present day activity of the shenzhen fault zone and its impact on the safety of a planned diversion tunnel in shenzhen china abstract in this paper after a study of present activity of shenzhen fault zone a numerical simulation of present three dimensional tectonic stress field is made to calculate the angle enclosed between the maximum horizontal principal compressive stress and the tunnel axis the tectonic stress states of four sectors at a depth of 200 250 m of the tunnel path and the present displacement rate of the shenzhen fault zone where the tunnel is planned to pass are also estimated by the modeling such data are considered necessary for the safety of the diversion tunnel construction

a multilayer model of time dependent deformation following an earthquake on a strike slip fault a multilayer half space model of the earth is used in finite element calculations of time dependent deformation and stress following an earthquake on an infinitely long strike slip fault the model involves shear properties of an elastic upper lithosphere a standard viscoelastic linear solid lower lithosphere a maxwell viscoelastic asthenosphere and an elastic mesosphere time dependent displacements strains and stresses are computed both at the surface of the earth and at depth the analysis includes both systematic variations of fault and layer depths and comparisons with simpler elastic lithosphere over viscoelastic asthenosphere calculations for conditions which may be appropriate for the earth the creep of both the lower lithosphere and the asthenosphere can contribute to the postseismic deformation the magnitude of the deformation is enhanced by a short distance between the bottom of the fault slip zone and the top of the creep layer but is less sensitive to the thickness of the creeping layer furthermore postseismic restressing is increased as the lower lithosphere becomes more viscoelastic but the tendency for the width of the restressed zone to grow with time is retarded

importance of pre existing oceanic conditions to upper ocean response induced by super typhoon hai tang 1 this study investigates upper ocean response to super typhoon hai tang the most intense typhoon in 2005 in the western north pacific using multi satellite microwave data typhoon data and argo profiles sequential merged microwave sea surface temperature sst images show that hai tang induced four separately cooling responses after its trail in the western north pacific during 11 18 july comparison between upper ocean responses of cooling sst and pre typhoon sea surface height suggests that these cooling responses of the upper ocean to hai tang are closely associated with pre existing cyclonic circulation denoted by pre existing negative sea surface features furthermore applying argo profiles the physical mechanism is confirmed to be associated with pre existing cyclonic circulations provide a relatively unstable thermodynamic structure and therefore cold water would be brought up readily this paper provides the rather convincing evidence that cyclonic flow and uplifted thermocline are the pre existing conditions for intensive cooling following the typhoon passage

short paper low grade metamorphism of the welsh basin lower palaeozoic succession an example of diastathermal metamorphism the lower palaeozoic succession of the welsh basin has suffered the effects of low grade metamorphism which shows features that do not accord with classical models of regional or burial metamorphism textural evidence demonstrates the development of greenschist facies metamorphism prior to deformation and cleavage development while mineralogical characters indicate a low pressure facies series in order to accommodate these features it is suggested that the area has suffered diastathermal metamorphism in which an enhanced thermal flux develops early in the tectonic cycle in response to an extensional setting temperatures remained relatively high during final closure associated with deformation of the basin sediments and consequent cleavage development such a model adequately resolves previously conflicting interpretations of burial and deformation related metamorphism

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

modelling natural stresses in the arc syncline and comparison with in situ measurements abstract in the provence coal basin the hbcm company mines lignite at great depths several measurements of the natural stresses were carried out using flat jack and hydraulic fracturing a strong stress anisotropy and high stresses were observed to interpret these measurements large scale numerical modelling is used this study presents a comparison between the predicted natural stresses and those measured during field investigations three models are successively developed and loaded on the basis of assumptions about current tectonics and historical tectonics calculations lead to a better explanation of the orientations and magnitudes of the principal stresses which are not in contradiction with the observation of a ne sw stress deviation in the vicinity of the area where the measurements were carried out it appears that the natural stresses in the arc depend mostly on the major geological structures in comparison to the loading type the results indicate that there is a relatively good match between predicted and measured stress magnitudes and orientations finally on the basis of a good estimation of the tectonic loading stages it is possible to explain the distribution of the natural stresses in the arc syncline even though the magnitudes of the tectonic stresses were unknown

three dimensional numerical simulation of the subduction dynamics in the sunda arc region southeast asia abstract the sunda arc has been modelled as a test subduction zone with geometrical complexities and varying dynamics we simulated spatially the structural heterogeneities of the subduction zone and carried out our computations by a three dimensional finite element approach the aim is to estimate the lateral variation in the effective horizontal compressive stress in the interplate region which can produce the present day observed seismogenic stress field in our simplified approach the stress field has been modelled by varying the traction ridge push while keeping the age dependent gravitational body force slab pull constant our result suggests that there is a considerable amount of stress dissipation in the decoupled segment of java at sumatra the required traction is much greater than that expected from the age dependency relations in our modelling the transition from the dominance of the horizontal compressive stress to that of the gravitational pull in the regional stress field occurs around a depth range of 150 200 km lateral variation in the subducted slab volume emerges crucial to the seismogenic stress field and the derived pattern of displacement subduction of the lighter oceanic rise material should enhance the stress level the effect of abrupt lateral variation in structure e g sudden changes in the slab dip or slab depth may also be significant on the regional stress field viscoelastic modelling attempted for estimating the temporal variation in subduction dynamics suggests that the thrust level will increase southward of the present day thrust zone in java while strike slip fault type stress field will be developed south of sumatra

40ar 39ar illite dating of late caledonian acadian metamorphism and cooling of k bentonites and slates from the welsh basin u k the vacuum encapsulation laser 40ar39ar technique allows extremely small 10 6 g samples of fine grained materials such as diagenetic clays to be dated here we show that the method can be extended to higher grade clay minerals the integration of transmission electron microscopic tem characterization with 40ar39ar dating of vacuum encapsulated samples permits the resolution of the timing of metamorphic growth cooling from the time of diagenesis we have applied this technique to well characterized lower paleozoic slates and k bentonites from the welsh basin which span the transition from anchizonal to epizonal grade which had been previously studied using rbsr and smnd dating tem observations of epizonal k bentonites and slate showed that illite in these samples is of 2m1 polytype of muscovite like composition and oriented parallel to cleavage suggesting that they are of metamorphic origin total gas ages equivalent to conventional kar ages for encapsulated epizonal k bentonites and slate 340 408 ma are considerably variable the ar retention ages calculated from 39ar and 40ar atoms retained in the sample after irradiation are more consistent 383 411 ma the 39ar recoil losses are minor for illites from whole rock samples of epizonal k bentonites but very significant for clay separates of epizonal slate plateaus in age spectra were observed in epizonal k bentonites and slate the plateau ages 414 421 ma and retention ages 383 411 ma can be correlated with the onset of acadian metamorphism and culmination of uplift and inversion of the welsh basin respectively these ages are significantly younger than the 450 ma ages previously reported for diagenetic clays using the same method suggesting that diagenetic history has been lost in these epizonal k bentonites and slate tem observations of anchizonal slates showed that there are two modes of illite the first mode is similar to that observed in epizonal samples suggesting a metamorphic origin the second mode consists of the 1md polytype has typical diagenetic illite composition and is oriented parallel to bedding suggesting a diagenetic origin total gas ages for encapsulated anchizonal slates vary considerably 361 422 ma the retention ages are more consistent 413 432 ma than the total gas ages the 39ar recoil losses are more significant than those for epizonal k bentonites and slate plateaus in age spectra are generally not observed however the consistent retention ages for the anchizonal slates correspond to the plateau ages for the epizonal samples and are inferred to represent the onset of acadian metamorphism these data when combined with our previously published results for diagenetic shales suggest that thermal conditions near the boundary of anchizonal and epizonal grades are necessary to completely reset ar systems in shales and slates

earthquakes gravity and the origin of the bali basin an example of a nascent continental fold and thrust belt we infer from the bathymetry and gravity field and from the source mechanisms and depths of the eight largest earthquakes in the bali region that the bali basin is a downwarp in the crust of the sunda shelf produced and maintained by thrusting along the flores back arc thrust zone earthquake source mechanisms and focal depths are inferred from the inversion of long period p and sh waves for all events and short period p waves for two of the events centroidal depths that give the best fit to the seismograms range from 10 to 18 km but uncertainties in depth allow a range from 7 to 24 km the p wave nodal planes that dip south at 13 to 35 7 strike roughly parallel to the volcanic arc and are consistent with thrusting of crust of the bali basin beneath it the positions of the earthquakes with respect to crustal features inferred from seismic and gravity data suggest that the earthquakes occur in the basement along the western end of the flores thrust zone the slip direction for the back arc thrust zone inferred from the orientation of the earthquake slip vectors indicates that the thrusting in the bali basin is probably part of the overall plate convergence as it roughly coincides with the convergence direction between the sunda arc and the indian ocean plate summation of seismic moments of earthquakes between 1960 and 1985 suggests a minimum rate of convergence across the thrust zone of 4 2 mm a the presence of back arc thrusting suggests that some coupling between the indian ocean plate and the sunda arc occurs but mechanisms such as continental collision or a shallow subduction of the indian ocean plate probably can be ruled out the present tectonic setting and structure of the bali basin is comparable to the early forelands of the andes or western north america in that a fold and thrust belt is forming on the continental side of an arc trench system at which oceanic lithosphere is being subducted the bali basin is flanked by the tertiary java basin to the west and the oceanic flores basin to the east and thus provides an actualistic setting for the development of a fold and thrust belt in which structure and timing of deformation can change significantly along strike on the scale a few hundred kilometers

