Local Magnitude (ML) Calibration in the Short-Distance Range—The Case of the Collalto Seismic Network and Surrounding Stations in Northeastern Italy

ABSTRACT We calibrate local magnitude equations for northeastern Italy, centered on the Collalto seismic network, using the International Association of Seismology and Physics of the Earth’s Interior (IASPEI) parametric formulation and a modified approach from Chovanová and Kristek (2018). Our goal is to develop an ML distance correction well constrained at short distances, adhering to the Richter definition. Using digital data from 2012 to 2023, we find initial parameters similar to those obtained by Bragato and Tento (2005) on an independent dataset. These values differ significantly from the literature, but their physical meaning is questionable. We then introduce an additional procedural step to reduce the bias likely caused by data oversampling in the 10–40 km range. The resulting equations are MLH=LogAH−(−1.545Log(R/100)+0.001357(R−100)−3+SiH) and MLV=LogAV+0.238−(−1.555Log(R/100)+0.000995(R−100)−3+SiV) for the horizontal and vertical components, respectively; the Si represent static corrections for each station and instrumentation configuration. Our new scale is valid for distances 7–200 km and magnitudes from −0.8 to 5.8. It aligns pretty well with the Hutton and Boore (1987) equation from 15 to 160 km, despite the different coefficients; below 50 km distance, where it is best constrained, it generally provides higher values than the other Italian laws and laws tuned for the shortest distances. Average residuals reveal a ± 0.1 perturbation between 40 and 170 km, closely matching a pattern found by Bragato and Tento as well. Our findings highlight the critical impact of distance oversampling and confirm a regional perturbation of wave propagation that affects the Wood–Anderson amplitudes. Our scale aligns linearly with independently calculated Mw and shows to be reliable for the investigated magnitude range across the whole regional area. Thus, it is usable by both local and regional networks in all of northeastern Italy.