2008年汶川8.0级地震前地下流体异常回顾与统计特征分析

Review and statistically characteristic analysis of underground fluid anomalies prior to the 2008 Wenchuan M_S8.0 earthquake

地震地下流体在地震预测研究与震情跟踪中发挥着重要作用.本文基于中国地震地下流体前兆观测台网所涉及的资料范围,系统收集了2008年四川汶川8.0级地震的可能地下流体前兆异常,分析了这些异常的空间展布、时空演化以及形态等总体变化特征.结果表明,收集到的68项异常均位于10-8应变量范围内,59项异常位于汶川8.0级地震3倍破裂区(约900km)范围内,占异常总数的87%,这与国内外已发表的地下流体前兆异常空间分布范围和震级之间的关系相符,说明汶川8.0级巨大地震的前兆观测范围至少包含该地震3倍破裂尺度甚至更大;异常出现时间总体呈现出临近地震异常数量增多的特点,但异常数量并非逐渐增多,而是在震前5个月和1个月突然增多;异常形态特征复杂,水氡和水位总体呈现出趋势性异常特征,水温总体表现出短临变化特征,主要表现为震前1—3个月突升、突降或波动异常变化.此外,本文还结合国内外已发表的地震地下流体前兆异常以及地震孕育理论,讨论了异常的空间分布、时间尺度与未来震中的关系,这对深入认识地下流体前兆异常及产生机理都具有重要的现实意义和科学价值.

Seismic underground fluid plays an important role in earthquake prediction research and seismic situation tracing.Based on the monitoring scope of the China earthquake underground fluid precursor observation network,underground fluid anomalies prior to the 2008 Wenchuan M_S8.0 earthquake were collected and the characteristics of spatial distribution,temporal evolution and pattern of these anomalies were analyzed.The results show that the 68 anomalies collected were all within the strain range of 10-8,and 59 anomalies were within three times of the rupture length of the Wenchuan MS8.0 earthquake（~900 km）,accounting for 87% of the total number of anomalies.This illustrates that precursor observation range of the Wenchuan M_S8.0 earthquake was at least three times larger than that of the earthquake rupture length.Number of the anomalies increases approaching to the occurrence of earthquake.However,the number of anomalies did not increase gradually,instead increased suddenly five months and one month prior to the earthquake,respectively.The anomalies show a complex pattern.Generally,water radon and water levels are characterized by long-term variations,and the water temperature show short-term changes,including a sudden rise,sudden drop or abnormal fluctuations 1—3 months before the earthquake.In addition,we discuss the relationship among spatial and temporal scale of the anomalies and earthquake magnitude combined with the published earthquake precursor anomalies and earthquake preparation theory.It is of great significance to identifying the precursory anomalies of underground fluid and understanding the mechanism of these anomalies.