晚寒武世早期河北北部柳江盆地发生的强地震

在河北省北部柳江盆地发现了距今5亿年前的地震记录。 震积岩发育在上寒武统崮山组中部。 沉积物变形构造主要有地裂缝、 地震楔、 砂脉、 砂层液化、 包卷层理等。 这些特征沉积构造是地震波作用于潮上—潮间带沉积环境中软沉积物而形成的。 在古沙垄顶部的内碎屑微晶灰岩中有沉积物液化产生扰动层理和流体逃逸形成的砂脉。 显微镜下可见沉积物液化流动显微构造和结构, 如微褶皱、 纹层断错、 液化颗粒等。 水面下在生物礁或沙垄斜坡局部可见重力流形成的包卷层理。 包卷层理核部为浅黄色和紫红色竹叶状灰岩, 中部为浅灰色破碎流化微晶灰岩碎屑, 外部为浅色和紫色内碎屑微晶灰岩。 崮山组的岩性组合和这些软沉积物变形构造表明, 崮山期柳江盆地构造活动剧烈, 沉积环境水深反复变化、 逐步变浅, 伴有地震发生, 估计产生震积岩的古地震的强度为7级左右。 该古地震事件的确定有益于研究地震活动和构造活动规律。

A Strong Earthquake Occurred in the Early Period of Late Cambrian in the Liujiang Basin,Northern Hebei Province

A strong earthquake occurred 500 million years ago in the early period of late Cambrian was recorded by seismite in the Liujiang basin, northern Hebei province. Seismites were discovered in the middle part of Gushan formation, upper Cambrian series. The soft-sediment deformation structures (SSDS) are mainly gravity flow structure, such as ground fissure, seismic wedge, sand vein, sand liquefaction and convolute bedding. The classic SSDSs were formed by the seismic waves acting on the soft-sediments in the sedimentary environment of supralittoral-mesolittoral zone. Disturbed bedding was produced by liquefaction of sediments, and sand veins were formed by fluid escaping in intramicrite at the top of paleo-longitudinal dune. The microstructure and structure of the liquefied flow of the sediments such as micro folds, fault lines, liquefied particles etc., were observed under a microscope. Convolute bedding was formed by gravity flow in the reef or sand ridge slope underwater. The core of the convolute bedding is buff and fuchsia edgewise conglomerate, the middle part is broken liquefied french grey micrite fragment, and the outside is light colored and purple intramicrite. Such lithological association and the SSDS in the Gushan formation suggested that the Liujiang basin experienced intense activity of tectonics and the water depth of sedimentary environment changed repeatedly and became gradually shallower during Gushanian Age. It was estimated that the magnitude of the paleo-earthquake was about 7.0. The determination of the seismic event is beneficial to the study the seismic activity and the regularity of tectonic activity in Qinhuangdao.