震级转换关系及其对地震活动性参数的影响研究

Research on Empirical Relationship of Earthquake Magnitude Scales and Its Influence on Seismicity Parameters

本文根据1990—2007年《中国地震年报》中同时给出MS和ML、且震源深度<70km的6577个浅源地震资料,经统计回归得到了全国和各地震区MS与ML之间的经验关系。新的震级转换关系接近于MS=ML,本文建议在需要进行震级标度转换时,对于没有测定MS的低震级地震可直接使用MS=ML进行转换。与目前广泛使用的震级转换关系相比,采用本文建议的震级转换方法后,全国各地震区5级以上地震的数量基本没有变化,但5级以下地震的数量有明显增加,导致由此统计得到的各地震区震级-频度关系中的b值有不同程度的增大。此外,用地震数量直接得到的4级以上地震的年平均发生率ν4有明显增高,且部分地震区甚至增加了50%以上。在高震级地震发生率不变的情况下,地震活动性参数的上述变化反映了对地震区地震活动水平的估计有提高,可能导致概率地震危险性分析结果的提高,对地震区划和工程场地地震安全性评价有重要意义。

The nation-wide and regional relationships between Ms and ME are empirically derived by 6577 shallow earthquake magnitude data from the ＂Annual Bulletin of Chinese Earthquakes＂ for 1990-2007. The data selection criteria are that the earthquake must have Ms and ME simultaneously and independently determined. The new relationship between Ms and ME is close to Ms=ML. It is suggested by this paper that, when the ME magnitude is need to convert to Ms for earthquakes which only have ML determined, the conversion equation be MS=ML. When the new conversion relation is applied the amount of M〉5 earthquakes is not affected while the M〈5 earthquakes increase significantly. Because of the application of new relationship the b value in every seismic region increases to some extent. The annual mean occurrence rate of M≥4 earthquakes v4 increases remarkably and in some seismic regions even increase over 50%. The variations of seismicity parameters due to the change of earthquake magnitude conversion relation may cause increase of probabilistic seismic hazard assessment result.