crustal shortening at the sierra pie de palo sierras pampeanas argentina near surface basement folding and thrusting the sierras pampeanas consist of basement uplifts that are mainly controlled by w verging crustal thrusts one of these the sierra pie de palo is controlled by a blind thrust whose kinematics is debated we analysed the reverse faults located along the eastern side of the range and show that these are characterized by small displacements few hundreds of metres of the neogene and quaternary deposits controlled by the inherited basement metamorphic foliation and shear zones at the sierra scale the inherited foliation pattern appears already folded before the cenozoic shortening forming a series of antiforms and synforms with various wavelengths we propose that this structure is amplified during the plio quaternary shortening resulting in a large asymmetrical basement cored anticline where foliation parallel slip can act as folding mechanism the overall kinematics is consistent with an e verging fault propagation fold this proposed kinematics is an alternative to the frequently suggested w verging fault bend fold model finally we propose a new alternative and speculative crustal and lithospheric structure for this region consisting of two e verging basement thrusts coeval to w verging pampean thrusts both rooting below the precordillera above an eclogitized lower crust

coseismic displacements and holocene slip rates for two active thrust faults at the mountain front of the andean precordillera 33 s 1 during the last few hundred years several destructive earthquakes occurred along the eastern margin of the andean precordillera where gps data reveal a shortening rate of 4 5 mm a we use fault scarp profiles and age determinations of deformed terraces t1 t4 to infer coseismic displacements and quantify slip rates for the penas and cal thrust faults near mendoza city scarps on the lowest terrace level t1 reveal vertical offsets of 0 8 1 0 m for both faults which are interpreted as coseismic displacements during the last earthquake together with the fault dip these offsets indicate that both faults are capable of producing magnitude mw 6 9 earthquakes which is corroborated by a magnitude ms 7 0 event on the cal fault that destroyed mendoza in 1861 at the penas thrust fault terrace t2 has an age of 3 3 ka and is offset by 1 9 m whereas the 12 ka old terrace t3 is displaced by 11 m combined with the fault dip of 25 the age and offset of terrace t3 define a shortening rate of 2 0 mm a on the penas fault i e about half of the present day shortening at the eastern margin of the precordillera at the cal fault terraces t2 to t4 have ages of 0 8 ka osl 3 9 ka 14c and 12 ka 10be and are vertically offset by 2 6 3 6 and 7 0 m respectively which implies that slip on the fault has recently accelerated hence the cal fault poses a serious seismic hazard to the one million inhabitants of mendoza

seismotectonic features from accurate hypocentre locations in southern central andes western argentina abstract a local seismic network over a five year period recorded about 450 earthquakes in western argentina in this region the geodynamics is controlled by the subduction of the nazca plate beneath the south american lithosphere which is characterized here by a sub horizontal path before reassuming its downward descent as accurate earthquake locations are of primary importance when studying the seismicity of a given area events recorded by the local seismic network enable in depth investigations into seismo tectonic patterns allowing to improve the earthquake source characterization and knowledge on the ongoing seismo tectonics of the region to this end we performed a simultaneous 1 d inversion of both the velocity structure and the hypocentre location the minimum 1 d model obtained is complemented by station corrections which lead to a first insight into the deeper 3 d structure in addition stability tests were performed to verify the robustness of our earthquake location results they reveal a fairly stable hypocentre determination demonstrating that the locations obtained by the inversion process are not systematically biased the results show that sierra pie de palo is characterized by a crustal seismogenic structure dipping west and extending from its eastern boundary to about 30 km of depth the study also provided new constraints on the geometry of the subducted slab we noted a great concentration of shallower seismicity compared to that of the surrounding areas of the wadati benioff zone at the expected position of the juan fernandez ridge jfr our hypocentres indicate that jfr certainly influences the subduction style along its strike leading to the formation of a bend in the slab geometry

