structure and deformation of the kermadec forearc in response to subduction of the pacific oceanic plate the tonga kermadec forearc is deforming in response to on going subduction of the pacific plate beneath the indo australian plate previous research has focussed on the structural development of the forearc where large bathymetric features such as the hikurangi plateau and louisville ridge seamount chain are being subducted consequently knowledge of the background forearc in regions of normal plate convergence is limited we report on an 250 km long multichannel seismic reflection profile that was shot perpendicular to the tonga kermadec trench at 28 s to determine the lateral and temporal variations in the structure stratigraphy and deformation of the kermadec forearc resulting solely from pacific plate subduction interpretation of the seismic profile in conjunction with regional swath bathymetry data shows that the pacific plate exhibits horst and graben structures that accommodate bending induced extensional stresses generated as the trenchward dip of the crust increases trench infill is also much thicker than expected at 1 km which we propose results from increased sediment flux into and along the trench pervasive normal faulting of the mid trench slope most likely accommodates the majority of the observed forearc extension in response to basal subduction erosion and a structural high is located between the mid and upper trench slopes we interpret this high as representing a dense and most likely structurally robust region of crust lying beneath this region sediment of the upper trench slope documents depositional hiatuses and on going uplift of the arc strong along arc currents appear to erode the kermadec volcanic arc and distribute this sediment to the surrounding basins while currents over the forearc redistribute deposits as sediment waves minor uplift of the transitional kermadec forearc observed just to the north of the profile appears to relate to an underlying structural trend as well as subduction of the louisville ridge seamount chain 250 km to the north relative uplift of the kermadec arc is observed from changes in the tilt of upper trench slope deposits and extensional faulting of the basement immediately surrounding the louisville ridge

teleseismic waveform complexities caused by near trench structures and their impacts on earthquake source study application to the 2015 illapel aftershocks central chile

morphology and geology of the continental shelf and upper slope of southern central chile 33 s 43 s the continental shelf and slope of southern central chile have been subject to a number of international as well as chilean research campaigns over the last 30 years this work summarizes the geologic setting of the southern central chilean continental shelf 33 s 43 s using recently published geophysical seismological sedimentological and bio geochemical data additionally unpublished data such as reflection seismic profiles swath bathymetry and observations on biota that allow further insights into the evolution of this continental platform are integrated the outcome is an overview of the current knowledge about the geology of the southern central chilean shelf and upper slope we observe both patches of reduced as well as high recent sedimentation on the shelf and upper slope due to local redistribution of fluvial input mainly governed by bottom currents and submarine canyons and highly productive upwelling zones shelf basins show highly variable thickness of oligocene quaternary sedimentary units that are dissected by the marine continuations of upper plate faults known from land seismic velocity studies indicate that a paleo accretionary complex that is sandwiched between the present relatively small active accretionary prism and the continental crust forms the bulk of the continental margin of southern central chile

fault trends on the seaward slope of the aleutian trench implications for a laterally changing stress field tied to a westward increase in oblique convergence 1 normal faults along the seaward trench slope sts commonly strike parallel to the trench in response to bending of the oceanic plate into the subduction zone this is not the circumstance for the aleutian trench where the direction of convergence gradually changes westward from normal to transform motion gloria side scan sonar images document that the aleutian sts is dominated by faults striking oblique to the trench west of 179 e and east of 172 w these images also show a pattern of east west trending seafloor faults that are aligned parallel to the spreading fabric defined by magnetic anomalies the stress strain field along the sts is divided into two domains west and east respectively of 179 e over the western domain sts faults and nodal planes of earthquakes are oriented oblique 9 46 to the trench axis and 69 90 to the magnetic fabric west of 179 e sts fault strikes change by 36 from the e w trend of sts where the trench parallel slip gets larger than its orthogonal component of convergence this rotation indicates that horizontal stresses along the western domain of the sts are deflected by the increasing obliquity in convergence an analytical model supports the idea that strikes of sts faults result from a superposition of stresses associated with the dextral shear couple of the oblique convergence and stresses caused by plate bending for the eastern domain most nodal planes of earthquakes strike parallel to the outer rise indicating bending as the prevailing mechanism causing normal faulting east of 172 w sts faults strike parallel to the magnetic fabric but oblique 10 26 to the axis of the trench on the basis of a coulomb failure criterion the trench oblique strikes probably result from reactivation of crustal faults generated by spreading

development of the accretionary prism along peru and material flux after subduction of nazca ridge results from ocean drilling program leg 112 indicated tectonic erosion of the peruvian convergent margin during subduction of the nazca ridge followed by renewed accretion against the erosional scar seismic images and swath mapping morphology show a regional geology shaped as subduction of nazca ridge migrated 800 km along the peru trench the accretionary prism is imaged at two stages of development since upper miocene time with depth migrated seismic records in which the back stop is well defined from these data material flux can be quantified after the ridge crest subducted and accretion dominated over erosion the prism grew rapidly to 10 to 15 km width during rapid growth the margin taper was large and about 60 percent of the sediment supply accreted as the prism growth slowed only 30 percent accreted despite an increased trench sediment supply since the convergence rate changed little the inverse relation between sediment supply and prism growth rate suggests that other processes allow most of the sediment input to be subducted the decreased prism growth may be partially controlled by varying the structure of the back stop the force required to activate faults cutting across the back stop is probably greater than the force allowed by basal friction across the plate boundary thus the shear stress transmitted across the plate boundary is insufficient to drive out of sequence thrust faults that would thicken the back stop or raise it beyond a limited height a constant back stop height limits accretion and the subduction window appears to open and accommodate increased trench sediment beneath the back stop rather than inducing a transport of sediment over the top of the back stop nazca ridge subduction affected the convergent margin mass flux for about 8 ma mass balancing indicates an order of magnitude increase in the current sediment volume subducted in the north compared to that near the ridge crest when growth of the accretionary prism slowed a greater terrigenous input through the trench was accommodated by sediment subduction the resulting range of material flux is estimated to increase the sediment interlayer between the plates about 500 m which may affect coupling

neotectonic deformation of the central chile margin deepwater forearc basin formation in response to hot spot ridge and seamount subduction 1 underlying the midslope valparaiso terrace the valparaiso basin constitutes a unique and prominent deepwater forearc basin on the central chile margin the basin contains a 3 3 5 km thick sediment fill of late cenozoic age the seismic stratigraphic architecture of the valparaiso basin fill shows that margin wide subsidence above the underthrusting juan fernandez ridge an aseismic volcanic swell crested with seamounts on the nazca plate characterized the basin evolution contemporaneously with margin subsidence and seaward rotation of the upper slope and shelf the outer part of the forearc was deformed by thrust faulting above the leading flank of underthrusting seamounts that rotated the middle slope to a more horizontal attitude and formed seaward vergent outer forearc highs subsidence of the margin above the underthrusting juan fernandez ridge is ascribed to processes of subduction erosion enhanced by factors related to subduction of the ridge that caused dismemberment of upper plate material along the shallow aseismic part of the plate interface and transfer of material to the downgoing nazca plate accelerated midslope subsidence 55 75 km landward of the trench coincides with the seaward limit of interplate seismicity suggesting that enhanced basal erosion is related to the transition from aseismic to stick slip subduction behavior since subduction of the juan fernandez ridge began near 33 s 10 ma ago the chile trench north of 33 s is inferred to have retreated 3 km myr and the midslope valparaiso basin subsided 0 3 0 5 km myr approximating analogous neogene rates for the peruvian margin

the west andean thrust the san ramon fault and the seismic hazard for santiago chile 1 the importance of west verging structures at the western flank of the andes parallel to the subduction zone appears currently minimized this hampers our understanding of the andes altiplano one of the most significant mountain belts on earth we analyze a key tectonic section of the andes at latitude 33 5 s where the belt is in an early stage of its evolution with the aim of resolving the primary architecture of the orogen we focus on the active fault propagation fold system in the andean cover behind the san ramon fault which is critical for the seismic hazard in the city of santiago and crucial to decipher the structure of the west andean thrust wat the san ramon fault is a thrust ramp at the front of a basal detachment with average slip rate of 0 4 mm yr young scarps at various scales imply plausible seismic events up to mw 7 4 the wat steps down eastward from the san ramon fault crossing 12 km of andean cover to root beneath the frontal cordillera basement anticline a range 5 km high and 700 km long we propose a first order tectonic model of the andes involving an embryonic intracontinental subduction consistent with geological and geophysical observations the stage of primary westward vergence with dominance of the wat at 33 5 s is evolving into a doubly vergent configuration a growth model for the wat altiplano similar to the himalaya tibet is deduced we suggest that the intracontinental subduction at the wat is a mechanical substitute of a collision zone rendering the andean orogeny paradigm obsolete

basin centered asperities in great subduction zone earthquakes a link between slip subsidence and subduction erosion 1 published areas of high coseismic slip or asperities for 29 of the largest circum pacific megathrust earthquakes are compared to forearc structure revealed by satellite free air gravity bathymetry and seismic profiling on average 71 of an earthquake s seismic moment and 79 of its asperity area occur beneath the prominent gravity low outlining the deep sea terrace 57 of an earthquake s asperity area on average occurs beneath the forearc basins that lie within the deep sea terrace in sw japan slip in the 1923 1944 1946 and 1968 earthquakes was largely centered beneath five forearc basins whose landward edge overlies the 350 c isotherm on the plate boundary the inferred downdip limit of the locked zone basin centered coseismic slip also occurred along the aleutian mexico peru and chile subduction zones but was ambiguous for the great 1964 alaska earthquake beneath intrabasin structural highs seismic slip tends to be lower possibly due to higher temperatures and fluid pressures kilometers of late cenozoic subsidence and crustal thinning above some of the source zones are indicated by seismic profiling and drilling and are thought to be caused by basal subduction erosion the deep sea terraces and basins may evolve not just by growth of the outer arc high but also by interseismic subsidence not recovered during earthquakes basin centered asperities could indicate a link between subsidence subduction erosion and seismogenesis whatever the cause forearc basins may be useful indicators of long term seismic moment release the source zone for cascadia s 1700 a d earthquake contains five large basin centered gravity lows that may indicate potential asperities at depth the gravity gradient marking the inferred downdip limit to large coseismic slip lies offshore except in northwestern washington where the low extends landward beneath the coast transverse gravity highs between the basins suggest that the margin is seismically segmented and could produce a variety of large earthquakes

