earthquake generation in different stress states abstract a method of numerical simulation of an earthquake sequence is presented based on the stress evolution equations and the new rebound theory a zone of earthquake preparation is modelled by a medium filled with parallel antiplane cracks some statistical properties of the numerically simulated sequences are discussed it follows that a complete representation of a cumulative energy diagram requires that energies released in the creep events should also be included

mechanisms by which gulf of guinea and eastern north atlantic sea surface temperature anomalies can influence african rainfall abstract the sensitivity of precipitation over west africa to sea surface temperature anomalies sstas in the gulf of guinea and the eastern north atlantic is studied using a gcm results from nine perpetual july simulations with various imposed sstas are presented and analyzed to reveal associations between the precipitation and sst fields via large scale circulation and atmospheric moisture anomalies rainfall increases over the guinean coast and decreases over the congo basin when warm sstas are present in the gulf of guinea these precipitation perturbations are related to the forcing of a kelvin and a rossby wave the former is associated with a weakening of the walker circulation while the latter strengthens the west african monsoon rainfall over west africa is less sensitive to cold sstas than to warm anomalies three contributing factors are identified as follows 1 latitude of the sst forcing 2 background flow and 3 nonlinearity of the clausius clapeyron equation no more than a 20 effec

structure and dynamics of the benguela low level coastal jet generations of coupled atmosphere ocean general circulation models have been plagued by persistent warm sea surface temperature sst biases in the southeastern tropical atlantic the sst biases are most severe in the eastern boundary coastal upwelling region and are sensitive to surface wind stress and wind stress curl associated with the benguela low level coastal jet bllcj a southerly jet parallel to the angola namibia coast however little has been documented about this atmospheric source of oceanic bias here we investigate the characteristics and dynamics of the bllcj using observations reanalyses and atmospheric model simulations satellite wind products and high resolution reanalyses and models represent the bllcj with two near shore maxima one near the angola benguela front abf at 17 5 s and the other near 25 27 5 s whereas coarse resolution reanalyses and models represent the bllcj poorly with a single broad more offshore maximum model experiments indicate that convex coastal geometry near the abf supports the preferred location of the bllcj northern maximum by supporting conditions for a hydraulic expansion fan intraseasonal variability of the bllcj is associated with large scale variability in intensity and location of the south atlantic subtropical high through modulation of the low level zonal pressure gradient

effects of wind stress and wind stress curl variability on coastal upwelling abstract aircraft measurements during the winter 1989 shelf mixed layer experiment smile and summer 1982 coastal ocean dynamics experiment code were used to characterize the spatial variation of the low level wind and wind stress over the northern california shelf the curl of the wind stress was calculated from directly measured turbulent stress components the accuracy of the computed curl was estimated to be adequate to map the spatial structure wintertime measurements showed a concentration of large positive curl over 1 pa 100 km 1 west of point arena regardless of wind direction due to the effects of the coastal topography on the wind fields results from summertime measurements showed a similar local maximum of positive curl west of point arena larger curl values over 3 5 pa 100 km 1 however were observed across a hydraulic jump propagating from stewarts point for highly supercritical marine boundary layer flow a two layer vertically integrated numerical model of coastal upwellin

subtropical anticyclones and summer monsoons the summer subtropical circulation in the lower troposphere is characterized by continental monsoon rains and anticyclones over the oceans in winter the subtropical circulation is more strongly dominated by the zonally averaged flow and its interactions with orography here the mechanics of the summer and winter lowertropospheric subtropical circulation are explored through the use of a primitive equation model and comparison with observations by prescribing in the model the heatings associated with several of the world s monsoons it is confirmed that the equatorward portion of each subtropical anticyclone may be viewed as the kelvin wave response to the monsoon heating over the continent to the west a poleward flowing low level jet into a monsoon such as the great plains jet is required for sverdrup vorticity balance this jet effectively closes off the subtropical anticyclone to the east and also transports moisture into the monsoon region the low level jet into north america induced by its monsoon heating is augmented by a remote response to the asian monsoon heating the rossby wave response to the west of subtropical monsoon heating interacting with the midlatitude westerlies produces a region of adiabatic descent it is demonstrated here that a local diabatic enhancement can lead to a strengthening of the descent longitudinal mountain chains act to block the westerly flow and also tend to produce descent in this region below the descent sverdrup vorticity balance implies equatorward flow that closes off the subtropical anticyclone to the west and induces cool upwelling in the ocean through ekman transport feedbacks involving for example sea surface temperatures may further enhance the descent in these regions the conclusion is that the mediterranean type climates of regions such as california and chile may be induced remotely by the monsoon to the east hence it can be argued that the subtropical circulation in summer comprises a set of weakly interacting monsoon systems each involving monsoon rains a low level poleward jet a subtropical anticyclone to the east and descent and equatorward flow to the west in winter it is demonstrated how the nonlinear interaction between the strong zonal mean circulation associated with the winter hadley cell and the mountains can define many of the large scale features of the subtropical circulation the blocking effect of the longitudinal mountain chains is shown to be very important subsequent diabatic effects such as a local diabatic enhancement would appear to be essential for producing the observed amplitude of these features

on the connection between benguela and equatorial atlantic ninos and the role of the south atlantic anticyclone for the eastern tropical atlantic two recurring el nino like phenomena with high interannual sst anomalies have been described one centered in the equatorial region as part of the atlantic zonal mode and one off angola referred to as benguela nino both events are supposed to be generated not locally but by a relaxation of the trade winds in the western equatorial atlantic here the connection between sst variations in the two regions is investigated with observational data sets as well as ocean model simulations they are correlated to such an extent that joint events should rather be viewed as one atlantic nino an intriguing feature counterintuitive in view of the remote forcing mechanism is that sst anomalies off angola precede those in the equatorial atlantic we show this behavior to be related to the difference in thermocline depths and a different seasonality of interannual sst variability in the two regions while benguela ninos peak in austral fall due to the angola benguela front being located furthest to the south and high interannual variability in coastal kelvin wave activity warm events at the equator are phase locked to austral winter when the thermocline is shallow perturbation experiments confirm the importance of remote forcing from the equator for sst variability off angola and demonstrate the leading role of wind stress in the generation of sst anomalies in the eastern tropical atlantic these wind stress variations are shown to be linked to fluctuations in the strength of the south atlantic anticyclone a connection that might be important with respect to the predictability of atlantic ninos

coupling and decoupling of crust and mantle in convergent orogens implications for strain partitioning in the crust topographic relief strain partitioning and crust mantle coupling within convergent orogens may be systematically related to the distribution of strength as a function of depth within continental crust this result is obtained by solving analytically for basally driven three dimensional flow within an idealized crust with a laterally invariant depth dependent newtonian viscosity two end member flow states are shown to occur in the first crustal flow is directly coupled to the underlying mantle and is controlled by stresses transmitted vertically through the crust in the second upper crustal to midcrustal flow is decoupled from the underlying mantle and is controlled by stresses transmitted horizontally through the crust this model is applied to deformation of continental crust above plate like mantle that undergoes oblique convergence across a narrow suture if no significant low viscosity zone develops within the crust crustal deformation is coupled to motion of the underlying mantle and a triangular mountain range develops surface shortening occurs at the foot of the mountains normal to the trend of the range while strike slip and extension are partitioned along the axis of the orogen if a low viscosity zone is initially absent but develops during crustal thickening a steep sided flat topped plateau forms crustal deformation is coupled to mantle motions on the plateau flanks but may be strongly decoupled beneath the plateau surface shortening normal to the plateau occurs at the foot of the plateau while surface extension normal to the plateau occurs at the plateau edge if a low viscosity zone is present in the lower crust prior to convergence a wide orogen with low topographic relief develops crustal deformation is decoupled from the mantle except at the edges of the deforming region and surface shortening is partitioned uniformly across the belt this study shows that horizontal and vertical partitioning of strain occur even in the absence of lateral strength variations and that shortening and extension may exist simultaneously in different parts of an orogen similar morphology and strain partitioning can be observed in many real orogenic belts including the eastern alps the tibetan plateau and the lachlan tasman fold belt

crustal rheology and faulting at strike slip plate boundaries 1 an analytic model this paper investigates how rheologic stratification within the crust affects the formation and long term evolution of fault systems at a strike slip plate boundary we present an analytic model of deformation at a strike slip plate margin within a two layer viscoelastic crust with fixed shear modulus but varying viscosity in each layer faulting is represented by static elastic dislocations of fixed shallow depth extent imposed at a high critical stress threshold for fracture of a new fault and a lower one for sliding on a preexisting fault to drive crustal deformation we impose basal velocity boundary conditions at the moho representing a narrow zone of high shear in the mantle in this study we restrict attention to deformation at the surface where simple analytic solutions exist for velocities and stresses our results suggest that when a primarily elastic brittle upper crust is underlain by a low viscosity lower crust the deformation zone in the upper crust widens in time at steady state the surface width of the deformation zone may be significantly greater than the prescribed narrow mantle shear zone the long term width of the deformation zone increases with the viscosity contrast within the crust and with the thickness of the low viscosity lower crust widening of the deformation zone is accompanied by the fracture of new faults in the upper crust leading to the formation of a system of parallel strike slip faults surrounding the plate boundary for fixed plate velocity and fracture criteria we find that the width of the brittle fault network within the total distributed deformation zone is primarily governed by strength properties of the upper crust and by the viscosity contrast within the crust

