Iio ( 1992, 1995) and Iio et al. ( 1999) showed that the emergent initial phase can be explained by models that predict slow slip velocities and/or rupture velocities immediately after rupture initiation, and that the initial slow phase is not a product of attenuation, but a source effect. Umeda ( 1990) found that the duration of the initial phase increases proportionately with the earthquake size. Results from different studies focused on this initial phase have revealed a variety of new insights about the rupture initiation. This initial phase is represented by a small emergent amplitude signal that is sometimes observed before the large impulsive amplitude onset of the P-waves (Figure 1). Two of the main types of precursory seismological observations are foreshocks (e.g., Dodge et al., 1996 Bouchon et al., 2011 Ellsworth & Bulut, 2018 Kato et al., 2012 Ruiz et al., 2014, 2017 Sánchez-Reyes et al., 2021 Cabrera et al., 2022 and references therein) and the seismic signals related to the initial part (i.e., over a few seconds or less) of the mainshock waveform, often known as the seismic nucleation phase (e.g., R.
However, direct measurements of the phases I and II in nature are hard, and the scientific community mostly relies on seismological observations, which are perhaps the most informative about the physical processes that precede large earthquakes. Nowadays, we know from laboratory experiments (e.g., Latour et al., 2013 McLaskey, 2019 Ohnaka & Shen, 1999) and numerical models (e.g., Ampuero & Rubin, 2008 Dascalu et al., 2000 Kaneko et al., 2016 Shibazaki & Matsu’ura, 1998) that earthquakes are preceded by different phases: a stable quasi-static deformation phase (phase I), which evolves into an unstable acceleration phase (phase II), after which the large dynamic rupture occurs (phase III). Therefore, it is crucial to detect and study signals that allow us to relate the rupture of an earthquake to precursory physical processes, if any exist. Understanding the physical processes and conditions that lead to the initiation of an earthquake is one of the major challenges of seismology, with implications for earthquake prediction and risk assessment. We also show that the parameters of the rupture initiation are representative of scale-dependent quantities for slip-dependent nucleation models. From the geometrical characterization and rupture parameters of this initial phase, we infer that the rupture struggled to initiate exhibiting a slow rupture velocity ( km/s) and low seismic efficiency ( ) due to a complex environment in the region where the rupture starts. From the detailed analysis of seismic waves recorded at several stations, we identify an ∼0.6-s signal preceding the large dynamic rupture. Here, we report on the rupture initiation of the M w 6.1 2009 L'Aquila earthquake.
However, obtaining observations of such or similar processes in nature is complex.
Laboratory experiments and numerical models have shown that earthquake nucleation has distinct phases: a quasi-static and an acceleration stage, followed by dynamic propagation. Understanding under which physical conditions large earthquakes begin, is a key question in Earth science.
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