permanent deformation caused by subduction earthquakes in northern chile earth s crust is thought to eventually rebound following an earthquake so that deformation is not permanent field analysis in the atacama desert northern chile however identifies numerous large cracks in the crust implying that the crust here has been permanently deformed by thousands of earthquakes that have occurred over the past million years

geometry and mechanism of faulting of the 1980 el asnam algeria earthquake from inversion of teleseismic body waves and comparison with field observations the el asnam earthquake of october 10 1980 ms 7 3 provided a wealth of geological and seismological data and is an ideal event for comparing geologically and seismologically derived models the event produced extensive surface faulting in addition to the main tectonic deformation which is clearly a thrust widespread secondary normal faulting was observed at the surface in the southwest region the surface break of a thrust fault could be traced for 24 km in the northeast part of the fault zone normal faults with throws of several meters were observed although no clear thrust type surface breaks were observed in this region geodetic measurements and aftershocks indicate that thrusting was nonetheless the main tectonic mode of deformation in this paper the rupture process of the el asnam earthquake is investigated by inversion of teleseismic p and sh waves the effort is concentrated on the late part of the waveforms which carries information about the faulting in the least understood north eastern segment of the fault zone during the earthquake rupture initiated at the southern terminus of the southwestern fault segment and propagated northward as indicated by the epicentral location and the observable azimuthal directivity of the body wave shapes and amplitudes the seismogenic faulting in the southwestern segment has a thrust mechanism with the following average parameters seismic moment 3 9 1019 n m depth 6 km strike 220 dip 46 and rake 72 the duration of rupture on this fault segment was approximately 10 s which together with the observed fault length of 24 km indicates a rupture velocity of 2 4 km s unconstrained inversions suggest normal faulting in the northeast part of the source region that began immediately following the arrival of the rupture to the northeast region the normal faulting mechanism is in agreement with that observed for the large surface fault scarps in this region guided by the evidence from aftershocks and geodetic measurements a solution with a thrust type mechanism was also sought using a priori constraints based on the field observations the observed strike and slip angle we obtain the following thrust source mechanism for the northeast segment seismic moment 3 6 1019 n m depth 6 km strike 230 dip 20 and rake 91 this solution requires that the thrusting in the northeast be initiated 10 s after the arrival of the rupture from the southwest suggesting that some time is required to overcome the geometric barrier marked by the abrupt change in the dip and azimuth inversions are nonunique with respect to the seismic moment associated with normal faulting the ambiguity being caused by the limited bandwidth of the observed body waves the moment estimated from the surface waves at periods about 300 s however favors thrust faulting to be dominant in the northeast region

asperities and barriers on the seismogenic zone in north chile state of the art after the 2007 mw 7 7 tocopilla earthquake inferred by gps and insar data the mw 7 7 2007 november 14 earthquake had an epicentre located close to the city of tocopilla at the southern end of a known seismic gap in north chile through modelling of global positioning system gps and radar interferometry insar data we show that this event ruptured the deeper part of the seismogenic interface 30 50 km and did not reach the surface the earthquake initiated at the hypocentre and was arrested 150 km south beneath the mejillones peninsula an area already identified as an important structural barrier between two segments of the peru chile subduction zone our preferred models for the tocopilla main shock show slip concentrated in two main asperities consistent with previous inversions of seismological data slip appears to have propagated towards relatively shallow depths at its southern extremity under the mejillones peninsula our analysis of post seismic deformation suggests that small but still significant post seismic slip occurred within the first 10 d after the main shock and that it was mostly concentrated at the southern end of the rupture the post seismic deformation occurring in this period represents 12 19 per cent of the coseismic deformation of which 30 55 per cent has been released aseismically post seismic slip appears to concentrate within regions that exhibit low coseismic slip suggesting that the afterslip distribution during the first month of the post seismic interval complements the coseismic slip the 2007 tocopilla earthquake released only 2 5 per cent of the moment deficit accumulated on the interface during the past 130 yr and may be regarded as a possible precursor of a larger subduction earthquake rupturing partially or completely the 500 km long north chile seismic gap

