the 2010 maule chile earthquake downdip rupture limit revealed by space geodesy radar interferometry from the alos satellite captured the coseismic ground deformation associated with the 2010 mw 8 8 maule chile earthquake the alos interferograms reveal a sharp transition in fringe pattern at 150 km from the trench axis that is diagnostic of the downdip rupture limit of the maule earthquake an elastic dislocation model based on ascending and descending alos interferograms and 13 near field 3 component gps measurements reveals that the coseismic slip decreases more or less linearly from a maximum of 17 m along strike average of 6 5 m at 18 km depth to near zero at 43 48 km depth quantitatively indicating the downdip limit of the seismogenic zone the depth at which slip drops to near zero appears to be at the intersection of the subducting plate with the continental moho our model also suggests that the depth where coseismic slip vanishes is nearly uniform along the strike direction for a rupture length of 600 km the average coseismic slip vector and the interseismic velocity vector are not parallel which can be interpreted as a deficit in strike slip moment release

shear stresses on megathrusts implications for mountain building behind subduction zones 1 shear stresses on a subduction megathrust play an important role in determining the forces available for mountain building adjacent to a subduction zone in this study the temperatures and shear stresses on megathrusts in 11 subduction zones around the pacific rim hikurangi tonga izu ogasawara western nankai northeastern japan aleutians western alaska cascadia northern chile southern chile and se asia northern sumatra have been determined the main constraint is that vertical normal stresses beneath the highlands behind the subduction zone are nearly equal to horizontal normal stresses in the plane of a trench or arc normal section for a typical brittle and ductile megathrust rheology frictional shear stress gz for depth z and ductile shear stress a exp b rt at temperature t where a b are rheological parameters treated as constants rheological constants common to all the megathrusts crust mantle b are determined by simultaneously solving for the force balance in the overlying wedge and megathrust thermal structure using a simplex minimization algorithm taking account of the induced mantle corner flow at depth 65 15 km 2 and constant radiogenic heating 0 65 0 3 w m 3 2 throughout the crust the a constants are solved individually for each subduction zone assuming that the maximum depth of interplate slip earthquakes marks the brittle ductile transition the best fit solution shows two groupings of megathrusts with most subduction zones having a low mean shear stress in the range 7 15 mpa crust 0 032 0 006 mantle 0 019 0 004 and unable to support elevations 2 5 km for a typical frictional sliding coefficient 0 5 the low effective coefficients of friction suggest high pore fluid pressures at 95 lithostatic pressure tonga and northern chile require higher shear stresses with crust 0 095 0 024 mantle 0 026 0 007 suggesting slightly lower pore fluid pressures at 81 lithostatic ductile shear in the crust is poorly resolved but in the mantle appears to show a strong power law dependency with b 36 18 kj mol 1 amantle values are sensitive to the precise value of b but are in the range 1 20 kpa the power law exponent n for mantle flow is poorly constrained but is likely to be large n 4 the brittle ductile transition in the crust occurs at temperatures in the range 370 c 512 c usually close to the base of the crust and in the mantle at much lower temperatures 180 c 300 c possibly reflecting a marked change in pore fluid pressure or quasi ductile and subfrictional properties in subduction zones where the subducted slab is older than 50 ma a significant proportion of the integrated shear force on the megathrust is taken up where it cuts the mantle and temperatures are 300 c in much younger subduction zones the stress transmission is confined mainly to the crust the shear stresses particularly in the crust may be kept low by some sort of lubricant such as abundant water rich trench fill which lowers the frictional sliding coefficient or effective viscosity and or raises pore fluid pressure the unusual high stress subduction zone in northern chile lacks significant trench fill and may be poorly lubricated with a mean shear stress 37 mpa required to support elevations 4 km in the high andes however where the crust is thin in sediment starved and poorly lubricated subduction zones such as tonga the mean shear stress will still be low sediment may lubricate megathrusts accommodating underthrusting of continental crust such as in the himalayas or eastern central andes which have a low mean shear stress 15 mpa

