**2. Methods**

To evaluate the possibility that the diurnal venting at the H2G site could be caused by solid earth tides, we computed the solid earth strain at the location studied in Cathles and Prinzhofer [4] using the theoretical tidal loading software SPOTL [9] and compared it to the variations in H2 concentration at 80 cm depth for sensor 6 in Figure 1b below. The theoretical earth tide strains determined with this software have often been used in the analysis of high-resolution borehole water level time-series measurements, since the tidal amplitude response and phase shift within a well or borehole are controlled by the formation permeability and specific storage [7,8].

**Figure 1.** (**a**) Comparison of changes in atmospheric pressure to observe changes in H2 concentration at 80 cm depth at the location studied by Cathles and Prinzhofer [4] from the 13th to the 16th of August 2018. (**b**) Comparison of changes in solid earth tidal strain at the same site over the same period.