deep lithospheric structures along the southern central chile margin from wide angle p wave modelling p crustal and upper mantle structures of the subduction zone in south central chile between 42 degrees s and 46 degrees s are determined from seismic wide angle reflection and refraction data using the seismic ray tracing method to calculate minimum parameter models three profiles along differently aged segments of the subducting nazca plate were analysed in order to study subduction zone structure dependencies related to the age that is thermal state of the incoming plate the age of the oceanic crust at the trench ranges from 3 ma on the southernmost profile immediately north of the chile triple junction to 6 5 ma old about 100 km to the north and to 14 5 ma old another 200 km further north off the island of chiloe remarkable similarities appear in the structures of both the incoming as well as the overriding plate the oceanic nazca plate is around 5 km thick with a slightly increasing thickness northward reflecting temperature changes at the time of crustal generation the trench basin is about 2 km thick except in the south where the chile ridge is close to the deformation front and only a small 800 m thick trench infill could develop in south central chile typically three quarters 1 5 km of the trench sediments subduct below the decollement in the subduction channel to the north and south of the study area only about one quarter to one third of the sediments subducts the rest is accreted above similarities in the overriding plate are the width of the active accretionary prism 35 50 km and a strong lateral crustal velocity gradient zone about 75 80 km landward from the deformation front where landward upper crustal velocities of over 5 0 5 4 km s su 1 su decrease seaward to around 4 5 km s su 1 su within about 10 km which possibly represents a palaeo backstop this zone is also accompanied by strong intraplate seismicity differences in the subduction zone structures exist in the outer rise region where the northern profile exhibits a clear bulge of uplifted oceanic lithosphere prior to subduction whereas the younger structures have a less developed outer rise this plate bending is accompanied by strongly reduced rock velocities on the northern profile due to fracturing and possible hydration of the crust and upper mantle the southern profiles do not exhibit such a strong alteration of the lithosphere although this effect may be counteracted by plate cooling effects which are reflected in increasing rock velocities away from the spreading centre overall there appears little influence of incoming plate age on the subduction zone structure which may explain why the m w 9 5 great chile earthquake from 1960 ruptured through all these differing age segments the rupture area however appears to coincide with a relatively thick subduction channel

deformation of erosive and accretive forearcs during subduction of migrating and non migrating aseismic ridges results from 3 d finite element models and application to the central american peruvian and ryukyu margins subduction of aseismic oceanic ridges causes considerable uplift and deformation of the upper plate and may lead for example to the indentation of the forearc the formation of marine terraces or distinct fault patterns in the upper plate depending on the orientation of the ridge relative to the plate convergence direction the ridge may either be stationary or migrate along the margin here we use three dimensional numerical models to investigate the tectonic evolution of forearcs affected by ridge subduction in different experiments we distinguish between migrating non migrating ridges and accretive erosive margins respectively our results reveal that displacement and strain fields above migrating and non migrating ridges are asymmetric with respect to the ridge axis unless both ridge and plate convergence direction are perpendicular to the trench as the asymmetric deformation pattern shifts along the margin through time uplift caused by the underthrusting ridge is followed by subsidence when the ridge crest passed by and regions initially experiencing shortening may subsequently undergo extension and vice versa if the forearc comprises an accretionary prism the ridge induced reentrant is larger than those in models with erosive forearcs and strain localizes in the frontal part of the wedge additional models with a setup adjusted to the cocos and gagua ridges provide constraints on the onset of their subduction at the central american and ryukyu margins at 2 ma and 1 ma respectively displacement and strain fields from a model for the nazca ridge collision zone show good agreement with geological data from marine terraces and quaternary faulting

combining seismic arrays to image the high frequency characteristics of large earthquakes summary this study investigates the source properties of the 2007 8 0 pisco peru 2007 8 4 and 7 9 mentawai islands indonesia 2009 8 1 samoa islands and 2010 8 8 maule chile earthquakes using a backprojection technique that utilizes data from multiple seismic arrays combining seismic arrays increases azimuth and distance coverage and improves lateral resolution the four subduction interface events show rupture propagation towards the bottom of the seismogenic zone in addition all of the earthquakes show evidence of multiple rupture segments with varying rupture speeds and directions relating these segments to the specific features of the subduction zones e g interseismic coupling suggests that asperity sizes may be controlled by subtle features of the subducted slab

outer rise stress changes related to the subduction of the juan fernandez ridge central chile 1 although outer rise seismicity is less common than interplate seismicity in subduction zones a significant level of seismicity has occurred between the nazca trench and juan fernandez ridge in central chile during the past 20 years we first study the 9 april 2001 mw 7 0 event and determine its focal mechanism depth and source time function by body waveform inversion from teleseismic broadband data the results indicate tensional faulting in the upper part of the mechanical lithosphere its strike 41 is similar to those observed in events down dip of the slab at about 100 km depth which could indicate that these earthquakes occur in preexisting structures formed at the trench compressive outer rise events have also occurred during the 1980s in front of the rupture zone of the 1985 mw 7 8 valparaiso earthquake to understand their relation with the state of stress of the lithosphere we construct yield stress envelopes of the oceanic lithosphere including static and dynamic stresses dynamic stresses are due either to slab pull ridge push resistive and drag forces we explain the sequence of compressive and tensional events by the accumulation of stress prior to 1985 when the subduction is assumed to be locked and after by the unlockage of the subduction by the valparaiso interplate event the yield stress envelope analysis enables us to quantify the accumulation of compressive forces before 1985 and the tensional force after

geothermal evidence for fluid flow through the gas hydrate stability field off central chile transient flow related to large subduction zone earthquakes fluids are suspected to play a major role in the nucleation and rupture propagation of earthquakes in chile seismological data were previously interpreted to indicate that fluids captured in the fault zone are released periodically during large underthrust earthquakes leading to post seismic fluid flow in central chile heat flow derived from the presence of a bottom simulating reflector bsr show a smooth trend across the margin bsr derived data are in excellent agreement with thermal subduction zone models over the young accretionary prism both bsr derived and measured surface heat flow support a common trend landwards of the backstop however measured heat flow triples over a distance of 20 30 km producing a profound discrepancy to the bsr derived data we suggest that this disparity is related to transient flow of warm fluids through the gas hydrate stability zone possibly caused by fluids released after large underthrust earthquakes such flow events may inherently affect the distribution of solid gas hydrates between the seafloor and the bsr

3d simultaneous seismic refraction and reflection tomography of wide angle data from the central chilean margin we present an application of three dimensional 3d simultaneous seismic refraction and reflection tomography for velocity and interface structure the inversion technique and method for developing the starting model are specifically designed for relatively sparse wide angle data acquired across strongly varying structure the data were recorded in a region of seamount subduction on the chilean margin and consist of seven receivers and ten intersecting airgun profiles over a 90 90 km area providing constraint to 25 km depth the tomographic method and the final model are assessed through a comparison with the large scale geologic features of the margin and a resolution test the 3d model shows the valparaiso forearc basin the accretionary wedge the subducting plate and possibly a subducted seamount our results show the potential of relatively sparse 3d wide angle data

crustal deformation dynamics and stress evolution during seamount subduction high resolution 3 d numerical modeling funding was received from the people programme marie curie actions of the european union s seventh framework programme fp7 2007 2013 under rea grant agreement 604713 zip zooming in between plates v s and c r r acknowledge financial support received through the hades project ctm2011 30400 c01 and ctm2011 30400 c02 funded by spanish mineco

lithospheric density structure of the southern central andes constrained by 3d data integrative gravity modelling the southern central andes sca between 27 s and 40 s is bordered to the west by the convergent margin between the continental south american plate and the oceanic nazca plate the subduction angle along this margin is variable as is the deformation of the upper plate between 33 s and 35 s the subduction angle of the nazca plate increases from sub horizontal 5 in the north to relatively steep 30 in the south the sca contain inherited lithological and structural heterogeneities within the crust that have been reactivated and overprinted since the onset of subduction and associated cenozoic deformation within the andean orogen the distribution of the deformation within the sca has often been attributed to the variations in the subduction angle and the reactivation of these inherited heterogeneities however the possible influence that the thickness and composition of the continental crust have had on both short term and long term deformation of the sca is yet to be thoroughly investigated for our investigations we have derived density distributions and thicknesses for various layers that make up the lithosphere and evaluated their relationships with tectonic events that occurred over the history of the andean orogeny and in particular investigated the short and long term nature of the present day deformation processes we established a 3d model of lithosphere beneath the orogen and its foreland 29 s 39 s that is consistent with currently available geological and geophysical data including the gravity data the modelled crustal configuration and density distribution reveal spatial relationships with different tectonic domains the crystalline crust in the orogen the magmatic arc and the main orogenic wedge is thicker 55 km and less dense 2900 kg m3 than in the forearc 35 km 2975 kg m3 and foreland 30 km 3000 kg m3 crustal thickening in the orogen probably occurred as a result of stacking of low density domains while density and thickness variations beneath the forearc and foreland most likely reflect differences in the tectonic evolution of each area following crustal accretion no clear spatial relationship exists between the density distribution within the lithosphere and previously proposed boundaries of crustal terranes accreted during the early paleozoic areas with ongoing deformation show a spatial correlation with those areas that have the highest topographic gradients and where there are abrupt changes in the average crustal density contrast this suggests that the short term deformation within the interior of the andean orogen and its foreland is fundamentally influenced by the crustal composition and the relative thickness of different crustal layers a thicker denser and potentially stronger lithosphere beneath the northern part of the sca foreland is interpreted to have favoured a strong coupling between the nazca and south american plates facilitating the development of a sub horizontal slab

short and long term effects of gps measured crustal deformation rates along the south central andes in this study we present the contemporary crustal deformation field along the central and southern andes 17 42 s estimated from four global positioning sys tem gps campaigns conducted in 1994 97 we find that the majority of the observed crustal deformation field is relatively homogenous roughly parallel to the plate conver gence direction with decreasing velocities away from the trench we attribute this type of deformation pattern to the inter seismic phase of an earthquake deformation cycle caused by 100 locking of the thrust interface between the subducting nazca and the overrid ing south american plates we have also detected a strong post seismic deformation sig nal in the vicinity of the 1995 mw8 0 antofagasta 22 26 s and 1960 mw9 5 chile 38 43 s earthquakes this type of deformation can be described as short term in nature compared to geological timescales the above conclusions are based on the results of the 3 d andean elastic dislocation model aedm by subtracting the aedm predicted deformation rates from the observations we obtained a residual velocity field that high lights the post seismic as well as more long term deformation eects for example we find a strong evidence for the continuing crustal shortening across the back arc reach ing its maximum 4 mm yr in the very north of our study area in addition between latitudes 29 34 s there is an indication of e w oriented extension within the fore arc in accordance with the recent geologic findings for the n s oriented normal faulting

episodic development of a convergent margin inferred from structures and processes along the southern chile margin seismic reflection data acquired in the vicinity of isla mocha across the southern coast of chile image structures formed along the continental margin and reveal an episodic history of accretion nonaccretion and possibly erosion structures formed at the toe of the continental slope suggest frontal accretion of to 1 km of trench fill seismic images also reveal that a small accretionary wedge 20 30 km wide abuts the truncated continental metamorphic basement that extends seaward from beneath the shelf the small size of the accretionary wedge on three profiles examined here is not consistent with a long history of accretion with the current deformational style as current rates of frontal accretion could have accumulated all of the existing accretionary wedge in less than 1 2 m y this is a small fraction of convergence history along this margin and the current accretionary mode has not been consistently maintained in the past the isla mocha region is located between the temperate climate of central chile and the glacial climate of southern chile and climatic conditions in this region have likely fluctuated sufficiently to cause significant variation in trench sediment supply accretionary and nonaccretionary or erosional episodes are probably linked to temporal variations in trench sediment thickness as suggested by observations along the chile margin currently thick trench sediment correlates with accretion along the southern chile margin and thin trench sediment correlates with nonaccretion tectonic erosion as near the chile ridge and from the juan fernandez ridge to northern chile the isla mocha region also lies 900 1000 km north of the chile triple junction and the chile ridge lies approximately 2000 km to the west and has not yet collided and affected the margin near isla mocha this part of the precollision zone provides an excellent reference to examine the effects of chile ridge collision in the development of the chile margin the most apparent effect of subduction of the buoyant young crust of the chile ridge is a shallow trench that is nearly devoid of sediment consequently the triple junction is undergoing nonaccretion or erosion and the accretionary complex near the triple junction remains smaller than to the north or south because the current phase of rapid accretion elsewhere in the trench has bypassed the triple junction region the interplay of subduction zone processes such as trench sedimentation and ridge collision has resulted in an episodic development of the margin and produced a discontinuous record of convergence history within the accretionary wedge

