Groundwater electrical conductivity and soil radon gas monitoring for earthquake precursory studies in Koyna, India

• Published in: Applied geochemistry Vol. 26; no. 5; pp. 731 - 737
• Main Authors: Reddy, D.V., Nagabhushanam, P.
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier Ltd 01.05.2011; Elsevier
• Subjects: Earth sciences; Earth, ocean, space; Earthquake prediction; earthquakes; Electrical conductivity; Electrical resistivity; Engineering and environment geology. Geothermics; Exact sciences and technology; freshwater; Geochemistry; Groundwater; mixing; Monitoring; Pollution, environment geology; porosity; radon; Resistivity; rocks; Signatures; soil; Soil (material); water table; India; Asia; Maharashtra India; Koyna; radon; strength; mixing; porosity; precursors; ground water; water table; concentration; earthquakes; anomalies; faults; boreholes; aquifers; pore pressure; electrical conductivity; soils; fractures
• ISSN: 0883-2927; 1872-9134
• DOI: 10.1016/j.apgeochem.2011.01.031

► It is the first hydrochemical precursory study in the Koyna region, India. ► Discrete conductivity measurements indicated progressive increase for 4years. ► Strong precursory EC change observed 40h before the M 5.1 earthquake. ► Precursory increase of soil Rn gas 20days earlier than earthquakes M 4.7 & 5.1. ► On-line monitoring of these parameters may help in earthquake forecast. Hourly monitoring of electrical conductivity (EC) of groundwater along with groundwater levels in the 210m deep boreholes (specially drilled for pore pressure/earthquake studies) and soil Rn gas at 60cm below ground level in real time, in the Koyna–Warna region (characterized by basaltic rocks, >1500m thick, and dotted with several sets of fault systems), western India, provided strong precursory signatures in response to two earthquakes (M 4.7 on 14/11/09, and M 5.1 on 12/12/09) that occurred in the study region. The EC measured in Govare well water showed precursory perturbations about 40h prior to the M 5.1 earthquake and continued further for about 20h after the earthquake. In response to the M 4.7 earthquake, there were EC perturbations 8days after the earthquake. In another well (Koyna) which is located 4km north of Govare well, no precursory signatures were found for the M 4.7 earthquake, while for M 5.1 earthquake, post-seismic precursors were found 18days after the earthquake. Increased porosity and reduced pressure head accompanied by mixing of a freshwater component from the top zone due to earthquakes are the suggested mechanisms responsible for the observed anomalies in EC. Another parameter, soil Rn gas showed relatively proportional strength signals corresponding to these two earthquakes. In both the cases, the pre-seismic increase in Rn concentration started about 20days in advance. The co-seismic drop in Rn levels was less by 30% from its peak value for the M 4.7 earthquake and 50% for the M 5.1 earthquake. The Rn anomalies are attributed to the opening and closing of micro-fractures before and during the earthquake. On line monitoring of these two parameters may be useful to check the entire chemistry change due to earthquake which may help to forecast impending earthquakes.