GNGTS 2013 - Atti del 32° Convegno Nazionale

factor. The magnitude of an earthquake is the average value of the magnitude calculated separately on each of the two horizontal components (Richter, 1958). Among the different types of magnitude developed afterwards, this magnitude is still a solid reference both for its simple definition and widespread usage worldwide. The sensor is formed by a copper cylinder with a diameter of 0.2 cm, 3.5 cm long and 0.944 g of weight suspended from a tungsten wire with a diameter of 0.02 mm (Anderson et al. , 1925) and immersed in an adjustable magnetic field. A small moving mirror adherent to the cylinder (with an angle of 22.5° respect to the front wall of the frame) reflected incident light, generated by an external bulb lamp, on a fixed spherical mirror (100 cm focal length), fixed on the instrument frame. The light was reflected back by the spherical mirror to the moving one that sent it to sheet of photosensitive paper placed over a rotating drum at one-meter physical distance from the sensor. The optical distance, because the double reflection, was four times greater (4 m). The damping is obtained through the magnetic field in which the cylinder is immersed: when the system is energized, the cylinder moves and generates Foucault currents proportional to its velocity, the resulting magnetic field contrasts that of the permanent magnet and the system is damped. It is possible to set period and idle position of the instrument bending it by the regulation screws situated on the base (changing the gravity component acting on the mass). The construction mechanics of the TS-220 instrument make the assessment of its static magnification very difficult simply from its mechanical characteristics (Uhrhammer et al. , 1990). In 1957 Gutenberg obtained, using a vibrating table, a value of 2800 +/- 500 (Gutenberg, 1957; Uhrhammer et al. , 1990). Fig. 1 – Overall data set of 1020 MAWs (circles proportional to the magnitude) recorded by the WA seismograph of the TRI station: A) (a) Friuli region, (b) Dinaric region, (c) Adriatic Sea, (d) Emilia region; B) distribution as function of epicentral distance; C) distribution as magnitude classes (bins equal to 0.25). 116 GNGTS 2013 S essione 1.1