subduction of the nazca ridge and the inca plateau insights into the formation of ore deposits in peru a large number of ore deposits that formed in the peruvian andes during the miocene 15 5 ma are related to the subduction of the nazea plate beneath the south american plate here we show that the spatial and temporal distribution of these deposits correspond with the arrival of relatively buoyant topographic anomalies namely the nazca ridge in central peru and the now consumed inca plateau in northern peru at the subduction zone plate reconstruction shows a rapid metallogenic response to the arrival of the topographic anomalies at the subduction trench this is indicated by clusters of ore deposits situated within the proximity of the laterally migrating zones of ridge subduction it is accordingly suggested that tectonic changes associated with impingement of the aseismic ridge into the subduction zone may trigger the formation of ore deposits in metallogenically fertile suprasubduction environments c 2005 elsevier b v all rights reserved

eocene propagating rifts in the southwest pacific and their conjugate features on the nazca plate we have mapped a 1200 km long nw se trending lineament in seasat and geosat radar altimeter data crossing a remote portion of the southwest pacific basin this lineament runs obliquely both to the fracture zones fzs and magnetic lineations between the austral and agassiz fzs by examining shipboard magnetics gravity and bathymetric profiles in this region and in the conjugate regions of the nazca plate we identify this feature as the site of a series of small ridge jumps highly asymmetrical spreading and small propagating rifts during the mid to late eocene chron c21 to chron c13 between the austral and agassiz fzs there is roughly 250 to 350 km of missing pacific plate crust which must have been transferred to the nazca plate by the propagating rifts and small ridge jumps on the nazca plate the conjugate area to the gravity lineament is now being subducted beneath the chile trench north of the challenger fz this area corresponds to a region of undecipherable magnetic anomalies south of the challenger fz there is evidence for a small 50 km ridge jump offshore of the chile trench this small ridge jump by itself is not sufficient to account for the missing crust on the pacific plate and we conclude that there must have been repeated small ridge jumps at the same site one consequence of these ridge jumps is that crust on the nazca plate entering the trench is not as old as would be predicted by simply mirror imaging the anomaly pattern on the pacific plate

active tectonics of the south chilean marine fore arc 35 s 40 s the south chilean marine fore arc 35 s 40 s is separated into four tectonic segments concepcion north concepcion south nahuelbuta and tolten from north to south these are each characterized by their individual tectonic geomorphology and reflect different ways of mechanical and kinematic interaction of the convergent nazca and south american plates splay faults that cut through continental framework rock are seismically imaged in both concepcion segments and the tolten segment additionally the concepcion south segment exhibits prominent upper plate normal faults normal faults apparently relate to uplift caused by sediment underthrusting at depth this has led to oversteepening and gravitational collapse of the marine fore arc there is also evidence for sediment underthrusting and basal accretion to the overriding plate in the tolten segment there uplift of the continental slope has created a landward inclined seafloor over a latitudinal distance of 50 km in the nahuelbuta segment transpressive upper plate faults aligned oblique to the direction of plate motion control the seafloor morphology based on a unique acoustic data set including 90 of bathymetric coverage of the continental slope we are able to reveal an along strike heterogeneity of a complexly deformed marine fore arc which had escaped attention in previous studies that only considered the structure along transects normal to the plate margin

tectonic control on sediment accretion and subduction off south central chile implications for coseismic rupture processes of the 1960 and 2010 megathrust earthquakes based on a compilation of published and new seismic refraction and multichannel seismic reflection data along the south central chile margin 33 46 s we study the processes of sediment accretion and subduction and their implications on megathrust seismicity in terms of the frontal accretionary prism fap size the marine south central chile fore arc can be divided in two main segments 1 the maule segment south of the juan fernandez ridge and north of the mocha block characterized by a relative large fap 20 40 km wide and 2 the chiloe segment south of the mocha block and north of the nazca antarctic south america plates junction characterized by a small fap 10 km wide in addition the maule and chiloe segments correlate with a thin 1 km thick and thick 1 5 km thick subduction channel respectively the mocha block lies between 37 5 and 40 s and is configured by the chile trench mocha and valdivia fracture zones this region separates young 0 25 ma oceanic lithosphere in the south from old 30 35 ma oceanic lithosphere in the north and it represents a fundamental tectonic boundary separating two different styles of sediment accretion and subduction respectively a process responsible for this segmentation could be related to differences in initial angles of subduction which in turn depend on the amplitude of the down deflected oceanic lithosphere under trench sediment loading on the other hand a small fap along the chiloe segment is coincident with the rupture area of the trans pacific tsunamigenic 1960 earthquake mw 9 5 while a relatively large fap along the maule segment is coincident with the rupture area of the 2010 earthquake mw 8 8 differences in earthquake and tsunami magnitudes between these events can be explained in terms of the fap size along the chiloe and maule segments that control the location of the updip limit of the seismogenic zone the rupture area of the 1960 event also correlates with a thick subduction channel chiloe segment that may provide enough smoothness at the subduction interface allowing long lateral earthquake rupture propagation

the 2015 illapel earthquake central chile a type case for a characteristic earthquake on 16 september 2015 the mw 8 2 illapel megathrust earthquake ruptured the central chilean margin combining inversions of displacement measurements and seismic waveforms with high frequency hf teleseismic backprojection we derive a comprehensive description of the rupture which also predicts deep ocean tsunami wave heights we further determine moment tensors and obtain accurate depth estimates for the aftershock sequence the earthquake nucleated near the coast but then propagated to the north and updip attaining a peak slip of 5 6 m in contrast hf seismic radiation is mostly emitted downdip of the region of intense slip and arrests earlier than the long period rupture indicating smooth slip along the shallow plate interface in the final phase a superficially similar earthquake in 1943 with a similar aftershock zone had a much shorter source time function which matches the duration of hf seismic radiation in the recent event indicating that the 1943 event lacked the shallow slip

the 2010 mw 8 8 chile earthquake triggering on multiple segments and frequency dependent rupture behavior 1 multi frequency back projection results of the february 27 2010 mw 8 8 chile earthquake reveal that this earthquake consists of distinct subevents with different slip characteristics the subevent south of the epicenter releases its energy at low frequencies 0 05 0 1 hz implying slow slip and has a rupture speed around 0 8 km s two subevents north of the epicenter are characterized by high frequency energy 1 5 hz release the first of these subevents appears to trigger slip on the second segment to the north which has higher amplitude energy release and a fast propagation speed of about 2 9 km s in addition to these rupture details high frequency energy release is observed at the rupture front followed by lower frequency energy release this observation suggests that the rupture of large earthquakes involves dynamic weakening of faults and hence their rupture properties may not be related easily to those of small earthquakes these distinct slip behaviors point out the need to consider data from a wide range of frequencies to fully assess the rupture process and associated hazards of giant earthquakes

time space mapping of easter chain volcanism abstract new 40ar 39ar and published k ar ages show that the locus of volcanism along the easter volcanic chain evc has shifted systematically from the nazca ridge at about 26 m y to the recently active sala y gomez island easter island region this indicates a plume rather than a hotline i e mantle roll origin for the evc the time space distribution of ages combined with published ages for the galapagos and juan fernandez volcanic chains is used to reconstruct nazca plate velocities over the past 26 m y a plume now located in the region of sala y gomez island is most compatible with these data west of the plume the evc records neither nazca nor pacific plate motions this section of the evc may be related to westward channeling of plume material to the pacific nazca spreading boundary region

geological and geophysical variations along the western margin of chile near lat 33 to 36 s and their relation to nazca plate subduction

monte vulture volcano basilicata italy an analysis of morphology and volcaniclastic facies the earliest activity of monte vulture central italy included ignimbrites but the bulk of the volcano was built up by plinian airfall deposits contemporaneous remobilisation of these deposits formed an apron of lahars around the base of the main cone the volcano was constructed on a ridge the valley to the east and tributaries to the north and south became sediment traps for volcaniclastic materials emplaced by fluvial reworking and directly from volcanic activity to the west the valley was swept clear by active downcutting instability of the west flank as a result of this erosion was probably a contributory cause of major gravitational sector collapse on the volcano s flank terminating the main cone building phase the resultant scar is an amphitheatre shaped hollow called here the valle dei grigi previous workers have attributed this feature to coalescing calderas formed by engulfment the last volcanic phase was the production of the monticchio calderas and associated phreatomagmatic explosions producing airfall and surge deposits because most of the activity at vulture has been repeated plinian eruptions producing similar assemblages of products detailed stratigraphy of the volcano is difficult to accomplish to characterise vulture in terms of its products various facies are identified and interpreted in relation to volcanic processes distance from vent and environmental conditions

assessment of crustal velocity models using seismic refraction and reflection tomography two tomographic methods for assessing velocity models obtained from wide angle seismic traveltime data are presented through four case studies the modelling inversion of wide angle traveltimes usually involves some aspects that are quite subjective for example 1 identifying and including later phases that are often difficult to pick within the seismic coda 2 assigning specific layers to arrivals 3 incorporating pre conceived structure not specifically required by the data and 4 selecting a model parametrization these steps are applied to maximize model constraint and minimize model non uniqueness however these steps may cause the overall approach to appear ad hoc and thereby diminish the credibility of the final model the effect of these subjective choices can largely be addressed by estimating the minimum model structure required by the least subjective portion of the wide angle data set the first arrival times for data sets with moho reflections the tomographic velocity model can be used to invert the pmp times for a minimum structure moho in this way crustal velocity and moho models can be obtained that require the least amount of subjective input and the model structure that is required by the wide angle data with a high degree of certainty can be differentiated from structure that is merely consistent with the data the tomographic models are not intended to supersede the preferred models since the latter model is typically better resolved and more interpretable this form of tomographic assessment is intended to lend credibility to model features common to the tomographic and preferred models four case studies are presented in which a preferred model was derived using one or more of the subjective steps described above this was followed by conventional first arrival and reflection traveltime tomography using a finely gridded model parametrization to derive smooth minimum structure models the case studies are from the se canadian cordillera across the rocky mountain trench central india across the narmada son lineament the iberia margin across the galicia bank and the central chilean margin across the valparaiso basin and a subducting seamount these case studies span the range of modern wide angle experiments and data sets in terms of shot receiver spacing marine and land acquisition lateral heterogeneity of the study area and availability of wide angle reflections and coincident near vertical reflection data the results are surprising given the amount of structure in the smooth tomographically derived models that is consistent with the more subjectively derived models the results show that exploiting the complementary nature of the subjective and tomographic approaches is an effective strategy for the analysis of wide angle traveltime data

