Anderson and Wood (1925) determined a magnification value equal to 2800 but they neglected the deformation of the tungsten wire under different equilibrium situations. Conversely, the deformation of the wire could be sufficient to reduce the magnification factor of 30%, increasing the moment of inertia. For this reason Uhrhammer and Collins (1990) and Uhrhammer et al. (1996) recomputed the instrument static magnification (GS) that was estimated equal to 2080 ± 60. Using 2800 instead of 2080 in the BB WA simulations leads to a magnitude error of +0.129 (e.g. Uhrhammer et al., 2011). To verify the GS value of the Trieste instrument, we adopted two different approaches. The first one involves a direct action on the instrument. According to Anderson …show more content…
Tilting the instruments of a known angle b and measuring the output voltage from the PSD, which is proportional to A, from Eq. (1) we calculate GS (Table 1). The measures made on the N-S component of the instrument are more reliable than those on the E-W one that was repaired at best by the OGS technical staff after the partial detachment of the moving mirror. The error associated to with the estimate is evaluated as the amplifier error, equal to 1% on the linearity of the response, plus the uncertainty on the voltmeter, equal to 0.05 …show more content…
The earthquakes that have been considered are 1152, those for which a location was found in the catalogues: for 956 of them it was possible to calculate also, as additional information, the equivalent WA magnitude (MLBB). Indeed, since October 22, 2004 a Guralp 40-T BB seismometer with a period extended to 60 s was placed very close to the WA one. In addition, for 134 events recorded in the period 2010-2013 the equivalent ML was estimated both by the BB instruments placed at on the surface (MLBB) and at the bottom (MLTRI) of the cave (see Introduction). To compute the equivalent ML we have first deconvolved the BB instrument transfer function to obtain a ground displacement record and then we have convolved the signal with the WA transfer function. From May 26, 2005 to March 5, 2010 the WA and BB instruments were temporarily moved from their historical site to a temporary location due to the restoration of the hosting building. The recordings of that period were discarded in this study because the temporary location was not on hard