nature and tectonic significance of co seismic structures associated with the mw 8 8 maule earthquake central southern chile forearc abstract the mw 8 8 maule earthquake on february 27 2010 affected the central southern chilean forearc of the central andes here we show the results of field investigations of surface deformation associated with this major earthquake observations were carried out within three weeks after the seismic event mostly in the central and northern part of the forearc overlying the rupture zone we provide a detailed field record of co seismic surface deformation and examine its implications on active andean tectonics surface rupture consisted primarily of extensional cracks push up structures fissures with minor lateral displacements and a few but impressive extensional geometries similar to those observed in analogical modeling of rift systems a major group of nw wnw striking fractures representing co seismic extensional deformation is found at all localities these appear to be spatially correlated to long lived basement fault zones the nw striking normal focal mechanism of the mw 6 9 aftershock occurred on march 11 demonstrates that the basement faults were reactivated by the mw 8 8 maule earthquake the co seismic surface ruptures show patterns of distributed deformation similar to those observed in mapped basement involved structures we propose that co seismic reactivation of basement structures play a fundamental role in stress release in the upper plate during large subduction earthquakes the fundamental mechanism that promotes stress relaxation is largely driven by elastic rebound of the upper plate located right above the main rupture zone

an integrated crustal velocity field for the central andes 1 we present an integrated velocity field for the central andes derived from gps observations collected between january 1993 and march 2001 that eliminates the velocity bias between the south america nazca plate project snapp and central andes gps project cap velocity fields published by norabuena et al 1998 and bevis et al 1999 the reference frame is realized by minimizing the motion of eight continuous gps stations and one rover gps station located in the stable core of the south american plate the rms horizontal motion of these stations is just 1 1 mm yr the amplitude of these residual motions is roughly compatible with expected levels of measurement error in our new solution five of the six snapp stations located just outside the orogenic belt are effectively stationary and the velocities for adjacent cap and snapp stations now agree at a level consistent with their formal uncertainties

active normal faulting and crustal extension a world wide review of fault plane solutions and focal depths for large normal faulting earthquakes on the continents shows that the overwhelming majority of such earthquakes nucleate in the depth range 6 15 km on faults dipping between 30 and 60 in the few cases where levelling or seismic data are good enough these normal faults are shown to be approximately planar from the surface to their nucleation depth at the base of the brittle crust there is evidence that in some cases as a result of the enormous transitory increase in strain rate during large earthquakes rupture continues into the normally ductile lower crust on surfaces with substantially gentler dips these low angle surfaces may be analogous to some of the detachments seen in metamorphic core complexes of the western usa but the nature of the motion on them depends on strain rate as well as on rheological contrasts between the detachments and the blocks on either side such contrasts however are unlikely to introduce substantial curvature to an originally planar shear zone if the observed spread in active normal fault dips is caused by rotation of the faults and the blocks they bound during extension then a maximum value of 1 7 can be accommodated by seismic activity on a single generation of normal faults with continued extension either a new generation of steeper faults cutting the rotated first faults is likely to form or the deformation will continue aseismically on faults dipping at less than 30 large earthquakes have not been observed to nucleate on very low angle 20 normal faults within the continental crust anywhere in the world such faults can of course move aseismically but are unlikely to do so on a large scale within the upper crust in areas where steep normal faults are seismically active thus extensional models that require concentrated simple shear on large sub horizontal faults within the brittle upper crust will also require a spatial separation between aseismic very low angle faulting and seismic highangle faulting since seismogenic high angle faults dominate the topography of extending regions upper crustal very low angle faulting if it occurs on a large scale presumably does so in areas that are relatively fiat

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

on the strength of interplate coupling and the rate of back arc convergence in the central andes an analysis of the interseismic velocity field 1 we interpret the interseismic crustal velocity field of the central andes using a simple three plate model in which the andean mountain belt is treated as a rigid microplate located between the nazca and south american soam plates we assume that the euler vectors associated with these plates are strictly coaxial and that the surface velocity field can be decomposed into an elastic loading field driven by locking of the main plate boundary and a smaller contribution associated with back arc convergence we obtain our best fit to the geodetic velocities if the main plate boundary is fully 100 locked between depths of 10 50 km and 8 5 of nazca soam plate convergence is achieved in the back arc by underthrusting of the brazilian shield beneath the subandean zone

