The seismic gap theory1 identifies regions of elevated hazard based on a lack of recent seismicity in comparison with other portions of a fault. It has successfully explained past earthquakes (see, for example, ref. 2) and is useful for qualitatively describing where large earthquakes might occur. A large earthquake had been expected in the subduction zone adjacent to northern Chile3,4,5,6, which had not ruptured in a megathrust earthquake since a M ∼8.8 event in 1877. On 1 April 2014 a M 8.2 earthquake occurred within this seismic gap. Here we present an assessment of the seismotectonics of the March–April 2014 Iquique sequence, including analyses of earthquake relocations, moment tensors, finite fault models, moment deficit calculations and cumulative Coulomb stress transfer. This ensemble of information allows us to place the sequence within the context of regional seismicity and to identify areas of remaining and/or elevated hazard. Our results constrain the size and spatial extent of rupture, and indicate that this was not the earthquake that had been anticipated. Significant sections of the northern Chile subduction zone have not ruptured in almost 150 years, so it is likely that future megathrust earthquakes will occur to the south and potentially to the north of the 2014 Iquique sequence.
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We thank R. Briggs for his comments in improving the manuscript. We thank the CSN, and member institutions of the IPOC network for their operation of seismic stations in northern Chile and for the contribution of waveform data and phase picks to this study. This study made use of broadband seismic data from globally distributed seismometers available to the USGS NEIC in real time or near real time (networks AE, BK, C, CN, CU, DK, G, GE, GT, IC, II, IU, IW and US) and archived in the NEIC Central Waveform Buffer and at the Incorporated Research Institutions for Seismology Data Management Center. RADARSAT-2 data were provided by the Canadian Space Agency and MDA Corporation. Bathymetry data come from GEBCO 2008 (ref. 30). Many of the figures were made with the Generic Mapping Tools software package31. National Science Foundation grant EAR-1153317 provided support to K.P.F. and M.W.H. for this research. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the US Government.
The authors declare no competing financial interests.
Extended data figures and tables
Relative misfit of W-phase inversions using fixed depths from 13.5 to 40.5 km. All solutions from 13.5 to 30.5 km are within 1% of the best solution at 25.5 km depth. Solutions to 40.5 km are within 3% of the best-fit solution.
Top: comparison of source inversions on a single-plane model with multi-plane models that gradually improve on their match to slab geometry, with models for one plane (left), three planes (middle) and five planes (right). As the fit to slab geometry improves (shown in the bottom panel), slip gradually migrates up-dip and west of the coastline. Bottom: cross-section of the subduction zone through the hypocentre, perpendicular to the strike of the source inversions. Slab geometry is shown with a red dashed line, and historical moment tensors with grey focal mechanisms. Green dots represent a projection of the single-plane model onto the cross-section, blue represent the three-plane model, and red the five-plane model.
Models are inverted from a single descending RADARSAT-2 interferogram (1 July 2011 to 4 April 2014; incidence angle 32°, azimuth −166°) consisting of five concatenated Multi-Look Fine frames, and displacements are determined from GPS station IQQE. a–f, For models inverted onto a single fault plane (a–c) and onto the Slab1.0 model geometry (d–f), slip distributions show the ‘best-fitting’ solution (a, d), as determined by the jRi criterion, as well as relatively rough (b, e) and smooth (c, f) models to demonstrate the dependence of up-dip slip on regularization. Dots indicate the relocated epicentres of the M 8.2 and M 7.7 events, respectively. g, Unwrapped interferogram (negative is motion away from the satellite, or subsidence). h, Down-sampled interferogram with GPS vectors from the M 8.2 (blue) and M 7.7 (black) events at station IQQE. i–k, Predicted surface displacements from the best-fitting geodetic (i, j) and teleseismic (k) finite fault models (synthetic interferogram plus synthetic GPS vector in red). The predicted displacements from teleseismic data include the displacements of both the M 8.2 and M 7.7 models.
a, Plot of earthquake latitude against time through the duration of the Iquique sequence. Foreshocks to the 1 April M 8.2 earthquake are plotted in red, aftershocks between 1 April and the M 7.7 event on 3 April in orange, and subsequent aftershocks in yellow. This temporal history of the sequence reveals northward (foreshock) followed by southward (early aftershocks) spatial migration across the subduction zone interface. b, Gutenberg–Richter relationship for this period of the 2014 Iquique sequence, including 600 earthquakes. Magnitude of completeness is about M 3.8, and the b value is 0.73. c, Triangle diagram for the sequence, used to display the distribution of best double-couple mechanisms of CMT solutions. These plots show the relative dominance of strike-slip (SS, top), normal (No, bottom left) and thrust (Th, bottom right) faulting for an earthquake mechanism. The regions between each vertex and the nearest red contour denote dominant faulting style. Symbols are coloured as in a. d, Plot of earthquake latitude against time through the duration of the Maule 2010 sequence. As in a, red circles represent foreshocks, orange the first 27 h of aftershocks, and yellow subsequent events. e, Gutenberg–Richter relationship for the 2010 Maule sequence, including 2,450 earthquakes. Magnitude of completeness is about M 3.7, and the b value is 0.83. f, Triangle diagram for Maule; symbols as in c.
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Hayes, G., Herman, M., Barnhart, W. et al. Continuing megathrust earthquake potential in Chile after the 2014 Iquique earthquake. Nature 512, 295–298 (2014). https://doi.org/10.1038/nature13677
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