摘要
地震液化阈值是一个非常重要的科学问题,一般认为M5地震不会形成大面积的液化。2009年12月14日,中国新疆哈密市发生了M5.1中等地震,震源深度仅4 km。地震砂脉网格在平面从几十厘米到2 m以上;砂脉纵断面呈楔形、倾斜(平均75.10°),分选较好。通常情况下,它们通过液化和流化分异作用发生在细粒丰富的盐渍砂砾层(SSGL,Salinization Sand—Gravel Layer)和盐粒砂砾层(SGSGL,Salt-Grain—Sand—Gravel Layer)中,尽管没有细粒盖层和源砂,但这些盐渍的砂砾层极易在盐溶后发生活化,颗粒之间的摩擦力骤降,液化上涌而形成砂脉。液化边界距震中可达80 km,甚至可能达到120 km,相当于M7.0~8.0级地震的液化最远距离。哈密地区之所以能在M5.1地震作用下形成远程砂脉,主要由于以下5个优势:(1)砂泥的盐溶液中细粒组分容易发生液化流化。浓盐水能够降低颗粒的剪切能力,平均降低25%~75%左右,使地震液化阈值降低到0.15~0.05 g(以0.2 g为一般阈值)。与此同时,浓盐水由于密度大,盐水可使淡水最小流化速度(Umf)降低12.51%~21.58%,有利于流化。(2)广泛分布的盐渍砂砾层和盐粒砂砾层。(3)震源极浅(深度仅4 km)。(4)基底极浅(深度0~3 m)。(5)表层盐屑混合盖+盐渍砂砾层+盐粒砂砾层+极浅基岩基底组成了特殊的三明治结构。通过对液化流化的形成机理研究表明,砂主要来自砂脉底部的砂砾层的流化分选,流化分选会在砂脉底部的砂砾层中形成一个分选晕。
Objectives:The earthquake liquefaction threshold is a very important scientific problem.It is generallybelieved that the earthquake of magnitude 5 will not result in large-scale liquefaction.On December 14,2009,amoderate 5.1 magnitude earthquake struck Hami City in western China’s Xinjiang Province,with a focal depth ofonly 4 kilometers(Figure 1).This paper is to discuss the mechanism of extensive liquefaction caused by Hami 5.1earthquake.Methods:The authors made a detailed field investigation in Hami,Xinjiang,and obtained a lot of valuablegeological information,especially sand dikes and obtained a lot of valuable salt solution activation data through thelaboratory fluidization experiment and salt solution activation experiment,which provided valuable data for furtherstudy of liquefaction activation of saline solution.Results:The liquefaction boundary can be 80 kilometers away from the epicenter,and may even reach 120km,which is equivalent to the epicenter distance of a 7.0~8.0 magnitude earthquake.It is undeniable that theformation of Hami sand dikes,especially the unusually significant liquefaction and fluidization.Seismic sand dikesgrid plane from tens of centimeters to more than 2 meters;The longitudinal section is wedge-shaped and oblique(average 75.10°),and the sorting is good.The dip Angle of sand dikes ranged from 52°to 90°,with an averageof 75.4°(250 sand dikes were taken as examples),and more than 96%of sand dikes had steep dip Angle(inclination>60°).The Hami sand dikes are convex in both plane and section,but are flat and even grooved insome places due to flood erosion.The protrusion height of hami sand dikes is generally 2~3 cm~5~6 cm,andthe maximum is 10 cm.The experimental results show that under the condition of salt water concentration of 23%and 9%~17%,the shear stress inside sand dikes can be greatly reduced by salt solution,which is 52.63%~85.20%and 12.51%~21.58%,respectively,which is conducive to the formation of sand dikes by seismicliquefaction and fluidization.An unusual assemblage of sedimentary facies and basement(sandwich-likeassemblage)is thought to facilitate large-scale liquefaction.In addition,solid salt dissolves to form a thinner fluidlayer that can form an overpressured fluid on its own without seismic liquefaction.Conclutions:①A large number of sand dikes are developed in the study area,which are continuouslydistributed,medium—small scale,polygon on the plane and wedge on the section;most of it leans.It is believedto have been a moderate earthquake of M5.1 on December 14,2009.②Due to the shallow basement,thin caplayer,and intense salinization,Hami,under the very limited shock impact of such a moderate intensity earthquakejust entering the liquefaction threshold(M5.1),formed a wide distribution of seismic liquefaction sands extendingat least 80 km outward from the epicenter,and possibly up to 120 km.③The good characteristics of dike sortingare definitely due to the liquefaction and fluidization sorting of SSGL and SGSGL,rather than a pure parent sandunit.Through fluidization experiment and calculation,fluidization distribution and fluidization boundary diagram offluidization sand with low particle size are obtained.④Experimental studies on the minimum fluidization velocity(Umf)of particles<0.125~40 M are reported,and two empirical formulas are proposed:Umf=6.612×D0.6277 andUmf=7.7443×D0.6293.The maximum fluid excess pore pressure and seismic shock pressure are calculated to beabout 513.448~637.29 kg/m2.They are quite different from the classical formulas.The maximum fluidoverpressure and seismic impact pressure(about 513.448~637.29 kg/m2)were calculated from the overpressurevalues of 40 M gravel and the elevation of sand dikes.⑤The study shows that the sand forming sand dikes mainlycomes from the fluidization sorting of the sand and gravel layer below the bottom of the dike.⑥The reason whyHami can form long distance sand veins under M5.1 earthquake is mainly due to the following six advantages:①The salt solution of sand mud sand is easy to liquefy and fluidize.Concentrated brine and semi-dissolved salt canreduce the shear capacity of particles by 25%~75%on average,and the seismic liquefaction threshold is reducedto 0.15~0.05 g(with 0.2 g as the general threshold).At the same time,with the increase of density,theminimum fluidization velocity(Umf)of fresh water can be reduced by 12.51%~21.58%,which is conducive tofluidization.②Widely distributed SSGL and SGSGL;③The source is very shallow(only 4km deep);④veryshallow basement(depth of about 0~3 m);⑤The special sandwich structure of the salt—debris-mixed-cover+SSGL+SGSGL+extremely shallow bedrock basement.Based on the excess pore pressure value of 40M gravel andthe height of sand veins above the ground,the maximum fluid excess pore pressure and seismic shock pressure arecalculated to be about 513.448~637.290 kg/m2.
作者
钟建华
倪良田
孙宁亮
曹梦春
ZHONG Jianhua;NI Liangtian;SUN Ningliang;CAO Mengchun(State Key Laboratory,China University of Mining and Technology(Beijing),Beijing,100083;School of Geoscience,China University of Petroleum,Qingdao,Shandong,266580;Research Institude of Exploration and Development,Hengli Oilfield Company,SINOPEC,Dongying,Shandong,257000;School of Resources and Material,Northeastern University at Qinhuangdao,Qinghuangdao,Hebei,066004)
出处
《地质论评》
CAS
CSCD
北大核心
2023年第4期1543-1563,共21页
Geological Review
基金
国家自然科学基金资助项目“哈密M5.1地震引起的砂砾岩远程液化特点和机理的研究”(编号:41572088)的成果~~。
关键词
砂脉
液化
盐溶活化
流化
地震
哈密
sand dike
liquefaction
salt-dissolution activation
fluidization
Hami
