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 theWorldwide 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 were used for teleseismic detection. The WWSSN seismometers were installed at the bottom (161 m above sea level) of Grotta Gigante, a giant cave of the Trieste karst, 12 km away from the city center. The WA seismometers were placed over a plinth in a darkroom at the surface (336 m above sea level, latitude 45.709° N, longitude 13.764° E). The daily processing of the photographic paper was quite expensive and very time consuming. This aspect also contributed to the abandonment of the Trieste WA recordings in April 1992.

In 2002, the WA instruments were recovered and upgraded by replacing the recording on photographic paper with an electronic device. From 17 December 2002 to 31 December 2013, the refurbished WA seismometer recorded 1252 events, with a break between May 2005 and March 2010 due to the restoration of the building where it was operated.

At present, the Trieste station concurrently acquires data from (1) the upgraded digitized WA seismometer, (2) the Güralp 40-T BB instrument placed at the top of the cave (since 2004), and (3) the BB Streckeisen STS-1 seismometers installed at the bottom of the cave (since 1995). The STS-1 instrument corresponds to station TRI of the MedNet network (Mazza et al., 1998).

In this paper, after describing the upgrade of the WA seismograph and verifying its static magnification (Gs), we reevaluate old estimates of ML and compile a new catalog of Trieste WA ML values updated to 2013. Finally, we compare the Trieste WA ML values with moment magnitudes.