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Continuingmegathrust earthquake potential in Chileafter the 2014 Iquique earthquakeGavin P. Hayes1, Matthew W. Herman2, William D. Barnhart1, Kevin P. Furlong2, Sebastian Riquelme3, Harley M. Benz1,Eric Bergman4, Sergio Barrientos3, Paul S. Earle1 & Sergey Samsonov5

The seismic gap theory1 identifies regions of elevated hazard basedon a lack of recent seismicity in comparisonwith other portions of afault. Ithas successfully explainedpast earthquakes (see, for example,ref. 2) and is useful for qualitatively describing where large earth-quakes might occur. A large earthquake had been expected in thesubduction zone adjacent to northern Chile36, which had not rup-tured in a megathrust earthquake since aM 8.8 event in 1877. On1 April 2014 a M8.2 earthquake occurred within this seismic gap.Herewepresent an assessmentof the seismotectonics of theMarchApril 2014 Iquique sequence, including analyses of earthquake reloca-tions,moment tensors, finite faultmodels,momentdeficit calculationsand cumulative Coulomb stress transfer. This ensemble of informa-tion allows us to place the sequence within the context of regionalseismicity and to identify areas of remaining and/or elevatedhazard.Our results constrain the size and spatial extent of rupture, and indi-cate that thiswasnot theearthquake thathadbeenanticipated. Signifi-cant sectionsof thenorthernChile subduction zonehavenot rupturedin almost 150 years, so it is likely that future megathrust earthquakeswill occur to the south and potentially to the north of the 2014 Iqui-que sequence.On 1 April 2014, aM 8.2 earthquake ruptured a portion of the sub-

ductionzone innorthernChileoffshoreof the cityof Iquique, amajorportand hub for the countrys coppermining industry. Peak shaking inten-sities reached MMI VIII on land, and a tsunami,2m high hit coastaltowns in southernPeru andnorthernChile. Six fatalitieswere attributedto the event, and at least 13,000 homeswere damaged or destroyed. Pre-liminary estimates suggest total economic losses close toUS$100million7.A megathrust earthquake in this region was not unexpected; 230

M.3.5 earthquakes occurredoffshoreof IquiquebetweenAugust 2013andMarch 2014, a 950% increase in the rate from January to July 2013(ref. 8). Over the threeweeks before the event, thereweremore than 80earthquakes betweenM 4.0 andM 6.7 (Fig. 1).Before the recent sequence,this subduction zone (between,19.5u S and 21u S) had been identifiedas a seismic gap3,4, last rupturing in aM,8.8 earthquake in 1877. The1April eventwas followedbya vigorous aftershock sequencewithmorethan100M$4 earthquakes, including aM 7.7 aftershocknear the south-ernmost extent of theM 8.2 rupture.The seismic moment of all 2014 earthquakes to date equates to an

event of justM,8.3, much smaller than the estimated size of the 1877earthquake and of the potential event that could fill the seismic gap5,6

(Methods). The earthquake sequence spans a section of the subductionzone about one-third of the size of the inferred 1877 rupture9. It remainsunknown how subduction zones behave over multiple seismic cyclesand whether any given section can be associated with a characteristicearthquake, making it unclear whether this seismic gap should behavein the twenty-first century as it did in thenineteenth.Observations sug-gest that enough strain has accumulated along this plate boundary seg-ment to host an earthquake close toM 9 (see, for example, ref. 5), andearthquakes of this size have occurred in the past. The expectation from

a seismic hazard perspective is that the fault can host another event ofa similar magnitude. Although a great-sized earthquake here had beenexpected, it is possible that this event was not it10.Sections of this subduction zone have ruptured since 1877 (Fig. 1),

