growth and collapse of the reunion island volcanoes this work presents the first exhaustive study of the entire surface of the reunion island volcanic system the focus is on the submarine part for which a compilation of all multibeam data collected during the last 20 years has been made different types of submarine features have been identified a coastal shelf debris avalanches and sedimentary deposits erosion canyons volcanic constructions near the coast and seamounts offshore criteria have been defined to differentiate the types of surfaces and to establish their relative chronology where possible debris avalanche deposits are by far the most extensive and voluminous formations in the submarine domain they have built four huge submarine bulges to the east north west and south of the island they form fans 20 30 km wide at the coastline and 100 150 km wide at their ends 70 80 km offshore they were built gradually by the superimposition and or juxtaposition of products moved during landslide episodes involving up to several hundred cubic kilometers of material about 50 individual events deposits can be recognized at the surface the landslides have recurrently dismantled piton des neiges les alizes and piton de la fournaise volcanoes since 2 ma about one third are interpreted as secondary landslides affecting previously emplaced debris avalanche deposits on land landslide deposits are observed in the extensively eroded central area of piton des neiges and in its coastal areas analysis of the present day topography and of geology allows us to identify presumed faults and scars of previous large landslides the submarine bulges are dissected and bound by canyons up to 200 m deep and 40 km long filled with coarse grained sediments and generally connected to streams onshore a large zone of sedimentary accumulation exists to the north east of the island it covers a zone 20 km in width extending up to 15 km offshore volcanic constructions are observed near the coast on both piton des neiges and piton de la fournaise volcanoes and are continuations of subaerial structures individual seamounts are present on the submarine flanks and the surrounding ocean floor a few seem to be young volcanoes but the majority are probably old eroded seamounts this study suggests a larger scale and frequency of mass wasting events on reunion island compared to similar islands the virtual absence of downward flexure of the lithosphere beneath the island probably contributes to this feature the increased number of known flank failure events has to be taken into consideration when assessing hazards from future landslides in particular the probability of landslide generated tsunamis

active deformation near the nicoya peninsula northwestern costa rica between 1996 and 2010 interseismic megathrust coupling 1 we use campaign and continuous gps measurements at 49 sites between 1996 and 2010 to describe the long term active deformation in and near the nicoya peninsula northwestern costa rica the observed deformation reveals partial partitioning of the cocos caribbean oblique convergence into trench parallel forearc sliver motion and less oblique thrusting on the subduction interface the northern costa rican forearc translates northwestward as a whole ridge block at 11 1 mm yr relative to the stable caribbean the transition from the forearc to the stable caribbean occurs in a narrow deforming zone of 16 km wide subduction thrust earthquakes take 2 3 of the trench parallel component of the plate convergence however surface deformation caused by interseismic megathrust coupling is primarily trench normal two fully coupled patches one located offshore nicoya centered at 15 km depth and the other located inland centered at 24 km depth are identified in nicoya with the potential to generate an mw 7 8 1950 type earthquake another fully coupled patch se of nicoya coincides with the rupture region of the 1990 nicoya gulf earthquake interface microearthquakes non volcanic tremor low frequency earthquakes and transient slow slip events generally occur in the intermediately to weakly coupled regions

salinity driven thermocline transients in a wind and thermohaline forced isopycnic coordinate model of the north atlantic abstract an isopycnic coordinate oceanic circulation model formulated with the aim of simulating thermodynamically and mechanically driven flow in realistic basins is presented special emphasis is placed on the handling of diabatic surface processes and on thermocline ventilation the model performance is illustrated by a 30 year spinup run with coarse horizontal resolution 2 mesh in a domain with north atlantic topography extending from 10 to 60 n latitude the vertical structure encompasses 10 isopycnic layers in steps of 0 2 units capped by a thermodynamically active mixed layer from an initially isohaline state with isopycnals prescribed by zonally averaged climatology the model is forced by seasonally varying wind stress radiative and freshwater fluxes and by a thermal relaxation process at the surface after a mechanical spinup time of about 15 years a quasi stationary pattern of mean circulation and annual variability ensues characterized by pronounced subtropical mode water formation

the 5 september 2012 nicoya costa rica mw 7 6 earthquake rupture process from joint inversion of high rate gps strong motion and teleseismic p wave data and its relationship to adjacent plate boundary interface properties 1 on 5 september 2012 a large thrust earthquake mw 7 6 ruptured a densely instrumented seismic gap on the shallow dipping plate boundary beneath the nicoya peninsula costa rica ground motion recordings directly above the rupture zone provide a unique opportunity to study the detailed source process of a large shallow megathrust earthquake using very nearby land observations hypocenter relocation using local seismic network data indicates that the event initiated with small emergent seismic waves from a hypocenter 10 km offshore 13 km deep on the megathrust a joint finite fault inversion using high rate gps strong motion ground velocity recordings gps static offsets and teleseismic p waves reveals that the primary slip zone slip 1 m is located beneath the peninsula the rupture propagated downdip from the hypocenter with a rupture velocity of 3 0 km s the primary slip zone extends 70 km along strike and 30 km along dip with an average slip of 2 m the associated static stress drop is 3 mpa the seismic moment is 3 5 1020 nm giving mw 7 6 the coseismic large slip patch directly overlaps an onshore interseismic locked region indicated by geodetic observations and extends downdip to the intersection with the upper plate moho at deeper depths below the upper plate moho seismic tremor and low frequency earthquakes have been observed most tremor locates in adjacent areas of the megathrust that have little coseismic slip a region of prior slow slip deformation to the southeast also has no significant coseismic slip or aftershocks an offshore locked patch indicated by geodetic observations does not appear to have experienced coseismic slip and aftershocks do not overlap this region allowing the potential for a comparable size rupture offshore in the future

a new seismically constrained subduction interface model for central america we provide a detailed seismically defined three dimensional model for the subducting plate interface along the middle america trench between northern nicaragua through to southern costa rica the model uses data from a weighted catalog of about 30 000 earthquake hypocenters compiled from nine catalogs to constrain the interface through a process we term the maximum seismicity method the method determines the average position of the largest cluster of microseismicity beneath an a priori functional surface above the interface this technique is applied to all seismicity above 40 km depth the approximate intersection of the hanging wall mohorovicic discontinuity where seismicity likely lies along the plate interface below this depth an envelope above 90 of seismicity approximates the slab surface because of station proximity to the interface this model provides highest precision along the interface beneath the nicoya peninsula of costa rica an area where marked geometric changes coincide with crustal transitions and topography observed seaward of the trench the new interface is useful for a number of geophysical studies that aim to understand subduction zone earthquake behavior geodynamic and tectonic development of convergent plate boundaries

interannual modulation of east african early short rains by the winter arctic oscillation in the present study we analyzed the interannual linkage between the boreal winter arctic oscillation ao and east african early short rains when the indian ocean dipole and el nino southern oscillation variance are excluded by linear regression the boreal winter ao index is significantly correlated with the october east african precipitation over the domain of 5 n 5 s and 35 45 e for the period 1979 2014 r 0 46 the upper ocean heat content likely acts as a medium that links the ao and east african precipitation significant subsurface warming and positive upper ocean heat content anomalies occur over the western indian ocean during the autumn following positive ao winters which enriches the atmospheric moisture intensifies convection and enhances precipitation oceanic dynamics play a key role in causing this subsurface warming winter ao related atmospheric circulation creates anomalous wind stress which forces a downwelling oceanic rossby wave between 60 75 e and 5 10 s where the thermocline significantly deepens this rossby wave propagates westward and accompanies significant subsurface warming along the thermocline the rossby wave arrives at the western indian ocean in the late summer significantly warming the region to the west of 55 e at a depth of 60 100 m this warming remains significant through october correspondingly the upper ocean heat content significantly increases by approximately 2 3 108 j m 2 in the region west of 60 e between 5 and 10 s the role of these oceanic dynamics in linking the winter ao and anomalous subsurface warming was tested by numerical experiments with an oceanic general circulation model the experiments were performed with the forcing of ao related wind stress anomalies over the indian ocean in the winter the oceanic rossby wave generated in the central indian ocean during boreal winter the consequent subsurface warming and the anomalous upper ocean heat content in october over the western indian ocean were all adequately reproduced the winter ao can serve as a precursor of east african short rain anomalies

