On 1 April 2014, Northern Chile was struck by a magnitude 8.1 earthquake following a protracted series of foreshocks. The Integrated Plate Boundary Observatory Chile monitored the entire sequence of events, providing unprecedented resolution of the build-up to the main event and its rupture evolution. Here we show that the Iquique earthquake broke a central fraction of the so-called northern Chile seismic gap, the last major segment of the South American plate boundary that had not ruptured in the past century1,2. Since July 2013 three seismic clusters, each lasting a few weeks, hit this part of the plate boundary with earthquakes of increasing peak magnitudes. Starting with the second cluster, geodetic observations show surface displacements that can be associated with slip on the plate interface. These seismic clusters and their slip transients occupied a part of the plate interface that was transitional between a fully locked and a creeping portion. Leading up to this earthquake, the b value of the foreshocks gradually decreased during the years before the earthquake, reversing its trend a few days before the Iquique earthquake. The mainshock finally nucleated at the northern end of the foreshock area, which skirted a locked patch, and ruptured mainly downdip towards higher locking. Peak slip was attained immediately downdip of the foreshock region and at the margin of the locked patch. We conclude that gradual weakening of the central part of the seismic gap accentuated by the foreshock activity in a zone of intermediate seismic coupling was instrumental in causing final failure, distinguishing the Iquique earthquake from most great earthquakes. Finally, only one-third of the gap was broken and the remaining locked segments now pose a significant, increased seismic hazard with the potential to host an earthquake with a magnitude of >8.5.
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Data used in this study come from the IPOC initiative (http://www.ipoc-network.org) operated by the GFZ – German Research Centre for Geosciences, Institut de Physique du Globe de Paris, Centro Sismológico National, Universidad de Chile, and Universidad Cátolica del Norte, Antofagasta, Chile. We also acknowledge the French–Chilean International Associated Laboratory (LIA) ‘Montessus de Ballore’ and the USA–Chilean Central Andean Tectonic Observatory Geodetic Array projects for giving access to data of several of their continuous GPS (cGPS) stations in Chile. Part of this work was made possible by the Hazard Assessment and Risk Team (HART) initiative funded by the GFZ and Hannover Re.
The authors declare no competing financial interests.
Extended data figures and tables
Extended Data Figure 1 Slip inversion scenarios employing different data sets and final waveform fits.
a, Final slip distribution of the 1 April mainshock obtained from the waveform inversion (left) of the teleseismic and local strong-motion seismograms, the inversion (middle) of static GPS displacement data, and the joint inversion (right) of the waveform and GPS data. b, The same for the 3 April aftershock. c, Data fit of the joint kinematic inversion for the 1 April mainshock. Top: observed and modelled teleseismic P waveforms. Station codes are marked on the seismogram and on the map. Bottom: comparison between the observed and modelled local strong-motion waveforms. Traces are scaled to a common maximum in each sub-plot. d, The same for the 3 April aftershock.
Extended Data Figure 2 Source time history and rupture velocity estimation from backprojection of high frequency teleseismic waves.
a, b, Time history of the peak semblance at each time frame (blue) (a) and the corresponding energy (red, arbitrary units) for the mainshock (left) and the Iquique aftershock (right) (b). Energy integrated over the whole grid is plotted as a black line. Whereas the red curves describe the energy time history of one (the principal) radiating point, the black lines take into account the seismic energy emitted from the whole source area. The time axis represents the central point of the sliding windows 8 s long; that is, the first onset of the event will affect the energy and semblance at nominal times up to 4 s before the physical onset of rupture. The semblance and energy peak at ∼160 s corresponds to an early aftershock. The area of the diamonds of Fig. 2b is scaled to the energy (red curves) shown in b (only solutions with semblance higher than 0.05 are shown in Fig. 2b). c, Distance of maximal semblance peaks to a reference profile (transects 1 and 2 for the mainshock (left) and the aftershock (right), respectively, plotted in Supplementary Videos 3 and 4). The figure is zoomed in the downdip migrations of the rupture fronts (about 0–30 s). The accelerated propagation can be identified in the interval 15–30 s. The area of the circles is scaled to the energy of the semblance maxima (red curves in b).
