中国大陆东部中-弱活动断层潜在地震最大震级评估的震级-频度关系模型

FREQUENCY-MAGNITUDE RELATIONSHIP MODELS FOR ASSESSMENT OF MAXIMUM MAGNITUDES OF POTENTIAL EARTHQUAKES ON MODERATELY AND WEAKLY ACTIVE FAULTS IN EASTERN CHINA MAINLAND

中国大陆东部中-弱活动断层小区的震级-频度关系符合特征地震模式,其指数函数部分(即G-R关系)的最大截距震级a/b明显偏小于当地的特征震级MC。为了能利用参数a/b间接估计断层小区潜在地震的最大震级,文中发展了综合历史与现代地震资料建立断层小区长期震级-频度关系的方法,计算出130个断层小区按t=500a归算的G-R关系参数at/b值。分析表明,断层小区已发生的最大地震震级Mmax与at/b值呈正相关,且随着at/b值的增大,最大地震震级Mmax的上限呈现较平整、单调上升的特点。已基于这种特点分别建立起华北、华东—华中、华南与东南沿海三大区域的最大地震震级上限Mmu-at/b值关系的3个经验公式,作为利用at/b值估算断层小区潜在地震最大震级的经验模型。文中应用新发展的方法与模型估计了若干断层小区的潜在地震最大震级。研究还揭示出中-小震群、余震和触发型地震序列、人为诱发地震等现代异常地震活动以及最小完整性震级的确定等均对断层小区的at/b值计算有影响。

In this paper, we make an effort to study the feasibility to assess magnitudes of maximum potential earthquakes in sub-areas of moderately and weakly active fauhs in eastern China mainland by using parameters of frequency-magnitude relationship, and develop the corresponding methodology. Data suggest that frequency-magnitude relationships of fault sub-areas in eastern China mainland accord with the characteristic earthquake model, meaning that for a fault sub-area, the ratio a/b of the constants, which is also called as the maximum intercept magnitude of the exponential portion （i. e. the G - R relationship） of the frequency-magnitude relationship, is obviously less than that of the characteristic magnitudes Mc. In order to make the ratio a/b be usable in indirectly assessing the magnitudes of maximum potential earthquakes in fault sub-areas, we develop a method to establish long-duration frequency-magnitude relationships for fault sub-areas by combining data of both historical and modern seismicity. In this method, event＇s numbers from the two sources of data are all normalized to a duration of t = 500years. We then calculate a,/b values of the normalized G - R relationships for 130 fault sub-areas. Our analyses reveal that maximum magnitudes, M of earthquakes occurring and recorded in the studied fault sub-areas are positively correlative with sizes of a1/b values, and with the increase of a1/b values the upper-limits of the maximum magnitudes, M show the feature of monotonously rising and relatively smooth variation. Based on such feature we develop three empirical formulae of the relations between the upper-limits Mmax of the maximum magnitudes, and a1/b values, for the three regions, i.e. North China, Central and East China, and South China and the southeastern coastal area, respectively, and take them as empirical models to estimate magnitudes of maximum potential earthquakes in fault sub-areas. By using the newly developed method and empirical models we estimate magnitudes of maximum potential earthquakes in several fault sub-areas. Our research also suggests that several types of the abnormal seismicity, such as the swarms of moderate and small size earthquakes, aftershocks and triggered earthquake sequences, and artificially induced seismicity, as well as the determination of minimum complete magnitudes, have influences to the calculation results of a1/b values, and that severely influenced a1/b values are overestimated and show deflecting to the right on Mmax -a1/b diagrams. The empirical models and method developed in this study can be applied to the assessment of magnitudes of maximum potential earthquakes for subareas of moderately and weakly active faults in eastern China mainland.