seismotectonics of sierra pie de palo a basement block uplift in the andean foreland of argentina we operated a digitally recording seismic network from september 1987 to may 1988 in the san juan province of northwestern argentina the network was located in the thin skinned precordillera and thick skinned sierras pampeanas tectonic provinces of the andean foreland our data provide a detailed view of the crustal seismicity of sierra pie de palo one of the most seismically active mountain blocks of the sierra pampeanas and the site of the destructive m 7 3 caucete earthquake in 1977 most of the well located shallow earthquakes we recorded occurred directly beneath the range between 9 and 30 km depth with a maximum activity around 25 km depth the morphology of the range seismicity pattern and focal mechanisms strongly suggest sierra pie de palo is cut into two blocks by a northeast southwest striking fracture in the northern block seismicity occurs along a well defined west dipping seismic zone while the southern block is characterized by east dipping seismic layers both dipping zones connect to a subhorizontal mid crustal detachment that we interpret as the lower boundary of the brittle failure domain the complex fault geometry and large variety of focal mechanism orientations we observe suggest that the ongoing seismic activity is a continuation of the aftershock sequence of the 1977 caucete earthquake we therefore interpret the two principal dipping zones to be buried thrusts activated during the complex rupture of the 1977 earthquake inversion of the focal mechanism data for the orientation of the regional deviatoric stress tensor shows the maximum compressive stress u oriented at an azimuth of n107 e this direction is approximately orthogonal to the trend of the sierra but rotated 27 clockwise with respect to the current nazca south america convergence direction this result suggests that local boundary effects within the sierras pampeanas province partially control the shortening direction in the crust it may also indicate that a component of sinistral strike slip motion exits within the precordillera to the west of the sierras pampeanas

surface deformation associated with the november 23 1977 caucete argentina earthquake sequence leveling data are combined with seismic observations from the november 23 1977 ms 7 4 caucete earthquake in western argentina in order to constrain models of crustal deformation in the sierras pampeanas tectonic province the spatial pattern and amount of uplift 1 m that accompanied this earthquake suggest 4 m of slip on a north south trending west dipping fault with a dip of 35 a downdip width of 24 km and a length of 80 km the top of the fault is 17 km below the surface these fault parameters are generally consistent with the seismic focal mechanism and moment of the main shock which occurred near the top of the fault the fault length is derived from the distribution of local and relocated teleseismic aftershocks and from analysis of the main shock a double event in which the two shocks occurred 21s apart and were separated by 65 km along the proposed fault plane strike the fault width is determined from the locations of the larger hypocenters along the preferred fault plane this fault is buried beneath the north south striking sierra pie de palo a laramide style crystalline basement uplift in the western part of the sierras pampeanas province the buried fault model is consistent with the lack of observed coseismic surface faulting and the broad east west symmetric character of the overlying sierra pie de palo by combining the 1977 earthquake observations with topographic relief and with estimates of the age of deformation for the sierras pampeanas basement uplifts we calculate a shortening rate of the order of 10 mm yr for the past 5 m y for this andean back arc province and estimate an average recurrence time of the order of 1000 years for earthquakes of this magnitude beneath the sierra pie de palo

modern crustal seismicity in the northern andean precordillera argentina abstract the precordillera in the andean backarc region of argentina has a long record of large and damaging earthquakes in the last century its northern part is not well known seismically neither its style of deformation in this work we study modern crustal seismicity occurring in the same epicentral area of the historical 1894 earthquake that earthquake caused deaths and severe damage even at large distances of about 500 km because of the unavailability of instrumental records to study this large earthquake not many details of its seismic source are known our study involves broadband and short period seismic data from modern seismicity to characterise the same epicentral area we used seismic records from the permanent argentine network inpres and from two temporary broadband experiments charge and siembra we determine 44 seismic locations and their focal depths which are in the range between 10 and 30 km and magnitudes 1 2 m 4 in addition we obtain 33 focal mechanism solutions for some of these earthquakes using p wave first motions and amplitude ratios between p and s waves for the same region five more moderate earthquakes have been ever observed globally and reported by the global centroid moment tensor catalogue in the last 55 years we note that epicentres are mainly distributed in the southern sector of the study region with a relatively greater concentration in the foothills of the frontal andes cordillera focal mechanisms show mainly reverse and in less proportion strike slip solutions overall these results agree with compression and shortening in an east west orientation of the andean northern precordilleran backarc region

