anomalous seismicity in the lower crust of the santiago basin chile abstract in central chile intraplate crustal seismicity is mostly constrained to the upper 20 km nevertheless sparse observations of earthquakes have been reported deeper without a definitive explanation in the lower crust of the santiago basin located within a compressional subduction environment we have observed a series of events organized in two clusters this work presents a detailed study of these earthquakes recorded from november 2002 to april 2005 with depths from 28 to 36 km and magnitudes ranging from 2 8 to 4 3 using a local network composed of vertical short periods broadbands and accelerometers we performed a time and spectral waveform analysis to determine their exact location and certain source parameters seismic moment corner frequency and seismic energy we found that each cluster presents extremely well correlated waveforms indicating that each event in the cluster ruptures the same patch in the same fashion even more we determined that representative events show a large apparent stress drop releasing six times more energy than interplate events for a given seismic moment the most plausible interpretation seems to be a decollement between the crust and the mantle where few but strong asperities are located in a broader region that continuously creeps we believe that this work will help improve our understanding of the processes involved in subduction zones as well as the assessment of the seismic hazard for the santiago metropolitan area

foreland shortening and crustal balancing in the andes at 30 s latitude excellent surface exposures known benioff zone geometry a dynamic morphology and the availability of industry seismic reflection data all make the andes at 30 s an excellent transect for investigating crustal scale balanced sections 150 170 km of horizontal shortening has occurred in three major belts located between the trench and the foreland the thin skinned east verging precordillera of western argentina accounts for 60 75 of the total shortening and formed mostly since major volcanism ceased at 10 ma industry seismic reflection data show that the decollement of the precordillera belt is located anomalously deep at 15 km the belt is dominated by fault propagation folds and contains several prominent out of sequence thrust faults seismic stratigraphie analysis shows that miocene strata in the iglesia valley located between the precordillera and the crest of the andes accumulated in a piggy back basin onlap relations on the western side indicate that the high cordillera was uplifted as a major fault bend fold over a buried ramp thrusting in the two western belts both in the high cordillera of chile formed during the waning stages of arc volcanism 11 16 ma and account for 25 40 of the shortening the observed shortening is probably greater than can be accounted for with reasonable crustal thicknesses indicating the possibility of continental truncation or erosion along the plate margin or an anomalously thick root held down by the nearly flat subducted nazca plate our preferred crustal geometry puts the ramp between upper and lower crustal deformation west of the high topography requiring crustal scale tectonic wedging to thicken the crust beneath the crest of the andes this non unique model provides a simple explanation of the first order morphology of the andes at this latitude

tectonic control of erosion in the southern central andes abstract landscape evolution modeling and global compilations of exhumation data indicate that a wetter climate mainly through orographic rainfall can govern the spatial distribution of erosion rates and crustal strain across an orogenic wedge however detecting this link is not straightforward since these relationships can be modulated by tectonic forcing and or obscured by heavy tailed frequencies of catchment discharge this study combines new and published along strike average rates of catchment erosion constrained by 10be and river gauge data in the central andes between 28 s and 36 s these data reveal a nearly identical latitudinal pattern in erosion rates on both sides of the range reaching a maximum of 0 27 mm a near 34 s collectively data on topographic and fluvial relief variability of rainfall and discharge and crustal seismicity suggest that the along strike pattern of erosion rates in the southern central andes is largely independent of climate but closely relates to the n s distribution of shallow crustal seismicity and diachronous surface uplift the consistently high erosion rates on either side of the orogen near 34 s imply that climate plays a secondary role in the mass flux through an orogenic wedge where the perturbation to base level is similar on both sides

sub aerial denudation of passive margins regional elevation versus local relief models abstract variations in denudation rates which are an important factor in the modification of the sub aerial topography of passive margins have frequently been modelled as being proportionate to regional elevation to be valid this assumption requires that mean slope gradient and the related property of local relief show a strong positive relationship to regional elevation since slope angle is a primary determinant of denudation rate analysis of the relationships between these variables based on the 1 250 000 scale topographic map coverage of southern africa demonstrates that regional modal elevation explains only 0 5 of the variance in mean sine slope gradient and only 0 14 of the variance in local relief measured over 15 15 areas these results demonstrate that regional elevation cannot be used to predict denudation rates for passive margins such as those of southern africa where a dissected coastal zone is separated from a high interior plateau by a major escarpment on the basis of a previous regression analysis relating local relief to sediment and solute transport by rivers the data for southern africa indicate denudation rates of 80 m ma 1 over much of the coastal zone and 1 across most of the elevated interior extrapolation of these results over geological time however necessitates either assumptions about the local relief of palaeotopography or independent evidence of long term denudation rates

estimating locations and magnitudes of earthquakes in southern california from modified mercalli intensities modified mercalli intensity mmi assignments instrumental moment magnitudes m and epicenter locations of thirteen 5 6 m 7 1 training set events in southern california were used to obtain the attenuation relation mmi 1 64 1 41 m 0 00526 h 2 63 log h where h is the hypocentral distance in kilometers and m is moment magnitude intensity magnitudes m i and locations for five 5 9 m 7 3 independent test events were consistent with the instrumental source parameters fourteen historical earthquakes between 1890 and 1927 were then analyzed of particular interest are the m i 7 2 9 february 1890 and m i 6 6 28 may 1892 earthquakes which were previously assumed to have occurred near the southern san jacinto fault a more likely location is in the eastern california shear zone ecsz these events and the 1992 m 7 3 landers and 1999 m 7 1 hector mine events suggest that the ecsz has been seismically active since at least the end of the nineteenth century the earthquake catalog completeness level in the ecsz is m 6 5 at least until the early twentieth century online material figures and table of empirical mmi site corrections

flexural constraints on the denudation of assymetric mountain belts the regional excavation of metamorphic rocks from mid and lower crustal levels is the consequence of erosion driven denudation of mountain belts that are in permanent isostatic equilibrium if the erosion process occurs at different rates in different parts of a topographically asymmetric mountain belt then the flexural properties of the lithosphere need to be considered in order to describe the features of the isostatic rebound such a situation is given in plateaulike mountain ranges which are likely to erode only at their steeply sloping edges whereas erosion on top stagnates this paper investigates the flexural isostatic response of a plate that is unloaded due to the continuous retreat of an erosional escarpment the isostatic response is tested for two limiting model cases 1 for a plate with a broken edge and 2 for a continuous plate with variable flexural rigidity it is shown that for both cases the rate of topography development on the exposed plate is not linear with the retreatrate of the escarpment this feature is utilized to predict the nature of the subsequent erosion of the newly formed topography and ultimately of the denudation of the plate geological limitations of the theoretical model and its relevance to plateaulike topographies in southern africa and the eastern highlands of australia are then discussed in these areas the origin of the apparently cyclic rebound forces creating stairlike topography at their continental margins and the old age of their present surface topography has previously been unexplained within the presented model these features can be explained satisfactorily two important conclusions may be drawn 1 the distance of flexural swells from great escarpments and the origin of subsequent stair like topographies may be explained by simple flexural considerations 2 the flexural rebound forces uplifting a plate exposed by a retreating escarpment are cyclic excavation times of metamorphic rocks under such mountain belts may therefore be prolonged not only by the nature of erosion of their initial topography but also by the nature of the subsequent isostatic rebound and ultimately their denudation history