the mw 8 0 antofagasta northern chile earthquake of 30 july 1995 a precursor to the end of the large 1877 gap abstract an m w 8 0 earthquake occurred on 30 july 1995 in the antofagasta region northern chile the main rupture corresponding to thrust faulting developed from 10 to 50 km in depth along the subduction interface between the nazca and the south american plates the 1995 earthquake took place just south of the large seismic gap where a great earthquake m 9 had occurred in 1877 most of the 1995 rupture was located within a local network consisting of nine short period stations that had been previously installed at the southern end of the 1877 gap and the aftershock sequence could be accurately monitored little destruction resulted from the 1995 earthquake in spite of its large size ground acceleration in antofagasta reached 29 of gravity a tsunami wave 2 to 2 5 m high was observed along the coast from mejillones to taltal one strong foreshock m w 6 2 occurred in the 1995 hypocentral region 6 months before the main event body wave modeling of broadband seismograms from the global network along with the analysis of the aftershock distribution allows us to propose a well constrained model for the whole rupture process some additional details of the rupture were obtained from an accelerometer record at antofagasta the main rupture started as a double even with thrust mechanism below the southern part of the mejillones peninsula and it propagated southward in a n200 e direction with an average velocity of 2 8 km sec it ended near the trench in normal faulting the total rupture area and seismic moment are 185 90 km 2 and 1 2 10 28 dyne cm respectively the aftershock distribution delineates a well defined rupture surface along the subduction interface the distribution of epicenters during the first 20 h of aftershock activity shows a sharp northern boundary beneath the mejillones peninsula hence the 1995 main rupture did not propagate north of the mejillones peninsula into the 1877 gap aftershocks during the following 2 weeks indicate a growth of the initial rupture zone toward the north the mechanisms of the strongest aftershocks are similar to that of the mainshock the down dip termination of the main rupture corresponds to the maximum depth 50 km of the region that had been identified as the locked part of the subduction interface from the analysis of the microseismicity recorded by the local network prior to the 1995 event a well constrained dislocation model is proposed for the 1995 main rupture which produces surface displacements in good agreement with available observations of coastal uplift and gps measurements the dislocation model as well as global positioning system gps measurements indicate that the 1995 earthquake generated e w extension in the coastal region of antofagasta the atacama fault located 40 to 50 km above the 1995 main rupture showed small fresh surface ruptures near sierra remiendos 70 km to the sse of antofagasta with a maximum vertical offset of 20 cm this offset corresponds to normal faulting which is in agreement with the e w co seismic extension the mejillones peninsula appears to be the surface expression of a barrier that interrupted the propagation of the 1995 rupture to the north into the region of the 1877 gap modeling of static stress changes induced by the antofagasta earthquake indicates an increase in compressive stresses along a direction transverse to the trench immediately to the north of the 1995 rupture surface thus the chances for the reactivation of the 1877 gap after this event are greater now

the andean subduction zone between 22 and 25 s northern chile precise geometry and state of stress abstract one year of seismicity recorded by a local network is used to obtain more precision about the geometry and the stress regime of the andean subduction between 22 and 25 s in the northern chile seismic gap a sharp image of the wadati benioff zone wbz is obtained down to 270 km in depth a seismically quasi quiescent zone is observed in the wbz below the volcanic arc between 150 and 210 km in depth hypocentres of distant intermediate depth earthquakes located with the local network are compared with worldwide seismic network hypocentres in order to evaluate the accuracy of the wbz image at depth greater than 100 150 km in depth no shallow microearthquakes have been observed in the continental crust but some seismic activity is likely to occur locally at the deep root of the atacama fault the stress field and the characteristics of faulting along the subducted slab are investigated underthrusting and localized reverse faulting earthquakes define the seismically coupled plate interface from 20 to 50 km in depth locked zone downdip intra slab normal faulting prevails tensile zone but some strike slip faulting is observed a transition between normal faulting with variable fault azimuth and normal faulting with nearly homogeneous nnw to nw oriented fault plane is found at about 80 km in depth it is found that the stress axes 1 and 3 in the locked zone are oriented in the convergence direction 75 80 e downdip in the tensile zone 3 has a mean azimuth 60 65 e there the slab is hence submitted to a tensional force slab pull oblique relatively to the convergence the transition between seismic underthrusting and intraplate normal faulting downdip occurs at the depth where the continental moho encounters the wadati benioff zone suggesting that a relationship exists between seismic coupling and the presence of continental crust at the plate interface the pre seismic state of this segment of the andean subduction zone is confirmed by the occurrence of strong earthquakes located by the global network around the presumed rupture area and by the stress regime found along the wadati benioff zone

automated fault model discretization for inversions for coseismic slip distributions 1 geoscientists increasingly rely on coseismic slip distributions inferred from geodetic observations to drive sophisticated models of the seismic cycle to date little work has been done on optimizing the parameterization of these fault models so that they reflect the resolving power of observed surface displacements the locations of noisy surface displacement observations are often widely scattered far from features we wish to analyze in the subsurface and result in highly variable model resolution with depth the few attempts to produce variably discretized fault planes are arduously constructed by hand and may not correctly reflect the ability of the data to resolve slip features motivated by the increasing size of geodetic data pools and the need for distributed slip models that accurately represent features the data can resolve we present a fully automated algorithm that iteratively adjusts the sizes of dislocations in a fault model we use the concept of smoothing scales derived from the model resolution matrix to resize dislocations so that each dislocation is sized appropriately given the area over which slip in that region of the fault would be smoothed we present a series of synthetic tests that utilize both sparse and dense data sets and we compare our variably discretized inversions to traditional regularly discretized inversions we also use our approach to invert for slip from geodetic observations of the 2004 mw 6 0 parkfield california earthquake and the 1995 mw 8 1 antofagasta chile earthquake

