Hydrochemical changes in thermal waters related to seismic activity: The case of the 2020–2021 seismic sequence in Granada (S Spain)

Abstract

Between the end of December 2020 and August 2021, 3110 earthquakes were recorded in the Granada Depression in southern Spain, with 353 of them being felt by the population. The highest concentration of earthquakes occurred between January 23rd and February 23rd, 2021, during which five out of six of the highest magnitude earthquakes (greater than 4.1 Mw) were recorded. This shallow seismicity is associated with NW-SE normal active faults, specifically the Santa Fe fault. Hydrochemical sampling was carried out at two deep, upwelling, and thermal boreholes located near the epicentral zone between January 27th, 2021 and July 18th, 2022, to analyze the response of a set of hydrochemical parameters during and after the earthquakes. The methodology employed different statistical tools, including Principal Components Analysis (PCA), PHREEQC modeling as well as Piper and Langelier-Ludwig diagrams, to determine the hydrochemical evolution during the seismic sequence and the parameter variations in relation to the number and magnitude of the earthquakes. The results showed significant variations in almost all of the parameters analyzed, with greater variations in SO4, Cl, Fe, Ca, Zn, and Al. During the days with the highest number of earthquakes of the highest magnitude, there was a rapid increase in Cl, SO4, SiO2, and Mg, while F, T, EC, and pH decreased. Peaks in SO4 seemed to anticipate earthquakes of greater magnitude that could be related to the preseismic dilatation which allows for an inflow of deep sulphated source water into the aquifer system. PCA and hydrochemical diagrams showed appreciable differences between the two boreholes, which could be related to the sampling distance from the epicentral zone. The results suggest the need for a hydrochemical monitoring network in the region, which has the highest seismic hazard in Spain.