Stability of fault plane solutions for Mw >= 4.8 in northern Italy in 2012

Abstract

We propose a critical analysis of the moment tensor solutions of the major seismic events that affected northern Italy in 2012. Inverting full waveforms at regional distance using the non-linear method named INPAR, we investigate period dependent resolution that affects in particular the solutions of shallow events. This is mainly due to the poor resolution of Mzx and Mzy components of the seismic tensor when inverting signals whose wavelengths significantly exceed the source depth. As a consequence, instability affects both source depth and fault plane solution retrieval, and spurious large Compensated Linear Vector Dipole components arise. The inversion performed at cutoff periods shorter than 20 s reveals in many cases different details of the rupture process, that are not resolved inverting at longer cutoff periods. Thus we conclude that inversion of full waveforms at cutoff period as short as possible should be preferred.