A Struggled Rupture Initiation of the Mw 6.1 2009 L’Aquila Earthquake

Understanding under which physical conditions large earthquakes begin, is a key question in Earth science. Laboratory experiments and numerical models have shown that earthquake nucleation has distinct phases: a quasi-static and an acceleration stage, followed by dynamic propagation. However, obtaining observations of such or similar processes in nature is complex. Here, we report on the rupture initiation of the Mw 6.1 2009 L'Aquila earthquake. From the detailed analysis of seismic waves recorded at several stations, we identify an ∼0.6-s signal preceding the large dynamic rupture. From the geometrical characterization and rupture parameters of this initial phase, we infer that the rupture struggled to initiate exhibiting a slow rupture velocity ((Formula presented.) km/s) and low seismic efficiency ((Formula presented.)) due to a complex environment in the region where the rupture starts. We also show that the parameters of the rupture initiation are representative of scale-dependent quantities for slip-dependent nucleation models.