Abstract:Precursory and coseismic anomalies of fault gas often appear prior to and following an earthquake.Following the Minxian-Zhangxian MS6.6 earthquake,an abnormal phenomenon occurred in Shendu,Minxian,and China,in which a large amount of white smoke was observed in a cellar.Physical evidence obtained recently demonstrates that deep fluid has a significant influence on earthquake gestation and that earthquakes involve a large number of chemical processes and may not be results of purely mechanical or physical processes.These deep fluids,which migrate to the ground surface,originate from the dehydration of hydrous minerals and contain a substantial amount of information about the crust and mantle.Thus,gases present in the soil offer the same information.Convenient,low-cost methods of measuring soil gases such as Hg and Rn that have been developed for wide application to active fault detection have also proven to be highly efficient for hazard assessment.The anomaly site is located in the southeastern Gansu area,which is at the northeast edge of the Tibetan Plateau in the middle of the North-South-trending seismic belt.Because this belt contains a group of active faults that produce intense tectonic activity in its boundary and interior,it is often used to study seismic hazards and the background field of southeastern Gansu.In this study,Rn and Hg soil gases collected near a cellar in Shendu,Minxian,following the Minxian-Zhangxian MS6.6 earthquake were tested on site.The results were combined with those of test samples examined by gas chromatography and a biological microscope to analyze the source,storage,transport,and physical and chemical mechanisms of the underground gases.The results reveal that following the earthquake,Rn and Hg values,in addition to those of CO2 and He,were higher;therefore,the deep gases migrated upward along ruptures and fractures during the earthquake.In addition,white smoke observed in the cellar and examined by a biological microscope was found to contain a large amount of sporopollen.We conclude that changes in the cellar condition led to the rapid reproduction and maturation of certain fungi growing on rotting wood at the top of cellar,and the resultant the white sporopollen floated in the escaping deep gas to resemble white smoke.Thus,the white smoke was related to the underground fault gas escaping from the deep strata.Therefore,because this cellar is located near a fault,a large amount of fault gases were released after the earthquake.The line between the epicenter and the anomaly site is the same as the long axis of intensity distribution,which indicates that the site is on the seismic fault.Therefore,it appears that this region is sensitive to fault activities and can thus be used as a long-term fault-gas monitoring site to study the aftershock sequence,fault activities,and earthquake prediction trend of southeastern Gansu.This method can lead to earlier warnings of large earthquakes,which may save thousands of lives.
