Hydrogeochemical changes before and during the 2016 Amatrice-Norcia seismic sequence (central Italy)

Abstract

Seismic precursors are an as yet unattained frontier in earthquake studies. With the aim of making a step towards this frontier, we present a hydrogeochemical dataset associated with the 2016 Amatrice-Norcia seismic sequence (central Apennines, Italy), developed from August 24^th, with an M_w 6.0 event, and culminating on October 30^th, with an M_w 6.5 mainshock. The seismic sequence occurred during a seasonal depletion of hydrostructures, and the four strongest earthquakes (M_w ≥ 5.5) generated an abrupt uplift of the water level, recorded up to 100 km away from the mainshock area. Monitoring a set of selected springs in the central Apennines, a few hydrogeochemical anomalies were observed months before the onset of the seismic swarm, including a variation of pH values and an increase of As, V, and Fe concentrations. Cr concentrations increased immediately after the onset of the seismic sequence. On November 2016, these elements recovered to their usual low concentrations. We interpret these geochemical anomalies as reliable seismic precursors for a dilational tectonic setting.

Figure 1

Geological setting. This figure has been drawn using CorelDRAW. (a) Map of central Apennines (see location in upper right inset) showing earthquake epicentres of the 2016–2017 sequence. Seismic data (including focal mechanisms) are from the INGV database (available online at http://cnt.rm.ingv.it/) between April 1^st, 2016, and March 30^th, 2017 (M_w ≥ 1.0) (Supplementary Table S1). Active faults (all extensional) are from the Ithaca database (available online at http://www.isprambiente.gov.it/en/projects/soil-and-territory/italy-hazards-from-capable-faulting). Base digital elevation model is from the ISPRA database SINAnet (available online at http://www.sinanet.isprambiente.it/it). Location of the well (PF 60.3) and springs monitored and analyzed in this work (S1-S8 in the Sulmona Plain Test Site and San Chiodo area) are displayed with blue and green squares, respectively (Supplementary Table S2). (b) Simplified stratigraphic logs from the Gargano (GA) and Puglia 1 (P1) deep wells drilled in the Apulian-Adriatic foreland of the Apennines fold-thrust belt. See well location in upper right inset of (a). Data are from the Videpi database (available online at http://unmig.sviluppoeconomico.gov.it/videpi/pozzi/pozzi.asp; see also ref.). (c) Interpretation of the CROP-11 deep seismic profile (modified after ref.). See A-B profile track in (a). Note the location and depth of the extensional Mt. Morrone Fault (red faults) that is adjacent to the well and springs considered in this paper (Sulmona Plain Test Site, SPTS).

Figure 2

Chebotarev’s classification diagram for groundwater. This figure has been realized using Grapher 7. All groundwater analyzed in this work falls within the calcium-bicarbonate quadrant. See ref. for the creation method of Chebotarev’s diagram.

Figure 3

Time series. This figure has been realized using Grapher 7. (a) Time series (August 1^st, 2016-March 31^st, 2017) of groundwater level recorded in the PF 60.3 well (100 m deep; Fig. 1a). Piezometric data were purposely recorded for this work. Occurrence of main earthquakes (belonging to the 2016–2017 central Apennines sequence; Supplementary Table S1) is shown with vertical red bars. (b) Time series (August 1^st, 2016–March 31^st, 2017) of magnitude (M_w) for earthquakes belonging to the 2016–2017 central Apennines sequence. Seismic data are from the INGV database (available online at http://cnt.rm.ingv.it/).

Figure 4

Coefficient of variation. This figure has been realized using Grapher 7. Coefficient of variation for chemical element concentrations and physical-chemical parameters (Supplementary Tables S3) measured in the Sulmona test site springs (Supplementary Table S2) between January 1^st, 2016 and March 31^st, 2017. The coefficient of variation corresponds to the ratio between the standard deviation (σ) and the absolute value of the mean (|μ|) of results from the chemical analyses realized on the water samples collected during the considered period (Supplementary Table S3).

Figure 5

Time series. This figure has been realized using Grapher 7. Time series (January 1^st, 2016–March 31^st, 2017) of element concentration, pH (Sulmona test site springs), and earthquake magnitude. Concentration of elements is shown as box-and-whisker plots representing interquartile ranges (25^th–75^th quartiles). Each plot includes geochemical data from the Sulmona test site springs (Supplementary Tables S2 and S3). Seismic data are from the INGV database (available online at http://cnt.rm.ingv.it/). (a) Iron, Fe, concentration. (b) Chrome, Cr, concentration. (c) Vanadium, V, concentration. (d) Arsenic, As, concentration. (e) Potential of hydrogen, pH. (f) Magnitude (M_w) of earthquakes belonging to the 2016–2017 central Apennines sequence.

Figure 6

Coefficient of variation. This figure has been realized using Grapher 7. Coefficient of variation for chemical element concentrations and δ¹⁸O from the San Chiodo spring (see location, S8, in Fig. 1a) between June 1^st, 2016 and March 31^st, 2017 (Supplementary Tables S3). The coefficient of variation corresponds to the ratio between the standard deviation (σ) and the absolute value of the mean (|μ|) of results from the chemical analyses realized on the water samples collected during the considered period (Supplementary Table S3).

Figure 7

Time series. This figure has been realized using Grapher 7. Time series (June 1^st, 2016–March 31^st, 2017) of element concentrations (San Chiodo spring, S8; Supplementary Tables S2 and S3) and earthquake magnitude. Seismic data are from the INGV database (available online at http://cnt.rm.ingv.it/). (a) Iron, Fe, concentration. (b) Chrome, Cr, concentration. (c) Vanadium, V, concentration. (d) Arsenic, As, concentration. (e) Magnitude (M_w) of earthquakes belonging to the 2016–2017 central Apennines sequence.