the 1985 central chile earthquake a repeat of previous great earthquakes in the region a great earthquake surface wave magnitude 7 8 occurred along the coast of central chile on 3 march 1985 causing heavy damage to coastal towns intense foreshock activity near the epicenter of the main shock occurred for 11 days before the earthquake the aftershocks of the 1985 earthquake define a rupture area of 170 by 110 square kilometers the earthquake was forecast on the basis of the nearly constant repeat time 83 9 years of great earthquakes in this region an analysis of previous earthquakes suggests that the rupture lengths of great shocks in the region vary by a factor of about 3 the nearly constant repeat time and variable rupture lengths cannot be reconciled with time or slip predictable models of earthquake recurrence the great earthquakes in the region seem to involve a variable rupture mode and yet for unknown reasons remain periodic historical data suggest that the region south of the 1985 rupture zone should now be considered a gap of high seismic potential that may rupture in a great earthquake in the next few tens of years

active faulting and heterogeneous deformation across a megathrust segment boundary from gps data south central chile 36 39 s 1 this study focuses on the present day deformation mechanisms of the south central chile margin at the transition zone between two megathrust earthquake segments defined from historical data the valdivia and concepcion sectors new gps data and finite element models with complex geometries constrained by geophysical data are presented to gain insight into forearc kinematics and to address the role of upper plate faults on contemporary deformation gps vectors are heterogeneously distributed in two domains that follow these two earthquake segments we find that models which simulate only interseismic locking on the plate interface fail to reproduce surface deformation in the entire study area in the concepcion domain models that include a crustal scale fault in the upper plate better reproduce the gps observations in the valdivia domain gps data show regional scale vertical axis rotations which could reflect postseismic deformation processes at the edge of the mw 9 5 earthquake that ruptured in 1960 and or activity of another crustal fault related to motion of a forearc sliver our study suggests that upper plate faults in addition to earthquake cycle transients may exert an important control on the surface velocity of subduction zone forearcs

interseismic coupling segmentation and mechanical behavior of the central chile subduction zone global positioning system gps measurements carried out in chile over the last two decades showed that an entire portion of the nazca south america subduction zone 38 s 24 s was locked over this period of time the induced accumulation of elastic deformation in the upper plate was not released until the recent maule earthquake of 27 february 2010 mw 8 8 that ruptured the southern part of this section locking or coupling between the two plates varies both with depth and along strike here we use our own gps data an updated solution of our extended network in central chile combined with other published data sets to quantify the spatial variations of the coupling that prevailed before the maule earthquake using a simple elastic model based on the back slip assumption we show that coupling variations on the subduction plane are sufficient to explain the observed surface deformation with no need of a sliver in central chile we identify four segments characterized by higher coupling and separated by narrow areas of lower coupling this segmentation is in good agreement with historical and recent seismicity in chile in particular the narrow zones of lower coupling seem to have stopped most large seismic ruptures including maule s these zones are often associated with irregular bathymetric or coastal features fracture zones or peninsulas finally coseismic and early post seismic slip distribution of the maule earthquake occurring either in previously highly or weakly coupled zones map a complex distribution of velocity weakening and velocity strengthening patches on the subduction interface

space geodetic estimation of the nazca south america euler vector abstract gps data from four sites in the nazca plate easter island galapagos robinson crusoe and san felix islands and from five sites in the stable core of the south american plate enabled us to estimate the euler vector of the nazca plate with respect to south america the observed velocities of easter island 6 6 cm yr at 102 3 galapagos 5 1 cm yr at 90 0 robinson crusoe 6 6 cm yr at 80 1 and san felix 6 0 cm yr at 82 1 are significantly slower than the global plate model nuvel 1a velocites for those four sites our estimated euler pole is located at 48 8 n 91 7 w with a rate of 0 59 m y the nuvel 1a and earlier space geodetic studies give rotation rates that are 20 faster

aftershock seismicity of the 2010 maule mw 8 8 chile earthquake correlation between co seismic slip models and aftershock distribution 1 the 27 february 2010 maule chile mw 8 8 earthquake is one of the best instrumentally observed subduction zone megathrust events here we present locations magnitudes and cumulative equivalent moment of the first 2 months of aftershocks recorded on a temporary network deployed within 2 weeks of the occurrence of the mainshock using automatically determined onset times and a back projection approach for event association we are able to detect over 30 000 events in the time period analyzed to further increase the location accuracy we systematically searched for potential s wave arrivals and events were located in a regional 2d velocity model additionally we calculated regional moment tensors to gain insight into the deformation history of the aftershock sequence we find that the aftershock seismicity is concentrated between 40 and 140 km distance from the trench over a depth range of 10 to 35 km focal mechanisms indicate a predominance of thrust faulting with occasional normal faulting events increased activity is seen in the outer rise region of the nazca plate predominantly in the northern part of the rupture area further down dip a second band of clustered seismicity showing mainly thrust motion is located at depths of 40 45 km by comparing recent published mainshock source inversions with our aftershock distribution we discriminate slip models based on the assumption that aftershocks occur in areas of rapid transition between high and low slip surrounding high slip regions of the mainshock

