1 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 GEOG129_proof ■ February 2017 ■ 1/10 Geodesy and Geodynamics xxx (2017) 1e10 Contents lists available at ScienceDirect Geodesy and Geodynamics journal homepages: www.keaipublishing.com/en/journals/geog; http://www.jgg09.com/jweb_ddcl_en/EN/volumn/home.shtml Rupture area analysis of the Ecuador (Musine) Mw ¼ 7.8 thrust earthquake on April 16, 2016, using GOCE derived gradients Q4 Q1 a, b, * Orlando Alvarez , Andres Folguera c, Mario Gimenez a, b gico Ing, Volponi, Universidad Nacional de San Juan, Argentina Instituto Geofísico y Sismolo Consejo Nacional de Investigaciones Científicas y T ecnicas, Argentina c gicas, FCEN, Universidad de Buenos Aires, Argentina INDEAN e Instituto de Estudios Andinos “Don Pablo Groeber”, Departamento de Cs Geolo a b a r t i c l e i n f o a b s t r a c t Article history: Received July 2016 Received in revised form 30 December 2016 Accepted 19 January 2017 Available online xxx The Ecuador Mw ¼ 7.8 earthquake on April 16, 2016, ruptured a nearly 200 km long zone along the plate interface between Nazca and South American plates which is coincident with a seismic gap since 1942, when a Mw ¼ 7.8 earthquake happened This earthquake occurred at a margin characterized by moderately big to giant earthquakes such as the 1906 (Mw ¼ 8.8) A heavily sedimented trench explains the abnormal lengths of the rupture zones in this system as inhibits the role of natural barriers on the propagation of rupture zones High amount of sediment thickness is associated with tropical climates, high erosion rates and eastward Pacific dominant winds that provoke orographic rainfalls over the Pacific slope of the Ecuatorian Andes Offshore sediment dispersion off the oceanic trench is controlled by a close arrangement of two aseismic ridges that hit the Costa Rica and South Ecuador margin respectively and a mid ocean ridge that separates the Cocos and Nazca plate trapping sediments Gravity field and Ocean Circulation Explorer (GOCE) satellite data are used in this work to test the possible relationship between gravity signal and earthquake rupture structure as well as registered aftershock seismic activity Reduced vertical gravity gradient shows a good correlation with rupture structure for certain degrees of the harmonic expansion and related depth of the causative mass; indicating, such as in other analyzed cases along the subduction margin, that fore-arc structure derived from density heterogeneities explains at a certain extent propagation of the rupture zones In this analysis the rupture zone of the April 2016 Ecuador earthquake developed through a relatively low density zone of the fore-arc sliver Finally, aftershock sequence nucleated around the area of maximum slips in the rupture zone, suggesting that heterogeneous density structure of the fore-arc determined from gravity data could be used in forecasting potential damaged zones associated with big ruptures along the subduction border © 2017 Institute of Seismology, China Earthquake Administration, etc Production and hosting by Elsevier B.V on behalf of KeAi Communications Co., Ltd This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Keywords: Gravity field and Ocean Circulation Explorer (GOCE) Vertical gravity gradient Ecuador earthquake Trench sediments Rupture zone Introduction * Corresponding author Ruta 12, Km 17, Jardín de los Poetas, Rivadavia, San Juan, Argentina E-mail addresses: orlando_a_p@yahoo.com.ar, orlando.alvarez@conicet.gov.ar (O Alvarez) Peer review under responsibility of Institute of Seismology, China Earthquake Administration Production and Hosting by Elsevier on behalf of KeAi During the last years Gravity field and Ocean Circulation Explorer (GOCE) data [1e4] have been used successfully to associate fore-arc density structure in subduction zones with internal displacement distribution in large rupture areas associated with thrust subduction earthquakes [5e8] This association is aimed to constitute a predictive tool in earthquake seismology following a premise in which rupture areas propagate underneath low density zones of the fore-arc after initiating in asperities located over the http://dx.doi.org/10.1016/j.geog.2017.01.005 