the source of benguela ninos in the south atlantic ocean 1 the intermittent occurrence of anomalous warm events in the upwelling regions of the pacific el ninos has been intensively studied pacific ninos have striking effects on the local ecosystem hence on the fisheries and on rainfall similar dramatic events have been observed in the south atlantic off the coasts of angola and namibia and named benguela ninos they tend not to occur in unison with their pacific counterpart and may thus have unrelated forcing mechanisms using an ocean general circulation model forced by real winds and verified with satellite data it is shown that benguela ninos are generated by specific wind stress events in the west central equatorial atlantic and progress from there as subsurface temperature anomalies that eventually outcropped only at the south west african coast these results suggest that it now may be possible to predict the occurrence of these disruptive events with a lead time of 2 months

on the dynamics of flow past a cylinder implications for ctd package motions and measurements during the collection of ship based observations heaving of the vessel may lead to variation in the descent rate of a ctd package this can result in the package being pulled upwards through previously sampled water leading to difficult to quantify errors due to the complex wake to reduce this problem to one of manageable stature we use the simple paradigm of two dimensional flow past a cylinder by using a tracer with a gradient along the flow we quantify the effect of the cylinder on its distribution and the impact of post processing at high reynolds numbers over 200 uniform translation leads to a small error in the tracer value this error is likely negligible at the much higher reynolds number of the ocean when the flow is oscillated longitudinally there are two main sources of error attached vortices may propagate around the cylinder and or shed vortices may translate into the path of the cylinder post processing by removing records from previous pressure levels removes much of the first error due to it occurring as the package ascends however the second source of error is more difficult to remove due to it occurring when the package is once again descending in general results indicate that long period oscillations are preferable whilst the magnitude of the errors are comparable to those from short period oscillations they are spread farther apart in time and space and the overall effect is to localise the errors in small regions of the final depth profile

evolution of fault systems at a strike slip plate boundary a viscoelastic model a viscoelastic model of crustal deformation suggests that the formation and evolution of strike slip fault systems are strongly influenced by rheologic contrasts between the upper and lower crust when deformation is driven by a narrow zone of high shear in the mantle the presence of a low viscosity lower crustal layer underlying a primarily elastic upper crust widens the deformation zone with time and promotes the formation of a broadly distributed network of interacting faults within the upper crust in contrast the deformation zone in a primarily elastic crust is narrow encompassing a single plate bounding fault patterns of surface strain rate and seismicity are thus significantly more complex in the presence of a low viscosity lower crust due to interactions between faulting in the upper crust at short time scales and viscous behavior in the lower crust at long time scales

oceanic kelvin waves and tropical atlantic intraseasonal variability 1 kelvin wave characterization 1 oceanic kelvin waves from the equator to the west african coast are investigated in the framework of tropical atlantic intraseasonal variability in order to better highlight the wave propagations a 25 95 day band pass filter was applied to the sea surface height ssh product derived from the topex poseidon altimeter and an ocean general circulation model simulation for the 1993 2000 period in addition to equatorial eastward propagations our analysis reveals recurrent and continuous propagations distinguishable over thousands of kilometers poleward along the coasts as far as about 10 15 latitude a novel result with altimeter data the variance of the filtered ssh signal goes from 1 cm at the equator to 2 cm at the african coast estimates of the phase speed range from 1 5 to 2 1 m s along the equator and the west african coastline such values are very close to those of equatorial kelvin wave propagations likely dominated by the first two baroclinic modes supporting the fact that the coastal propagations are coastally trapped kelvin waves in order to simplify the description of these kelvin waves we present an intraseasonal climatology which reveals regular boreal autumn winter equator to coast propagations an improved description is achieved thanks to the computation of an extended empirical orthogonal function for the boreal autumn winter propagations lag correlation of ssh signals allows for a twofold quantification the phase speed and the importance of remote forcing along the coast the remote forcing effect of intraseasonal kelvin waves is clearly evidenced over coastal upwelling regions as far as 10 15 latitude the physical mechanism associated with the forcing of the kelvin waves and its impacts will be investigated in a future paper

the brittle plastic transition and the depth of seismic faulting a simple rheological model of shearing of the lithosphere that has gained wide acceptance is a two layer model with an upper brittle zone in which deformation takes place by frictional sliding on discrete fault surfaces and a lower plastic zone in which deformation takes place by bulk plastic flow the two are separated by an abrupt brittle plastic transition which is assumed to be indicated by the lower limit of seismicity experimental studies however as well as the deformation structures of mylonites indicate that a broad transitional field of semi brittle behavior lies between these extremes this is a field of mixed mode deformation with a strength that can be expected to be considerably higher than that predicted from the extrapolation of high temperature flow laws for quartzofeldspathic rocks the semi brittle field lies between t1 the onset of quartz plasticity at about 300 c and t2 feldspar plasticity at about 450 c a model is presented in which the transition t1 does not correspond to a transition to bulk flow but to a change from unstable velocity weakening friction to stable velocity strengthening friction t1 thus marks the depth limit of earthquake nucleation but large earthquakes can propagate to a greater depth t3 t3 t2 which corresponds to the lower limit of dynamic frictional behavior in the semi brittle field and approximately to the peak in strength the zone between t1 and t3 is one of alternating behavior with flow occurring m the interseismic period and with co seismic dynamic slip occurring during large earthquakes this zone is characterized by mylonites interlaced by pseudotachylytes and other signs of dynamic faulting the transition t1 is also marked by a change in the generation mechanism of fault rocks from abrasive wear above which produces cataclastites to adhesive wear below which is proposed as an important generation mechanism of mylonites in the upper part of the semi brittle field

on the origin of equatorial atlantic biases in coupled general circulation models many coupled ocean atmosphere general circulation models gcms suffer serious biases in the tropical atlantic including a southward shift of the intertropical convergence zone itcz in the annual mean a westerly bias in equatorial surface winds and a failure to reproduce the eastern equatorial cold tongue in boreal summer the present study examines an ensemble of coupled gcms and their uncoupled atmospheric component to identify common sources of error it is found that the westerly wind bias also exists in the atmospheric gcms forced with observed sea surface temperature but only in boreal spring during this time sea level pressure is anomalously high low in the western eastern equatorial atlantic which appears to be related to deficient excessive precipitation over tropical south america africa in coupled simulations this westerly bias leads to a deepening of the thermocline in the east which prevents the equatorial cold tongue from developing in boreal summer thus reducing atmospheric model errors during boreal spring may lead to improved coupled simulations of tropical atlantic climate

influence of asperities along subduction interfaces on the stressing and seismicity of adjacent areas abstract dmowska r and lovison l c 1992 influence of asperities along subduction interfaces on the stressing and seismicity of adjacent areas in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 23 43 we have investigated the influence of large scale fault inhomogeneities in large subduction earthquakes on the style of deformation and seismic behavior of the incoming oceanic plate and slab at intermediate depths during the earthquake cycle the zones of the large subduction events of rat islands 1965 alaska 1964 and valparaiso 1985 have been searched for earthquakes with m b 5 0 if available and for time periods as long as possible it has been found that in general the seismicity in the incoming oceanic plate clusters in front of asperities areas of highest seismic moment release and strongest locking and is positioned relative to them in the direction of plate motion it is usually lacking in areas adjacent to non asperities that is to zones that slip during the main event but with appreciably smaller seismic moment release and possibly slip seismically aseismically during the whole cycle similar behavior occurs in the downgoing slab at intermediate depths where seismicity during the cycle clusters but less strongly than in the oceanic crust next to asperities and down dip from them we infer that the locking of asperities causes higher stresses associated with the earthquake cycle itself to occur in areas adjacent to asperities both up dip and down dip from them along the direction of plate motion and that such stressing is much less pronounced in the areas adjacent to non asperities this opens the possibility of identifying the areas of highest seismic moment release in future subduction earthquakes and carries implications for where the highest deformation and possibly precursory phenomena and or nucleation of a future event might occur