slab pull and the seismotectonics of subducting lithosphere this synthesis links many seismic and tectonic processes at subduction zones including great subduction earthquakes to the sinking of subducted plate earthquake data and tectonic modeling for subduction zones indicate that the slab pull force is much larger than the ridge push force interactions between the forces that drive and resist plate motions cause spatially and temporally localized stresses that lead to characteristic earthquake activity providing details on how subduction occurs compression is localized across a locked interface thrust zone because both the ridge push and the slab pull forces are resisted there the slab pull force increases with increasing plate age thus because the slab pull force tends to bend subducted plate downward and decrease the force acting normal to the interface thrust zone the characteristic maximum earthquake at a given interface thrust zone is inversely related to the age of the subducted plate the 1960 chile earthquake mw 9 5 the largest earthquake to occur in historic times began its rupture at an interface bounding oceanic plate 30 m y old however this rupture initiation was associated with the locally oldest subducting lithosphere weakest coupling the rupture propagated southward along an interface bounding progressively younger oceanic lithosphere terminating near the subducting chile rise prior to a great subduction earthquake the sinking subducted slab will cause increased tension at depths of 50 200 km with greatest tension near the shallow zone resisting plate subduction plate sinking not only leads to compressional stresses at a locked interface thrust zone but may load compressional stresses at plate depths of 260 350 km provided that the shallow sinking occurs faster than the relaxation time of the deeper mantle this explains k mogi s observations of m 7 thrust earthquakes at depths of 260 350 km immediately downdip and within 3 years prior to five great shallow earthquakes of northern japan the slab pull model explains the lower layer of double seismic zones as due to tension from the deeper sinking plate and the upper layer as due to localized in plate compression as plate motion is resisted by the bounding mantle just downdip of the interface thrust zone there occurs an aseismic 20 50 dip increase of subducted plate this slab bend reflects the summed slab pull force of deeper plate and probably is at the crustal basalt to eclogite phase change resistance to subduction provided by a continually developing slab bend may be an important factor in the size of slab pull force delivered to an interface thrust zone

intermediate and deep seismicity and lateral structure of subducted lithosphere in the circum pacific region in this paper we present a region by region review of the wadati benioff zone structure of most of the world s seismically active subduction zones focusing primarily on the intermediate and deep seismicity lateral changes in wadati benioff zone structure are common in every major subduction zone in this study we use these changes to define possible boundaries between portions or segments of lithosphere with differing subduction geometries although earthquake data seldom have the resolution to show conclusively whether these boundaries separate independent blocks of lithosphere the available data indicate that the active process at most of these segment boundaries is ductile deformation of the subducting plate rather than tearing we found the strongest evidence for the existence of tears where tears are geometrically necessary such as where a transform boundary terminates a trench as at the new hebrides tonga and south sandwich trenches weak evidence suggesting other tears does exist in some regions such as taiwan the japan izu bonin corner and the philippines the causes of these changes in structure are in most cases unclear only about 34 of the possible segment boundaries coincide with subducting bathymetric features some boundaries occur where there is apparent lateral strain caused by anomalous trench geometry we have designed a simple modeling procedure which incorporates published plate motion and the observed geometry of trenches and wadati benioff zones to estimate the lateral strain in subduction zones throughout the circum pacific region although no observed subduction zone has a perfectly strain free geometry there is a broad range of geometries for which the lateral strain is small indeed the observed geometry of most subduction zones involve relatively little lateral strain comparison with centroid moment tensor focal mechanisms indicates that in zones where the modeling predicted little lateral strain the mechanisms of intermediate and deep earthquakes show no effects of lateral stress and downdip stresses are clearly dominant in regions such as the mariana arc where the model predicts very large lateral extension lateral tension is very evident in the focal mechanisms in regions such as the hokkaido corner where the modeling predicts large compressional strains the plate appears to buckle and bending stresses parallel to the trench are evident in general this study finds that subducted lithosphere is remarkably cohesive and rigid and only rarely deforms by breaking or stretching

stress reconstruction and lithosphere dynamics along the sumatra subduction margin abstract gravity modeling and stress reconstructions over the sumatra subduction margin between indian australian and asian plates provide insights into the geodynamic setting and kinematics of the area based on ridge trench interaction plate obliquity plate age and seismicity the area was divided into two sectors northern and southern for gravity modeling along two profiles the stress data with magnitude mw 5 0 and focal depth 100 km for the subducting plate were compiled for six zones i vi and were isolated under different time domains for high resolution study the reconstructed gravity models were used to isolate the events for subducting oceanic indian australian and overriding continental asian plates the data sets for the subducting lithosphere were used for stress inversion to determine the predominant stress regimes for pre earthquake and post earthquake phases of the sumatra earthquake of december 2004 the results were interpreted in terms of operative intra plate stress fields and for reconstruction of kinematics and dynamics along the sumatra subduction margin analysis shows both sparse and intense seismic activities identified over different time segments in the six zones under pre and post december 2004 phases with respect to the incidence of mw 9 2 off sumatra mega earthquake of 2004 the stress regime of the 2004 mega event and subsequent release of strain energy through cyclic changes of the stress obliquities in the northern part of the study area account for heterogeneous coupling caused by rough topography of the subducting indian australian plate reduction of compressive stress regime in the deeper segment of the subducting plate in the northern sector triggered twin great strike slip dominated earthquakes in 2012 in the floor of the northeastern indian ocean the weak zones of a spreading ridge subducting below the asian plate possibly suffered strain localization and weakening the less transmission of co and post seismic strain energy from northern to southern part of the margin was possibly prohibited near the zone of ridge trench interaction the trench orthogonal tearing slab window at the edge of the subducting plate in this area cannot be ruled out the slab window caused by the subduction of the young buoyant ridge and the trench ward movement of the upper plate has transformed the central segment into compressional regime

response of a preexisting cyclonic ocean eddy to a typhoon abstractthe three dimensional responses of a cyclonic ocean eddy coe for 1 2 months following a typhoon were investigated using altimeter data and numerical experiments two significant features were found 1 the cyclonic eddy was enhanced and the three dimensional structure was changed and 2 the cyclonic eddy underwent two processes elliptical deformation and reaxisymmetrization in the horizontal plane these two features are generally associated with typhoon induced upwelling and the dynamic processes of eddy adjustment these results imply that the local ocean processes can be affected by a typhoon through low frequency response

on the origin of the cocos nazca spreading center to investigate the mechanism underlying the break up of the farallon plate into the cocos plate and nazca plate we analyze the state of stress in the farallon plate at about 30 m y b p just prior to the fragmentation to this purpose we use finite element methods and a reconstruction of regional plate boundaries appropriate for 30 m y b p a key role in the model is played by the dependence of two important plate tectonics forces slab pull and ridge push on the age of oceanic lithosphere the results show a highly tensional stress field with maximum principal stresses of 5 to 6 kbar north south tension in the vicinity of present day panama is proposed to have been the cause for the fragmentation of the farallon plate and the inception of spreading along the new cocos nazca plate boundary because the kind of plate interaction that gave rise to the high level of tensional stresses in the farallon plate is not restricted to the area of this study the proposed mechanism seems to shed light on the problem of fragmentation of oceanic plates in general

controls of the structure of subducted slabs numerical simulations of subducting slabs are formulated in which the shape and dip of the slab are determined by the dynamics of the flow rather than imposed a priori the dip of slabs is a function of the time since the initiation of subduction slabs fold develop a kink in dip and thicken on entry into a high viscosity lower mantle comparison of the simulations with seismic observations suggest that the lower mantle is at least 10 30 times more viscous than the upper mantle

stress and flow beneath island arcs summary the flow pattern stress distribution topography and gravity anomalies were computed from numerical models having density and viscosity distributions resemblant to the aleutian arc the results were compatible with the hypothesis that the excess density of the slab drives its descent and that hydrodynamic forces are responsible for topographic and gravity highs over the outer rise seaward of the trench and the frontal arc and lows over the trench in models with simple distributions of rheological parameters the force from the slab was transmitted directly upward producing a negative gravity anomaly over the arc material with low resistance to flow was needed along the fault plane above the slab or within the crust of the frontal arc and within the wedge of asthenosphere above the slab to reduce that force and to allow the horizontal lithosphere to move with the slab models with the resistance to flow thus reduced had outer rises deep trenches horizontal tension seaward of the trench horizontal compression under the trench and downdip tension in the slab free air gravity anomalies which are the sum of between deflections of the free surface due to hydrodynamic forces and direct attractions from the masses driving the flow were not fit excellently by any of the models in part because the coarse grid used precluded accurate representation of the fault zone above the slab and the frontal arc an alternate to the hypothesis that about 5 kb of stress on the fault plane is needed to produce an outer rise is offered by these models shear stress between the slab and the island arc was always below 700 bars in the more successful models if the density distribution was scaled to match the topography of the trench this is much less than the 2000 bars stresses needed if frictional heating causes island arc volcanism

