怀4井地下流体异常与地震的关系

RELATIONSHIP BETWEEN THE ANOMALIES OF UNDERGROUND FLUID IN THE HUAILAI NO.4 WELL AND EARTHQUAKE

怀 4井为国家重点观测井,在多次地震前都有明显的异常出现,对监视北京西北地区的地震起着重要的作用。文中分析了 5次中强地震前怀 4井地下流体测项的异常情况。统计了自 1998年以来周围 300km范围内 4级以上地震前怀 4井流体测项的映震情况。认为怀 4井对其周围 260km范围内≥4 0级的地震反应比较灵敏。并归纳出怀 4井流体测项前兆异常的如下特征: 1)水汞前兆异常形态为突跳型,异常结束后发震; 2)断层气二氧化碳前兆异常形态为多点高值起伏型,异常转折下降中发震; 3)水氡前兆异常形态为阶变型,异常中发震; 4)氢气前兆异常形态为突跳型,异常结束后发震。通过本研究,有助于今后对怀 4井流体异常的识别,根据怀 4井各测项对地震孕育各阶段反映的差异,更容易判定未来地震的发震时刻。

The Huailai No.4 well is one of the national key observation wells. Obvious anomalies have been observed in this well before the occurrence of many earthquakes. Therefore, the well has played an important role in the monitoring of earthquake activity in Northwest Beijing region. This paper describes in detail the anomalies of underground fluid in the well before the occurrences of 5 moderate-strong earthquakes. In addition, we have also collected the earthquake-reflecting anomalies of the underground fluid observed in the Huailai No.4 well before the occurrence of earthquakes of magnitude above 4.0 in Huailai and its neighboring area since 1998. The research shows that the Huailai No.4 well may sensitively reflect M≥4.0 earthquakes that occur within the range of ≤260km around the well. The main features of precursory anomalies of underground fluid observed in the Huailai No.4 well can be summarized as follows: ⑴ The precursory anomalies of water mercury appear as abrupt variations. They occur several days to 2 months before the occurrence of an earthquake. The amplitude of variation might be several to tens times as high as the background value. The higher the amplitude of anomalies the greater the magnitude of earthquake will be. The earthquakes occur after the ending of anomalies; ⑵ The precursory anomalies of carbon dioxide in fault gas appear as sharp variations at many points. They occur 16 days to 47 days before the occurrence of an earthquake. The amplitude of variation might be several times to ten times as high as the background value. The higher the amplitude of anomalies, the greater the magnitude of earthquake will be. The earthquakes occur when the anomalies fall off; ⑶ The precursory anomalies of water radon appear as stepped variation. They occur several days to 2 months before the occurrence of an earthquake. The amplitude of anomalies might be of about 15% of the background value. The earthquakes occur during the growing period of anomalies; ⑷ The precursory anomalies of hydrogen escaped from groundwater appear as abrupt variations. The amplitude of variation of anomalies might be 3 times as high as the background value. The earthquakes occur after the ending of anomalies. All these results may shed light on the earthquake-related behavior of the underground fluids in this well, and may enhance the capability of earthquake prediction in the near future.