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

seismological analyses of the 2010 march 11 pichilemu chile mw 7 0 and mw 6 9 coastal intraplate earthquakes on 2010 march 11 a sequence of large shallow continental crust earthquakes shook central chile two normal faulting events with magnitudes around m w 7 0 and m w 6 9 occurred just 15 min apart located near the town of pichilemu these kinds of large intraplate inland crustal earthquakes are rare above the chilean subduction zone and it is important to better understand their relationship with the 2010 february 27 m w 8 8 maule earthquake which ruptured the adjacent megathrust plate boundary we present a broad seismological analysis of these earthquakes by using both teleseismic and regional data we compute seismic moment tensors for both events via a w phase inversion and test sensitivities to various inversion parameters in order to assess the stability of the solutions the first event at 14 hr 39 min gmt is well constrained displaying a fault plane with strike of n145 e and a preferred dip angle of 55 sw consistent with the trend of aftershock locations and other published results teleseismic finite fault inversions for this event show a large slip zone along the southern part of the fault correlating well with the reported spatial density of aftershocks the second earthquake 14 hr 55 min gmt appears to have ruptured a fault branching southward from the previous ruptured fault within the hanging wall of the first event modelling seismograms at regional to teleseismic distances 10 is quite challenging because the observed seismic wave fields of both events overlap increasing apparent complexity for the second earthquake we perform both point and extended source inversions at regional and teleseismic distances assessing model sensitivities resulting from variations in fault orientation dimension and hypocentre location results show that the focal mechanism for the second event features a steeper dip angle and a strike rotated slightly clockwise with respect to the previous event this kind of geological fault configuration with secondary rupture in the hanging wall of a large normal fault is commonly observed in extensional geological regimes we propose that both earthquakes form part of a typical normal fault diverging splay where the secondary fault connects to the main fault at depth to ascertain more information on the spatial and temporal details of slip for both events we gathered near fault seismological and geodetic data through forward modelling of near fault synthetic seismograms we build a kinematic k 2 earthquake source model with spatially distributed slip on the fault that to first order explains both coseismic static displacement gps vectors and short period seismometer observations at the closest sites as expected the results for the first event agree with the focal mechanism derived from teleseismic modelling with a magnitude m w 6 97 similarly near fault modelling for the second event suggests rupture along a normal fault m w 6 90 characterized by a steeper dip angle dip 74 and a strike clockwise rotated strike 155 with respect to the previous event

the 1960 chile earthquake inversion for slip distribution from surface deformation summary a total of 166 observations of sea level change 130 measurements of elevation difference and 16 determinations of horizontal strain provide an excellent view of the quasi static source process of the great 1960 chilean earthquake these surface deformation data were employed in classical uniform slip fault models as well as more recently developed models that allow spatial variability of slip the best uniform slip planar usp model is 850km long 130km wide and dips 20 seventeen metres of fault displacement contributed to a usp moment of 9 4 times 1022 n m the variable slip planar vsp model concentrates slip on a 900 km long 150 km wide band parallel to the coast several peaks of slip with dimensions of 50 100 km appear in this band and are thought to represent major subduction zone asperities important fractures of the oceanic lithosphere bound the 1960 rupture and are offered as a potential source of fault segmentation within the chilean subduction zone the vsp moment for 1960 earthquake totals 9 5 times 1022 n m about one fifth of the value estimated for the foreshock mainshock sequence from seismic methods except for areas out to sea geodetic resolution on the fault is fairly uniform thus it is unlikely that slip missed by the network could increase the vsp moment much beyond 1 8 times 1023 n m several patches of moment isolated from the main body at 80 110 km depth are found down dip in the vsp model and are presumably indicative of aseismic slip one patch at the northern end of the rupture is probably associated with the initiation phase of the mainshock although the time sequence of the relationship is unknown tide gauge records suggest that another patch between 40 and 43 s responsible for the observed strain and uplifts inland at those latitudes is not of coseismic origin but derives from in place post seismic creep over several years apparently great 1960 type events are not typical members of the 128 yr earthquake cycle in south central chile the nazca south america boundary here is characterized by a variable rupture mode in which major asperities are completely broken by great earthquakes only once in four or five earthquake cycles the more frequent large earthquakes that geographically overlap the great events fill in between the locked zones

seismicity and shape of the subducted nazca plate an updated compilation of earthquake locations and focal mechanism solutions from the international seismological centre and preliminary determination of earthquakes is the basis of a comprehensive study of the geometry of the wadati benioff zone beneath western south america the new data support previous mapping of a sharp flexure rather than a tear in the subducted nazca plate beneath southern peru and provide evidence for a similar flexure in the southward transition from nearly horizontal subduction to a slab with 30 dip at latitude 33 s in contrast the transition from 30 slab dip beneath bolivia to a nearly horizontal dip in the region between 28 s 32 s is more gradual occurring over several hundred kilometers of along strike distance between 20 s and 32 s this southward flattening corresponds to a broadening of a horizontal benchlike part of the subducted plate formed between 100 and 125 km depth the transition in continental tectonic style near 27 s 28 s from a wide volcanically active plateau to a narrow nonvolcanic cordillera appears not to be associated with the main slab flattening which begins to the north of these latitudes but with a more abrupt change in curvature of the subducted slab from convex upward to concave upward immediately below the plate boundary interface the concept of gaussian curvature is applied to slab bending to explain how subduction geometry is affected by the shape of the south american plate we hypothesize that the polarity of vertical curvature in the subducting slab is governed by the orientation of lateral curvature of the plate margin focal mechanism solutions for intermediate and deep earthquakes are grouped by geographic region and inverted for the orientation and relative magnitudes of the principal stresses results of the inversion indicate that downdip extension dominates in the slab above 350 km while downdip compression dominates at greater depths

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

crustal deformation in the south central andes backarc terranes as viewed from regional broad band seismic waveform modelling summary the convergence between the nazca and south america tectonic plates generates a seismically active backarc region near 31 s earthquake locations define the subhorizontal subducted oceanic nazca plate at depths of 90 120 km another seismic region is located within the continental upper plate with events at depths 1 80 th 45 55 km than those of the eastern sierras pampeanas vp 6 0 6 2 km s 1 vp vs 1 70 th 27 35 km in addition we observed an apparent distribution of reverse crustal earthquakes along the suture that connects those terranes finally we estimated average p and t axes over the charge period the entire region showed p and t axis orientations of 275 and 90 plunging 6 and 84 respectively

