【作者】 闵伟；

【导师】 邓起东； 程绍平；

【作者基本信息】 中国地震局地质研究所 ， 地震地质， 1998， 博士

【摘要】 古地震研究是活动断裂研究内容的一部分,具有非常重要的意义。到目前为止,几乎所有的古地震研究都是针对一条断裂带或断裂段进行的,而没有开展一定区域的古地震研究,现有的地震危险性分析也是原地重复地震的危险性性分析。 在大陆内部地震原地重复时间尺度内,对于一定区域可能有多次地震发生,相邻地震之间的时间间隔远远小于原地重复地震之间的时间间隔,等于“缩短”了地震重复间隔,如果一定区域内强震活动在时间和空间上都有一定规律,那么,研究这种规律有可能找到一条以区域强震为基础的适合于大陆内部的地震危险性分析模型。 研究区地处青藏高原东北边缘与华北构造区西缘过渡部位,活动断裂十分发育,历史地震和现代地震活动十分活跃,在距今约400年前到公元1920年,先后发生5次M≥7级地震,提供了区域强震丛集的实例。因此,本文以历史强震丛集为线索,在对区内主要断裂活动特征及其相互关系详细研究的基础上,通过大量探槽资料,剖析了研究区一万年以来古地震活动在时、空、强分布上的特征,并从动力学的角度出发,解释了形成该区域古地震复发模型的机制。 以往,牛首山一罗山断裂被认为是华北和青藏高原两大构造区的边界断裂,人们一直把牛首山一罗山断裂划归青藏高原区。通过对研究区主要活动断裂晚第四纪以来活动特征的研究,我们认为最新活动的罗山东麓断裂向北已不与牛首山断裂相连,而应是黄河断裂向南扩展的结果,属于华北构造区。 海原断裂带一般分为东、中、西三段,中段和西段为两个独立的破裂段落。本次研究中,对东段通过槽探研究了其古地震,结果表明东段不是一个独立破裂段落,只是中段部分破裂事件的影响段,因此,从破裂分段的角度出发,可将海原断裂带以干盐池为界,分为东段和西段,同时破裂东、西段的地震事件称为破裂全断裂的事件。一万年以来,破裂全断裂的事件只有两次,分别发生于距今8534±80年和78年,间隔大于8千年;西段共发生5次古地震(包括上述两次全带破裂事件),规律性较好,平均重复间隔2116年,最大重复间隔2387年,最小1930年;东段共发生7次古地震事件(同样包括上述两次全带破裂事件),重复间隔平均为1410年,但规律性不如西段,最大重复间隔1890年,最小只有845年。海原断裂带古地震事件具有丛集特征。第一次和最后一次古地震都为全带破裂事件,其它古地震大致可分为4个丛集期,每个丛集期由分别破裂西段和东段的两次地震组成。 中卫一同心断裂以西梁头和双井子为界,可以分为西、中、东三段,东段为挤压构造变形区,全新世以来没有破裂事件分布,因此,从破裂分段的角度出发,将中卫一同心断裂带以西梁头为界,分为东西两段。l万年以来的古地震只有五次,东段三次,西段两次,都为破裂次级段落的地震,没有破裂全断裂的事件。最大间隔为3212年,最小为1085年,平均重复间隔为2070年。 罗山东麓断裂全长约60km,主要由五条次级断层组成,公元1561年71/4级历史地震破裂全断裂,表明它没有次一级破裂分段。探槽揭露该断裂l万年以来共发生4次破裂事件,分别为距今8200±600、5020±70、3331±120年和公元1561年历史地震,最大重复间隔3180年,最小1670年,平均2588年,中间两次地震的重复间隔明显小于其他几次地震的间隔。 根据前人的研究结果,贺兰山东麓断裂l万年以来共发生五次8级左右的地震,其时间分别为距今8240±170、6330±80、4760±80、2675±70年和公元1739年历史地震,全新世早期和晚期重复间隔长,一般大于2千年,全新世中期的两次地震重复间隔短,更多还原

