Abstract
Due to frequent large earthquakes in the Lanping-Simao fault basin—located in China’s Yunnan Province—the Simao observation well has observed groundwater discharge, as well as Ca2+, Mg2+, and HCO3− concentrations every day between 2001–2018. Over 18 years of observations, M ≥ 5.6 earthquakes within a radius of 380 km from the well were seen to cause hydrochemical variations. In this study, we investigated CO2 release and groundwater mixing as possible causes of regional earthquake precursors, which were caused by the characteristics of the regional structure, lithology, water-rock reactions, and a GPS velocity field. Precursory signals due to CO2 injection are normally short-term changes that take two months. However, groundwater mixing linked to earthquakes was found to take, at the earliest, 15 months. The proportion of shallow water that contributes to mixing was found to significantly increase gradually with the stronger regional strain. These finding delineate the two mechanisms of earthquake-induced hydrochemical variations in an observation well, and would contribute to a better understanding of chemical changes before events in the Simao basin.
Highlights
Earthquake-induced geochemical processes are complex natural phenomena
We investigated CO2 release and groundwater mixing as possible causes of regional earthquake precursors, which were caused by the characteristics of the regional structure, lithology, water-rock reactions, and a GPS velocity field
We focused on longterm hydrochemical characteristics caused by two genetic mechanisms: CO2 injection and groundwater mixing
Summary
Earthquake-induced geochemical processes are complex natural phenomena. Many earthquakes occur in areas where underground fluids are inaccessible; the hydrochemical constituents are important parameters that can provide precursory information [1,2]. There are many reasons for earthquake precursors at sites both proximal and distal to an epicenter, including the increased injection of deep-trapped CO2 into aquifers, the mixing of groundwater components in response to crustal dilation, dilation due to increasing numbers and sizes of microcracks, the squeezing of gas-rich pore fluids out of the rock matrix and into aquifers, increased rock/water interactions, and permeability dominant with increasing lithostatic load [12,13,14,15,16,17,18,19,20]. It is necessary to research the repeated response before strong events to better understand chemical change mechanisms in long-term continuous hydrochemical observations. Geochemical observations are known to be sensitive to pre-seismic CO2 release and groundwater mixing [4,32]. We focused on longterm hydrochemical characteristics caused by two genetic mechanisms: CO2 injection and groundwater mixing. We attempted to decipher possible hydrochemical precursory signals in Simao basin
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