strong ground motion from the michoacan mexico earthquake the network of strong motion accelerographs in mexico includes instruments that were installed under an international cooperative research program in sites selected for the high potenial of a large earthquake the 19 september 1985 earthquake magnitude 8 1 occurred in a seismic gap where an earthquake was expected as a result there is an excellent descripton of the ground motions that caused the disaster

long term interactions between intermediate depth and shallow seismicity in north chile subduction zone we document interactions between intermediate depth and interplate seismicity in the north chile subduction zone over a 25 year period 1990 2015 we show that the 2005 mw 7 8 tarapaca slab pull earthquake was followed by 9 years of enhanced deep and shallow seismicity together with the decrease of eastward average gps velocities and associated interplate coupling eventually leading to the 2014 mw 8 1 iquique megathrust earthquake in contrast megathrust ruptures e g mw 8 0 antofagasta in 1995 or mw 8 1 iquique in 2014 initiate several years of silent background seismicity in the studied area both at shallow and intermediate depths the plunge of a rigid slab into a viscous asthenospheric mantle could explain the observed synchronization between deep and shallow seismicity and their long term interactions

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

trench parallel shortening in the northern chilean forearc tectonic and climatic implications in the central andes the frictional coupling between south america and the subducting nazca plate occurs beneath the coastal cordillera of northern chile one of the most distinctive characteristics of the coastal cordillera is a suite of ew topographic scarps located between 19 and 21 6 s latitude these scarps are associated with predominantly south dipping reverse faults that have almost pure dip slip and produce shortening parallel to the plate boundary limited geochronology as well as more regional studies indicate that the scarps formed during the late miocene and early pliocene though some show activity extending into the quaternary in several areas the scarps dammed local drainages producing internally drained basins that accumulated evaporites this relationship indicates that the coastal cordillera was probably moister during the late miocene and pliocene than it is today and also indicates that the coastal cordillera has been significantly uplifted or that the coastal escarpment of northern chile has advanced significantly eastward since the pliocene the limited latitudinal extent of the ew scarps and their location symmetrically about the axis of topographic and wadati benioff zone symmetry suggest that they owe their origin to the concave seaward shape of the continental margin due to prior formation of the bolivian orocline

displacement history of the atacama fault system 25 00 s 27 00 s northern chile in the cordillera de la costa of the central andes in northern chile mesozoic arc complexes are cut by a trench parallel strike slip fault system the atacama fault system brittle faulting in the atacama fault system is superposed on steeply dipping foliations in ductile shear belts between 25 s and 27 s the western part of the fault system was active in early cretaceous time as an upper amphibolite facies down to the east dip slip ductile shear zone in the eastern part of the fault system ductile deformation is of similar early cretaceous age but occurred under lower grade metamorphic conditions at the greenschist lower amphibolite facies transition the mylonites in the eastern part of the fault system were formed by sinistral strike slip displacement the dip slip and sinistral strike slip displacements are contemporary with the development of a magmatic arc and they imply that the tectonic environment in this part of the arc was transtensional the ductile deformation was partitioned spatially into a dip slip component associated with the emplacement of magmas and a sinistral strike slip component brittle fault zones in the el salado segment of the atacama fault system define large scale sidewall ripout structures subhorizontai slickenlines ripout asymmetry and s c type fabrics in fault gouge indicate that brittle deformation involved sinistral strike slip displacements the transition from ductile to brittle sinistral strike slip displacements may have occurred due to cooling in mid cretaceous time when the magmatic arc was abandoned

active faulting in northern chile ramp stacking and lateral decoupling along a subduction plate boundary two large features parallel to the coastline of northern chile have long been suspected to be the sites of young or active deformation 1 the 700 km long coastal scarp with average height above sea level of about 1000 m 2 the atacama fault zone that stretches linearly for about 1100 km at an average distance of 30 50 km from the coastline new field observations combined with extensive analysis of aerial photographs demonstrate that both the coastal scarp and the atacama fault are zones of quaternary and current fault activity little degraded surface breaks observed in the field indicate that these fault zones have recently generated large earthquakes m 7 8 normal fault offsets observed in marine terraces in the coastal scarp at mejillones peninsula require tectonic extension roughly orthogonal to the compressional plate boundary strike slip offsets of drainage observed along the salar del carmen and cerro moreno faults atacama fault system imply left lateral displacements nearly parallel to the plate boundary the left lateral movement observed along the atacama fault zone may be a local consequence of e w extension along the coastal scarp but if also found everywhere along strike left lateral decoupling along the atacama fault zone would be in contradiction with the right lateral component of nazca south america motion predicted by models of present plate kinematics clockwise rotation with left lateral slicing of the andean orogen south of the arica bend is one way to resolve this contradiction the coastal scarp and the atacama fault zone are the most prominent features with clear traces of activity within the leading edge of continental south america the great length and parallelism of these features with the subduction zone suggest that they may interact with the subduction interface at depth we interpret the coastal scarp to be a west dipping normal fault or flexure and propose that it is located over an east dipping ramp stack at the subduction interface the similar flexure at the western edge of the altiplano may have the same origin ramp stacking along the subduction zone a mechanism disregarded so far may be an important thickening process in the andes and perhaps the basic cause of the uplift of the altiplano