plate tectonics and island arcs the plate tectonic concepts that developed rapidly in the late l960s made possible the understanding of island arcs before that time mobilistic concepts evolved slowly hindered particularly in the united states by an obstructionist geoscience establishment the volcanic belts of island arcs form about 100 km above subducting plates convergent plate boundaries evolve complexly with time and at any one time vary greatly along their lengths seismicity defines positions but not trajectories of descending slabs which sink more steeply than they dip and are overridden by advancing upper plates subduction occurs beneath only one side at a time of an internally rigid plate and the common regime in an overriding plate behind a surficial accretionary wedge is extensional except where a collision is underway back arc basin lithosphere is built behind or by migrating island arcs which lengthen and increase their curvatures a collision can involve two active arcs in which case intervening lithosphere sinks beneath both of them or an active margin and a passive one either type of collision generally is followed by the breaking through of new subduction beneath the composite mass of light crust from a new trench on the outside of the aggregate conversely a new subduction system commonly is a by product of collision a strip of back arc basin crust is in many cases left attached to the aggregate in front of the new trench and becomes the basement for a fore arc basin the leading edge of which is raised as melange is stuffed under it sedimentation in trenches is predominantly longitudinal and can be from distant sources accretionary wedges are dynamic being thickened at both toes and bottoms by tectonic accretion and thinned by gravitational forward flow melange is largely a product of tectonic imbrication and flowage driven by these conflicting processes not of submarine sliding high pressure metamorphic rocks form beneath overriding plates not within wedges in front of them arc magmas incorporate much material from the lithosphere through which they rise and vary correspondingly with the evolving composition of that lithosphere arc crust is inflated into geanticlines by intrusive rocks and thermal expansion submarine island arc volcanic rocks are widely spilitized with na enrichment and ca depletion by hydrothermal reaction with sea water the lower crust of mature island arcs consists of granulite facies rocks of mafic intermediate and felsic intermediate compositions the mohorovicic discontinuity may be primarily a constructional boundary representing the shallow limit off crystallization of voluminous rocks of ultramafic composition or plagioclase free mineralogy

long term seismic observations along myanmar sunda subduction margin insights for 2004 m w 9 0 earthquake the present study investigates the temporal variation of few seismic parameters between the myanmar zone i andaman nicobar northwest sumatra zone ii southeast sumatra west indonesia zone iii and east indonesia zone iv converging boundaries in reference to the generation of 26 december 2004 mw 9 0 off sumatra mega earthquake event the four segments are distinguished based on tectonics parameters distinct geological locations great earthquake occurrences and the wadati benioff zone characteristics two important seismic parameters such as seismic energy and b values are computed over a time window of 6 month period during the entire 1976 2013 period for these segments the b values show a constant decrease in zones ii iii and iv whereas the zone i does not show any such pattern prior to the 2004 mega event the release of seismic energy was also gradually decreasing in zones ii and iii till the 2004 event and little similar pattern was also noted in zone iv this distinct observation might be indicating that the stress accumulation was dominant near the sumatra java area located towards southeast of zone ii and northwest of zone iii the released strain energy during the 2004 event was subsequently migrated towards north rupturing 1300 km of the boundary between the northwest sumatra and the north andaman the occurrence of 2004 mega event was apparently concealed behind the long term seismic quiescence existing near the sumatra and nicobar margin a systematic study of the patterns of seismic energy release and b values and the long term observation of collective behaviour of the margin tectonics might have had given clues to the possibility of the 2004 mega event

low level kinematic thermodynamic and reflectivity fields associated with hurricane bonnie 1998 at landfall during 11 h on 26 august 1998 two noaa wp 3d aircraft deployed 85 global positioning system gps dropwindsondes within 2 of latitude of the circulation center of hurricane bonnie as it made landfall in north carolina about 75 of the sondes successfully collected data which were used to create a series of storm relative horizontal maps of kinematic and thermodynamic variables from 10 m to 2 km reflectivity fields were analyzed from the weather surveillance radar 1988 dopplers wsr 88ds located at wilmington and morehead city north carolina and the tail and lower fuselage radars aboard the wp 3ds gps sonde performance and deployment spacing is adequate to identify several aspects of the vortex these include 1 warm dry stable air in the offshore flow that results in reduced equivalent potential temperatures entering the southern portion of the eyewall 2 cooler air collocated with the upwelled water in the right rear quadrant and under the eyewall and 3 an atypical radial wind pattern with strong inflow southwest of the circulation center and outflow northeast of the center the strongly asymmetric structure found at 10 m becomes much more homogeneous by 2 km altitude no intense rainbands developed over land in the onshore flow nor did the bands in the onshore flow undergo any significant changes once they made landfall beyond the eyewall the offshore flow contained much less precipitation than the onshore portion of the storm characteristics beyond the eyewall appear to have been modulated by the proximity to land but hurricane intensity did not vary the authors infer that the lower energy content of the inflow was offset by the contraction of the eyewall

lithospheric deformation under pre and post seismic stress fields along the nicobar sumatra subduction margin during 2004 sumatra mega event and its tectonic implications abstract a high resolution study was carried out under pre seismic i e static and post seismic stress fields i e dynamic in a space time frame along the nicobar sumatra margin the study reveals that the descending lithosphere records minimum stress obliquity predominant thrust movement and down dip least compressive stress axis under static stress field in northwest sumatra sector iii the imbalance between down dip component of slab pull force and viscous resistive force possibly caused cyclic stress loading in compressive field around the flexing zone 25 km and that undergone brittle failure through generation of mega thrust event on 26th december 2004 a sharp decrease in stress obliquity towards north sector ii redressing of least compressive stress axes from horizontal to down dip direction and increasing thrust movements under dynamic stress field account for continued upward shortening of the lithosphere the weak thinner zone i e between 159 and 217 km depth an age discontinuity portion possibly was collapsed through rapid enhancement of stress induced weakening and strain localization following the 2004 sumatra mega shock it is also well appreciated in the literature that such shallow disruption of the lithosphere is inevitable in the upper mantle if the slab is weakened or broken there and this phenomenon is not uncommon below the sumatra

controls of subduction geometry location of magmatic arcs and tectonics of arc and back arc regions most variation in geometry and angle of inclination of subducted oceanic lithosphere is caused by four interdependent factors combinations of 1 rapid absolute upper plate motion toward the trench and active overriding of the subducted plate 2 rapid relative plate convergence and 3 subduction of intraplate island seamount chains aseismic ridges and oceanic plateaus anomalously low density oceanic lithosphere cause low angle subduction under conditions of low angle subduction the upper surface of the subducted plate is in contact with the base of the overlying plate the wedge of low density asthenosphere is replaced by subducted lithosphere and the width of the arc trench gap either is significantly increased or a magmatic arc is not developed within the overlying plate the fourth factor is age of the subducting lithosphere subduction of young lithosphere produces two opposing tendencies 1 low angle subduction and increased arc trench distance owing to its low density and 2 decreased arc trench distance owing to its higher temperature two factors of secondary importance contribute to variation in subduction zone geometry and arc trench distance accretion of sediment in trenches depresses the upper portion of the subducting oceanic plate and causes the trench axis to migrate seaward prolonged subduction thickens the upper plate depresses the isotherms in the subducted plate and may create a broader arc both factors increase the arc trench gap the four primary factors also control development of other tectonic elements such as regional subsidence for example the amazon basin and a portion of the cretaceous interior seaway of western united states intra arc extension for example the basin and range province foreland fold and thrust belts and laramide style tectonics

the dynamics of boundary layer jets within the tropical cyclone core part i linear theory abstract observations of wind profiles within the tropical cyclone boundary layer have until recently been quite rare the recent massive increase in observations due to the operational implementation of the global positioning system dropwindsonde has emphasised that a low level wind speed maximum is a common feature of the tropical cyclone boundary layer here is proposed a mechanism for producing such a maximum whereby strong inward advection of angular momentum generates the supergradient flow the processes that maintain the necessary inflow against the outward acceleration resulting from gradient wind imbalance are identified as being i vertical diffusion ii vertical advection and iii horizontal advection two complementary tools are used to diagnose the properties and dynamics of the jet the first presented here is a linear analytical model of the boundary layer flow in a translating tropical cyclone it is an extension of the classical ekman boundary layer model as well as of earlier w

the driving mechanism of plate tectonics relation to age of the lithosphere at trenches sea floor age depth and age trench depth relations suggest that the oceanic lithosphere continues to thicken and subside with age beyond 80 ma if so slab pull fs and ridge push fr forces can be calculated from the age of the sea floor at trenches t and if the shear stress acting on the base of the plate increases with absolute velocity a the approximate force balance equation is dva2 svat32 rt where d s and r are constants related to drag slab pull and ridge push respectively the relation between velocity and age is va2 s vat12 r we have tested this model by linear regression using rate and age data for 15 trenches va mm a 2 9 20 32 vat12 26 250 the strength of this correlation r 0 98 is strong evidence in favor of the validity of this simple model these results suggest that ridge push is just sufficient to overcome drag at the base of the plate and does not contribute significantly to the motions of oceanic plates though the value of r is consistent with the motions of plates not attached to subducted slabs

