The Seismic Sequence of the 16 September 2015M_w 8.3 Illapel, Chile, Earthquake

Abstract

On 16 September 2015, the M w 8.3 Illapel, Chile, earthquake broke a large area of the Coquimbo region of north-central Chile. This area was well surveyed by more than 15 high-rate Global Positioning System (GPS) instruments, installed starting in 2004, and by the new national seismological network deployed in Chile. Previous studies had shown that the Coquimbo region near Illapel was coupled to about 60%. After the M w 8.8 Maule megathrust earthquake of 27 February 2010, we observed a large-scale postseismic deformation, which resulted in a strain rate increase of about 15% in the region of Illapel. This observation agrees with our modeling of viscous relaxation after the Maule earthquake. The area where upper-plate GPS velocity increased coincides very well with the slip distribution of the Illapel earthquake inverted from GPS measurements of coseismic displacement. The mainshock started with a small-amplitude nucleation phase that lasted 20 s. Backprojection of seismograms recorded in North America confirms the extent of the rupture, determined from local observations, and indicates a strong directivity from deeper to shallower rupture areas. The coseismic displacement shows an elliptical slip distribution of about 200 km × 100 km with a localized zone where the rupture is deeper near 31.3°S. This distribution is consistent with the uplift observed in some GPS sites and inferred from field observations of bleached coralline algae in the Illapel coastal area. Most of aftershocks relocated in this study were interplate events, although some of the events deeper than 50 km occurred inside the Nazca plate and had tension (slab-pull) mechanisms. The majority of the aftershocks were located outside the 5 m contour line of the inferred slip distribution of the mainshock.

Reawakening of large earthquakes in south central Chile: The 2016 Mw 7.6 Chiloé event

“…The slip inversion using teleseismic data [Kikuchi et al, 1993] shows a simple rupture patch of 20 km radius and short rupture duration and a seismic moment equivalent to M w 7.5 ( Figures S5 and S6). We inverted the static displacements obtained from GPS receivers in GUAG, MELK, QLLN, and RMBA, using a uniform isotropic medium with rigidity 45 GPa and Poisson's ratio 0.25 and the slab model proposed by Tassara and Echaurren [2012]. This slip distribution (Figure 4) agrees well with the aftershock distribution area; the maximum slip that is larger than 4.0 m. Finally, we performed a kinematic inversion using the regional strong motion data.…”

“…Before the 2010 Maule earthquake, surface deformation near the 1960 Valdivia rupture zone was dominated by viscoelastic relaxation processes following this megaearthquake [Hu et al, 2004] as well as by a heterogeneous degree of plate locking throughout the rupture zone [Moreno et al, 2011]. The 2010 Maule megaearthquake produced changes in the GPS velocities of sites adjacent to the rupture zone of that event at a continental scale [Klein et al, 2016;Ruiz et al, 2016;Melnick et al, 2017] (Figure 2). Melnick et al [2017] proposed that a superinterseismic phase was triggered by the Maule event in its adjacent segments, increasing the constant interseismic shear stress at the plate interface and bringing the megathrust closer to failure.…”

“…For deeper events it has been suggested that the plate interface may recover its interseismic locked state rapidly, as, for example, after the Pisco 2007 M w 8.0 and Tocopilla 2007 M w 7.8 events [Remy et al, 2016;Weiss et al, 2016]. Recent observations following the Valdivia 1960 M w 9.5 and Maule 2010 M w 8.8 megathrust earthquakes suggest that the relocking process may be heterogeneous in space and time [Moreno et al, 2011;Métois et al, 2012] and accompanied by a prolonged phase of postseismic relaxation of the mantle [see, e.g., Wang et al, 2012;Ruiz et al, 2016, Bedford et al, 2016Klein et al, 2016]. Therefore, to characterize the first indications of the seismic reawakening after the Valdivia megathrust earthquake and its associated lag time, it is essential to improve the understanding of the recurrence of earthquakes in the plate interface.…”

“…The M w 8.8 Maule 2010 megathrust earthquake [Vigny et al, 2011;Moreno et al, 2012] generated continentalscale deformation, accompanied by strain rotation and enhanced the coupling on segments adjacent to the rupture [Klein et al, 2016;Ruiz et al, 2016;Melnick et al, 2017;Loveless, 2017]. This process is described as a superinterseismic phase that increases shear stress away from the rupture zone of a great earthquake, bringing these areas closer to failure.…”

“…This process is described as a superinterseismic phase that increases shear stress away from the rupture zone of a great earthquake, bringing these areas closer to failure. It is very likely that a mechanical connection between the 2010 earthquake and the 2015 Illapel [Ruiz et al, 2016;Melnick et al, 2017] and 2016 Chiloé earthquakes may exist, triggering seismicity in the adjacent segments to the north and south of the 2010 rupture. In this study, we characterize the spatiotemporal evolution of surface deformation detecting velocity changes in continuous GPS time series after the 2010 Maule earthquake.…”