accurate source depths and focal mechanisms of shallow earthquakes in western south america and in the new hebrides island arc synthetic seismograms are matched to long period p waveforms in order to obtain accurate depths of shallow earthquakes with known focal mechanisms accurate depths are obtained for a large sample of moderate sized 6 ms 7 events which produced relatively simple p waveforms and do not require intensive analysis the estimated error in the determination of depth is about 5 km or less for most of the events considered the procedure is applied to two suites of events one in south america 83 events and one in the new hebrides 61 events in these two areas of contrasting styles of subduction the accurate depths provide new information on the bending of descending plates near trenches the seismicity and tectonics of the upper plate and the geometry and structure of the zone of contact between the upper and descending plates depths of suboceanic earthquakes which occurred near the trench and which have either tensional and compressional horizontal stress axes agree well with chapple and forsyth s 1979 preferred model of a bending elastic perfectly plastic plate however an unusually deep event occurred beneath the fiji plateau in an area of young ocean floor supposed to have been formed by sea floor spreading processes only since late miocene time the calculated depth of the event 48 km places it below the lithosphere asthenosphere boundary derived from thermal models vertical cross sections through the shallow parts of the subduction zones show that most of the earthquakes with thrust faulting focal mechanisms can be interpreted to be interplate events located in a thin 10 km thick curved zone of contact dipping arcward or landward from the trench axis however a few events with focal mechanisms similar to the interplate events do not fit on the inferred thin zone of contact these events would be accommodated by an interplate zone 15 km thick in both subduction zones interplate events occur at depths between about 15 and 50 km however the downdip width of the south american interplate zone is 2 to 3 times larger than that in the new hebrides most of the intraplate events near the interplate boundary are located in the descending plate in the upper plate seismicity is more pronounced in the back arc regions of the two subduction zones thus our results support the generality of a relatively aseismic zone in the upper plate located trenchward of the magmatic arc depths of shallow earthquakes in the eastern andes and sub andean regions of south america range between about 9 and 36 km indicating that both the upper and middle crust are seismically active about 75 of the events occurred in the upper 25 km furthermore the focal mechanisms indicate that from ecuador to argentina the crust of the eastern side of the andean cordilleran is in a state of east west compression oriented nearly parallel to the direction of convergence between the nazca and south american plates the depths the orientations of the earthquake nodal planes and the close relationships to quaternary tectonics imply basement involved crustal shortening in much of the eastern andes and sub andean zones of peru and argentina

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

paleogeography and andean structural geometry northwest argentina neogene tectonic segmentation of the andean cordillera in northwestern argentina is a function of both the dip of the subducted plate and of major paleogeographic structures and sedimentary basins vergence fault dip and lateral continuity of neogene structures can be related to the character and pre existing geometries and structures of the materials in which they form south of 27 s the thick skinned sierras pampeanas which occur mostly over a flat segment of the subducted nazca plate formed where little sedimentary cover overlay crystalline basement north of 24 s the thin skinned subandean belt overlying an inclined slab segment formed within a thick elongate eastward tapering trough of paleozoic and mesozoic sediments that may represent a complex of pre andean foreland basin deposits the transitional region between 24 27 s the santa barbara system and cordillera oriental is characterized paleogeographically by complex cross trending structures and irregular basin geometries and the neogene structures are equally complex and variable the paleogeographic features of nw argentina are a product of compressional tectonism during much of the paleozoic that resulted in an orogenic belt oriented obliquely to the modern andes this earlier orogen was disrupted by several zones of transverse structures and by cretaceous extensional or pull apart basins these features clearly influence the geometry of the superposed andean structures more intriguing however is the possibility that preexisting structures of the continental lithosphere may in some way help to modify the geometry of the subducted plate