the nicoya convergent margin a region of exceptionally low heat flow over 100 measurements of seafloor heat flow reveal that the accretionary complex adjacent to the nicoya peninsula is characterized by remarkably low heat flow values over the accretionary prism average 28 mw m and values in the trench and the ocean crust seaward of the trench average 14 mw m we attribute the low heat flow to effective hydrothermal cooling of the upper crust on the subducting plate and suggest that extensional faults created by flexure of the lithosphere enhance hydrothermal circulation thermal models show that subduction of low temperature crust combined with significant frictional heating at the decollement can explain the low and uniform heat flow disparity between heat flow values observed on the lower trench slope with model results suggests upward advection of heat by porewater flux through broadly distributed conduits

impact of mascarene high variability on the east african short rains the interannual variability of east african short rains easr and its link with the mascarene high mh variation are explored using observations and reanalysis data correlation and composite analyses for flood and drought events reveal that the easr variability is strongly linked to the mh zonal displacement in particular the zonal movement of the mh eastern ridge when the mh eastern ridge is anomalously displaced to the west east of its normal position the south east se trade winds over the south indian ocean sio anomalously strengthen weaken this enhances reduces the relatively cool and dry se trade winds and induces cold warm sea surface temperature anomaly in the sio as a result convection over the western equatorial sio is suppressed enhanced and leads to rainfall deficits excess over east africa droughts in east africa are associated with a westward migration of the mh eastern ridge while the relationship is less clear for flood events and their link to an eastward migration of the mh therefore the zonal migration of the mh eastern ridge provides a novel indicator for the easr extremes especially droughts this revelation has immense social application for rainfall forecast over east africa where rainfall deficits have become more prevalent against the background of deteriorating conventional forecasts for easr droughts

indian ocean capacitor effect on indo western pacific climate during the summer following el nino significant climate anomalies persist through the summer june august after el nino dissipates in spring over the equatorial pacific they include the tropical indian ocean tio sea surface temperature sst warming increased tropical tropospheric temperature an anomalous anticyclone over the subtropical northwest pacific and increased mei yu baiu rainfall over east asia the cause of these lingering el nino effects during summer is investigated using observations and an atmospheric general circulation model gcm the results herein indicate that the tio warming acts like a capacitor anchoring atmospheric anomalies over the indo western pacific oceans it causes tropospheric temperature to increase by a moist adiabatic adjustment in deep convection emanating a baroclinic kelvin wave into the pacific in the northwest pacific this equatorial kelvin wave induces northeasterly surface wind anomalies and the resultant divergence in the subtropics triggers suppressed convection and the anomalous anticyclone the gcm results support this kelvin wave induced ekman divergence mechanism in response to a prescribed sst increase over the tio the model simulates the kelvin wave with low pressure on the equator as well as suppressed convection and the anomalous anticyclone over the subtropical northwest pacific an additional experiment further indicates that the north indian ocean warming is most important for the kelvin wave and northwest pacific anticyclone a result corroborated by observations these results have important implications for the predictability of indo western pacific summer climate the spatial distribution and magnitude of the tio warming rather than simply whether there is an el nino in the preceding winter affect summer climate anomalies over the indo western pacific and east asia

importance of the indian ocean for simulating rainfall anomalies over eastern and southern africa the relative contributions of the indian ocean and pacific ocean sea surface temperatures ssts to the rainfall variability over eastern central and southern africa during the austral spring summer are examined the variability of african rainfall is statistically related to both oceans but the variability in the two oceans is also related to separate the effects of the indian and pacific oceans a suite of numerical model simulations is presented goga the atmosphere is forced by observed ssts globally ioga the atmosphere is forced by observed ssts only in the indian ocean basin and poga the atmosphere is forced by observed ssts only in the tropical pacific basin while the sst variability of the tropical pacific exerts some influence over the african region it is the atmospheric response to the indian ocean variability that is essential for simulating the correct rainfall response over eastern central and southern africa analyses of the dynamical response s seen in the numerical experiments and in the observations indicate that the pacific and indian oceans have a competing influence over the indian ocean african region this competition is related to the influence of the two oceans on the walker circulation and the consequences of that variability on low level fluxes of moisture over central and southern africa finally given the high correlation found between sst variability in the indian and pacific oceans with the pacific leading by 3 months we speculate on an approach to long lead dynamical climate prediction over central east and southern africa

ocean rossby waves as a triggering mechanism for primary madden julian events the madden julian oscillation mjo is sporadic with episodes of cyclical activity interspersed with inactive periods however it remains unclear what may trigger a madden julian mj event which is not immediately preceded by any mjo activity a primary mj event a combination of case studies and composite analysis is used to examine the extent to which the triggering of primary mj events might occur in response to ocean dynamics the case studies show that such events can be triggered by the arrival of a downwelling oceanic equatorial rossby wave which is shown to be associated with a deepening of the mixed layer and positive sea surface temperature sst anomalies of the order of 0 5 1 c these sst anomalies are not attributable to forcing by surface fluxes which are weak for the case studies analysed furthermore composite analysis suggests that such forcing is consistently important for triggering primary events the relationship is much weaker for successive events due to the many other triggering mechanisms which operate during periods of cyclical mjo activity this oceanic feedback mechanism is a viable explanation for the sporadic and broadband nature of the mjo additionally it provides hope for forecasting mj events during periods of inactivity when mjo forecasts generally exhibit low skill copyright c 2011 royal meteorological society

how did the 2013 lushan earthquake ms 7 0 trigger its aftershocks insights from static coulomb stress change calculations the lushan earthquake ms7 0 occurred at 08 02 beijing time 00 02 utc on 20 april 2013 located on the eastern margin of the tibetan plateau where the 2008 ms8 0 wenchuan earthquake took place the other strong shock following the wenchuan earthquake occurred on the longmen shan thrust fault a large number of aftershocks were recorded after the mainshock to investigate how the aftershocks were triggered we firstly relocated the 6 month aftershock sequence by the double difference algorithm next we calculated coulomb failure stress changes imparted by the lushan mainshock both on the optimally oriented plane and on the aftershock nodal planes then we examined the correspondence between the coulomb failure stress changes and the spatial distribution of the aftershocks the computed results show that most of the aftershock hypocenters did not occur on the region with the positive coulomb stress changes caused by the lushan mainshock moreover the majority of aftershock nodal planes were brought away from failure by coseismic coulomb stresses this implies that coseismic static coulomb stress changes may not be the governing process for aftershock genesis in contrast postseismic stress transfer by processes of afterslip and pore fluid flow are assumed to play an important part in aftershock triggering based upon the spatiotemporal distribution of the aftershocks

circumglobal teleconnections the jet stream waveguide and the north atlantic oscillation abstract monthly and seasonally averaged upper tropospheric northern hemisphere winter fields are examined to determine whether the waveguiding effect of the time averaged tropospheric jets on low frequency disturbances that is predicted by theory does affect the behavior of these disturbances it is found that indeed disturbances in the vicinity of the mean jets particularly the jet that stretches across south asia are fundamentally different from those that reside in regions where the mean winds have weaker meridional gradients like the mid pacific patterns of variability in the jets tend to be smaller scale and to consist of zonally oriented chains of anomalies while variability in the mid pacific is composed of patterns with distinct meridional orientation because they are meridionally trapped and zonally elongated patterns associated with the jet stream waveguide connect activity at points that are much farther apart than do patterns in other regions of the globe within the south asian waveg