most notably in 1967, in aM 7.4 event between,21.5u S and 22u S, andin the 2007M 7.7Tocopilla earthquakebetween,22u S and23.5u S. Slipduring these events was limited to the deeper extent of the seismogeniczone, leaving shallower regions unruptured6,11. Farther south, the 1995M 8.1Antofagasta earthquakebroke the seismogenic zone immediatelysouthof theMejillonesPeninsula, a feature argued to beapersistent bar-rier to rupture propagation11,12. Adjacent to the southern coast of Peru,a seismic gap associatedwith the 1868M 8.8 rupturewas partly filledbythe 2001 M 8.4 Arequipa earthquake. Coupling models5 indicate thatstrain accumulationmay remain to the southeast of theArequipa event,towards the northernmost edge of the 2014 rupture in Chile (a section,200 km long).However,within that zone adjacent toArica, at the pro-nounced bend in the subduction zone andPeruChile Trench, couplingis low and may not support throughgoing rupture.The National Earthquake Information Center (NEIC) and Centro

SismologicoNacional (CSN)W-phase centroidmoment tensor (CMT)solutions for the 1April 2014 earthquake align with the slab interface13

and indicate a seismicmoment of (1.002.35)3 1021Nm (Mw 5 8.078.18). Our finite fault solution14 (Methods and Fig. 2) describes a rup-ture area in the deeper portion of the seismogenic zone, with a peak slipof ,8m to the southeast of the hypocentre at depths of ,3040 km.Shallower slip to the north is not well resolved but may account for thegeneration of a local tsunami. Slip extendedonly,50 kmalong the inter-face from the hypocentre, a very compact rupture area for an earth-quake of this size15. The location of peak slip in thismodel is consistentwithW-phase CMT inversions (Extended Data Fig. 1), slab geometry(Extended Data Fig. 2) and the centroid location of an updated CMT(Methods).Thismodel alsomatches inversionsof regional geodeticdata (Methods

andExtendedData Fig. 3), which donot uniquely resolve slip up-dip ofthe hypocentre but place strong constraints on the location and extentof slip between the hypocentre and the coast, and on its down-dip edge.Geodeticmodels show that slip during themainshock endedwest of thecoastline, in agreementwith (perhaps slightlyup-dipof) seismicmodels.Tsunamimodels16 place better constraint on shallowslip, and show littlemotion up-dip and west of the hypocentre.Earthquakes in this sequencewere relocatedbyusing amultiple-event,

hypocentroidaldecomposition17 algorithm,using seismicphasedata fromlocal, regional and global stations. This allows us to interpret locationswithin a consistent, regionally anchored and absolute framework2,18.Beginningon16Marchwith aM 6.7 earthquake, the foreshock sequencegenerated more than 80M$4 earthquakes (Fig. 2), showing a north-wardmigration towards the epicentreof the1AprilM8.2 event (ExtendedDataFig. 4).Over the followingweek, theNEICrecorded140M$4after-shocks, including a M 7.7 event on 3April, 27 h after the mainshock.

1National Earthquake InformationCenter, UnitedStatesGeological Survey,Golden,Colorado80401,USA. 2DepartmentofGeosciences,PennsylvaniaStateUniversity, UniversityPark, Pennsylvania16802,USA. 3CentroSismologicoNacional, UniversidaddeChile, BlancoEncalada2002, Santiago8370449,Chile. 4Global Seismological Services, Golden,Colorado80401,USA. 5CanadaCentre forMapping andEarth Observation, Natural Resources Canada, Ottawa, Ontario K1A0E4, Canada.

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Between the mainshock and this largest aftershock, earthquake loca-tions migrated southwards, towards the epicentre of theM 7.7 event.Since then, aftershocks have been scattered up-dip of the rupture areasof the two largest events.Analyses of regional moment tensors19 (RMTs; Methods) of fore-

