GNGTS Sessione Amatrice

Transcription

1 Mapping of surface faulting and geochemical monitoring of groundwater following the August 24, 2016 Amatrice earthquake: preliminary results and work in progress G. Binda, F. Brunamonte, M.F. Ferrario, C. Frigerio, F. Livio, R. Gambillara, A.M. Michetti, A. Pozzi, S. Terrana Dipartimento di Scienza e Alta Tecnologia, Università degli Studi dell Insubria, Como, Italy Introduction. On August 24, 2016, a Mw 6.0 earthquake hit central Italy between Norcia and Amatrice, some tens of kilometers NW of L Aquila, struck by a slightly larger earthquake in 2009 (Mw 6.3). The epicenter of the 2016 normal faulting event was located near the village of Accumoli at the border of Umbria, Marche and Latium regions. One hour later, a strong Mw 5.3 aftershock followed, with epicenter close to Norcia. This region hosts several Quaternary normal faults included in the ITHACA database, that is, the inventory of capable faults in Italy, managed by ISPRA ( According to preliminary seismological, geodetic and geologic data, the seismogenic structure ruptured across a relay zone between two major NNW trending normal faults: the Vettore and Laga faults. Total rupture length is ca km and rupture width ca km. The Insubria University group of Environmental Geology and Chemistry conducted analysis of coseismic environmental effects since August 25, Fieldwork is presently in progress to assess the post-seismic evolution of faulting (i.e., afterslip) and trends in spring hydrogeochemistry. Some preliminary results are described in the following, focusing on specific issues which might be relevant for seismic hazards assessment and the vulnerability of the physical environment in the Central Apennines geological and hydrogeological setting. Surface faulting: a new case study for a long-debated issue. We started surveying the day after the mainshock, and our observations were collected quite continuously for almost 15 days, and work is still in progress, including data on after-slip on mapped ground ruptures. The fieldwork is being conducted in collaboration with ISPRA, INGV, University of Leeds, Birkbeck University of London, CNR Firenze, University of Camerino (Central Italy Earthquake WG, 2016; EMERGEO WG, 2016). Here we present some preliminary results regarding the Vettore fault (i.e., Cordone del Vettore ). The western slope of Mt. Vettore (Fig. 1) is the morphologic expression of a NNWtrending, SW dipping major normal fault segment (total length 30 km), cutting through pelagic carbonates of the Umbria-Sabina meso-cenozoic succession with a total stratigraphic offset of ca m. A set of ground ruptures was observed on the E slope of Mt. Vettoretto. These ruptures, showing normal displacement (slip vector ca. N240 ) with minor left-lateral component, strike ca. N150, and commonly show en-echelon right-stepping. Along the Cordone del Vettore we record an average strike of N134 and the slip vector is ca. N230. The fractures generally affect colluvium and soil close to the bedrock fault plane or, less frequently, at a distance of a few meters. In the first days, the observed throw ranged from 2 to 25 cm, while heave reached max. 10 cm. These ruptures run continuously, crosscutting the road SP34, up to the end of Mt. Vettoretto slope, where the ruptures bend slightly downslope, for a total distance of at least 1.7 km. In the uphill sectors, ground ruptures were found along the fault scarp, for a length of about 2.4 km. The end-to-end length of this surface rupture zone is thus, at least, 4.1 km. Repeated field surveys have clearly pointed out a post-seismic evolution of the rupture, whose offset is continuing to increase with time. The phenomenon can be clearly observed where the fault crosses the SP34 road near Forca di Presta. The assessed primary surface faulting along the Vettore Fault is significant to earthquake geology and paleoseismology, and therefore to seismic hazard assessment, based on geological investigation. This earthquake clearly confirms that the threshold for surface faulting during extensional events in the Apennines is between Mw 5.5 and 6.0. The comparison between field mapping, InSAR remote sensing analyses, and instrumental 13