earthquake relocation and focal mechanism determination using waveform cross correlation nicoya peninsula costa rica the nicoya peninsula in costa rica directly overlies the seismogenic zone of the middle america trench making it an ideal location for geophysical investigations of shallow subduction zone earthquake processes as part of the collaborative costa rica seismogenic zone experiment crseize a seismic network consisting of 20 land and 14 ocean bottom seismometers recorded small magnitude local earthquakes along the nicoya peninsula from december 1999 to june 2001 previous studies have used these data to compute local earthquake locations and 3d velocity structure to identify plate boundary seismicity and to investigate seismogenic behavior here we utilize waveform cross correlation and clustering techniques in an attempt to improve earthquake relocations and determine first motion focal mechanisms to validate refine and expand on existing models due to the high quality of the original locations and the small cross correlation p wave arrival time adjustments large differences between the previously determined and the cross correlated earthquake locations are not observed however focal mechanism determinations using cross correlated p waveforms are significantly enhanced approximately 90 of the focal mechanisms computed for events previously identified as interplate earthquakes are consistent with underthrusting focal mechanisms for continental intraplate events indicate dextral strike slip motion in the central region and normal faulting at the southern tip of the peninsula these motions may be associated with oblique convergence and seamount subduction respectively within the subducting plate steep p and t axes of earthquakes below 50 km depth are consistent with unbending of the slab

modified frequency wavenumber algorithm for regional seismograms using filon s quadrature modelling of lg waves in eastern north america summary the main focus of this study is to develop an efficient frequency wavenumber code to synthesize high frequency say 10 hz regional seismograms up to a distance of more than 1000 km in a medium consisting of a large number of crustal layers and investigate effects of regional waveguides on lg waves in this newly developed code the frequency wavenumber response is evaluated by using the filon s quadrature at high frequency and the polynomial approximation of bessei s functions implementing trapezoidal integration at low frequency this has reduced the computation time by several folds compared to the time needed for the trapezoidal integration using this algorithm both broad band seismograms at harvard and short period ectn eastern canadian telemetered network seismograms from 1988 november 25 saguenay earthquake were successfully modelled a moment magnitude of 5 9 was used initially an average waveguide was calibrated to successfully excite the regional pnl and snl waves at hrv its near surface structure constrained by matching the recorded frequency content the crust mantle transition zone was constrained by modelling the strength of the broad band snl waves consisting of sn ssmsnsmssms and ssmssms phases relative to the initial pnl waves the model was modified to consist of thin layers with an alternating high and low velocity distribution to match the high frequency ectn lg waves observed from 300 to 600 km away in addition the effect of a near surface irregular receiver structure on the duration of lg waves was also investigated by convolving its response with the regional lg seismograms initial study suggests this method as viable for modelling the entire lg waves including the coda numerical studies suggest that the shapes and peak amplitudes on lg seismograms depend more strongly on the source depth anelasticity and crustal waveguide than do the pnl seismograms

the synchronous occurrence of shallow tremor and very low frequency earthquakes offshore of the nicoya peninsula costa rica 1 the occurrence of transient shallow slow slip at seismogenic zones has important implications for earthquake and tsunami hazards here we provide evidence that a tremor and slow slip event occurred at shallow depth offshore of the nicoya peninsula costa rica in august 2008 the temporal coincidence of offshore tremor very low frequency earthquakes vlfes motions consistent with slow slip on the plate interface on western coastal gps stations and a pressure transient in an iodp borehole all indicate slow slip occurring at shallow depths large ocean loading stresses on the shallow plate interface modulate tremor activity with the peak coulomb stress forced by semi diurnal ocean tides correlating with tremor productivity based on beamforming data we constrain that the vlfe activity occurs in the same region as the tremor and slow slip the presence of slow slip at shallow depth has important implications for the up dip extent of earthquake rupture the proximity of the 5 september 2012 mw 7 6 megathrust earthquake to slow slip tremor and vlfe activity in the 2008 event suggests abrupt frictional transitions from locked to conditionally stable behavior on the plate interface offshore of the nicoya peninsula

atmospheric hydrospheric mechanisms of climate anomalies in the western equatorial indian ocean atmosphere ocean mechanisms of rainfall anomalies at the coast of eastern africa are studied using long term ship observations in the indian ocean surface current measurements subsurface casts upper air analyses by the european centre for medium range weather forecasts rain gauge series in eastern africa and india and an index of the southern oscillation so the high so phase being defined by anomalously high low pressure at tahiti darwin the causalities of precipitation anomalies at the coast of eastern africa differ for the two rainy seasons centered on april may and october november and only the latter is strongly related to the so in the high so phase april may pressure is low over the entire indian ocean domain whereas in october november pressure is high in the west and low in the east concomitantly surface waters are anomalously cold in the west and strong westerlies sweep the equatorial zone of the indian ocean eastern african rainfall anomalies are related to the so through a combination of cooperative mechanisms that function most effectively in the boreal autumn rainy season of eastern africa 1 equatorial westerly winds are conducive to lower tropospheric divergence over equatorial east africa and in the high so phase these are accelerated especially in october november owing to the anomalous eastward pressure gradient 2 the equatorial westerly winds drive the eastward equatorial jet in the upper hydrosphere which entails cold water upwelling in the western extremity of the basin where sea surface temperature further hydrostatically affects the zonal pressure gradient and thus feeds back into the equatorial westerly winds 3 in addition cold water anomalies in the western indian ocean most pronounced in october november during the high so phase also suppress convection 4 in the high so phase the indian summer monsoon tends to be strong leaving behind an anomalously cold western indian ocean which in turn feeds into mechanisms 1 to 3 the eastward equatorial jet thus has a role to play in feedback mechanisms contributing to the anomalies of the boreal autumn rains at the coast of eastern africa

annular modes in the extratropical circulation part i month to month variability the leading modes of variability of the extratropical circulation in both hemispheres are characterized by deep zonally symmetric or annular structures with geopotential height perturbations of opposing signs in the polar cap region and in the surrounding zonal ring centered near 458 latitude the structure and dynamics of the southern hemisphere sh annular mode have been extensively documented whereas the existence of a northern hemisphere nh mode herein referred to as the arctic oscillation ao has only recently been recognized like the sh mode the ao can be defined as the leading empirical orthogonal function of the sea level pressure field or of the zonally symmetric geopotential height or zonal wind fields in this paper the structure and seasonality of the nh and sh modes are compared based on data from the national centers for environmental prediction national center for atmospheric research reanalysis and supplementary datasets the structures of the nh and sh annular modes are shown to be remarkably similar not only in the zonally averaged geopotential height and zonal wind fields but in the mean meridional circulations as well both exist year round in the troposphere but they amplify with height upward into the stratosphere during those seasons in which the strength of the zonal flow is conducive to strong planetary wave mean flow interaction midwinter in the nh and late spring in the sh during these active seasons the annular modes modulate the strength of the lagrangian mean circulation in the lower stratosphere total column ozone and tropopause height over mid and high latitudes and the strength of the trade winds of their respective hemispheres the nh mode also contains an embedded planetary wave signature with expressions in surface air temperature precipitation total column ozone and tropopause height it is argued that the horizontal temperature advection by the perturbed zonal mean zonal wind field in the lower troposphere is instrumental in forcing this pattern a companion paper documents the striking resemblance between the structure of the annular modes and observed climate trends over the past few decades