1674-9847/© 2017 Institute of Seismology, China Earthquake Administration, etc Production and hosting by Elsevier B.V on behalf of KeAi Communications Co., Ltd This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Please cite this article in press as: O Alvarez, et al., Rupture area analysis of the Ecuador (Musine) Mw ¼ 7.8 thrust earthquake on April 16, 2016, using GOCE derived gradients, Geodesy and Geodynamics (2017), http://dx.doi.org/10.1016/j.geog.2017.01.005 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 GEOG129_proof ■ February 2017 ■ 2/10 O Alvarez et al / Geodesy and Geodynamics xxx (2017) 1e10 subducted oceanic floor [9e13] Then an acceptable to remarkable match, in some cases, is noticed in some of the major earthquake subduction phenomena (M > 8) such as the Valdivia (1960), Arequipa (2001), Pisco (2007), Maule (2010), Pisagua-Iquique (2014) and Illapel (2015) large earthquake events with vertical gradient gravity anomalies of GOCE satellite data [6e8] This work is aimed to test such relation in the recent Ecuador Mw ¼ 7.8 thrust earthquake on April 17, 2016, 28 km SSE of Muisne (Fig 1) For this purpose GOCE data are corrected by either their topographical as trench-sedimentary effects, isolating crustal component linked to the heterogeneous density structure of the fore-arc zone Additionally, vertical gravity gradient from GOCE is discomposed by cutting-of at different degrees of the spherical harmonic expansion in order to isolate contributions from mass heterogeneities at different depths and thus to find a term that best fit measured displacements into the rupture area and gravity data The Ecuador Mw ¼ 7.8 thrust earthquake on April 16, 2016 has ruptured an area similar to a rupture zone developed in 1942 with an earthquake magnitude of Mw ¼ 7.8 at similar latitudes, suggesting that seismic segmentation depends on mechanical properties of the interplate medium that is affected by co- and postseismic displacements However, transform faults associated with the Cocos-Nazca mid ocean ridge that usually determine barriers to rupture propagation, dispose parallel to the Ecuadorsouth Colombia subduction zone The arrangement of the plate interface with respect to the subduction zone, summed to the fact that no aseismic ridge impacts north of the Carnegie aseismic ridge (CR) that could have limited both the 1942 and 2016 rupture propagation zones, suggests that upper plate density structure could be playing a role in earthquake segmentation, as suggested in previous works for other subduction segments along the Peruvian-Chilean trench [14] Therefore, this work explores density structure of the fore-arc zone and its potential relation to seismic segmentation in the interplate zone through processing of satellite gravity data Seismotectonic setting The Ecuador Mw ¼ 7.8 thrust earthquake on April 17, 2016 is part of a series of rupture zones that have filled partially a large gap of approximately 500 km (Fig 2) that had not been totally ruptured in one single event since the Mw ¼ 8.8 1906 earthquake In 1979 an Mw ¼ 8.2 earthquake ruptured to the North, while the 1958 Mw ¼ 7.7 earthquake [20] filled the central region of this seismic gap In particular, the analyzed 2016 rupture (Fig 2) would constitute a reactivation of the 1942 rupture area of such gap [20e23] Ye et al [25] pointed out the similarity of the 1942 and Fig Ecuador and Colombia subduction zone with indication of the Northern Volcanic zone comprehended between the Perú and Bucaramanga arc gaps and location of the Mw ¼ 7.8 earthquake epicenter on April 17, 2016 (red star) with corresponding focal mechanism Note the high complexity of this subduction segment that involves collision of the Carnegie aseismic ridge at the Guayaquil Gulf and subduction of a mid ocean ridge separating Nazca and Cocos plates, transversally disposed respect to the trench Relief is from ETOPO1 [15], triangles indicate the current position of the active volcanic arc [16] References: G-Fz: Gijalva Fz, A-Fz: Alvarado Fz, S-Fz: Sarmiento Fz, YG: Yaquina Graben Please cite this article in press as: O Alvarez, et al., Rupture area analysis of the Ecuador (Musine) Mw ¼ 7.8 thrust earthquake on April 16, 2016, using