upper mantle structure in the south central chilean subduction zone 30 to 36 s 1 regional p and s wave travel time data were used to obtain three dimensional seismic tomography models for vp vs and vp vs above the subducting slab in central chile and argentina in this region there is an abrupt change from a normal subduction geometry south of 33 s to a flat subduction geometry to the north we find low vp low vs and high vp vs ratios in the southern half of our study area directly beneath the modern active volcanic arc which we interpret as localized pockets of melt in the northern half of our study area above where the subducting nazca plate flattens at 100 km depth we find low vp high vs and low vp vs ratios these unusual results point to a lack of melt or hydrated mineralogies such as serpentine both of which are characterized by high vp vs values the only mantle rocks that have low vp vs and high vs are mg rich compositions such as dehydrated serpentinite or orthopyroxenite we suggest that significant portions of the mantle overlying the flat slab consist of orthopyroxenite formed by a transient fluxing of silica rich fluids such fluids may have come from sediments that were subducted during the initiation of flat subduction at this latitude at 10 ma this would imply that the hydration of mantle material above a flat slab can be a transient phenomenon which leaves little residual free water behind but significantly alters the mantle chemistry

submarine weathering of silicate minerals and the extent of pore water freshening at active continental margins in order to investigate how submarine weathering processes may affect the water balance of sediments at convergent plate margins six sediment cores were retrieved off central chile at water depth between 800 and 4000 m the sediment solid phase was analyzed for its major element composition and the pore fluids were analyzed for dissolved sulfate sulfide total alkalinity major cations chloride bromide iodide hydrocarbons as well as the carbon isotopic composition of methane because of negligible weathering on land surface sediments off central chile are rich in reactive silicate minerals and have a bulk composition similar to volcanic rocks in the adjacent andes deep sourced fluxes of alkalinity cations and chloride indicate that silicate minerals are subject to weathering in the forearc during burial comparison of deep sourced signals with data from nearby ocean drilling program sites reveals two different types of weathering processes in shallow tens of meters methanic sediments of slope basins with high organic carbon burial rates reactive silicate minerals undergo incongruent dissolution through reaction with co2 from methanogenesis at greater burial depth hundreds of meters silicate weathering is dominated by authigenic smectite formation this process is accompanied by uptake of water into the clay interlayers thus leading to elevated salinities in the surrounding pore water deep seated smectite formation is more widespread than shallow silicate dissolution as it is independent from the availability of co2 from methanogenesis although solute transport is not focused enough to form cold seeps in the proper sense tectonically induced diffuse fluid flow transfers the deep seated signal of smectite formation into the shallow sediments the temperature controlled conversion of smectite to illite is considered the most important dehydration process in marine forearc environments depth of kilometers however in agreement with other studies at active margins e g aleutians cascadia nankai trough and despite ubiquitous evidence for smectite formation little evidence for seafloor seepage of dehydration fluids could be found off central chile we argue that the circular process of pore water uptake during smectite formation and release upon illitization implies a balanced freshwater budget and therefore a rather limited potential for net pore water freshening on a margin wide scale according to this rationale pore water freshening at seafloor seeps preferentially occurs at lower latitudes central america barbados mediterranean ridge where terrestrial weathering is more intense thus leading to external i e detrital smectite and thus freshwater inputs to the subduction system

three dimensional finite element models on the deformation of forearcs caused by aseismic ridge subduction the role of ridge shape friction coefficient of the plate interface and mechanical properties of the forearc abstract geological and geophysical data show that the forearc of subduction zones experiences strong deformation during the subduction of aseismic oceanic ridges in order to better understand ridge related forearc deformation patterns we performed a series of three dimensional finite element models in which we varied the ridge shape the friction coefficient of the plate interface and the mechanical strength of the forearc experiments were carried out for migrating non migrating ridges and accretive erosive margins respectively our results show that the subducting ridge uplifts the forearc and induces horizontal displacements that alter the strain regime of both erosive and accretive forearcs generally shortening prevails in front of the ridge while domains of shortening and extension exist above the ridge models with stationary ridges show high uplift rates only above the ridge tip whereas the forearc above migrating ridges experiences uplift above the leading ridge flank and subsequent subsidence above the trailing flank the height and width of the ridge as well as the friction coefficient of the plate interface have the largest effect on the forearc deformation patterns whereas the mechanical strength of the forearc plays a lesser role forearc indentation at the trench is largest for high and broad ridges high friction coefficients and or weak forearc material shortening and extension of the forearc above the ridge are more intense for high and narrow ridges our model results provide information about the distribution of ridge induced displacements and strain fields and hence help to identify deformation patterns caused by subducting aseismic ridges in nature

elastic thickness structure of the andaman subduction zone implications for convergence of the ninetyeast ridge abstract we use the bouguer coherence morlet isostatic response function technique to compute the spatial variation of effective elastic thickness te of the andaman subduction zone the recovered te map resolves regional scale features that correlate well with known surface structures of the subducting indian plate and the overriding burma plate the major structure on the india plate the ninetyeast ridge ner exhibits a weak mechanical strength which is consistent with the expected signature of an oceanic ridge of hotspot origin however a markedly low strength 0

plate reconstructions aseismic ridges and low angle subduction beneath the andes low angle subduction segments beneath peru and chile are believed to represent interaction of the subduction zone with the aseismic nazca and juan fernandez ridges on the basis of symmetric sea floor spreading models and plate hotspot reconstructions predicted continuations of the relatively buoyant ridges correspond well with the limits of the low angle subduction segments further the history of interaction of the aseismic ridges with the subduction zone as predicted by plate reconstructions is not inconsistent with available information on volcanic episodes of the andes if the contemporary correlation of volcanic gaps and low angle subduction is also applicable to the rest of the late cenozoic the occurrence of deep seismicity beneath the eastern flanks of the andes is also believed to reflect the effects of low angle subduction the deep seismic segments are adjacent to segments which are presently or were recently experiencing low angle subduction thus the deep seismicity is interpreted as representative of remnants of normal oceanic lithosphere subducted before interaction of aseismic ridges with the subduction zone other geologic phenomena are associated with low angle subduction beneath south america including landward shift in loci of tectonism surficial subsidence and laramide style deformation the aseismic ridge buoyancy hypothesis appears to be a valuable predictive tool for the interpretation of such geologic effects

fault gouge analyses k ar illite dating clay mineralogy and tectonic significance a study from the sierras pampeanas argentina we introduce a method for the detailed interpretation of k ar illite fine fraction ages of fault gouges from non sedimentary host rocks ages are cross evaluated with several independent parameters e g illite crystallinity illite polytype quantification grain size mineralogical observations k ar muscovite and biotite host rock cooling ages as well as low temperature thermochronological data aft ahe zhe this interpretation approach is applied to a regional study in order to constrain the deformation path of the eastern sierras pampeanas in nw argentina in the course of this study a large number of gouge bearing fault zones were systematically sampled and analysed obtained k ar illite fine fraction ages range from devonian to cretaceous times documenting a long lasting brittle fault activity in this region ages 320 ma are synchronous with a period of intra carboniferous compressional tectonism whereas permo triassic ages are contemporaneous to a flat slab subduction episode of the farallon plate beneath the south american plate middle to late permian and early triassic ages as well as early jurassic to middle cretaceous ages correlate with extensional tectonics in this region additionally k ar illite ages reveal a propagation of brittle deformation from north to south in the sierras de cordoba and san luis data integrity and consistency with other chronometers and geological evidence show that the here suggested interpretation is valid and can provide a powerful tool to evaluate cooling and deformation histories despite of that we could show that the reliability of fault gouge data strongly depends on the regional cooling

lithospheric 3 d flexure modelling of the oceanic plate seaward of the trench using variable elastic thickness summary whendescribingthemechanicalbehaviourofthelithospheremodelledasathinplate themost important parameter corresponds to itsflexural rigidity which is commonly expressed through the effective elastic thickness te this parameter is a measure of the stiffness of the plate and defines the maximum magnitude and wavelength of those surface loads that can be supported without suffering unelastic deformation realistic 3 d models of the flexural response of the lithosphere near the trench are scarce because of the mathematical and computational complexity we present a method for determining the flexure of the lithosphere caused by the combined effect of 3 d seamount loading and bending of the lithosphere near the trench our method consists on solving numerically the flexure equations of the reissner mindlin thin plate theory including variable thickness using the finite element method with mesh adaptation the method was applied to study the flexure of the oceanic nazca lithosphere beneath the o higgins seamount group which lies 70km seaward of the chile trench the results show that an elastic thickness te of 5km under the seamounts a te of 15km far from the trench and a te of 13km near the trench can explain both the down deflection of the oceanic moho and bending of the oceanic lithosphere observed in seismic and gravity profiles in order to study the impact of high trench curvature on the morphology of the outer rise we apply the same methodology to study and model the flexure of the lithosphere in the arica bend region 14 s 23 s results indicate that the te values are overestimated if the 3 d trench curvature is not included in the modelling

seismic structure of the north central chilean convergent margin subduction erosion of a paleomagmatic arc we study the erosive convergent margin of north central chile at similar to 31 degrees s by using high resolution bathymetric wide angle refraction and multichannel seismic reflection data to derive a detailed tomographic 2 d velocity depth model in the overriding plate our velocity model shows that the lowermost crustal velocities beneath the upper continental slope are 6 0 6 5km s which are interpreted as the continental basement composed by characteristic metamorphic and igneous rocks of the coastal cordillera beneath the lower and middle continental slope however the presence of a zone of reduced velocities 3 5 5 0km s is interpreted as the outermost fore arc composed of volcanic rocks hydrofractured as a result of frontal and basal erosion at the landward edge of the outermost fore arc the bathymetric and seismic data provide evidence for the presence of a prominent trenchward dipping normal scarp similar to 1km offset which overlies a strong lateral velocity contrast from similar to 5 0 to similar to 6 0km s this pronounced velocity contrast propagates deep into the continental crust and it resembles a major normal listric fault we interpret this seismic discontinuity as the volcanic continental basement contact of the submerged coastal cordillera characterized by a gravitational collapse of the outermost fore arc subduction erosion has most likely caused large scale crustal thinning and long term subsidence of the outermost fore arc

tectonometamorphic evolution of the aracena metamorphic belt sw spain resulting from ridge trench interaction during variscan plate convergence 1 the aracena metamorphic belt is a high temperature low pressure ht lp band located at the southern end of the european variscan chain it marks a suture between armorica and avalonia this belt is characterized by 1 the presence of mid ocean ridge basalt derived metabasites that were affected by an inverted ht lp metamorphism related to sw verging thrusting whose thermal peak diachronously migrated eastward 2 the occurrence of a ht lp metamorphism related to an extensional event affecting continental rocks belonging to the ossa morena zone with peak temperatures that are 150 c higher than that recorded in the oceanic metabasites and 3 the presence of near trench magmatism with high mg andesite composition these and other characteristics can be interpreted in terms of a ridge trench trench triple junction that migrated along the armorican margin during variscan oblique convergence

appraisal of waveform repeatability for crosshole and hole to tunnel seismic monitoring of radioactive waste repositories countries worldwide are seeking solutions for the permanent removal of high level radioactive waste from the environment surrounding the waste with multiple engineered barriers and emplacement in deep geological repositories is widely accepted as a safe means of isolating it from the biosphere for the necessary 105 106 years as a precautionary measure society demands that repositories be monitored for 100 300 years after they are backfilled and sealed effective monitoring that does not compromise the engineered and natural barriers is challenging to address this issue we investigate the viability of crosshole and hole to tunnel seismic methods for remotely monitoring high level radioactive waste repositories measurements are made at two underground rock laboratories in switzerland one within granitic rock and one within clay rich sediments numerical simulations demonstrate that temporal changes of the monitored features i e bentonite plug excavation damage zone sand filled microtunnel sh