zonal circulations over the equatorial indian ocean abstract the ncep ncar 1958 97 upper air dataset and surface observations have been analyzed for evidence of zonal vertical circulations along the indian ocean equator and their role in climatic variability the long term mean upper tropospheric circulation is dominated in boreal winter by divergent outflow from the southern indian ocean northwestward into southern asia and in summer from southern asia southwestward into the southern hemisphere in boreal autumn only divergent easterlies blow from indonesia along the equator into an upper tropospheric convergence band over east africa and only then a closed zonal vertical circulation cell materializes along the indian ocean equator between the centers of ascending motion over indonesia and of subsidence over equatorial east africa and featuring westerlies in the lower layers the boreal autumn zonal vertical circulation varies interannually a regime of intense circulation features accelerated equatorial surface westerlies enhanced subsidence and d

geographical variability of the first baroclinic rossby radius of deformation global 1 83 18 climatologies of the first baroclinic gravity wave phase speed c1 and the rossby radius of deformation l1 are computed from climatological average temperature and salinity profiles these new atlases are compared with previously published 5 83 58 coarse resolution maps of l1 for the northern hemisphere and the south atlantic and with a 1 83 18 fine resolution map of c1 for the tropical pacific it is concluded that the methods used in these earlier estimates yield values that are biased systematically low by 5 15 owing to seemingly minor computational errors geographical variations in the new high resolution maps of c1 and l1 are discussed in terms of a wkb approximation that elucidates the effects of earth rotation stratification and water depth on these quantities it is shown that the effects of temporal variations of the stratification can be neglected in the estimation of c1 and l1 at any particular location in the world ocean this is rationalized from consideration of the wkb approximation

asian jet waveguide and a downstream extension of the north atlantic oscillation abstract anomalous atmospheric fields associated with the north atlantic oscillation nao are analyzed on interannual and intraseasonal time scales in order to examine the extent to which the nao is a regional phenomenon analyses on the interannual time scale reveal that the nao signal is relatively confined to the euro atlantic sector in december while it extends toward east asia and the north pacific in february the difference is most clearly seen in the meridional wind anomaly which shows a wave train along the asian jet collocated with an anomalous vorticity source near the jet entrance diagnoses using a linear barotropic model indicate that this wave train is interpreted as quasi stationary rossby waves trapped on the asian jet waveguide and effectively excited by the anomalous upper level convergence over the mediterranean sea it is found that when the nao accompanies the mediterranean convergence mc anomaly most frequently seen in february the nao indeed has a much wider horizontal str

rupture processes of the 2012 september 5 mw 7 6 nicoya costa rica earthquake constrained by improved geodetic and seismological observations

interseismic megathrust coupling beneath the nicoya peninsula costa rica from the joint inversion of insar and gps data the nicoya peninsula costa rica was struck by a long anticipated and gap filling mw 7 6 earthquake in 2012 to study interseismic strain accumulation on the megathrust beneath the nicoya peninsula we present an improved interseismic coupling model by integrating interferometric synthetic aperture radar insar and gps data our model reveals three strongly coupled patches the first strongly coupled patch locates beneath the nicoya peninsula and ruptured during the 2012 earthquake the second strongly coupled patch locates offshore the central nicoya peninsula and remained largely unbroken however this region is close to and possibly intermingled with shallow slow slip and tremor suggesting that accumulated strain in this region may be released both seismically and aseismically the third strongly coupled patch offshore of the southeastern end of nicoya overlaps part of the coseismic rupture of the 1990 mw 7 0 nicoya gulf earthquake indicating that significant strain has re accumulated since this event incorporating insar data provides a more refined interseismic coupling model than using gps alone and allows for a more reliable comparison with local seismic and aseismic activities this comparison indicates that strongly locked regions during the interseismic stage are the loci of coseismic slip and deep slow slip and low frequency earthquakes occur in regions of low coupling or transition zones from low to high coupling while shallow slow slip and tremor commingle with strongly coupled regions our study demonstrates that insar data can be used to recover small long wavelength deformation signals with refined resolution in challenging subduction zone environments when integrated with gps observations

diagnosing the 2005 drought in equatorial east africa abstract equatorial east africa suffered severe drought during its 2005 short rains centered on october november the circulation mechanisms of such precipitation anomalies are examined using long term upper air and surface datasets and based on diagnostic findings from earlier empirical investigations the steep eastward pressure gradient is conducive to fast westerlies over the central equatorial indian ocean surface manifestation of a powerful zonal circulation cell with subsidence over east africa and ascending motion over indonesia with fast westerlies rainfall in east africa is deficient and they tend to be accompanied by anomalously cold waters in the northwestern and warm anomalies in the southeastern extremity of the equatorial indian ocean basin without any seesaw between these domains in october november 2005 pressure in the west was anomalously high entailing a steep eastward pressure gradient along the equator conducive to fast westerlies and further symptomatic of the zonal ci

seismicity and state of stress in guerrero segment of the mexican subduction zone 1 we take advantage of a relatively dense network of seismic stations in the guerrero segment of the mexican subduction zone to study seismicity and state of stress in the region we combine our results with recent observations on the geometry of the subducted cocos plate imaged from receiver function rf analysis an ultraslow velocity layer mapped in the upper crust of the subducted slab and episodic slow slip events sses and nonvolcanic tremors nvts reported in the region to obtain a comprehensive view of the subduction process seismicity and focal mechanisms confirm subduction of the cocos plate below mexico at a shallow angle reaching a depth of 25 km at a distance of 65 km from the trench the plate begins to unbend at this distance and becomes horizontal at a distance of 120 km at a depth of 40 km some of the highlights of the inslab seismicity are as follows 1 a cluster of earthquakes in the depth range of 25 45 km immediately downdip from the strongly coupled part of the plate interface revealing both downdip compressional and extensional events this seismicity extends from 80 to 105 km from the trench and may be attributed to the unbending of the slab 2 the slab devoid of seismicity in the distance range of 105 160 km 3 sparse inslab seismicity revealing downdip extension in the distance range of 160 240 km the nvts are also confined to this distance range the episodic sses occur on the horizontal segment in the distance range of 105 240 km where an ultraslow velocity layer in the upper crust of the slab has been mapped from waveform modeling of converted sp phases thus in the distance range of 105 160 km sses occur but nvts and inslab earthquakes are absent this suggests that metamorphic dehydration reactions in the subducting oceanic crust and upper mantle begin at a distance of 160 km giving rise to both the inslab earthquakes and nvts no inslab earthquake occurs beyond 240 km the receiver function images and p wave tomography suggest that the slab begins a steep plunge at a distance of 310 km reaching a depth of 500 km around 340 km from the trench the negative buoyancy of such a slab should give rise to large extensional stress in the slab yet inslab seismicity is remarkably low which may be explained by a slab that is not continuous up to a depth of 500 km but is broken at a shallower depth the resulting slab window may permit subslab material to flow through the gap this may provide an explanation for the recent rift related basalts found near mexico city the fore arc upper plate seismicity which during the period of study 1995 2007 consisted of a moderate earthquake mw5 8 near the coast h 12 km its numerous aftershocks and two shallow events farther inland demonstrates a trench normal extension in the upper plate near this convergent margin a state of stress that may be explained by tectonic erosion and or seaward retreat of the trench seismicity location of the mantle wedge and rupture areas of mexican earthquakes suggest that the downdip limit of rupture during large great earthquakes in guerrero may be 105 15 km shallow dipping interplate thrust earthquakes are not the only type of events that affect the seismic hazard in the region the magnitudes of inslab downdip compressional and extensional earthquakes that occur within 20 km inland from the coast in the depth range of 25 45 km may reach 6 5 and 7 5 respectively in addition we now identify normal faulting earthquakes in the upper plate these sources need to be taken into account in the hazard estimation

melange rheology and seismic style shear displacements in crustal fault zones are accommodated by a range of seismic styles including standard earthquakes non volcanic tremor and continuous and transitory aseismic slip subduction channel shear zones containing highly sheared fluid saturated trench fill sediments intermingled with fragments of oceanic crust are commonly inferred to occur along active subduction megathrusts if this interpretation is correct these plate boundary faults are not discrete planes but may resemble the melange shear zones commonly found in exhumed subduction related rock assemblages melange deformation appears to depend critically on the ratio of competent to incompetent material with shear surfaces localized along lithological contacts or within competent domains while matrix flow accommodates shearing by distributed strain if the style of strain and or displacement accommodation in a melange reflects the partitioning between aseismic and seismic slip the proportion of competent material seems likely to be a significant factor affecting seismic style within subduction channel shear zones and along comparable mixed lithology fault zones