shocks indicate thatmany represent thrust faulting on or near the plateinterface (Fig. 2).However, about 20%havewell-constraineddepths tooshallow to involve interface slip, have focal mechanisms inconsistentwith thrust faulting, or have nodal planes severely rotated with respectto local slab structure.Although constraining the absolute depths of off-shore earthquakes in subduction zones is difficult, the spread of solu-tions (which are all subject to similar uncertainties) and the systematicrotation of many mechanisms with respect to slab geometry indicatethat upper plate faulting was involved in the foreshock sequence. Thisis particularly true of theM 6.7 event on16March,whose depth (20 km,NEIC; 20.6 km, CSN; 12 km, global centroid moment tensor project(global CMT)20; 15.5 km, NEIC W-phase) is substantially shallowerthan the slab, andwhose shallownodalplane is rotated 60u (globalCMT;CSN W-phase) to 70u (NEIC W-phase) anticlockwise with respect tothe slab, perhaps indicating that this foreshockoccurred above the plateinterface alonga splay fault.AftershockRMTs showthat the vastmajorityoccurred along the megathrust interface (Fig. 2), surrounding regionsof largest co-seismic slip. TheM 7.7 aftershock ruptured a compact por-tion of the seismogenic zone ,50 km south and directly along strike

from themainshock asperity. Almost all 2014 events have been locatedup-dip of the rupture zones of the two biggest quakes.Recentmegathrust earthquake sequences (see, for example, refs 21, 22)

have demonstrated the need for integrative real-time monitoring andassessments thatmap seismic cycles intomodels of strain accumulationnear the source regions of large earthquakes. The Iquique sequence is anideal case study involving the integration of geodetic, geodynamic andseismological constraints to improve the quantification and assessmentof anearthquake sequence as it evolves.Organizing seismotectonic infor-mation for major global plate boundaries2,23 is crucial for understandingthe spectrumof expectedbehaviourof a fault zone after amajor event hasoccurred (and indeed beforehand), and is the foundation of any frame-work for better communicationof time-dependent earthquake hazardsto affected communities24.To understand the scale of this earthquake in the context of theChil-

ean subduction zone, it is useful to compare the Iquique sequence withthe2010M 8.8Maule earthquake2, theChileanmargins last greatmega-thrust event. Although both occurred in recognized seismic gaps, theirevolution, behaviour and characteristicswere quite different (ExtendedData Fig. 4). In contrast to Iquique,Maule did not have any recognizedforeshocks. Itsmainshock nucleated in themiddle of the SouthCentralChile Seismic Gap1 and ruptured bilaterally beyond the extent of the gapinto the 1985M 8.0and1960M 9.5 rupture zones to thenorth andsouth,respectively.The Iquiqueearthquakenucleatedwithin theNorthernChileSeismic Gap, but in a region that had slippedmore recently thanmuch ofthe gap, and it ruptured an areamuch shorter than the gaps recognizedextent5,6,11,25. Over the twomonths surrounding theMaule mainshock,earthquakesdemonstrated a typicalGutenbergRichter relationship,witha b value of,0.85 (ExtendedData Fig. 4). The Iquique sequence, on theotherhand, is deficient inmoderate-to-largeevents (b< 0.73, in contrastwith b< 1.02 over the preceding year). Co-seismic2 and post-seismic26

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Figure 1 | Tectonic setting of 2014 Iquique earthquake sequence. Ruptureareas of large historical earthquakes are indicated by grey (modelled) andwhite(estimated) outlines. Relocated 2014 earthquakes are shown by colour:foreshocks in red; those between the mainshock (largest orange circle) on1April 2014 and the largest aftershock (M 7.7) on 3April in orange; and morerecent events in yellow. Rupture areas of theM 8.2 andM 7.7 events arecoloured and contoured at 2.0-m intervals. The extent of the northern Chileseismic gap is indicated with arrows. Bathymetric data are taken from theGEBCO_08 grid30.

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slip during the Maule sequence indicated rupture of most of the seis-mogenic zone, resulting in aftershockmechanisms that spannedabroadrange of the faulting spectrum (Extended Data Fig. 4). In contrast, theIquique sequencehasnot elicitedaclearupperplateorouter rise response.Future studiesof regionalGPSdata27may reveal the extentofpost-seismicinterface slip and its relation to the 2014 Iquique sequence.WecanuseCoulombfailure stress (DCFS) analysis28 to assesswhether

