GNGTS 2018 - 37° Convegno Nazionale

32 GNGTS 2018 S essione 1.1 As mentioned earlier the total stress at any point and epoch in the study area can be approximated as the sum of the timewise calibrated regional stress and the stress transferred by previous events in the area. Fig. 3 shows the map of the Coulomb stress before and after the 2009 l’Aquila event. On the left we plot the regional stress calibrated as described earlier, plus the stress released by historical earthquakes in the neighborhoods, notably l’Aquila (1315 and 1461) and Montereale (1703); on the right we further account for the stress change caused by the 2009 event. The total stress is resolved into a normal and tangential component to the Paganica fault plane and the Coulomb linear combination is contoured with a friction coefficient equal to 0.4. It is interesting to note the role of the ‘stress reservoir’ near the Isola del Gran Sasso fault, and the fact that the 2009 event seems to load the adjacent Montereale and San Pio delle Camere faults. While the Montereale fault ruptured in 1349 and 1703, almost alternating with the Aquila events of 1315 and 1461, there is no historical evidence of rupture in the San Pio delle Camere fault. Hence both faults could be reasonable candidates for rupture in the future, according to this reconstruction. It is finally remarkable that the 1461 Aquila event, if at the same hypocenter as the 2009 event as assumed in this reconstruction, would have generated a state of negative Coulomb stress making the 2009 event unlikely. This suggests that hypocenters of the 1461 and 2009 events are likely to be displaced to each other, taking into account that according to the CPTI15 v.1.5 historical database the epicentral Latitude Longitude coordinates of the 2009 and 1461 events (entries #4368 and #178, respectively) are (42.309, 13.510) and (42.308, 13.543), with uncertainties of 0.02 deg (2009) and 0.10 deg (1461). Southwest of the Fucino fault there seems to be a wide area of negative Coulomb stress. The absence of mapped seismogenic faults, the lack of historical seismicity and the small horizontal gradient of GPS velocities in this area are independent indications supporting a status of low Coulomb stress in this particular area. Conclusions. The combination of the regional (steady state) and transferred (episodic) stress from historical events is attempted by requiring the minimum regional stress ensuring, at the depth of the 2009 event at l’Aquila, that the total stress is tensional both before and after the event. The minimum tensional stress satisfying this criterion is in the order of nearly 50 MPa at 6.5 km depth. This stress is about 50% of the tractional horizontal stress one might expect on an Andersonian fault dipping at 45° with a friction coefficient of 0.4. Hence, this analysis is made in the frame of a ‘weak crust’ approach. Doubling the regional stress would bring to a picture more in keeping with the ‘strong crust’ approach, at the expenses of the role of the episodic seismicity becoming less evident in the total stress budget. References Basili R., Valensise G., Vannoli P., Burrato P., Fracassi U., Mariano S., Tiberti M. M. and Boschi E.; 2008: The database of individual seismogenic sources (DISS), version 3: Summarizing 20 years of research on Italy’s earthquake geology . Tectonophysics, 453 , 20-43, doi:10.1016/j.tecto.2007.04.014. Caporali A., Barba S., Carafa M. M., Devoti R., Pietrantonio G. and F. Riguzzi; 2011: Static stress drop as determined from geodetic strain rates and statistical seismicity. J. Geophys. Res., 116 , B02410, doi:10.1029/2010JB007671. Caporali A., Braitenberg C., Montone P., Rossi G., Valensise G., Viganò A. and Zurutuza J.; 2018: A quantitative approach to the loading rate of seismogenic sources in Italy.  Geophysical Journal International , 213 , 2096– 2111,  doi.org/10.1093/gji/ggy112. D’Agostino N.; 2014: Complete seismic release of tectonic strain and earthquake recurrence in the Apennines (Italy) . Geophys. Res. Lett., 41 , doi.org/10.1002/2014GL059230. DISS Working Group; 2015: Database of Individual Seismogenic Sources (DISS), Version 3.2.0: a compilation of potential sources for earth- quakes larger than M 5.5 in Italy and surrounding areas. © INGV 2015 - Istituto Nazionale di Geofisica e Vulcanologia, <http://diss.rm.ingv.it/diss/ >, doi:10.6092 /INGV.IT -DISS3.2.0. Galli. P., Galderisi A., Ilardo, I., Piscitelli S., Scionti V., Bellanova J. And Calzoni F.; 2018: Holocene paleoseismology of the Norcia fault system (Central Italy ). Tectonophysics, 745 , 154–169, doi.org/10.1016/j. tecto.2018.08.008. Guidoboni E., Ferrari G., Mariotti D., Comastri A., Tarabusi G., Sgattoni G. and Valensise G. L.; 2018: CFTI5Med, Catalogo dei Forti Terremoti in Italia (461 a.C.-1997) e nell’area Mediterranea (760 a.C.-1500). Istituto Nazionale di Geofisica e Vulcanologia <http://storing.ingv.it/cfti/cfti5/>