changes in static stress on southern california faults after the 1992 landers earthquake the magnitude 7 5 landers earthquake of 28 june 1992 was the largest earthquake to strike california in 40 years the slip that occurs in such an earthquake would be expected to induce large changes in the static stress on neighbouring faults these changes in stress should in turn affect the likelihood of future earthquakes stress changes that load faults towards failure have been cited as the cause of small1 5 moderate6 and large7 earthquakes conversely those that relax neighbouring faults have been related to a decrease in seismicity5 here we use an elastic half space model8 to estimate the stress changes produced by the landers earthquake on selected southern california faults including the san andreas we find that the estimated stress changes are consistent with the triggering of four out of the five aftershocks with magnitude greater than 4 5 and that the largest changes 1 10 bar occurring on part of the san bernardino segment of the san andreas fault may have decreased the time to the next magnitude 8 earthquake by about 14 years

crustal rheology and faulting at strike slip plate boundaries 2 effects of lower crustal flow we present a numerical model of deformation at a strike slip plate boundary within a linear viscoelastic crust which is driven by far field plate motions and basal mantle velocities the crust is assumed to have uniform elastic properties but continuously varying viscosity as a function of depth brittle faulting is represented by static elastic dislocations that are imposed when stresses exceed a critical threshold for fracture or frictional sliding the locations and depth extents of faults in this model are not prespecified but instead are governed by stress evolution within the crust we find that when a primarily elastic upper crust is underlain by a low viscosity lower crustal layer the deformation zone broadens in time to encompass many parallel strike slip faults in an interacting network in contrast when the entire crust behaves elastically the deformation zone remains narrow and focused on a single plate bounding fault reflecting imposed mantle motions surface strain rate patterns within the interacting fault network are complex and reflect significant faulting related strain rate perturbations that decay over timescales of postseismic relaxation in the lower crust 10 100 years the fault network has a characteristic spacing with complex fault interactions and with the depth extents of faults increasing with time to a maximum depth governed by crustal rheology the maximum depth of faults is limited by stress relaxation and large scale viscous flow in the lower crust which confines brittle failure to shallow and midcrustal levels

dynamics of a zone of four parallel faults a deterministic model we consider a two dimensional model for quasi static evolution of four parallel faults the model is based on the physics of continuum with slips along the faults long range interactions a distribution of barriers and asperities a velocity dependent friction healing and reactivation of cohesive forces and a mutual causality between faulting and tectonic forces all taken into account both the long term time series of seismic activity and details of seismic and aseismic crustal deformations during each phase of the earthquake cycle are produced by the model it is shown that the system whose evolution is described by a set of deterministic differential equations may exhibit periodic or quite irregular despite its assumed symmetry behavior depending on model parameters

land atmosphere ocean interactions in the southeastern atlantic interannual variability land atmosphere ocean interactions in the southeastern south atlantic and their connections to interannual variability are examined using a regional climate model coupled with an intermediate level ocean model in austral summer zonal displacements of the south atlantic subtropical high sash can induce variations of mixed layer currents in the benguela upwelling region through surface wind stress curl anomalies near the namibian coast and an eastward shifted sash is related to the first pacific south american mode when the sash is meridionally displaced mixed layer vertically integrated ekman transport anomalies are mainly a response to the change of alongshore surface wind stress the latitudinal shift of the sash tends to dampen the anomalous alongshore wind by modulating the land sea thermal contrast while opposed by oceanic diffusion although the position of the sash is closely linked to the phase of el nino southern oscillation enso and the southern annular mode sam in austral summer an overall relationship between benguela upwelling strength and enso or sam is absent during austral winter variations of the mixed layer ekman transport in the benguela upwelling region are connected to the strength of the sash through its impact on both coastal wind stress curl and alongshore surface wind stress compared with austral summer low level cloud cover change plays a more important role although wintertime sea surface temperature fluctuations in the equatorial atlantic are strong and may act to influence variability over the northern benguela area the surface heat budget analysis suggests that local air sea interactions dominate

the south atlantic anticyclone as a key player for the representation of the tropical atlantic climate in coupled climate models the key role of the south atlantic anticyclone saa on the seasonal cycle of the tropical atlantic is investigated with a regionally coupled atmosphere ocean model for two different coupled domains both domains include the equatorial atlantic and a large portion of the northern tropical atlantic but one extends southward and the other northwestward the saa is simulated as internal model variability in the former and is prescribed as external forcing in the latter in the first case the model shows significant warm biases in sea surface temperature sst in the angola benguela front zone if the saa is externally prescribed these biases are substantially reduced the biases are both of oceanic and atmospheric origin and are influenced by ocean atmosphere interactions in coupled runs the strong sst austral summer biases are associated with a weaker saa which weakens the winds over the southeastern tropical atlantic deepens the thermocline and prevents the local coastal upwelling of colder water the biases in the basins interior in this season could be related to the advection and eddy transport of the coastal warm anomalies in winter the deeper thermocline and atmospheric fluxes are probably the main biases sources biases in incoming solar radiation and thus cloudiness seem to be a secondary effect only observed in austral winter we conclude that the external prescription of the saa south of 20 s improves the simulation of the seasonal cycle over the tropical atlantic revealing the fundamental role of this anticyclone in shaping the climate over this region

evolution of interannual warm and cold events in the southeast atlantic ocean extreme warm episodes in the southeast atlantic ocean known as benguela ninos have devastating envi ronmental impacts and have been shown to be remotely forced to place these extreme events into perspective the investigation is here extended to minor warm events as well as to cold episodes to this end different sets of observations have been combined with outputs from a numerical simulation of the tropical atlantic for the period 1982 99 it is shown that both warm and cold surface events develop regularly in the same specific region along the coast of angola and namibia some cold events compete in magnitude with major warm episodes local sea air heat flux exchanges do not seem to precondition the sea surface in the angola benguela region prior to the arrival of an event most warm and cold episodes are large scale events despite their limited surface signature they appear to be generated by wind anomalies in the western and central equatorial atlantic in the same way as benguela ninos seasonal fluctuations of the depth and shape of the tropical thermocline seem partly to control the way subsurface anomalies eventually impact the surface during austral summer surface anomalies create an identifiable pool centered near 158s whereas in winter they show an elongated pattern along the coast stretching toward the equator local upwelling or downwelling favorable wind regimes as well as local net heat fluxes may modulate the surface expression of events

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

evidence for chaotic fault interactions in the seismicity of the san andreas fault and nankai trough interactions between fault segments are one of the main sources of complexity in the seismicity of active tectonic regions such interactions are likely to influence the spatial and temporal patterns of earthquakes here we examine the dynamical behaviour introduced by fault interactions using a simple spring loaded slider block model with velocity weakening friction the model consists of two slider blocks coupled to each other and to a constant velocity driver by elastic springs for an asymmetric system in which the frictional forces on the two blocks are not equal the solutions exhibit chaotic behaviour the system s behaviour over a range of parameter values seems to be generally analogous to that of weakly coupled segments of an active fault we see similarities between our model simulations and observed patterns of seismicity on the south central san andreas fault in california and in the nankai trough along the coast of southwestern japan

patterns of seismic activity preceding large earthquakes we analyze the patterns of seismic activity which precede large events in a mechanical model of a fault the model generates a statistical distribution of events similar to that observed for a single fault with a scaling region consistent with the gutenberg richter law at small and moderate magnitudes and an excess of events at large magnitudes we find only slight variation in the scaling behavior during a loading cycle however we do observe systematic variations in space and time of the overall rate of activity in the model the activity accelerates dramatically preceding a large event and is usually a maximum hi the neighborhood of the future epicenter these results are compared to california seismicity data where we find that activity patterns vary regionally looking at patterns of activity in the san francisco bay area since 1948 we find an increase of activity on the calaveras fault near san jose beginning in the 1980s which if our model is relevant would forecast a large earthquake in that region the 1989 loma prieta earthquake occurred on the san andreas fault within 30 km of the section of the calaveras fault showing increased activity

the impact of enso on wave breaking and southern annular mode events abstract this study examines the relationship between intraseasonal southern annular mode sam events and the el nino southern oscillation enso using daily 40 yr ecmwf re analysis era 40 data the data coverage spans the years 1979 2002 for the austral spring and summer seasons the focus of this study is on the question of why positive sam events dominate during la nina and negative sam events during el nino a composite analysis is performed on the zonal mean zonal wind eliassen palm fluxes and two diagnostic variables the meridional potential vorticity gradient a zonal mean quantity that is used to estimate the likelihood of wave breaking and the wave breaking index wbi which is used to evaluate the strength of the wave breaking the results of this investigation suggest that the background zonal mean flow associated with la nina el nino is preconditioned for strong weak anticyclonic wave breaking on the equatorward side of the eddy driven jet the type of wave breaking that is found

origins and variability of the benguela current a subset of the benguela sources and transports best 1992 1993 data is analyzed to study the magnitude and variability of the large scale transports in the area the data consist of inverted echo sounder series and conductivity temperature depth stations the mean 16 month transport values for the upper 1000 m indicate that of the 13 sv northward transport within the benguela current 50 is derived from the central atlantic which from geometry may be chiefly south atlantic water 25 comes from the indian ocean which may be chiefly agulhas water and the remaining 25 may be a blend of agulhas and tropical atlantic water a simple schematic of the transport pattern with a somewhat restricted corridor for agulhas eddies translation is envisioned to the west of the eddy corridor flows the south atlantic source for the benguela current to the east is the agulhas indian ocean source the corridor is breached by south atlantic and indian water as the transient eddy field stirs these water masses

