Moment magnitude and local magnitude of small earthquakes nucleating along a low angle normal fault in the Upper Tiber Valley (Italy)

Abstract

The computation of the moment magnitude of small earthquakes (MW < 3) allows the investigation of key aspects of the physics of the seismic source, like the scaling properties of earthquakes. In order to do that, we analyse the crustal propagation of seismic waves in the Upper Tiber Valley (Northern Apennines, Italy) using 38,000 high-resolution broadband seismograms from 1192 well-located micro-earthquakes that occurred between 2010 and 2014, in the local magnitude range -1.0 ≤ ML ≤ 3.8. Because we use weak-motion data, we maximize the signal-to-noise ratios by applying a complex technique based on Random Vibration Theory (RVT). Our analysis of the data produced two main results: i) we are able to calculate the seismic moment (and moment magnitude) for very small events, down to at least MW = -1.5. ii) we determined a relationship between MW and ML , and use RVT to show that ML ~ log10 (M0) for small earthquakes.