Abstract: Estimation of sizes of potential earthquakes is required in long-term seismic hazard assessment.Such estimation is usually related to a single active fault segment with specific scale,and can be carried out by using an empirical relationship between magnitude and rupture-scale of earthquake.This type of empirical relationships published already for China mainland is established on data of surface ruptures along seismogenic active faults in West China.While,the special active tectonic environment in the region of North China makes it impossible to establish the same type of empirical relationships that are suitable for the region by using the surface-rupture data.Therefore,for North China,we should make an effort to develop the empirical relationships between earthquake magnitude and(sub-surface)source rupture-scale.In the latest 40 years,4 major earthquakes of magnitudes ≥7.0,7 strong ones of magnitudes 6.0 to 6.9 and tens of moderate ones(including aftershock events)of magnitudes 5.0 to 5.9,occurred in North China.For a part of these earthquakes,data or distribution patterns of aftershock sequences with good or high precision locations are available,and for many of these earthquakes,studies on source rupture processes and parameters have been carried out.These make it possible to establish preliminarily the empirical relationships between magnitude and rupture-scale for North China.For this purpose,from data of earthquakes that occurred in North China since 1965,we systematically collected and compiled the relevant rupture-scale parameters obtained from methods of analyzing seismic wave spectrum,coseismic crustal deformation and aftershock distribution,or available from published researches,including rupture length L,downdip rupture width W,and rupture area A(A=L×W).We also re-determined the rupture-scale parameters for part of these earthquakes basing on aftershock distributions.In order to deal with uncertainties in the source rupture-scale parameters due to the use of various methods,and to take the main factors influencing the uncertainties into account,we broungt forward 8 criterions for determining the reliable parameters through further analyses and identification.With synthetic analyses basing on these criterions,we obtained reliable rupture-lengths of 34 earthquakes and reliable rupture-areas of 20 of the 34 earhtquakes.Focal mechanism data suggest that the absolute majority of the 34 ruptures are of strike-slip type.By using the least-square method,we further established two regression relationships between surface-wave magnitude M_S and rupture-length L,and between magnitude M_S and rupture-area A for seismogenic active faults.They are M_S=3.821+1.860lg(L)and M_S= 4.134+0.954lg(A),respectively.A comparative analysis with previous empirical relationships of the same type suggests that the two new empirical relationships developed in this research are suitable very well for estimation of sizes of potential earthquakes on seismogenic active strike-slip faults in North China and in the urban area of Beijing and its surroundings.

Key words: seismogenic active fault, magnitude-rupture length relationship, magnitude-rupture area relationship, North China

摘要： 为了建立适用于华北地区地震活断层的地震震级-震源破裂尺度经验关系,从1965年以来华北发生的地震中,系统整理出已由地震波谱、地形变、余震分布等方法获得和发表的震源破裂尺度参数,包括破裂长度L、破裂下倾宽度W以及破裂面积A(A=L×W),同时还基于余震分布重新确定出部分地震的破裂尺度参数.针对来源于不同方法的破裂尺度参数的不确定性及其主要影响因素,提出通过进一步分析和鉴别来确定出可靠参数的8条原则.依据这些原则的综合分析,共获得34次地震的可靠破裂长度以及其中20次的破裂面积.震源机制解反映这些地震破裂以走滑型占绝大多数.我们进一步采用最小二乘法,分别建立起华北地区的面波震级M_S-破裂长度L的回归关系式M_S=3.821+1.860lg(L) 以及面波震级M_S-断层破裂面积A的回归关系式M_S=4.134+0.954lg(A).与已有同类关系式的比较分析表明:文中新建的2个关系式可较好地适用于华北以及首都圈地区的走滑型地震活断层的潜在地震强度评价.

关键词: 地震活断层, 震级-破裂长度关系, 震级-破裂面积关系, 华北