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

why large earthquakes do not nucleate at shallow depths although by definition tectonic earthquakes may nucleate anywhere within the seismogenic layer in almost all cases those earthquakes that become large nucleate near the base of the seismogenic layer because frictional strength and stress drop should increase with depth and in situ stress measurements suggest that the ambient stress increases with depth we have examined spontaneous rupture models with gradients of both average stress drop and average strength relative to ambient stress to determine whether this explains the observed phenomena we report here that ruptures that are nucleated with in the low stress drop shallow regions of the model are inhibited from propagating but those that are nucleated within the high stress drop regions can propagate over the entire fault plane

an aftershock study of the el asnam algeria earthquake of 1980 october 10 summary an array of 28 portable seismic stations was operated in the region of el asnam following the magnitude 7 3 ms thrust earthquake of 1980 october 10 locations of 494 events are presented in this paper and provide an indication of the overall form of the aftershock distribution tests to establish location accuracy particularly depth reduce this set to 277 events which it is argued are well constrained p waves alone are used in this study as a consequence of a debate about the reliability of reading s phases from the reduced set of 277 events 81 events provide well constrained focal mechanisms the locations are presented in the form of maps and cross sections and discussed in relation to information already derived from field mapping of surface breaks and teleseismic studies of the waveforms of the main event the zone of surface faulting including secondary normal faulting extended for 35 km but the aftershock distribution extends for twice this distance along the part of the fault which experienced substantial displacement in the main shock the fault plane itself appears to be devoid of aftershocks although many lie in the footwall beneath the fault at junctions between segments of thrust faulting strike slip motion occurs this is apparent in the aftershock focal mechanisms and in the surface ruptures in one place the large number of aftershocks in the north east area appears to be due to the reactivation of a fan like system of smaller reverse faults associated with surface folding activity at the south west end is considerably less than that in the north east and is not obviously associated with recognizable geological or morphological features

multihazard simulation and cyberinfrastructure the integration of multihazard simulations and remotely sensed observations is providing enormous benefits to earthquake and tsunami research integrating data and models through cyberinfrastructure is enabling understanding of earthquake and tsunami generation mechanisms in the asia pacific region and improving assessment strategies for mitigating risk earthquake rupture processes occur on all scales from microns to global and from sub seconds to millions of years earthquakes cause damage but also generate tsunamis which create additional damage remotely sensed observations coupled with geologic field measurements and simulations contribute to our understanding of earthquake processes which is necessary for mitigating loss of life and property from these damaging events remotely sensed observations play a unique role in the mitigation of natural hazards measurements of surface motions can be used to infer strain accumulation and transfer within interacting fault systems as well as the mechanisms of earthquakes which is an important input to tsunami generation models and disaster response remotely sensed geodetic imaging data include interferometric synthetic aperture radar insar the global positioning system gps and other techniques such as lidar or optical imaging geodetic imaging can be used to understand aspects of earthquakes including the tectonic plate motions that drive earthquakes gps regional and local crustal deformation associated with faults or other sources gps insar lidar optical and detailed motions associated with earthquakes at a resolution of 1 hz gps the united states hosted the 8th international symposium of the apec cooperation of earthquake simulation aces october 22 26 2012 in maui hawaii the workshop focused on assimilation of remotely sensed observations to advance multihazards simulation the apec cooperation for earthquake simulation aces is a multilateral grand challenge science research cooperation of apec the asia pacific economic cooperation aces aims to develop realistic simulation models for the complete earthquake generation process and to assimilate observations into such models this capability provides a powerful virtual laboratory to probe earthquake behavior and the earthquake cycle hence it offers a new opportunity to gain understanding of the earthquake nucleation process precursory phenomena and space time seismicity patterns needed for breakthrough advances in earthquake forecasting and hazard quantification the project represents a grand scientific challenge because of the complexity of phenomena and range of scales from microscopic to global involved in the earthquake generation process aces symposia provide unique opportunities for the apec economies to work together on the topics important for the hazard mitigation of natural disasters this volume follows earlier pageoph topical volumes based on aces symposia see references below this topical volume reflects the 4 day workshop which consisted of plenary talks technical sessions working sessions and poster sessions it brought together an interdisciplinary set of researchers it brings together remote sensing experts modelers and computer scientists the symposium and this