GOCE derived gradients, Geodesy and Geodynamics (2017), http://dx.doi.org/10.1016/j.geog.2017.01.005 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 GEOG129_proof ■ February 2017 ■ 3/10 O Alvarez et al / Geodesy and Geodynamics xxx (2017) 1e10 Fig Fore-arc sliver detached through the arc zone associated with oblique convergence of the Nazca Plate against South American Plate Seismic structure along the Ecuadorsouthern Colombia fore-arc zone taken from Ref [17], DGFZ: DoloreseGuayaquil fault zone [18] Superimposed slip distributions (dashed ellipses) of the main earthquakes: 1906 Mw ¼ 8.8; 1942 Mw ¼ 7.8; 1958 Mw ¼ 7.7, 1979 Mw ¼ 8.2 and 2016 Mw ¼ 7.8 Colored stars indicate location of Mw > 7.5 epicenters and dashed ellipses delineate the approximate rupture area with the same color as corresponding epicenter [19e25] Convergence rate is from Ref [26]) Please cite this article in press as: O Alvarez, et al., Rupture area analysis of the Ecuador (Musine) Mw ¼ 7.8 thrust earthquake on April 16, 2016, using GOCE derived gradients, Geodesy and Geodynamics (2017), http://dx.doi.org/10.1016/j.geog.2017.01.005 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 GEOG129_proof ■ February 2017 ■ 4/10 O Alvarez et al / Geodesy and Geodynamics xxx (2017) 1e10 2016 events in an observed heterogeneous pattern of interseismic coupling of the plate interface and rupture of a similar subset of asperities, which probably also ruptured as part of the 1906 event These authors suggested that the seismic asperities are probably associated with persistent spatial variations of frictional properties of the megathrust over successive ruptures Chlieh et al [17] found a heterogeneous locking of the plate interface by modeling interseismic geodetic strain Patches with high interseismic coupling are close to the rupture areas of the 1942, 1958 and 1979 earthquakes, supporting the notion of some persistent segmentation of the plate boundary The inferred rupture of several asperities beneath the coastline by Ref [25] is consistent with the patchy interseismic locking pattern and location of the large slip deficit patches from Ref [17] The last authors reported that the characteristic recurrence time for these events is of 575 ± 100 yr (1906), 140 ± 30 yr (1942), 90 ± 20 yr (1958) and 153 ± 80 yr (1979), at the actual long-term moment deficit accumulation rate Historical ruptures are limited to the south by the zone of inception of the Carnegie aseismic ridge into the subduction zone (Figs and 2) since none of these events appears to have ruptured across the ridge [27] These ruptures developed along a heavily sedimented trench due to the tropical location of the northern EcuadoreColombia Andes and Pacific dominant winds that provoke orographic rains on the western Andean slope and provoke a strong rain gradient from the Peruvian to the Ecuatorian western Andean slope Particularly, along the north Ecuador-south Colombia margin, the trench shows a thick sedimentary infill (2e3 km) probably due to recent and massive turbiditic intakes, via canyons, in association with hemipelagic sedimentation [24,28e30] This sediment supply fills the trench and is trapped by an intricate ocean floor morphology constituted by two nearly perpendicular aseismic ridges (Fig 1) produced from the Galapagos hot spot and a mid ocean ridge positive morphology that separates Nazca and Cocos plates [31,32] On the CR and on its southern flank, the sedimentary cover and trench fill are thinner (0.5e1.0 km) The prominent topographic feature of the CR, which acts as a barrier and influences marine current trajectories, sedimentary flux, deposition and the erosive power of strong marine currents [27,33e35], explain this lack of sediment accumulation [30] Additionally, the coastal area close to the CR subduction has been uplifted [36] inhibiting contribution for trench sediment influx from rivers coming from the Andes [27,33e35] To the south of the CR, the Guayaquil submarine basins exhibit up to km of sediments [37e39] Graindorge et al [40] found an over thickened (14 km) oceanic crust for the CR and reported that the plate interface dips 4 e10 east from the trench to a depth of 15 km, revealed by on-shore off-shore