2 Fig. 1 Trace of continuous surface faulting along Mt. Vettore slope; the arrow shows intersection with the SP34 road near Forca di Presta; note that the fault rupture at this site downthrows the uphill side of the road. seismology investigations triggered some discussion whether the observed ground breaks were generated by tectonics or, conversely, landslides or deep seated mass movements (e.g., Gruppo di Lavoro INGV, 2016). This reminds us a long-debated issue, following the September 26, 1997, Colfiorito earthquake, a very similar seismic event in terms of both source parameters and structural framework (Cello et al., 1998; Mildon et al., 2016). Similarly to what happened for the Amatrice event, surface ruptures associated with the Colfiorito earthquake showed coseismic surface slip occurs along pre-existing Quaternary faults responsible for the recent tectonic evolution of the area. Along the reactivated fault segments, surface offset and slip vectors were remarkably constant over tens to several hundreds of metres, while the thickness of slope deposits varies from a few centimetres to tens of metres. All these indicate that the observed ground displacement was related to fault slip rather than ground failure due to shaking. It was also important to note that coseismic displacement of 4 to 8 cm over lengths of few kilometres was exactly what one should have expect in average for Mw = 5.7 to 6.0 earthquakes, based on the empirical relations (e.g., Wells and Coppersmith, 1994). Therefore, following the Amatrice earthquake, we believe it is useful to quote what was written after the Colfiorito event: the discussion on the amount of tectonic versus gravity component in the surface displacement in similar conditions is just useless [ ]. The reactivation of faults already mapped as having potential for coseismic surface displacement, because of the close resemblance with bedrock fault scarps and slickensides ruptured during the Fucino, 1915 [ ], and the Irpinia, 1980 [ ] earthquakes is the demonstration that these faults positively fit the definition of capable faults [ ]. This is what is needed in order to define geological criteria 14

3 Fig. 2 a) Coseismic rupture of the September 26, 1997, M6 Colfiorito earthquake on the Costa limestone fault scarp (Cello et al., 1998); b) Vettore fault rupture in the August 24, 2016, earthquake; c) Coseismic rupture corresponding to the fault plane exposed in the road cut at La Pintura site, Colfiorito basin (Cello et al., 1998); d) Coseismic rupture corresponding to the fault plane exposed in the road cut along the Vettore fault near Forca di Presta. for mapping surface faulting potential along tectonic structures in Italy, a basic complement to earthquake hazard reduction programmes [ ]. (Cello et al., 1998). The Colfiorito surface ruptures, and now the Amatrice earthquake, definitely prove that the so-called nastri di faglia (Segre, 1950) are themselves evidence of very recent strong paleoseismicity (in contrast to what was concluded by some authors; i.e., Bosi et al., 1993; Fubelli et al., 2009) and can be safely used for capable fault mapping and seismic source characterisation. The evidence discussed in this note also stresses that surface faulting should be regarded as a typical phenomenon for > IX (in the MM, MSK, MCS, and ESI 2007 scale; e.g., Serva et al., 2016). Hydrochemical relationships with the earthquake. Changes in hydrogeochemistry before and after earthquakes have been linked to temporal changes of permeability structure, fluid flow, and/or fluid-rock interaction (Claesson et al., 2004). In collaboration with University of Nevada at Reno, we sampled the Peschiera, Vicenna Riara and Santa Susanna springs near Rieti; the Uccelletto spring (SN sampling point, Nerea S.p.A. drinking water facility; Fig. 3) 15