tropical atlantic variability modes 1979 2002 part i time evolving sst modes related to west african rainfall abstract this work presents a description of the 1979 2002 tropical atlantic ta sst variability modes coupled to the anomalous west african wa rainfall during the monsoon season the time evolving sst patterns with an impact on wa rainfall variability are analyzed using a new methodology based on maximum covariance analysis the enhanced climate prediction center cpc merged analysis of precipitation cmap dataset which includes measures over the ocean gives a complete picture of the interannual wa rainfall patterns for the sahel dry period the leading ta sst pattern related to the atlantic el nino is coupled to anomalous precipitation over the coast of the gulf of guinea which corresponds to the second wa rainfall principal component the thermodynamics and dynamics involved in the generation development and damping of this mode are studied and compared with previous works the sst mode starts at the angola benguela region and is caused by alongshore wind anomalies it then propagates wes

the south atlantic current in this paper we use the historical hydrographic data base for the south atlantic ocean to investigate i the hydrographic boundary between the subtropical gyre and the antarctic circumpolar current acc the sub tropical front stf and ii the southern current band of the gyre which is called the south atlantic current sac the stf begins in the west in the brazil falkland malvinas confluence zone but at locations at and west of 45 w this front is often coincident with the brazil current front east of 45 w the stf appears to be a distinct feature to at least the region south of africa whereupon it continues into the indian ocean the associated current band of increased zonal speed is the sac which except for one instance is found at or north of the surface stf until indian ocean water from the agulhas retroflection is reached a reversal of baroclinicity in the stf is observed south of a highly saline agulhas ring causing the sac to separate from the stf and turn north into the benguela current zonal flow south of the stf is generally weak and serves to separate the south atlantic and circumpolar currents in the argentine basin the sac has a typical volume transport of 30 sv 1 sv 106m3s 1 in the upper 1000 m relative to a deep potential density surface 4 45 87 kg m 3 and can be as high as 37 sv it is thus comparable to or stronger than the brazil current in the cape basin the transport of the sac is reduced to about 15 sy before it turns north to feed the benguela current in late 1983 this flow was joined by about 8 sv of water from the agulhas current

postseismic strain following the 1989 loma prieta earthquake from gps and leveling measurements postseismic deformation in the 5 years following the 1989 loma prieta earthquake has been measured with the global positioning system and precise leveling postearthquake velocities at distances greater than 20 km from the coseismic rupture are not significantly different from those observed in the 20 years prior to the earthquake however velocities at stations within 20 km of the rupture exceed preearthquake rates and exhibit unanticipated contraction normal to the strike of the san andreas fault system a combination of forward modeling and nonlinear optimization suggests that the observed postseismic deformations were caused by aseismic oblique reverse slip averaging 2 9 cm yr on the san andreas fault and or the loma prieta rupture zone and 2 4 cm yr reverse slip along a buried fault within the foothills thrust belt the best fitting sources of postseismic deformation are all located at depths of less than 15 km we find no evidence for accelerated flow or shear below the loma prieta rupture in the first 5 years following the earthquake the inferred postseismic slip is likely to have been caused by the coseismic stress change updip of the 1989 rupture

geodesy tracks plate motion our current understanding of the large scale movements of the earth s great crustal plates is based mainly upon offsets of old geological features on land as well as on the ocean floor

models of recurrent strike slip earthquake cycles and the state of crustal stress numerical models of the strike slip earthquake cycle assuming a viscoelastic asthenosphere coupling model are examined the time dependent simulations incorporate a stress driven fault which leads to tectonic stress fields and earthquake recurrence histories that are mutually consistent single fault simulations with constant far field plate motion lead to a nearly periodic earthquake cycle and a distinctive spatial distribution of crustal shear stress the predicted stress distribution includes a local minimum in stress at depths less than typical seismogenic depths the width of this stress trough depends on the magnitude of crustal stress relative to asthenospheric drag stresses the models further predict a local near fault stress maximum at greater depths sustained by the cyclic transfer of strain from the elastic crust to the ductile asthenosphere models incorporating both low stress and high stress fault strength assumptions are examined under newtonian and non newtonian rheology assumptions model results suggest a preference for low stress a shear stress level of about 10 mpa fault models in agreement with previous estimates based on heat flow measurements and other stress indicators

equivalent strike slip earthquake cycles in half space and lithosphere asthenosphere earth models by virtue of the images used in the dislocation solution the deformation at the free surface produced throughout the earthquake cycle by slippage on a long strike slip fault in an earth model consisting of an elastic plate lithosphere overlying a viscoelastic half space asthenosphere can be duplicated by prescribed slip on a vertical fault embedded in an elastic half space for the case in which each earthquake ruptures the entire lithosphere thickness h the half space equivalent slip rate is as follows depth interval 0 h slip identical to that in lithosphere asthenosphere model i e abrupt coseismic slip and no subsequent slip depth interval 2n 1 h to 2n 1 h n 1 2 slip rate uniform in space and dependent upon time as fn t exp t a where fn is a n 1 degree polynomial in t a is twice the asthenosphere relaxation time and t is measured from the instant after the preceding earthquake the slip rate averaged over the seismic cycle in each depth interval equals the secular rate of relative plate motion for reasonable values of a slip rates below 5h do not vary much from that mean value and can be treated as constant thus the surface deformation due to the earthquake cycle in the lithosphere asthenosphere model can be calculated very simply from the half space model with time dependent slip in the two depth intervals h 3h and 3h 5h and uniform slip at a rate equal to the secular relative plate velocity below depth 5h inversion of 1973 1988 geodetic measurements of deformation across the segment of the san andreas fault in the transverse ranges north of los angeles for the half space equivalent slip distribution suggests no significant slip on the fault above 30 km and a uniform slip rate of 36 mm yr below 30 km one equivalent lithosphere asthenosphere model would have a 30 km thick lithosphere and an asthenosphere relaxation time greater than 33 years but other models are possible

a southern hemisphere wave response to enso with implications for southern africa precipitation ensemble gcm simulations with an imposed idealized warming of the eastern pacific ocean reveal two wave anomalies in the southern hemisphere one in the eastern and one in the western hemisphere both are statistically significant at the 99 confidence level application of a steady state linear model and a rossby wave source analysis is used to diagnose the causes of the waves the western hemisphere wave is forced by the advection and stretching of planetary vorticity by the divergent flow from the southern hemisphere component of the central pacific twin anticyclones that straddle the equator during warm events the eastern hemisphere wave is a result of the northeastward shift of the south indian convergence zone sicz that in turn is forced from the upper troposphere by convergence to the north an upper level convergence maximum over the equatorial indian ocean induces divergence to the south encouraging vertical motion and precipitation to the northeast of the sicz s normal position the resulting anomalous upper level convergence in the climatological position of the sicz as well as the anomalous vorticity flux convergence by the transients associated with an equatorward shift of the storm track behind the sicz force the eastern hemisphere rossby wave since a shift of the sicz is a fairly robust observed consequence of enso events these results suggest the mechanism by which drought conditions develop over southern africa at the height of many warm events seasonal prediction capabilities in this region can be improved by monitoring and understanding the details and consequences of the adjustment of the walker circulation near the equator outside of the pacific ocean basin

theory of the benguela upwelling system the paper gives a theoretical study of the benguela upwelling system by means of a relatively simple conceptual model that allows an analytical treatment the model consists of a stratified flat bottomed coastal ocean the coast is idealized by a straight wall the model ocean is forced by an alongshore wind band extending from the area near cape town to the border of angola and namibia the wind varies alongshore and cross shore wind stress curl and changes periodically in time the response of the coastal ocean is governed by coastal jets upwelling and kelvin waves as well as by a current system driven by the wind stress curl the model is able to reproduce several observed features such as the poleward undercurrent the northward coastal currents in most of the wind band and a southward coastal current near the northern edge of the wind band export of coastal jets out of the forcing area by coastal kelvin waves provide a mechanism to guide energy into the area off cape town the southward propagating kelvin waves may bend around the southern tip of africa and provide a dynamic linkage between the benguela coastal current and the agulhas current