【Abstract】 The paleoearthquake research is a part of active tectonic research, it has great significance. We have obtained much paleoearthquake research data of a fault or segments, we have not studied recurrence behavior of paleoearthquakes in a region. The probabilistic seismic risk is to evaluate the earthquake that will occur in the same place.The recurrence interval intraplate is as long as several thousand years or over ten thousand years. The difference of recurrence interval intraplate is several hundred years or thousand years, forecasting using such data is insufficient.There are many earthquakes taking place in a region in the interval that two earthquakes occurred in the same place intraplate. The interval between two neighbor earthquakes in a region is greatly less than the intervals between two earthquakes in the same place, it means "shortening" recurrence interval and also reduces indetermination of recurrence interval. It is possible to find a new seismic hazard assessment model that is suitable to intraplate.The studying area locates in the northeastern margin of Tibet and the western margin of Huabei. Historic earthquakes and modem seismicity are very strong in thin area, five (M>7)earthquakes have occurred between A.D 1561 and A.D 1920, it is a real regional clustering example.Earthquake is a form of modern tectonic movement. The distribution of modern seismicity has been used to study modern crust movement, similarly, region paleoearthquake recurrence image can be used to study Holocene tectonic movement and make up shortage of geological and geomorgraphic methods.We got following main consequences based on a lot of field geologic working and trenching: (1)Nuishoushan—Luoshan fault was regarded as the boundary fault between Tibet and Huabei, and belonged to Tibet. Through studying the active characteristics of main faults since late Quaternary, we believed Luoshan fault is not linked with Nuishoushan fault but Huanghe fault, and belong to Huabei tectonic area now.(2)Haiyuan fault ruptured 10 times since 10,000 years B.P, average recurrence interval is 940a, the longest interval is 1582a,the shortest interval is 382a, the recurrence behavior has clustering characteristics. Two paleoearthquakes ruptured entire fault .which occurred 8534±80a B.P and A.D 1920, the interval is longer than 8,000 years.Five paleoevents ruptured western segment, average recurrence interval is 2116a, the longest interval is 2387a, the shortest interval is 1930a.Seven paleoevents ruptured eastern segment, average recurrence interval is 1410a, the longest interval is 1890a, the shortest interval is 845a.(3)Zhongwei—Tongxin fault ruptured only five times since 10,000 years B.P Three events ruptured eastern segments, two events ruptured western segments, there is not rupturing entire fault events. The longest recurrence interval is 3212a, the shortest recurrence interval is 1085a, average recurrence interval is 2090a.(4)Luoshan fault can’t be divided into sub rupture segments. Four paieoevents ruptured entire fault since 10,000years B.P ,the longest recurrence interval is 3180a, the shortestrecurrence interval is 1670a, average recurrence interval is 2588a.(5)Helanshan fault also can’t be divided into sub rupture segments. Five paleoevents ruptured entire faults, they occurred 82401170a, 6330±80a, 4760±80a, 2675±70a B.P and A.D 1739, average recurrence interval is 1570a.(6)The region paleoearthquake recurrence behavior is clustering. There are six clustering periods since 10,000years B.P , they can be divided into two types. The first continued about 300a, located beginning and last of the six clustering periods. Ecery fault rupture once and migration direction is not definite. The second sustained about 1000a, the paleoearthquake migration direction is definite, from Haiyuan fault to Zhongwei— Tongxin fault to Luoshan fault to Helanshan fault, then returned to Haiyuan fault. Every period, Haiyuan fault ruptured twice, other faults ruptured once.(7)Region paleoearthquake active image reflected the regional crust movement process: 8,000a B.P ,a strong moving force continued about 300years, subsequently, a stable and relatively week moving force continued about 7,000a B.P ,the recent strong moving force continued about 300a 400years B.P .The regional crust movement process studying by tectonic geography : the regional crust uplifted rapidly early Holocene, 7m high terrace formed; subsequently, the regional crust steadily uplifted, four terraces 3-4m high formed.It is consistent that the regional crust movement process studied by regional paleoearthquake images and tectonic geomorgraphy.(8)The important progresses :?proposing regional paleoearthquake recurrence model through real working; it is possible to find a new probabilistic seismic risk method which is suitabl to intraplate;?giving a relative perfact paleoearthquake catalogue in a region;(Dthe behavior of paleoevent recurrence is concerned with the fault texture .The fault texture is simple ,its paleoevent recurrence is quasi-period; the fault texture is complex, its paleoevent recurrence behavior is clustering; ? analysing the structure movement through regional paleoevent recurrence 10,000years B.P, the regional tectonic movement style studied from regional paleoevent recurrence behaviro and tectonic geomorgraphy is consistent since 10,000 years B.P.(9)Remaining problems: ? regional paleoearthquake recurrence begins recently .there are not practical region separation principled is needed to study different tectonic region; ?some fault segment paleoearthquake is not complete, it perhaps influenced the final results;<Dquantitative tectonic geomorgraphy research is insufficient, there are not more dating data.更多还原