【摘要】 利用地震烈度资料定量估算历史地震震中区域和震级的方法是由Bakun和Wentworth于1997年首先提出的.该方法定量程度高,对烈度数据较少或发生在近海的历史地震的定位和震级估算尤为有效.按照Bakun和Wentworth给出的思路,笔者尝试了对我国华北地区的历史和近代地震的震中和震级进行初步分析.首先,选取20世纪以来发生在该地区的10次有仪器记录的地震(5.3≤MS≤7.8),对该地区的烈度-震级-震中距衰减关系进行标定并给出了烈度衰减模型,表明华北地区烈度随震中距增大而衰减的速率明显小于美国加州地区(约50%).在此衰减模型的基础上,提出了确定震级和震中区域的网格搜索试算方法(GSTSL),并给出了适用于华北地区的圈定震中区域和烈度震级的等值线置信值.最后,讨论了计算震中区域等值线时所引进的权因子Wi及其中参数b对震中区域等值线圈闭性的影响.利用该方法,对发生在1668年莒县—郯城地震,1679年三河—平谷地震,以及1966年隆尧地震和1969年渤海地震进行了分析.需要指出的是,该方法也可推广应用于我国其它历史地震资料丰富的地区.更多还原

【Abstract】 A strategy has been proposed by Bakun and Wentworth in 1997 for bounding the earthquake epicentral region and magnitude from historical intensity data only.This technique is quantitative and particularly appropriate for historical earthquakes which have few intensity data or occurred near offshore.With this technique we have focused our study region on North China,where there have been a lot of moderate or large historical and present-day earthquakes for last a few centuries.First,nine earthquakes with modern instrumental records in magnitude range from MS 5.3 to 7.8 in this region were used to develop an intensity-magnitude-epicentral distance attenuation relationship and the intensity magnitude MI is calibrated to surface wave magnitude MS.The intensity attenuation relation predicts that,with the same earthquake magnitude,the intensity decrease rate with distance in this region is about 50% of that predicted by California intensity attenuation model developed by Bakun and Wentworth.Based on our intensity attenuation model,we have proposed a method of grid searching of trial source location(GSTSL)for estimating earthquake magnitude and bounding epicentral region by calculating confidence value of rms[MI]=rms(MI-Mi)-rms0(MI-Mi)and the contours of MI=mean(Mi),where rms is the root mean square,rms0(MI-Mi)is the minimum rms over a grid of assumed epicenter,and Mi(i=1,2,3,…)are the discrete values derived from the intensity attenuation model.Furthermore,we have discussed the size effect of b-value,a parameter used in weight function Wi in order to precisely compute rms[MI] on the closure of rms[MI] contours.Finally,in order to demonstrate the technical strategy we have developed in estimating historical earthquake magnitude and bounding its epicentral region,two typical earthquakes,the 1679 Sanhe-Pinggu event and 1969 Bohai event,are used in our analysis.The result shows that,by minimizing earthquake magnitude within a 90% confidence region of rms[MI],the intensity magnitudes of MI are 7.8 and 6.9 for the Sanhe-Pinggu and Bohai event,respectively.It needs to be pointed out that the technique we have developed here can also be used for bounding the epicentral region and earthquake magnitude in other regions with abundant historical earthquake documents in China.更多还原