crustal structure of the south central andes cordillera and backarc region from regional waveform modelling summary we investigate the crustal structure in the andes cordillera and its backarc region using regional broadband waveforms from crustal earthquakes we consider seismic waveforms recorded at regional distances by the chile argentina geophysical experiment charge during 2000 2002 and utilize previous seismic moment tensor inversion results for each single station earthquake pair we fixed the source parameters and performed forward waveform modelling using ray paths that sample the crust of the highest elevation cordillera and the accreted terranes in the backarc region our investigation indicates that synthetic seismograms for our earthquake station geometry are most sensitive to crustal parameters and less sensitive to mantle parameters we performed a grid search around crustal thickness p wave seismic velocity vp and p t os wave seismic velocity ratio vp vs fixing mantle parameters we evaluated this waveform analysis by estimating an average correlation coefficient between observed and synthetic data over the three broadband components we identified all acceptable crustal models that correspond to high correlation coefficients that provide best overall seismogram fits for the data and synthetic waveforms filtered mainly between 10 and 80 s our results indicate along strike variations in the crustal structure for the north south high cordillera with higher p wave velocity and thickness in the northern segment north of 33 s and persistently high vp vs ratio 1 85 in both segments this is consistent with a colder mafic composition for the northern segment and a region of crustal thickening above the flat slab region in contrast the results for the current volcanic arc south of 33 s agree with a warmer crust consistent with partial melt related to quaternary volcanism presumably of an intermediate to mafic composition a distinctive feature in the backarc region is the marked contrast between the seismic properties of the cuyania and pampia terranes that correlates with their heterogeneous crustal composition the cuyania terrane composed of mafic ultramafic rocks exhibits high vp high vp vs and a thicker layered crust versus the thinner more quartzrich crust of the eastern sierras pampeanas associated with low vp and low vp vs these differences may have some effect on the mechanism that unevenly generates crustal seismicity in the upper 30 km in this active compressional region in particular the seismic properties of the cuyania terrane which shows evidence for a high vp and high vp vs crust may be reflecting the complex tectonic evolution history of this terrane including accretion rifting and re accretion processes since the palaeozoic that promote a high level of crustal seismicity in the upper 30 km enhanced by the flat slab subduction in the segment between 31 s and 32 s another possible mechanism could be directly related to the presence of a strong lower crust above the flat slab that efficiently transfer stresses from the slab to the upper crust generating higher seismicity in the cuyania terrane between 30 s and 34 s

source characterization of the san juan argentina crustal earthquakes of 15 january 1944 mw 7 0 and 11 june 1952 mw 6 8 abstract the backarc region of the andes in the vicinity of san juan argentina is one of the most seismically active fold and thrust belt regions in the world four large damaging crustal earthquakes 1894 1944 1952 and 1977 occurred during the last 111 yr between 30 s and 32 s we have determined the source parameters for two of these important earthquakes the 1944 and 1952 events using historic seismic records the earthquake on 15 january 1944 had an epicentral location between the eastern thin skinned precordillera fold and thrust belt and the thick skinned sierras pampeanas basement cored uplifts the 11 june 1952 earthquake occurred in the eastern precordillera about 35 km southwest of the 1944 epicenter location the p wave first motions long period teleseismic p waveform modeling and sv sh amplitude ratio indicate a thrust focal mechanism for the 1944 event strike n45 e dip 35 to the southeast and rake 110 with m0 3 01 1019 n m and mw 7 0 the 1952 earthquake focal mechanism solution indicates a more oblique mechanism strike n40 e dip 75 to the southeast and rake 30 with m0 2 20 1019 n m and mw 6 8 both the 1944 and 1952 earthquakes have focal depths

the denudation history of the onshore continental margin of se brazil inferred from apatite fission track data a suite of 68 new apatite fission track analysis results from the onshore rift margin in southeast brazil are presented some of the results reflect resetting related to the eruption of the parana continental flood basalt 130 ma and four others are directly related to late cretaceous magmatism 80 ma the majority of the results were obtained from precambrian basement rocks and sediments from the parana basin the fission track ages broadly increase inland from 60 90 ma on the coastal plain to 300 ma in the continental hinterland the track length distributions show a characteristic trend being negatively skewed at the coast and showing increasing bimodality inland except for the oldest samples which are more unimodal these trends do not reflect rift related heating and resetting but are the result of protracted denudation since the opening of the south atlantic the results are consistent with more than 3 km of exhumation on the coastal plain but little more than 1 km in the hinterland the topographic morphology in the northern region of the study area is complex and is clearly influenced by structure and bedrock lithology for the region further south model calculations of the isostatic response to the denudation suggest that isostatic rebound makes a minor contribution to the long wavelength topography unless the flexural rigidity is more than 1025 n m in this case the initial elevation of the rift margin would need to be 2 5 km in order to accommodate the amount of denudation inferred from the fission track data however it is possible that the effective flexural rigidity varies across the margin becoming greater in the continental interior alternatively the broad scale wavelength topography may be the result of magmatic underplating related to the parana continental flood basalt event if so estimates of the volume of magma generated would need to be revised upward by as much as a factor of 5

upper plate deformation measured by gps in the coquimbo gap chile abstract since the mw 7 3 punitaqui earthquake in 1997 the area between 30 s and 32 s coquimbo illapel section of the chilean subduction has been the locus of a decennial seismic swarm a dense network of 30 benchmarks have been installed in this area and surveyed six times with high precision gps over the last 3 years surface deformation here is compatible with elastic loading due to partial locking on the subduction interface at depth here we show that in this area only 40 45 of the total convergence rate between nazca and south america plates gives way to accumulation of elastic deformation in the upper plate the remaining 60 55 being dissipated by free or aseismic slip the cumulative slip due to the seismic swarm explaining no more than 1 3rd to 1 4th of it we also find that the accumulation decreases northward to reach almost zero around 30 s la serena tongoy whether this is a steady state or only a transient pattern a steady decrease of coupling is not clear since our measurements span only 3 years and since early measurements 10 years ago were sparse and differ only marginally from ours

post gondwana geomorphic evolution of southwestern africa implications for the controls on landscape development from observations and numerical experiments the relationship between morphology and surficial geology is used to quantify the denudation that has occurred across southwestern africa since the fragmentation of gondwana during the early mesozoic two main points emerge significant denudation of the order of kilometers is widespread except in the kalahari region of the continental interior the denudation is systematically distributed so that the continental exterior catchment draining directly to the cape basin is denuded to a greater depth than the interior catchment inland of the great escarpment the analysis also implies that the majority of the denudation occurred before the beginning of the cenozoic for both the exterior and interior catchments existing models of landscape development are reviewed and implications of the denudation chronology are incorporated into a revised conceptual model this revision implies that the primary effect of rifting on the subsequent landscape evolution is that it generates two distinct drainage regimes a marginal upwarp or rift flank uplift separates rejuvenated rivers that drain into the subsiding rift from rivers in the continental interior that are deflected but not rejuvenated the two catchments evolve independently unless they are integrated by breaching of the marginal upwarp if this occurs the exterior baselevel is communicated to the interior catchment that is denuded accordingly denudation rates generally decrease as the margin evolves and this decrease is reinforced by the exposure of substrate that is resistant to denudation and or a change to a more arid climate the observations do not reveal a particular style of smaller scale landscape evolution such as escarpment retreat that is responsible for the differential denudation across the region it is proposed that numerical model experiments which reflect the observational insights at the large scale may identify the smaller scale controls on escarpment development if the model and natural systems are analogous four numerical experiments are presented in which the roles of antecedent topography resistant substrate climate change and lowering the baselevel of the interior catchment are investigated for an initially high elevation margin bordered by an escarpment the model results suggest several styles of landscape evolution that are compatible with the observations escarpments may retreat in a regular manner but they also degrade and are destroyed only to reform at the drainage divide between exterior and interior catchments