rupture process of the andreanof islands earthquake of may 7 1986 the andreanof islands earthquake of may 7 1986 ms 7 7 is the largest earthquake to have occurred along the aleutian arc in the past 20 yr source parameters of the mainshock three large aftershocks 6 1 ms 6 7 and a foreshock ms 6 0 are determined by inverting digitally recorded teleseismic waveforms and amplitudes long period p and sh waves are inverted simultaneously to determine the strike dip rake centroid depth source time function and seismic moment of each earthquake complexities in the rupture process of the mainshock are modeled using broadband 5 to 0 01 hz p wave displacement waveforms modeling the mainshock as a propagating line source the long period and broadband waveforms indicate that the locus of largest moment release propagated bilaterally along the arc with a well resolved velocity of 1 5 km sec the source mechanism is consistent with faulting on a low angle 23 1 thrust fault the fault s strike 260 10 and the rake of the slip vector on the fault 93 8 are less well resolved and trade off with each other acceptable centroid depths range between 10 and 25 km within this range of depths the misfit of the synthetic to the observed long period and broadband waveforms varies by only 6 per cent while moment estimates vary by as much as 64 per cent for a centroid depth of 20 km the estimated seismic moment is 1 1 1028 dyne cm 85 per cent of the moment estimated from surface wave analysis ninety per cent of the moment release was generated by three patches on the fault surface all located within 90 km of the nucleation point very little moment however was released near the epicenter of the mainshock the epicentral locations source mechanisms and depths of the foreshock and one of the aftershocks modeled by us are consistent with faulting on the same thrust plane as the mainshock the other two modeled aftershocks are located north of the island arc and have source depths of less than 15 km indicating that they occurred within the overriding plate the focal mechanisms of these events indicate almost pure strike slip faulting but the waveform data do not contain sufficient information to distinguish which of the couples of the double couple mechanism corresponds to the fault plane joint relocations of several of the aftershocks from one of these strike slip earthquakes suggests that right lateral strike slip faulting occurred on a northeasterly trending fault plane

assessment of the size of large and great historical earthquakes in peru a critical reappraisal of great historical earthquakes in peru is made in order to obtain quantitative estimates of rupture length seismic moment tsunami magnitude mt and moment magnitude mw newly collected information concerning several historical earthquakes permits the evaluation of macroseismic zones and from the size of these zones the estimation of length of rupture seismic moment and magnitude mw it is found that local tsunami data provide good quantitative estimates of mt and therefore an independent way of verifying macroseismic estimates of rupture length and seismic moment a space time diagram of great historical earthquakes shows the existence of three zones roughly corresponding to the segmentation of the nazca plate hasegawa and sacks 1981 subducting under peru the northern zone looks aseismic the central one is very complex it breaks either completely or in smaller segments and it may go through long quiescence periods the recurrence time for the southern segment the more regular one is of the order of one century

rupture process of the 1995 antofagasta subduction earthquake mw 8 1 a finite source rupture model of the july 30 1995 m w 8 1 antofagasta northern chile subduction earthquake is developed using body and surface waves that span periods from 20 to 290 s a long period 150 290 s surface wave spectral inversion technique is applied to estimate the average finite fault source properties deconvolutions of broadband body waves using theoretical green s functions and deconvolutions of broadband fundamental mode surface waves using empirical green s functions provided by a large aftershock yield effective source time functions containing periods from 20 to 200 s for many directivity parameters the source time functions are used in an inverse radon transform to image a one dimensional spatial model of the moment rate history the event produced a predominantly unilateral southward rupture yielding strong directivity effects on all seismic waves with periods less than a few hundred seconds the aftershock information spectral analysis and moment rate distribution indicate a rupture length of 180 200 km with the largest slip concentrated in the first 120 km a rupture azimuth of 205 10 along the chilean coastline and a rupture duration of 60 68 s with a corresponding average rupture velocity of 3 0 3 2 km s the overall rupture character is quite smooth accentuating the directivity effects and reducing the shaking intensity however there are three regions with enhanced moment rate distributed along the rupture zone near the epicenter 50 to 80 km south of the epicenter and 110 to 140 km south of the epicenter