the 2014 Iquique earthquake sequence followed a spatial and temporalmigration pattern dictated by the stress changes caused by previousearthquakes.DCFScalculations (Fig. 3) showthat 18of the20 foreshockswith associated RMTs ruptured the megathrust interface where it hadbeen positively stressed by previous events, and loaded the hypocentralregion of the subsequent M 8.2 event by 0.04MPa. It thus seems thatthe northward migration of foreshocks responded to cascading DCFS,ultimately leading to themainshock.Thehypocentral regionof theM 7.7aftershock was loaded 0.25MPa by themainshock and the first 27 h ofaftershocks. Aftershocks have generally nucleated in areas of increasedDCFS, surrounding themain slip patches of the largest events. Overall,the hypocentres of ,70% of relocated aftershocks (94 of 138 events)occurred inareasofpositiveDCFS. Ifuncertainties inrelocatedhypocentres(623km)are considered (Fig. 3),more than80%of aftershocksoccurredin regions of positiveDCFS, lending support to the real-timeuse ofDCFSmodelling as earthquake sequences unfold to aid in anticipating likelylocations for subsequent events.Analysis ofmoment accumulation and release along theplatemargin

(Fig. 4) shows that themain asperity ruptured in theM 8.2 earthquakepartly filled a historical gap in moment relative to adjacent sections ofthe arc. TheMarch 2014 foreshocks began near the northern extent ofthe 1933 event, and moved northwards. Despite surrounding regionsof largermomentdeficit, the southernportionof theM 8.2 and theM 7.7aftershock ruptured approximately the same region as the 1933 event.Moment fromboth large 2014earthquakes, and fromall historical events,falls off rapidly to the south between 20u S and 21u S; here, a 5080-kmsection of arc has seen little to no seismic activity over the past centuryormore, until thenorthernextentof the1967M7.4 earthquake is reached.

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accurate rupture areas are used8. Red showsmoment for historical earthquakes;blue for 2014 seismicity (dark blue up to 3April 2014; light blue since then);green represents all summed moment. Horizontal dashed lines representmoment accumulation levels given constant coupling percentages of 50%, 75%and 100%.

Figure 3 | Coulomb failure (DCFS) stress changes for foreshocks andaftershocks. DCFS is resolved onto the subduction zone interface13 1 daybefore the mainshock (a) and after the largest aftershock (b). Earthquakes arecoloured by the localDCFS resulting fromprevious earthquakes, at their time ofoccurrence; light grey symbols indicate negative stress changes, and darkgrey symbols, positive. The slip models of the mainshock and theM 7.7aftershock are shown with white contours (1 m intervals). The dashed boxin b represents the spatial extent of the region in a.

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Continuing south, parts of the seismogenic zonehave been ruptured bythis 1967event and the2007M 7.7Tocopilla earthquake, althoughmuchof the shallowportion of the subduction zone here has remained unrup-tured since at least 1877.These analyses imply that there is cause for concern for one ormore

further megathrust earthquakes in northern Chile. Large earthquakessince1877, in1967and2007 in the south, andnowwith this2014 sequencein thenorth, have combined to rupture a fraction of theNorthernChileSeismicGap, only partly releasing accumulated strain (Fig. 4). Thehighlycoupled section of the subduction zone,5080 km to the south of the2014 events, and themoderately coupled interface for a similar distanceto the north, have beenpositively stressedby the 2014 Iquique sequence(Fig. 3).Neither sectionhashosted significant events in almost 150 years.Given that earthquakes in this subductionzonehaveoccurred in responseto stress transfer in the past, there is now an increased probability ofmegathrust earthquakes occurring to the south or north of the April2014 events in the future.Comparisons between the 2014 Iquique and 2010 Maule sequences

highlight the broad range of seismotectonic behaviour that is possiblealong the same subduction zone, leadingup to and in response tomega-thrust ruptures. Until we understand these differences, our ability toaccurately predict the future behaviour of surrounding megathrustregions29 will remain elusive. Nevertheless, it is likely that the subduc-tion zone to the south of theMarchApril 2014 events, between Iquiqueand Antofagasta, will host great-sizedmegathrust events in the future.A smaller section to the north, towards Arica, may also be capable ofhosting aM$8 event. Chilean and global seismologists now face thedifficult task of communicating this uncertain yet perhaps elevatedhazard, without appearing alarmist.

