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 (Vr=0.9±0.2 $Vr=0.9\pm 0.2$ km/s) and low seismic efficiency (η=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. Plain Language Summary: Understanding the process leading to a large earthquake is a key question in Earth science with implications for earthquake prediction and risk assessment. Although results from laboratory experiments and numerical simulations show that earthquake nucleation is composed of several preliminary stages, these stages and the associated processes are very difficult to observe in natural earthquakes. In this work we study the Mw 6.1 2009 L'Aquila earthquake, and show that it started with a slow rupture, where most of the energy was used to propagate the rupture. Our results show that the rupture struggled to initiate, and it did it slowly probably due to the conditions in which it starts. In addition, our results show similarities with previous work done for earthquakes in different regions of the world and also using theoretical models. Key Points: The initiation of the L'Aquila earthquake is characterized by a small amplitude signal before the onset of large amplitude P‐wavesWe evidence a slow rupture velocity during the rupture initiation of the L'Aquila earthquakeLow seismic efficiency indicates that most of the energy budget was used to initiate the rupture (fracture energy ∼76%) [ABSTRACT FROM AUTHOR]