GNGTS 2019 - Atti del 38° Convegno Nazionale

124 GNGTS 2019 S essione 1.1 catalog by adopting the Kijko-Sellevoll approach (Kijko and Singh, 2011), a method usually applied on regions where only a limited number of the largest magnitudes are available. Regarding data processing, we analysed an extensive GNSS dataset which, covering 20 years of observations (from 1999 up to 2019), includes more than 300 continuous GNSS sites and various networks developed on the Iberian Peninsula by local institutions and agencies mainly for mapping, engineering and cadastral purposes. In addition, we also included data from 25 episodic GNSS sites located in Morocco (Koulali et al. , 2011). The GNSS phase observations were processed by using the GAMIT/GLOBK 10.7 software (http://www-gpsg.mit.edu ), following the strategy described in Palano (2015). As a final processing step, a consistent set of positions and velocities into a fixed Nubian reference frame has been computed. In a next step, the horizontal strain-rates have been estimated on a regular 0.5° x 0.5° grid over the investigated area by adopting the method reported in Shen et al. (2015). Finally, the geodetic moment-rate (and its standard error) was estimated for each seismic source zone by adopting the Savage and Simpson (1997) formulation. The moment-rate estimate from geodetic strain-rates is proportional to the chosen seismogenic thickness. In order to choose appropriate values, the depth distributions of earthquakes with magnitude M≥1 are analyzed for each seismic source zone. After some preliminary estimations, all the seismic source zones have been merged into 3 main regions, defining approximate seismogenic thicknesses of 15, 18 and 25 km for Betics, Rif-western Atlas and Tell Atlas, respectively. Results. The geodetic moment-rate is a measure of both elastic and anelastic loading rates while the seismic moment-rate is a measure of the elastic unloading rate. The SCC values usually lie between 0% and 100%: the more the value close to 100, the more the larger part of the deformation is released by brittle deformation (earthquakes). A low ratio instead suggests an apparent deficit of seismic moment-rate. This may be due to regions characterized by the presence of faults with aseismic behavior or seismic gaps. Another cause can be attributed to an incomplete seismic catalog or to a seismic catalog covering a very short time interval if compared with the seismic cycle of the investigated region. Moreover, value larger than 100% can be also observed, although less frequently. Such a case would occur if the estimation of the seismic moment-rate has been computed soon after a large earthquake, especially if the time interval covered by geodetic data appears to be insufficient to capture the spectrum of seismic and aseismic components. Another explanation can be attributed to a bad geometry of the geodetic network due to a highly heterogeneous distribution of GPS stations. Taking into account the considerations above, achieved results highlight as most of the studied area is characterized by an apparent seismic moment-rate deficit, in according to previous estimations. A large number of these seismogenic source zones (western Betics, western Rif, High, Middle and Saharan Atlas) is characterized by SCC values lower than 23%, evidencing its prevailing aseismic behaviour. Intermediate SCC values (between 30% and 60%) are also obtained in some seismogenic sources, where well-known faults have hosted moderate earthquakes (eastern Betics, central Rif and Middle Atlas), indicating as crustal seismicity accounts only for a moderate fraction of the total deformation-rate budget. Finally, high SCC values (>95%) are observed along most of the Tell Atlas, suggesting that crustal deformation is mostly released through earthquakes, but also depicting a potential seismic gap in the Eastern Tellian Atlas. It must be considered that the comparison between seismic and geodetic moment-rates involves a large number of assumptions and simplifications. Nonetheless, it provides significant insights into the seismic hazard of regions subjected to relevant tectonic deformation, essentially for time-dependent seismic hazard assessments. References Hyndman R. D., Weichert D. H.; 1983: Seismicity and rates of relative plate motion on the plate boundaries of western North America . Geophys. J. R. Astron. Soc., 72 , 59–82, DOI 10.1111/j.1365-246X.1983.tb02804.x.