the mw 8 3 illapel earthquake chile preseismic and postseismic activity associated with hydrated slab structures the accumulated stress in subduction zones is discharged with earthquake and aseismic activity the latter is hosted in rheological complex regions characterized by high pore fluid pressure and is often accompanied by repeated earthquakes and earthquake swarms the spatiotemporal analysis of seismic activity can reveal the presence of aseismic transients associated with large earthquakes here we study 20 years of seismicity prior to and after the m w 8 3 earthquake that occurred in a d 2015 in central chile we identified several earthquake swarms before the main shock and repeating aftershocks at the border of the main slip area spatial clustering of the seismic activity shares similar orientation with the main fracture zones observed on the outer rise of the subducting nazca plate our findings suggest that the fracture zones enclosing the rupture are playing a major role in accommodating the pre and post main shock stress evolution we further recognize how fracture regions have acted as barriers to the propagation of large earthquakes in the region

intense foreshocks and a slow slip event preceded the 2014 iquique mw 8 1 earthquake the subduction zone in northern chile is a well identified seismic gap that last ruptured in 1877 the moment magnitude mw 8 1 iquique earthquake of 1 april 2014 broke a highly coupled portion of this gap to understand the seismicity preceding this event we studied the location and mechanisms of the foreshocks and computed global positioning system gps time series at stations located on shore seismicity off the coast of iquique started to increase in january 2014 after 16 march several mw 6 events occurred near the low coupled zone these events migrated northward for 50 kilometers until the 1 april earthquake occurred on 16 march on shore continuous gps stations detected a westward motion that we model as a slow slip event situated in the same area where the mainshock occurred

the mw 7 7 tocopilla earthquake of 14 november 2007 at the southern edge of the northern chile seismic gap rupture in the deep part of the coupled plate interface the slip distribution of the mw 7 7 tocopilla earthquake was obtained from the joint inversion of teleseismic and strong motion data rupture occurred as underthrusting at the base of the seismically coupled plate interface mainly between 35 and 50 km depth from the hypocenter located below the coast 25 km south of the town of tocopilla the rupture propagated 50 km northward and 100 km southward overall the slip distribution was dominated by two slip patches one near the hypo center and the other 70 km to the south where slip reached its maximum value 3 m an additional branch of moderate slip propagated at shallower depth toward the west near the northern tip of the mejillones peninsula rupture velocity remained close to 2 8 km sec with a total rupture duration of 45 sec the first 2 weeks of aftershocks located with a local seismic network display a strong correlation with the slip distri bution the 2007 rupture ended below the mejillones peninsula where the 1995 an tofagasta rupture also ended ruegg et al 1996 delouis et al 1997 pritchard et al 2006 this corroborates the role of barrier played by this structure the downdip end of the seismically coupled zone at 50 km depth evidenced by previous studies for the 1995 event is also confirmed the 2007 tocopilla earthquake contributed only mod erately to the rupturing of the great northern chile seismic gap which still has the capacity for generating a much larger underthrusting event

determination of the friction law parameters of the mw 6 7 michilla earthquake in northern chile by dynamic inversion 1 we perform a full dynamic inversion at low frequencies of the 16 december 2007 mw 6 7 northern chile earthquake that we model as a simple elliptical patch we use two different stress friction end member models asperities and barriers finding similar results the inversions are performed for strong motion data filtered between 0 02 and 0 5 hz eleven geometrical and stress and friction parameters are inverted using the neighbourhood algorithm the optimum solutions have relative errors lower than 0 21 the earthquake rupture has duration of less than 5 s and propagates at sub shear speed the rupture area is similar to that of the aftershock distribution and the seismic moment is 0 95 10 19 nm we derive the friction law parameters from the models situated close to the optimum solution using a monte carlo technique the results show a strong trade off between applied stress and frictional resistance we find that the distribution of friction models collapses into a finite zone of the space of moment and non dimensional parameter k we conclude that it is possible to determine the friction law from near field seismograms but there is a strong trade off between friction and initial stress

