The Quaternary Activity Characteristics Of Faults In Changliu Of Northern Hainan Island

The northern Hainan island is characterized by high-frequency earthquakes and volcanic activities,and developed active faults widely.Active faults are closely related to earthquakes and geological disasters,and they pose potential threats to urban construction and human life.Surface dislocation caused by seismic faults is the most serious displacement belt and destruction belt.To find out the activity of the faults since the Quaternary in northern Hainan island can provide geological basis for seismic risk assessment,seismic fortification of buildings across the faults and construction of free trade zone of Hainan island.In this paper,the stratigraphic sequence shelf in Changliu of northern Hainan island is established by applying geological and geomorphic investigation,high-density electrical detection,shallow artificial seismic prospecting,cross-section validation and the Optically Stimulated Luminescence?OSL?and ¹⁴C dating;and the geometric and kinematic characteristics of major active faults are systematically studied through investigating the spatial distribution characteristics,modes of movement and the Quaternary activity,and the formation mechanism of NNE trending fault is discussed through the transformation of stress field in northern Hainan island.Obtained the following understanding and results:1.The late Cenozoic strata in Changliu of northern Hainan island were determined.These include Pliocene Haikou Formation,early Pleistocene Xiuying Formation,middle Pleistocene Beihai Formation and Duowen Formation,late Pleistocene Daotang Formation and Basuo Formation,Holocene Qiongshan formation and Yandun Formation.It lays a foundation for the study of Quaternary activity of faults in the study area.2.The main faults were investigated in the field and their Quaternary activity were studied in northern Hainan island.The western segment of Maniao-Puqian Fault is composed of three near-E-W trending subparallel stepped normal faults,which had experienced two events in 31¹6 ka B.P.:an earlier N-/S-dipping normal faulting with a 3.3 m cumulative vertical displacement,followed by an intensive S-dipping normal faulting.Qionghua-Liantang Fault consists of two normal faults with opposite inclinations,F_6-1-1 is a normal fault with an inclination to southwest,F_6-2-2 is a normal fault with an inclination to northeast,and the latest event is no later than 98.9 ka B.P.3.The NNE trending Laocheng-Xixiu Fault was found in northern Hainan island,and the activity time and the formation mechanism of the fault were studied.This fault was composed of four NNE trending subparallel stepped normal faults,and there is an echelon form reverse dip fault locally.Among them,F_15–25–2 was acting in8.90⁶.45 ka and F_15–4 was acting in 6.70⁵.41 ka B.P.Also,the tapering stratigraphic displacement over time indicates that the fault that was formed in Pliocene at least accompanied multiple activities.The formation of NNE trending fault was derived from a tectonic field in northern Hainan island.Accordingly,the Holocene activity of the Laocheng-Xixiu Fault indicates that Neocathaysian structural system stress field may still be working nowadays.