comparison of high frequency seismic sources at the grimsel test site central alps switzerland in august 1995 various high frequency seismic sources were tested at the grimsel test site gts located inside a crystalline rock body in the central swiss alps these source tests were designed to facilitate future tomographic studies of potential radioactive waste disposal sites the principal objective was to identify borehole and tunnel seismic sources capable of generating powerful high frequency signals such that frequencies up to 1000 hz can be observed over distances of 1000 m in crystalline or consolidated sedimentary rocks seismic sources were situated in water filled boreholes sparker two piezoelectric sources explosives and at or near the tunnel wall accelerated weight drop minivibrator bolt gun buffalo gun explosives to evaluate and compare the source characteristics the direct p wave generated by the various seismic sources was investigated for the decay of its s n and dominant frequency with offset and for the maximum distance at which first arrivals could be picked of the

cased borehole effects on downhole seismic measurements both approximate and exact formulations for the interaction of an incident elastic wave with a cased borehole are presented in the approximate method simple and explicit formulae are derived for the pressure in fluid at low frequencies in the exact method elastic potentials in each annulus are presented as a superposition of fundamental solutions to the helmholtz equations continuity of displacements and stresses across layer boundaries are used to determine unknown coefficients a global matrix algorithm is employed to compute simultaneously these coefficients in individual layers calculations show that in cased boreholes the borehole effects on downhole seismic measurements are more significant than in open boreholes a strong resonance occurs in the fluid for sv wave incidence from a soft formation this resonance is prominent even at very high frequencies because the tube wave velocity is raised well above the formation shear velocity by the steel pipe at a particular angle of incidence of a plane p wave the pressure in the fluid is near zero at low frequencies the cased borehole screening phenomenon for hard formations and frequencies above 1 khz the cased borehole influence on a downhole geophone measurement is significant especially at grazing incidence for soft formations both the pressuremore in the fluid and the solid displacement on the borehole wall show strong dependence on frequency and angle of incidence even at low frequencies less

the nature of the subduction wedge in an erosive margin insights from the analysis of aftershocks of the 2015 mw 8 3 illapel earthquake beneath the chilean coastal range abstract aftershocks of the 16 september 2015 m8 3 illapel earthquake in central chile were recorded for a period of one year by the chile illapel aftershock experiment chillax seismic network an initial catalog of about 100 000 events was generated by a novel automated picking algorithm that combines an auto regressive detection onset estimation method with some recently developed windowing techniques we combine arrival times of p and s waves from about 9 000 of the best recorded of these events with relative arrival times of teleseismic p waves and phase delays of rayleigh waves recovered from ambient noise to generate a three dimensional image of p and s wavespeeds in that part of the andean margin located beneath the network hypocenters of aftershocks located in the final model appear to be grouped into four distinct zones two lower parallel zones that dip about 20 to the east an upper highly active zone that dips about 30 to the east and a diffuse zone of smaller events located between this upper zone and the surface nearly all of the events with m 4 are located at the bottom edge of the highly active zone and define a plane dipping about 26 to the east most of these events have almost identical thrust mechanisms and are separated from the lower zones by a steep gradient in increasing wavespeeds the lower end of the upper zone at about 55 km depth is marked by another velocity gradient dipping to the west that bounds the diffuse shallow activity and projects to the surface west of the high andes we postulate that the triangular region delimited by these two high wavespeed gradients is a subduction wedge generated by the removal of material from the leading edge of the upper plate by subduction erosion and subsequently underplated back on to the overriding plate at depth this model explains many of the features we observe in the seismicity and wavespeeds and is consistent with geologic observations of uplift and extension in this part of the chilean coastal margin characteristics of this wedge such as its shape and the lack of correlation with the coeval back arc tectonic setting suggest that high andes are not directly coupled with the behavior of the subduction interface

seismic investigations of the o higgins seamount group and juan fernandez ridge aseismic ridge emplacement and lithosphere hydration the o higgins seamount group is a cluster of volcanic domes located 120 km west of the central chilean trench on the crest of the juan fernandez ridge this aseismic hot spot track is subducting under south america triggering a belt of intraslab earthquake hypocenters extending about 700 km inland the juan fernandez ridge marks the southern boundary of a shallow subduction segment subduction of oceanic basement relief has been suggested as a cause for the flat slab segments characterizing the andean trench system the juan fernandez ridge however shows only moderate crustal thickening inadequate to cause significant buoyancy in 2001 wide angle seismic data were collected along two perpendicular profiles crossing the o higgins group we present tomographic images of the volcanic edifices and adjacent outer rise trench environment which indicate a magmatic origin of the seamounts dominated by extrusive processes high resolution bathymetric data yield a detailed image of a network of syngenetic structures reactivated in the outer rise setting a pervasive fault pattern restricted to the hot spot modified lithosphere coincides with anomalous low upper mantle velocities gained from a tomographic inversion of seismic mantle phases reduced uppermost mantle velocities are solely found underneath the juan fernandez ridge and may indicate mineral alterations enhanced buoyancy due to crustal and upper mantle hydration may contribute an additional mechanism for shallow subduction which prevails to the north after the southward migration of the juan fernandez ridge

shallow intraplate seismicity related to the illapel 2015 mw 8 4 earthquake implications from the seismic source abstract the september 16 2015 mw 8 4 illapel chile earthquake is the first large event occurring in north central chile after the 1943 earthquake filling a known seismic gap in the region the earthquake took place in a complex tectonic region nearby an area where transition from erosive to accretionary margin occurs due to the collision of juan fernandez ridge jfr along the chilean margin we inverted the kinematic rupture process of the 2015 mw 8 4 illapel earthquake from the joint inversion of teleseismic body waves and near field data the relative weighting between datasets and the weighting of spatial temporal constraints are objectively estimated by applying the akaike s bayesian information criterion the coseismic slip model yields a total seismic moment of 4 92 1021 nm occurred over 120 s the rupture shows both downdip and updip propagation with slip extending along the thrust interface from 50 km depth to shallow near trench depths

tectonic and magmatic responses to the subduction of high bathymetric relief subduction of high bathymetric relief such as aseismic ridges and magmatic plateaus is considered to be responsible for dramatic changes in the dynamics and kinematics of the subduction zone for example the buoyancy of high bathymetric relief is thought to flatten the dip of the subducting slab modifying the structural and magmatic evolution of the overriding plate and terminating arc volcanism in addition the effect of ridge subduction in retreating plate boundaries can inhibit subduction rollback a process that could locally pin the subduction hinge and lead to the development of cusps and slab tearing here we discuss the tectonic response to subduction of high bathymetric relief using examples from the circum pacific subduction systems we demonstrate that flattening of the subduction dip angle is only significant in the eastern pacific where the average slab dip angle is relatively shallow in the western pacific in contrast the average subduction dip angle is steeper and there is no significant flattening of the dip angle in areas of ridge subduction subduction of high bathymetric relief in the circum pacific is commonly associated with reduced arc volcanism and in many cases the area of ridge subduction coincides with a volcanic gap in the overriding plate ridge subduction is associated with pronounced changes in the style of deformation involving uplift reactivation of basement thrusts development of orogen perpendicular tear faults and block rotations leading to oroclinal bending the discussed characteristic patterns associated with ridge subduction provide important guidelines for reconstructing past plate tectonic processes and could help constraining the geodynamics of ancient subduction systems

morphology of san antonio submarine canyon on the central chile forearc a multibeam survey was conducted over san antonio submarine canyon near valparaiso chile in april and may 1993 using the seabeam 2000 system on the r v melville the bathymetric data from this survey reveal a canyon with an overall sinuosity of 1 25 a broad roughly u shaped cross section along most of its length and an almost constant channel slope above the forearc structural high the course of the canyon is deflected to the north by a prominent structural high opposite the town of san antonio seabeam 2000 side scan sonar data reveal high backscatter material on the floor of the canyon with a longitudinal fabric reminiscent of stream braiding and point bars formed on the inside of channel bends we interpret this high backscatter material to be coarse sediment transported down the canyon as turbidity currents the source of these turbidity currents is probably the rio maipo which enters the ocean near the head of the canyon

goce derived vertical gravity gradient delineates great earthquake rupture zones along the chilean margin abstract in the south central andes region the nazca oceanic plate that subducts beneath the south american plate is characterized by a rough topography derived from different oceanic features that collide against the continental margin these features determine an important segmentation of both the margin and of the interplate zone the chilean subduction margin has been characterized by megathrust earthquakes affecting the plate interface with large rupture areas reaching hundreds of kilometers parallel to the trench the occurrence of these phenomena has been linked among other causes to the subduction of sediments that fill the trench and their spatial relation to the relatively prominent oceanic features we calculated the topography corrected vertical gravity gradient from goce satellite data and from egm2008 model in order to delineate mass heterogeneities related to density variations along the south central chile subduction zone obtained results show a spatial relation between the subduction of the nazca oceanic highs and associated along strike segmentation of the vertical gravity gradients over the interplate zone we compared our results with the different rupture areas and found a good correspondence with the ellipses for the main earthquakes such as the valdivia 1960 and maule 2010 ones then we compared vertical gravity gradients with slip distribution obtained from different models finding that they are actually correlated with high slip over negative vertical gradient the goce derived gradient adjusts better with the main slip distribution contour since its signal has a characteristic high wavelength instead the egm2008 model presents a better performance in defining the high frequency anomalies however the last results need to be considered only in those regions where the statistical comparison with goce data shows a good performance this is because egm2008 model data present varying quality of the original terrestrial data while the quality of the goce data is locally homogeneous

supraslab earthquake clusters above the subduction plate boundary offshore sanriku northeastern japan seismogenesis in a graveyard of detached seamounts 1 thousands of offshore repeating earthquakes with low angle thrust focal mechanisms occur along the subduction plate boundary of ne japan double difference relocation methods using p and s wave arrivals reveal clusters of events above these repeating events to assure good depth control we restrict our study to events that are close to seismic stations these supraslab earthquake clusters are regional features at depths of 25 to 50 km and most of these clusters are below the depth of the forearc moho which we determined from converted waves seismicity over this depth range does not occur under the inland area of ne japan except just below the vicinity of the arc volcanoes re entrants in the inner trench slope indicate that repeated collisions of seamounts have occurred in the past our preliminary interpretation of supraslab clusters is that they represent seismicity in seamounts detached from the pacific plate during slab descent driven by the resistance of seamounts to subduction detachment during slab descent probably occurs on the sedimented and hydrothermally altered seafloor on which seamounts were originally constructed since these are known as zones of weakness during active island growth high fluid pressure produced during dehydration of clay minerals and other low temperature hydrous minerals could enable detachment at depths seamount crust is thus accreted to forearcs possibly leading to a long term component of near coastal uplift supraslab earthquake clusters may be our most direct evidence of the fates of seamounts and suggest that tectonic underplating is actively occurring in this subduction system

