Abstract

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 M-w 6.1 2009 L'Aquila earthquake. From the detailed analysis of seismic waves recorded at several stations, we identify an similar to 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 (Vr=0.9 +/- 0.2 $Vr=0.9\pm 0.2$ km/s) and low seismic efficiency (eta=0.24 $\eta =0.24$) 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.