断层崖研究与地震危险性估计——以贺兰山东麓断层崖为例

FAULT SCARPS RESEARCH AND EARTHQUAKE RISK ESTIMATION-EXAMPLE IN EASTERN HOLANSHAN FAULT SCARPS

摘要:

本文研究了断层崖的形成条件及演化过程,提出了必须对断层崖同时进行几何学和沉积学的研究,并根据我们对贺兰山山前断裂带的断层崖的研究结果,初步总结了断层崖崩积楔的若干特征及其在大地震重复性研究中的意义。
根据对宁夏红果子沟和苏峪口断层崖几何学和沉积学的研究结果,确定了贺兰山山前断裂自全新世以来曾发生过4次快速错动事件.从老至新分别命名为 A、B、C、D 事件。由于第二期崩积楔下部物质的 C¹⁴年龄为距今5745±90年,所以,B 事件大约发生在距今约6000年左右。第四次事件(D)发生在距今400年以内,以致使明代长城发生错动,西错断点的垂直断距为0.35米,东错断点为0.95米。由此估计4次断层错动事件的重复间隔约为2000-2500年。根据三期古崩积楔高度估计三次古错动的垂直位移幅度分别为:红果子沟西部断层崖为0.25-0.5米,东部断层崖为0.9-1.2米,苏峪口为0.8-1.6米,它们均与串件 D 相当。若假定事件 D 与1739年平罗地震相关,则上述4次断层错动的重复间隔即为7-7(1)/(2)级以上地震的重复间隔。此数据与根据相同地段断层滑动速率计算的大地震平均重复间隔相当。
宁夏北部贺兰山东麓断层为右旋走滑正断层,全新世以来垂直滑动速率北段为0.2-0.25毫米/年,中段为0.5-0.63毫米/年。而宁夏南部南、西华山断裂第四纪以来为左旋走滑断裂,20000年以来的走滑速率最大可达28.65毫米/年。这是因为二者分属于华北和青藏两个不同的构造区,因而具有不同的构造活动性及地震重复率。

Abstract:

This paper deals with the formation and evoluting processes of fault scarps,and proposes that research on fault scarps should be not only on geometry but also on sedimentology.On the basis of research on the eastern Holanshan frontal fault scarps,we realized several characters of fault colluvial wedge and the important sense in studying earthquake occurrenee.
According to geometry and sedimentary research on fault scarps in Hogguozi valley and Suyukou,four times rapid dislocation events have been indicated in eastern Holan Shan frontal fault since Holocene These four events can be named as A.B.C.D from the oldest to the youngest.The C'₁ age of material beneath the second colluvial wedge(B) ls adout 5745±90from now,so event B may occur 6000 years ago.The last event(D)occured within 400 years from now,it displaced the Great Wal-which built 400 years ago.There are two places where Great wall have been displaced, the vertical displacement is 0.35 meter in weastern place and 0.95 meter in eastcrn place. The occurrence interval of four offset ev-ents is estimated as 2000 to 2500 years.According to the hight of ancient colluvial wedge,we estimate the amplitude of vertical displacement as 0.52 to 0.5 meter in western scarp and 0.9 to 1.2 meter in eastern scarp of Hong guozi valley,0.8 to 1.6 meter in Suyukou.obviously they coincide with the event D.If we suppose that event D is associated with Pingluo earthquake in1739,the occurrence interval of the four fault offsets can be seen as the acurrence interval of earthquake with magnitude 7(1)/(2).This occurrence interval coincides with the one which can be calculated by using fault slip rate in same segment of the fault.
The eastern Holanshan fault in northern Ninxia is a right lateral strike slip to normal fault.Since Holocene its vertical slip rate is 0.2-0.25 mm/year in northern segment,and 0.5-0.6mm/year inmiddle segment.How ever in south of Ninxia,the Southern Huashan to Western Huashan fault is a left lateral strike-slip fault,the horizontal strike-slip rate can be as large as 28.65mm/year since 20000 years.That is because these two faults belong to North China and Qinzang tectonic regions respeetively,so they have different active tectonics and earthquake occurrence.