关键词: 中-弱活动断层, 断层小区, G-R关系, 潜在最大地震, 震级评估

Abstract: 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 faults 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 M_C.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_t/b values of the normalized G-R relationships for 130 fault sub-areas.Our analyses reveal that maximum magnitudes,M_max,of earthquakes occurring and recorded in the studied fault sub-areas are positively correlative with sizes of a_t/b values,and with the increase of a_t/b values the upper-limits of the maximum magnitudes,M_max,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 M_mu of the maximum magnitudes,and a_t/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 a_t/b values,and that severely influenced a_t/b values are overestimated and show deflecting to the right on M_max-a_t/b diagrams.The empirical models and method developed in this study can be applied to the assessment of magnitudes of maximum potential earthquakes for sub-areas of moderately and weakly active faults in eastern China mainland.

Key words: moderately and weakly active fault, fault sub-area, G-R relationship, maximum potential earthquake, assessment of earthquake magnitude