Online Content Methods, along with any additional Extended Data display itemsandSourceData, are available in theonline versionof thepaper; referencesuniqueto these sections appear only in the online paper.

Received 24 April; accepted 10 July 2014.

Published online 13 August 2014.

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8. CentroSismologicoNacional, UniversidaddeChile,http://www.sismologia.cl/..9. Kausel, E. Los terremotos deAgostode1868 yMayode1877que afectaron el Sur

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22. Hayes, G. P., Furlong, K. P., Benz, H. M. & Herman, M. W. Triggered aseismic slipadjacent to the 6 February 2013Mw 8.0 Santa Cruz Islands megathrustearthquake. Earth Planet. Sci. Lett. 388, 265272 (2014).

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AcknowledgementsWe thank R. Briggs for his comments in improving themanuscript. We thank the CSN, andmember institutions of the IPOC network for theiroperation of seismic stations in northern Chile and for the contribution of waveformdata and phase picks to this study. This study made use of broadband seismic datafromglobally distributed seismometers available to the USGSNEIC in real time or nearreal time (networksAE, BK, C, CN, CU, DK, G,GE,GT, IC, II, IU, IWandUS) andarchived inthe NEIC Central Waveform Buffer and at the Incorporated Research Institutions forSeismology Data Management Center. RADARSAT-2 data were provided by theCanadian Space Agency and MDA Corporation. Bathymetry data come from GEBCO2008 (ref. 30).Manyof the figuresweremadewith theGenericMappingTools softwarepackage31. National Science Foundation grant EAR-1153317 provided support toK.P.F. and M.W.H. for this research. Any use of trade, product, or firm names is fordescriptive purposes only and does not imply endorsement by the US Government.

Author ContributionsG.P.H., K.P.F. andW.D.B. wrote themanuscript. G.P.H. generatedFigs 1, 2 and4;M.W.H. generated Fig. 3. G.P.H., M.W.H. andW.D.B. generated ExtendedData figures. G.P.H. conducted seismic fault inversions and moment deficitcalculations.M.W.H. conducted stress transfer calculations.W.D.B. conducted geodeticfault inversions. G.P.H. and W.D.B. were jointly responsible for fault models. S.R. andS.B. contributed regional real-time analyses and data used in seismic inversions andearthquake relocations. E.B. and H.M.B. conducted earthquake relocations. P.S.E.contributed to the real-time analysis of the earthquake sequence and edited themanuscript. S.S. scheduled the acquisition of RADARSAT-2 data andperformed InSARanalysis.

Author Information Reprints and permissions information is available atwww.nature.com/reprints. The authors declare no competing financial interests.Readers are welcome to comment on the online version of the paper. Correspondenceand requests for materials should be addressed to G.P.H. ([email protected]).

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METHODSAt the latitude of this sequence, the Nazca plate subducts eastwards beneath SouthAmerica at a rate of,73mmyr21 (ref. 33). Between the PeruChile Trench,wheresubduction begins, and the coastline (which approximates the eastern limit of seis-mic coupling atdepth), dips increase from,10u to,20u. The resulting seismogenicwidth for this portion of the subduction zone has been calculated as,140km (see,for example, ref. 13). Estimates of seismogenic coupling using geodetic data5,25 showthat the interface is nearly fully locked (coupling coefficient a$ 0.75). This impliesthat the northern Chile subduction zone had stored enough strain between 1877and 2014 to host an event as large asM,8.9.Approximately 360 events were analysed for RMTs, following the approach of

ref. 19.Thismethod solves for the source depth,momentmagnitude, strike anddipand rake angles of a shear-dislocation source bymeans of a time-domain inversionscheme. The specifics of the approach and its use to study earthquake sequencesare discussed further in ref. 2.We model the earthquake source by using teleseismic data as a rupture front of