statistical relations among earthquake magnitude surface rupture length and surface fault displacement abstract in order to refine correlations of surface wave magnitude fault rupture length at the ground surface and fault displacement at the surface by including the uncertainties in these variables the existing data were critically reviewed and a new data base was compiled earthquake magnitudes were redetermined as necessary to make them as consistent as possible with the gutenberg methods and results which make up much of the data base measurement errors were estimated for the three variables for 58 moderate to large shallow focus earthquakes regression analyses were then made utilizing the estimated measurement errors the regression analysis demonstrates that the relations among the variables magnitude length and displacement are stochastic in nature the stochastic variance introduced in part by incomplete surface expression of seismogenic faulting variation in shear modulus and regional factors dominates the estimated measurement errors thus it is appropriate to use ordinary least squares for the regression models rather than regression models based upon an underlying deterministic relation in which the variance results primarily from measurement errors significant differences exist in correlations of certain combinations of length displacement and magnitude when events are grouped by fault type or by region including attenuation regions delineated by evernden and others estimates of the magnitude and the standard deviation of the magnitude of a prehistoric or future earthquake associated with a fault can be made by correlating m s with the logarithms of rupture length fault displacement or the product of length and displacement fault rupture area could be reliably estimated for about 20 of the events in the data set regression of m s on rupture area did not result in a marked improvement over regressions that did not involve rupture area because no subduction zone earthquakes are included in this study the reported results do not apply to such zones

repeated folding during late holocene earthquakes on the la cal thrust fault near mendoza city argentina in 1861 one of the most destructive earthquakes in the history of argentina destroyed the city of mendoza currently 1 million inhabitants the magnitude m s 7 0 earthquake is inferred to have occurred on the 31 km long la cal thrust fault which extends from mendoza to the north where it offsets an alluvial fan and small inset terraces along a well preserved fault scarp a trench excavated on a terrace that is vertically offset by 2 5 m exposes two main stratigraphic units separated by an erosional unconformity the coarse grained upper unit is deformed by three east vergent folds f 1 f 3 retrodeformation of these folds yields total displacements of 2 0 m 2 4 m and 0 5 m on the underlying fault splays respectively the displacement of 2 0 m recorded by fold f 1 is interpreted as the result of the fault rupture that caused the 1861 earthquake f 2 and f 3 were presumably generated during the penultimate event with an inferred magnitude of m w 7 0 although formation during two distinct ruptures cannot be excluded finite element modeling shows that coseismic folding above the tip of a blind thrust fault is a physically plausible mechanism to generate these folds a published luminescence age of 770 76 years which is interpreted to date the formation of the deformed terrace indicates that the two or possibly three scarp forming events occurred during the last 800 years the fine grained sediments below the erosional unconformity that contain evidence for at least one older earthquake are dated at 12 kyr our results indicate that elastic strain energy which is accumulating at the front of the precordillera today as shown by global positioning system gps data was repeatedly released during earthquakes on the la cal fault in the past hence the la cal thrust fault poses a serious threat to the city of mendoza

lateral velocity variations in the andean foreland in argentina determined with the jhd method abstract lateral velocity variations in the andean foreland near san juan province argentina were detected by analyzing the station corrections computed as part of the joint location of local earthquake data the events recorded by a portable network panda have intermediate and shallow depths about 100 km and less than 40 km respectively the jhd technique was applied to intermediate depth events and to two subsets of shallow events that concentrate on two distinct geologic units precordillera and pampean ranges the computed station corrections have a strong dependence on the data processed analysis of the different patterns of corrections shows that the corrections are consistent with the presence of large scale lateral velocity variations bounded by major fault systems based on these corrections a qualitative velocity model is proposed that has relatively high velocities between the eastern precordillera and a westerly dipping plane on the eastern edge of pie de palo one of the mountain blocks of the pampean ranges velocities are lower immediately east of this plane and between the central and eastern precordillera this model has important tectonic implications because it suggests that there is a major boundary between the central and eastern precordillera and that the pampean basement whose extent is unknown underlies the eastern precordillera the existence of large velocity variations across the precordillera is consistent with a westward shift of almost 10 km of the relocated intermediate depth events and with the removal of an apparent westward dip opposite to the direction of subduction observed in the wadati benioff zone as determined from single event location the analysis of the intermediate depth events also exemplifies the deleterious effect on the jhd results introduced by the cancellation of small singular values in the computation of the station corrections