seismic subduction of the nazca ridge as shown by the 1996 97 peru earthquakes by rupturing more than half of the shallow subduction interface of the nazca ridge the great november 12 1996 peruvian earthquake contradicts the hypothesis that oceanic ridges subduct aseismically the mainshock s rupture has a length of about 200 km and has an average slip of about 1 4 m its moment is 1 5 1028 dyne cm and the corresponding m w is 8 0 the mainshock registered three major episodes of moment release as shown by a finite fault inversion of teleseismically recorded broadband body waves about 55 of the mainshock s total moment release occurred south of the nazca ridge and the remaining moment release occurred at the southern half of the subduction interface of the nazca ridge the rupture south of the nazca ridge was elongated parallel to the ridge axis and extended from a shallow depth to about 65 km depth because the axis of the nazca ridge is at a high angle to the plate convergence direction the subducting nazca ridge has a large southwards component of motion 5 cm yr parallel to the coast the 900 1200 m relief of the southwards sweeping nazca ridge is interpreted to act as a rigid indenter causing the greatest coupling south of the ridge s leading edge and leading to the large observed slip the mainshock and aftershock hypocenters were relocated using a new procedure that simultaneously inverts local and teleseismic data most aftershocks were within the outline of the nazca ridge a three month delayed aftershock cluster occurred at the northern part of the subducting nazca ridge aftershocks were notably lacking at the zone of greatest moment release to the south of the nazca ridge however a lone foreshock at the southern end of this zone some 140 km downstrike of the mainshock s epicenter implies that conditions existed for rupture into that zone the 1996 earthquake ruptured much of the inferred source zone of the m w 7 9 8 2 earthquake of 1942 although the latter was a slightly larger earthquake the rupture zone of the 1996 earthquake is immediately north of the seismic gap left by the great earthquakes m w 8 8 9 1 of 1868 and 1877 the m w 8 0 antofagasta earthquake of 1995 occurred at the southern end of this great seismic gap the m w 8 2 deep focus bolivian earthquake of 1994 occurred directly downdip of the 1868 portion of that gap the recent occurrence of three significant earthquakes on the periphery of the great seismic gap of the 1868 and 1877 events among other factors may signal an increased seismic potential for that zone

determination of seismogenic interplate contact zone and crustal seismicity around antofagasta northern chile using local data summary during september and october 1988 a microseismic field experiment was carried out around the city of antofagasta northern chile with 29 portable analogue and digital stations a total of 197 reliable microearthquake locations and 19 focal mechanisms were determined it is proposed that it is possible to estimate the maximum depth of the coupled uncoupled transition of the subducting lithosphere using local data defined by the depth of the expected change of the stress field from compressional to tensional along the slab this change is observed at about 70 km depth in the antofagasta field work two estimations of the width of the seismogenic interplate contact are discussed 1 the maximum depth of the coupled zone defined by the observed maximum depth of the shallow dipping thrust events recorded during the experiment of 47 km corresponding to a width of the seismogenic contact zone of about 90 km and 2 the maximum depth of the coupled zone defined by the depth of the observed change from a compressional to tensional stress field which is 70 km and corresponds to a width of the seismogenic contact zone of about 130 km with both values the maximum magnitude ms estimated for the region varies between 8 6 and 8 7 no shallow event associated with the atacama fault system was observed during the experiment

gps derived deformation of the central andes including the 1995 antofagasta mw 8 0 earthquake in order to study both the interplate seismic loading cycle and the distribution of intraplate deformation of the andes a 215 site gps network covering chile and the western part of argentina was selected monumented and observed in 1993 and 1994 a dense part of the network in northern chile and northwest argentina comprising some 70 sites was re observed after two years in october november 1995 the m w 8 0 antofagasta north chile earthquake of 30th july 1995 took place between the two observations the city of antofagasta shifted 80 cm westwards by this event and the displacement still reached 10 cm at locations 300 km from the trench three different deformation processes have been considered for modeling the measured displacements 1 interseismic accumulation of elastic strain due to subduction coupling 2 coseismic strain release during the antofagasta earthquake and 3 crustal shortening in the sub andes eastward displacement of the sites to the north and to the south of the area affected by the earthquake is due to the interseismic accumulation of elastic deformation assuming a uniform slip model of interseismic coupling the observed displacements at the coast require a fully locked subduction interface and a depth of seismic coupling of 50 km the geodetically derived fault plane parameters of the antofagasta earthquake are consistent with results derived from wave form modeling of seismolog ical data the coseismic slip predicted by the variable slip model reaches values of 3 2 m in the dip slip and 1 4 m in the strike slip directions the derived rake is 66 our geodetic results suggest that the oblique nazca south american plate convergence is accommodated by oblique earthquake slip with no slip partitioning the observed displacements in the back arc indicate a present day crustal shortening rate of 3 4 mm year which is significantly slower than the average of 10 mm year experienced during the evolution of the andean plateau

