2018年5月28日中国吉林松原M5.7级地震引起的液化构造*

2018年5月28日,吉林松原市宁江区毛都站镇牙木吐村发生M5.7级地震(45°16'12″N,124°42'35″E),震源深度13 km,震中位于郯庐断裂带西北侧的扶余/松原—肇东断裂带、第二松花江断裂带和扶余北断裂带交汇处。地震诱发震中距3 km范围内普遍的液化和地表裂缝,给当地居民带来严重灾害。可见液化构造以砂火山为主,其次为液化砂堆、液化砂脉和液化砂席等。液化砂火山又可分为有火山口型砂火山、无火山口型砂火山和无砂型(水)火山。地震液化伴生软沉积物变形构造有变形层理、负载构造和火焰构造、滑塌褶皱、碟状构造和包卷层理等。地震诱发液化砂火山形成过程包括液化层内超孔隙流体压力形成、上覆低渗透层破裂和水、砂喷出地表后砂涌3个阶段。液化和流化砂体在上涌过程中会注入低渗透黏土层形成各种形态的砂脉、砂席和多种类型的变形构造。垂向上地震液化结构可划分为底部松散可液化层、下部液化变形层、上部液化变形层和地表砂火山4层结构。液化层埋深2~5 m,液化层厚度2 m。松原M5.7级地震发震机制为NE-SW(35°~215°)方向挤压应力使断层活跃,推测扶余/松原—肇东断裂是主要的发震断层。松原地震液化构造研究为现代地震活动区和灾害易发区预测提供依据,为地震引发的现代软沉积物变形构造研究提供丰富的素材,兼具将今论古意义,为揭示本世纪以来郯庐断裂带北段进入了一个强断裂和地震活跃阶段提供了最新的实际资料。

Liquefaction structures induced by the M5.7 earthquake on May 28,2018 in Songyuan area,Jilin Province,China

An earthquake of magnitude M5.7 occurred in Yamutu village,Songyuan City,Jilin Province of Northeast China(45°16'12″N,124°42'35″E)on May 28,2018,with a focal depth of 13 km. The epicenter is located at the intersection of the Fuyu/Songyuan-Zhaodong fault,Second Songhua River fault and Fuyu north fault which lie west of Tanlu Fault. The earthquake-induced widespread liquefication structures and ground cracks within 3 km from the epicenter,bringing serious disasters to the local surroundings. The visible liquefied structures are mainly composed of sand volcanos,followed by liquefied sand mounds,sand dikes and sand sills. The liquefied sand volcanoes can be divided into volcanoes with craters,volcanoes without craters and water volcanoes(sand-free). Other soft-sediment deformation structures(SSDSs)induced by earthquakes include deformation lamination,load structures and flame structures,deformation folds,dish structures,convolute bedding and some water-escape structures. The formation process of the sand volcanos comprises building up excess pore-fluid pressure in the liquefied layer,cracking of the low-permeability overlying layer and sand-water venting out of the ground surface inthree stages. During the upwelling,the liquefied sand is injected into the low-permeability layer to form sand veins,sand sills and various types of deformation structures. The vertical seismic liquefaction structure can be divided into the four parts of the bottom clean unconsolidated liquefiable sand,the lower liquefied deformation layer,the upper liquefied deformation layer and the ground surface liquefied sand volcano layer. The liquefaction occurred at a buried depth of 2-5 m,causing a thickness of 2 m of sand to liquefy. NE-SW(35°~215°)trending compressive stress that caused the fault to reactivate may be the seismogenic mechanism of the Songyuan M5.7 earthquake,and the Fuyu/Songyuan-Zhaodong fault was conjectured to be the major seismogenic fault. The study of the Songyuan seismic liquefaction structures provides guidance for the prediction of modern earthquake activity areas and disaster-prone areas and provides abundant basis material for studying earthquakes induced SSDSs in modern sediments,which will be of great significance in interpreting the theories of “the present is the key to the past”,which provides the latest practical data to reveal that the northern part of tanlu fault zone has entered into a stage of strong faults and seismicity since the beginning of this century.