high resolution relocation and mechanism of aftershocks of the 2007 tocopilla chile earthquake summary we study the distribution of the aftershocks of tocopillamw 7 7 earthquake of 2007 november 14 in northern chile in detail this earthquake broke the lower part of the seismogenic zone at the southern end of the northern chile gap a region that had its last megathrust earthquake in 1877 the aftershocks of tocopilla occurred in several steps the first day they were located along the coast inside the co seismic rupture zone after the second day they extended ocean wards near the mejillones peninsula finally in december they concentrated in the south near the future rupture zone of the michilla intermediate depth earthquake of 2007 december 16 the aftershock sequence was recorded by the permanent ipoc integrated plate boundary observatory in chile network and the temporary task force network installed 2 weeks after the main event a total of 1238 events were identified and the seismic arrival times were directly read from seismograms initially we located these events using a single event procedure and then we relocated them using the double difference method and a crosscorrelation technique to measure time differences for clusters of aftershocks we tested a 1 d velocity model and a 2 d one that takes into account the presence of the subducted nazca plate relocation significantly reduced the width of the aftershock distribution in the inland area the plate interface imaged by the aftershocks is thinner than 2km the two velocity models give similar results for earthquakes under the coast and a larger difference for events closer to the trench the surface imaged by the aftershocks had a length of 160km it extends from 30 to 50km depth in the northern part of the rupture zone and between 5 and 55km depth near the mejillones peninsula we observed a change in the dip angle of the subduction interface from 18 to 24 at a depth of 30km we propose that this change in dip is closely associated with the upper limit of the rupture zone of the main event we also studied the focal mechanisms of the aftershocks most of them were thrust events like the mainshock as the aftershock activity was significantly reduced on 2007 december 13 an ml 6 1 event occurred offshore of the mejillones peninsula reactivating the seismicity three days later the michilla intraslab earthquake of mw 6 8 ruptured an almost vertical fault with slab push mechanism the aftershocks locations of this event define a planar zone about 11km in depth situated right bellow the subduction interface

andean structural control on interseismic coupling in the north chile subduction zone segmentation can influence the extent of earthquake rupture and event magnitude large megathrust earthquakes result from total rupture of relatively continuous segments of the subduction interface segmentation is attributed to variations in the frictional properties of the seismogenic zone or to topographic features on the down going plate structures in the overriding plate may also influence segmentation but their importance has been dismissed here we investigate the links between interface segmentation at the north chile seismic gap and a crustal scale fault structure in the overriding plate that forms a coastal scarp of about 1 km in height we use satellite interferometric synthetic aperture radar insar and global positioning system gps data to measure interseismic surface deformation between 2003 and 2009 and compare the deformation with rupture extent during well documented earthquakes from these data we infer the degree of coupling and segmentation at depth we find that along a 500 km long segment the base of the strongly coupled seismogenic zone correlates with the line of the surface coastal scarp and follows the outline of the mejillones peninsula this correlation implies that large scale structures in the overriding plate can influence the frictional properties of the seismogenic zone at depth we therefore suggest that the occurrence of megathrust earthquakes in northern chile is controlled by the surface structures that build andean topography

the 2007 m7 7 tocopilla northern chile earthquake sequence implications for along strike and downdip rupture segmentation and megathrust frictional behavior 1 in 2007 a m7 7 earthquake occurred near the town of tocopilla within the northern chile seismic gap main shock slip derived from coseismic surface deformation was confined to the depth range between 30 and 55 km we relocated 1100 events during six months before and one week after the main shock aftershock seismicity is first congruent to the main shock slip and then it spreads offshore west and northwest of mejillones peninsula mp waveform modeling for 38 aftershocks reveals source mechanisms that are in the majority similar to the main shock however a few events appear to occur in the upper plate some with extensional mechanisms juxtaposing the tocopilla aftershocks with those following the neighboring 1995 antofagasta earthquake produces a striking symmetry across an ew axis in the center of mp events seem to skirt around mp probably due to a shallower moho there we suggest that the seismogenic coupling zone in northern chile changes its frictional behavior in the downdip direction from unstable to mostly conditionally stable for both earthquake sequences aftershocks agglomerate in the conditionally stable region whereas maximum inter seismic slip deficit and co seismic slip occurs in the unstable region the boundary between the unstable and conditionally stable zones parallels the coastline we identify a similar segmentation for other earthquakes in chile and peru where the offshore segments break in great m 8 earthquakes and the onshore segments in smaller m 8 earthquakes using critical taper analysis we demonstrate a causal relationship between varying slip behavior on the interface and forearc wedge anatomy that can be attributed to spatial variations in the rate dependency of friction

