Typical earthquake-induced soft-sediment deformation structures in the Mesoproterozoic Wumishan Formation, Yongding River Valley, Beijing, China and interpreted earthquake frequency

The Mesoproterozoic Wumishan Formation, composed of dolomite is a widely distributed stratigraphic unit in the Beijing area. It was formed over a long period of time in the Yan-Liao aulacogen, a stable peritidal environment that was ideal for recording earthquakes in the form of soft-sediment deformation structures (SSDS). Numerous examples occur in the upper part of the Wumishan Formation, along the Yongding River Valley. In addition, brittle structures include intrastratal fault and seismically cracked breccias. The soft-sediment deformation structures include liquefied features (diapirs, clastic dykes, convolute bedding), compressional deformation features (accordion folds, plate-spine breccias, mound-and-sag structures), and extensional plastic features (loop-bedding). Based on the regional geological setting and previous research, movements along the main axial fault of the Yan-Liao aulacogen are considered as the triggers for earthquakes since the Early Mesoproterozoic. The number and distribution of the SSDS suggest the major earthquake frequency in the Wumishan Formation of 20 to 32 thousand years.