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

persistent tremor within the northern costa rica seismogenic zone 1 we identify tremor using a spectral detection method and characterize its occurrence over a period of four years 2006 2009 in the vicinity of the nicoya peninsula costa rica although a few major tremor events accompanied by geodetic slow slip occur much of the tremor record consists of minor episodes with short duration and no detectable geodetic slip its persistent occurrence suggests that some portion of the interface is experiencing slow slip nearly continuously driving small patches to fail in accelerated slip locations indicate that much of the tremor occurs at shallow depth in freely slipping regions of the seismogenic zone this result is significant in that locations of slow slip and tremor at other subduction zones are largely limited to the downdip frictional transition tremor locations may help to refine the heterogeneous distribution of locked and freely slipping patches within the costa rica seismogenic zone

nicoya earthquake rupture anticipated by geodetic measurement of the locked plate interface part of the subduction zone plate interface beneath costa rica was previously locked which allowed strain to accumulate analyses using gps and geomorphic data show that almost the entire locked region ruptured during a megathrust quake in 2012 implying that plate interface mapping towards the end of the earthquake cycle can aid seismic hazard assessments

hydrologic control of forearc strength and seismicity in the costa rican subduction zone the subduction zone beneath costa rica experiences infrequent large earthquakes in its northwestern part whereas slow slip dominates in the southeast seismic data reveal a disparity in fluid accumulation in the overriding continental crust that correlates with this change in seismic behaviour implying that spatial gradients in fluid content may control subduction zone seismicity

along strike variability in the seismogenic zone below nicoya peninsula costa rica 1 at the subduction zone in northwestern costa rica the seismogenic zone lies directly beneath the nicoya peninsula allowing for near source seismic studies of earthquake activity we located 650 earthquakes along the seismogenic plate interface using a dense seismic network in the vicinity of the nicoya peninsula using these data we constrained the updip limit of the seismogenic zone there and found a transition in depth 10 km in the south to 20 km in the north that occurs where the subducting oceanic crust changes from warmer cocos nazca spreading center cns origin to colder east pacific rise epr origin we argue that the temperature of the incoming oceanic crust controls the seismogenic updip limit beneath nicoya costa rica subducting colder oceanic crust deepens the seismogenic updip limit

multiscale postseismic behavior on a megathrust the 2012 nicoya earthquake costa rica the nicoya peninsula in northwest costa rica overlies a section of the subduction megathrust along the middle america trench on 5 september 2012 a moment magnitude 7 6 megathrust earthquake occurred beneath a dense network of continuous gps and seismic stations many of the gps stations recorded the event at high rate 1 hz or better we analyze the temporal and spatial evolution of surface deformation after the earthquake our results show that the main rupture was followed by significant afterslip within the first 3 h following the main event the behavior of the surface displacement can be represented by relaxation processes with three characteristic times 7 70 and more than 400 days we assume that the long relaxation time corresponds to viscoelastic relaxation and the intermediate relaxation time corresponds to afterslip on the main fault the short relaxation time may represent a combination of rapid afterslip poroelastic adjustment in the upper crust or other processes during the first few months that followed the earthquake afterslip likely released a significant amount of slip deficit still present following the coseismic rupture in particular updip of the rupture afterslip seems to be bounded updip by regions affected by slow slip events prior to the earthquake suggesting that the two processes are influenced by different frictional properties

recurrent large scale landsliding on the west flank of la palma canary islands a large area of debris avalanche deposits has been discovered on the western submarine flanks of the island of la palma multibeam bathymetry and its derivative backscatter data towed ocean bottom instrument tobi sidescan sonar images and 3 5 khz and airgun seismic reflection data have been used to identify at least two and possibly as many as four major landslide events the youngest of the events the cumbre nueva debris avalanche extends onshore into the valleys bounded by the caldera de taburiente and cumbre nueva ridge which mark the degraded collapse scars radiometric dating of the volcanic flows in the headwall indicate an age of between 536 and 125 ka for this landslide the debris avalanche covers an area of 780 km2 has a maximum thickness of 500 m and has an estimated volume of 95 km3 older deposits collectively referred to as the playa de la veta debris avalanche complex are probably as the name indicates an amalgamation of at least two or three events rather than the result of a single catastrophic failure the playa de la veta debris avalanche complex is associated onshore with an unconformity dated as late matuyama 1 ma to 800 ka it covers an area of 1200 km2 has a maximum thickness of 1300 m and may represent a total volume of up to 650 km3 the greater thicknesses and limited areas occupied by debris avalanches on the western flank of la palma compared to other landslides in the canary archipelago suggest that the la palma landslide masses have relatively low mobility the different debris avalanche lobes formed by each landslide event are separated by channels 2 2 5 km wide the clear relationship between channel position and the boundaries of each debris avalanche lobe indicates that debris avalanches control later channel formation and pathways the relief of the submarine flanks of the la palma volcanoes in the areas of island slope unaffected by landslides is mainly the result of constructional volcanic processes however the older submarine slopes such as in the northern taburiente volcano may also have been modified by smaller scale submarine mass wasting and sediment flows

the relationship between the north atlantic jet and tropical convection over the indian and western pacific oceans abstractboreal winter jet variability over the north atlantic is investigated using 40 yr european centre for medium range weather forecasts re analysis era 40 data where the variability is defined by the first eof of the zonal wind on seven vertical levels the principal component time series of this eof is referred to as the jet index a pattern correlation analysis indicates that the jet index more accurately describes intraseasonal north atlantic zonal wind variability than does the north atlantic oscillation nao a series of composite calculations of the jet index based on events of intraseasonal convective precipitation over the tropical indian and western pacific oceans reveals the following statistically significant relationships 1 negative jet events lead enhanced indian ocean precipitation 2 positive jet events lag enhanced indian ocean precipitation 3 positive jet events lead enhanced western pacific ocean precipitation and 4 negative jet events lag enhanced western pacific ocean p

frictional afterslip following the 2005 nias simeulue earthquake sumatra continuously recording global positioning system stations near the 28 march 2005 rupture of the sunda megathrust moment magnitude m w 8 7 show that the earthquake triggered aseismic frictional afterslip on the subduction megathrust with a major fraction of this slip in the up dip direction from the main rupture eleven months after the main shock afterslip continues at rates several times the average interseismic rate resulting in deformation equivalent to at least a m w 8 2 earthquake in general along strike variations in frictional behavior appear to persist over multiple earthquake cycles aftershocks cluster along the boundary between the region of coseismic slip and the up dip creeping zone we observe that the cumulative number of aftershocks increases linearly with postseismic displacements this finding suggests that the temporal evolution of aftershocks is governed by afterslip