teleseismic inversion for rupture process of the 27 february 2010 chile mw 8 8 earthquake the 27 february 2010 chile m w 8 8 earthquake is the fifth largest earthquake to strike during the age of seismological instrumentation the faulting geometry slip distribution seismic moment and moment rate function are estimated from broadband teleseismic p sh and rayleigh wave signals we explore some of the trade offs in the rupture process estimation due to model parameterizations limited teleseismic sampling of seismic phase velocities and uncertainty in fault geometry the average slip over the 81 500 km 2 rupture area is about 5 m with slip concentrations down dip up dip and southwest and up dip and north of the hypocenter relatively little slip occurred up dip offshore of the hypocenter the average rupture velocity is 2 0 2 5 km s

estimating groundwater recharge in hebei plain china under varying land use practices using tritium and bromide tracers summary tritium and bromide were used as applied tracers to determine groundwater recharge in hebei plain north china to evaluate the impacts of different soil types land use irrigation and crop cultivation practice on recharge additional objectives were to evaluate temporal variability of recharge and the effect on results of the particular tracer used thirty nine profiles at representative locations were chosen for investigation average recharge rates and recharge coefficient determined by tritium and bromide tracing for different sites were 0 00 1 05 mm d and 0 0 42 5 respectively the results showed relative recharge rates for the following paired influences items within each pair are listed with the influence producing greater recharge first flood irrigated cropland and non irrigated non cultivation land flood irrigation 0 42 0 58 mm d and sprinkling irrigation 0 17 0 23 mm d no stalk mulch 0 56 0 80 mm d and stalk mulch 0 44 0 60 mm d vegetable e g chinese cabbage and garlic 0 70 mm d and wheat maize 0 38 mm d peanut 0 51 mm d and peach 0 43 mm d the results also showed greater recharge for the first year of tracer travel than for the second because total precipitation and irrigation were greater in the first year than in the second this may reflect temporal variability of recharge the method may not be applicable where the water table is shallow less than 3 m a comparison of the near ideal tritium tracer with the more common but less ideal bromide showed that bromide moved approximately 23 faster than tritiated water perhaps because of anion exclusion

permanent fore arc extension and seismic segmentation insights from the 2010 maule earthquake chile 1 geologists have long known that young normal faults are an important structural element of the andean coastal cordillera but their relationship to the subduction seismic cycle is still unclear some of the largest aftershocks of the 2010 mw 8 8 maule earthquake in central chile were nucleated on upper plate normal faults including the mw 6 9 and 7 0 events of the pichilemu earthquake sequence we use the available coseismic gps displacements moment tensor sums and slip distribution models for the maule earthquake to compute the static strain and stress fields imposed on the upper plate by slip on the subduction interface the extensional strains calculated from coseismic gps and from a moment tensor sum of the pichilemu events have similar orientations and orders of magnitude the normal coulomb stress increment csi on the pichilemu fault has maximum positive stresses as high as 4 9 mpa regionally the maule event produced a semi elliptical radial pattern of static extension and deviatoric tension csi 1 5 mpa along the coastal cordillera enclosing the rupture area this elliptical pattern mimics the trends of the major upper crustal structures the static deformation field produced by a great subduction earthquake is an effective mechanism for generating permanent extension above the seismogenic zone reactivating suitably oriented long lived normal faults we suggest that the semi elliptical outline of the first order structures along the coastal cordillera may define the location of a characteristic long lived megathrust segment this observation implies a persistence at least over the quaternary of great subduction ruptures along the maule segment