development of a coupled regional model and its application to the study of interactions between the west african monsoon and the eastern tropical atlantic ocean abstract a regional ocean atmosphere coupled model is developed for climate variability and change studies the model allows dynamic and thermodynamic interactions between the atmospheric boundary layer and an ocean mixed layer with spatially and seasonally varying depth prescribed from observations the model reproduces the west african monsoon circulation as well as aspects of observed seasonal sst variations in the tropical atlantic the model is used to identify various mechanisms that couple the west african monsoon circulation with eastern atlantic ssts by reducing wind speeds and suppressing evaporation the northward migration of the itcz off the west coast of africa contributes to the modeled spring sst increases during this period the westerly monsoon flow is expanded farther westward and moisture transport on to the continent is enhanced near the end of the summer upwelling associated with this enhanced westerly flow as well as the solar cycle lead to the seasonal cooling of the ssts over

atmosphere ocean coupled variability in the south atlantic abstract the climate variability of the south atlantic region is determined from 40 yr 1953 92 of comprehensive ocean atmosphere data set monthly sea surface temperature sst and sea level pressure slp data using the empirical orthogonal function eof and the singular value decomposition svd analysis methods the eof method is applied to each field separately whereas the svd method is applied to both fields simultaneously the significance of the atmosphere ocean interaction is revealed by a strong resemblance between individual eof and coupled svd modes of sst and slp the three leading modes of coupled variability on interannual and interdecadal timescales are discussed in some detail the first coupled mode which accounts for 63 of the total square covariance represents a 14 16 yr period oscillation in the strength of the subtropical anticyclone accompanied by fluctuations of a north south dipole structure in the sst the atmosphere ocean coupling is strongest during the southern summe

application of fracture mechanics to the simulation of seismicity and recurrence of characteristic earthquakes on a fault we present a fault model consisting of a large number of collinear cracks and intervening asperities for the simulation of seismicity and recurrence of large earthquakes on a fault we apply fracture mechanics to the analysis of the interaction of cracks and to the consideration of fracture condition our fault model gives rise to a complex frequency distribution of event sizes even if no externally imposed spatial inhomogeneity is assumed for the distribution of cracks and asperities and a homogeneous statistical distribution is assumed for fracture strengths the frequency distribution of event sizes is found to be bimodal with no intermediate size event in all the calculations for the smaller events the frequency distribution shows a power law behavior whereas the larger size events denote ones that rupture the whole fault statistically the same frequency distributions are obtained for inhomogeneous distributions of crack lengths and fracture strengths the occurrence of events that rupture the whole fault is shown to be periodic when the fault is isolated the magnitudes of these large events generally increase with increasing recurrence period for given fault length and rate of remote loading the events having a longer recurrence period have a shorter fault length for given magnitude and rate of remote loading if there are fault interactions the periodicity in the recurrence of the large events that would be observed for the isolated fault disappears and the recurrence of the large events becomes complex

scaling of ground motion parameters state of stress and focal depth the ground motion parametersrv and ra where r is hypocentral distance v is peak velocity is density and a is peak acceleration are found observationally to be strong functions of stress state and linearly related to focal depth z if the stress state and focal depth are properly taken into account then the ground motion parameters depend on the seismic moment m0 as expected from basic scaling principles that is rv m01 3 and ra is independent of earthquake size regression lines fitted to observations covering broad ranges in seismic moment and focal depth indicate that for extensional and compressional tectonic regimes corresponding to normal and thrust or reverse faulting earthquakes respectively peak acceleration is given by ra 1 08 mpa 3 06 mpa km z normal and ra 5 65 mpa 8 76 mpa km z thrust and for peak velocity rv m01 3 10 4 m2 s n m 1 3 3 00 0 69 km 1 z normal and rv m01 3 10 4 m2 s n m 1 3 4 63 1 82 km 1 z thrust for strike slip earthquakes the data currently are insufficient to define regression fits but such lines would definitely lie between those for the normal and thrust stress regimes these equations are appropriate for ground motion at small hypocentral distance as recorded in a whole space it follows that for comparable hypocentral depth peak acceleration in a compressional tectonic regime for example much of the eastern united states is a factor of about 3 greater than in an extensional regime such as nevada and for peak velocity a similar comparison yields a factor of 2 difference the similarity in behavior of the peak acceleration parameter to crustal strength estimated from byerlee s law of friction with respect to depth and stress state indicates that crustal strength is probably the factor governing the seismic source processes that give rise to the high frequency peak ground motion analysis of the peak acceleration data in the context of crustal strength suggests bounds on this ground motion parameter as recorded at a typical surface site that depend on stress state specifically a normal 0 5 g and a thrust 1 9 g thus the state of stress as well as the focal depth clearly is an important factor to be taken into account in the prediction of seismic ground motion

near field transpressive deformation along the san andreas fault zone in southern california based on exhumation constrained by u th he dating low temperature thermochronometry reveals that a narrow crustal sliver trapped within strands of the san andreas fault zone in southern california has experienced recent rapid exhumation eight apatite u th he ages from a 1 km relief section along yucaipa ridge in the san bernardino mountains range from 1 4 to 1 7 ma the minimal change in age with elevation implies exhumation of 5 7 mm yr 1 sustained for at least several hundred thousand years three titanite helium ages from the ridge are much older ranging from 57 to 82 ma these show a steep gradient with elevation representing either an exhumed partial retention zone or slow cooling through much of the tertiary these data imply that a total exhumation of 3 to 6 km has occurred since 1 8 ma it is uncertain whether this exhumation terminated as early as 1 ma or has continued up to the present at a decelerated rate we surmise that this exhumation represents rock uplift in the absence of major surface uplift in that it kept pace with tectonic uplift as the narrow fault block maintained steady state relief the record of sedimentation in adjacent basins is consistent with the implied magnitude of erosion such rapid large magnitude exhumation within the strands of the san andreas fault zone is important for models of transpressional tectonics it is consistent with a strain partitioning model which predicts that pure shear dominated fault zones experience significant vertical strain however it is inconsistent with a stress partitioning model which predicts that fault zone weakness limits pure shear deformation to the borderlands of the master strike slip fault in addition a concentration of secondary contraction within the fault zone may require modification of coupling models between strong upper mantle and brittle upper crust via the weak lower crust these models predict that transpressional deformation will either be uniformly distributed across the plate boundary or be limited to the far field borderlands rather than concentrated in the near field alternatively the exhumation of yucaipa ridge may have been driven by the nearby restraining bend in the san andreas fault at san gorgonio pass in which case it represents local fault geometry rather than accommodation of far field plate motion

the congo basin walker circulation dynamics and connections to precipitation the existence seasonality and variability of a congo basin walker circulation are investigated in reanalyses and connections with rainfall are explored a zonal overturning circulation along the equator connects rising motion in the congo basin and sinking in the eastern atlantic during june through october this timing is out of phase with precipitation over equatorial africa which greatest during spring and fall and does not correlate with the seasonality of land temperatures rather the zonally overturning circulation only occurs when the atlantic cold tongue has formed although the cold tongue formation is essential for setting up the congo basin walker circulation variations in equatorial eastern atlantic sea surface temperatures are not associated with interannual variability in the strength of the circulation when cold tongue ssts are anomalously cool warm evaporation from the ocean surface is reduced enhanced and the westerly flow advects less more moisture into the base of the congo basin walker circulation this reduces increases the release of latent heat in the upbranch and weakens strengthens the walker circulation this process dominates the pure dry dynamical response to enhanced land sea temperature differences which has an opposite sign a positive correlation connects low level vertical velocity in the congo basin with low level vertical velocity and precipitation over west africa a wave response to anomalous vertical velocity in the congo basin in several reanalyses suggests a teleconnection into west africa such that an anomalously strong weak upbranch is associated with anomalously strong weak rainfall over the guinean coast and southern sahel

order and diversity in the modes of circum pacific earthquake recurrence recurrence characteristics of great circum pacific earthquakes and determinations of spatial distribution of seismic moment release are surveyed in order to delineate their general features and relate earthquake slip distribution to models of recurrent rupture as noted by others the pattern of moment release is typically very irregular with strong concentrations in a few isolated regions of a much larger aftershock zone despite this complexity rupture nucleation is notably systematic with mainshock epicenter showing a strong tendency to locate in or immediately adjacent to identified regions of high moment release this generalization suggests that earthquake recurrence is more likely to be controlled by maximum rather than average fault slip well documented case histories from twelve plate boundary regions demonstrate that seismic strain release tends to be temporally well ordered while source dimensions slip and cumulative moment release vary considerably from cycle to cycle on none of the segments studied do earthquakes consistently recur in nearly identical events instead individual great earthquakes differ significantly from cycle to cycle or rerupture takes place in a sequence of two or more smaller events despite these differences the duration of the seismic cycle is approximately uniform furthermore when strain release occurs in a sequence of large earthquakes these events take place towards the end of the cycle and occur with increasing rupture length and magnitude these ordered and irregular features of earthquake recurrence argue for the existence of corresponding elements on plate boundary faults the ordered characteristics are identified with zones of concentrated moment release that slip comparable amounts in each cycle and have high shear strength the irregular features are associated with intervening weaker regions that move in response to the stress concentration of dynamic rupture and slip by differing amounts in each event although these mechanistic associations are indirect and tentative the features of recurrent behavior documented here have implications for long term earthquake hazard assessment that are not dependent on the models proposed to explain them chief among these are the cycle to cycle differences among gap filling events and the absence of shocks that fill major slip deficient regions early in the seismic cycle

