GNGTS 2018 - 37° Convegno Nazionale

GNGTS 2018 S essione 2.3 505 The strongest seismic event ever recorded in Italy in an area with geothermal energy production was the M w 4 earthquake (2000) at Mt Amiata, with epicenter near Piancastagnaio. Relocation of the event by using an updated crustal velocity model, as well as an independent hypocentral depth estimation lead to a focal depth similar to the production level. Considering however, that historical earthquakes in the area reached magnitudes up to M5.3, it cannot be decided if this event was of anthropogenic or natural origin. A further example concerns the seismicity observed during the preparatory phase of a future CO 2 extraction from a deep well (PSS1) in the Upper Tiber Valley (UTV). For the three main events (2.9<M L <3.2) recorded in 2010 thrust fault mechanisms were calculated. These events are located in the vicinity of the active low-angle Alto Tiberina Normal Fault, thus thrust faults are geologically unexpected. As they occurred before starting the CO 2 production excludes any anthropogenic influence. In the framework of a medium-enthalpy geothermal exploitation project, seismicity and soil gas emissions have been monitored in the area of Castel Giorgio-Torre Alfina (central Italy) since 2014. Seismicity recorded by a local network at Torre Alfina before the energy production shows that the M L estimations are poorly constrained and mostly incompatible with magnitudes determined by the National Seismic Network (Braun et al. , 2018a). Such differences are due to inaccurate attenuation laws and correction factors, especially for stations at local distances. Thus, the question comes up, which magnitude should be calculated (M L , M d , M w ) and how significant is generally the use of the magnitude for the application of the ILG-traffic light. A final example concerns the upper crustal seismicity observed near the Montedoglio reservoir, situated in the UTV, and investigates potential effects of stress variations due to water level variations. Beyond seasonal oscillations - only significant long-term water level changes lead to important seismic events, especially after previous pressure levels have been exceeded (‘’stress shadow effect’’, Kaiser, 1950). Even a decrease by 14 m of the dry line, after the rupture of the dam crest, caused no significant associable earthquake. Crustal stress variations expected from the static load of the water column have been modeled and compared to the local seismicity, in order to understand the elastic and undrained response. The result suggests that the induced instantaneous stress changes are rather small at the distances where larger earthquakes occur (Braun et al. , 2018b). References Astiz, L., Dieterich, J., Frohlich, C., Hager, B., Juanes, R., Shaw, J. (2014). On the Potential for Induced Seismicity at the Cavone Oilfield: Analysis of Geological and Geophysical Data, and Geomechanical Modeling. Technical Report. Report for the Laboratorio di Monitoraggio Cavone. http://labcavone.it/documenti/32/ allegatrapporto- studiogiacimento.pdf. Braun, T., Caciagli, M., Carapezza, M., Famiani, D., Gattuso, A., Lisi, A., Marchetti, A., Mele, G., Pagliuca, N.M., Ranaldi, M., Sortino, F., Tarchini, L., Kriegerowski, M., and Cesca, S. (2018a). The seismic sequence of 30 th May - 9 th June 2016 in the geothermal site of Torre Alfina (central Italy) and related variations in soil gas emissions. J. Volcanol. Geotherm. Res. 359, pp. 21-36. 10.1016/j.jvolgeores.2018.06.005. Braun, T., Cesca, S., Kühn, D., Martirosian-Janssen,A. and Dahm., T. (2018b).Anthropogenic seismicity in Italy and its relation to tectonics: State of the art and perspectives. Anthropocene, 21, pp.80-94. 10.1016/j.ancene.2018.02.001. Caciagli, M., Camassi, R., Danesi, S., Pondrelli, S., Salimbeni, S. (2015). Can we consider the 1951 Caviaga (Northern Italy) earthquakes as non-induced events? Seismol. Res. Lett. 86 doi :http://dx.doi.org/10.1785/0220150001 10 pp.. Cesca, S., Braun, T., Maccaferri, F., Passarelli, L., Rivalta, E., and Dahm, T. (2013). Source modelling of the M5-6 Emilia-Romagna, Italy, earthquakes (2012 May 20-29). Geophys. J. Int., 193, 1658-1672, doi:10.1093/gji/ggt069. Dahm, T., Cesca, S., Hainzl, S., Braun, T. and Krüger, F. (2015). Discrimination between induced, triggered and natural earthquakes close to hydrocarbon reservoirs: A probabilistic approach based on the modeling of depletion-induced stress changes and seismological source parameters. J. Geophys. Res. B2 120, pp. 2491-2509. 10.1002/2014JB011778. Dialuce, G., Chiarabba, C., Di Bucci, D., Doglioni, C., Gasparini, P., Lanari, R., Priolo, E., Zollo, A. (2014). Indirizzi e linee guida per il monitoraggio della sismicità, delle deformazioni del suolo e delle pressioni di poro nell’ambito delle attività antropiche. GdL MISE, Roma. unmig.mise.gov.it/unmig/agenda/upload/85_238.pdf.