spatial and temporal variations in the andean subduction zone a comprehensive global study of the process of subduction of oceanic lithosphere has made it evident that a major role is played by the age of the subducted lithosphere the age dependence of subduction is of special interest in an understanding of the late cenozoic history of the andean subduction zone many of the complexities of this zone result from the peculiar circumstance that the zone is in transition from an early stage of consumption of old lithosphere 70 ma producing deep earthquakes towards the latest stage of consumption of young lithosphere without deep earthquakes this transition has induced an evolutionary pattern with significant temporal variations notably changes in dip and length of the subducted slab moreover the various segments of the south american subduction zone are in different stages of this evolution in comparison with the other segments the northern chile segment is in the earliest stage and therefore shows a much closer resemblance to the classic deep subduction zones in the western pacific than do other parts of the south american zone a roughly n s symmetry in development is found around northern chile with the ecuadorian and southern chilean near 40 45 s segments representing the latest stage to a large extent the temporal and spatial variations in lithospheric age at the trench originate from the reorganization in plate configuration which started with the break up of the farallon plate into the cocos plate and the nazca plate at about 25 ma

a thin viscous sheet model for continental deformation summary for the purposes of describing its large scale and long term deformation the continental lithosphere is regarded as a continuum obeying a newtonian or a power law rheology the flow of a thin sheet of power law material overlying an inviscid substrate is studied under the assumption that vertical gradients of the horizontal velocity are negligible a numerical model is used to investigate the deformation of such a sheet under conditions approximating those of continent continent collision the material flows in response to forces applied to its boundaries for example the indenting of one continent by another and to forces in its interior arising from gradients in crustal thickness the horizontal divergence of the flow produces changes in the crustal thickness and hence a time dependent form to the flow itself for a given set of boundary conditions the flow depends on the stress exponent in the power law rheology n and on the argand number which is a measure of the ratio between the stress arising from crustal thickness contrasts and the stress required to deform the material at the ambient strain rates when the effective viscosity of the medium is very high ar o crustal thickness variations do not influence the flow if the material is newtonian n l the deformation associated with an influx of material approximating an indenter is of much greater lateral dimension than the width of the indenter whereas when material has a power law rheology n 3 5 are used the deformation is confined to a region of lateral extent comparable to that of the indenter as ar increases the forces arising from crustal thickness contrasts exert more influence on the flow and the maximum crustal thickness that can be sustained by a given influx of material is related by a simple expression to the effective viscosity of the medium at the ambient strain rates in the limit of a very weak medium ar 10 the lithosphere is unable to sustain appreciable crustal elevation contrasts the results of these numerical experiments show that systems in which the effective viscosities are such that the maximum deviatoric stresses are between 1 kbar

plate reconstructions aseismic ridges and low angle subduction beneath the andes low angle subduction segments beneath peru and chile are believed to represent interaction of the subduction zone with the aseismic nazca and juan fernandez ridges on the basis of symmetric sea floor spreading models and plate hotspot reconstructions predicted continuations of the relatively buoyant ridges correspond well with the limits of the low angle subduction segments further the history of interaction of the aseismic ridges with the subduction zone as predicted by plate reconstructions is not inconsistent with available information on volcanic episodes of the andes if the contemporary correlation of volcanic gaps and low angle subduction is also applicable to the rest of the late cenozoic the occurrence of deep seismicity beneath the eastern flanks of the andes is also believed to reflect the effects of low angle subduction the deep seismic segments are adjacent to segments which are presently or were recently experiencing low angle subduction thus the deep seismicity is interpreted as representative of remnants of normal oceanic lithosphere subducted before interaction of aseismic ridges with the subduction zone other geologic phenomena are associated with low angle subduction beneath south america including landward shift in loci of tectonism surficial subsidence and laramide style deformation the aseismic ridge buoyancy hypothesis appears to be a valuable predictive tool for the interpretation of such geologic effects

joints as neotectonic plate signatures abstract it is the contention of the present author that surface joints are signatures of the plate tectonic stress field according to the writer s view joints are shear features caused by plate tectonic stresses with the maximum compression axis lying in the smaller quadrant formed by the regional sets these contentions are substantiated by new joint measurements geotectonic lines in europe are reflected in the joint orientations and the subduction direction in the philippines is indicated by them other instances of joints being plate tectonic signatures which have been published previously are referred to

application of continuum models to deformation of the aleutian island arc continuum models were constructed to describe large scale deformation of the aleutian island arc over the past 5 m y these models consider the island arc as a continuum in the horizontal plane with the velocity boundary condition at the pacific edge stated as a fraction of pacific plate convergence transferred to the arc first a simple model of uniformly distributed strain is formulated to illustrate the mechanics of continuous deformation lineaments along the arc massif rotated about a vertical axis are matched by small element rotation calculated from the model however this model does not predict across arc variations in deformation and produces an unrealistic amount of crustal thickening after 5 m y a physically more meaningful model of deformation is the thin viscous sheet model based on averages of stress and rheology throughout the lithosphere the amount of motion transferred from the pacific plate to the arc is constrained by the rotated lineaments while the effective stress strain exponent n and the ability the lithosphere has to sustain crustal thickness contrasts the argand number are independent variables primarily bathymetry earthquake focal mechanisms and styles of faulting are used to evaluate the models the preferred model is one where the amount of motion transferred from the pacific plate is greater in an arc parallel direction than in an arc normal direction producing stresses consistent with strike slip faulting at the far western end of the arc and tensional stresses consistent with transverse normal faulting elsewhere in the arc massif this model agrees with observations of slip vectors by ekstr6m and engdahl 1989 who conclude that a portion of the arc parallel component of relative plate motion is taken up in the overriding plate this model implies that compressive stress transferred to the arc is small in comparison to along arc shear stress and that stresses conducive to strikeslip faulting are prevalent throughout the arc

accretion and lateral variations in tectonic structure along the peru chile trench abstract extensive marine geophysical data sets on the deformational characteristics of the oceanic crust and of sediments in and near the peru chile trench notably those presented by schweller kulm and co workers have established the presence of significant north south variations along the trench in this paper we investigate the physical basis for these variations by studying the stress field in the nazca plate with emphasis on the region near the trench finite element methods were employed to calculate the stress distribution the resulting regional stress field near the trench is characterized by compression normal to the trench at latitudes between 1 s and approximately 16 s tension in the range 16 s 27 s and again compression from 27 s to 45 s many conspicuous and hitherto unexplained variations along the trench system can be understood by taking into account these lateral changes a close correlation appears to exist between the latitude ranges with compression and tension normal to the trench and the accreting and non accreting provinces of the peru chile trench respectively the basis for this correlation must be sought in the role which grabens in the seaward trench slope play in the subduction or accretion of sediments deep grabens can trap the sediments and carry them down into the subduction zone leaving little or no material to be accreted onto the margin the regional stress field is superimposed on the stresses generated in bending the nazca plate prior to subduction tension in the direction normal to the trench as inferred for the nazca plate off southern peru and northern chile 16 s 27 s lowers the neutral plane in the bending plate and increases the tensional stresses near the surface this promotes the formation of grabens and consequently the subduction of sediments similarly compression normal to the trench counteracts the formation of deep grabens in the latitude ranges 1 s 16 s and 27 s 45 s thus favouring accretion of sediments from this study we infer that the regional stress field may have a significant effect on the topography of the seaward trench slope taking into account the role of regional stress in graben formation not only explains first order latitudinal variations in accretion and subduction of sediments along the peru chile trench but it is also expected to contribute significantly to understanding similar variations observed in other subduction zones

finite strain calculations of continental deformation 2 comparison with the india asia collision zone numerical experiments on a thin viscous sheet model for deformation of continental lithosphere subjected to an indenting boundary condition yield distributions of crustal thickness of stress and strain rate and of latitudinal displacements that may be compared with observations in the india asia collision zone a simple indenting boundary condition applied to initially laterally homogeneous sheets obeying a power law rheology produces results that are in broad agreement with the observations provided that the power law exponent is three or greater and the sheet can support vertically integrated stress differences of 2 1013 5 1012 n m 1 in regions in front of the indenter under these conditions the calculated deformation shows accommodation of convergence primarily by crustal thickening to produce a plateau in front of the indenter palaeomagnetic data from india and tibet and the observed distribution of topography suggest that much of the post eocene convergence of india with asia has been taken up by deformation within asia that involved crustal thickening the principal difference between calculation and observation is the absence from the calculated strain rate fields of east west extension of the plateau in front of the indenting boundary the calculations show that once such a plateau is formed the buoyancy force associated with the crustal thickness contrast inhibits further thickening and the plateau strains at less than half the rate of its immediate surroundings seismically determined regional strain rates exhibit a similar distribution with the tibetan plateau straining at about one quarter the rate of the tien shan and ningxia gansu regions calculated principal compressive stress orientations and regional strain rates agree with the seismically determined quantities in the mongolia baikal tien shan tibet and ningxia gansu regions of asia to within the uncertainty of the latter the vertically integrated stresses that are calculated for the viscous sheet are comparable with those that can be supported by a theologically stratified continental lithosphere obeying laboratory determined flow laws we suggest that the thin viscous sheet model described in this paper and its companion gives a simple and physically plausible description of the observed deformation in central asia in this description the predominant mechanism of accommodation of continental convergence is diffuse crustal thickening with shear on vertical planes playing a subsidiary role once large crustal thickness contrasts have been established

the boundary layer of tropical cyclone kerry 1979 abstract the boundary layer structure of tropical cyclone kerry 1979 is investigated using composite analysis of research aircraft surface ship and automatic weather station observations the boundary layer was moist convective and strongly confluent to the east of the tropical cyclone center but was dry subsident and diffluent to the west the vertical momentum transport in the eastern convective sector of kerry was around two to three times the surface frictional dissipation in contrast the stable boundary layer in the western sector consisted of a shallow mixed layer capped by an equivalent potential temperature minimum and a low level jet which underwent a marked diurnal oscillation three mechanisms appear to have contributed to the observed asymmetry 1 a general zonal distortion arose from cyclonic rotation across a gradient of earth vorticity 2 a westerly environmental vertical shear produced forced ascent on the east side of the storm and subsidence on the west side throughout the