in the shadow of 1857 the effect of the great ft tejon earthquake on subsequent earthquakes in southern california the great 1857 fort tejon earthquake is the largest earthquake to have hit southern california during the historic period we investigated if seismicity patterns following 1857 could be due to static stress changes generated by the 1857 earthquake when post 1857 earthquakes with unknown focal mechanisms were assigned strike slip mechanisms with strike and rake determined by the nearest active fault 13 of the 13 southern california m 5 5 earthquakes between 1857 and 1907 were encouraged by the 1857 rupture when post 1857 earthquakes in the transverse ranges with unknown focal mechanisms were assigned reverse mechanisms and all other events were assumed strike slip 11 of the 13 earthquakes were encouraged by the 1857 earthquake these results show significant correlations between static stress changes and seismicity patterns the correlation disappears around 1907 suggesting that tectonic loading began to overwhelm the effect of the 1857 earthquake early in the 20th century

earthquake rupture dependence on hypocentral location along the nicoya peninsula subduction megathrust abstract although interseismic locking distributions have been used in qualitatively evaluating the future earthquake potential quantitatively estimating how an earthquake may rupture through the locked interface is a more useful tool for quantifying both seismic and tsunami hazards here we investigate rupture scenarios from interseismic locking models along the megathrust interface below nicoya peninsula costa rica using spontaneous rupture simulations we first estimate initial stress from locking then initiate spontaneous ruptures at different nucleation points and observe the eventual earthquake magnitudes and slip distribution we find that 40 of nucleations tested develop into large earthquakes of m w 7 2 based on present interseismic locking models of these events those nucleated from deeper depths have a tendency for larger amplitude shallow slip suggesting increased tsunami potential furthermore irrespective of the input locking models we do not observe rupture scenarios of earthquakes with intermediate magnitudes between 6 and 7 a result consistent with observations in nicoya the results of hypocentre dependent earthquake magnitudes and tsunamigenic potential not only pose challenges in estimating rupture extents from locking models but also underscore the significance of quantitatively evaluating seismic and tsunami hazard in subduction zones

the ocean atmosphere response to wind induced thermocline changes in the tropical south western indian ocean in the indian ocean basin the sea surface temperatures ssts are most sensitive to changes in the oceanic depth of the thermocline in the region of the seychelles dome observational studies have suggested that the strong sst variations in this region influence the atmospheric evolution around the basin while its impact could extend far into the pacific and the extra tropics here we study the adjustments of the coupled atmosphere ocean system to a winter shallow doming event using dedicated ensemble simulations with the state of the art ec earth climate model the doming creates an equatorial kelvin wave and a pair of westward moving rossby waves leading to higher sst 1 2 months later in the western equatorial indian ocean atmospheric convection is strengthened and the walker circulation responds with reduced convection over indonesia and cooling of the sst in that region the pacific warm pool convection shifts eastward and an oceanic kelvin wave is triggered at thermocline depth the wave leads to an sst warming in the east equatorial pacific 5 6 months after the initiation of the seychelles dome event the atmosphere responds to this warming with weak anomalous atmospheric convection the changes in the upper tropospheric divergence in this sequence of events create large scale rossby waves that propagate away from the tropics along the atmospheric waveguides we suggest to repeat these types of experiments with other models to test the robustness of the results we also suggest to create the doming event in june so that the east pacific warming occurs in november when the atmosphere is most sensitive to sst anomalies and el nino could possibly be triggered by the doming event under suitable conditions

global seismicity of 2003 centroid moment tensor solutions for 1087 earthquakes abstract centroid moment tensor cmt solutions are presented for 1087 earthquakes that occurred during 2003 the solutions are obtained using the method of dziewonski et al dziewonski a m chou t a woodward j h 1981 determination of earthquake source parameters from waveform data for studies of global and regional seismicity j geophys res 86 2825 2852 and applying corrections for aspherical earth structure as represented by the whole mantle shear velocity model sh8 u4l8 of dziewonski and woodward dziewonski a m woodward r l 1992 acoustic imaging at the planetary scale in emert h harjes h p eds acoustical imaging vol 19 plenum press new york pp 785 797 the model of inelastic attenuation of durek and ekstrom durek j j ekstrom g 1996 a radial model of inelasticity consistent with long period surface wave attenuation bull seism soc am 86 144 158 is used to predict the decay of the waveforms the focal mechanisms of the largest or otherwise significant earthquakes of 2003 are reviewed

a dynamical ocean feedback mechanism for the madden julian oscillation composite analysis is applied to study the dynamical ocean response to madden julian mj events measured by anomalies in sea surface height from the merged topex poseidon european remote sensing satellite altimetry dataset in each of the tropical ocean basins significant equatorial waves are forced which are shown to modulate the sea surface temperature sst by 0 2 0 3 degc in the absence of strong surface heat fluxes in the indian ocean there is a clear dynamical response which may play a significant role in generating later mj events surface westerly winds associated with the active phase of the madden julian oscillation mjo force an eastward propagating oceanic downwelling equatorial kelvin wave which on reaching the eastern boundary at sumatra forces reflected downwelling equatorial rossby waves and coastal kelvin waves the coastal kelvin waves propagate southwards towards northern australia and northwards into the bay of bengal and will be important for local physical chemical and biological processes the equatorial rossby waves propagate westward across the indian ocean arriving in the western indian ocean approximately 80 100 days after the initial kelvin wave was generated the arrival of these waves generates positive sst anomalies which leads to convection and may trigger the next but one mj event or amplify the low frequency tail of the mjo this constitutes a coupled feedback mechanism from the ocean dynamics onto the mjo somewhat similar to the delayed oscillator mechanism for the el nino southern oscillation

spring arctic oscillation east asian summer monsoon connection through circulation changes over the western north pacific in the present study the links between spring arctic oscillation ao and east asian summer monsoon easm was investigated with focus on the importance of the north pacific atmospheric circulation and sea surface temperature sst to reduce the statistical uncertainty we analyzed high pass filtered data with the inter annual time scales and excluded the el nino southern oscillation signals in the climate fields using a linear fitting method the significant relationship between spring ao and easm are supported by the changes of multi monsoon components including monsoon indices precipitation and three dimensional atmospheric circulations following a stronger positive spring ao an anomalous cyclonic circulation at 850 hpa appears in southeastern asia and the western north pacific in summer with the easterly anomalies spanning from the pacific to asian continent along 25 n 30 n and the westerly anomalies south of 15 n at the same time the summer western north pacific subtropical high becomes weaker consistently the positive precipitation anomalies are developed over a broad region south of 30 n stretching from southern china to the western pacific and the negative precipitation anomalies appear in the lower valley of the yangtze river and southern japan the anomalous cyclone in the western north pacific persisting from spring to summer plays a key role in modulating easm and monsoon precipitation by a positive air sea feedback mechanism during spring the ao associated atmospheric circulation change produces warmer ssts between 150 e 180 near the equator the anomalous sensible and latent heating in turn intensifies the cyclone through a gill type response of the atmosphere through this positive feedback the tropical atmosphere and sst patterns sustain their strength from spring to summer that consequently modifies the monsoon trough and the western north pacific subtropical high and eventually the easm precipitation moreover the sst response to ao circulation is supported by the numerical simulations of an ocean model and the anomalous atmospheric circulation over the western north pacific is also reproduced by the dedicated numerical simulations using the coupled atmosphere ocean model the observation evidence and numerical simulations suggest the spring ao can impact the easm via triggering tropical air sea feedback over the western north pacific

