关键词: 古地震, 岩溶洞穴, 地震预报, 工程建筑

Abstract: Karst caves are an ideal place for palaeoseismic research.This paper introduces at first the basic principle and method for the research of palaeosecsmic traces in karst caves.Under normal condition,stalactite,drips and stalagmite in cave are arranged as a system following a common vertical line.During earthquake they are dislocated,and the vertical median line among them is displaced;the stalagmite is deformed(e.g. broken off),while after the earthquake a new stalagmite may grow on the remaining base of old stalagmite,on the fallen stalagmite or on new place,and then a new system of stalactite,drip and stalagmite will be aligned along a new vertical line again.With the alternation of earthquake and non-earthquake processes,the time-space correlation of the varied vertical lines may constitute"a natural pendulum",which swing successively,just like a"seismograph"recording permanently palaeoseismic events occurred near the cave in different times.The investigation of the time-space correlation among these cave deposits may provide valuable information about palaeoseismic events in that region. Furthermore,the main characteristics of palaeoseismic traces and the indicators of palaeoseismic events in karst caves are demonstrated in this paper.When an earthquake occurs,the landscape of the caves may be destroyed and deformed.The distribution of the traces of paleoseismic events in the cave shows statistically a preferred orientation.During earthquake the vertical median line between the stalactite and stalagmite may be displaced or deformed laterally,in contrast to the normal phenomenon in karsts cave.The vertical projection of the collapsed roof on the floor of the cave is not coincident with the extent of colluvial deposits,which are displaced laterally.The distribution of slightly sorted colluvial deposits on the floor of the branch caves shows a preferred orientation.These earthquake-produced characteristics differ significantly from those of gravity collapse deposits in the cave. This study shows that the indicators of paleoseismic events in karst cave can be enumerated as follows:the dislocation of karsts cave and/or its corridor along vertical,horizontal or oblique fault planes;displacement of rock mass,existence of earthquake fault and earthquake groove(line or fracture);rock splitting and lateral dislocation or pull-apart of stone column;lateral displacement of vertical median axis between stalactite and stalagmite;deformation of stalagmite(dislocation,riving,dumping and tilting)and earthquake-induced deformation of cave deposits. It is worth noting that the investigation of paleoseismic traces in karst caves should be carried out at the beginning of the exploration of the karst caves,so that geological phenomena generated by aseismic factors in the cave can be distinguished.

Key words: paleoseismology, karsts cave, earthquake prediction, engineering construction