Groundwater flow changes in response to extensional earthquakes: a case study from the 2016-17 seismic sequence in Central Italy

Changes in groundwater flow in response to strong earthquakes are widely described in many tectonic environments. For example, a post-seismic discharge variation is often attributed to an increase of bulk permeability due to co-seismic fracturing and/or to a change in the role of faults in acting as conduits/barrier to groundwater flow.

We take as an example the fractured aquifer of the Mts. Sibillini carbonate massif, in Central Italy, which were affected by a strong and prolonged extensional seismic sequence in 2016-17. The sequence was characterized by an M=6.5 event (mainshock), an M=6 event, an M=5.9 event, up to 60 M>4 events and several M>5 earthquakes. The strongest events caused rupturing of the topographic surface for a cumulative length in the order of 30 km and an important portion of aftershocks occurred at depths where groundwater is stored.

As a response to the seismic sequence, the main NNW-directed groundwater flow was diverted to the west and a discharge deficit was observed at the foot-wall of the activated fault system with a relevant discharge increase, accompanied by geochemical variations, at the fault system hanging-wall.

By integrating geo-structural reconstructions, seismological and ground deformation data, artificial tracer tests results and a 4-years discharge and geochemical monitoring campaign data, we show that the observed groundwater variations are due to a combination of permeability increase along the activated fault systems and hydraulic conductivity increase of the hanging-wall block due to fracturing, extension and subsidence, which determined a fast aquifers emptying. Seismicity temporarily triggered a change of the pre-existing predominant along-faults-strike NNW-SSE oriented regional flow to a west-directed flow, perpendicular to faults strike. We discuss the position of the aquifer with respect to the activated faults and how this affected the observed phenomena.

 

REFERENCE

Cambi, C., Mirabella, F., Petitta, M., Banzato, F., Beddini, G., Cardellini, C., ... & Valigi, D. (2022). Reaction of the carbonate Sibillini Mountains Basal aquifer (Central Italy) to the extensional 2016–2017 seismic sequence. Scientific Reports, 12(1), 1-13. DOI: 10.1038/s41598-022-26681-2