motion on upper plate faults during subduction zone earthquakes case of the atacama fault system northern chile 1 motion on the atacama fault system afs in northern chile is driven by andean subduction zone processes we use two approaches observational and theoretical to evaluate how the afs and other forearc faults responded to coseismic stress induced by one well studied megathrust earthquake the 1995 mw 8 1 antofagasta event we use synthetic aperture radar interferometry insar to search for small scale coseismic and postseismic deformation on individual faults the insar data are ambiguous some images show offset consistent with coseismic faulting on the paposo segment of the afs and others lack such signal the fact that we do not observe the fault like displacement in all coseismic interferograms suggests that atmospheric contamination not tectonic deformation is responsible for the signal to explore the capacity of the earthquake to trigger motion on upper plate faults we use seven published slip maps constrained by geodetic and or seismic data to calculate static and dynamic coulomb stress change csc on faults in the antofagasta region the static csc field varies between models and depends on the distribution of coseismic interplate slip on the basis of the csc distribution predicted by our preferred model constrained by all available data we suggest it was unlikely that the antofagasta earthquake directly triggered normal motion on the afs and the insar data are consistent with this null result field reports of normal faulting related to the earthquake may reflect recent but not coseismic motion or highly localized behavior not representative of the regional coseismic stress field

crustal motion in the southern andes 26 36 s do the andes behave like a microplate 1 a new global positioning system gps derived velocity field for the andes mountains 26 36 s allows analysis of instantaneous partitioning between elastic and anelastic deformation at the orogen s opposing sides adding an andes microplate to the traditional description of nazca south america plate convergence provides the kinematic framework for nearly complete explanation of the observed velocity field the results suggest the oceanic nazca boundary is fully locked while the continental backarc boundary creeps continuously at 4 5 mm yr the excellent fit of model to data 1 7 mm yr rms velocity misfit and the relative aseismicity of the upper crust in the interior andean region in comparison with its boundaries supports the notion that the mountains are not currently accruing significant permanent strains additionally the model implies permanent deformation is not accumulating throughout the backarc contractional wedge but rather that the deformation is accommodated only within a narrow deformational zone in the backarc

the 2010 mw 8 8 maule megathrust earthquake of central chile monitored by gps large earthquakes produce crustal deformation that can be quantified by geodetic measurements allowing for the determination of the slip distribution on the fault we used data from global positioning system gps networks in central chile to infer the static deformation and the kinematics of the 2010 moment magnitude m w 8 8 maule megathrust earthquake from elastic modeling we found a total rupture length of 500 kilometers where slip up to 15 meters concentrated on two main asperities situated on both sides of the epicenter we found that rupture reached shallow depths probably extending up to the trench resolvable afterslip occurred in regions of low coseismic slip the low frequency hypocenter is relocated 40 kilometers southwest of initial estimates rupture propagated bilaterally at about 3 1 kilometers per second with possible but not fully resolved velocity variations

three dimensional viscoelastic finite element model for postseismic deformation of the great 1960 chile earthquake 2 5 10 19 pa s for the continental upper mantle the model well explains the gps observations numerical tests suggest that the continental mantle viscosity value is reasonably well constrained the model shows the prolonged postseismic seaward motion of the inland area to be a unique feature of earthquakes with very long rupture along strike and large coseismic fault slip for short rupture and small coseismic slip the motion will stop very quickly after the earthquake explaining why this phenomenon is not more commonly observed with an oceanic mantle viscosity of 10 20 pa s the model also provides an explanation for tide gauge constrained postseismic uplift 200 km from the trench that had previously been explained using a model of prolonged afterslip of a deep segment of the chile subduction fault index terms 1236 geodesy and gravity rheology of the lithosphere and mantle 8160 1242 geodesy and gravity seismic deformations 7205 3230 mathematical geophysics numerical solutions 7209 seismology earthquake dynamics and mechanics 7260 seismology theory and modeling keywords postseismic deformation viscoelastic stress relaxation gps finite element modeling chile subduction zone upper mantle viscosity

circum pacific seismic potential 1989 1999 the seismic potential for 96 segments of simple plate boundaries around the circum pacific region is presented in terms of the conditional probability for the occurrence of either large or great interplate earthquakes during the next 5 10 and 20 years i e 1989 1994 1989 1999 and 1989 2009 this study represents the first probabilistic summary of seismic potential on this scale and involves the comparison of plate boundary segments that exhibit varying recurrence times magnitudes and tectonic regimes presenting these data in a probabilistic framework provides a basis for the uniform comparison of seismic hazard between these differing fault segments as well as accounting for individual variations in recurrence time along a specific fault segment and uncertainties in the determination of the average recurrence time

