GNGTS 2013 - Atti del 32° Convegno Nazionale

The Wood-Anderson instrument digitization and recalibration. The modernization of the instrument consisted in the removal of the fixed cylindrical mirror and its support creating a small side window; through this window a red laser visible beam (Flexpoint model FP- 65/5 AE-AW-SD5-GL47, 650 nm wavelength, Power 5 mW) hits the moving mirror and then, once reflected, a Sitek 1L20 position-sensing detector (PSD) few centimeters far from the instrument. By removing the cylindrical mirror the ray undergoes a single reflection changing the optical leverage from 4 to 2 times. The PSD is a 1D semiconductor device sensitive to visible radiation. The sensor has two anodes (Y1 and Y2) and a cathode (bias) and provides an analogue output directly proportional to the position of the spotlight on its surface (20x3 mm of active area). It offers high resolution and linearity: it is enough to stay inside the 80% of his surface to preserve a 0.1% of linearity. The y centroid position measured from the center of the sensor surface is calculated by: (2) where L is the length of the PSD and y 1 and y 2 are the distances of the beam spot from Y 1 and Y 2 respectively. An ON-TRAK OT-301SL amplifier, that provides in output a voltage directly proportional to the beam spot centroid position, drives the PSD. Presently, after an antialiasing filtering, the traces are recorded by a 16 bit system at 100 sps. The two Lehner Griffith-TS-220 (N-S and E-W oriented respectively) were disassembled and cleaned, and then they were completely recalibrated. On the base of the two instruments there are two reference marks. Once the period and damping are correctly settled, moving the relative index over one of the two marks, a displacement of 50 mm over the photographic paper is produced. Dividing this value by the resulting number of counts, the sensitivity of the instruments is determined and has to be set in the data acquisition code. In this way we will have in output directly the width in mm of the tracks, equivalent to that we would have on the photographic paper. Tab. 1 reports the parameters for the first calibration operated. A few comments about it are reported in a devoted paragraph in the following. Tab. 1 – The first calibration results after the new assemblage. Component Period (s) Damping N-S 0.792 0.787 E-W 0.796 0.818 Data available. First period: 1971-1992. During the period of operation of the original WA (1971-1992), the calculation of the local magnitude was performed following the Richter’s formula (Richter, 1935), using the table of corrections factor unmodified from those calibrated for California and without station correction applied (Finetti, 1972). However the WA am- plitudes were computed as vector sum (at the same instant of time) rather than as arithmetic average of the horizontal components, resulting in a systematic overestimation. The TRI monthly paper bulletins reported only the phases recorded, the local magnitude and epicentral distance estimated by the time lag between P and S arrivals. On May 6, 1977 (exactly one year after the Friuli earthquake, whose TRI was the nearest station) OGS activated a stable network of 4 permanent stations that later expanded to cover a large part of Friuli first and north-eastern Italy later. Since 1977, then, OGS has been producing the seismological bulletin of its network. The bulletin locations have also undergone one major revision (Renner, 1995) and a number of minor revisions aimed at correcting the errors and maintaining, as possible, the homogeneity of the data over the years. At present, this bulletin is published in electronic format only, and it is accessible at the INTERNET address “http://www.crs.inogs.it/ bollettino/RSFVG/” (complete access data from 1977 to the present). 117 GNGTS 2013 S essione 1.1