slab1 0 a three dimensional model of global subduction zone geometries 1 we describe and present a new model of global subduction zone geometries called slab1 0 an extension of previous efforts to constrain the two dimensional non planar geometry of subduction zones around the focus of large earthquakes slab1 0 describes the detailed non planar three dimensional geometry of approximately 85 of subduction zones worldwide while the model focuses on the detailed form of each slab from their trenches through the seismogenic zone where it combines data sets from active source and passive seismology it also continues to the limits of their seismic extent in the upper mid mantle providing a uniform approach to the definition of the entire seismically active slab geometry examples are shown for two well constrained global locations models for many other regions are available and can be freely downloaded in several formats from our new slab1 0 website http on doi gov d9arbs we describe improvements in our two dimensional geometry constraint inversion including the use of average active source seismic data profiles in the shallow trench regions where data are otherwise lacking derived from the interpolation between other active source seismic data along strike in the same subduction zone we include several analyses of the uncertainty and robustness of our three dimensional interpolation methods in addition we use the filtered subduction related earthquake data sets compiled to build slab1 0 in a reassessment of previous analyses of the deep limit of the thrust interface seismogenic zone for all subduction zones included in our global model thus far concluding that the width of these seismogenic zones is on average 30 larger than previous studies have suggested

crustal motion north and south of the arica deflection comparing recent geodetic results from the central andes 1 an intercomparison of recent velocity solutions for the global positioning system gps networks constructed by the south american nazca plate project snapp based in bolivia and peru and our central andes gps project cap based in chile and argentina indicates a velocity discontinuity of order 10 mm yr near the boundary between these networks we suggest that this velocity jump manifests measurement bias in the snapp velocity field our results indicate that no major slip partioning occurs within the forearc of northern chile in response to the obliquity of subduction but that nazca south america plate convergence is partitioned between the forearc and the backarc regions the present rate of shortening across the southern part of the sub andean belt in argentina is 8 9 1 6 mm yr

ridge subduction at an erosive margin the collision zone of the nazca ridge in southern peru the 1 5 km high obliquely subducting nazca ridge and its collision zone with the peruvian margin have been imaged by wide angle and reflection seismic profiles swath bathymetry and gravity surveying these data reveal that the crust of the ridge at its northeastern tip is 17 km thick and exhibits seismic velocities and densities similar to layers 2 and 3 of typical oceanic crust the lowermost layer contributes 10 12 km to the total crustal thickness of the ridge the sedimentary cover is 300 400 m thick on most parts of the ridge but less than 100 m thick on seamounts and small volcanic ridges at the collision zone of ridge and margin the following observations indicate intense tectonic erosion related to the passage of the ridge the thin sediment layer on the ridge is completely subducted the lower continental slope is steep dipping at 9 and the continental wedge has a high taper of 18 tentative correlation of model layers with stratigraphy derived from ocean drilling program leg 112 cores suggests the presence of eocene shelf deposits near the trench continental basement is located 15 km landward of the trench normal faults on the upper slope and shelf indicate extension a comparison with the peruvian and northern chilean forearc systems currently not affected by ridge subduction suggests that the passage of the nazca ridge along the continental margin induces a temporarily limited phase of enhanced tectonic erosion superposed on a long term erosive regime

analysis of the illapel mw 8 3 thrust earthquake rupture zone using goce derived gradients satellite gravimetry has proven to be a useful tool to identify mass anomalies along a subduction interface interpreted as heterogeneities related to the rupture process during megathrust earthquakes in the last years different works reinforced with data derived from satellite gravity missions as grace and now goce have analyzed not only the static component of the earth gravity field but also its temporal variations and relation to the seismic cycle in particular during the last decade the chilean margin has been affected by three megathrust earthquakes withmw 8 maule 2010 mw 8 8 pisagua 2014 mw 8 2 and recently the mw 8 3 illapel event then the recently completed goce mission november 2009 to november 2013 offered a unique opportunity to study the maule february 2010 and pisagua april 2014 events by means of gravity gradients directly measured at satellite height altitudes which allowed mapping density heterogeneities with greater detail than the gravity anomaly which has been used in most studies up to now in the present work we use the last goce model go cons gcf 2 dir r5 the one of higher spatial resolution n 300 2 66 km derived from satellite only data the methodology used is the same as that to study the previous events with the addition that now we derived a relation between the associated depths of a causative mass with a determined degree of the spherical harmonic expansion this allowed to decompose the gravimetric signal by cutting off the degree order of the harmonic expansion as depth increases from this analysis we found that prominent oceanic features such as the challenger fracture zone and the juan fernandez ridge played a key role in latitudinal seismic segmentation for the illapel earthquake rupture zone acting as barriers attenuators to the seismic energy release we compared the slip model from tilmann et al geophysical research letters 43 574 583 doi 10 1002 2015gl066963 2016 for the illapel earthquake with vertical gravity gradient with and without sediment correction and at different degree order of the harmonic expansion from this analysis we inferred that prominent oceanic features over the subducting nazca plate play a key role in seismic segmentation not only at heavily sedimented trenches but also at sediment starved segments

subducted seafloor relief stops rupture in south american great earthquakes implications for rupture behaviour in the 2010 maule chile earthquake great subduction earthquakes cause destructive surface deformation and ground shaking over hundreds of kilometres their rupture length is limited by the characteristic strength of the subduction plate interface and by lateral variations in its mechanical properties it has been proposed that subduction of topographic features such as ridges and seamounts can affect these properties and stop rupture propagation but the required relief and physical mechanisms of topographic rupture limitation are not well understood here we show that the rupture limits of thirteen historic great earthquakes along the south america nazca plate margin are strongly correlated with subducted topography with relief 1000 m including the juan fernandez ridge the northern limit of rupture in the mw 8 8 maule chile earthquake of 27 february 2010 is located where this ridge subducts analysis of intermediate magnitude earthquakes shows that in most places the subduction of high seafloor relief creates weak aseismic zones at the plate interface which prevent rupture propagation but that the juan fernandez ridge is associated with a locally strong plate interface the maximum rupture length and thus magnitude of great subduction earthquakes is therefore determined by the size and lateral spacing of topographic features where they are present on the subducting plate

two stage composite megathrust rupture of the 2015 mw8 4 illapel chile earthquake identified by spectral element inversion of teleseismic waves the mw8 4 illapel earthquake occurred on 16 september was the largest global event in 2015 this earthquake was not unexpected because the hypocenter was located in a seismic gap of the peru chile subduction zone however the source model derived from 3 d spectral element inversion of teleseismic waves reveals a distinct two stage rupture process with completely different slip characteristics as a composite megathrust event the two stages were temporally separated rupture in the first stage with a moment magnitude of mw8 32 built up energetically from the deeper locked zone and propagated in the updip direction toward the trench subsequently the rupture of the second stage with a magnitude of mw8 08 mainly occurred in the shallow subduction zone with atypical repeating slip behavior the unique spatial temporal rupture evolution presented in this source model is key to further in depth studies of earthquake physics and source dynamics in subduction systems

geometry and brittle deformation of the subducting nazca plate central chile and argentina summary we use data from the chile argentina geophysical experiment charge broad band seismic deployment to refine past observations of the geometry and deformation within the subducting slab in the south american subduction zone between 30 s and 36 s this region contains a zone of flat slab subduction where the subducting nazca plate flattens at a depth of 100 km and extends 300 km eastward before continuing its descent into the mantle we use a grid search multiple event earthquake relocation technique to relocate 1098 events within the subducting slab and generate contours of the wadati benioff zone these contours reflect slab geometries from previous studies of intermediate depth seismicity in this region with some small but important deviations our hypocentres indicate that the shallowest portion of the flat slab is a

compressional and transpressional stress pattern for pliocene and quaternary brittle deformation in fore arc and intra arc zones andes of central and southern chile kinematic analysis of fault slip data for stress determination was carried out on late miocene to quaternary rocks from the fore arc and intra arc regions of the chilean andes between 33 and 46 south latitudes studies of neogene and quaternary infilling the central depression as well as plutonic rocks of the north patagonian batholith along the liquine ofqui fault zone have revealed various compressional and or transpressional states of stress in the pliocene the maximum compressional stress 1 was generally oriented east west during the quaternary the deformation was partitioned into two coeval distinctive states of stress in the fore arc zone the state of stress was compressional with 1 oriented in a n s to nne ssw direction in the intra arc zone the state of stress was transpressional with 1 striking ne sw along the coast in one site 37 30 s the quaternary strain deformation is extensional with an e w direction which can be explained by a co seismic crustal bending readjustment

magnetic anomaly interpretation across the southern central andes 32 34 s the role of the juan fernandez ridge in the late tertiary evolution of the margin marine and terrestrial magnetic surveys have been integrated to study the tectonic structure of the convergent margin of chile between 32 34 s three magnetic domains have been identified oceanic continental margin and subaerial the oceanic domain has seafloor spreading anomalies 16 37 ma to 18 39 5 ma disturbed by anomalies of the juan fernandez hot spot chain in the continental margin the most prominent fabric are e w anomalies in the upper slope corresponding to onshore e w anomalies of large intrusive bodies onshore a n s lineament of short wavelength anomalies defines the roots of a cretaceous volcanic arc a resembling lineament offshore indicates a submerged older volcanic arc and that continental basement extends to 50 km landward of the trench absolute cenozoic plate motion for nazca and south american plates and dating of the juan fernandez chain provide a kinematic model of ridge continent collision the reconstruction indicates rapid southward migration of the collision point along 1400 km of the margin from 20 to 11 ma 20 cm yr 1 from 11 ma to present the collison point has migrated at a slower rate along 375 km of the margin 3 5 cm yr 1 the predicted location of the subducted portion of the juan fernandez chain coincides with the south edge of the southward migrating flat slab segment of the subducted lithosphere and with a cluster of deep earthquakes indicating a causal relationship in the last 10 myr the ridge has separated a sediment starved trench to the north where subduction erosion may dominate from a sediment filled trench to the south where recent sediment accretion dominates these observations indicate that subduction of the juan fernandez chain plays a major role in arc forearc tectonics

simulation of late cenozoic south american flat slab subduction using geodynamic models with data assimilation abstract the formation mechanisms of flat slabs in south america remain unclear to quantitatively evaluate the earlier proposed mechanisms we simulate the post 100 ma subduction history below south america using 4 d geodynamic models by progressively incorporating plate kinematics seafloor ages and key tectonic features including the buoyant oceanic crust continental cratons oceanic plateaus i e the inferred inca plateau subducting nazca ridge and juan fernandez ridge as well as deformable trench profiles according to recent geological reconstructions we find that in the absence of an overriding plate and subducting buoyancy features the seafloor age affects slab dip angle by controlling the slab s mechanical strength i e the resistance to bending and negative buoyancy integrated positive density anomaly that enhances bending our models show that slab strength dominates its buoyancy at age 30 ma and the opposite for younger ages the existence of a thick overriding plate reduces the slab dip by increasing dynamic suction and individual cratonic roots further lead to along trench variations of dip angle reduction while dynamic suction from the overriding plate generates a permanent reduction of the long wavelength slab dip angle it is the final addition of subducting oceanic plateau and aseismic ridges that produces the transient and localized flat slabs as observed these results suggest that all mechanisms except the buoyancy features affect the slab dip only at large spatial scales our best fit model with all the above tectonic features included provides a good match to both the upper mantle benioff zones and the temporal evolution of volcanic arcs since the mid miocene the imperfect match of the peruvian flat slab is likely associated with the uncertain 3 d configuration of the amazonian craton