structural evolution of the argentine precordillera the rio san juan section abstract the argentine precordillera ap represents a high level fold and thrust belt which was formed during the andean tertiary crustal shortening in a w e transect studied mostly e directed imbrications are combined with folding and involve a pile of cambrian to tertiary sediments the thrust belt is detached above a main decollement within the ordovician to lower devonian strata to the east it is bounded by a back thrust zone directed westwards restored sections allow an estimate of crustal shortening and also provide insight into the structural development during pre tertiary times carbonate platform sequences of cambrian to ordovician age grade into a volcani clastic succession with transitional melanges representing deposits along a western continental slope in the western and easternmost parts of the ap cambrian to silurian deposits are affected by a first folding which to the west is combined with a slight metamorphic overprint during carboniferous to permian times intensified mobility along the eastern rim and in the western parts of the ap leads to crustal extension which partly was triggered by the formation of synsedimentary normal faults in the central ap no angular unconformities at the base of carboniferous permian deposits were found there was no field evidence found to postulate a terrane boundary within the ap area and an onset of thrust tectonics during cretaceous to early tertiary times

analysis of neotectonic structures in the eastern precordillera of argentina in relation to seismic hazard by the application of integrated geophysical methods abstract the city of san juan in the central western region of argentina has been the target of very destructive superficial earthquakes some of which have not been associated to a clear structural source up to this date the city is constantly growing outside the valley where it is located towards the area of eastern precordillera which is currently having an increased socio cultural activity thus this study is focused on increasing the geological knowledge of the latter by studying the eastern flank of sierra chica de zonda eastern precordillera whose proved neotectonic activity represents a geohazard on the basis of the general geological setting the neotectonic structures in the study area are related to a major active synclinal folding located just under the western sector of the san juan city geophysical potential methods gravimetric and magnetometric surveys were used to recognize contacts by contrast of density and magnetic susceptibility in order to reduce the ambiguity of these methods the gravi magnetometric results were constrained by using seismic and electrical tomographies these contacts where geophysical properties abruptly change were interpreted as faults despite many of them not having a superficial expression the latter being of great importance to asses the seismic hazard of the study area

aftershocks of the western argentina caucete earthquake of 23 november 1977 some tectonic implications an aftershock survey using a network of eight portable and two permanent seismographs was conducted for the western argentina caucete earthquake ms 7 3 of november 23 1977 monitoring began december 6 almost 2 weeks after the main shock and continued for 11 days the data set includes 185 aftershock hypocenters that range in the depth from near surface to more than 30 km the spatial distribution of those events occupied a volume of about 100 km long 50 km wide 30 km thick the volumnar nature of the aftershock distribution is interpreted to be a result of a bimodal distribution of foci that define east and west dipping planar zones efforts to select which of those zones was associated with the causal faulting include special attention to the determination of the mainshock focal depth and dislocation theory modeling of the coseismic surface deformation in the epicentral region our focal depth 25 35 km and modeling studies lead us to prefer an east dipping plane as causal a previous interpretation by other investigators used a shallower focal depth 17 km and similar modeling calculations in choosing a west dipping plane our selection of the east dipping plane is physically more appealing because it places fault initiation at the base of the crustal seismogenic layer rather than in the middle of that layer which requires fault propagation to be updip rather than downdip