source study and tectonic implications of the historic 1958 las melosas crustal earthquake chile compared to earthquake damage abstract the 4 september 1958 earthquake has been the largest event recorded at shallow depths in the western flank of the andes on chilean territory new estimates of fault orientation depth and size have been carried out using modern techniques of body wave modeling two possible fault planes solutions with right lateral displacement on an east west fault or left lateral displacement on a north south fault nucleated at 5 9 km depth produce the best fit to teleseismic recordings a seismic moment m0 of 0 227 1019 n m associated with a moment magnitude mw of 6 3 has been estimated with these techniques which is a more reliable estimation of earthquake size than the 0 4 0 7 units larger surface wave magnitude ms earlier reported although no surface rupture for the 1958 las melosas crustal earthquake was reported the displacement along east west structures like that one suggested for one of the fault plane in our focal mechanism solution seems to be an efficient mechanism to accommodate differences in shortening from north to south in the high andean cordillera reports on damage landslide effects as well as re analysis of intensities associated with the new seismic source estimations for the 1958 las melosas earthquake are presented to further estimate the hazard to which this zone and others along the western foothills of the andes is exposed

equating the basal unconformity with the palaeoplain a model for passive margins abstract the palaeoplain is the terrestrial continental surface little changed from the landsurface that existed before continental breakup the basal unconformity occurs offshore separating older continental rocks from younger post rift sediments this paper considers the palaeoplain and the basal unconformity to be the same surface it presents a hypothesis for continental passive margins that integrates the geomorphology near the continental boundary with the offshore geomorphology by recognising the identity of the palaeoplain with the basal unconformity the hypothesis links the terrestrial story to the offshore story many details that were separated in earlier models come together in a single model continental rifting starts with a rift valley stage at this stage the pre breakup landsurface the palaeoplain at the edge of the continent is downwarped on the seaward side post rift sediments accumulate on the submerged palaeoplain which thus becomes the basal unconformity this model explains the parallel structures of continental divide present coast line and continental edge it also explains the creation of the basal unconformity in a short time and the comparative volumes of onshore erosion and offshore sedimentation onshore the model explains the modifications of drainage and the great escarpments that are characteristic of passive margins it also explains oldlands observed near the coast

cenozoic potassium rich mafic volcanism in the western u s a its relationship to deep subduction cenozoic potassium rich volcanism characterized by k 2 o na 2 o and sio 2 60 wt follows a northeast to southwest younging trend in the western u s a correlation of this trend with the outer limits of arc related magmatism based on variable dip subduction models suggests that potassium rich volcanism may be related to deep subduction rather than intraplate tectonics

along strike segmentation of the andean foreland causes and consequences abstract thrust belts of tertiary age characterize the east flank of the andes and part of the adjacent foreland from venezuela to the southern tip of south america this foreland deformation comprises three basically different structural styles 1 thin skinned thrust belts detached within the sedimentary cover 2 thick skinned thrust belts with an inferred basal detachment in the basement at 10 20 km depth and 3 foreland basement thrusts which possibly affect the entire crust alternating foreland structural styles along the orogen often with sharp boundaries produce a segmentation of the andean foreland the changes in structural style are interpreted to be primarily controlled by inherited stratigraphic and structural features of the south american plate for instance thin skinned thrust belts appear to require a more or less conformable sedimentary cover at least some 3 km thick over a metamorphic crystalline basement unaffected by mesozoic extension in areas of mesozoic rifting normal faults have been reactivated during tertiary contraction resulting in the formation of thick skinned thrust belts the basement becomes involved even if potential decollement levels are present deep seated foreland basement thrusts tend to form over basement arches with a thin sedimentary cover the three basic structural styles are unequally efficient in terms of shortening thin skinned belts are typically shortened by 40 70 thick skinned belts by 20 35 and areas of foreland thrusts by less than 10 the structural segmentation of the foreland therefore implies strong along strike variations in the amount of shortening available estimates of shortening along the central andes actually suggest that shortening does not vary steadily but has several maxima and minima strongly and weakly shortened foreland segments correlate poorly with the segmentation of the subducted plate in steeper and subhorizontal segments flat slabs but closely with lithospheric structure the most strongly shortened segment coincides with thick mantle lithosphere suggesting that shortening of the edge of the south american plate has resulted in substantial underthrusting of lithospheric mantle beneath part of the andes this underthrusting of thick lithosphere may have caused steepening of a formerly flat slab in the arica bend region to the normal 25 30 dip presently observed

tarapaca intermediate depth earthquake mw 7 7 2005 northern chile a slab pull event with horizontal fault plane constrained from seismologic and geodetic observations 1 a large mw 7 7 intermediate depth earthquake occurred on 13 june 2005 in the tarapaca region of the northern chile seismic gap source parameters are inferred from teleseismic broadbands strong motions gps and insar data relocated hypocenter is found at 98 km depth within the subducting slab the 21 days aftershock distribution constrained by a postseismic temporary array indicates a sub horizontal fault plane lying between the planes of the double seismic zone and an upper bound of the rupture area of 60 km 30 km teleseismic inversion shows a slab pull down dip extension mechanism on a nearly horizontal plane total seismic and geodetic moments are 5 5 10 20 n m with an averaged slip of 6 5 m from geodesy the earthquake rupture is peculiar in that the effective velocity is slow 3 5 km s a1 for a high stress drop 21 30 mpa we propose that rupture was due to the reactivation by hydraulic embrittlement of a inherited major lithospheric fault within the subducting plate the stress drop suggests that the region of the slab between planes of the double seismic zone can sustain high stresses citation peyrat s et al 2006 tarapaca intermediate depth earthquake mw 7 7 2005 northern chile a slab pull event with horizontal fault plane constrained from seismologic and geodetic observations geophys

isopach and borehole data as indicators of rifted margin evolution in southwestern africa abstract isopach and borehole data are used to document post rifting sediment accumulation along the western margin of southern africa the isopach data reveal an increasing rate of sediment accumulation up to the end of the eocene but the borehole data generally show the opposite trend an integrated analysis of both types of data however is consistent with a general model of post rifting drainage development in which the area supplying sediment expanded significantly after breaching of a linear upwarp along the margin allowed a dominant drainage network to develop the borehole data suggest that such a major sediment outlet from the interior of the sub continent the palaeo orange became established in the late cretaceous coniacian in the vicinity of the present orange river mouth after conversion to equivalent rock volumes the isopach data can be combined with the general drainage model to infer the likely spatial and temporal patterns of erosion over southern africa during the accumulation of four offshore sedimentary sequences beginning with the onset of rift related sedimentation and extending up to the present this analysis using three variant forms of the general model to encompass a wide range of drainage development possibilities indicates that the average rate of erosion for catchment areas draining to the western margin has declined from 41 82 m ma 1 during the first sedimentary sequence 152 to 113 ma to about 7 9 m ma 1 during the last sequence 37 to 0 ma maximum depths of post rifting erosion of several kilometres appear to have occurred in a narrow zone bordering the margin primarily during its early development while depths of erosion in some interior areas have probably been an order of magnitude less the average depth of post rifting erosion for the present day catchment as a whole is estimated to be about 1 8 km and the erosion rate over this period has averaged about 12 m ma 1

