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Conclusion • The semi-analytical model shows that poroelastic interaction may have indeed caused changes in CFS. Observed seismicity in positive CFS changes, although the values are smaller than few decimal of bar. A rate-and-state model is able to predict peaks of seismicity, while overestimating the rate at later time of observation. Earthquakes interaction may have played a role in reducing the Coulomb stress. • The use of a numerical model highlights the importance of accounting for heterogeneities, which could enahnced and focus the stress variation at observed seismogenetic region. This research is funded by a Reasearch agreement between SED and INGV. Funding to A. P. Rinaldi are pro- vided by a Swiss National Science Foundation (SNSF) Ambizione Energy grant (PZENP2_160555). Easting (km) -10 -5 0 5 10 Northing (km) -5 0 5 10 -10 Effect of heterogeneities W N Quaternary Mélange Carbonate Density (kg/m 3 ) 2300 2300 2300 1-30.0 30.0 30.0 Poisson’s ratio α Biot 0.9 B Skempton Permeability (m 2 ) 5 . 10 -15 Young’s modulus (GPa) 1.00 1.00 1.00 5 . 10 -15 5 . 10 -17 0.9 0.9 0.25 0.25 0.25 } } } Quaternary (weak, permeable) 500m thickness Melange (stiff, low permeable) -500 m to -1800m Carbonate (stiff, permeable) OPENGEOSYS simulator - Lake: time-dependent boundary condition. Vertical load and pore pressure - Open pressure boundary - Fixed displacement boundary - 30 days loading/ protracted response Easting (km) -10 -5 0 5 10 Northing (km) -5 0 5 10 -10 -0.8 -0.4 0 0.4 0.8 CFS (bar) Days Water level Focal plane: 330, 60, -80 Focal plane: 120, 30, -120 Days Water level Easting (km) -10 -5 0 5 10 Northing (km) -5 0 5 10 -10 Coulomb and Rate-and-State: homogeneous model -0.8 -0.4 0 0.4 0.8 CFS (bar) -8 -4 Easting (km) -15 -10 -5 0 Northing (km) 2003 0 8 4 4 km depth (seismicity between 2 km and 4 km) 6 km depth (seismicity between 4 km and 6 km) -8 -4 Easting (km) -15 -10 -5 0 Northing (km) 2009 0 8 4 -8 -4 Easting (km) -15 -10 -5 0 Northing (km) 1967 0 8 4 -8 -4 Easting (km) -15 -10 -5 0 Northing (km) 2003 0 8 4 -8 -4 Easting (km) -15 -10 -5 0 Northing (km) 2009 0 8 4 -8 -4 Easting (km) -15 -10 -5 0 Northing (km) 1967 0 8 4 1972 1992 1982 2002 2012 initial filling 440 480 520 time (year) water level (m.a.s.l.) Rate-and-State SEMI-ANALYTICAL MODEL - Poroelastic model to compute changes in loading and pressure diffusion due to water level changes - Resulting CFS changes are inputed to a Rate-and-State model to compute seismicity rate (for fault oriented similar to observation - Valoroso et al., 2009) Diffusivity (m 2 /s) 0.05 0.5 Poisson’s ratio Friction 0.6 B Skempton 0.25 50 60 70 80 90 Water level (m) 0 1 3 5 7 Time (years from 01/01/2001) 0 0.05 0.10 0.15 R mod (#ev/volume) 0 2 4 6 8 0 2 4 6 R obs (#ev/volume) 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 modelled observed 0.20 9 11 10 12 100 8 0.0009 1280.7 0.058 Aσ (MPa) t a (days) r (#/day) Rate-and-State Conceptual model Days Water level Days Pressure Days Pressure Hydraulic effect The bottom of the lake is pressurized by water co- lumn weigth: no volumetric strains, pressure evolution depends on permeability k and specific storage S e Mechanical effect The bottom of the lake is deformed by water-co- lumn weigth: volumetric and shear strains genera- te the pressure evolution, which depends on perme- ability k and specific sto- rage S e , as well as on po- roelastic properties. M to H coupling effect Additional pressure response Additional stress changes H to M effect Time dependent DCFS Mitigate pore pressure drop Effective stress reduction Undrained response Introduction A water reservoir affects the underlying crust stress state through the poroelastic response to the weight of the water volume stored and by the consequent fluid movement. The artificial lake of Pertusillo in Val d’Agri (Italy) is one of the known water reservoirs showing protracted seismic activity for several years after the initial filling in 1963. More than 800 small magnitude events (ML <3; Mc=1.1) were located between 2001 and 2013 by a monitoring network of a local industry operator supplemented by permanent and temporary stations of the Istituto Nazionale di Geofisica e Vulcanologia. Hypocenters concentrate at a shallow depth (< 5 km) to the south of the lake. During the same period the reservoir water level fluctuated in average of tens of meters between summer and winter periods. The observed seismicity rate positively correlates with these seasonal oscillations. 15.8˚ 15.9˚ 16˚ 16.1˚ 40.2 ˚ 40.3˚ 3 km 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 Year Lat. Long. CM2 Dam Figures from Valoroso et al. (2009) Seismicity induced by seasonal variation of reservoir level: the case of Pertusillo lake, Val D’Agri (Italy) Rinaldi A. P. (1) , Catalli F. (2) , Urpi L. (1) , Improta L. (3) , Buttinelli M. (3) (1) Swiss Seismological Service, ETH Zürich, Switzerland; (2) Deutsches GeoForschung- Zentrum, GFZ – Potsdam, Germany; (3) Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Roma, Italy