neogene sedimentary and mass wasting processes on the continental margin off south central chile inferred from dredge samples the active continental margin off south central chile 36 to 40 s is transitional between the tectonically erosive empty trench margin north of juan fernandez ridge and the accretionary trench filled margin south of the chile triple junction the small width of the presently active accretionary wedge maximum width of 25 to 50 km argues for past phases of tectonic erosion at present this sector shows indications of contemporaneous accretion subduction and underplating of sediment as well as readjustment of the slope by various mass wasting processes in this context this study aims to examine the neogene sedimentary processes on the continental margin from dredge samples recovered during r v sonne cruise so161 5 within this transitional domain using lithology sandstone petrology shale mineralogy and analysis of sedimentary structures our results yield that the principal transport of material occurs in high energy turbidity currents and debris flows via submarine canyons deeply cutting the continental slope whereas sediment on the shelf is transported by strong coast parallel bottom currents and trapped by submarine canyons cutting into the shelf a wide range of mass wasting processes including slumping debris flows evolving to low density turbidity currents and mud flows rework the slope sediments in contrast thick undisturbed sequences of mostly hemipelagic sediments accumulate in active slope basins which are largely protected from mass movements xrd analyses revealed early diagenetic lithification and overall burial depths of up to 230 mbsf suggesting a shallow subsurface cycle of sedimentation subsidence diagenesis uplift erosion and resedimentation the composition of sandstones is dominated by volcanic rock fragments of andean provenance along strike modal changes reflect a southward increase in glacially denudation and rainfall the combination of which caused more intense erosion of volcanic rocks and exposure weathering and as a result increased fluvial transport of metamorphic and plutonic rocks to the sea

andean tectonics related to geometry of subducted nazca plate seismological and geological data show that tectonic segmentation of the andes coincides with segmentation of the subducted nazca plate which has nearly horizontal segments and 30 east dipping segments andean tectonics above a flat subducting segment between 28 s to 33 s are characterized by from west to east 1 a steady topographic rise from the coast to the crest of the andes 2 no significant quaternary and possibly neogene magmatism 3 a narrow belt of eastward migrating apparently thin skinned neogene to quaternary shortening of the andes and 4 plio pleistocene uplift of the crystalline basement on reverse faults in the pampeanas ranges from about 15 to 24 s over a 30 dipping subducted plate a west to east andes cross section includes 1 a longitudinal valley east of coastal mountains 2 an active neogene and holocene andesitic volcanic axis 3 the altiplano puna high plateau 4 a high neogene but inactive thrust belt eastern cordillera and 5 an active eastward migrating subandean thin skinned thrust belt tectonics above a steeply subducting segment south of 33 s are similar west of the volcanic axis but quite different to the east early cenozoic tectonics of western north america were quite similar to the neogene andes however duration of segmentation was longer and the width of deformation was greater in the western united states patterns of crustal seismicity are systematically related to plio quaternary structural provinces implying that current deformational processes have persisted since at least the pliocene horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench above flat subducting segments crustal seismicity occurs over a broad region whereas over steep segments it is confined to the narrow thrust belt strain patterns in the forearc region are complex and perhaps extensional and a broad region of the altiplano puna and eastern cordillera appears to be aseismic

a high frequency sparker source for the borehole environment for tomographic investigations of shallow subsurface features of limited lateral extent a high frequency low cost borehole seismic source would be highly desirable particularly for investigators with limited budgets we constructed a simple arc discharge seismic source from off the shelf items this source consists of a salt water filled bottle containing exposed conductors of a coaxial cable across which 100 to 300 joules of electrical power were discharged this source produced a seismic pulse with a dominant frequency in the neighborhood of 1 5 khz and a half power bandwidth in excess of 1 khz repeatability of seismic signatures in a variety of environmental settings was excellent sufficient power was generated to observe seismic signals with at least a 35 db signal to noise ratio at horizontal borehole separations of 100 m for a borehole separation of 33 2 m signals with at least a 35 db signal to noise ratio were observed at angular ranges in the vertical plane to 68 degrees the hydrostatic

tectonics and geology of spreading ridge subduction at the chile triple junction a synthesis of results from leg 141 of the ocean drilling program an active oceanic spreading ridge is being subducted beneath the south american continent at the chile triple junction this process has played a major part in the evolution of most of the continental margins that border the pacific ocean basin a combination of high resolution swath bathymetric maps seismic reflection profiles and drillhole and core data from five sites drilled during ocean drilling program odp leg 141 provide important data that define the tectonic structural and stratigraphic effects of this modern example of spreading ridge subduction a change from subduction accretion to subduction erosion occurs along strike of the south american forearc this change is prominently expressed by normal faulting forearc subsidence oversteepening of topographic slopes and intensive sedimentary mass wasting overprinted on older signatures of sediment accretion overthrusting and uplift processes in the forearc data from drill sites north of the triple junction sites 859 861 show that after an important phase of forearc building in the early to late pliocene subduction accretion had ceased in the late pliocene since that time sediment on the downgoing oceanic nazca plate has been subducted site 863 was drilled into the forearc in the immediate vicinity of the triple junction above the subducted spreading ridge axis here thick and intensely folded and faulted trench slope sediments of pleistocene age are currently involved in the frontal deformation of the forearc early faults with thrust and reverse kinematics are overprinted by later normal faults the chile triple junction is also the site of apparent ophiolite emplacement into the south american forearc drilling at site 862 on the taitao ridge revealed an offshore volcanic sequence of plio pleistocene age associated with the taitao fracture zone adjacent to exposures of the pliocene aged taitao ophiolite onshore despite the large scale loss of material from the forearc at the triple junction ophiolite emplacement produces a large topographic promontory in the forearc immediately after ridge subduction and represents the first stage of forearc rebuilding

regional relative tectonic activity of structures in the pampean flat slab segment of argentina from 30 to 32 s abstract the pampean flat slab segment of the central andes in argentina is a broad active deformation zone subject to both plate boundary related and intraplate deformation to the west plate boundary related deformation is dominated by thin skinned fold and thrusts of the precordillera while to the east intraplate deformation involves thick skinned reverse faults of the sierras pampeanas gps data and absolute dating of displaced geomorphic features suggest that most of the active permanent deformation occurs between these two regions at the andean orogenic front and that there is a west to east trend of decreasing shortening rates however gps stations and sites with geomorphic slip rate measurements are currently too sparse and cover too short a time period to make reliable inferences about the longer term distribution and relative rates of deformation mountain range scale geomorphic indices on the other hand reflect tectonic activity since at least the pleistocene in this study we measured geomorphic indices hypsometric integral and curves basin elongation ratio basin volume to area ratio valley floor width to height ratio mountain front sinuosity and normalized steepness indices of rivers from 10 different n s striking range bounding faults that span both the precordillera and sierras pampeanas regions from west to east we use these data to assess the regional relative tectonic activity of major quaternary deformation features in the pampean segment of the central andes even when variations in climate and geology are considered mean values for each geomorphic index suggests uplift rates for the 10 mountain range bounding faults display local trends that are closely associated with the structural style and tectonic evolution of these structures although vertical uplift rates vary little horizontal shortening rates are higher for the precordillera fold and thrusts in the west than the thick skinned reverse faults of the sierras pampeanas in the east due to shallower fault dips for precordilleran structures the similarity with decadal slip rate gradients from gps studies could possibly indicate that the stress field has been constant since at least the pleistocene but further slip rate studies over broader time intervals in the late quaternary are necessary to confirm this variations observed in relative uplift rates along each fault point to sites that warrant further detailed study of slip rates and evaluation of associated seismic hazards

tectonic control of the subducting juan fernandez ridge on the andean margin near valparaiso chile near the latitude of valparaiso chile a fundamental change in configuration of the benioff zone volcanic arc activity and the structure of the continental margin occurs opposite the subducting juan fernandez ridge upper plate tectonics related to subduction of the ridge were studied by an international group of geoscientists in the two degree segment offshore valparaiso extending from the shelf edge seaward across the eastern end of juan fernandez ridge near the o higgins group of seamounts the juan fernandez ridge strikes northeast rather than continuing its east west trend across the pacific basin the ridge uplifts the upper plate and sediments of the valparaiso basin are deformed against its southern flank this deformation is consistent with the southward migration required by an oblique trending ridge and the nearly trench normal vector of plate convergence in the trench axis the ridge forms a basement barrier behind which sediments 2 5 km deep have ponded the lower slope over the ridge appears eroded whereas the margin not yet affected by ridge subduction is fronted by an accretionary prism about 25 km wide nazca plate relief clearly influences tectonism of the margin where it subducts beneath thin continental crust its relation to deeper processes segmenting the andean orogen appears to involve prior tectonic events

crustal seismicity in the andean precordillera of argentina using seismic broadband data abstract in this study we analyze 100 crustal precordilleran earthquakes recorded in 2008 and 2009 by 52 broadband seismic stations from the siembra and esp two temporary experiments deployed in the pampean flat slab region between the andean cordillera and the sierras pampeanas in the argentine andean backarc region in order to determine more accurate hypocenters focal mechanisms and regional stress orientations we relocated 100 earthquakes using the jhd technique and a local velocity model the focal depths of our relocated events vary between 6 and 50 km we estimated local magnitudes between 0 4 ml 5 3 and moment magnitudes between 1 3 mw 5 3 focal mechanisms were determined from new hypocenter relocations and first motion p wave polarities our solutions exhibit a majority of the earthquakes with reverse faulting mechanism regional stress tensor from the inversion of p and t axis orientations shows a maximum stress axis 1 almost horizontal with a strike of 85 and a minimum stress axis 3 almost vertical we correlate this small to moderate magnitude seismicity with the presence of large basement structures beneath the iglesia calingasta basin in the west and the eastern precordillera in the east the nucleation of deep earthquakes beneath the iglesia basin could be related to the presence of a major ramp accommodating the crustal shortening between the frontal cordillera and the precordillera the crustal seismicity beneath the precordillera seems to correlate with west dipping structures rooting deep into the cuyania basement suggesting a thick skinned basement deformation system beneath the precordillera and its shallow thin skinned fold and thrust belt

imaging rupture process of the 2015 mw 8 3 illapel earthquake using the us seismic array we study the rupture process of the mw 8 3 illapel chile earthquake that occurred on 16 september 2015 using the us seismic network as a large aperture array we apply the back projection technique using two frequency bands 0 1 0 5 and 0 25 1 hz both frequency bands reveal that this event is characterized by rupture of three patches the higher frequency band shows an earlier burst of seismic radiation and illuminates a relatively down dip patch of energy radiation on the other hand the lower frequency band shows a more up dip rupture and matches well with the slip inversion model in other studies the illapel earthquake ruptures about 100 km along strike and shows 40 km up dip and 40 km down dip movement along the subduction megathrust fault the earthquake first ruptures around the epicenter with a relatively low level of seismic radiation then it propagates northeast along the juan femandez ridge jfr to rupture a patch down dip accompanied by strong higher frequency seismic radiation finally it ruptures to the northwest of the epicenter and terminates south of the challenger fracture zone cfz releasing a burst of strong lower frequency seismic radiation most of the aftershocks are either within or at the edge of the rupture patch a region characterized by high coupling in central chile the rupture is bounded along strike by two fractures zones to the north and south the jfr to the south of the rupture zone may have acted as a barrier along strike leaving the area south of the mainshock vulnerable for a large damaging earthquake in the near future

