Hydrogeological study of the effects of 2016-2017 seismic sequence on the Paccar's d'Arquata spring (Central Italy)

Since August 2016 Central Italy experienced a long seismic sequence including nine Mw 5.0-6.5 events. The strongest shock (Mw 6.5) occurred on 2016 October 30th about 5 km NNE of Norcia Town, 9 km below the surface, as a result of upper-crust normal faulting on Mt. Vettore-Mt. Bove fault system, in the Sibillini mountains. This affected the dynamic of groundwater systems of the area, both east and west of the Sibillini Mountains. The longest and most reliable series of daily discharge measurements are available for the Pescara d'Arquata spring, located south-east of Mount Vettore, starting from 1985. In the short term, the 2016 seismic sequence caused a discharge increase, lasting from October 2016 to February 2017. In this period the surplus outflow (2.33x106m3) has been about 30% of the average annual amount of groundwater flowed before the seismic sequence. Later, the discharge series analysis showed that in 2017 the minimum discharge at the end of the depletion period has been about 25 L/s aslow as in 2007 only. In order to determine whether this very low discharge could be linked to climatic conditions similar to those which caused low discharges in other years, the SPI was calculated on the Arquata del Tronto rainfall data in 1985-2018 interval. It was foundthat in 2017 a discharge lower than 50 L/s corresponded to an SPI value between -1 and -1.5 (moderately dry). On the contrary, similarly low discharges recorded in other years were associated with SPI values closeto or lower than -2 (extremely dry). Recession periods in the 2003 − 2018 interval were analyzed and found tofit the Maillet equation. It was observed that the average Maillet depletion coefficient α between 2003 and 2016 has been 6.35x10−3 d−1. After the seismic sequence, the α coefficient has increased, with an average value of 1.27x10−2 d−1.Post-seismic tracer test showed that the connection of the spring to the Mergani sinkhole, located in the Pian Grande area (about 1300 m asl) wasno longer found. The δ18O content recorded after the seismicsequence is more depleted than it was before the earthquake.Data shows both short and long term effects.Short term effect is mainly the surplus outflow between September 2016 and February 2017. The long term effects are a likely permanent change of the aquifer geometry and/or the recharge area which may be occurred and this factseems to be supported by the δ18O variation, suggesting an increase of the mean apparent isotopic elevation. This could be linkedto the fact that a portion of Pian Grande does no longer contribute tothe spring recharge. The variation of the Maillet α coefficientsseems to confirm this hypothesis.