morphotectonic evolution of rifted continental margins inferences from a coupled tectonic surface processes model and fission track thermochronology we use a numerical model to study the topographic evolution and erosional history of rifted continental margins the model combines a kinematic description of lithospheric extension tectonic model with a surface processes model that includes short range hillslope and long range fluvial transport the tectonic model predicts the evolution of the lithospheric temperature and strength elastic thickness distribution as well as the tectonic uplift through time this is input into the surface processes model which calculates the degradation of topography and associated isostatic rebound modeled denudation histories across the margin are used to predict apatite fission track age and length patterns modeling results indicate that depending on the adopted parameters an uplifted rift flank should degrade by erosion within 50 100 m y without significant retreat of the topographic elevation maximum the development of an escarpment system at rifted continental margins is in itself not an indication of tectonic rift flank uplift but results from the existence of a high elevation interior plateau erosional base level lowering as a consequence of rifting and regional isostatic response to erosion of the margin however apatite fission track thermochronology reveals that the areas seaward of the escarpment at a number of rifted margins have been exhumed from several kilometers depth such amounts of denudation cannot be accommodated with isostatic rebound alone and require additional tectonic uplift of the rift flanks modeling of apatite fission track patterns suggests that fission track thermochronology dates the onset of rapid erosion which coincides with the initiation of strong relief i e initiation of rifting fission track ages which are younger than the age of rifting thus cannot be unambiguously interpreted as excluding prerift uplift the timing of margin uplift can be established only by careful track length analysis and integration with regional stratigraphic data the model is applied to the saudi arabian red sea margin and the southeastern australian highlands where it is constrained by present day topography and apatite fission track data as well as seismic and gravity data for the saudi arabian red sea margin synrift regional plateau uplift with a magnitude of approximately 1 km is inferred possibly as a result of asthenospheric upwelling flexurally induced tectonic uplift of the rift flank with a magnitude of 2 km is superimposed on this regional uplift the relatively high elevation of the southeastern australian highlands their steep front and the relatively high amounts of erosion suggest that apart from mesozoic synrift flexural uplift tertiary rejuvenation of topography has occurred possibly as a result of renewed lithospheric thinning and underplating the low elevation of the australian interior would inhibit the evolution of a major escarpment in the absence of such renewed uplift

thick skinned and thin skinned thrusting in the la ramada fold and thrust belt crustal evolution of the high andes of san juan argentina 32 sl abstract the cenozoic structural evolution of the la ramada fold and thrust belt in the high andes of argentina between 32 00 s and 32 30 s shows in time and space the interaction of thin skinned and thick skinned deformations based on geometric and kinematic constraints three different deformation stages have been recognized the first began with thin skinned nnw trending structures detached in upper jurassic gypsum in the second stage the basement was involved by tectonic inversion of triassic normal faults producing a thick skinned fold and thrust belt this inversion combined with the ductile rheology of the jurassic gypsum resulted in refolding of older structures the uplift of the basement by high angle reverse faults at the ramada espinacito massif produced a sticking point in the foreland propagation of the thrust belt this was responsible for the third stage of deformation characterized by thin skinned nnw out of sequence thrusts developed in the westernmost sector the total andean shortening calculated in the area was 21 24 km near 30 the structure of the la ramada fold and thrust belt was integrated into a crustal scale cross section at 32 s latitude based on gravimetric models and surface geology a crustal model is proposed for the andes of chile and argentina a shortening of 180 km was calculated between the chile trench and the sierra de pie de palo this implies a mean deformation rate of 2 8 mm year 1 during cenozoic times an initial crustal thickness of ca 33 km was computed a two layer model with a middle detachment level is proposed to explain the spatial decoupling between maximum shortening at the surface and maximum crustal thickening the upper layer was balanced based on surface structural information the lower layer is interpreted to have been constructed by an imbricate thrust system controlled by the mercedario rift master shear and the precordillera chilenia suture

denudation rates for the southern drakensberg escarpment se africa derived from in situ produced cosmogenic 36c1 initial results cosmogenic 36 c1 concentrations in basalt samples from the drakensberg escarpment on the se african passive margin give quantitative estimates of denudation and scarp retreat rates over the 10 4 10 6 year timespan addressed by these data the calculated escarpment retreat rate has been 50 95 m ma 1 and the average summit denudation rate 6 m ma 1 the scarp retreat rate is an order of magnitude less than previously suggested the rate of summit lowering is sufficient to prevent the long term intact survival of erosion cycle surfaces formed in the mesozoic that were previously inferred for this region

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

analysis of the january 26th 1985 mendoza argentina earthquake effects and of their possible correlation with the recorded accelerograms and soil conditions abstract an analysis was made of the distribution of effects produced by mendoza argentina earthquake of january 26 1985 mb 6 0 which affected one of the most important urban agglomerates of western argentina housing more than 800 000 inhabitants the epicenter was located at distances of 15 and 50 km from the closest and farthest ends of the urban area respectively damage distribution was not uniform some small areas reached a maximum intensity of viii mm damage indexes were defined from the information provided by damage surveying these indexes relate the degree of damage suffered by adobe constructions with respect to the total number of them also the distribution of damaged constructions as a function of their age was analyzed many strong motion records were obtained during this earthquake five accelerograms were recorded at sites located in the highly populated area several seismoscope records were also obtained some of them at the same sites as the accelerograms peak ground accelerations in gran mendoza varied between 0 10 and 0 41 g the analysis of all these records gave information on spectral acceleration spectral amplification and prevailing periods two types of subsoil conditions prevail in the area studied shallow stiff soils and deep soils the former are soils with less than 20 m of depth to the high shear wave velocity materials while the latter have more than 20 m to that material the spatial distribution of both types of soils was accurately known from the information provided by many drillings and several surface geophysical measurements this knowledge allowed the soil types to be relate with damage distribution and strong motion record parameters the results obtained show no relation between damage intensities and subsoil conditions the best example of this conclusion was observed in the two places where higher intensities were recorded in agreement with the higher values of peak accelerations while the subsoil conditions were completely different however a fairly good relation was obtained between intensity of damage and age of the construction

change in the pattern of crustal seismicity at the southern central andes from a local seismic network abstract shallow seismicity in the southern central andes is associated with interplate earthquakes due to the subduction of the nazca plate beneath the south american plate and neotectonic activity mainly located in the retro arc region however this pattern changes drastically south of 34 s within the transition zone at the southern central andes where crustal seismicity associated with mountain building processes concentrates at the fore arc and intra arc region in order to define more accurately this transition we used data from a high density seismic network over the chilean fore arc and axial andean sector 33 34 5 s we obtained a constraint data set of 77 seismic events located mostly in the principal cordillera western flank in the first 10 km of the upper crust this cluster implies an abrupt change in the pattern of seismicity at the southern central andes with a set of structures in the fore arc and intra arc accommodating shortening this change in the locus of crustal seismicity and particularly its location on the fore arc and intra arc south of 34 s is discussed on the light of different hypotheses among which changes in the precipitation pattern and erosion along the andes were favored focalized erosion associated with direction of prevailing pacific winds south of 34 s could determine subcritical conditions that could be adjusted by out of sequence deformation causing crustal earthquakes in the fore arc region becoming the retro arc zone nearly fossilized from a deformational point of view additionally trench sediments associated with this change in the precipitation pattern could also favor decoupling of the subduction zone inhibiting retro arc seismicity although it does not explain activation of fore arc structures south of 34 s and their absence north of this latitude finally inhomogeneous distribution of seismicity through the fore arc zone south of 34 s is discussed on the light of variable elastic thicknesses