paleoseismic inferences from a high resolution marine sedimentary record in northern chile 23 s abstract the active margin of northern chile is characterized by strong seismic events which induce gravity instability for sedimentary sequences located along the outer forearc of the andean range even though the narrow continental platform of this area limits the accumulation of marine sediments a detailed paleoceanographic reconstruction using high resolution sedimentological and geochronological techniques of a holocene sedimentary sequence in mejillones bay 23 s provides important tools to infer the occurrence of past great subduction earthquakes and or associated tsunamis from the analyses of short cores retrieved in this shallow marine basin we infer the occurrence of two strong seismic events during historical times the first one dated between the years 1409 and 1449 ad produced an angular unconformity and associated lenticular coarse grained deposits generated after local reworking of shallower material the second one dated between the years 1754 and 1789 ad caused slumping erosive processes and local reworking of material the analysis of seismic profile data acquired in this bay allowed the recognition of a major slump deposit within the upper sedimentary sequence tentatively dated as of middle holocene age this event might record the greatest earthquake and tsunami affecting the basin within the last few thousand years we interpret that destabilization of sediments occurs most probably in response to local reactivation of faults within the mejillones peninsula during strong earthquakes especially the mejillones fault

the kinematic and geodynamic significance of the atacama fault zone northern chile abstract the atacama fault zone afz is the dominant feature in the structure of the north chilean coastal cordillera left lateral displacement took place along its system of longitudinal faults during the jurassic and early cretaceous this development was contemporaneous with arc magmatism and was later reactivated resulting in a steep normal fault strike slip movements along the afz consist of two sets of ductile shear zones of different ages one jurassic formed under amphibolite facies conditions the other early cretaceous with greenschist facies mylonites structural asymmetries point to a sinistral sense of shear in both sets the afz can be interpreted as a magmatic arc structure which accommodated the oblique subduction of an oceanic plate trench linked strike slip fault the sinistral sense of shear is consistent with reconstructions of late jurassic to early cretaceous plate configurations in the se pacific

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

the ms 8 tensional earthquake of 9 december 1950 of northern chile and its relation to the seismic potential of the region abstract the only known great m s 8 intermediate depth earthquake localized downdip of the main thrust zone of the chilean subduction zone occurred landward of antofagasta on 9 december 1950 in this paper we determine the source parameters and rupture process of this shock by modeling long period body waves the source mechanism corresponds to a downdip tensional intraplate event rupturing along a nearly vertical plane with a seismic moment of m 0 1 10 28 dyn cm of strike 350 dip 88 slip 270 m w 7 9 and a stress drop of about 100 bar the source time function consists of two subevents the second being responsible for 70 of the total moment release the unusually large magnitude m s 8 of this intermediate depth event suggests a rupture through the entire lithosphere the spatial and temporal stress regime in this region is discussed the simplest interpretation suggests that a large thrust earthquake should follow the 1950 tensional shock considering that the historical record of the region does not show large earthquakes a slow earthquake can be postulated as an alternative mechanism to unload the thrust zone a weakly coupled subduction zone within an otherwise strongly coupled region as evidenced by great earthquakes to the north and south or the existence of creep are not consistent with the occurrence of a large tensional earthquake in the subducting lithosphere downdip of the thrust zone the study of focal mechanisms of the outer rise earthquakes would add more information which would help us to infer the present state of stress in the thrust region

the giant coastal landslides of northern chile tectonic and climate interactions on a classic convergent plate margin abstract documented for the first time are an extensive suite of late neogene giant terrestrial coastal landslides along the classic convergent margin of western south america 18 to 24 south these are remarkable in terms of their unusual abundance and atypical setting such failures previously being linked with oceanic volcanic edifices or over steepened glaciated coastlines located within the hyper arid coastal cordillera of the atacama desert of northern chile we report the presence of more than 60 individual large scale landslides with individual volumes up to 9 km 3 developed over a horizontal coastline distance of some 650 km these landslides were emplaced as a combination of rock avalanches and multiple rotational failures the majority terminated directly into the pacific likely generating significant tsunami hazard to the chilean and south peruvian coastline in a region which is today considered to be part of a notorious seismic gap the proliferation and scale of these late neogene giant landslides in this actively uplifting hyperarid terrain suggests they are the main geomorphic agent for relief reduction probably triggered by megathrust earthquakes and potentially providing a unique palaeoseismic archive the temporal and spatial distribution of these giant landslides corresponds with a period of surface steepening of the forearc wedge in the central andes and south to north differential uplift associated with factors such as aseismic ridge subduction the resulting surface gradient increases combined with the persistent climatic aridity of the region have served to limit effective relief reducing geomorphic processes in this oversteepened terrain to large scale landsliding the phenomena documented here geospatially link previously recognised large scale slope failures from the off shore environment and higher altitude areas of the andean forearc suggesting that large scale landsliding is capable of transferring sediment on a regional scale to the off shore peru chile trench this has implications for the friction of the subducting nazca plate and associated seismicity and uplift

