GNGTS 2017 - 36° Convegno Nazionale

88 GNGTS 2017 S essione 1.1 We employed the following two innovative approaches to determine the duration and the number of events of the selected aftershock sequences: 1) Tangents method. We adopted a completeness (threshold) magnitude (Mc) of 2.5 as, below this threshold, the number of recorded seismic events strongly depends on the sensitivity of the seismic network, which varies from place to place. The adoption of a completeness magnitude allowed us to remove the seismic noise. Our methodology is based on the build-up of cumulative curves for each seismic sequence, reporting the days elapsed from the mainshock on the x-axis versus the cumulative number of earthquakes on the y-axis. We distinguish two different parts in the graphs: one indicates a non-linear increase of the cumulative number and the other one a linear increment. The first non-linear trend suggests that the seismic sequence related to the mainshock event is still active, whereas the linear increment represents the ground seismicity that affect an active seismic region. We consider the point where the tangent to the linear increment departs from the cumulative curve as indicative of the end of the aftershock sequence. 2) Mandelbrot method. We examined faulting and fragmentation processes using the fractals theory (e.g. Turcotte, 1986; Mandelbrot, 1989). The fractal geometries are related to fragmentation processes caused by earthquake nucleation and, therefore, the variation of fractal parameters can be indicative of the evolutions of the fragmentation processes along a fault system in time and space. We analysed the seismological data, fit the same data with a linear regression and obtained the fractal dimension and the related coefficient of determination (i.e., R squared). This method allows the representation of the magnitude-frequency distribution of earthquakes. In particular, we realized semi-logarithmic graphs for each seismic sequences, in which we compared the number of earthquakes occurred in certain magnitude ranges. The fitting straight line represents a simple linear regression according to the following equation, which also define a fractal set: N i = Cr i − D where N i is the number of objects with a characteristic linear dimension r i , C is a constant of proportionality, and D is the fractal dimension. The fractal dimension value represents the level of irregularity of the selected fractal set (e.g., Turcotte, 1997) and is indicative of the fragmentation process occurred during the mainshock and the following aftershocks. If D =0, it represents the classical Euclidean dimension of a point; if D =1, it represents the dimension of a line segment; if D =2, it represents the dimension of a surface and, finally, if D =3, it represents the dimension of a volume. According to the here proposed Tangents method, the average duration of aftershocks sequences within extensional tectonic settings is about 390 days, i.e. about 270 days longer than the duration (about 120 days) of aftershocks sequences within contractional tectonic settings. Furthermore, aftershock sequences within extensional tectonic settings are characterized by a larger number of seismic events (1045 aftershocks on average) than those within contractional earthquakes (790 aftershocks on average). According to the Mandelbrot method, the average duration of aftershocks sequences within extensional tectonic settings is about 430 days, which is about 295 days longer than that of aftershocks sequences within contractional tectonic settings (about 135 days). Furthermore, aftershock sequences within extensional tectonic settings are characterized by a larger number of seismic events (1056 aftershocks on average) than those within contractional earthquakes (795 aftershocks on average). The fractal dimension ( D ) values calculated for extensional and contractional seismic sequences are also different. For extensional earthquakes, the fractal dimension varies from ca. 2 to ca. 3. Conversely, for contractional earthquakes, the fractal dimension varies from 1 to ca. 2. The fractal dimension is indicative of the geometrical fragmentation process. Therefore, the analysis of the fractal dimension shows that extensional seismic sequences tend to occupy a volume with time; on the other hand, contractional seismic sequences tend to develop along a surface with time. Our analyses strongly support the conclusion that, irrespective of the magnitude of the mainshocks, extensional seismic sequences are longer than compressional sequences. We