slip distribution of the february 27 2010 mw 8 8 maule earthquake central chile from static and high rate gps insar and broadband teleseismic data 1 the shallow depth underthrust earthquake of february 27 2010 mw 8 8 ruptured the subduction plate interface in central chile between 34 s and 38 s we retrieve the spatial and temporal distribution of slip during this mega earthquake through a joint inversion of teleseismic records insar and high rate gps hrgps data additionally our model is shown to agree with broadband surface waves rupture initiated at about 32 km depth and propagated bilaterally resulting in two main slip zones located ssw and nne of the hypocenter nucleation did not take place within or at the edge of one of these main asperities but in between during the first 30s slip propagated predominantly southwards later on the rupture evolved more slowly and more symmetrically eventually the northern asperity became predominant with maximum slip reaching about 20 m most of the seismic moment was released within 110s a relatively short time explained by the bilateral propagation the overall average rupture velocity is 2 6 km s but propagation occurred initially faster towards the south 3 2 km s large slip did not reach the trench a result consistent with the moderate size of the tsunami down dip rupture stopped at about 50 km depth in agreement with the lower limit of the locked zone inferred by ruegg et al 2009 from pre seismic gps data

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

large extensional aftershocks in the continental forearc triggered by the 2010 maule earthquake chile summary the mw 8 8 maule earthquake occurred off the coast of central chile on 2010 february 27 and was the sixth largest earthquake to be recorded instrumentally this subduction zone event was followed by thousands of aftershocks both near the plate interface and in the overriding continental crust here we report on a pair of large shallow crustal earthquakes that occurred on 2010 march 11 within 15 min of each other near the town of pichilemu on the coast of the o higgins region of chile field and aerial reconnaissance following the events revealed no distinct surface rupture we infer from geodetic data spanning both events that the ruptures occurred on synthetic sw dipping normal faults the first larger rupture was followed by buried slip on a steeper fault in the hangingwall the fault locations and geometry of the two events are additionally constrained by locations of aftershock seismicity based on the international maule aftershock data set the maximum slip on the main fault is about 3 m and consistent with field results the onshore slip is close to zero near the surface satellite radar data also reveal that significant aseismic afterslip occurred following the two earthquakes coulomb stress modelling indicates that the faults were positively stressed by up to 40 bars as a result of slip on the subduction interface in the preceding megathrust event in other words the pichilemu earthquakes should be considered aftershocks of the maule earthquake the occurrence of these extensional events suggests that regional interseismic compressive stresses are small several recent large shallow crustal earthquakes in the overriding plate following the 2011 mw 9 0 tohoku oki earthquake in japan may be an analogue for the triggering process at pichilemu

the recent isla san luis volcanic centre petrology of a rift related volcanic suite in the northern gulf of california mexico abstract isla san luis one of the most recent eruptive centres in the gulf of california presents a complete magma evolution trend from basaltic andesites to rhyolites the less evolved lavas are palagonite tuffs related to surtseyan type activity which characterized the emergent stage of the island subaerial lava flows and later high energy hydromagmatic eruptions are dacites which make up the tuff rings of the southeastern corner of the island rhyolites are the latest erupted products and the development of the younger dome in the centre of the island was preceded by ash and pumice fallout containing quenched bombs with an obsidian crust and a pumiceous core basaltic andesites contain olivine fo87 80 calcium rich plagioclase and sparse clinopyroxene microphenocrysts the typical mineralogy of dacitic and rhyolitic lavas is plagioclase two pyroxenes major and trace elements vary regularly with mgo taken as a differentiation index isla san luis lavas are enriched in lile and depleted in nb ta and ti their characteristics are intermediate between tholeiitic and calc alkaline lavas they are also lree enriched and the la yb n ratios steepen slightly with differentiation such an evolution is better explained by combined fractional crystallization and assimilation processes