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

seismic coupling along the chilean subduction zone underthrusting at subduction zones can cause large earthquakes at shallow depths but is accommodated by aseismic creep below a certain depth the maximum depth of the seismically coupled zone or seismogenic zone is a transition from unstable to stable sliding we have determined the maximum depth of the coupled zone and its variability along the chilean subduction zone the maximum depth of seismic coupling is defined by the depth of large m 6 underthrusting earthquakes that have occurred at the downdip edge of the coupled plate interface earthquake depth is determined with omnilinear waveform inversion of long period p waves the statistical uncertainty in the depth is estimated using bootstrapping omnilinear inversion formally accounts for the scaling incompatibility between the p waveforms and decreases the uncertainty in the depth estimate we have found the depths of 27 earthquakes in the time period from 1961 to 1987 seismic coupling in chile extends down to 48 53 km there is a resolvable change in the maximum depth of coupling around 28 s the region immediately north of this latitude has a coupled zone that extends no deeper than 36 41 km while south of this latitude coupling extends down to 48 53 km this transition is not simply related to the oceanic lithospheric age and plate convergence rate but coincides with a change in several geophysical and geological phenomena with respect to shallower north and deeper south coupling there is a change from active volcanos to no volcanos the dip of the deep slab changes from steep to shallow the thickness of trench sediments changes from thin to thick and the depth of the oceanic basement changes from deeper to shallower at 28 s due to the multiplicity of changes at 28 s there is no clear unique interpretation of what physical mechanism controls the variability in the maximum depth of the seismically coupled zone in chile

quantifying passive margin denudation and landscape development using a combined fission track thermochronology and cosmogenic isotope analysis approach abstract a new approach to empirically constraining numerical models of denudation and landscape development across passive continental margins is presented involving a combination of apatite fission track thermochronology and cosmogenic isotope analysis this integrated strategy is applied to the namibian sector of the south west african margin where the conventional landscape evolution model of inland retreat of an escarpment initiated at the coast at break up implies a mean rate of retreat of 1 km ma 1 the thermochronological and cosmogenic isotope data are incompatible with a simple escarpment retreat model since they show that denudation rates oceanward of the present escarpment position have been low 1 since the end of the eocene and the estimated rate of escarpment retreat has been only 10 m ma 1 this low rate of retreat is however consistent with numerical landscape evolution models where the escarpment is pinned at an inland drainage divide

rupture distribution of the 1977 western argentina earthquake abstract teleseismic p and sh body waves are used in a finite fault waveform inversion for the rupture history of the 23 november 1977 western argentina earthquake this double event consists of a smaller foreshock mo 5 3 1026 dyn cm followed about 20 s later by a larger main shock mo 1 5 1027 dyn cm our analysis indicates that these two events occurred on different fault segments with the foreshock having a strike dip and average rake of 345 45 e and 50 and the main shock 10 45 e and 80 respectively the foreshock initiated at a depth of 17 km and propagated updip and to the north the main shock initiated at the southern end of the foreshock zone at a depth of 25 to 30 km and propagated updip and unilaterally to the south the north south separation of the centroids of the moment release for the foreshock and main shock is about 60 km the apparent triggering of the main shock by the foreshock is similar to other earthquakes that have involved the failure of multiple fault segments such as the 1992 landers california earthquake such occurrences argue against the use of individual mapped surface fault or fault segment lengths in the determination of the size and frequency of future earthquakes

distinguishing intraplate from megathrust earthquakes using lacustrine turbidites subduction zone seismicity arises from megathrust crustal and intraslab earthquakes and understanding the recurrence patterns of each type is crucial for hazard assessments lake sediments can record earthquakes from all three seismogenic sources here we studied the turbidite record of lo encanado an andean lake located in central chile we show that lo encanado turbidites can be attributed to 1 subaquatic slope failure by earthquake shaking coseismic phase 2 floods or human impact and 3 postseismic catchment response all historical events with shaking intensities vi modified mercalli intensity have triggered coseismic turbidites but only the intraplate earthquakes triggered subaerial slope failures followed by postseismic turbidites we argue that this contrasting result is due to different spectra of seismic waves from these earthquake sources higher frequency accelerations from intraplate earthquakes are hardly attenuated in rocks around the lake whereas lower frequency accelerations from megathrust earthquakes are amplified in soft lake sediments we tested our findings by comparing acceleration response spectra of recent and historical intraslab and megathrust earthquakes along a longitudinal profile results suggest that the location of andean lakes is ideal to distinguish earthquake sources

early tectonic extension between the agulhas bank and the falkland plateau due to the rotation of the lafonia microplate abstract along the southeast african continental margin a deep sedimentary basin and marginal ridge bordering the agulhas fracture zone formed coevally with south american counterparts underlying the falkland plateau extension in both the southern outeniqua and the falkland basin is related to large angle 100 rotation of the lafonia falkland islands block in middle late jurassic times during attempted propagation of the proto indian ocean across the active circum pacific margin of gondwanaland in the early cretaceous the jurassic microplate boundaries were partly reactivated and partly transected by a right lateral shear along the 1200 km long falkland agulhas transform fault preceded by substantial vorticity of the surrounding crustal blocks this boundary segment separating the south american and african plates was probably generated by complex geodynamic interactions between oceanic plate subduction continental to back arc rift propagation and the impingement of deep mantle plumes on the lithosphere

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

modeling postbreakup landscape development and denudational history across the southeast african drakensberg escarpment margin 1 we employ a numerical surface processes model to study the controls on postbreakup landscape development and denudational history of the southeast african margin apatite fission track data presented in the companion paper suggest that the drakensberg escarpment formed by rapid postbreakup river incision seaward of a preexisting drainage divide located close to its present position and subsequently retreated at rates of only 100 m m y 1 numerical modeling results support such a scenario and show that the prebreakup topography of the margin has exerted a fundamental control on subsequent margin evolution the rheology of the lithosphere lithological variations in the eroding upper crust and inland base level falls provided secondary controls a relatively low flexural rigidity of the lithosphere te 10 km is required to explain the observed pattern of denudation as well as the observed geological structure of the southeast african margin lithological variations have contributed to the formation of flat topped ridges buttressing the main escarpment as well as major fluvial knickpoints both these features have previously been interpreted as supporting significant cenozoic uplift of the margin an inland base level fall possibly related to back cutting of the orange river drainage system and occurring 40 50 m y after breakup explains the observed denudation inland of the escarpment as well as the development of inland drainage parallel to the escarpment our model results suggest that in contrast to widely accepted inferences from classical geomorphic studies the southeast african margin has remained tectonically stable since breakup and escarpment retreat has been minimal 25 km

