the philippine sea new survey results reveal the structure and the history of the marginal basins the hydrographic department of japan initiated a long term geological and geophysical survey of the philippine sea in 1983 we here summarize the bathymetry and magnetic anomalies of this area focusing on the structure and the evolutionary process of the marginal basins the shikoku and parece vela basins developed as one backarc system in the later phase of their formation the significant difference between the two basins however is clearest near the extinct spreading centers the steep oki daito escarpment located in the northern west philippine basin was mapped for the first time during this survey the n s trending spreading fabric north of the escarpment is in striking contrast to the nw se to e w lineament in the south our data reveal a more complicated history of the western philippine sea than has been proposed in previous studies

the 3d numerical modeling of subduction dynamics plate stagnation and segmentation and crustal advection in the wet mantle transition zone 1 water content in the mantle transition zone mtz has been broadly debated in the earth science community as a key issue for plate dynamics in this study a systematic series of three dimensional 3d numerical simulation are performed in an attempt to verify a hypothesis regarding the behavior of subducted oceanic plate and the role of crust in the mtz under wet conditions this hypothesis is based on the seismic observations of structural variations associated with large scale flattened high velocity anomalies i e stagnant slabs in the mtz in our model weak low viscosity fault zones wfzs which presumably correspond to the fault boundaries of large subduction earthquakes are imposed on the top part of subducting plates the phase transitions of olivine to wadsleyite and ringwoodite to perovskite magnesiowustite under both dry and wet conditions are considered based on recent high pressure experiments the effect of viscosity reduction of wet garnet is also considered for oceanic crust in the mtz results show that there is a substantial difference in the behaviors of subducting plate and the trace of crustal materials between the models under dry and wet conditions under wet conditions the subducting plate tends to stagnate with a maximum lateral extent of over 1000 km the weaker and denser crustal material garnet rich is fed into the wfzs and is advected faster than the plate main body of peridotite olivine rich thus the viscosity and density contrast between the crustal materials and the peridotite permits the plate segmentation under a wet mtz condition

spatiotemporal distribution of interplate coupling in southwest japan from inversion of geodetic data 1 we determine the spatiotemporal distribution of slip or slip deficit on the subduction interface of the nankai trough over an entire earthquake cycle using geodetic data including leveling triangulation and trilateration sea level and gps surveys obtained during the past 100 years in southwest japan we develop a new inversion method that accounts for long term crustal deformation coseismic earthquake displacements and stress relaxation of the viscoelastic asthenosphere from this analysis we obtain a model that shows postseismic afterslip on the deeper part of the plate interface following the 1946 nankaido earthquake significant afterslip is found beneath central shikoku that totals about 0 8 m the slip deficit rate during the interseismic period is 5 6 cm year in the n50 w n60 w direction which is consistent with the relative plate motion between the philippine and amurian plates a fully locked region is found in the shallower portion 30 km and the slip deficit rate has a maximum at a depth of about 20 km the plate interface deeper than 30 km is slowly slipping the amount of slip deficit reaches about 3 3 m off shikoku and about 2 m off the kii peninsula 50 years after the 1946 earthquake this suggests that fault healing and full interplate coupling has occurred by this time along the nankai trough

comparison of smectite and illite rich gouge frictional properties application to the updip limit of the seismogenic zone along subduction megathrusts along plate boundary subduction thrusts the transformation of smectite to illite within fault gouge at temperatures of 150 c is one of the key mineralogical changes thought to control the updip limit of seismicity if correct this hypothesis requires illite rich gouges to exhibit frictionally unstable velocity weakening behavior here we report on laboratory experiments designed to investigate the frictional behavior of natural and synthetic clay rich gouges we sheared 5 mm thick layers of commercially obtained pure ca smectite a suite of smectite quartz mixtures and natural illite shale grain size ranging from 2 to 500 m in the double direct shear geometry to shear strains of 7 30 at room humidity and temperature xrd analyses show that the illite shale contains dominantly clay minerals and quartz within the clay sized fraction 40 mpa our data specifically the velocity strengthening behavior of illite shale under a wide range of conditions do not support the hypothesis that the smectite illite transition is responsible for the seismic aseismic transition in subduction zones we suggest that other depth and temperature dependent processes such as cementation consolidation and slip localization with increased shearing may play an important role in changing the frictional properties of subduction zone faults and that these processes in addition to clay mineralogy should be the focus of future investigation