wide-angle seismic profiles The regional pattern of seismicity and volcanism shows a high degree of segmentation of the Andes along strike, as early noted by Ref [27] In particular, along the Peru-Ecuador-Colombia margin segment a steep slab subduction regime alternates with segments of shallower subduction angles [27], [41] Increased interplate coupling related to the subduction of the thick, buoyant CR may account for an apparent local increased recurrence interval between great interplate earthquakes [40] Many authors have proposed a link between high sediment thickness along the subducting margin and large ruptures associated with great megathrust earthquakes [6,42e45] These works propose that the subduction interface becomes smoothened when high volumes of sediments are subducted, resulting in a homogenous plate interface that allows seismic ruptures to overcome bathymetric barriers favouring trench-parallel propagation Important changes in morphology of the subduction zone along the northern Perú-Ecuador and Colombia subduction zone have been attributed to a non-linearity of the subduction margin and subduction of bathymetric features with high-floatability in the subducted ocean floor While south of the Guayaquil Gulf a downward flexure has been attributed to an ocean ward convex subduction margin [41], a mild shallowing of the Nazca Plate since 30 Ma north of it, determined from the consequent arc expansion to the plate interior, is attributed to the subduction of the CR [46] A high obliquity between Nazca and South American plates decouples inland a forearc sliver through the arc zone that defines a strike slip system associated with a strain partitioned regime, named the North Andean Sliver [26,47e49], (Fig 2) Methods 3.1 GOCE derived gravity data We performed a direct modeling from satellite-only GOCE model GO_CONS_GCF_2_DIR_R5 [50], a full combination of GOCESGG (Satellite Gravity Gradiometer), GOCE-SST (Satellite-to-Satellite Tracking), GRACE (Gravity Recovery and Climatic Experiment) and LAGEOS (Laser Geodynamics Satellite) data, leading to an excellent performance of the long as well as of the short wavelengths processing details are given in Refs [2,3] This satellite only model obtained by the direct approach method, presents homogeneous precision and it is the one of maximum degree/order (N ¼ 300) from satellite-only data The half-wavelength resolution is of approximately 67 km according to l/2 ¼ pR/Nmax [51e53], with R being the mean Earth radius and Nmax the maximum degree/order of the harmonic expansion The observed potential is obtained from the global gravity field model Then, the disturbing potential (T) is derived by subtracting the potential field of the reference ellipsoid from the first [54] The gravity gradient tensor (Marussi tensor) is composed by five independent elements and is obtained as the second derivative of the disturbing potential [52] We calculate the second derivative of the disturbing potential in the radial direction, or vertical gravity gradient (Tzz), from the spherical harmonic coefficients [54] on a € tvo €s regular grid of 0.05 grid cell size The Tzz is expressed in Eo (10À4 mGal/m) and represents a better theoretical resolution than the gravity vector itself for some geophysical features [51], allowing to determine the location of anomalous masses with better detail and accuracy [55] This methodology has already been used in Refs [6e8,56,57], with a detailed description presented in Refs [58,59] 3.2 Topographic and sediment corrections The topographic effect must be removed from the satellite observations [60] in order to eliminate the correlation with the topography The effect generated by the topographic masses on the gravity field and its derivatives is calculated according to Newton's law of universal gravitation To remove the topographic effect from the vertical gravity gradient we performed the topographic correction by discretizing a digital elevation model ETOPO1, using spherical prisms of constant density [15,61e64] By Please cite this article in press as: O Alvarez, et al., Rupture area analysis of the Ecuador (Musine) Mw ¼ 7.8 thrust earthquake on April 16, 2016, using GOCE derived gradients, Geodesy