finitewidth propagating on a series of two-dimensional planar fault segments,wheretheprescribedorientationsmatch local slab geometry13.Weuse a simulated-annealingalgorithm14 to invert azimuthally distributed P, SH and surface waves for the com-binations of slip amplitude, rake angle (77u137u; W-phase moment tensor rake630u), rupture velocity (0.53.5 km s21) and rise time at each sub-fault elementthat best explains the teleseismic records.The source time function for the earthquake34 indicates weak initial seismic

radiation, with little activity in the first 20 s after origin time, and themain asperityslip occurring after,30 s. This delay in rupture can lead to artefacts in fault-slip inver-sionmodels with most standard modelling algorithms, if constant rupture move-out at a fixed rupture velocity from a fixed hypocentre is imposed. Here we allowlong sub-fault rise times and slow rupture velocities to minimize artefacts causedby such modelling assumptions. Allowing longer rise times produces models thatlocate the main slip patch just north of 20u S. Tests with a variety of fault geomet-ries (Extended Data Fig. 3) also show that the location of the modelled peak slip isheavily dependent on the dip of the assumed fault plane(s). This leads us to favourmodels with multiple connected planes where the dips better match the observedcurved slab geometry in the vicinity of the rupture10, therebyminimizing artefactsresulting frommismatches in geometry betweenmodel and reality. These assump-tions also lead to fault-slip solutions that bettermatch preliminary results from theinversion of on-land geodetic data (Extended Data Fig. 2).

We perform a suite of W-phase inversions35 for the mainshock at fixed depthsranging from13.5 to 40.5 km, using 130high-quality groundmotion recordings from88 globally distributed seismic stations, bandpass filtered from 200 to 1,000 s. Ourbest-fitting solution indicates a nearly pure double-couplewith a geometry (W 5352u,d 5 12u, l 5 97u) very similar to the rapid USGS W-phase CMT, a moment of2.33 1021Nm (Mw5 8.17), and a centroid time shift of 45 s. Inversions show a lessthan 1% change in misfit from 13.5 to 30.5 km, and only a 3% change to 40.5 km(Extended Data Fig. 1).We explore fault slip constrained from geodetic observations using a single

RADARSAT-2 interferogram (1 July 2011 to 4April 2014) and 5-min GPS pointsolutions for IGS station IQQEprocessed by theNevadaGeodetic Laboratory36. Bothdata sets span theMw 5 8.2mainshock andM 7.7 aftershock. InSARdata process-ing follows the approach of ref. 37. Two faultmodels are generated (ExtendedDataFig. 2). The first assumes a single planar fault where the geometry (strike5 358u,dip5 13u) is constrained by means of an inversion for uniform slip on a plane ofunknown orientation and size, using the Neighbourhood Algorithm38. We thenapply an iterative, variable discretization algorithm that produces a triangular faultmeshwhere the sub-fault sizes reflectmodel resolution39. The secondmodel is con-structed from Slab1.0 (ref. 10), uniformly meshed with triangular dislocations. Inboth approaches we solve for distributed slip in a uniform elastic halfspace40, applyminimummoment regularization and choose a smoothing constant using the jRicriterion39.

33. DeMets, C., Gordon, R. G. & Argus, D. F. Geologically current plate motions.Geophys. J. Int. 181, 180 (2010).

34. USGS. M8.294km NW of Iquique, Chile (BETA), 2014-04-01 23:46:47 UTC,http://earthquake.usgs.gov/earthquakes/eventpage/usc000nzvd#scientific_finite-fault..

35. Duputel, Z., Rivera, L., Kanamori, H. & Hayes, G. W phase source inversion formoderate to large earthquakes (19902010). Geophys. J. Int. 189, 11251147(2012).

36. Nevada Geodetic Laboratory.,http://geodesy.unr.edu/..37. Barnhart, W. D., Hayes, G. P., Samsonov, S. V., Fielding, E. J. & Seidman, L. E.

Breaking the oceanic lithosphere of a subducting slab: the 2013 Khash, Iranearthquake. Geophys. Res. Lett. 41, 3236 (2014).

