Gradual unlocking of plate boundary controlled initiation of the 2014 Iquique earthquake

Abstract

Several major earthquakes (Mw>7) have occurred in this gap since 1850 (Fig. 1); the largest until now was the Mw 7.7 Tocopilla earthquake in 2007, which broke the southern rim of this segment beneath and north of Mejillones Peninsula along a total length of 150 km. Only the downdip end of the locked zone slipped in this event, and the total slip in the rupture area was less than 2.6 m 6,7 leaving most of the past slip deficit of c. 8-9 m accumulated since 1877 3 approaches. First, we performed waveform modelling of local strong motion seismograms and teleseismic body waves to constrain the kinematic development of the rupture towards the final displacement in a joint inversion with continuous GPS data of static displacements (Fig. 1, 2a). Second, we use the backprojection technique applied to stations in North America to map the radiation of high frequency seismic waves (HFSR; 1-4 Hz) 9,10 . The latter technique is not sensitive to absolute slip amplitudes, but rather to changes in slip and rupture velocity.During the first 35-40s the rupture propagated downdip with increasing velocity, nearly reaching the coastline (Fig. 2a,b). Surprisingly, towards the end of the rupture, the area near the epicenter was reactivated. In spite of the relatively complicated kinematic history of the rupture the cumulative slip shows a simple 'bull's eye' pattern with a peak coseismic slip of (Fig. 3a). The Iquique main shock nucleated at the 4 northwestern border of a locked patch and ruptured towards its center (Fig. 2a, 3a). The downdip end of the main shock as well as for the large Mw 7.6 aftershock rupture mapped both by the HFSR and co-seismic slip agrees quite accurately with the downdip end interseismic coupling (Fig. 2a,c 3a). The accelerated downdip rupture propagation for both earthquakes closely followed the gradient towards higher locking. Therefore, the Iquique event and its largest aftershock appear to have broken the central, only partly locked segment of the Northern Chile Southern Peru seismic gap releasing part of the slip deficit accumulated here since 1877 (cf. Fig. 1).The seismicity before the Iquique earthquake also concentrates in this zone of intermediate locking at the fringe of the highly locked -high slip patch (Fig. 3a). Starting in July 2013, three foreshock clusters with increasingly larger peak magnitudes and cumulative seismic moment occurred here (Fig. 2c, 3a,c). The mainshock rupture started at the northern end of the foreshock zone, inside the region of intermediate locking (Fig. 2c, 3a). Interestingly, the second foreshock cluster (January 2014) is associated with a weak transient deformation, whereas the third cluster (March 2014) shows a very distinct transient signal. GPS displacement vectors calculated over the times spanning these foreshock clusters point towards the cluster epicentres (Extended Data Figure 4). Deformation for both transients is entirely explained by the cumulative coseismic displacement of the respective foreshock clusters (Fig. 3d inset, Extended Data Figure 4). The ar...

An 8 month slow slip event triggers progressive nucleation of the 2014 Chile megathrust

“…The earthquake ruptured añ 150 km long portion of the subduction zone [Schurr et al, 2014;Ruiz et al, 2014], in an area that was partially locked before the earthquake [Bejar Pizarro et al, 2013;Metois et al, 2016]. The earthquake was preceded by a series of earthquake swarms beginning in July 2013 [Schurr et al, 2014;Ruiz et al, 2014].…”

“…Previous studies focused mostly on the 20 days immediately preceding the earthquake when a strong transient signal occurred [Schurr et al, 2014;Ruiz et al, 2014;Lay et al, 2014;Bedford et al, 2015;Cesca et al, 2016]. Nevertheless, a debate remains on the mechanisms leading to this foreshock activity, notably on the existence or not of aseismic slip preceding the earthquake.…”

“…Some earthquakes have been preceded by an intense foreshock activity [Bouchon et al, 2013;Schurr et al, 2014;Ruiz et al, 2014;Lay et al, 2014;Bedford et al, 2015;Cesca et al, 2016;Kato et al, 2016;Meng et al, 2015;Hasegawa and Yoshida, 2015;Kato et al, 2012;Ozawa et al, 2012, Sato et al, 2013Bouchon et al, 2011] raising the possibility that earthquake forecasting may be achieved through a better understanding of precursory mechanisms. Two concurrent models have been proposed to explain the initiation of seismic rupture [Dodge et al, 1996].…”

“…Nevertheless, a debate remains on the mechanisms leading to this foreshock activity, notably on the existence or not of aseismic slip preceding the earthquake. Apart from the study of the foreshock sequence [Schurr et al, 2014;Ruiz et al, 2014;Lay et al, 2014;Bedford et al, 2015;Cesca et al, 2016;Kato et al, 2016;Meng et al, 2015], very little is known about any potential long-term precursors, in particular in terms of deformation.…”

Revision of earthquake hypocentre locations in global bulletin data sets using source-specific station terms

“…The first database is provided by the IPOC (Integrated Plate Boundary Observatory Chile) network (GFZ German Research Centre for Geosciences; Institut des Sciences de l'Univers-Centre National de la Recherche CNRS-INSU 2006) shown by triangles in Figs 7(a) and (b) [catalogue provided by Bernd Schurr as personal communication, 2015; a subset of this catalogue was published in Schurr et al (2014)]. Events roughly within the network and with magnitudes M ≥ 4 are used as comparison data set.…”