云南地区中短期前兆场异常与强震关系研究

The Relation between Medium and Short-term Anomaly of Precurso r Field and Strong Earthquakes in Yunnan

通过对云南地区观测连续、稳定的近 2 0 0个台项前兆资料全时空的中短期异常信息分析研究 ,发现有规律的前兆变化只是一种理想的模式。在强区域应力场的作用下 ,会导致观测环境发生变化 ,使观测值的规律性发生变化 (这类变化通常发生在强震前后 ) ,从而异常的判别标准需有变化 ,才能提高预报效能。由于常常存在多个应力集中点 ,用前兆预测地点很难达到现行预报要求的精度 ,但用场上的群体特征判别云南地区的活动水平 ,有较好的预测预报效能。云南强震前前兆的群体异常特征很显著 ,当有 4 0 %以上的前兆观测项目出现异常 ,可能有 7级地震发生。笔者认为目前观测到的大部分异常是区域应力场增强的结果 ,是场上的信息 ,而不完全是震源的应力集中引起的。在区域应力场的作用下 ,一定范围内可以有多个应力集中点 ,这些点中因结构和介质的不同 ,有的积累了应变 ,逐渐发展成震源 ,有的以应力应变的形式表现出来 ,被前兆仪器观测到就成了人们认为的异常 ,地震和异常都是区域应力场增强的结果 ,二者是伴生关系 ,而不是因果关系。

Analysis and research on overall spatial and temporal information of moderate-t erm and short-term anomalies of 200 continuous and stable precursor statoions(or items)in Yunnan show that regular changes are only an ideal model of precursor. Strong regional stress can result in variety of observation circumstances, there by result in regular changes of observation value,which usually occur before and after strong earthquakes. Only when the criteria of anomalies are changed, can the prediction efficiency be improved. Because there usually exist several stres s-concentration points, in term of location prediction using precursory informat ion, it is difficult to meet the need of precision in current practice of earthq uake prediction. But prediction efficiency will be satisfactory if the seismicit y level in Yunnan is estimated with group feature of precursor field. The group feature of precursor is obvious before the occurrence of strong earthquakes in Y unnan. If more than 40% of the precursory observation items are anomalous, maybe an earthquake withM≥7 will occur. The author considers that most anomalies observed so far are the result of the strengthening of regional stres s field rather than the result of the concentration of the stress of the epicent er. Perhaps there are several stress-concentration points in a certain area infl uenced by regional stress field. Because of the difference of structure and medi um, some points accumulate strain and gradually evolve into epicenter while othe rs show the stress and strain. The points observed by instruments are anomalies. Both earthquakes and anomalies are the results of strengthening of regional stre ss field. They are of companionship rather than the causal relationship.