revised tectonic boundaries in the cocos plate off costa rica implications for the segmentation of the convergent margin and for plate tectonic models the oceanic cocos plate subducting beneath costa rica has a complex plate tectonic history resulting in segmentation new lines of magnetic data clearly define tectonic boundaries which separate lithosphere formed at the east pacific rise from lithosphere formed at the cocos nazca spreading center they also define two early phase cocos nazca spreading regimes and a major propagator in addition to these sharply defined tectonic boundaries are overprinted boundaries from volcanism during passage of cocos plate over the galapagos hot spot the subducted segment boundaries correspond with distinct changes in upper plate tectonic structure and features of the subducted slab newly identified seafloor spreading anomalies show oceanic lithosphere formed during initial breakup of the farallon plate at 22 7 ma and opening of the cocos nazca spreading center a revised regional compilation of magnetic anomalies allows refinement of plate tectonic models for the early history of the cocos nazca spreading center at 19 5 ma a major ridge jump reshaped its geometry and after 14 5 ma multiple southward ridge jumps led to a highly asymmetric accretion of lithosphere a suspected cause of ridge jumps is an interaction of the cocos nazca spreading center with the galapagos hot spot

do subducting seamounts generate or stop large earthquakes seamount subduction is a common process in subduction zone tectonics contradicting a widely held expectation that subducting seamounts generate large earthquakes seamounts subduct largely aseismically producing numerous small earthquakes on rare occasions when they do produce relatively large events the ruptures tend to be complex suggesting multiple rupture patches or faults we explain that the seismogenic behavior of these seamounts is controlled by the development and evolution of an adjacent fracture network during subduction and cannot be described using the frictional behavior of a single fault the complex structure and heterogeneous stresses of this network provide a favorable condition for aseismic creep and small earthquakes but an unfavorable condition for the generation and propagation of large ruptures

an observed connection between the north atlantic oscillation and the madden julian oscillation abstract based on the bivariate madden julian oscillation mjo index defined by wheeler and hendon and 25 yr 1979 2004 of pentad data the association between the north atlantic oscillation nao and the mjo on the intraseasonal time scale during the northern hemisphere winter season is analyzed time lagged composites and probability analysis of the nao index for different phases of the mjo reveal a statistically significant two way connection between the nao and the tropical convection of the mjo a significant increase of the nao amplitude happens about 5 15 days after the mjo related convection anomaly reaches the tropical indian ocean and western pacific region the development of the nao is associated with a rossby wave train in the upstream pacific and north american region in the atlantic and african sector there is an extratropical influence on the tropical intraseasonal variability certain phases of the mjo are preceded by the occurrence of strong naos a significant change of upper zonal

seismogenic zone structure beneath the nicoya peninsula costa rica from three dimensional local earthquake p and s wave tomography the subduction plate interface along the nicoya peninsula costa rica generates damaging large mw 7 5 earthquakes we present hypocenters and 3 d seismic velocity models vp and vp vs calculated using simultaneous inversion of p and s wave arrival time data recorded from small magnitude local earthquakes to elucidate seismogenic zone structure in this region interseismic cycle microseismicity does not uniquely define the potential rupture extent of large earthquakes plate interface microseismicity extends from 12 to 26 and from 17 to 28 km below sea level beneath the southern and northern nicoya peninsula respectively microseismicity offset across the plate suture of east pacific rise derived and cocos nazca spreading center derived oceanic lithosphere is 5 km revising earlier estimates suggesting 10 km of offset interplate seismicity begins downdip of increased locking along the plate interface imaged using gps and a region of low vp along the plate interface the downdip edge of plate interface microseismicity occurs updip of the oceanic slab and continental moho intersection possibly due to the onset of ductile behaviour slow forearc mantle wedge p wave velocities suggest 20 30 per cent serpentinization across the nicoya peninsula region while calculated vp vs values suggest 0 10 per cent serpentinization interpretation of vp vs resolution at depth is complicated however due to ray path distribution we posit that the forearc mantle wedge is regionally serpentinized but may still be able to sustain rupture during the largest seismogenic zone earthquakes

structural asperity focusing locking and earthquake slip along the nicoya megathrust costa rica on 5 september 2012 a moment magnitude 7 6 earthquake occurred along the locked megathrust interface directly beneath the nicoya peninsula in costa rica the event was anticipated given the history of frequent large earthquakes and recent gps results strain accumulation observed before and coseismic slip observed during the earthquake create a novel and important data set offering a unique look at the strain and locking behavior around a large megathrust environment given this geodetic data and a newly developed regional 3 d subduction interface we develop a regionally appropriate finite element model to evaluate the role of interface topography in generating interseismic locking and ultimately coseismic slip following the 2012 earthquake we find that the interface is most strongly coupled in a patch immediately beneath the central portion of nicoya in an area where a topographic high in the downgoing cocos plate is resisting further subduction this zone is the dominant slip environment in the 2012 earthquake reevaluation of the interseismic locking over 62 years before the most recent event finds that the total accumulated moment potential m0 3 48 1020 n m well matches the total moment release in the 2012 rupture found here m0 3 73 1020 n m mw 7 68 with rupture extending to the se outside of the strongly locked region possibly as early afterslip in contrast to a prior study little locking is required immediately offshore nicoya a conclusion important for understanding the rupture area of the 2012 event regional tsunami potential and relation with regionally observed shallow slow slip events

observations of static coulomb stress triggering of the november 2011 m5 7 oklahoma earthquake sequence in november 2011 a m5 0 earthquake occurred less than a day before a m5 7 earthquake near prague oklahoma which may have promoted failure of the mainshock and thousands of aftershocks along the wilzetta fault including a m5 0 aftershock the m5 0 foreshock occurred in close proximity to active fluid injection wells fluid injection can cause a buildup of pore fluid pressure decrease the fault strength and may induce earthquakes keranen et al 2013 links the m5 0 foreshock with fluid injection but the relationship between the foreshock and successive events has not been investigated here we examine the role of coseismic coulomb stress transfer on earthquakes that follow the m5 0 foreshock including the m5 7 mainshock we resolve the static coulomb stress change onto the focal mechanism nodal plane that is most consistent with the rupture geometry of the three m 5 0 earthquakes as well as specified receiver fault planes that reflect the regional stress orientation we find that coulomb stress is increased e g fault failure is promoted on the nodal planes of 60 of the events that have focal mechanism solutions and more specifically that the m5 0 foreshock promoted failure on the rupture plane of the m5 7 mainshock we test our results over a range of effective coefficient of friction values hence we argue that the m5 0 foreshock induced by fluid injection potentially triggered a cascading failure of earthquakes along the complex wilzetta fault system

potential predictability of the sea surface temperature forced equatorial east african short rains interannual variability in the 20th century in this article the predictability of the 20th century sea surface temperature sst forced east african short rains variability is analyzed using observational data and ensembles of long atmospheric general circulation model agcm simulations to our knowledge such an analysis for the whole 20th century using a series of agcm ensemble simulations is carried out here for the first time the physical mechanisms that govern the influence of sst on east african short rains in the model are also investigated it is found that there is substantial skill in reproducing the east african short rains variability given that the ssts are known consistent with previous recent studies it is found that the indian ocean and in particular the western pole of the indian ocean dipole iod play a dominant role for the prediction skill whereas ssts outside the indian ocean play a minor role the physical mechanism for the influence of the western indian ocean on east african rainfall in the model is consistent with previous findings and consists of a gill type response to a warm cold anomaly that induces a westerly easterly low level flow anomaly over equatorial africa and leads to moisture flux convergence divergence over east africa on the other hand a positive el nino southern oscillation enso anomaly leads to a spatially non coherent reducing effect over parts of east africa but the relationship is not strong enough to provide any predictive skill in our model the east african short rains prediction skill is also analyzed within a model derived potential predictability framework and it is shown that the actual prediction skill is broadly consistent with the model potential prediction skill low frequency variations of the prediction skill are mostly related to ssts outside the indian ocean region and are likely due to an increased interference of enso with the indian ocean influence on east african short rains after the mid 1970s climate shift