current sierra nevada north america motion from very long baseline interferometry implications for the kinematics of the western united states we use geodetic measurements from very long baseline interferometry to estimate the motion of the sierra nevadan microplate which is composed of the sierra nevada and the great valley the motion of the sierra nevadan microplate relative to the north american plate is described by a right handed rotation of 0 61 m y about lat 32 n long 128 w this euler pole lies only 10 southwest of the sierra nevadan microplate and predicts a significant counterclockwise rotation about a local vertical axis it further predicts a velocity of the eastern edge of the sierra nevada at 38 0 n 119 3 w relative to stable north america of 11 1 mm yr toward n36 3 w quoted uncertainties are plus or minus one standard error which accounts for about one fourth of the velocity between the pacific and north american plates and is 25 clockwise of many prior estimates the velocity nearly parallels the boundary between the sierra nevada and the great basin which implies that current motion within the great basin results in a rotational noncoaxial deformation we use this velocity to estimate how motion is distributed across the broad deforming zone taking up pacific north america plate motion we find that the vector sum of strike slip along the san andreas fault and motion of the sierra nevada relative to stable north america taken up by deformation within the great basin differs little from the pacific north america plate velocity the difference can be described at 36 n along the san andreas fault by a vector of 6 mm yr directed toward n20 w this vector resolves into components of 5 mm yr parallel to the fault and 2 mm yr perpendicular to the fault with 95 confidence intervals of 0 to 10 mm yr and 1 to 5 mm yr respectively the component perpendicular to the fault is several times smaller than found in prior studies and places a small upper bound on fault perpendicular shortening we conclude that motion previously inferred to be taken up by deformation other than strike slip along the san andreas fault or deformation within the great basin is much smaller than previously thought

space geodetic measurement of crustal deformation in central and southern california 1984 1992 we estimate the velocity field in central and southern california using global positioning system gps observations from 1986 to 1992 and very long baseline interferometry vlbi observations from 1984 to 1991 our core network includes 12 gps sites spaced approximately 50 km apart mostly in the western transverse ranges and the coastal borderlands the precision and accuracy of the relative horizontal velocities estimated for these core stations are adequately described by a 95 confidence ellipse with a semiminor axis of approximately 2 mm yr oriented roughly north south and a semimajor axis of approximately 3 mm yr oriented east west

marine notches in coastal cliffs indicators of relative sea level change perachora peninsula central greece abstract in several sites in the complex faulted western part of perachora peninsula central greece three principal marine notches in rocky coasts are present in each site no holocene notches occur above the upper of the three notches only subaerial weathering affected the rocks lateral variation in the heights of notches demonstrates differential uplift due to fault control and illustrates the complexity of application of marine notches in interpretation of tectonic history the lower undated notch is prominent in places coincides with modern sea level and is interpreted to be forming now it suggests that the western part of the peninsula has been stable for a considerable time the middle and upper notches vary in height between sites and may not be time equivalent but represent earlier sea level positions in the harbour adjacent to the heraion archaeological site the middle and upper notches are preserved in most places as notch roofs so that gradual uplift led to removal of notch floors by physical chemical and biotic processes in exposed locations however within a few hundred metres west of the harbour four uplifted plus a fifth forming now notches have been illustrated by pirazzoli et al tectonophysics 229 1994 201 that cannot be readily correlated to the harbour site and indicate differential uplift by fault displacement in a more complex situation at mylokopi 6 km ne of heraion three different notch profiles occur in three blocks separated by two faults however within the central block of the three two very different profiles are preserved only tens of metres apart beneath one continuous upper notch roof at the same height this complex arrangement interpreted as variability of exposure to marine processes on different parts of this irregular embayment such that one profile containing three notches was controlled largely by abrasion and formed notches that are vertically more extensive while the other profile containing five narrow notches formed by dissolution and or biological erosion at the same time in the protected east end of the block however if only one or the other profile was visible very different tectonic interpretations would result the bedrock of perachora peninsula also shows important variations in resistance to erosion affecting the resulting notch profiles and some sites are influenced by structurally controlled differential subaerial weathering forming ridges and grooves giving the false impression of extra minor notches the distinction between dissolution controlled and structure controlled minor notches may not be possible in some places the interpretations of notch profiles can be highly complex and influence views of tectonic history in a given region overall the occurrence of three principal notches beneath one uplifted notch roof in several sites indicates that a holocene uplift rates outpaced post glacial sea level rise b there were three principal periods of relative sea level stillstand affecting the perachora peninsula and c uplift of the entire western part of the perachora peninsula is an important component of its tectonic processes probably more important than fault movement within the peninsula

on the triggering of benguela ninos remote equatorial versus local influences 1 the relative importance of remote and local influences in the development of benguela ninos is examined using observations and a coupled general circulation model capable of simulating interannual variability in the tropical atlantic while previous studies have emphasized the role of equatorially excited kelvin waves the present study finds that meridional wind anomalies along the southwest african coast contribute substantially both observations and model output indicate that sea surface temperatures along the southwest african coast respond rapidly to changes in meridional wind stress these wind anomalies form part of a basin scale weakening of the subtropical anticyclone that extends to the equator as the weakening begins three months before the peak of the event it might have some predictive potential results also indicate that the close correlation between benguela and atlantic ninos in observations might result from the large spatially coherent wind stress anomalies associated with the weakened anticyclone

a simplified spring block model of earthquakes the time interval between earthquakes is much larger than the actual time involved during slip in an individual event the authors have used this fact to construct a cellular automaton model of earthquakes this model describes the time evolution of a 2 d system of coupled masses and springs sliding on a frictional surface the model exhibits power law frequency size relations and can exhibit large earthquakes with the same scatter in the recurrence time observed for actual earthquakes

decadal change of the south atlantic ocean angola benguela frontal zone since 1980 high resolution simulations with a regional atmospheric model coupled to an intermediate level mixed layer ocean model along with multiple atmospheric and oceanic reanalyses are analyzed to understand how and why the angola benguela frontal zone abfz has changed since 1980 a southward shift of 0 05 0 55 latitude decade 1 in the annual mean abfz position accompanied by an intensification of 0 05 to 0 13 k 100 km decade 1 has occurred as ocean mixed layer temperatures have warmed cooled equatorward poleward of the front over the 1980 2014 period these changes are captured in a 35 year model integration the oceanic warming north of the abfz is associated with a weakening of vertical entrainment reduced cooling associated with vertical diffusion and a deepening of the mixed layer along the angola coast these changes coincide with a steady weakening of the onshore atmospheric flow as the zonal pressure gradient between the eastern equatorial atlantic and the congo basin weakens oceanic cooling poleward of the abfz is primarily due to enhanced advection of cooler water from the south and east increased cooling by vertical diffusion and shoaling of the mixed layer depth in the atmosphere these changes are related to an intensification and poleward shift of the south atlantic sub tropical anticyclone as surface winds hence the westward mixed layer ocean currents intensify in the benguela upwelling region along the namibian coast with a few caveats these findings demonstrate that air sea interactions play a prominent role in influencing the observed decadal variability of the abfz over the southeastern atlantic since 1980

model for intermediate term precursory clustering of earthquakes a causal sequence of an earthquake swarm extended quiescence foreshocks and main shock can be understood in terms of an aggregation model of crack growth and ultimate fusion on a multiplanar earthquake fault system the rheological model is one of accelerated crack growth under conditions of slip weakening coupled with a slow recovery of static friction after fracture once a slip feature has been generated that spans a large part of the available geometry a stress shadow is cast on other neighboring cracks that strongly retards and inhibits crack growth thus initiating the quiescent phase abruptly the success of the model depends on the assumption that nonlinear rheology regulates the rate of slip and that earthquake faults are not simply connected surfaces but instead have many strands

markers of mis 5 sea level fluctuations in the bergeggi marine cave nw mediterranean sea regional setting the onshore territory of the liguria region fig 1 is represented by an arch shaped mountain ridge presenting a steep seaward slope and with its watershed very close to the mediterranean sea from a geological point of view the region can be divided into two parts the western part of the arch west of genova ne sw oriented is considered to be the southern continuation of the western alps ligurian alps vanossi et al 1994 in this part provencal flysch subbrianconneese brianconneese piedmonteese and metaophiolitic units and related metamorphic covers outcrop west of genova fig 2 an ophiolitic complex named the voltri group marks the transition between the alpine and the apenninic domains cortesogno haccard 1984 the latter stretching nw se where ligurian subligurian and tuscan units outcrop gianmarino et al 2002 fig 1 geographical position of the liguria region in the italian peninsula

