GNGTS 2013 - Atti del 32° Convegno Nazionale

earthquakes retrieved from the TRI official paper bulletins) and re-computed MAW according to the original Richter (1935) formula obtaining a catalogue of 1102 events. The MAW s reported in the TRI paper bulletins are, on average, higher than the re-computed ones by 0.25. It has been asked to three experts to re-read the WA waveforms on the original photographic sheets, and we had the confirmation that the past WA magnitude overestimation was due to the wrong method of its calculation: the WA amplitudes were computed as vector sum rather than arithmetic average of the horizontal components. For comparative purpose, we considered 833 common events recorded also by a Guralp 40-T BB seismometer installed close to the WA instrument. The WA transfer function, according to Anderson and Wood (1925) should have a GS value of 2800. As first result the MAW s calculated by the BB seismometer are higher than the WA MAW s on average 0.11. In order to check the actual WA GS value, we considered also a method involving a direct action on the instrument. The result suggests that the GS value depends on the waveform amplitude recorded: decreases with increasing amplitude, reaching a value approximately constant in Fig. 3 – Actual WA GS as function of the amplitude of the seismograms recorded by the BB seismometer for: A) the N-S and B) the E-W components; uncertainty in the magnitude estimation applying a GS equal to 2800 as function of the amplitude of the waveform recorded on C) the N-S and D) the E-W component of the WA. 122 GNGTS 2013 S essione 1.1