invited review paper seismicity along the south american subduction zone review of large earthquakes tsunamis and subduction zone complexity abstract earthquakes along the shallow south american subduction zone exhibit heterogeneous rupture characteristics going back several centuries of the earthquake record in this area this heterogeneity is manifest in several ways such as changes in rupture mode from magnitude 8 events during one century followed by smaller ones in other time periods as well as unusual tsunami events there is also an apparent interaction between earthquake rupture and subducting plate complexity in this region significant complexity exists on the subducting nazca plate including fracture zones and ridges such as the large nazca ridge several large magnitude earthquakes have occurred in the region of ridge subduction but no earthquake rupture has ruptured through these features into adjacent regions suggesting that these subducting features act to segment the margin other features such as fracture zones and variable sediment thickness on the nazca plate appear to influence earthquake behavior over a wide range of magnitude scales upper plate features such as crustal faults also lead to heterogeneous earthquake behavior here i provide an overview of seismicity along the margin since 1850 in the context of the subduction zone structures this includes great earthquakes such as the 1906 ecuador and 1960 chile events i also present results showing increased rupture complexity in moderate magnitude earthquakes that can also be linked to certain nazca plate features

the mw 8 1 antofagasta north chile earthquake of july 30 1995 first results from teleseismic and geodetic data a strong mw 8 1 subduction earthquake occurred on july 30 1995 in antofagasta northern chile this is one of the largest events during this century in the region it ruptured the southernmost portion of a seismic gap between 18 s and 25 s in 1992 we had used gps to survey a network with about 50 benchmarks covering a region nearly 500 km long n s and 200 km wide e w part of these marks were re surveyed with gps after the 1995 earthquake comparison with 1992 positions indicate relative horizontal displacement towards the trench reaching 0 7 m the inland subsided several decimeters the mejillones peninsula was uplifted by more than 15 cm teleseismic body wave modelling of vbb records gives a subduction focal mechanism and source time function with three distinct episodes of moment release and southward directivity modelling the displacement field using a dislocation with uniform slip in elastic half space suggests a rupture zone with 19 24 eastward dip extending to a depth no greater than 50 km with n s length of 180 km and an average slip of about 5 m the component of right lateral slip inferred both from the teleseismic and geodetic data does not require slip partitioning at the plate boundary that the well constrained northern end of the 1995 rupture zone lies under the southern part of the mejillones peninsula increases the probability for a next rupture in the gap north of it

normal and reverse faulting driven by the subduction zone earthquake cycle in the northern chilean fore arc 1 despite its location in a convergent tectonic setting the coastal cordillera of northern chile between 21 s and 25 s is dominated by structures demonstrating extension in the direction of plate convergence in some locations however normal faults have been reactivated as reverse faults complicating the interpretation of long term strain in order to place these new observations in a tectonic context we model stress changes induced on these faults by the subduction earthquake cycle our simulations predict that interseismic locking on the plate boundary encourages normal slip on fore arc faults which may result from elastic rebound due to interplate earthquakes or from seismic or aseismic motion that takes place within the interseismic period conversely stress generated by strong subduction zone earthquakes such as the 1995 mw 8 1 antofagasta event provides a mechanism for the reverse reactivation we document here upper plate fault slip in response to the low magnitude stress changes induced by the subduction earthquake cycle suggests that the absolute level of stress on these faults is very low furthermore seismic hazard analysis for northern chile requires consideration of not only the plate boundary earthquake cycle but also the cycle on fore arc faults that may or may not coincide with the interplate pattern though the relationships between permanent strain and deformation calculated using elastic models remain unclear the compatibility of modeled stress fields with the distribution of fore arc faulting suggests that interseismic strain accumulation and coseismic deformation on the subduction megathrust both play significant roles in shaping structural behavior in the upper plate

new constraints on the 1922 atacama chile earthquake from historical seismograms we recently found the original omori seismograms recorded at hongo tokyo of the 1922 atacama chile earthquake m s 8 3 in the historical seismogram archive of the earthquake research institute eri of the university of tokyo these recordings enable a quantitative investigation of long period seismic radiation from the 1922 earthquake we document and provide interpretation of these seismograms together with a few other seismograms from mizusawa japan uppsala sweden strasbourg france zi ka wei china and de bilt netherlands the 1922 event is of significant historical interest concerning the cause of tsunami discovery of g wave and study of various seismic phase and first motion data also because of its spatial proximity to the 1943 1995 and 2015 great earthquakes in chile the 1922 event provides useful information on similarity and variability of great earthquakes on a subduction zone boundary the 1922 source region having previously ruptured in 1796 and 1819 is considered to have significant seismic hazard the focus of this paper is to document the 1922 seismograms so that they can be used for further seismological studies on global subduction zones since the instrument constants of the omori seismographs were only incompletely documented we estimate them using the waveforms of the observed records a calibration pulse recorded on the seismogram and the waveforms of better calibrated uppsala wiechert seismograms comparison of the hongo omori seismograms with those of the 1995 antofagasta chile earthquake m w 8 0 and the 2015 illapel chile earthquake m w 8 3 suggests that the 1922 event is similar to the 1995 and 2015 events in mechanism i e on the plate boundary megathrust and rupture characteristics i e not a tsunami earthquake with m w 8 6 0 25 however the initial fine scale rupture process varies significantly from event to event the g1 and g2 and r1 and r2 of the 1922 event are comparable in amplitude suggesting a bilateral rupture which is uncommon for large megathrust earthquakes

