Modeling of the gaseous and dissolved CO2 emissions in Central Nepal subject to a major earthquake

– Internship mentors: Frédéric Girault (IPGP), Jean Vandemeulebrouck (ISTerre), Frédéric Perrier (IPGP)

On April 25, 2015 Central Nepal was severely shaken by the deadly Mw7.8 Gorkha earthquake. In the Main Central Thrust shear zone of Central Nepal, located along the northern border of the seismic surface rupture, several hot springs and gaseous emissions (Diffuse Degassing Structures) of carbon dioxide (CO2) were documented before the event. These hot springs and CO2 emissions were strongly affected by the earthquake. In particular, in the upper Trisuli Valley located 60 km north to Kathmandu and at epicentral distance of 50–60 km, a significant increase of CO2 emission, associated with increases of water temperature and dissolved CO2 content, were observed after the earthquake. These manifestations persisted more than 2.5 years after the earthquake (Girault et al., Nature Communications, 2018).
The M2 internship will focus on the modeling of the CO2 emissions in the Syabru-Bensi Hydrothermal System, Central Nepal. The first task will be to define the geometry of the system, based on available aerial photographs (taken from a drone), geological map, geophysical soundings (ERT, GPR and MT) and geochemical mapping (maps of CO2 flux). The second task will be devoted to the assessment and modeling of different CO2 transport scenarios, compatible with available CO2 flux data on the ground, dissolved inorganic carbon content in hot springs, soil-air and springs temperature records. Once a steady-state model will be available, the third step will be to study different hypotheses accounting for the observed earthquake-induced changes, such as for example transient permeability changes, CO2 source variations and physical modifications of the shallow CO2 reservoir by seismic waves.
The use of numerical codes, available at ISTerre Chambéry (HYDROTHERM and TOUGH2-3) will be strongly encouraged. TOUGH 2 and 3 are numerical codes for simulating non-isothermal flows of multi-component (here water and CO2) multi-phase fluids in 2-D or 3-D fractured and porous media. This model-based physical assessment would provide essential materials for a future PhD thesis. This work will also give important information about the necessary critical parameters in order to constrain an observation potentially related to earthquake-induced changes. The tools developed during this M2 internship will also be applied to other sites in Nepal, where earthquake-induced changes in CO2 emissions were also observed, and possibly to other sites in other contexts such as in the Cascades (CA, USA), or to other published pre-, co-, and post- seismic changes in CO2 emissions.