stress in the indo australian plate cloetingh s and wortel r 1986 stress in the ind austr ian plate in b johnson and a w bally editors intraplate deformation characteristics processes and causes tectonophysics 132 49 67 we modelled the state of stress in the indo australian plate in order to investigate quantitatively variations observed in tectonic style the numerical procedure incorporates the dependence of slab pull and ridge push on the age of the oceanic lithosphere estimates are presented for the average net resistive forces at the himalayan collision zone the suction force acting on the overriding indo australian plate segment at the tonga kermadec trench and the drag at the base of the lithosphere our modelhng shows a concen ation of compressive stresses of the order of 3 5 lcbar in the ninetyeast ridge area the effects of the compressive resistance associated with himalayan collision and subduction of young lithosphere off the northern part of the sunda arc are focused in this region the stress field as calculated gives a consistent explanation for the observed concentration of seismic activity stein and okal 1978 and significant deformation in the oceanic crust weissel et al 1980 mcadoo and sandwell 1985 in the area the calculated stress field in the area adjacent to the southeast and central india ridges is characterized by tension parallel to the spreading axis this explains the concentration of near ridge normal faulting seismicity with t axes subparaile1 to the spreading ridge in the indian ocean as recently observed by bergman et al 1984 and wiens and stein 1984 the regional stress field along the strike of the sunda arc varies from compression seaward of and parallel to the sumatra trench segment to tension perpendicular to the java flores segment this explains the selective occurrence of well developed grabens seaward off the java flores segment of the trench observed by hilde 1983 our modelling shows that the observed rotation of the stress field denham et al 1979 in the australian continent is mainly the consequence of its geographic position relative to the surrounding trench segments and the variations of the forces acting on the down going slab in each of these the state of compression in west and central australia is induced by the action of resistive forces at the himalayan and banda arc collision zones the joint occurrence of high levels of compression in the plate s interior and normal faulting seismicity in the near ridge areas is a transient feature unique to the present dynamic situation of the indo australian plate

subduction zone stresses constraints from mechanics and from topographic and geoid anomalies abstract there are in principle direct relations between several important phenomena associated with subduction zones the depth of oceanic trenches the magnitude of the net force from trenches acting on subducting plates the distribution and fault plane orientations of earthquakes the magnitude of stresses on subduction faults the bathymetry of back arc regions and the magnitudes of gravity and geoid anomalies these phenomena are related through the stresses transmitted through surface and subducted lithosphere and are associated with the mass anomaly of the subducted lithosphere quantitative estimates suggest that observed trench depths imply a trench pull force on subducting plates which is comparable to the ridge push force but much less than the excess weight of the subducted lithospheric slab it is further suggested that either the mass anomaly of subducted lithosphere is much less than would be expected on the basis of conventional thermal and compositional models or that a a large resistance acts on the upper part of slabs due to high stress corner flow and b the mass anomaly of the slab is 70 90 compensated either by a broad 1 km deep back arc depression or a low density mantle wedge above the slab or both

factors affecting seismic moment release rates in subduction zones the amount of shallow seismic activity in subduction zones varies greatly from region to region we quantify this seismicity by calculating seismic moment release rates and seismic slip rates for 24 subduction zones to calculate the moment release we sum the seismic moment for all interplate thrust type events with surface wave magnitude ms 7 0 occurring from 1904 to 1980 we present a time history plot of the seismic moment release for each subduction zone these exhibit the differences in the seismic release patterns for subduction zones where the time window of our study is not representative the total moment release is corrected using information on repeat times the moment release rates are compared with various subduction parameters in order to determine which factors influence the degree of coupling these parameters include the age of the subducting lithosphere absolute velocities of the upper and subducting plates convergence velocity and length maximum depth and dip of the wadati benioff zone the moment release rate decreases as the age of the subducting lithosphere increases when the zones belonging to a single subducting plate are considered this age versus moment release relation is consistent for the zones in which the pacific cocos philippine sea and indian plates are subducting the moment release rates for the subduction zones in which the pacific plate is subducting are much higher than for zones of other plates with similar age the age versus moment release relation holds among the zones which belong to one subducting plate however zones with similar ages but belonging to different plates do not have the same moment release rates this suggests that within a single plate the age is the dominating factor affecting the strength of seismic coupling but that each plate as a whole has a characteristic moment release budget zones with retreating upper plates tend to have lower moment release rates the moment release rate does not increase with convergence velocity no simple relationship was found between these two parameters the moment release rate depends most clearly on the age of the subducting lithosphere and the absolute velocity of the upper plate these are the two independent subduction zone parameters among the variables considered the other variables depend on these two parameters

tectonic features of the southern sumatra western java forearc of indonesia 1 multichannel reflection seismic profiles along the active sunda arc where the indo australian plate subducts under the overriding eurasian margin revealed two accretionary wedges the inner wedge i is of assumed paleogene age and the outer wedge ii is of neogene to recent age the inner wedge i is composed of tectonic flakes stretching from southeast sumatra across the sunda strait to northwest java implying a similar plate tectonic regime in these areas at the time of flake development during upper oligocene today wedge i forms the outer arc high and the backstop for the younger outer wedge ii the missing outer arc high of the southern sunda strait is explained by a combination of neogene transtension due to a clockwise rotation of sumatra with respect to java and by arc parallel strike slip movements the rotation created transtensional pull apart basins along the western sunda strait semangka graben as opposed to transpression and inversion on the eastern sunda strait within the new detected krakatau basin the arc parallel transpressional mentawai strike slip fault zone mfz was correlated from the sumatra forearc basin to the northwest java forearc basin off the sunda strait northward bending branches of the mfz are connected with the right lateral sumatra fault zone sfz along the volcanic arc segment on sumatra it is speculated that the sfz was attached to the cimandiri pelabuhan ratu strike slip fault of java prior to the presumed rotation of sumatra and that since the late lower miocene the main slip movement shifted from the volcanic arc position to the forearc basin area due to increasingly oblique plate convergence

seismotectonics of subduction and back arc rifting under the andaman sea abstract active underthrusting of the indian plate below the burma plate produces seismic activity under the andaman and burmese arcs it is suggested that significant differences exist in the configuration of the benioff zone underlying the two arcs several parallel and linearly extensive fault zones are outlined by epicentral distribution in the andaman arc the andaman back arc spreading ridge is defined by a relatively narrow shallow seismic zone that orients nne to sse in north and south parts of the andaman sea respectively the spreading ridge is offset by active transform faults on the far north the spreading ridge appears to connect to a major transform fault in burma across the burmese coast focal mechanism study provides evidence for tensional stress near the inner wall of the andaman trench dominant compressive deviatoric stress within the overriding burma plate and tensional stress under the back arc spreading ridge

spherical shell tectonics effects of sphericity and inextensibility on the geometry of the descending lithosphere the shape of a deep seismic zone is thought to represent that of the descending slab of lithosphere the lithosphere before subduction is a spherical shell and the shape of the descending slab is the result of the deformation of the spherical lithosphere at the subduction zone upon bending a spherical shell often deforms in a very different way from a simple plate we examine whether the actual shape of the descending slab can be explained by a simple bending of an inextensible spherical shell which shows little surface deformation under moderate stress this examination is made region by region for most of the subduction zones in the world by means of an analogue method the lithosphere is simulated by an inextensible spherical shell made of polyvinyl chloride resin the wadati benioff zone is shaped by plaster by referring to the reliable hypocentral data selected from the international seismological centre isc bulletins the spherical shell is forced to fit the miniature of the wadati benioff zone fitting is first attempted only by bending if a good fit is not attainable and if a discontinuity or gap in seismic activity is observed in the relevant region the spherical shell is torn along this discontinuity or gap and the goodness of fit is reexamined the results of the analysis are summarized as follows 1 the shape of the wadati benioff zone can be simulated largely by a simple bending of a spherical shell without surface deformation 2 in almost all of the regions of poor fit with bending a good fit can be achieved by tearing the spherical shell along the trace of low seismicity the sphericity of the lithosphere and the inextensibility upon deformation are the two essential factors in controlling the slab shape this means that the lateral constraint is most important for understanding the geometry of the downgoing slab of lithosphere and the stress state within it further several problems related to the deformational characteristics of the spherical lithosphere are also reviewed and discussed in connection with subduction tectonics

three dimensional numerical modeling of detachment of subducted lithosphere recent seismological studies suggest that slab detachment has occurred in the mediterranean and the new hebrides subduction zones subducted slabs in these regions are recognized to be torn at depths ranging from 100 to 300 km presumably caused by the lateral migration of the tear along the strike of the slab to investigate the physical mechanism of the slab detachment and in particular its migration we constructed a viscoelastic three dimensional finite element model and introduced a small initial tear from one side of the slab we investigated spatio temporal variations in the state of stress within the slab as a function of tear length rheology and a variety of force distribution our results show that an area of high shear stress concentration of the order of several hundred megapascals forms near the tip of the tear inside the slab which is probably sufficient to cause further lateral migration of the tear the stress concentration increases with the length of the tear and lower viscosity values of the surrounding mantle and increases with downdip tension from our modeling we conclude that favorable conditions for slab detachment are characterized by a high interplate frictional force at a subduction zone and a low convergence rate forming in plate tensional stress at intermediate depths such a condition is indeed observed in the dinarides hellenic and the new hebrides subduction zone