the updip and downdip limits to great subduction earthquakes thermal and structural models of cascadia south alaska sw japan and chile this study examines thermal and structural controls of the updip and downdip rupture limits of great subduction thrust earthquakes data on past great earthquake seismic limits have been compiled for four continental subduction zones cascadia sw japan south alaska and chile these limits have then been compared to the predictions of several models for what constrains great earthquake rupture temperatures on the subduction thrusts have been estimated by finite element numerical models the landward limits of the observed updip aseismic zones correspond to the position where the thrust temperature reaches about 100 c that is depths of about 2 to 10 km for the subduction zones studied this temperature agrees with the dehydration of stable sliding smectite clays to illite chlorite the temperatures in this region are controlled mainly by the thickness of sediment on the incoming crust and by the crustal age and thus heat flow the downdip limits correspond to the depth on the thrust where either 1 the temperature reaches about 350 c which corresponds to thermally activated stable sliding behavior for crustal rocks with a transition to 450 c or 2 about 40 km depth if 350 c is reached at greater depth depths of about 40 km approximately correspond to the intersection of the thrust with the continental forearc moho and this downdip limit may be a consequence of stable sliding serpentinite or talc and other hydrated forearc mantle rocks the primary temperature controls on the downdip region are the age of the subducting oceanic plate and the thrust dip profile secondary control comes from the thickness of incoming sediment the convergence rate and the radioactive heat generation in the overlying forearc the 100 c updip limit occurs near the trench for young subducting plates with a thick sediment section such as cascadia 6 8 ma and up to 80 km landward for older oceanic crust such as south alaska 50 ma the 350 c downdip thermal limit is applicable for young oceanic plates e g cascadia and nankai whereas the forearc mantle limit applies for older plates e g south alaska and chile except near the chile rise for the margins studied that have experienced great earthquakes there is generally good agreement between the postulated thermal and moho limits and the rupture or seismogenic zone as defined by the distribution of aftershocks and by waveform tsunami and dislocation modeling the downdip limit of the interseismic locked zone from dislocation modeling is also in agreement with these limits

source tomography by simulated annealing using broad band surface waves and geodetic data application to the mw 8 1 chile 1995 event summary we invert surface wave and geodetic data for the spatio temporal complexity of slip during the mw 8 1 chile 1995 event by simulated annealing this quasi global inversion method allows for a wide exploration of model space and retains the non linearity of the source tomography problem complex source spectra are obtained from 5 to 45 mhz from first and second orbit fundamental mode rayleigh waves using an empirical green s function cross correlation technique coseismic displacement vectors were measured at 10 gps sites near antofagasta they are part of a french chilean experiment which monitors the northern chile seismic gap the spectra together with the geodetic data are inverted for the moment distribution on a 2 d dipping fault under the physical constraints of slip positivity and causality marginal a posteriori distributions of the model parameters are obtained from several independently inverted solutions in general features of the slip model are well resolved data are well fitted by a purely unilateral southward rupture with a nearly uniform velocity around 2 5 3 0 km s 1 and a total duration of 65 s several regions of moment release were imaged one near the hypocentre a major one 80 km south of it and a minor one 160 km south of it the major patch of moment release seemed to have propagated to relatively shallow depths near the trench 100 km ssw of the epicentre the region of major slip is located updip of the 1987 mw 7 5 earthquake suggesting a causal relationship most of the slip occurred updip of the hypocentre 36 km but the entire coupled plate interface 20 40 km ruptured during the chile 1995 event

seismogenic plane of the northern andean subduction zone from aftershocks of the antofagasta chile 1995 earthquake 1 a temporary seismological network which was located in the antofagasta northern chile region recorded a large part of the aftershocks from a mw 8 0 earthquake in 1995 results from a tomographic inversion show that the aftershocks were concentrated in a plane approximately 3 km thick spatial zone comparison of our results with earlier processed wide angle reflection data shows clearly that this zone is positioned at the top of the oceanic crust therefore it represents the shearing boundary between the subducting oceanic and the continental plate and gives an in situ estimation of the thickness of the boundary outside of this zone events were detected in the continental crust as well as within the oceanic mantle

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