aftershocks of the 2012 mw 7 6 nicoya costa rica earthquake and mechanics of the plate interface abstract subduction of the cocos plate beneath the nicoya peninsula costa rica generates large underthrusting earthquakes with a recurrence interval of about 50 yrs the most recent of these events occurred on 5 september 2012 m w 7 6 a vigorous sequence of more than 6400 aftershocks was recorded by a local seismic network within the first four months after the mainshock we determine locations and focal mechanisms for as many aftershocks as possible with m 1 5 occurring within the first nine days of the mainshock all aftershocks with m 3 through the end of 2012 and all events with m 4 through the end of 2015 we determine faulting geometries using regional full waveform moment tensor mt inversion for the largest events m 4 and p wave first motion polarities for smaller events producing a mechanism catalog with 347 earthquakes sixty percent of these events are identified as underthrusting and their locations are compared with spatial distributions of mainshock slip afterslip prior interplate seismicity and slow slip phenomena to better understand the mechanical behavior of the plate interface most of the aftershocks on the megathrust occur up dip of the coseismic slip where afterslip is large and between coseismic slip and shallow slow slip patches the pattern of interplate seismicity during the interseismic period is similar to that for the aftershocks but does not extend to as great a depth the coseismic slip extends even deeper than the interplate aftershocks suggesting that the mainshock ruptured a strongly locked patch driving down dip slip into the conditionally stable part of the deep plate interface that also hosts slow slip about 80 of the aftershocks have one nodal plane oriented favorably to promote failure from static stress changes following the mainshock and early afterslip whereas most others occur in regions of large afterslip electronic supplement tables of hypocenter location and focal mechanisms and selection criteria for underthrusting events and figures showing the gutenberg richter distribution regional moment tensor inversion spatial distribution of earthquake activity and temporal distribution of underthrusting events

enigmatic upper plate sliver transport paused by megathrust earthquake and afterslip abstract how does deformation after an earthquake affect megathrust stresses five years of surface velocities following the 2012 moment magnitude 7 6 nicoya costa rica earthquake uniquely capture the lithospheric recovery during a four year period seaward afterslip transitions to relocked landward interseismic motion that matches the velocity field seen before the 2012 event locking reinitiates temporarily but is interrupted by late 2014 and is followed soon by a period of never before described exclusively trench parallel motion associated with a slow slip event and no resolvable megathrust locking we present a conceptual model in which low postseismic megathrust coupling little locking generates partitioned slip trench normal motion on the megathrust during afterslip and trench parallel motion during this never before seen transient high coupling strong locking during the interseismic period drives oblique convergent surface motions this challenges the paradigm that megathrusts are either always partitioned or always oblique contradicts the tectonic escape hypothesis in central american and introduces a new time dependent megathrust fault behavior given that most subduction zones are oblique explaining these observations is critical to characterizing stress accumulation

the geometry of the wadati benioff zone under southern central america and its tectonic significance results from a high resolution local seismographic network abstract we present here a detailed geometry of the wadati benioff zone under costa rica obtained from seismicity recorded by a dense local seismographic network jointly operated by the costa rica volcanological and seismological observatory national university and the charles f richter seismological laboratory university of california santa cruz underneath the nicaragua costa rica border the wadati benioff zone smoothly contorts from steep to shallow dip angles nw to se but does not show evidence of a brittle tear as postulated by others however further to the se ne of puerto caldera costa rica the wadati benioff zone does show a segmentation the quesada sharp contortion at intermediate depths h 70km nw of this sharp contortion the deepest portion of the seismically active slab dips at about 80 and reaches maximum depths ranging from 200 km near the nicaragua costa rica border to 135 km under ciudad quesada to the se the deeper portion of the wadati benioff zone dips at about 60 and the seismicity does not extend below depths ranging from 125 km behind the volcanic arc to 50 km east of quepos in southern costa rica east of 83 55 w we find no evidence of the wadati benioff zone deeper than 50 km the obtained geometry and other known tectonic features related to the subduction of the cocos plate under the caribbean plate along the southern terminus of the middle america trench nicaragua and costa rica correlate well with along trench variations in age of the subducted cocos plate some of these tectonic features are 1 the shallowing of middle america trench bathymetry from nw to se 2 variations in the energy release within the subducted slab 3 differences in coupling between cocos and caribbean plates 4 the termination of the central america volcanic chain in central costa rica 5 distinct stress field variations on the overriding caribbean plate the subduction of the cocos ridge under southern costa rica is partially responsible for some of these features shallowing of the middle america trench in southern costa rica and for the high uplift rates of the outer arc however as the presence of the panama fracture zone limits the subducted extension of the cocos ridge to less than 100 km from the trench we propose that the overall geometry of the wadati benioff zone is controlled by abrupt along trench changes in the age of the subducted cocos plate

far field triggering of foreshocks near the nucleation zone of the 5 september 2012 m w 7 6 nicoya peninsula costa rica earthquake abstract on 5 september 2012 a moment magnitude mw 7 6 earthquake occurred directly beneath the nicoya peninsula an area with dense seismic and geodetic network coverage the mainshock ruptured a portion of a previously identified locked patch that was recognized due to a decade long effort to delineate the megathrust seismic and aseismic processes in this area here we conduct a comprehensive study of the seismicity prior to this event utilizing a matched filter analysis that allows us to decrease the magnitude of catalog completeness by 1 unit we observe a statistically significant increase in seismicity rate below the nicoya peninsula following the 27 august 2012 mw 7 3 el salvador earthquake about 450 km to the northwest and 9 days prior to the nicoya earthquake additionally we identify a cluster of small magnitude

diagnosing kenya rainfall in boreal autumn further exploration abstracta powerful zonal vertical circulation cell along the indian ocean equator controls the boreal autumn rains in kenya with a tight negative correlation between surface westerlies ueq and rainfall ueq is favored by a steep eastward pressure gradient pwe and slow winds in the downstream portion of the south indian ocean trade winds siw the high phase of the southern oscillation so favors weak siw lower pressure in the east and thus steeper pwe but that is also affected by pressure in the west in 1958 97 circulation causalities were most distinct in the regime of abundant rain and slow ueq with the so in the low phase in the regime of deficient rain and fast ueq relationships were less distinct although the so was in the high phase almost throughout in the decade 2001 10 ueq had a weaker stronger correlation with pwe siw the so was more active and contributing in the proper sense to pressure in the east but pwe was mainly controlled by pressure in the west ueq strongly contr

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

corrugated megathrust revealed offshore from costa rica exhumed faults are rough often exhibiting topographic corrugations oriented in the direction of slip such features are fundamental to mechanical processes that drive earthquakes and fault evolution however our understanding of corrugation genesis remains limited due to a lack of in situ observations at depth especially at subducting plate boundaries here we present three dimensional seismic reflection data of the costa rica subduction zone that image a shallow megathrust fault characterized by corrugated and chaotic and weakly corrugated topographies the corrugated surfaces extend from near the trench to several kilometres down dip exhibit high reflection amplitudes consistent with high fluid content pressure and trend 11 18 oblique to subduction suggesting 15 to 25 mm yr 1 of trench parallel slip partitioning across the plate boundary the corrugations form along portions of the megathrust with greater cumulative slip and may act as fluid conduits in contrast weakly corrugated areas occur adjacent to active plate bending faults where the megathrust has migrated up section forming a nascent fault surface the variations in megathrust roughness imaged here suggest that abandonment and then reestablishment of the megathrust up section transiently increases fault roughness analogous corrugations may exist along significant portions of subduction megathrusts globally mature parts of the shallow megathrust beneath costa rica are characterized by striking corrugations that may channel fluids according to seismic images nascent sections of the subduction zone plate boundary appear only weakly corrugated