Seismic-tsunami sequence and its geological features of Mesoproterozoic Wumishan Formation in Fangshan Global Geopark, Beijing, China: a case study on Yesanpo scenic district.

Yesanpo scenic district was distributed in the eastside of the south branch of the central axial fault in the south Yanshan-Liaoning aulacogen during Mesoproterozoic (1800~1200 Ma B.P.). The seismic sequence (seismites) of the 2nd member of Wumishan Formation in Yesanpo, which is dominantly made up of dolomite, can be subdivided into four units from bottom to top: A. step faults and autoclastic breccia deformation unit; B. microfold-microfault deformation unit; C. liquefied homogenization deformation unit; D. liquefied convolute lamination deformation unit. They represent respectively the deformation features of the consolidated, sub-consolidated and weakly consolidated submarine deposits within the depth of 10 meters beneath seafloor. All of these four deformation units had undergone the effect of foreshock and principal phase of earthquake (P-wave and S-wave) in various degrees. The tsunami sequence ( tsunamites ) can be subdivided into three units from bottom to top: E. tsunamic hummocky layer; F. groove-mound structure layer; G. homogeneous particle size layer. Tsunami always happens later tens of seconds than principal phase of earthquake and it makes seafloor instantaneous uplift in large amplitude, and then suddenly subsides, which causes the inrush of outer seawater and induced the tsunami associated with the formation of hummocky bed. The F and G units are formed by the event of oscillating currents and gravitation deposit respectively in the after shock stage. The central axial fault in the study area (paleo-Zijingguan fault), as a submarine vertical fault zone, stimulates and controls the seismic and tsunami as well as the deposit processes in the 2nd member of Wumishan Formation. The authors conclude that the epicenter of this submarine earthquake was located at Baishishan in the west border of central axial fault zone with magnitude of the earthquake ranging 7.0~7.5 in Richter scale and shallow seismic source.