gradual unlocking of plate boundary controlled initiation of the 2014 iquique earthquake a long foreshock series unlocked the south american plate boundary until eventually initiating the m 8 1 iquique chile earthquake two groups publishing in this issue of nature analyse the seismic context of the iquique earthquake that occurred off the coast of northern chile on 1 april 2014 in a seismic zone that had been quiescent since a significant event in 1877 gavin hayes et al identify areas of remaining or elevated earthquake hazard along the megathrust fault in the region and conclude that the 2014 iquique event was not the earthquake that had been anticipated given that significant sections of the northern chile subduction zone have not ruptured in almost 150 years they suggest that it is likely that future megathrust earthquakes will occur south and potentially north of the 2014 iquique sequence bernd schurr et al show that the april 2014 earthquake broke a central fraction of the northern chile seismic gap the last major segment of the south american plate boundary that had yet to rupture in the past century from july 2013 up to the april earthquake they identify three seismic clusters along this part of the plate boundary each lasting a few weeks with earthquakes of increasing peak magnitudes they conclude that these seismic clusters and their slip transients reflect a gradual weakening of the central part of the seismic gap that was instrumental in initiating the final failure 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

detection of earthquake swarms at subduction zones globally insights into tectonic controls on swarm activity earthquake swarms are characterized by an increase in seismicity rate that lacks a distinguished mainshock and does not obey omori s law at subduction zones they are thought to be related to slow slip events sses on the plate interface earthquake swarms in subduction zones can therefore be used as potential indicators of slow slip events however the global distribution of earthquake swarms at subduction zones remains unclear here we present a method for detecting such earthquake sequences using the space time epidemic type aftershock sequence etas model we applied this method to seismicity m 4 5 recorded in the anss catalog at subduction zones during the period 1995 2009 we detected 453 swarms which is about 6 7 times the number observed in a previous catalog foreshocks of some large earthquakes are also detected as earthquake swarms in some subduction zones such as at ibaraki oki japan swarm like foreshocks and ordinary swarms repeatedly occur at the same location given that both foreshocks and swarms are related to sses on the plate interface these regions may have experienced recurring sses we then compare the swarm activity and tectonic properties of subduction zones finding that swarm activity is positively correlated with curvature of the incoming plate before subduction this result implies that swarm activity is controlled either by hydration of the incoming plate or by heterogeneity on the plate interface due to fracturing related to slab bending

interseismic coupling megathrust earthquakes and seismic swarms along the chilean subduction zone 38 18 s the recent expansion of dense gps networks over plate boundaries allows for remarkably precise mapping of interseismic coupling along active faults the interseismic coupling coefficient is related to the ratio between slipping velocity on the fault during the interseismic period and the long term plates velocity but the interpretation of coupling in terms of mechanical behavior of the fault is still unclear here we investigate the link between coupling and seismicity over the chilean subduction zone that ruptured three times in the last 5 years with major earthquakes maule mw 8 8 in 2010 iquique mw 8 1 in 2014 and illapel mw 8 4 in 2015 we combine recent gps data acquired over the margin 38 18 s with older data to get the first nearly continuous picture of the interseismic coupling variations on the subduction interface here we show that at least six low coupling zones lcz areas where coupling is low relatively to the neighboring highly coupled segments can be identified we also find that for the three most recent mw 8 events co seismic asperities correlate well with highly coupled segments while lczs behaved as barriers and stopped the ruptures the relation between coupling and background seismicity in the interseismic period before the events is less clear however we note that swarm sequences are prone to occur in intermediate coupling areas at the transition between lcz and neighboring segments and that the background seismicity tends to concentrate on the downdip part of the seismogenic locked zone thus highly coupled segments usually exhibit low background seismicity in this overall context the metropolitan segment that partly ruptured during the 2015 illapel earthquake appears as an outlier since both coupling and background seismicity were high before the rupture raising the issue of the remaining seismic hazard in this very densely populated area

influence of lithosphere asthenosphere interaction on the overriding lithosphere in a subduction zone numerical modeling 1 several factors influence the topography of the overriding plate in a subduction zone mantle wedge flow caused by motion of the descending slab lithosphere elasticity and interaction between the solid lithosphere and the viscous asthenosphere this paper presents the results of new subduction modeling that incorporates both elastic elastoplastic deformation in the overriding plate and viscous deformation in the asthenosphere the effects of fluid solid coupling and interaction were considered using an arbitrary lagrangian eulerian technique that solves for the continuous deformation of fluid flow in contact with the rigid plate modeling results show that thicker lithosphere which is harder to deform contributes to the development of broadly shallower back arc basins as lithospheric strength decreases its ability to withstand outside force fluid load resulting from flow in the mantle wedge also declines leading to the development of deep and narrow basins large scale deformation of the lithosphere modifies both the mantle wedge geometry and the wedge flow pattern which in turn increases the fluid load and thereby enhances topographic variation generally the effect of interaction is enhanced with lithospheric deformation the elastoplastic model also leads to a weaker lithosphere and a correspondingly larger deflection in the lithosphere similarly to previous studies back arc basins are deeper when mantle viscosity is higher for a 50 km thick elastic lithosphere the lithosphere asthenosphere interaction is negligible when the asthenospheric viscosity is less than 4 1019 pa s increasing the rate of subduction or decreasing the dip angle will also lead to the development of deeper basins