from oceanic plateaus to allochthonous terranes numerical modelling abstract large segments of the continental crust are known to have formed through the amalgamation of oceanic plateaus and continental fragments however mechanisms responsible for terrane accretion remain poorly understood we have therefore analysed the interactions of oceanic plateaus with the leading edge of the continental margin using a thermomechanical petrological model of an oceanic continental subduction zone with spontaneously moving plates this model includes partial melting of crustal and mantle lithologies and accounts for complex rheological behaviour including viscous creep and plastic yielding our results indicate that oceanic plateaus may either be lost by subduction or accreted onto continental margins complete subduction of oceanic plateaus is common in models with old 40 ma oceanic lithosphere whereas models with younger lithosphere often result in terrane accretion three distinct modes of terrane accretion were identified depending on the rheological structure of the lower crust and oceanic cooling age frontal plateau accretion basal plateau accretion and underplating plateaus complete plateau subduction is associated with a sharp uplift of the forearc region and the formation of a basin further landward followed by topographic relaxation all crustal material is lost by subduction and crustal growth is solely attributed to partial melting of the mantle frontal plateau accretion leads to crustal thickening and the formation of thrust and fold belts since oceanic plateaus are docked onto the continental margin strong deformation leads to slab break off which eventually terminates subduction shortly after the collisional stage has been reached crustal parts that have been sheared off during detachment melt at depth and modify the composition of the overlying continental crust basal plateau accretion scrapes oceanic plateaus off the downgoing slab enabling the outward migration of the subduction zone new incoming oceanic crust underthrusts the fractured terrane and forms a new subduction zone behind the accreted terrane subsequently hot asthenosphere rises into the newly formed subduction zone and allows for extensive partial melting of crustal rocks located at the slab interface and only minor parts of the former oceanic plateau remain unmodified oceanic plateaus may also underplate the continental crust after being subducted to mantle depth u hp terranes are formed with peak metamorphic temperatures of 400 700 c prior to slab break off and subsequent exhumation rapid and coherent exhumation through the mantle along the former subduction zone at rates comparable to plate tectonic velocities is followed by somewhat slower rates at crustal levels accompanied by crustal flow structural reworking and syndeformational partial melting exhumation of these large crustal volumes leads to a sharp surface uplift

structural variability of the tonga kermadec forearc characterized using robustly constrained geophysical data subducting bathymetric anomalies enhance erosion of the overriding forearc crust the deformation associated with this process is superimposed on pre existing variable crustal and sedimentary structures developed as a subduction system evolves recent attempts to determine the effect and timescale of louisville ridge seamount subduction on the tonga kermadec forearc have been limited by simplistic models of inherited overriding crustal structure that neglect along strike variability synthesis of new robustly tested seismic velocity and density models with existing datasets from the region highlight along strike variations in the structure of the tonga kermadec subducting and overriding plates as the subducting plate undergoes bend faulting and hydration throughout the trench outer rise region observed oceanic upper and mid crustal velocities are reduced by 1 0 km s 1 and upper mantle velocities by 0 5 km s 1 in the vicinity of the louisville ridge seamount chain lrsc the trench shallows by 4 km and normal fault throw is reduced by 1 km suggesting that the subduction of seamounts reduces plate deformation we find that the extinct eocene frontal arc defined by a high velocity 7 0 7 4 km s 1 and density 3 2 g cm 3 lower crustal anomaly increases in thickness by 6 km from 12 to 18 km over 300 km laterally along the tonga kermadec forearc coincident variations in bathymetry and free air gravity anomaly indicate a regional trend of northward increasing crustal thickness that predates lrsc subduction and highlight the present day extent of the eocene arc between 32 s and 18 s within this framework of existing forearc crustal structure the subduction of seamounts of the lrsc promotes erosion of the overriding crust forming steep gravitationally unstable lower trench slopes trench slope stability is most likely re established by the collapse of the mid trench slope and the trenchward side of the extinct eocene arc which within the framework of forearc characterisation implies seamount subduction commenced at 22 s

foundation evaluation of underground metro rail station using geophysical and geotechnical investigations abstract excavations for underground space developments in a thickly populated urban area are sometimes hindered by unexpected formidable consequences like formation of sinkholes or uneven settlement of foundations such troublesome incidences occasionally damage structures in the vicinity of excavations although adequate design guidelines and codes of practices are adopted the disturbance of subsurface regime due to excavation or construction activities often leads to undesired stability issues with resultant appearance of weak zones the reinforcement of weak zones by grouting is one of the common remedies without demarcating the weak or the affected zones becomes counter productive excavation at a location in chennai mega city for metro rail was severely affected by soil piping with development of sinkholes and weakening of the surrounding strata subsequently development of cracks in the adjacent buildings and caused tilting of the nearby taller structures the mandated the geophysical and geotechnical investigations were carried out to demarcate the affected area to avoid further deterioration of the site cross hole seismic p and s wave velocities were estimated between10 pairs of boreholes up to 20 m depth on both the sides of the excavation two electrical resistivity tomography ert profiles each of 141 m in length were carried out in the same location cross hole s wave velocity profiles and resistivity tomograms clearly indicate the presence of weak zones and possible sinkhole locations site specific empirical relationship was established between s wave velocity and standard penetration test spt n value this relationship was further used for determination of spt n values from the measured vs values to compare the subsurface conditions identified under pre and post excavations the results confirmed the presence of weak zones which are responsible for the weakening of the surrounding strata in the foundation geophysical and geotechnical investigations similar to present investigation can be useful for designing metro rail projects in a subsurface environment comprising alluvium sand clay mixed with silt pebbles and boulders lying near a region of ground water pocket in a thickly populated urban area

mantle subducting slab structure in the region of the 2010 m8 8 maule earthquake 30 40 s chile we present a new tomographic model of the mantle in the area of the 2010 m8 8 maule earthquake and surrounding regions increased ray coverage provided by the aftershock data allows us to image the detailed subducting slab structure in the mantle from the region of flat slab subduction north of the maule rupture to the area of overlapping rupture between the 1960 m9 5 and the 2010 m8 8 events to the south we have combined teleseismic primary and depth phase arrivals with available local arrivals to better constrain the teleseismic earthquake locations in the region which we use to conduct nested regional global tomography the new model reveals the detailed structure of the flat slab and its transition to a more moderately dipping slab in the maule region south of the maule region a steeply dipping relic slab is imaged from 200 to 1000 km depth that is distinct from the moderately dipping slab above it and from the more northerly slab at similar depths we interpret the images as revealing both horizontal and vertical tearing of the slab at 38 s to explain the imaged pattern of slab anomalies in the southern portion of the model in contrast the transition from a horizontal to moderately subducting slab in the northern portion of the model is imaged as a continuous slab bend we speculate that the tearing was most likely facilitated by a fracture zone in the downgoing plate or alternatively by a continental scale terrane boundary in the overriding plate

rupture process of the april 24 2017 mw 6 9 valparaiso earthquake from the joint inversion of teleseismic body waves and near field data abstract the central chilean margin 32 33 s is characterized by the subduction of the juan fernandez ridge jfr beneath the continental south american plate the jfr corresponds to a hotspot track composed by seamounts typically 3 3 5 km high above the surrounding seafloor with a ridge trench collision zone underlying the prominent valparaiso forearc basin vfb this region has been affected by several large and mega earthquakes where the last event corresponds to a complex seismic sequence that took place at the southern edge of vfb in april 2017 the spatio temporal distribution of the seismic events is characterized by a predominant southeast migration of the seismicity an mw 6 9 earthquake triggered two days after the sequence started and occurred at the northern end of the rupture area of the 1985 mw 8 0 valparaiso earthquake we compute the kinematic rupture process of the 2017 mw 6 9 valparaiso earthquake from the joint inversion of teleseismic body waves and near field data the akaike s bayesian information criterion was used to objectively estimate both the relative weighting between datasets and the weighting of spatial and temporal constraints used as a priori information the coseismic slip is distributed over an area of dimensions 35 10 km2 with a maximum slip of 1 5 m the rupture propagated unilaterally downdip the source duration from the moment rate solution is 20 s with a total seismic moment of 3 05 1019 nm mw 6 9 the analysis of the seismicity shows that most of the events occurred along the plate interface foreshock clustered northern from the mainshock epicenter and the aftershocks occurred to the southeast at a deeper location the inverted regional moment tensors show similar faulting mechanism than the mainshock the seismic sequence started two days before the mainshock and lasted for about two weeks and a migration pattern of the seismicity was observed the rupture of the 2017 mw 6 9 earthquake nucleated where the san antonio seamount belonging to the jfr is subducting and propagated downwards along a zone that presents high interseismic coupling the complex seismic sequence might be explained by an aseismic slip transient in the zone and the influence of the downdip migration of fluids from the accretionary prism along the subduction channel the erosive and tunneling effect left by the sudden slip of the subducting seamount might provide the cavity for downdip migration of fluids and subsequent swarm seismicity

aftershock seismicity and tectonic setting of the 2015 september 16 mw 8 3 illapel earthquake central chile powerful subduction zone earthquakes rupture thousands of square kilometers along continental margins but at certain locations earthquake rupture terminates to date detailed knowledge of the parameters that govern seismic rupture and aftershocks is still incomplete on 16 september 2015 the mw 8 3 illapel earthquake ruptured a 200 km long stretch of the central chilean subduction zone triggering a tsunami and causing significant damage here we analyze the temporal and spatial pattern of the co seismic rupture and aftershocks in relation to the tectonic setting in the earthquake area aftershocks cluster around the area of maximum coseismic slip in particular in lateral and downdip direction during the first 24 hours after the mainshock aftershocks migrated in both lateral directions with velocities of approximately 2 5 and 5 km h at the southern rupture boundary aftershocks cluster around individual subducted seamounts that are related to the downthrusting juan fernandez ridge in the northern part of the rupture area aftershocks separate into an upper cluster above 25 km depth and a lower cluster below 35 km depth this dual seismic aseismic transition in downdip direction is also observed in the interseismic period suggesting that it may represent a persistent feature for the central chilean subduction zone

structure of the subduction transition region from seismic array data in southern peru data from three seismic arrays installed in southern peru were analysed using receiver functions from p pp and pkp wave phases in order to image the subducted nazca slab the arrays cover the transition region from flat slab subduction in central peru to normal subduction with an angle of about 30 further south a previous study used data from the first array in the normal subduction region to image the moho depth and slab and showed the existence of a mid crustal structure at 40 km depth that is suggested to be a possible underthrusting of the brazilian shield here we discuss new observations from the other two arrays that span the transition between the two subduction regimes and the flat subduction region the results provide an image of the flattened slab from the coast to approximately 300 km inland and also across the transition region from flat to 30 subduction which appears to be a bend rather than a tear in the slab in the flat slab region the slab is well defined near the coast and flattens out at 100 km depth beneath the altiplano the slab appears to start flattening some 400 km in advance of the subduction of the nazca ridge and the flattening is maintained for 1300 km after its passage the moho begins at a depth of around 30 km near the coast and has a maximum depth of 75 km beneath the altiplano consistent with the results of the other arrays both arrays also show a positive impedance mid crustal structure at 40 km depth which if explainable by underthrusting of the brazilian shield would add further support to the observations from the normal subduction region and show the northward and westward extent of the signal the underthrusting hypothesis would explain the missing crust from the shortening budget needed to support the altiplano the vp vs ratios for both arrays exhibit average values between 1 73 and 1 75 indicating a lesser likelihood that there is a high degree of partial melting or magma bodies at depth in this region the receiver function results provide new imaging of the flat slab and transition from normal to flat slab subduction which allows for comparison of different subduction regimes