a simple global model of plate dynamics and mantle convection cooling and thickening of lithospheric plates with age and subduction result in large scale horizontal density contrasts tending to drive plate motions and mantle flow we quantify the driving forces associated with these density contrasts to determine if they can drive the observed plate motions first two dimensional models are computed to evaluate the effects of assumed rheologies and boundary conditions we are unable to obtain platelike behavior in viscous models with traction free boundary conditions the piecewise uniform velocities distinctive of plate motion can be imposed as boundary conditions and the dynamic consistency of the models evaluated by determining if the net force on each vanishes if the lithosphere has a newtonian viscous rheology the net force on any plate is a strong function of the effective grid spacing used leading to ambiguities in interpretation incorporating a rigid plastic lithosphere which fails at a critical yield stress into the otherwise viscous model removes these ambiguities the model is extended to the actual three dimensional spherical plate geometry the observed velocities of rigid plastic plates are matched to the solution of the viscous stokes equation at the lithosphere asthenosphere boundary body forces from the seismically observed slabs from the thickening of the lithosphere obtained from the actual lithospheric ages and from the differences in structure between continents and oceans are included interior density contrasts such as those resulting from upwellings from a hot bottom boundary layer are assumed to occur on a scale small compared to plate dimensions and are not included the driving forces from the density contrasts within the plates are calculated and compared to resisting forces resulting from viscous drag computed from the three dimensional global return flow and resistance to deformation at converging boundaries the rms residual torque is 30 of the driving torque the density contrasts within the plates themselves can reasonably account for plate motions body forces from convection in the interior may provide only a small net force on the plates at converging boundaries the lithosphere has a yield stress of 100 bars drag at the base of the plates is 5 bars and resists plate motion the net driving forces from subducting slabs and collisional resistance are localized and approximately balance driving forces from lithospheric thickening are distributed over the areas of the plates as is viscous drag the approximate balance of these two forces predicts plate velocities uncorrelated with plate area as observed the model represents a specific case of boundary layer convection the dynamical results are consistent with either upper mantle or mantle wide convection

effects of preexisting cyclonic eddies on upper ocean responses to category 5 typhoons in the western north pacific 1 this study examines the impacts of preexisting mesoscale cyclonic eddies pces on successive enhanced sea surface cooling in response to the passage of super typhoon hai tang in the western north pacific in 2005 using numerical simulation methods we have done two numerical experiments one with the presence of these pces resolved by the hybrid coordinate ocean model navy coupled ocean data assimilation system exphyc and another with world ocean atlas 2001 climatology as initial conditions expwoa the results show that the cooling response simulated by expwoa is only half of that simulated by exphyc which is close to satellite observations this suggests that an accurate representation of the upper dynamic conditions is required to estimate the sea surface cooling to a typhoon accurately subsequently the effects of the pces on successive cooling response to most major typhoons are evaluated by conducting a systematical analysis with a focus on category 5 typhoons occurring in the region from 2003 to 2008 satellite altimeter sea surface height anomaly data and merged tropical rainfall measuring mission microwave imager advanced microwave scanning radiometer for eos microwave sea surface temperatures sst are used to characterize pces and cooling responses to those typhoons the results identify the relationship between pces and successive enhanced sst cooling for most strong typhoons in the western north pacific

subduction back arc spreading and global mantle flow the shapes and orientations of benioff zones beneath island arcs interpreted as marking the location of subducted lithosphere provide the best presently available constraints on the global convective flow pattern associated with plate motions this global flow influences the dynamics of subduction subduction zone phenomena therefore provide powerful tests for models of mantle flow we compute global flow models which while simple include those features which are best constrained namely the observed plate velocities applied as boundary conditions and the density contrasts given by thermal models of the lithosphere and subducted slabs two viscosity structures are used for one flow is confined to the upper mantle while for the other flow extends throughout the mantle instantaneous flow velocity vectors match observed benioff zone dips and shapes for the model which allows mantle wide flow but not for the upper mantle model which has a highly contorted flow pattern the effect of trench migration on particle trajectories is calculated it is not important if subduction velocities are greater than migration rates two dimensional finite element models show that including a coherent high viscosity slab does not change these conclusions a coherent high viscosity slab extending deep into the upper mantle would significantly slow subduction if flow were confined to the upper mantle the maximum earthquake magnitude m w for island arcs correlates well with the age of the subducted slab and pressure gradient between the trench and back arc region for the whole mantle but not the upper mantle flow model the correlations with orientations of benioff zones and seismic coupling strongly suggest that the global return flow associated with plate motions extends below 700 km for both models regions of back arc spreading have asthenospheric shear pulling the back arc toward the trench regions without back arc spreading have the opposite sense of shear suggesting global flow strongly influences back arc spreading

the 1977 sumba earthquake series evidence for slab pull force acting at a subduction zone the great 1977 sumba earthquake occurred at the eastern sunda trench just west of the collision of australian continental lithosphere with the island arc the length of the aftershock zone of this normal faulting earthquake is about 200 km aftershocks are concentrated 65 115 km east of the main shock epicenter with very few aftershocks in a 50 km long segment that spans the main shock epicenter relocated hypocenters and focal mechanism data are consistent with normal faulting throughout the upper 28 km of the oceanic lithosphere there is no evidence for thrust faulting of the deeper aftershocks these data imply that the neutral bending surface must be at least 35 40 km deep a second aftershock zone about 180 km northwest of the main shock became active immediately following the main shock but events were concentrated during days 50 52 this zone is a 70 km long lineation that trends toward the main shock epicenter and reflects right lateral strike slip faulting within the subducted oceanic plate seismicity exists to a depth of about 650 km in the very old plate beneath the sunda banda arc and that plate s negative buoyancy causes very large slab pull forces great interface thrust earthquakes are absent at the sumba region and slab pull forces are inferred to have partially decoupled the subducted plate from the overriding plate this decoupling permits slab pull stresses to be guided updip to the region of the sumba main shock such shallow acting slab pull provides a bending moment at the trench and explains the deformation and timing observed for the entire sumba earthquake series in this model slab pull forces stretch the subducted plate until the increasing stresses at the shallow subduction zone lead to a subduction zone earthquake postseismically the released oceanic plate undergoes a pulse of downdip strain returning the plate to a less extended state the moment of this downdip plate motion could exceed the seismic moment of the main shock

age dependence of oceanic intraplate seismicity and implications for lithospheric evolution we have determined depths for 16 oceanic intraplate earthquakes using body wave modeling a data set composed of these and 11 other well constrained depths shows that the maximum depth of seismicity deepens with increasing lithospheric age and appears to be bounded by a 700 800 c isotherm the maximum depth to which intraplate earthquakes occur is approximately equal to the flexural elastic thickness but much less than the seismic thickness from surface wave dispersion the maximum faulting depth is consistent with the predictions of rapid weakening at high temperatures for dry olivine rheologies determined by experimental rock mechanics oceanic intraplate earthquakes can occur at greater depths and temperatures than has previously been reported for continental crustal events presumably because oceanic lithosphere is stronger at high temperatures both the number of earthquakes and the cumulative seismic moment decrease with lithospheric age the cause of this trend is unclear possibilities include nonrandom location of preexisting weak zones with respect to the age provinces fault strengthening with age or a decrease in the stress level a decrease in intraplate stress with age is opposite that expected from ridge push stresses which increase with age but may result either from thermoelastic stresses which are greatest in young lithosphere or near ridge stress concentrations due to local heterogeneities in the spreading process

heterogeneities of the instrumental seismicity catalog 1904 1980 for strong shallow earthquakes we analyze the world wide consistency of teleseismic reporting completeness of the seismicity record and homogeneity of magnitude determination for strong shallow earthquakes magnitude m 6 depth h 70 km in the period 1904 to 1980 for earthquakes with magnitude m 7 we used the catalog given by abe 1981 which lists the surface wave magnitude ms of every reported large event that occurred between 1904 and 1980 according to the original formulation of gutenberg 1945 under the postulate that the rate of earthquake occurrence for the entire world as a whole is consistent on a time scale of decades and that since the installation of the world wide standardized seismograph network in the early 1960s the earthquake catalog for strong m 6 shocks is complete and the seismicity rates are typical of all periods in the century we find that for instrumental related reasons the ms of large events prior to 1908 have been systemically overestimated by at least 0 5 magnitude unit and the ms of events in the period 1908 to 1948 have been consistently overestimated by 0 2 unit relative to the ms assigned to shocks occurring after 1948 when these corrections are taken into account the catalog of events with ms 7 0 is shown to be largely complete i e nearly all the large shallow shocks which occurred in the earth are listed for most of the world since early in the century for earthquakes as the magnitude 6 level we used the international seismological center isc tape and regional catalogues of earthquakes this file compiles the earthquake data given by gutenberg and richter 1954 for the first half of the century and data from various international agencies after 1945 we find a highly incomplete seismicity catalog at this magnitude level for the first half of the century mainly due to numerous increases and decreased in the earthquake detection and reporting capabilities a more complete but extremely inhomogeneous catalog for the period 1950 to 1963 due to the use of different formulations and criteria to calculate and report the parameter magnitude by the various seismological agencies and a largely complete and homogeneous in ms record since 1964