and Geodynamics (2017), http://dx.doi.org/10.1016/j.geog.2017.01.005 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 GEOG129_proof ■ February 2017 ■ 5/10 O Alvarez et al / Geodesy and Geodynamics xxx (2017) 1e10 using a spherical approximation instead of a planar one, we considered the Earth's curvature [65], avoiding considerable errors as the region under study is large enough [52,58,59,64,66] We performed calculation of the topography generated Tzz using the software Tesseroids [59,65] Adopted densities are mean standard values of 2.67 g/cm3 for masses above sea level and a 1.03 g/cm3 for sea water The calculation height is of 7000 m to ensure that all values are above the topography The topographic €tvo €s, with higher positive correction amounts up to tens of Eo values over the Andes and maximum negative values over the lowest relief such as the trench (Fig 3a) The topographic effect was filtered by using a 4th order Butterworth filter at 133 km wavelength in order to reduce satellite data at comparable wavelengths (Fig 3b) The sediment correction was performed using the same method considering a mean density of 2.4 g/cm3 (Fig 4a and b) Sediment thicknesses were obtained from NGDC's global ocean sediment thickness grid from Ref [67], an updated version of the NGDC's original ocean sediment thickness grid from Ref [68] The topography- and sediment-corrected vertical gravity gradient is shown in Fig and in Fig slip distribution is superimposed (preliminary model taken from http://earthquake.usgs gov/earthquakes/eventpage/us20005j32#finite-fault) Fig a) Computed direct topographic effect over the vertical gravity gradient signal b) Filtered topographic effect over the vertical gravity gradient signal Fig a) Computed sedimentary effect over the submerged accretionary prism and ocean floor (sediment thicknesses taken from NOAA) b) Filtered offshore sediment effect over the gravity signal Please cite this article in press as: O Alvarez, et al., Rupture area analysis of the Ecuador (Musine) Mw ¼ 7.8 thrust earthquake on April 16, 2016, using GOCE derived gradients, Geodesy and Geodynamics (2017), http://dx.doi.org/10.1016/j.geog.2017.01.005 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 GEOG129_proof ■ February 2017 ■ 6/10 O Alvarez et al / Geodesy and Geodynamics xxx (2017) 1e10 3.3 Harmonic decomposition There is an approximate relationship between the associated depths of a causative mass with a determined degree of the spherical harmonic expansion [69] By cutting-off the degree/ order of the harmonic expansion allows to decompose the gravimetric signal as causative mass depth increases Featherstone [69] related the depth (Zl) of a causative mass with a determined degree of the spherical harmonic expansion (N) by performing a spectral analysis of the geoid and gravity anomalies In a recent work, we derived a similar equation (Eq (1)) but relating Zl with a determined N for gravity anomalies and vertical gravity gradient (see Ref [8]) In this work, we calculated Tzz up to different degree/orders of the harmonic expansion in order to analyze the response with increasing depths Fig Vertical gravity gradient from GOCE data up to degree/order N ¼ 300 after removing the filtered topographic and offshore sediment effects Red star indicates the position of the epicenter associated with the Mw ¼ 7.8 thrust earthquake on April 17, 2016 and white triangles indicate the volcanic arc as a reference Fig Detail of the vertical gravity gradient from GOCE data up to degree/order N ¼ 300 after removing topographic and offshore sediment effects, where displacements (red numbers) in the rupture zone are depicted with solid white line contours (preliminary slip model taken from http://earthquake.usgs.gov/earthquakes/ eventpage/us20005j32#nite-fault) Red star indicates epicenter Zl ẳ RE ỵ Hc ịN 1ị N ỵ 2ịN ỵ 1ị (1) where RE is the Earth's radius, HC is the Tzz calculation height and N is the selected degree/order of the harmonic expansion Higher orders are associated with shallower sources (low Zl), while decreasing orders are related to deeper mass anomalies (higher Zl) Table shows the used degree/orders, the corresponding depth Zl and spatial resolution, using RE ¼ 