38. Sambridge, M. Geophysical inversion with a neighbourhood algorithmI. Searching a parameter space. Geophys. J. Int. 138, 479494 (1999).

39. Barnhart,W.D.&Lohman,R.B.Automatedfaultmodeldiscretizationfor inversionsfor coseismic slip distributions. J. Geophys. Res. Solid Earth 115, B10419 (2010).

40. Okada, Y. Internaldeformationdue to shear and tensile faults inahalf-space.Bull.Seismol. Soc. Am. 82, 10181040 (1992).

LETTER RESEARCH


http://earthquake.usgs.gov/earthquakes/eventpage/usc000nzvd#scientific_finite-faulthttp://earthquake.usgs.gov/earthquakes/eventpage/usc000nzvd#scientific_finite-faulthttp://geodesy.unr.edu/

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solutions from 13.5 to 30.5 km are within 1% of the best solution at 25.5 kmdepth. Solutions to 40.5 km are within 3% of the best-fit solution.

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subduction zone through the hypocentre, perpendicular to the strike of thesource inversions. Slab geometry is shownwith a red dashed line, and historicalmoment tensors with grey focal mechanisms. Green dots represent aprojection of the single-plane model onto the cross-section, blue represent thethree-plane model, and red the five-plane model.

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Extended Data Figure 3 | Geodetic fault modelling of the Iquiquemainshock and largest aftershock. Models are inverted from a singledescending RADARSAT-2 interferogram (1 July 2011 to 4April 2014;incidence angle 32u, azimuth 2166u) consisting of five concatenatedMulti-Look Fine frames, and displacements are determined from GPS stationIQQE. af, For models inverted onto a single fault plane (ac) and onto theSlab1.0 model geometry (df), slip distributions show the best-fitting solution(a, d), as determined by the jRi criterion, as well as relatively rough (b, e) andsmooth (c, f) models to demonstrate the dependence of up-dip slip on

regularization. Dots indicate the relocated epicentres of theM 8.2 andM 7.7events, respectively. g, Unwrapped interferogram (negative is motion awayfrom the satellite, or subsidence). h, Down-sampled interferogram with GPSvectors from theM 8.2 (blue) andM 7.7 (black) events at station IQQE.ik, Predicted surface displacements from the best-fitting geodetic (i, j) andteleseismic (k) finite fault models (synthetic interferogram plus synthetic GPSvector in red). The predicted displacements from teleseismic data include thedisplacements of both theM 8.2 andM 7.7 models.

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distribution of best double-couple mechanisms of CMT solutions. These plotsshow the relative dominance of strike-slip (SS, top), normal (No, bottom left)and thrust (Th, bottom right) faulting for an earthquake mechanism. Theregions between each vertex and the nearest red contour denote dominantfaulting style. Symbols are coloured as in a.d, Plot of earthquake latitude againsttime through the duration of the Maule 2010 sequence. As in a, red circlesrepresent foreshocks, orange the first 27 h of aftershocks, and yellowsubsequent events. e, GutenbergRichter relationship for the 2010 Maulesequence, including 2,450 earthquakes. Magnitude of completeness is aboutM 3.7, and the b value is 0.83. f, Triangle diagram for Maule; symbols as in c.

LETTER RESEARCH


TitleAuthorsAbstractReferencesMethodsMethods ReferencesFigure 1 Tectonic setting of 2014 Iquique earthquake sequence.Figure 2 Source processes of events in the March-April 2014 Iquique earthquake sequence.Figure 3 Coulomb failure (DCFS) stress changes for foreshocks and aftershocks.Figure 4 Moment deficit along the northern Chile subduction zone.Extended Data Figure 1 Depth resolution of W-phase inversion.Extended Data Figure 2 The effect of fault geometry on teleseismic source inversions.Extended Data Figure 3 Geodetic fault modelling of the Iquique mainshock and largest aftershock.Extended Data Figure 4 Characteristics of the 2010 Maule and 2014 Iquique earthquake sequences.