hydrologic control of forearc strength and seismicity in the costa rican subduction zone the subduction zone beneath costa rica experiences infrequent large earthquakes in its northwestern part whereas slow slip dominates in the southeast seismic data reveal a disparity in fluid accumulation in the overriding continental crust that correlates with this change in seismic behaviour implying that spatial gradients in fluid content may control subduction zone seismicity

accurate hypocentre determination in the seismogenic zone of the subducting nazca plate in northern chile using a combined on offshore network the coupled plate interface of subduction zones commonly called the seismogenic zone has been recognized as the origin of fatal earthquakes a subset of the after shock series of the great antofagasta thrust type event 1995 july 30 mw 8 0 has been used to study the extent of the seismogenic zone in northern chile to achieve reliable and precise hypocentre locations we applied the concept of the minimum 1 d model which incorporates iterative simultaneous inversion of velocity and hypocentre parameters the minimum 1 d model is complemented by station corrections which are influenced by near surface velocity heterogeneity and by the individual station elevations by relocating mine blasts which were not included in the inversion we obtain absolute location errors of 1 km in epicentre and 2 km in focal depth a study of the resolution parameters ale and dspr documents the importance of offshore stations on location accuracy for offshore events based on precisely determined hypo centres we calculate a depth of 46 km for the lower limit of the seismogenic zone which is in good agreement with previous studies for this area for the upper limit we found a depth of 20 km our results of an aseismic zone between the upper limit of the seismogenic zone and the surface correlates with a detachment zone proposed by other studies the results are also in agreement with thermal studies for the antofagasta forearc region

variations of fluid pressure within the subducting oceanic crust and slow earthquakes 1 we show fine scale variations of seismic velocities and converted teleseismic waves that reveal the presence of zones of high pressure fluids released by progressive metamorphic dehydration reactions in the subducting philippine sea plate in tokai district japan these zones have a strong correlation with the distribution of slow earthquakes including long term slow slip ltss and low frequency earthquakes lfes overpressured fluids in the ltss region appear to be trapped within the oceanic crust by an impermeable cap rock in the fore arc and impede intraslab earthquakes therein in contrast fluid pressures are reduced in the lfe zone which is deeper than the centroid of the ltss because there fluids are able to infiltrate into the narrow corner of the mantle wedge leading to mantle serpentinization the combination of fluids released from the subducting oceanic crust with heterogeneous fluid transport properties in the hanging wall generates variations of fluid pressures along the downgoing plate boundary which in turn control the occurrence of slow earthquakes

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

a narrowly spaced double seismic zone in the subducting nazca plate 1 high precision relocations of intermediate depth earthquakes 80 130 km below the central andes reveal a fine scale double layered wadati benioff zone wbz upper and lower band of seismicity are separated by about 9 km and occur at the top of the oceanic crust and in the uppermost oceanic mantle respectively analysis of focal mechanisms and waveform similarities indicate that fluid processes are causing the events earthquakes in the oceanic crust occur on pre existing normal faults due to hydraulic embrittlement from metamorphic dehydration and on subvertical faults that connect the two layers in a narrow depth range extensional faulting predominates in both layers indicating that slab pull forces are the dominant stress source superseding possible unbending forces in this segment of the nazca plate

