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.
Extended data figures
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).
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).
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.
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.
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.
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Scientific Reports (2019)