夏垫断裂荣家堡探槽揭示的断裂活动特征及未来地震危险性概率评价

THE ACTIVITY FEATURES OF XIADIAN FAULT ZONE REVEALED BY RONGJIABAO TRENCH AND ITS PROBABILISTIC SEISMIC HAZARD EVALUATION

时间相依的活动断裂地震危险性概率评价以地震地质学定量研究为基础,采用随离逝时间增长的发震概率和预测震级共同描述断裂段的地震危险性,对地震预防和灾害管理具有重要的现实意义。传统概率方法中泊松分布(Poisson)模型的时间不相关性与活动断裂上大地震的活动特征不符,不能直接用于计算上次大地震离逝时间较短的活动断层的地震危险性。曾于1679年发生过三河-平谷8级大地震的夏垫断裂即是典型实例,现有的泊松模型可能高估了该断裂的潜在地震风险,应考虑该地震之后的应力积累和时间因素来评估其地震危险性。文中基于野外探槽古地震和地貌测量等调查工作,揭示该断裂晚全新世以来发生过2次古地震事件:事件E1即1679年三河-平谷大地震,距今341a;另一次古地震事件E2的年龄限定为距今(4.89±0.68) ka,平均同震位移约为(1.4±0.1) m。与前人的研究数据进行对比可发现,最近一次大地震的离逝时间约341a,预测未来的最大震级为8.0级。同时,文中采用时间相依的布朗模型(BPT)、随机特征滑动模型(SCS)和通用模型(NB)表述断层破裂源上特征型地震活动的时间相依特征,综合计算了该断裂带未来30a的强震发震概率,并与泊松模型的计算结果进行了对比。研究结果表明,该断裂未来30a的强震发震概率较低,原来所采用的时间不相关的泊松模型所计算的发震概率值相对偏保守。所得结果有助于科学评价该断裂的地震潜势,同时有助于讨论时间相依的概率方法如何更好地适用于东部地区活动断裂的地震危险性评价。

The time-dependent probabilistic seismic hazard assessment of the active faults based on the quantitative study of seismo-geology has the vital practical significance for the earthquake prevention and disaster management because it describes the seismic risk of active faults by the probability of an earthquake that increases with time and the predicted magnitude.The Poisson model used in the traditional probabilistic method contradicts with the activity characteristics of the fault,so it cannot be used directly to the potential earthquake risk evaluation of the active fault where the time elapsing from the last great earthquake is relatively short.That is to say,the present Poisson model might overestimate the potential earthquake risk of the Xiadian active fault zone in North China because the elapsed time after the historical M8 earthquake that occurred in 1679 is only 341 a.Thus,based on paleoearthquake study and geomorphology survey in the field,as well as integrating the data provided by the previous scientists,this paper reveals two paleo-events occurring on the Xiadian active fault zone.The first event E1 occurred in 1679 with magnitude M8 and ruptured the surface from Sanhe City of Hebei Province to Pinggu District of Beijing at about 341 a BP,and the other happened in(4.89±0.68)ka BP(E2).Our research also found that the average co-seismic displacement is^(1.4±0.1)m,and the predicted maximum magnitude of the potential earthquake is 8.0.In addition,the probabilistic seismic hazard analysis of great earthquakes for Xiadian active fault zone in the forthcoming 30 a is performed based on Poisson model,Brownian time passage model(BPT),stochastic characteristic-slip model(SCS)and NB model to describe time-dependent features of the fault rupture source and its characteristic behavior.The research shows that the probability of strong earthquake in the forthcoming 30 a along the Xiadian active fault zone is lower than previously thought,and the seismic hazard level estimated by Poisson model might be overestimated.This result is also helpful for the scientific earthquake potential estimation and earthquake disaster protection of the Xiadian active fault zone,and for the discussion on how to better apply the time-dependent probabilistic methods to the earthquake potential evaluation of active faults in eastern China.