GNGTS 2016 - Atti del 35° Convegno Nazionale

GNGTS 2016 S essione 1.2 237 We use daily observations from permanent GPS stations in south-central Apennines, most of which belong to the Rete Integrata Nazionale GPS network (RING; Avallone et al. , 2010). Time series of vertical and horizontal components (in a Tyrrhenian reference frame and rotated in N45E and N135E directions, respectively perpendicular and parallel to the long-axis of the Apennines) are shown in Fig. 1 progressively ordered from the Tyrrhenian (to the west) to the Adriatic (to the east) coasts. The horizontal components, in particular in the N45E direction, show that the interseismic ≈3 mm/yr extensional opening in the axial part of the Apennines (D’Agostino, 2014), is modulated by annual and multi-year (≈4 years) signals that modify the intensity of the tectonic signal. Both the annual and multi-year horizontal signals show consistent spatial patterns displaying a symmetrical behavior with respect to an axis running along the crest of the Apennines. In particular, two multi-year episodes (each 2.5 year-long) of inward (T1, 2006.4-2008.9) and outward (T2, 2008.95-2011.45) motion of the stations in the axial part of the Apennines have been highlighted (Figs. 1 and 2). The behavior of the vertical component, highlighted by stacking multiple GPS time series from several sites covering all the studied area, shows that a consistent annual seasonal signal exists in the vertical component (peak of uplift in July-August), but also evidences two peaks of multi-year upward motion in 2007 and 2012 (Fig. 3). The Mediterranean climatic features and their temporal variability strongly control the amount of surface water and the recharge processes of the karst aquifers in Southern Apennines (e.g., De Vita et al., 2012). We consider daily time series of rainfall data from two rain-gauges (Gioi Cilento and Senerchia, from Protezione Civile Regione Campania, Fig. 2), which, thanks to their record completeness and location, are assumed as representative of the precipitation trend in the last decade in southern Apennines. Maximum precipitations are generally concentrated in October-February showing significant inter-seasonal variability and a significant reduction in drought years 2007-2008 and 2012 (Fig. 3). The previously defined T1 and T2 intervals correspond to periods of anomalously low (T1) or high (T2) precipitations. Further evidences of inter-annual hydrological variability are also visible in the discharge time series of one of the largest karst springs in the Apennines (Fiorillo et al. , 2015) located near Caposele (Sorgente Sanità, 417 m a.s.l.; Figs. 2 and 3). The Caposele spring drains the Cervialto massif and is strictly controlled by the climate trends. The maximum discharge at Caposele usually occurs in spring, some (4-5) months after the period of maximum precipitation, whereas the minimum discharge takes place during the rainy months. The smooth shape of the hydrograph and the delayed response to rainfall indicate that the spring discharge is not affected by single events of rainfall, but it responds to the precipitation cumulated over several months (Fiorillo and Doglioni, 2010). The GRACE satellite provides estimations of the TWS variations. The 300-km resolution of the GRACE solution does not allow to spatially define the source of mass variability at the scale of the study area, and likely the GRACE mass solution is an average of TWS in the Apennines and in the nearby regions (Wahr et al., 1998). This kind of data therefore provides information about the temporal variability of the TWS and about the influence of long wavelength changes of the TWS on the observed GPS signal. GRACE TWS time series (Fig. 3), show that higher annual TWS values generally occur in December-February. The drought years evident in rainfall and spring discharge time series (i.e. 2007-2008 and 2012) show lower TWS peaks in winter months with respect to the long-term average. T1 and T2 intervals correspond to TWS values which are, respectively, significantly below and above the 2002-2015 average and, consistently with rainfall and spring discharge data, indicate a significant multi-year variation of surface water and groundwater storage. Discussion and conclusions. The GPS and hydrological time series reveal clear similarities (Fig. 3). GPS horizontal time series at stations close to the karst aquifers are highly correlated with the discharge time series of the Caposele spring (Fig. 3). These observations confirm the