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Hydrochemical Variations in the Pu’er Well Associated With the 5 Earthquakes (M5.9-7.2) Since 2001 to 2018, SW China

Session: New Insights Into the Preparatory Phase of Earthquakes From Tectonic, Field and Lab Experiments III

Type: Oral

Date: 4/19/2021

Presentation Time: 06:00 PM Pacific

Description: 

Long-term observations of deep-well water ion concentrations record the precursory characteristics of earthquakes. In this study, we analyzed 18 years of continuous Ca²⁺, Mg²⁺, HCO₃^-, and pH data from an observation well in the Lanping-Simao fault basin, Yunnan province, China. The ion contents and pH values of the Pu'er observation well show clear precursory responses to earthquakes of M≥ 5.9 within a radius of 400 km. Prior to five such earthquakes, all ion concentrations and pH values experienced a sudden fall; they then either recovered or remained low. These changes were controlled by two different genetic mechanisms: (1) CO₂ injection related to epicentral distance and/or (2) mixing due to crustal dilation. CO₂ injection occurred before earthquakes with an epicentral distance of > 90 km, but not before earthquakes with epicentral distances of < 80 km. Mixing due to crustal dilation was observed before all earthquakes. Groundwater chemistry was found to be controlled by two aquifers and the mixing proportion of the upper aquifer increased gradually with time, independent of epicentral distance or earthquake magnitude; the mixing proportion may have been controlled by a gradual increase in stress. The results indicate that deep CO₂ injection and water-rock-gas reactions along new interfaces before an earthquake can rapidly change the water chemistry. This study provides potential hydrochemical indicators for earthquake monitoring based on high frequency well water data from active tectonic zones.

Presenting Author: Zhi H. Zhou

Student Presenter: No

Authors