pre and postseismic slow slip surrounding the 2011 tohoku oki earthquake rupture abstract slow aseismic slip that accommodates part of the long term plate motion on subduction megathrusts is thought to be strongly related to the occurrence of large earthquakes on the same fault zone however the temporal evolution and spatial distribution of the aseismic slip before major earthquakes and of accelerated postseismic afterslip are largely unconstrained we estimate cumulative offsets of small repeating earthquakes that are interpreted to reflect the in situ aseismic slip history on the subduction zone offshore northeastern japan these data reveal contrasting aseismic slip patterns between the coseismic rupture area of the mw 9 0 tohoku oki earthquake and surrounding portions of the subduction thrust the rupture area is characterised by low and variable slip rates before 2008 and the slip stopped almost completely after the earthquake the region surrounding the rupture area exhibited higher aseismic fault slip rates before the earthquake and clear postseismic slip of up to 1 6 m within 9 months following the main shock the frictional fault properties and complete relief of ambient stress in the central rupture zone of the main shock probably control the observed distribution the postseismic slip shows a more abrupt increase in the region closer to the source suggesting outwards propagation of afterslip small but distinct increases in the slip rate in the 3 yr before the earthquake near the area of large coseismic slip suggests preseismic unfastening of the locked area in the last stage of the earthquake cycle

the effects of weathering on the physical and mechanical properties of a granitic saprolite less research has been carried out on the changes induced by weathering to the mechanical properties of reconstituted and intact soils that originate from chemical decomposition of rocks saprolite

invited review paper neogene to quaternary tectonics of the coastal cordillera northern chile abstract the extreme aridity of the northern chilean coastal cordillera enables the complete preservation of permanent deformation related to the coupling between nazca and south america the region between antofagasta and arica is characterized by four types of late cenozoic structures ew striking reverse faults ns striking normal faults sparse wnw and nnw striking right lateral strike slip faults and unimodal and bimodal populations of surface cracks the ew reverse faults occur only between 19 and 21 40 s latitude and have been active since at least 6 ma to present a march 2007 earthquake demonstrates that this deformation can occur during the interseismic phase of the plate boundary seismic cycle the ns normal faults are most abundant near antofagasta and mejillones and diminish in significance northward several of these faults in both the antofagasta and the salar grande area have been reactivated as reverse faults causing minor topographic inversion we suggest that normal faults move during both interseismic and coseismic deformation and are fundamentally very weak surface cracks are ubiquitous throughout the region and form by both non tectonic and tectonic mechanisms a significant proportion forms during coseismic deformation and thus may be used to identify long lived rupture segments on the plate boundary overall forearc deformation is very slow with permanent strain rates of 1 5 nstrain year and fault slip rates less than 0 5 mm year

locking of the chile subduction zone controlled by fluid pressure before the 2010 earthquake large subduction zone earthquakes are thought to occur where the down going and overriding tectonic plates are strongly locked analysis of geodetic and seismic data collected in the decade before the 2010 chile earthquake shows that variations in pore fluid pressure correlate with the degree of plate interface locking and may therefore control earthquake rupture

are the lower planes of double seismic zones caused by serpentine dehydration in subducting oceanic mantle in a number of subduction zones earthquakes at 50 200 km depth define two dipping planes separated by 20 40 km that appear to merge downdip upper plane earthquakes are inferred to occur within the subducting oceanic crust whereas lower plane earthquakes occur in the subducting oceanic mantle beneath northeast japan the results of a new finite element heat transfer model suggest that the lower seismic plane cuts across isotherms at a shallow angle lower plane earthquakes occur at 550 800 c at 100 km depth and at 350 600 c at 160 km depth these conditions coincide with the dehydration reaction antigorite serpentine forsterite enstatite h 2 o which suggests that lower plane earthquakes may be triggered by dehydration embrittlement which in turn suggests that the subducting oceanic mantle is partially hydrated serpentinization may occur in the trench outer rise region where faulting may promote infiltration of seawater several tens of kilometers into the oceanic lithosphere if this hypothesis is correct current subduction zone h 2 o budgets may significantly underestimate the amount of bound h 2 o entering the subduction factory

historical and recent large megathrust earthquakes in chile abstract recent earthquakes in chile 2014 mw 8 2 iquique 2015 mw 8 3 illapel and 2016 mw 7 6 chiloe have put in evidence some problems with the straightforward application of ideas about seismic gaps earthquake periodicity and the general forecast of large megathrust earthquakes in northern chile before the 2014 iquique earthquake 4 large earthquakes were reported in written chronicles 1877 1786 1615 and 1543 in north central chile before the 2015 illapel event 3 large earthquakes 1943 1880 1730 were reported and the 2016 chiloe earthquake occurred in the southern zone of the 1960 valdivia megathrust rupture where other large earthquakes occurred in 1575 1737 and 1837 the periodicity of these events has been proposed as a good long term forecasting however the seismological aspects of historical chilean earthquakes were inferred mainly from old chronicles written before subduction in chile was discovered here we use the original description of earthquakes to re analyze the historical archives our interpretation shows that a priori ideas like seismic gaps and characteristic earthquakes influenced the estimation of magnitude location and rupture area of the older chilean events on the other hand the advance in the characterization of the rheological aspects that controlled the contact between nazca and south american plate and the study of tsunami effects provide better estimations of the location of historical earthquakes along the seismogenic plate interface our re interpretation of historical earthquakes shows a large diversity of earthquakes types there is a major difference between giant earthquakes that break the entire plate interface and those of mw 8 0 that only break a portion of it