moisture transport between the south atlantic ocean and southern africa relationships with summer rainfall and associated dynamics moisture exchange between the south atlantic and southern africa is examined in this study through zonal moisture transport along the west coast of southern africa a multivariate analysis of the zonal flow of moisture computed from ncep doe amip ii re analyses reveals a primary mode of variability typical of variations in intensity and of the latitudinal migration of the circulation associated with the midlatitude westerlies and the south atlantic anticyclone in austral summer january february this mode referred to as the south atlantic midlatitude mode is found to be well correlated with rainfall over southern africa i e to the south of the upper lands surrounding the congo basin its positive negative phases are found to correspond with surface pressures changes over the south atlantic region in austral summer when the south atlantic anticyclone is shifted northward southward respectively such changes are accompanied by dipole like sst anomalies in the midlatitude south atlantic ocean while simultaneous sst anomalies with a similar structure are also found over south indian ocean regions in january february positive negative events linked to the south atlantic midlatitude mode are marked by meridional shifts northward southward and weakening strengthening of the itcz over the southern tropics together with modulations in intensity weakened sustained of the angola low which could act as a tropical source of moisture for tropical temperate troughs ttts in association with a strengthened weakened zonal component of the southern extension of the african easterly jet aej this could modulate the meridional transfer of moisture south of 15 s to the advantage detriment of angolan coastal regions where above below rainfall are expected variations in the latitudinal position northward southward of the south atlantic anticyclone and thus of the midlatitude westerlies are also found to reduce favour moisture advection towards southern africa subtropics allowing the southern indian trades to penetrate less more over the subcontinent south of 25 s this would create a situation where convection processes are inhibited supported within the sicz ttts region resulting in drier wetter conditions locally for positive negative events respectively

crustal deformation in great california earthquake cycles periodic crustal deformation associated with repeated strike slip earthquakes is computed for the following model a depth l h extending downward from the earth s surface at a transform boundary between uniform elastic lithospheric plates of thickness h is locked between earthquakes it slips an amount consistent with remote plate velocity vpl after each lapse of earthquake cycle time tcy lower portions of the fault zone at the boundary slip continuously so as to maintain constant resistive shear stress the plates are coupled at their base to a maxwellian viscoelastic asthenosphere through which steady deep seated mantle motions compatible with plate velocity are transmitted to the surface plates the coupling is described approximately through a generalized elsasser model we argue that the model gives a more realistic physical description of tectonic loading including the time dependence of deep slip and crustal stress buildup throughout the earthquake cycle than do simpler kinematic models in which loading is represented as imposed uniform dislocation slip on the fault below the locked zone parameters of the model are chosen to fit seismic and geologic constraints and the apparent time dependence of surface strain rates along presently locked traces of the 1857 and 1906 san andreas ruptures we fix vpl 35 mm yr tcy 160 years and l 9 11 km based on earthquake nucleation depths the geodetic data are then found to be described reasonably within the context of a model that is locally uniform along strike and symmetric about a single san andreas fault strand by lithosphere thickness h 20 30 km and elsasser relaxation time tr 10 16 years we therefore suggest that the asthenosphere appropriate to describe crustal deformation on the earthquake cycle time scale lies in the lower crust and perhaps crust mantle transition zone and has an effective viscosity between about 2 1018 and 1019 pa s depending on the thickness assigned to the asthenospheric layer predictions based on the chosen set of parameters are also consistent with data on variations of contemporary surface strain and displacement rates as a function of distance from the 1857 and 1906 rupture traces although the fit is degraded by asymmetry relative to the fault and by slip on adjacent fault strands

the benguela upwelling system quantifying the sensitivity to resolution and coastal wind representation in a global climate model abstractof all the major coastal upwelling systems in the world s oceans the benguela located off southwest africa is the one that climate models find hardest to simulate well this paper investigates the sensitivity of upwelling processes and of sea surface temperature sst in this region to resolution of the climate model and to the offshore wind structure the community climate system model version 4 is used here together with the regional ocean modeling system the main result is that a realistic wind stress curl at the eastern boundary and a high resolution ocean model are required to well simulate the benguela upwelling system when the wind stress curl is too broad as with a 1 atmosphere model or coarser a sverdrup balance prevails at the eastern boundary implying southward ocean transport extending as far as 30 s and warm advection higher atmosphere resolution up to 0 5 does bring the atmospheric jet closer to the coast but there can be too strong a wind stress curl the most

multimodel study of tropical atlantic variability and change 1 interannual variability associated with the zonal and the meridional mode in the tropical atlantic is studied in nine coupled ocean atmosphere models for twentieth century climate conditions tc and the sres a1b scenario for future greenhouse gas concentrations for tc the subtropical part of the meridional mode is reasonably well simulated in contrast to the deep tropical part of the meridional mode and the zonal mode a common model bias is that the onset of the meridional mode is preceded by the presence of a zonal mode in boreal fall that extends towards the western boundary of the atlantic basin and which initiates a wind evaporation sst feedback as a result of this there is a spuriously strong interaction between the zonal and the meridional mode the models that seem to best represent the meridional mode show a weakening for future climate conditions biases in the zonal mode are too strong to assess changes citation breugem w p w hazeleger and r j haarsma 2006 multimodel study of tropical atlantic variability and change geophys res lett 33 l23706 doi 10 1029 2006gl027831

finite element models of stress orientations in well developed strike slip fault zones implications for the distribution of lower crustal strain finite element models are used to examine the effects of strike slip earthquakes on stresses in an elastic layer overlying a finite width viscoelastic shear zone in the lower crust the overall dimensions of the model are 300 km wide 400 km long and 50 km deep three geometries for the lower crustal shear zone are considered 1 a viscoelastic half space approximation with a shear zone that extends to the model boundaries 300 km in width 2 a wide shear zone model 70 km in width and 3 a narrow shear zone model 10 km in width earthquakes are simulated with a fault plane that slips without friction at the desired time step and is centered above the shear zone extending to a depth of 15 km and running the length of the mesh far field plate velocity boundary conditions are enforced at the model edges so that stress on the fault evolves naturally a coulomb type failure criterion based on the average shear stress on the fault is set such that the earthquake cycle is 250 years the models are run until a limit cycle is reached and transient stresses have decayed away we focus on the maximum changes in the stress field during the earthquake cycle by examining stresses before and immediately after each earthquake in addition to comparisons of the separate components of the stress tensor we present results in the form of maximum compressive stress orientations plunge angles of the principal stress axes and beach ball diagrams that facilitate visualizing the full tensor stresses are concentrated in the upper crust where it overlies the finite width viscoelastic shear zone which causes the plunge angles of the principal stress axes to rotate from andersonian orientations of 90 and 0 to angles that approach 45 in the lower crust our results suggest that an examination of stress orientations in the upper and middle crust from borehole breakouts and focal mechanisms may provide insight as to the distribution of strain in the lower crust and may eventually allow us to distinguish between localized and distributed deformation models for the lower crust in active strike slip zones

the south indian convergence zone and interannual rainfall variability over southern africa abstract the south indian convergence zone sicz is identified in this paper as a region of enhanced precipitation extending off the southeast coast of southern africa during austral summer unlike the south pacific convergence zone the sicz is a land based convergence zone lbcz with position and intensity at least partially determined by surface conditions over southern africa an idealized gcm simulation is used to explore the basic dynamics of lbczs in the tropics and subtropics output from a realistic gcm simulation and the national centers for environmental prediction national center for atmospheric research 40 year reanalysis are analyzed to apply this basic dynamical framework to the case of the sicz in contrast to the intertropical convergence zone where column moisture convergence is primarily due to meridional wind convergence in the moist environment precipitation within the sicz and the lbczs in general is also supported by zonal wind convergence moisture convergence by transient eddy

cloud sst feedback in southeastern tropical atlantic anomalous events 1 using satellite based cloud measurements for 1984 2004 the interannual variability of the low level cloud cover over the tropical atlantic ocean in austral winter june july august jja is examined it is found that the leading pattern of the low cloud anomalies in this season is a modulation of the climatological center of the cloud cover over the southeastern tropical atlantic ocean off the angola and namibia coasts the fluctuation of cloud amount there occurs on both interannual and longer timescales the relationship between this low cloud anomalous pattern and basinwide ocean atmosphere anomalies is studied through a composite analysis based on the objectively selected major low cloud deficit and excess years for the composites we intentionally use data sets mainly based on satellite measurements for the past one to two decades to minimize the potential influences of the bias in the model based ocean atmosphere analyses the composites show that the jja anomalous cloud pattern is strongly influenced by the sea surface temperature sst anomalies of the equatorial and southeastern tropical atlantic ocean in the previous summer the anomalous surface warm events off the southwestern coast of africa near 15 s in january and february are usually initiated dynamically by remote forcing from the westerly wind anomalies over the western equatorial atlantic preceding the cloud anomalies during the next few months the warm water is spread into the southeastern tropical atlantic ocean and is conducive to deficit low cloud cover in subsequent jja the reduced low cloud cover in turn forces positive sst tendency in a larger area of the southeastern atlantic ocean by changing the amount of the local solar radiation reaching the sea surface in june and july this process moves the major center of the sst anomalies away from the coast and closer to the equator when the coastal process weakens the low cloud radiation sst feedback also plays a role in the slow westward expansion of the sst anomalies in late austral winter and spring overall the influence of the cloud fluctuation is an important component in the evolution of the southeastern tropical atlantic anomalous events