4 Fig. 3 Sampling localities for hydrochemical analyses in the Vettore Fault vicinity and in the Rieti basin. at Castelsantangelo sul Nera; and three minor springs near Forca di Presta along the Vettore fault (FP01, FP02 and FP03 sampling points). For most of these springs we will compare preearthquake and post-earthquake chemical and isotope geochemistry features. Analyses for collected water samples include in situ characterization of parameters as ph, electrical conductivity and temperature, major ions concentrations through ionic chromatography and trace elements evaluation through inductively coupled plasma- mass spectrometry (ICPMS). Some samples will be analyzed from the Reno University lab to obtain data for stable isotopes ratio (sulfur, oxygen and hydrogen). Pre-earthquake data was obtained from the Reno University for the sampling points in the Rieti basin. For the Uccelletto spring in Valnerina, these data will be obtained through the analysis of water samples from the stock of drinking water bottles, thanks to the collaboration of Nerea S.p.A. Bottled water remains very stable regarding physico-chemical parameters, and permit to observe data before the earthquake (see for instance, the case of the Kobe earthquake; Tsunogai and Wakita, 1995). We have no pre-earthquake data for the two minor springs in Forca di Presta, and we analyze these springs to observe the post-earthquake effect through time on local near-fault aquifers with small hydrogeological basin. First results for the Rieti basins samples shows a decreasing trend, especially for the Peschiera spring, which fed the Roma aqueduct. We observe the reduction of electrical conductivity after the earthquake, and the decrease of some transition metals (Fe, Ni, Cr, Cu) from the beginning of September. This trend must be obviously confirmed based on the results of ongoing analyses. Acknowledgements. We are grateful to Dr P. Mariani, Nerea S.p.A., for the kind availability of water samples from the Uccelletto Spring. References Bosi, C., Galadini, F., Messina, P.,1993: Neotectonic significance of bedrock fault scarps: case studies from the LazioAbruzzi Apennines (Central Italy). Z. Geomorph., Suppl.- Bd. 94, pp

5 Cello G., Deiana G., Mangano P., Mazzoli S., Tondi E., Ferreli L., Maschio L., Michetti A.M., Serva L. & Vittori E.; 1998: Evidence for surface faulting during the september 26, 1997, Colfiorito (Central Italy) earthquakes. Journal of Earthquake Engineering, 2, pp. 1-22, Imperial College Press, U.K. Central Italy Earthquake Working Group, 2016: The August 24, 2016, Amatrice earthquake (Mw 6.0): field evidence of on-fault effects. Preliminary report, 31p. ; Claesson L., Skelton A., Graham C., Dietl C., Mörth M., Torssander P., Kockum I.; 2004: Hydrogeochemical changes before and after a major earthquake. Geology, 32, pp EMERGEO Working Group; 2016: The 24 August 2016 Amatrice Earthquake Coseismic Effects. Version of , 7p; Fubelli, G., Gori, S., Falcucci, E., Galadini, F., & Messina, P.; 2009: Geomorphic signatures of recent normal fault activity versus geological evidence of inactivity: Case studies from the central Apennines (Italy). Tectonophysics, 476, pp Gruppo di Lavoro INGV sul terremoto di Amatrice; 2016: Secondo rapporto di sintesi sul Terremoto di Amatrice Ml 6.0 del 24 Agosto 2016 (Italia Centrale). doi: /zenodo Mildon, Z. K., Roberts, G. P., Walker, J. P. F., Wedmore, L. N., & McCaffrey, K. J.; 2016: Active normal faulting during the 1997 seismic sequence in Colfiorito, Umbria: Did slip propagate to the surface?. Journal of Structural Geology, 91, pp Segre A. G.; 1950: Sulla struttura dell arco abruzzese interno. La Ricerca Scientifica, Suppl. Contr. Sc. Geol, 20, CNR, Roma, pp Serva L., Vittori E., Comerci V., Esposito E., Guerrieri L., Michetti A.M., Mohammadioun B., Mohammadioun G.C., Porfido S., Tatevossian R.E.; 2016: Earthquake Hazard and the Environmental Seismic Intensity (ESI) Scale. Pure and Applied Geophysics (PAAG), 173, pp , doi: /s Tsunogai U. and Wakita, H.; 1995: Precursory chemical changes in ground water: Kobe earthquake, Japan. Science, 269, pp Wells, D. L. and Coppersmith, K. J.; 1994: New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement. Bulletin of the seismological Society of America, 84, pp