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...

Viscoelastic effects potentially play an important role during all phases of the earthquake cycle in subduction zones. However, most current models neglect such effects in the interseismic deformation pattern. Here we use finite element method (FEM) models to investigate the control of viscoelasticity on interseismic deformation and to highlight the pitfalls of interpreting the data with purely elastic models for both the forward and inverse problems. Our results confirm that elastic models are prone to overestimating the interseismic locking depth, a crucial parameter for estimating the maximum possible earthquake magnitude. The application of the viscoelastic model improves the fit to the interseismic deformation, especially in the inland area. Additionally, we construct 3‐D FEM models constrained by geophysical and GPS data and apply our methodology to the Peru‐North Chile subduction zone. Our results indicate that viscoelastic effects contribute significantly to the observed GPS data. The signals interpreted as back‐arc shortening in the elastic model can be alternatively explained by viscoelastic deformation, which, in turn, dramatically refines the interseismic locking pattern in both dip and strike directions. Our viscoelastic locking map exhibits excellent correlation with the slip distributions of previous earthquakes, especially the recent 2014 Mw 8.1 Iquique earthquake. The incorrect elastic assumptions affect the analysis of interseismic deformation with respect to slip deficit calculations. Our results thus suggest that it is necessary to thoroughly reevaluate existing locking models that are based on purely elastic models, some of which attribute viscoelastic deformation to different sources such as microplate sliver motions.

Editor: P. ShearerKeywords: subduction post-seismic megathrust afterslip GPS MauleThe excellent spatial coverage of continuous GPS stations in the region affected by the Maule 8.8 2010 earthquake, combined with the proximity of the coast to the seismogenic zone, allows us to model megathrust afterslip on the plate interface with unprecedented detail. We invert post-seismic observations from continuous GPS sites to derive a time-variable model of the first 420 d of afterslip. We also invert co-seismic GPS displacements to create a new co-seismic slip model. The afterslip pattern appears to be transient and non-stationary, with the cumulative afterslip pattern being formed from afterslip pulses. Changes in static stress on the plate interface from the co-and post-seismic slip cannot solely explain the aftershock patterns, suggesting that another process -perhaps fluid related -is controlling the lower magnitude aftershocks. We use aftershock data to quantify the seismic coupling distribution during the post-seismic phase.Comparison of the post-seismic behaviour to interseismic locking suggests that highly locked regions do not necessarily behave as rate-weakening in the post-seismic period. By comparing the inter-seismic locking, co-seismic slip, afterslip, and aftershocks we attempt to classify the heterogeneous frictional behaviour of the plate interface.

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The postseismic deformation captured with continuous Global Positioning System (cGPS) monitoring following many recent megathrust events has been shown to be a signal composed of two dominant processes: afterslip on the plate interface and viscoelastic relaxation of the continental and oceanic mantles in response to the coseismic stress perturbation. Following the south central Chile 2010 Maule Mw 8.8 earthquake, the time series from the regional cGPS network show a distinct curvature in the pathway of the horizontal motion that is not easily fit by a stationary decaying pattern of afterslip in combination with viscoelastic relaxation. Here we show that with realistic assumptions about the long‐term decay of the afterslip signal, the postseismic signal can be decomposed into three first‐order contributing processes: plate interface relocking, plate interface afterslip, and mantle viscoelastic relaxation. From our analyses we conclude that the plate interface recovers its interseismic locking state rapidly (model space ranges between an instant recovery and a period of 1 year), a finding that supports laboratory experimental evidence as well as some recent studies of aftershocks and postseismic surface deformation. Furthermore, relocking is the main cause of the curvature in the cGPS signal, and this study presents a plausible range of geodetic relocking rates following a megathrust earthquake.