Hydrological response to multiple large distant earthquakes in the Mile well, China

Groundwater level changes of up to 0.716 m and temperature changes of up to 0.0708°C have been observed in the Mile well, Yunnan Province, China, in response to earthquakes with a seismic energy density exceeding 1 × 10^-3 Jṡm^-3 from 2004 to 2012. Sustained water level changes, however, only occurred in earthquakes with a surface wave magnitude M_s ≥ 8.0. Groundwater temperature and permeability changes also only occurred following such large earthquakes. This indicates that larger earthquakes (with M_s ≥ 8.0) are more effective at triggering hydrologic responses than smaller earthquakes. The amplitudes of changes in groundwater level and groundwater temperature were positively correlated, indicating that temperature changes are possibly caused by advection-diffusion and mixing following groundwater inflows into the well. We used a tidal response as a proxy to investigate aquifer properties, showing that the changes in groundwater level and temperature correlate well with phase shift and tidal factor. Based on the aquifer parameters calculated from the tidal response using the M₂ wave, we found that amplitudes of inferred changes in transmissivity and storage coefficient have a positive relationship with the groundwater level and temperature changes. The scale of transmissivity change was much larger than that of the storage coefficient. We interpret the co-seismic changes in groundwater level and temperature in the Mile well to be due to changes of permeability in the aquifer due to unclogging of fractures induced by seismic waves.