Hot springs disturbances induced by the 2015 Gorkha earthquake in Nepal

The Main Central Thrust zone of the Nepal Himalayas exhibits numerous hydrothermal systems characterized by hot springs and carbon dioxide (CO₂) emissions. Water has a meteoric origin while CO₂ is produced at >5 km depth by metamorphic reactions. These natural geosystems located in the Earth's largest seismogenic orogen are particularly important assets to better understand fluid-earthquake interplay.

The M_w=7.8 April 25, 2015 Gorkha earthquake hit Central Nepal over a 120-km-long segment east from the epicenter, and was followed by an intense aftershock sequence. This seismic crisis, the most important in Nepal since 1934, greatly affected the hydrothermal systems located in Central Nepal (Girault et al., 2018). Following the earthquake, spectacular outbursts of CO₂ were observed at several sites separated by more than 10 km. Other sites showed a post-seismic decrease in CO₂ emission. Hot springs disturbances were also noticed in several valleys of Central Nepal, such as mainly post-seismic changes in temperature, pH, flow rate and dissolved inorganic carbon content, and even including a complete cessation of water flow. Possible pre-seismic cooling was also observed at four hot springs in the same valley.

Here we complement these observations by an exhaustive water chemistry data-set of Nepalese hot springs to study potential chemical changes around the time of the Gorkha earthquake. Post-seismic changes in hot springs chemistry are exhibited at several locations. Also, intriguing pre-seismic chemical disturbances are observed. These observations reveal that Himalayan hydrothermal waters are highly sensitive to post- and possibly pre- seismic deformation, and that not only the pathways are affected, but also the chemical interactions. Thus, as a large earthquake is still pending in the Himalayas, Himalayan hot springs appear as promising systems to study the relationship between water chemistry and the earthquake cycle, and shed light on previously reported groundwater changes before earthquakes, for example in Japan and China.