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

dynamic numerical investigation of a stepped planar rockslide in the central andes chile abstract in this paper static and dynamic analyses were performed considering the reconstituted geometry of a prehistoric 0 1 km2 rockslide with an exposed stepped planar failure surface using a distinct element code udec the rockslide occurred on the northern slope of the cerro catedral mount located in the headers of the maipo river drainage basin main range of the andes cordillera at the latitude of the santiago city chile between the late pleistocene and the holocene the remaining failure surface consists of two distinct sliding planes dipping at an angle of 37 and connected by a sub vertical step the original geometry of the rock slope was estimated from a terrestrial laser scan elevation model lidar the deposit that remained in the valley and the hill surface topography the laser scan of the sliding surfaces and schmidt hammer field measurements allowed the estimation of the sliding surface mechanical properties whereas laboratory test results were used to estimate the rock mass material properties results from 2d static numerical analyses indicate that the rock slope requires the presence of at least one rock bridge at the sliding surface connecting step in order to be statically stable furthermore we studied the effect of different acceleration time histories recorded during shallow crustal earthquakes compatible with the regional seismicity in the rockslide response the dynamic numerical analyses show that the input earthquake records induce a pronounce topographic amplification in the rock slope that the rockslide displacement highly correlates with the arias intensity the peak velocity and the peak acceleration calculated in the rock slope center of gravity and that the rockslide may have been triggered by horizontal accelerations larger than 0 6 g in the free field finally the slope s response is evaluated under the coupled action of horizontal and vertical accelerations

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

the 1960 chilean earthquakes an unusual foreshock sequence that began with an earthquake of moment 2 0 1021 n m and rupture length of about 150 km preceded the great 1960 chilean earthquake earthquake relocations of the sequence show a progression of seismic activity toward the initiation of the main shock the rupture length of the great 1960 chilean earthquake is estimated from the distribution of aftershocks and crustal deformation to be 920 100 km the source mechanisms of a foreshock and two aftershocks are determined and used to constrain the source mechanism of the main shock the first event in the sequence ruptured the segment of the peru chile trench between 37 03 s and 38 74 s whereas the great 1960 chilean main shock ruptured the adjoining segment to the south the great 1960 chilean main shock initiated within the rupture area of the first event and terminated near the intersection of the chile ridge with the peru chile trench 46 5 s

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

the october 15 1997 punitaqui earthquake mw 7 1 a destructive event within the subducting nazca plate in central chile abstract the 1943 illapel seismic gap central chile 30 32 s was partially reactivated in 1997 1998 by two distinct seismic clusters on july 1997 a swarm of offshore earthquakes occurred on the northern part of the gap along the coupled zone between nazca and south american plates most of the focal mechanisms computed for these earthquakes show thrust faulting solutions the july 1997 swarm was followed on october 15 1997 by the punitaqui main event mw 7 1 which destroyed the majority of adobe constructions in punitaqui village and its environs the main event focal mechanism indicates normal faulting with the more vertical plane considered as the active fault this event is located inland at 68 km depth and it is assumed to be within the oceanic subducted plate as are most of the more destructive chilean seismic events aftershocks occurred mainly to the north of the punitaqui mainshock location in the central eastern part of the illapel seismic gap but at shallower depths with the two largest showing thrust focal mechanisms the seismicity since 1964 has been relocated with a master event technique and a joint hypocenter determination jhd algorithm using teleseismic and regional data along with aftershock data recorded by a temporary local seismic network and strong motion stations these data show that the 1997 seismic clusters occurred at zones within the illapel gap where low seismicity was observed during the considered time period the analysis of p and t axis directions along the subduction zone using the harvard centroid moment tensor solutions since 1977 shows that the oceanic slab is in a downdip extensional regime in contrast the punitaqui mainshock is related to compression resulting from the flexure of the oceanic plate which becomes subhorizontal at depths of about 100 km analog strong motion data of the punitaqui main event show that the greatest accelerations are on the horizontal components the highest amplitude spectra of the acceleration is in the frequency band 2 5 10 hz in agreement with the energy band responsible for the collapsed adobe constructions the isoseismal map derived from the distribution of observed damage show that a high percentage of destruction is due to the proximity of the mainshock the poor quality of adobe houses and probably local site amplification effects

a possible mechanism for epeirogenic uplift continental geology is dominated by vertical movements some of these are the result of crustal shortening and extension associated with large horizontal displacements involving processes which are now understood at least in outline however both uplift and subsidence can occur without associated shortening or extension of the continental crust most investigations of such epeirogenic movements have been concerned with subsidence partly because of the economic importance of the resulting sedimentary basins and partly because of the existence of a simple model for the process1 which can account for the development of several basins in some detail2 4 however uplift has been relatively neglected and cannot be produced by the same mechanism the purpose of this paper is to examine whether epeirogenic uplift can result from the intrusion of large thicknesses of basic magma into the lower part of continental crust this suggestion was considered as a possible mechanism for the regional uplift of the colorado plateau by gilbert5 more than 100 years ago and also by holmes6 though both rejected the idea however a recent discussion of no less than 14 different mechanisms for generating uplift7 did not include this process even though it seems able to produce the observed vertical motions and also to account for some of the features of the evolution of sedimentary basins that cannot be explained by the uniform stretching model3 8 9 although the suggestion is not new the argument used here to support it is and depends on an understanding of the relationship between ocean island volcanoes and mantle circulation

escarpment evolution on high elevation rifted margins insights derived from a surface processes model that combines diffusion advection and reaction experiments with a surface processes model of large scale 1 1000 km long term 1 100 m y erosional denudation are used to establish the controls on the evolution of a model escarpment that is related to the rifting of a continent the model describes changes in topographic form as a result of simultaneous short and long range mass transport representing hillslope diffusive processes and fluvial transport advection respectively fluvial entrainment is modeled as a first order kinetic reaction which reflects the credibility of the substrate and therefore the fluvial system is not necessarily carrying at capacity one dimensional and planform models demonstrate that the principal controls on the evolution of an initially steep model escarpment are 1 antecedent topography drainage 2 the timescale or equivalently a length scale in the fluvial entrainment reaction 3 the flexural response of the lithosphere to denudation and 4 the relative efficiencies of the short and long range transport processes when rainfall and substrate lithology are uniform a significant amount of discharge draining over the escarpment top causes it to degrade only when the top of the model escarpment coincides with a drainage divide can escarpment retreat occur for these conditions an additional requirement for retreat of a model escarpment without decline is a long reaction time scale for fluvial entrainment this corresponds to a substrate that is hard to detach by fluvial erosion and therefore to fluvial erosion that is not transport limited continuous backtilting of an escarpment due to flexural isostatic uplift in response to denudational unloading helps maintain the scarp top as a divide it is essential if the escarpment gradient is to be preserved during retreat in a uniform lithology low flexural rigidities promote steep and slowly retreating escarpments for given rainfall and substrate conditions the morphology of a retreating model escarpment is determined by the ratio of the short range diffusive and long range advective transport efficiencies a low ratio which is interpreted to correspond to a relatively arid climate and weathering limited conditions promotes steep sharp topped escarpments with straight main slopes and escarpment retreat occurs over a wide range of height scales a high ratio interpreted to correspond to a more humid temperate climate produces a convex upper slope and concave lower slope morphology and only major escarpments are predicted to preserve a high scarp gradient lithological contrasts in the model produce more complex morphologies and predict the formation of scarps crowned by an erosionally resistant caprock however resistant caprocks are not an essential requirement for model scarps to retreat we conclude that the inferred controls and model behavior are both consistent with the present day morphology of rifted continental margins and with modern conceptual models of landscape evolution

