1976年唐山MS7.8地震余震序列持续时间及对地震危险性分析的意义

仲秋史保平

摘要: 1976年7月28日唐山MS7.8大地震对唐山及其周边地区造成了重大的人员伤亡和财产损失. 主震之后约15小时滦县又发生了MS7.1地震；同年11月15日宁河也发生了MS6.9地震. 唐山MS7.8主震后的余震一直持续至今，使该区域至今保持了与主震前相比具有较高的地震活动性．如何估计余震的持续时间，并进一步将余震从主震目录中去除，一直是地震学中所关注的问题．该文通过对数线性回归和理论计算，从不同角度求取并讨论了1976年唐山MS7.8大地震的余震持续时间．结果表明，由对数线性回归计算得到的余震持续时间约为80 a．而基于Dieterich的余震触发理论所得到的余震持续时间则与区域剪应力变化率有关．区域剪应力变化率可有几种不同方法求得： ① 根据剪应力变化率和静态应力降Delta;tau;e及地震回复周期tr之间的关系求取应力变化率，该方法所得到的余震持续时间约为70——100 a；② Ziv和Rubin对Dieterich的方法进行了修正，给出了通过远场加载速率和断层宽度求取应力变化率，该方法得到的余震持续时间约为80 a；③ 由背景场地震活动性求取远场剪应力速率，可以得到该地区二维分布式的余震持续时间，此方法得到的研究区域内余震持续时间为130——160 a．综上，唐山地区余震持续时间约为70——140 a，据此, 该地区现今所发生的地震仍为MS7.8唐山地震所触发的余震．

关键词:

1976唐山地震　余震持续时间　地震触发　剪应力变化率

Abstract: The disastrous MS7.8 earthquake in Tangshan, China, on 28 July 1976 caused great damage and significant casualties. The mainshock was followed by two large aftershocks: the MS7.1 eventto the northeast near Luanxian 15 h after the mainshock, and the MS6.9 event on 15 Novembersouthwest of Tangshan near Ninghe. The aftershock sequence lasts to date, making regional seismic activity around the Tangshan main fault much higher than that of before the mainshock. It is always difficult to accurately determine the duration time of aftershock sequences and identify the aftershocks from main event catalog. In this study, by using theoretical inference and empirical relation, we tried to derive the time length of aftershock sequences of the MS7.8 Tangshan earthquake. The estimated aftershock duration from a log-log regression is about 120 years according to the empirical Omorirsquo;s relation (Toda et al). Based on Dietrich theory, it has been claimed that the aftershock duration is a function of remote shear stressing rate, normal stress acting on the fault plane, and fault frictional constitutive parameters (Dieterich). In general, shear stressing rate could be estimated in three ways: ① Shear stressing rate could be written as a function of static stress drop and a mean earthquake recurrence time (Dieterich). In this case, the time length of aftershock sequences is about 70mdash;100 years. However, the recurrence time inherits a great deal of uncertainty. ② Ziv and Rubinderived a general function between shear stressing rate, fault slip speed and fault width with a consideration that aftershock duration does not scale with mainshock magnitude. Therefore, from Zivrsquo;s consideration, the aftershock duration is about 80 years. ③ Shear stressing ratecan also be described as a function of background seismicity rate (Console et al). From this approach, a two-dimensional spatial distribution of the aftershock duration can be obtained, and the resultant time length of aftershock sequences is about 130mdash;160 years, which is much longer than that given by previous two approaches. The conclusive noting gives that, for the 1976 MS7.8 Tangshan earthquake, the triggered aftershocks would last at least 70 years, implying that the current earthquakes which occurred in Tangshan region after the 1976 mainshock are the aftershocks of the main event.

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