Extended Data Figure 3 Interseismic GPS data corrections, slip deficit estimation and sensitivity tests for interseismic locking inversion.
a, Demonstration of the effect of sliver and shortening corrections on the interseismic GPS data. The left plot shows the data in the stable South American reference frame. Red vectors indicate the stations that the corrections were applied to. All stations underwent the sliver correction. Stations in the northeast and southeast underwent shortening corrections. b, Slip deficit estimation. The left panel shows our locking model; the central panel shows a compilation of events since 1877 according to ref. 1 plus the Antofagasta and Tocopilla earthquakes of 1995 and 2007 (refs 4, 7), as well as our solutions for the Pisagua mainshock and largest aftershock. The right panel shows moment density along the trench projected on latitude. The total accumulated moment corresponds to a M 8.97 event. This is about one-sixth of the moment released during the 1960 Valdivia M 9.5 event further south, but sixfold that released in our region of interest between 1877 and now according to the events listed in the central panel summing to a magnitude of 8.41. Even though the Pisagua sequence released a significant amount of the moment in the northernmost part, the remaining moment would still correspond to M 8.92. c, Model smoothness plotted against residual. The optimal smoothing factor of 0.05 in the corner of the L-curve resulted in a residual of 0.17 cm yr−1. d, A selection of solutions with different smoothing factors. The central solution is the one we prefer. Black lines are 1-m isolines of the co-seismic slip distribution of the mainshock and the largest aftershock. e, Checkerboard tests of locking. Top: forward models consisting of three and two rows of locked patches. Lower panels: inverted locking patterns using the signal from the forward models at the GPS station positions applying the same uncertainties as in the actual observation data. For three locking rows, the trenchward row is clearly missed, whereas the areas closer to the station positions (magenta) are captured fairly well, the resolution being about 40 km.
a, Map showing stations used for common-mode filtering (black triangles) and those to which the correction signal is applied (green triangles). b, East and north displacement time series of the detrended, common-mode filtered data are plotted with blue crosses. The green lines are the cumulative GPS displacements predicted by the forward modelling of elastic displacements for events in the seismic foreshock catalogue. Black vertical dashed lines indicate the onsets of the two clusters of 2014. The red dashed line shows the zero positions of the GPS after detrending. A significant departure of the data from this zero position is an indication of transient motion at that station. c, The two panels show the GPS data displacements (blue) and the forward modelled GPS displacements of the seismically related slip (red) during the periods shown above each panel. Both the data and the predictions have been smoothed with a nine-day moving-average filter. Error ellipses are shown for the data displacements. The black dashed line is the trench and the solid black lines are the coastline and political borders. Events from the foreshock catalogue for days within the specified periods (also considering length of smoothing window) are plotted in dark grey. For the first 2014 cluster (left panel), GPS stations of interest in the south move towards a common source. For the second 2014 cluster (right panel), GPS vectors point towards the eventual Mw 8.1 rupture zone.
Extended Data Figure 5 Magnitude histogram of analysed catalogue and frequency-magnitude distributions for different data subsets.
a, Frequency–magnitude distribution of earthquakes within latitude 17.0°–21.0° S and longitude 70.0°–72.0° W, used in our b-value analysis and ETAS modelling. The histogram of the overall seismicity is shown by grey boxes, and thin lines refer to the cumulative distributions of foreshocks, aftershocks and the overall activity. Bold lines refer to the data used for the analysis above the magnitude threshold (Mc 3) ignoring periods of incomplete recordings after larger earthquakes. b, Frequency–magnitude distributions at different times before the mainshock. The distributions correspond to the b values shown in grey in Fig. 3e.
Animation of the April 1 Mw 8.1 mainshock rupture process (cumulative fault slip). The upper left inset shows the source time function (moment rate history). (MOV 268 kb)
Animation of the April 3 Mw 7.6 aftershock rupture process (cumulative fault slip). The upper left inset shows the source time function (moment rate history). (MOV 233 kb)
Time sequence of the spatial distribution of the radiated energy for the mainshock. The yellow star marks the epicenter adopted for calibrating the static corrections. White dashed 1 m contours show the co-seismic slip. (MP4 1153 kb)
Time sequence of the spatial distribution of the radiated energy for the aftershock. The yellow star marks the epicenter adopted for calibrating the static corrections. White dashed 0.5 m contours show the co-seismic slip. (MP4 314 kb)
Animation showing the evolution of horizontal displacements at coastal GPS stations near to the Pisagua segment leading up to the mainshock of April 1st 2014.
The dashed line is the trench and the solid black lines are the coastline and Chilean borders. Blue arrows show cumulative GPS displacements (the deviation from the zero position after de-trending and common mode filtering). Forward modeled GPS displacements of the seismically related slip are shown with red vectors. Both the data and the predictions have been smoothed with a 9-day long moving average filter. Events from the foreshock catalogue for days within the smoothing average window (+/- 4 days) are plotted in dark grey. (MP4 490 kb)
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Schurr, B., Asch, G., Hainzl, S. et al. Gradual unlocking of plate boundary controlled initiation of the 2014 Iquique earthquake. Nature 512, 299–302 (2014). https://doi.org/10.1038/nature13681
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