multi trace deconvolution with unknown trace scale factors omnilinear inversion of p and s waves for source time functions seismic source and wave propagation theories allow seismologists to compute long period synthetic seismograms that commonly match even subtle details of observed waveshapes but the overall trace amplitudes must be scaled by arbitrary factors to emphasize the excellent waveshape correspondence this unexplained amplitude scatter causes considerable difficulties when formally inverting observed waveforms for seismic source parameters omnilinear inversion simultaneously determines the linear model parameters and trace scaling factors to minimize the mismatch between observed and synthetic seismograms omnilinear inversion is applied to source time function deconvolution from long period p and sh waves a simulation with synthetic data shows that omnilinear inversion finds the proper scaling factors and the source time function and focal depth are better determined than for standard linear inversion omnilinear inversion is then applied to a data set of seven p waves and one sh wave from the underthrusting earthquake of july 5 1968 ms 6 6 in northern honshu japan omnilinear inversion produces a best fit focal depth of 36 km with a single pulse time function of 6 s duration and seismic moment of 4 1018 n m mw 6 4 this focal depth supports the notion that the seismically coupled plate interface extends no deeper than 40 km in northern honshu

tertiary palaeosurfaces of the sw deccan western india implications for passive margin uplift two genetically distinct lateritized palaeosurfaces of different ages are recognized in the southwest deccan traps region of western india using a combination of geochemical topographical and satellite image data the deccan traps were erupted at the cretaceous tertiary boundary c 65 ma and comprise a huge area of originally near horizontal basalt lavas covering much of northwest peninsular india and topographically forming the coast parallel western ghats escarpment and elevated maharashtra plateau to the east remnants of the older palaeosurface currently exist as a series of isolated laterite capped plateaux forming the highest elevations along the western ghats 15 30 18 15 n this surface is of late cretaceous early tertiary age and originally developed upon flows which lay at or near to the top of the lava sequence this lateritization phase was terminated by a period of uplift and extensive erosion in lower to mid tertiary times during which the low lying low relief coastal konkan plain developed through the eastward recession of the ghats scarpline a second phase of lateritisation occurred upon this coastal pediplain during mid to late tertiary times since the earlier uplift had gently deformed the lava pile prior to the development of the pedimented surface the low level konkian laterite lies with marked angular unconformity upon the lava stratigraphy both surfaces have been subject to further large scale distortion resulting from continuing uplift effects development and evolution of these deccan palaeosurfaces is important since together they provide a record of uplift effects in western india moreover they offer a datum against which the uplift erosional history may be further constrained and demonstrate that uplift effects have acted upon the indian margin throughout the tertiary since such longevity of uplift is difficult to reconcile with the commonly cited thermal and dynamic post rift mechanisms known to act upon passive margins the morphological and structural evolution of the rifted deccan margin is better described in terms of denudational isostasy

active basement uplift of sierra pie de palo northwestern argentina rates and inception from10be cosmogenic nuclide concentrations quaternary tectonic and denudation rates are investigated for an actively growing basement anticline the sierra pie de palo range which belongs to the andean foreland of northwestern argentina 28 s 33 s in this study a detailed morphometric analysis of the topography is combined with in situ produced cosmogenic10be concentrations measured in 1 surface boulders abandoned on alluvial terraces affected by fault activity along the north bounding fault and growth of the basement fold along the southeastern border 2 bedrock outcrops corresponding to an exhumed and folded regional erosion surface and 3 fluvial sediments sampled at the outlets of several watersheds along the eastern and northern borders of the range incision and uplift rates have been estimated at approximately 0 5 and 1 mm yr when integrated on holocene and pleistocene time scales in close agreement with both long term structural and basin evolution data and short term gps derived velocity field analyses cosmogenic derived denudation and uplift rates combined with geomorphic characteristics of watersheds and river channels allows estimating the onset of the uplift at 4 6 ma followed by a more recent period of topographic rejuvenation at roughly 1 2 ma probably synchronous with steepening of the eastern and northern flanks of the anticline

geologic constraints on bedrock river incision using the stream power law denudation rate in unextended terranes is limited by the rate of bedrock channel incision often modeled as work rate on the channel bed by water and sediment or stream power the latter can be generalized as ka m s n where k represents the channel bed s resistance to lowenng whose variation with lithology is unknown a is drainage area a surrogate for discharge s is local slope and m and n are exponents whose values are debated we address these uncertainties by simulating the lowering of ancient river profiles using the finite difference method we vary m n and k to match the evolved profile as closely as possible to the corresponding modern river profile over a time period constrained by the age of the mapped paleoprofiles we find at least two end member incision laws ka 0 3 0 5 s for australian rivers with stable base levels and k f a 0 1 0 2 s n for rivers in kauai subject to abrupt base level change the long term lowering rate on the latter expression is a function of the frequency and magnitude of knickpoint erosion characterized by k f incision patterns from japan and california could follow either expression if they follow the first expression with m 0 4 k varies from 10 7 10 6 m 0 2 yr for granite and metamorphic rocks to 10 5 10 4 m 0 2 yr for volcaniclastic rocks and 10 4 10 2 m 0 2 yr for mudstones this potentially large variation in k with lithology could drive strong variability in the rate of long term landscape change including denudation rate and sediment yield

cenozoic tectonics of the high andes of west central argentina 30 36 s latitude abstract the structure of the central andes shows three distinctive segments characterized by different geometries these geometries are superimposed on the present large scale plate tectonic setting characterized by distinct subduction segments the northern la ramada segment is a thick skinned fold and thrust belt formed by tectonic inversion of a late triassic rift the central aconcagua segment consists of a thin skinned fold and thrust belt while the southern malargue segment like the first one is a thick skinned fold and thrust belt developed by tectonic inversion of a late triassic early jurassic rift system during late cenozoic times the amount of shortening gradually decreases from north to south as indicated by the crustal roots of the central andes the different geometries along the principal cordillera controlled the abrupt changes in the shortening among segments the structure of precordillera and sierras pampeanas has also been considered in order to account for the total shortening in the la ramada segment the main shortening occurred in the precordillera in the aconcagua segment in the principal cordillera while in the malargue segment the shortening is widely distributed in a broader principal cordillera because south of the flat slab subduction segment the precordillera and sierras pampeanas are missing

the control of pre existing extensional structures on the evolution of the southern sector of the aconcagua fold and thrust belt southern andes abstract the aconcagua fold and thrust belt located in the andean mountains at 32 30 to 34 s has been described as a classic model of a thin skinned thrust belt however new structural data from its southern sector have shown that it has a complex structural framework reflected in multiple mesozoic extensional phases overprinted by structural inversion as well as thin and thick skinned tectonics two major superimposed extensional structural styles have been identified for the mesozoic characterized by distinctly oriented stress fields a key role in the evolution of this part of the fold and thrust belt was played by a late triassic to early jurassic depocentre and by late jurassic block faulting shortening was accommodated by a combination of inversion of pre existing normal faults development of footwall short cuts and both thin and thick skinned thrusting synrift and postrift sedimentary rocks were uplifted by reactivation of normal faults with further shortening along newly formed thin skinned thrust faults the geometry of thin skinned fault systems is controlled by the architecture of the rift basin competent footwalls forming barriers to the lateral propagation of detachments

the crustal seismicity of the western andean thrust central chile 33 34 s implications for regional tectonics and seismic hazard in the santiago areathe crustal seismicity of the western andean thrust central chile 33 34 s