Abstract: 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.

Key words: Paleo-earthquake, Late Cambrian, Ground fissure, Sand liquefaction, Liujiang Basin