6371 km as mean Earth radius Results from this harmonic decomposition tool (by truncating the harmonic expansion) allow analyzing Tzz response with increasing depths of the causative masses (Fig 7) For the Musine earthquake, the best fit (between Tzz and slip distribution) is obtained with N between 175 (approximately 36 km depth) and 200 (approximately 31 km depth), while contrastingly, for the Illapel earthquake the best fit had been obtained for N between 225 and 250 [8] that would preliminarily be interpreted as a relatively deeper rupture Ye et al [25] inferred a minor deep asperity (at a depth of approximately 30 km) at the southeastern end of their slip model, being consistent with the causative mass depth found in this work (for the best fit between Tzz and slip distribution) (see Fig 8) Q2 Table Associated depth (Zl) of a causative mass with a determined degree of the spherical harmonic expansion for Tzz Degree/order N Spatial resolution l/2 ¼ pR/Nmax (km) Zl (km) for Tzz Eq (1) (Hc ¼ km) 300 275 250 225 200 175 150 125 100 66.72 72.78 80.06 88.95 100.07 114.37 133.43 160.12 200.15 20.98 22.86 25.11 27.85 31.26 35.62 41.40 49.42 61.29 Please cite this article in press as: O Alvarez, et al., Rupture area analysis of the Ecuador (Musine) Mw ¼ 7.8 thrust earthquake on April 16, 2016, using GOCE derived gradients, Geodesy and Geodynamics (2017), http://dx.doi.org/10.1016/j.geog.2017.01.005 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 GEOG129_proof ■ February 2017 ■ 7/10 O Alvarez et al / Geodesy and Geodynamics xxx (2017) 1e10 Fig Topography and sediment corrected Tzz slices calculated at different degrees of the harmonic expansion Downwards: as degree/order decreases, exploration depths increase Please cite this article in press as: O Alvarez, et al., Rupture area analysis of the Ecuador (Musine) Mw ¼ 7.8 thrust earthquake on April 16, 2016, using GOCE derived gradients, Geodesy and Geodynamics (2017), http://dx.doi.org/10.1016/j.geog.2017.01.005 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 GEOG129_proof ■ February 2017 ■ 8/10 O Alvarez et al / Geodesy and Geodynamics xxx (2017) 1e10 Fig Detail of the vertical gravity gradient of GOCE data up to N ¼ 175 after removing topographic and offshore sediment effects, with internal displacements in the rupture zone (solid white line) Note the good correspondence (match) between low Tzz lobe with slip On the right corner (zoom) the aftershock sequence is plotted (up to June 6, 2016) corresponding to colored circles over seismicity (grey circles) from USGS Catalog Note how this post-earthquake sequence traces lines of iso-displacements into the rupture zone, surrounding patch of maximum slip A similar observation is arrived for the Illapel earthquake in central Chile [70] Conclusions Vertical gravity gradients calculated from GOCE satellite data corrected by sediment and topographic effects show a correlation with the rupture area of the Mw ¼ 7.8 April 16, 2016 Ecuador earthquake, for certain degrees of the harmonic expansion (N ¼ 175/200) and related depth (Zl z 35/31 km) of the causative mass This implies that heterogeneous density structure of the decoupled Ecuador fore-arc could explain propagation of the rupture zone In particular, the rupture zone of the Mw ¼ 7.8 April 16, 2016 Ecuador earthquake developed through a relatively low density zone of the fore-arc sliver, such as for other cases along the South American subduction zone has been recently noted Finally, aftershock sequence nucleated around the area of maximum slips in the rupture zone Recent works [25] suggest that asperities can be persistent features determined by the spatial variations of the mechanical properties of the subduction megathrust This observation implies that heterogeneous density structure of the fore-arc determined from gravity data could be used in forecasting potential damaged zones Acknowledgments Authors acknowledge the use of the GMT-mapping software of reference [71] The authors would like to thank to CONICET References [1] R Floberghagen, M Fehringer, D Lamarre, D Muzi, B Frommknecht, ~ eiro, A Costa, Mission design, operation and exploitation of the C 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