the java margin revisited evidence for subduction erosion off java the eastern sunda margin off indonesia from central java to sumba island remains a little investigated subduction zone contrary to its well studied northwestern segment whereas large portions of the sunda margin are considered a classical accretionary zone subduction characteristics along the central java sector indicate erosive processes as the dominant mode of mass transfer the tectonic framework of the central java margin with a convergence rate of 6 7 cm yr insignificant sediment input and a pronounced seafloor roughness where the oceanic roo rise is subducting underneath java facilitates subduction erosion evidence for erosion comes from newly acquired geophysical data off central java local erosive processes in the wake of seamount subduction are documented by a high resolution bathymetric survey and result in an irregular trend of the deformation front sculpted by seamount collision scars subduction of oceanic basement relief leads to large scale uplift of the forearc as recorded on a reflection seismic profile and to a dismemberment of the previous outer forearc high giving way to isolated topographic elevations the broad retreat of the java trench and deformation front above the leading edge of the roo rise has exposed an area of approximately 25 000 km2 of deeper seafloor formerly covered by the previous frontal prism frontal erosion coincides with a steepening of the lower slope angle in the central java sector compared to the neighbouring segments in global compilations the key geological parameters of the central java margin lie in the erosive regime reflecting the interplay of basement relief subduction negligible sediment supply and a high convergence rate on the evolution of the margin

the dynamics of boundary layer jets within the tropical cyclone core part ii nonlinear enhancement abstract observations of wind profiles within the tropical cyclone boundary layer until recently have been quite rare however the recent spate of observations from the gps dropsonde have confirmed that a low level wind speed maximum is a common feature of the tropical cyclone boundary layer in part i a mechanism for producing such a maximum was proposed whereby strong inward advection of angular momentum generates the supergradient flow the processes that maintain the necessary inflow against the outward acceleration due to gradient wind imbalance were identified as being i vertical diffusion ii vertical advection and iii horizontal advection and a linear analytical model of the boundary layer flow in a translating tropical cyclone was presented and used to diagnose the properties of the jet and the near surface flow a significant shortcoming was that the jet was too weak which was argued to be due to the neglect of vertical advection here a high resolution dry hydrostatic numerical

regional stress field of the indian plate we have calculated the regional stress field in the indian plate implementing dependence of slab pull and ridge push on the age of the oceanic lithosphere in the finite element procedure the high level of the calculated stress field order of a few kbar and the dominance of compression in the plate are consequences of the unique dynamic situation of the present day indian plate the calculated stress field explains the concentration of intraplate deformation in the region of the ninetyeast ridge and yields insight into the variations in stress directions in the australian continent

seismic coupling and uncoupling at subduction zones seismic coupling has been used as a qualitative measure of the interaction between the two plates at subduction zones kanamori 1971 introduced seismic coupling after noting that the characteristic size of earthquakes varies systematically for the northern pacific subduction zones a quantitative global comparison of many subduction zones reveals a strong correlation of earthquake size with two other variables age of the subducting lithosphere and convergence rate the largest earthquakes occur in zones with young lithosphere and fast convergence rates while zones with old lithosphere and slow rates are relatively aseismic for large earthquakes results from a study of the rupture process of three great earthquakes indicate that maximum earthquake size is directly related to the asperity distribution on the fault plane asperities are strong regions that resist the motion between the two plates the zones with the largest earthquakes have very large asperities while the zones with smaller earthquakes have small scattered asperities this observation can be translated into a simple model of seismic coupling where the horizontal compressive stress between the two plates is proportional to the ratio of the summed asperity area to the total area of the contact surface while the variation in asperity size is used to establish a connection between earthquake size and tectonic stress it also implies that plate age and rate affect the asperity distribution plate age and rate can control asperity distribution directly by use of the horizontal compressive stress associated with the preferred trajectory i e the vertical and horizontal velocities of subducting slabs are determined by the plate age and convergence velocity indirect influences are many including oceanic plate topography and the amount of subducted sediments all subduction zones are apparently uncoupled below a depth of about 40 km and we propose that the basalt to eclogite phase change in the down going oceanic crust may be largely responsible this phase change should start at a depth of 30 35 km and could at least partially uncouple the plates by superplastic deformation throughout the oceanic crust during the phase change

seismic history and seismotectonics of the sunda arc historic records of the last 300 years reveal two great interplate earthquakes 1833 mw 8 1861 mw 8 8 which ruptured major segments of the sumatra fore arc in western indonesia a significant percentage of interplate slip along this portion of the plate boundary appears to occur seismically the ends of these rupture zones are coincident with clusters of large ms 7 and moderate 6 ms 7 shocks and with heterogeneities in the plate interface as inferred from geologic and geophysical data the northern extent of the 1833 rupture zone is coincident with a group of moderate and large earthquakes a structural arch in the fore arc and a fracture zone on the subducted plate the southern portion is coincident with a very prominent cluster and the along strike extension of a sediment filled trough on the subducted plate prior to this study sumatra was characterized as relatively aseismic as inferred from the lack of great earthquakes in the instrumental record of this century java and the lesser sunda islands had major ms 6 earthquakes in the historic record but none are of the same magnitude as the great events near sumatra there is no clear evidence that shocks in the java fore arc were interplate events yet geophysical data indicate recent uplift of the trench slope break and a change in strike of the trench axis where the northernmost flank of the roo rise a prominent bathymetrie high appears to be interacting with the fore arc this feature was formed at a ridge crest and is more buoyant and thus more difficult to subduct than the surrounding seafloor there is no increase in seismicity associated with this tectonism however that may indicate a lack of contact between the crystalline portions of the plates at this time we infer that a majority of slip on the plate interface near java occurs aseismically the variation in occurrence of great interplate earthquakes along the sunda arc may be interpreted in terms of a model that attributes variation in interplate coupling to the age of the subducted lithosphere great interplate earthquakes occur near sumatra where the age of the youngest crust is 46 ma the characteristics of the sunda arc and analogies with pacific arcs imply that the entire length of sumatra has the potential to produce great thrust earthquakes its seismic potential should be considered high as there has been no recurrence of the great events of 1833 and 1861 in contrast no such events have been reported off java and the lesser sunda islands where the oldest crust is 152 ma the plate interface near java and the lesser sunda islands should be considered to have low seismic potential

active tectonics of the eastern sunda and banda arcs the tectonics of the eastern sunda and banda arcs of eastern indonesia are inferred from the centroid depths seismic moments and fault plane solutions of 63 large earthquakes that occurred between 1962 and 1984 source parameters are estimated by inverting long period teleseismic p and sh waves earthquakes show that the collision of the australian continent with the banda arc shortens the upper plate beneath the forearc and back arc basin over the broad area between the timor and seram troughs along the eastern end of the sunda arc and along the southern banda arc earthquakes reveal that the arc and forearc respond to collision by shortening in the direction of convergence by elongating in the direction perpendicular to convergence and by thrusting over the back arc basin shallow thrust and strike slip earthquakes beneath the banda basin the back arc basin demonstrate that deformation within the back arc accommodates some of the northward motion of the australian continent the rate of north south shortening of the entire upper plate at the longitude of timor calculated from the seismic moments of the earthquakes over the 22 year period is roughly 20 of the predicted convergence rate between australia and southeast asia strike slip faulting within the banda basin results also in eastward motion of the banda arc with respect to western indonesia and australia eastward extrusion of the banda arc results in thrusting at the aru trough earthquakes due to thrusting of the back arc basin beneath the volcanic arc are too shallow to corroborate that the arc s polarity has reversed although the wetar back arc thrust zone at present has a high seismic slip rate earthquakes and their mechanisms provide evidence that the lithosphere of the weber basin a 7500 m deep forearc basin rests on the subducting lithosphere without intervening asthenosphere so that the weber basin subsides in response to sinking of the subducting lithosphere closure of the banda basin is geometrically incompatible with the australian plate subducting simultaneously beneath both the timor and seram troughs instead the bird s head which subducts beneath seram is decoupled from the australian plate in western new guinea and probably moves west or southwest with respect to australia in the eastern indonesian example strike slip faulting plays a major role in the evolution of the collision zone and clearly demonstrates the complexity in three dimensions of continental accretion and mountain building by arc continent collision

the geometry of back arc thrusting along the eastern sunda arc indonesia constraints from earthquake and gravity data the flores earthquake of december 23 1978 represents the first seismological evidence for active back arc thrusting behind the eastern sunda arc this mb 5 8 earthquake occurred north of flores island where seismic reflection profiling has revealed back arc thrusting interpreted to be a reaction to compression of the arc following collision with the australian continent we investigate the source mechanism and depth of the flores earthquake by inversion of long period p waveforms and relocate the earthquake s epicenter incorporating arrival time data from local stations we find that this event occurred beneath the accretionary prism south of the back arc thrust zone at a depth of 11 5 km 7 km below the seafloor the best fitting fault plane solution is consistent with active southward thrusting of the floor of the flores basin beneath the volcanic arc along a 30 dipping fault plane the morphology of the thrust zone and the free air gravity profile over the flores basin in the epicentral region resemble those of oceanic trenches and the observed gravity field is best interpreted if the crust of the flores basin dips smoothly below the accreted wedge at an average angle of about 6 the position of the earthquake hypocenter south of the flores thrust and the fault plane solution suggest that the 1978 earthquake is analogous to thrust events at subduction zones and represents slip between the subducting and overriding plates the flores thrust is thus the surface expression of a deep seated thrust zone and may represent the initial stage of polarity reversal of the eastern sunda arc

