HydroQuakes, central Apennines, Italy: Towards a hydrogeochemical monitoring network for seismic precursors and the hydro-seismo-sensitivity of boron

Hydrogeochemical changes before Mw >= ~5.0 earthquakes are hoped to be applicable as seismic precursors. It is now necessary to pass from single experiences to structured networks, systematically monitoring large seismic areas for long periods. Our aim is to provide a pilot methodological experience for how such networks could be realized and the types of results they could deliver. We present the method and some initial results to document the method's applicability. In 2017, we started the HydroQuakes project in seismically active central Italy, with the Sulmona and Matese areas selected as the two monitoring nodes. Each node includes 5-6 monitored springs, and for each spring, we performed laboratory analyses of major and trace elements and some isotopes. We installed in situ probes for continuous monitoring. Between December 2017 and January 2020, the most energetic earthquakes were three events with Mw of 3.9, 4.1, and 4.4. They were not preceded by significant hydrogeochemical anomalies, except for the Mw 4.1 event. Despite its low Mw, this earthquake occurred over a large normal fault at a great depth in the crust (~17 km) and was preceded by weak but detectable hydrogeochemical anomalies of boron and delta11B that were similar but milder than the hydrogeochemical anomalies before the 2016 Mw 6.0 earthquake in the central Apennines. This and previous evidence show the B and delta11B hydro-seismo-sensitivity and indicates that pre-seismic hydrogeochemical anomalies could arise mostly before Mw >= ~5.0 earthquakes, with weak exceptions occurring in specific conditions such as long and deep faults.