GNGTS 2013 - Atti del 32° Convegno Nazionale

The Wood Anderson magnitude of the Trieste station (TRI - NE Italy): a new dataset D. Sandron, G.F. Gentile, S. Gentili, A. Rebez, M. Santulin, D. Slejko National Institute of Oceanography and Experimental Geophysics – OGS, Trieste and Udine, Italy Introduction. The standard torsion Wood Anderson (WA) seismograph owes its fame to the fact that historically it has been used for the definition of the magnitude of an earthquake (Richter, 1935). With the progress of the technology, digital broadband (BB) seismographs replaced it. However, for historical consistency and homogeneitywith the old seismic catalogues, it is still important continuing to compute the so called Wood Anderson magnitude. It has been proven that synthetic seismograms WA equivalent can be simulated convolving the waveforms recorded by a BB instrument with a suitable transfer function. The seismological station of Trieste (TRI), belonging to the World-Wide Standardized Seismographic Station Network (WWSSN) and sited in Borgo Grotta Gigante, about 12 km out of town, and managed by the National Institute of Oceanography and Experimental Geophysics (OGS), was equipped on September 1971 with two horizontal WA seismometers Lehner-Griffith TS-220 for the computation of local magnitude (Finetti and Morelli, 1972). However, the start of the WWSS Trieste (code TRI-117 of the global WWSS network) recordings dates back to July 29, 1963. The short-period seismograph comprised three Benioff seismometers, while Ewing-Press seismometers were employed for the teleseismic detection. The recording room was located at the surface (268 m above the sea level) while the seismometers were installed at the bottom (161 m above the sea level) of the Grotta Gigante: a giant cave of the Trieste Karst with its central cavern 107 m high, 65 m wide and 130 m long. The WA instrument was located at the surface in a darkroom. The daily development of the photographic paper required a lot of time, not to mention that the photo paper procedure itself was very expensive. This uncomfortable configuration, along with the progress of the technology, contributed over the time to the abandonment of the WA recording, which occurred in April 1992. In 1995 the station was enhanced by the installation of a very BB Streckeisen STS-1 seismometer at the bottom of the cave. The WA seismograph, after a period of inactivity, was recovered and modernized by replacing the recording on photographic paper with an electronic device. From December 17, 2002 to the present, with a period of interruption due to the property renovation of the building where it is located (May 2005 - March 2010), the instrument is fully operational and has recorded 783 events till August 9, 2012. Since 2004, next to the WA (few decimeters apart), a Guralp 40-T BB seismometer was installed, with a proper period extended to 60 s. Since then, the following signals have been acquired at the same time: i) the data of the digitized original WA, ii) the data of WA simulated by the BB close to it, and iii) those of the broad band installed at the bottom of the cave. From April 1992, the WA magnitude ( MAW ) estimates, regularly reported in the seismic station bulletins up to 1992, had no more official publication. Aim of the present work is twofold: from one side to recover the whole data set of MAW values recorded until 2012, and from the other side to verify the WA static magnification GS , so that to apply it in the WA simulation from waveforms recorded by broadband (BB) seismometers. The Wood-Anderson instrument. The WA torsion seismograph, with its natural period of 0.8 s, 0.8 damping and GS value equal to 2800, was the instrument with which Richter (1935) defined the local magnitude as: M L = log A – log A 0 + dM (1) where A is the maximum excursion (mm) of the WA seismograph, A 0 is an empirical function that depends on the epicentral distance of the station, dM is the station-specific corrections 115 GNGTS 2013 S essione 1.1