon from the fault zone plays an important role in earthquake monitoring and prediction.

Key words: Seismicity, Babaoshan fault zone, Geochemistry, Radon, Fault gas