A new MD-ML relationship for Mt. Etna earthquakes (Italy)

A homogenous database of magnitude observations is a basic requirement for seismic hazard estimation and other seismic studies. Unfortunately, the magnitude reported in the seismic catalogue of Mt. Etna is not homogenous. Until 2005 only the duration magnitude (MD) is available, though since then the more stable local magnitude (ML) has also been calculated. To overcome this limitation, earthquake data recorded at Mt. Etna during the period 2005 – 2014 were used to derive a new relationship between local and duration magnitude, by applying the General Orthogonal Regression (GOR) which is an alternative to least squares when the ratio between errors on the independent and the dependent variables can be estimated. The relationship obtained is: ML = 1.164 (± 0.011) * MD - 0.337 (± 0.020) The new relationship allows to back-extend the local magnitude dataset to cover a period of about 15 years