microearthquake seismicity and fault plane solutions related to arc continent collision in the eastern sunda arc indonesia the collision of australia with the eastern sunda and banda arcs of indonesia represents a modern example of the early stages of arc continent collision in order to obtain a more detailed view of the tectonics of the collision zone a microearthquake survey was conducted around the savu sea the region that encompasses the transition from subduction of indian ocean lithosphere on the west to collision or subduction of australian continental litho sphere on the east intermediate depth earth quakes were concentrated near pantar island the easternmost active volcano of the sunda arc and outline a northwest dipping zone that strikes n65oe from 70 to 150 km depth the seismic zone probably marks the western edge of the collision zone and its more northerly strike and the orientations of nodal planes from fault plane solutions indicate a 25 o bend in the subducted slab to a depth of at least 150 km because the convergence rate and the timing of collision from geologic observations on timor suggest that the oart of the slab now at 150 km depth was at the trench when australia first collided with timor we infer that continental crust or at least rifted continental margin crust caused the bend in the convergent margin and has been subducted to 150 km depth fault plane solutions of several events show nearly vertical nodal planes that trend parallel to the strike of the seismic zone along which a northwest side down sense of dis placement is indicated these events are concentrated at the southern top side of the seismic zone and suggest that the subducted lithosphere is presently detaching in the 50 100 km depth range beneath the eastern savu sea fault plane solutions for earthquakes within the upper part of the detaching slab 70 100 km depth have p and t axes whose trends lie parallel to the strike of the seismic zone and horizontal southeast trending b axes these earthquakes are interpreted as due to stresses caused by the bend in the subducting plate

a high resolution prediction study of two typhoons at landfall abstract the landfall on the china coast of two typhoons has been studied using both observational diagnostics and a 30 km 19 level prediction system the typhoons vary in size and in the amount of rainfall they produced after landfall use is made of a synthetic typhoon insertion scheme and the relatively dense observational network around china to provide objective analyses the authors illustrate that in the forecast model as the typhoon s outer circulation impinges on the china coast an outer cloud band develops offshore parallel to the coast extending cyclonically north and east during this time the inner circulation weakens as the outer cloud band strengthens the center of the circulation approaches it from the east and there is a short term intensification of both the inner region vertical motion and vorticity following landfall rapid dissipation of the core occurs using detailed diagnostics from the model integration the changing thermal structure of the forecast vortex is illustrated

hurricane forced upwelling and chlorophyll a enhancement within cold core cyclones in the gulf of mexico 1 clear skies subsequent to hurricane ivan s passage across the gulf of mexico in september 2004 provided a unique opportunity to investigate upper ocean responses to a major hurricane oceanic cyclonic circulation was rapidly intensified by the hurricane s wind field 59 62 m s 1 maximizing upwelling and surface cooling 3 7 c in two large areas along ivan s track upward isothermal displacements of 50 65 m computed from wind stress and sea surface height changes caused rapid ventilation of thermoclines and nutriclines leading to phytoplankton blooms with peak concentrations 3 4 days later wind speed changes along ivan s track demonstrated that the cool waters 20 26 c provided immediate negative feedback to the hurricane s intensity although our study focused on a relatively small ocean area it revealed that mesoscale cyclones in addition to warm anticyclones may play an important role in producing along track hurricane intensity changes

transient modulation of kuroshio upper layer flow by directly impinging typhoon morakot in east of taiwan in 2009 this study deals with the modulation of the kuroshio upper layer flow kulf in response to the passage of typhoon morakot in 2009 using regional oceanic modeling system roms and in situ measurements from argos drifters and argo floats the analysis of the simulated current fields near the typhoon track revealed an intermittency phenomenon of the kulf which was almost shut down for at least 6 hours the process begun 2 days prior to the approach of typhoon center due to blockage of the kulf by the steadily northerly winds and lasted for more than 2 days simultaneously shifting the kuroshio main stream kms path when the morakot gradually moved closely to the kuroshio the kms shifted vertically from the surface layer to deeper layer of 50 100 m depth and the maximum current speed in the kms decreased from more than 1 3 m s 1 to less than 1 1 m s 1 when the morakot center approached about 100 km to the original position of the kuroshio the kulf spread eastward for 1 5 degrees at 24 n when the morakot center moved to the original position of the kms the kuroshio abruptly rushed with a maximum speed near 1 4 m s 1 meanwhile an offshore cool jet originating from southeastern tip of taiwan was generated and extended northward along the kuroshio in the cool jet the lowest temperature reached about 5 c lower than the ambient waters modeled current variations and the cool jet during the morakot passage were validated by in situ measurements

back arc thrusting in the eastern sunda arc indonesia a consequence of arc continent collision the structure of the eastern sunda back arc region is dominated by two large north directed thrusts the wetar and flores thrusts and one or more minor thrusts which may represent early stages of subduction polarity reversal of the arc the relationships between thrusting and magmatic activity surface slopes cross arc faulting uplift forearc structure and collision by the australian continental margin provide constraints on the likely driving mechanisms of thrust formation driving mechanisms are considered in two broad categories one set of possible mechanisms originates within the upper plate and includes gravity sliding gravity spreading and magmatic intrusion the other set focuses on stress propagation between the upper and lower plates and for this arc system the major mechanism is that of continental collision while no one mechanism explains the range of observations we see in the back arc region all of these aspects play some role in the development of the thrusts we can eliminate upper plate mechanisms as primary driving forces because slope effects are similar in areas of both thrusting and nonthrusting in addition the wetar thrust shows a negative relation between thrusting and volcanism and side looking airborn radar images show no evidence of postvolcanic rifting that could accompany intrusion without volcanism collision between the arc and the australian continent provides the clearest driving mechanism but it does not explain the discontinuous nature or the particular location of the thrusts we suggest a sequence that appears to account for the data and to provide insight into the controversies often associated with other such thrust belts thrusting was initiated in those areas where the crust of the forearc region was thick a factor that facilitated stress propagation across the arc and where the crust of the back arc was thin which facilitated back arc thrusting thrusting concentrated at the base of the slope where the slope stress is maximum the early thrusting may have been helped also by thermal weakening of the crust due to volcanism as convergence proceeded volcanism waned then ceased in the eastern part of the arc the crust then strengthened and thrusting required higher stress to maintain and propagate this later stage should show higher seismic energy release which appears to be the case for the wetar thrust region displacement on the flores thrust appears to be matched by deflection of the volcanic arc we have estimated shortening in the back arc from interpretation of reflection profiles and for the flores thrust the value of approximately 30 km is consistent with that estimated for deflection of the arc compressional deformation continues west of the collision zone across the bali basin and into java it is reported in ne sumatra as well this deformation involves much less convergence than the eastern sunda thrusts and it may be related to magmatic processes to subduction of oceanic plateaus such as the roo rise or in the case of the bali basin to efficient lateral propagation of the flores thrust

subduction of the eastern panama basin and seismotectonics of northwestern south america spreading along the cocos nazca plate boundary since the breakup of the farallon plate in the miocene has resulted in the formation of the panama basin and a complex interaction of plates in and near northwestern south america current plate boundaries have been defined and segments of subducted lithosphere identified through selection of hypocentral locations of earthquakes considering only well located events and through focal mechanism determinations the existence of relict plate boundaries bathymetric features and the panamanian isthmus has affected the subduction process of the nazca plate beneath south america and determined the present day configuration of the subducting lithospheric plate there is no single triple junction separating the caribbean south american and nazca plates instead the panamanian isthmus and surrounding areas are accommodating east west compression and a lesser degree of north south compression along a series of thrust faults striking nw to ne and the andean ranges of ecuador colombia and venezeula are moving as a block nne relative to the rest of the south american plates along a system of faults following the front of the eastern cordillera the subducted portions of the panama basin and old farallon plate have become segmented into three pieces recognized in this study from north to south they are 1 a bucaramanga segment continuous with the caribbean seafloor northwest of colombia 2 a cauca segment continuous with oceanic crust nazca plate currently being subducted beneath south america at the colombia ecuador trench and 3 an ecuador segment at the northern end of the subducted lithospheric plate which is dipping at a small angle to the east beneath northern peru the segmentation of the subducted plate can be explained by the buoyancy of bathymetric features which have been partially subducted

passive margin evolution initiation of subduction and the wilson cycle we have constructed finite element models at various stages of passive margin evolution in which we have incorporated the system of forces acting on the margin depth dependent rheological properties and lateral variations across the margin we have studied the interrelations between age dependent forces geometry and rheology to decipher their net effect on the state of stress at passive margins lithospheric flexure induced by sediment loading dominates the state of stress at passive margins this study has shown that if after a short evolution of the margin time span a few tens of million years subduction has not yet started continued aging of the passive margin alone does not result in conditions more favourable for transformation into an active margin although much geological evidence is available in support of the key role small ocean basins play in orogeny and ophiolite emplacement evolutionary frameworks of the wilson cycle usually are cast in terms of opening and closing of wide ocean basins we propose a more limited role for large oceans in the wilson cycle concept