THE CHARACTERISTICS AND MECHANISM OF FLUID ANOMALIES IN THE DAZHAI OBSERVATION WELL OF PU'ER,YUNNAN PROVINCE BEFORE THE M5.9 MOJIANG EARTHQUAKE ON SEPTEMBER 8,2018
The precursors before earthquakes are very useful to earthquake prediction,and fluid anomalies before earthquakes are very important to precursory observations.This paper reviews the characteristics of hydrochemical ions and well-aquifer permeability anomalies of the Dazhai observation Well in Pu'er,which is in the Yunnan-Southwestern region of China,for all M≥ 5.5 earthquakes since 2004.We find that both the chemical ions and physical parameters before the Mojiang M5.9 earthquake exhibited the largest magnitude of changes since observation,and the abnormal state was much stronger than that of previous historical earthquakes,but the magnitude of the earthquake was below 6.About 1.5-2a before the M5.9 Mojiang earthquake,the composition of hydrogen and oxygen isotopes of the water samples in the Dazhai observation Well showed a significant deviation,accompanied by a continuously increasing concentration of fluoride ions from sources at deeper depths.This might suggest that the deep material in the earthquake source area began to be active.At the same time,starting one year before the earthquake,the phase lag of the water level in the wellhole changed from negative to positive,indicating that the source and pathway of well water recharge have been changed.In addition,around half a year before the earthquake,the continuously observed water chemical ions at shallow depths in the wellhole began to show a dramatic change.Moreover,macroscopic anomalies of hot spring water volume increased sharply before the earthquake,showing a remarkable evolution process from deep to shallow,from background to short-term,and from micro anomalies to macro anomalies before the earthquake.To investigate the causes and mechanisms of this phenomenon,we attempt to discuss the abnormal evolution process before the M5.9 Mojiang earthquake from the aspects of regional deep material activity and regional stress level.The abnormal concentration of the hydrochemical ions and the change of aquifer permeability observed continuously at the Dazhai observation well before the M5.9 Mojiang earthquake were caused by the continuous increase in shear stress in the region,which caused the aquifer to be compressed,resulting in a vertical fluid recharge and ultimately the alternation and mixing of different aquifer water bodies.In addition to being controlled by the continuous increase in regional vertical shear stress,the abnormal formation process was also accompanied by the intense activity of deep-sourced chemical elements such as helium isotope and fluoride ion.The abnormal evolution process showed a remarkably coupled process of migration from deep to shallow,which may be the reason why the shallow ion anomaly before the M5.9 Mojiang earthquake was the most significant among all the observed cases.Therefore,the evolution process of fluid activity starting from the deep and continuously transmitting to the surface with the accumulation of regional stress is essential to the abnormal evolution of the hydrological phenomenon before the M5.9 Mojiang earthquake.The regional stress and the process of deep material activity are the biggest differences between the M5.9 Mojiang earthquake and other historical earthquake cases in the study area,which will be the two main factors to be considered when similar ion changes occur again in the future.Our study provides insight into a comprehensive understanding of the predictive significance of underground fluid anomalies in the Dazhai well and the coupled evolution process of deep-shallow fluid anomalies before the earthquake.
characteristics of fluid anomaliesdeep-shallow couplingDazhai observation well of Pu'erM5.9 Mojiang earthquake
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维普