the geometry of the chilean continental wedge tectonic segmentation of subduction processes off chile abstract based on high resolution bathymetry and geophysical observations the precise continental wedge geometry along the chilean margin is analyzed the data show complex patterns in continental wedge geometry that challenge the most frequently used classification methodology for the convergent margin tectonics a detailed modeling of the parameters involved in the non cohesive coulomb wedge theory reveals a tectonic latitudinal segmentation of the chilean offshore subduction zone this segmentation is characterized by a sequence of broad segments with different basal effective friction coefficient and or internal fluid pressure conditions which are limited by the presence of bathymetric oceanic highs fracture zones and peninsulas the results suggest a general increase of the fluid pressure inside the continental wedge north of 33 s which is interpreted as a result of a more pervasive fracturing due to tectonic erosion at the base and within the continental wedge the tectonic segmentation proposed here shows a close spatial relation with the short term deformation process associated to the coseismic ruptures of large earthquakes in the chilean margin

upper plate reverse fault reactivation and the unclamping of the megathrust during the 2014 northern chile earthquake sequence after 137 years without a great earthquake the mw 8 1 pisagua event of 1 april 2014 occurred in the central portion of the southern peru northern chile subduction zone this megathrust earthquake was preceded by more than 2 weeks of foreshock activity migrating 3 5 km day toward the mainshock hypocenter this foreshock sequence was triggered by an mw 6 7 earthquake on a reverse fault in the upper plate that strikes at a high angle to the trench similar to well documented reverse faults onshore these margin oblique reverse faults accommodate north south shortening resulting from subduction across a plate boundary that is curved in map view reverse slip on the crustal fault unclamped the subduction interface precipitating the subsequent megathrust foreshock activity that culminated in the great pisagua earthquake the combination of crustal reverse faults and a curved subduction margin also occurs in cascadia and northeastern japan indicating that there are two additional localities where great megathrust earthquakes may be triggered by upper plate fault activity

background seismicity rate at subduction zones linked to slab bending related hydration tectonic properties strongly control variations in seismicity among subduction zones in particular fluid distribution in subduction zones influences earthquake occurrence and it varies among subduction zones due to variations in fluid sources such as hydrated oceanic plates however the relationship between variations in fluid distribution and variations in seismicity among subduction zones is unclear here we divide earth s subduction zones into 111 regions and estimate background seismicity rates using the epidemic type aftershock sequence model we demonstrate that background seismicity rate correlates to the amount of bending of the incoming oceanic plate which in turn is related to the hydration of oceanic plates via slab bending related faults regions with large bending may have high seismicity rates because a strongly hydrated oceanic plate causes high pore fluid pressure and reduces the strength of the plate interface we suggest that variations in fluid distribution can also cause variations in seismicity in subduction zones

kinematic rupture process of the 2007 tocopilla earthquake and its main aftershocks from teleseismic and strong motion data summary we study a large m w 7 6 earthquake that occurred on 2007 november 14 in the northern chile seismic gap near the city of tocopilla using a variety of seismic data we show that this earthquake ruptured only the lower part of the interplate seismic zone and generated a series of plate interface aftershocks two large aftershocks on 2007 november 15 ruptured the interplate zone oceanwards of the mejillones peninsula a major geographical feature in the antofagasta region on 2007 december 16 a large m w 6 8 aftershock that occurred near the southern bottom of the fault plane of the main event is shown to be a slab push earthquake located inside the subducted nazca plate and triggered by along slab compression aftershocks of this event demonstrate that it occurred on an almost vertical fault the tocopilla earthquake took place just after the installation of a new seismological network by chilean german and french researchers the accelerometric data combined with far field seismic data provide a quite complete and consistent view of the rupture process the earthquake broke a long 130 km and narrow about 30 50 km zone of the plate interface just above the transition zone using a non linear kinematic inversion method we determined that rupture occurred on two well defined patches of roughly elliptical shape we discuss the consequences of this event for models of gap filling earthquakes in chile proposed in the 1970s