antarctic temperature and global sea level closely coupled over the past five glacial cycles sea level has varied by over one hundred metres across glacial interglacial cycles over the past 520 000 years an extended sea level reconstruction shows a strong coupling between these sea level changes and antarctic surface temperatures over the past five glacial cycles

south atlantic variability arising from air sea coupling local mechanisms and tropical subtropical interactions abstract interannual variability in the southern and equatorial atlantic is investigated using an atmospheric general circulation model agcm coupled to a slab ocean model som in the atlantic in order to isolate features of air sea interactions particular to this basin simulated covariability between sea surface temperatures ssts and atmosphere is very similar to the observed non enso related covariations in both spatial structures and time scales the leading simulated empirical coupled mode resembles the zonal mode in the tropical atlantic despite the lack of ocean dynamics and is associated with baroclinic atmospheric anomalies in the tropics and a rossby wave train extending to the extratropics suggesting an atmospheric response to tropical sst forcing the second non enso mode is the subtropical dipole in the sst with a mainly equivalent barotropic atmospheric anomaly centered on the subtropical high and associated with a midlatitude wave train consistent with atmospheric forcing of the sub

uranium series chronology and environmental applications of speleothems an increasing number of scientists recognize the value of speleothems1 as often extremely well preserved archives of information about past climate vegetation hydrology sea level nuclide migration water rock interaction landscape evolution tectonics and human action well constrained data are required to document past changes reconstruct past patterns and predict future responses of the earth system at a wide range of spatial and temporal scales speleothems are particularly useful in this regard because they can be found in many locations of the globe sampled at high resolution and reliably dated using high precision uranium series techniques speleothems are bodies of mineral material formed in caves as the result of chemical precipitation from groundwater flowing or dripping in a cave most speleothems are composed of calcite formed by slow degassing of co2 from supersaturated groundwater but aragonite and gypsum forms are also common particularly near cave entrances where evaporative effects are important a host of different speleothem types decorate the walls ceilings and floors of caves and their mineralogy and morphology is a function of fluid flow and chemistry of waters feeding a particular location as well as the ambient conditions temperature chemistry light in the air or water filled void subaerial forms include the familiar stalagmites stalactites draperies flowstones subaqueous forms include rimstone pools rafts mammillary calcite wall coatings and dog tooth spar for an extensive review of the types of speleothem that have been observed see hill and forti 1997 speleothems are used in a multitude of ways to explore past environmental conditions perhaps the most fundamental of which is their very presence or absence deposition of speleothems relies on sufficient water supply and soil co2 to enable dissolution and transport of reactants in the vadose zone to underlying caves in arid or glacial times conditions may not have been favorable for speleothem formation

new earthquake rebound theory abstract the model adopted in this paper assumes that a medium is permeated with a continuum field of defects and that a flow of defects describes an advanced state of deformation leading to fracturing the obtained system of equations describes a field of internal stresses and a relative flow velocity of cracks the form of equations shows that before an earthquake a field of internal stresses reaches its maximum and that this moment corresponds to a specific silent gap expressed by the disappearance of the relative velocity field of cracks later on there starts a rebound process with an opposite sense of relative velocities with decrease of defect density joining of cracks and with a main process of earthquake release

what determines the position and intensity of the south atlantic anticyclone in austral winter an agcm study abstract the south atlantic anticyclone is a major feature of the austral winter climatology an atmospheric general circulation model agcm is used to study the dynamics of the south atlantic anticyclone by means of control simulations and experiments to investigate sensitivity to prescribed orography sea surface temperatures and soil wetness the south atlantic anticyclone in the first control simulation is unrealistically zonally elongated and centered too far west errors typical of coupled ocean atmosphere gcms results of the sensitivity experiments suggest that these deficiencies are associated with another family of systematic model errors the overprediction of convection over the tropical land surfaces particularly over eastern tropical africa and india and the concurrent large scale westward shift in the divergence center at upper levels and the convergence center at lower levels the results also confirm the important role of south american and african orography in localizing the south atl

zonal movement of the mascarene high in austral summer temporal variations in the mascarene high mh defined by the sea level pressure slp maximum within the region of 40 120 e 50 10 s in austral summer november january were investigated using atmospheric reanalysis datasets the mh longitudinal position has a dominant timescale at about 6 years which appears to be independent of the mh intensity variation the mh longitudinal movement is caused by a combination of slp variations in the eastern south indian ocean esio and in the western south indian ocean wsio as follows 1 pressure variations in the esio region are confined to the lower atmosphere up to the mid troposphere and are associated with the el nino southern oscillation events 2 pressure variations in the wsio region characterized by a quasi barotropic structure throughout the troposphere are related to the meridional movement of storm track sea surface temperature anomalies associated with the longitudinal movement of the mh show a southwest northeast dipole structure but this is shifted 10 westward in longitude compared with that associated with the mh intensity

sea level and deep water temperature changes derived from benthic foraminifera isotopic records we show that robust regressions can be established between relative sea level rsl data and benthic foraminifera oxygen isotopic ratios from the north atlantic and equatorial pacific ocean over the last climatic cycle we then apply these regressions to long benthic isotopic records retrieved at one north atlantic and one equatorial pacific site to build a composite rsl curve as well as the associated confidence interval over the last four climatic cycles our proposed reconstruction of rsl is in good agreement with the sparse rsl data available prior to the last climatic cycle we compute bottom water temperature changes at the two sites and at one southern indian ocean site taking into account potential variations in north atlantic local deep water 18o our results indicate that a last glacial maximum lgm enrichment of the ocean mean oxygen isotopic ratio of 0 95 is the lowest value compatible with unfrozen deep waters in the southern indian ocean if local deep water 18o did not increase during glacials with respect to present such a value of the lgm mean ocean isotopic enrichment would impose a maximum decrease in local bottom water 18o at the north atlantic site of 0 30 during glacials

numerical study of convection observed during the winter monsoon experiment using a mesoscale two dimensional model abstract a two dimensional version of the pennsylvania state university mesoscale model has been applied to winter monsoon experiment data in order to simulate the diurnally occurring convection observed over the south china sea the domain includes a representation of part of borneo as well as the sea so that the model can simulate the initiation of convection also included in the model are parameterizations of mesoscale ice phase and moisture processes and longwave and shortwave radiation with a diurnal cycle this allows use of the model to test the relative importance of various heating mechanisms to the stratiform cloud deck which typically occupies several hundred kilometers of the domain frank and cohen s cumulus parameterization scheme is employed to represent vital unresolved vertical transports in the convective area the major conclusions are ice phase processes are important in determining the level of maximum large scale heating and vertical motion because there is a strong anvil componen

earthquake mechanism and predictability shown by a laboratory fault slip events generated in a laboratory fault model consisting of a circulinear chain of eight spring connected blocks of approximately equal weight elastically driven to slide on a frictional surface are studied it is found that most of the input strain energy is released by a relatively few large events which are approximately time predictable a large event tends to roughen stress distribution along the fault whereas the subsequent smaller events tend to smooth the stress distribution and prepare a condition of simultaneous criticality for the occurrence of the next large event the frequency size distribution resembles the gutenberg richter relation for earthquakes except for a falloff for the largest events due to the finite energy storage capacity of the fault system slip distributions in different events are commonly dissimilar stress drop slip velocity and rupture velocity all tend to increase with event size rupture initiation locations are usually not close to the maximum slip locations

earthquake recurrence and risk assessment in circum pacific seismic gaps the development of the concept of seismic gaps regions of low earthquake activity where large events are expected has been one of the notable achievements of seismology and plate tectonics its application to long term earthquake hazard assessment continues to be an active field of seismological research here i have surveyed well documented case histories of repeated rupture of the same segment of circum pacific plate boundary and characterized their general features i find that variability in fault slip and spatial extent of great earthquakes rupturing the same plate boundary segment is typical rather than exceptional but sequences of major events fill identified seismic gaps with remarkable order earthquakes are concentrated late in the seismic cycle and occur with increasing size and magnitude furthermore earthquake rup ture starts near zones of concentrated moment release suggesting that high slip regions control the timing of recurrent events the absence of major earthquakes early in the seismic cycle indicates a more complex behaviour for lower slip regions which may explain the observed cycle to cycle diversity of gap filling sequences