北京房山世界地质公园中元古界雾迷山组地震-海啸序列及地质特征——以野三坡园区为例

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

野三坡园区中元古界(1800～1200Ma B.P)位于燕辽裂陷槽南部中轴断层南支(古紫荆关断裂)之东盘。野三坡雾迷山组笫二段白云岩中的地震序列自下而上分为:A.阶梯状断层与震碎角砾岩变形单元;B.微褶皱-微断层变形单元;C.液化均一变形单元;D.液化卷曲变形单元。这4个变形单元分别代表海底之下深度不足10m内已固结成岩—半固结—弱固结沉积层的变形特征,都经历了前震及主震期(P波和S波)不同程度的影响。海啸系列(海啸岩)自下而上为:E.津浪丘状层单元;F.丘-槽构造层单元和G.粒序均一层单元。海啸是在主震期数十秒后发生的。海底瞬时大幅度抬升,然后突然大幅度下降,使外海海水涌入,引发海啸,形成丘状层及有关变形。余震阶段的震荡流与一次沉降事件分别形成F和G单元。本区中轴断层(古紫荆关断层)是一条海底直立的断层带,对雾迷山组二段中的地震-海啸及沉积过程均起到激发与控制作用。此次震中位于中轴断层带西缘的白石山,震级为7.0～7.5里氏级,是震源浅、裂度大的海底地震。

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 （ tsunanfites ） can be subdivided into three units from bottom to top： E. tsunamic hummocky layer; F. groove-mound struc- ture 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 tsunanfi 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.