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

earthquake deformation cycle in the andean back arc western argentina repeated leveling surveys conducted in 1938 1967 1975 1976 1978 1979 1980 and 1981 provide information on preseismic coseismic and postseismic movements associated with the ms 7 4 1977 caucete argentina earthquake sequence the spatial pattern and amount of uplift 1 m that accompanied the earthquake suggest 4 m of slip on a north south trending west dipping fault buried 17 km below the surface with a dip of 35 a downdip width of 24 km and a length of 80 km these fault parameters are generally consistent with the seismic focal mechanism the moment of the double main shock and the distribution of aftershocks vertical movements during the first preseismic period 1938 1967 while an order of magnitude smaller have a spatial pattern similar to the observed coseismic movements the second period of preseismic deformation 1967 1976 is characterized by movements opposite those which accompanied the earthquake 2 1 cm of relative subsidence during the first postseismic interval 1978 1980 deformation continued in the same sense as the coseismic movements an additional 10 1 cm of relative uplift subsequent postseismic movements 1980 1981 were opposite in sense to the coseismic deformation 12 1 cm of relative subsidence we suggest that the preseismic movements for the 1938 1967 time period result from reverse slip 50 cm on a downdip section of the 1977 earthquake fault this slip may have been induced by a ms 7 5 earthquake that occurred near san juan in 1944 or may represent coseismic slip for a m 6 5 earthquake that occurred in 1941 deformation during the second preseismic period 1967 1976 is interpreted as being due to steady strain accumulation on the fault 9 mm yr 1 the initial postseismic movements 1978 1980 are roughly consistent with afterslip on the 1977 earthquake fault 40 cm in the same sense as the coseismic slip the second phase of postseismic deformation may represent the period of rapid postseismic recovery i e strain accumulation observed previously in interplate areas there is some evidence that strain accumulation following the earthquake began on the updip section of the fault this process of strain accumulation may be responsible for the fact that the 1977 main shock initiated at the updip end of the coseismic fault assuming that the time predictable model of earthquake occurrence is applicable to this area we estimate that the next earthquake on the 1977 earthquake fault will occur in 200 600 years

lipid geochemistry of late pleistocene lacustrine sediments from burland cheshire u k abstract extractable lipids isolated from a lacustrine sediment sequence 50 ka old were analysed by gc and c gc ms to determine the persistence of certain biological markers and to characterize the depositional environment the central peat deposit gave unimodal distributions of homologous n alkanes n alkanoic acids alkan 1 ols and alkyl esters typical of higher plants triterpenoids of similar origin including partial aromatic and de a ring hydrocarbon derivatives were detected throughout the profile as were prokaryote derived hopanoids two sections overlying the peat also contained more labile lipids including c14 c18 fatty acids mainly unsaturated c30 c38 alkyl esters and triacylglycerols preservation of which is attributed to rapid burial a geolipid input to the older section was recognized by the presence of hopanes algal biomarkers in these sediments include c37 alkenones 4 methyl steroids and 2 6 10 trimethyl 7 3 methylbutyl dodecane the last two biomarkers also occurred in sediment below the peat the profile reflects slow deposition of sediment within a mesotrophic lake succeeded by more rapid input arising initially from erosion of marginal peat and later from erosion of mineral soils as a result of climatic deterioration

thick skinned tectonics and basement involved fold thrust belts insights from selected cenozoic orogens defining the structural style of fold thrust belts and understanding the controlling factors are necessary steps towards prediction of their long term and short term dynamics including seismic hazard and to assess their potential in terms of hydrocarbon exploration while the thin skinned structural style has long been a fashionable view for outer parts of orogens worldwide a wealth of new geological and geophysical studies has pointed out that a description in terms of thick skinned deformation is in many cases more appropriate this paper aims at providing a review of what we know about basement involved shortening in foreland fold thrust belts on the basis of the examination of selected cenozoic orogens after describing how structural interpretations of fold thrust belts have evolved through time this paper addresses how and the extent to which basement tectonics influence their geometry and their kinematics and emphasizes the key control exerted by lithosphere rheology including structural and thermal inheritance and local regional boundary conditions on the occurrence of thick skinned tectonics in the outer parts of orogens

earthquakes and multi hazards around the pacific rim vol 1 introduction the seismic belt along the pacific rim is the greatest earthquake zone in the world generating more than 80 of the world s largest earthquakes https earthquake usgs gov learn topics megaqk facts fantasy php it is also susceptible to tsunamis and volcanic eruptions which could generate serious multihazards since the beginning of the twenty first century most countries along the pacific rim have suffered from tremendous multi hazards especially earthquakes and tsunamis

