Resumen de The Wood–Anderson of Trieste (Northeast Italy): one of the last operating torsion seismometers

Denis Sandron, Giovanni Francesco Gentile, Stefania Gentili, Angela Saraò, Alessandro Rebez, Marco Santulin, D. Slejko

• The Wood–Anderson (WA) torsion seismograph, used by Richter (1935) for the definition of the local magnitude (ML) of an earthquake, has been abandoned over time due to the cumbersome nature of its use. With the progress of technology, modern digital broadband (BB) instruments have replaced older instruments such as the WA, and the equivalent ML, obtained from simulated WA seismograms after convolution of the recorded BB data with a proper transfer function (Bormann, 2002a,b), has replaced the WA ML.

  Despite the paucity of WA instruments today, the ML in its original form remains relevant for continuity with old earthquake catalogs and as a long-standing reference for all other magnitude scales up to approximately ML 6.5. For larger earthquakes, the ML scale progressively underestimates the actual energy release and ML is said to saturate (Kanamori, 1983). Even so, ML is a good predictor of structural damage caused by earthquakes because many buildings have resonant periods close to that of the WA seismograph (0.8 s).

  In Trieste, located in northeastern Italy, there is one of the few stations equipped with an original pair of WA instruments that are still operating. The two horizontal WA seismometers (Lehner-Griffith TS-220) were installed in September 1971 and have been managed since then by the Osservatorio Geofisico Sperimentale, presently the Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS). The Trieste station was part of the Worldwide Standardized Seismographic Station Network (WWSSN) with the code TRI-117, and it dates its operation back to 29 July 1963. At that time, three Benioff seismometers were employed as short-period seismographs, and three Ewing-Press seismometers …