on the relationships between slab dip back arc stress upper plate absolute motion and crustal nature in subduction zones 1 statistical analysis of modern oceanic subduction zone parameters such as the age of a downgoing plate or the absolute plate motions is performed in order to investigate which parameter controls the dip of a slab and conversely what the influence of slab geometry is on upper plate behavior for that purpose parameters have been determined from global databases along 159 transects from all subduction zones that are not perturbed by nearby collision or ridge plateau seamount subduction on the basis of tomographic images slabs that penetrate through or lie on the 670 km discontinuity are also identified the results of the statistical analysis are as follows 1 back arc stress correlates with slab dip i e back arc spreading is observed for deep dips deeper than 125 km larger than 50 whereas back arc shortening occurs only for deep dips less than 30 2 slab dip correlates with absolute motion of the overriding plate the correlation is even better when the slab lies on or even more penetrates through the 670 km discontinuity 3 slabs dip more steeply by about 20 on average beneath oceanic overriding plates than beneath continental ones 4 slabs dip more steeply on average by about 10 near edges 5 slab dip does not correlate with the magnitude of slab pull the age of subducting lithosphere at the trench the thermal regime of the subducting lithosphere the convergence rate or the subduction polarity east versus west the present study provides evidence that the upper plate absolute motion plays an important role on slab dip as well as on upper plate strain retreating overriding plates are often oceanic ones and thus may partially explain the steeper slab dips beneath oceanic upper plates one can infer that low slab dips correlate well with compression in continental advancing upper plates whereas steep dips are often associated with extension in oceanic retreating upper plates excess weight of old slabs is often counterbalanced by other forces probably asthenospheric in origin such as lateral mantle flow near slab edges or anchor forces to determine slab dip

central chile finally breaks chile is the site of some of the largest earthquakes in the world on average a magnitude 8 earthquake occurs there every 10 years or so these earthquakes take place in the subduction zone either as interplate ruptures at the interface between the south american and nazca plates or as intraplate events within the subducted nazca plate a few times in every century massive plate interface earthquakes break several hundred kilometers in a single shock this is what happened on 27 february 2010 when a major earthquake magnitude 8 8 occurred in the maule and biobio regions in central chile see the first figure this region had last experienced a major subduction earthquake in 1835 when darwin 1 visited the area as part of his voyage on the beagle his description of the earthquake inspired many seismologists and historians of chilean earthquakes 2 3

widespread seismicity excitation throughout central japan following the 2011 m 9 0 tohoku earthquake and its interpretation by coulomb stress transfer 1 we report on a broad and unprecedented increase in seismicity rate following the m 9 0 tohoku mainshock for m 2 earthquakes over inland japan parts of the japan sea and izu islands at distances of up to 425 km from the locus of high 15 m seismic slip on the megathrust such an increase was not seen for the 2004 m 9 1 sumatra or 2010 m 8 8 chile earthquakes but they lacked the seismic networks necessary to detect such small events here we explore the possibility that the rate changes are the product of static coulomb stress transfer to small faults we use the nodal planes of m 3 5 earthquakes as proxies for such small active faults and find that of fifteen regions averaging 80 by 80 km in size 11 show a positive association between calculated stress changes and the observed seismicity rate change 3 show a negative correlation and for one the changes are too small to assess this work demonstrates that seismicity can turn on in the nominal stress shadow of a mainshock as long as small geometrically diverse active faults exist there which is likely quite common

young displacements on the atacama fault system northern chile from field observations and cosmogenic 21ne concentrations 1 we present the first numerical age constraint for young deformation of the atacama fault system afs in northern chile the young activity of the afs is expressed by several fault scarps which affects alluvial fan sediments of the eastern side of the coastal cordillera 23 30 23 42 s detailed mapping of alluvial fans reveals a complex relationship between fault motion erosion and alluvial fan development an older group of alluvial fans became inactive prior to the scarp formation younger alluvial fans arising directly from feeder channels and entrenched in the fault scarps posts date the scarp formation the youngest slip on the afs is recorded by headward eroding channels entrenched across the scarp which are in turn displaced vertically 0 3 0 5 m by the fault quartz fragments in four sites on the older inactive fan group were analyzed for cosmogenic 21ne concentrations yielding an average age of 424 151 ka the upper limit for the recent activity of the fault combined with the height of fault scarp we calculate a 0 01 mm yr minimum vertical fault slip rate thus young displacement on the afs is quaternary in age and confined to the late pleistocene