pie de palo argentina a cataclastic diapir the term cataclastic diapirism is proposed for the low temperature extrusion of highly fractured rocks through more competent strata to produce domed topographies at the surface the process is illustrated by reference to the geomorphology neotectonics and microseismicity of the pie de palo an elongated ridge in the western sierras pampeanas of argentina composed of shattered and sheared lower palaeozoic rocks and subject to coseismic uplift the pie de palo is conventionally interpreted as a fault driven basement fold linked to low angle eastward subduction of the nazca plate beneath south america the diapiric model implies instead that deformation is powered by regional compression from west verging near surface crustal shortening which results ultimately from atlantic spreading

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

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

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

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

the evolution of the gulf of corinth greece an aftershock study of the 1981 earthquakes summary a preliminary study of the aftershocks of three earthquakes that occurred near to corinth greece in 1981 is combined with observations of the morphology and faulting to understand the evolution of the eastern gulf of corinth the well located aftershocks form a zone 60km long and 20km wide they do not lie on the main fault planes and are mostly located between the north dipping faulting on which the first two earthquakes occurred and the south dipping faulting associated with the third event a cluster of aftershocks also lies in the footwall of the eastern end of the south dipping fault of the third event morphologically it is observed that in the evolution of the eastern gulf of corinth antithetic faulting apparently predates the appearance of the main faulting at the surface this evolution can be explained by motion on a deep seated shallow angle aseismic normal fault a model based on such a fault also accounts for the aftershock distribution of the 1981 earthquakes

seismic hazard reappraisal from combined structural geology geomorphology and cosmic ray exposure dating analyses the eastern precordillera thrust system nw argentina because earthquakes on large active thrust or reverse faults are not always accompanied with surface rupture paleoseismological estimation of their associated seismic hazard is a difficult task to improve the seismic hazard assessments in the andean foreland of western argentina san juan province this paper proposes a novel approach that combines structural geology geomorphology and exposure age dating the eastern precordillera of san juan is probably one of the most active zones of thrust tectonics in the world we concentrated on one major regional active reverse structure the 145 km long villic um pedernal thrust where this methodology allows one to 1 constrain the quaternary stress regime by inversion of geologically determined slip vectors on minor or major fault planes 2 analyse the geometry and the geomorphic characteristics of the villic um pedernal thrust and 3 estimate uplift and shortening rates through determination of in situ produced 1 0 be cosmic ray exposure cre ages of abandoned and uplifted alluvial terraces from a structural point of view the villic um pedernal thrust can be subdivided into three thrust portions constituting major structural segments separated by oblique n40 e trending fault branches along the three segments inversion of fault slip data shows that the development of the eastern precordillera between 31 s and 32 s latitude is dominated by a pure compressive reverse faulting stress regime characterized by a n110 10 e trending compressional stress axis 1 a geomorphic study realized along the 18 km long las tapias fault segment combined with cre ages shows that the minimum shortening rate calculated over the previous 20 kyr is at least of the order of 1 mm yr 1 an earthquake moment tensor sum has also been used to calculate a regional shortening rate caused by seismic deformation this analysis of the focal solutions available for the last 23 yr shows that the seismic contribution may be three times greater than the shortening rate we determined for the las tapias fault i e 3 mm yr 1 suggesting that the san juan region may have experienced a seismic crisis during the 20th century moreover the ramp that controls the development of the eastern precordillera appears to be one of the main seismic sources in the san juan area particularly the 65 km long villic um las tapias segment a first order evaluation of the seismic hazard parameters shows that this thrust segment can produce a maximum earthquake characterized by a moment magnitude of 7 3 0 1 and a recurrence interval of 2 4 1 5 kyr this part of the villic um pedernal ramp may have ruptured during the ms 7 4 1944 san juan earthquake producing very few surface ruptures and only distributed flexural slip deformation on to the neogene foreland bedding planes between the eastern precordillera and pie de palo