competing influences of plate width and far field boundary conditions on trench migration and morphology of subducted slabs in the upper mantle abstract seismic tomography of the mantle provides a snapshot view of the present day structure of subducted tectonic plates slabs in the mantle however a correct interpretation of how such features developed requires knowledge of both the history of plate motions in particular the locations of subduction zones over time as well as an understanding of how slabs deform and interact with the lower mantle the understanding of seismic tomography requires a detailed knowledge of the motion of the trench with time as well as the time evolution of subduction zones and their associated slabs we use 3 d numerical models to investigate two effects which can strongly influence the kinematics of subduction in particular the migration of the trench the width of the plate and the far field boundary conditions of the subducting plate these models study the full range of plate widths 300 6000 km and are subjected to the full range of boundary conditions fixed free and pushed representative of the tectonic settings found on earth additionally our results indicate that trailing edge boundary conditions dominate over the effect of plate width each boundary condition is identified to produce a particular trench migration behavior with an associated upper mantle slab morphology 1 fixed edges result in retreating trenches and retrograde motion of the slab which ends up lying flat atop the lower mantle 2 free trailing edges tend to exhibit quasi stationary trenches with slabs developing into folded piles atop the lower mantle and 3 pushed trailing edges generally result in advancing trenches that overrun previously subducted material to form forward draping slabs lying prone atop the lower mantle this range of behavior can be observed in seismic tomography and an example of each is provided within each suite of models with similar boundary conditions plate width has a subsidiary role in the observed slab morphology but three distinct regimes of plate width can be distinguished in regards to trench migration thus plate width is an important effect on trench migration and does result in notable differences in slab morphologies between narrow and wide plate widths

three dimensional numerical modeling of temperature and mantle flow fields associated with subduction of the philippine sea plate southwest japan we investigated temperature and mantle flow distributions associated with subduction of the philippine sea phs plate beneath southwest japan by constructing a three dimensional parallelepiped model incorporating a past clockwise rotation the bathymetry of the philippine sea plate and distribution of the subducting velocity within its slab the geometry of the subducting plate was inferred from contemporary seismic studies and was used as a slab guide integrated with historical plate rotation into the 3 d simulation using the model we estimated a realistic and high resolution temperature field on the subduction plate interface which was constrained by a large number of heat flow data and attempted to clarify its relationship with occurrences of megathrust earthquakes long term slow slip events l sses and nonvolcanic low frequency earthquakes lfes results showed that the oblique subduction coupled with the 3 d geometry of subducting phs plate was a key factor affecting the interplate and intraplate temperature distributions leading to a cold anomaly in the plate interface beneath western shikoku the bungo channel and the kii peninsula temperatures in the slab core in these regions at a depth near the continental moho were nearly 200 c lower than that in eastern shikoku indicating a high thermal lateral heterogeneity within the subducting plate the geothermal control of the lfes beneath western shikoku was estimated to be within a range from 400 to 700 c and the interplate temperature for the l sses with a slip larger than 15 cm beneath the bungo channel was estimated to be approximately 350 500 c a large horizontal temperature gradient of 2 5 c km was present where the lfes occurred repeatedly the steep temperature change was likely to be related to the metamorphic phase transformation from lawsonite or blueschist to amphibolite of hydrous minerals of the mid ocean ridge basalt of the subducting phs plate

interplate coupling and deformation in the accretionary prism in the southwest japan subduction zone frictional coupling at the plate interface will control seismicity and tectonics in subduction zones in this study temperature structure and deformation in the accretionary prism in the southwest japan subduction zone are simulated using a two dimensional isoviscous corner flow model using surface heat flow data the average interplate frictional stress and the effective viscosity of the prism are estimated to be 2 30mpa and 1020 21pa s respectively the low stress value is probably due to high pore pressures caused by dehydration reactions in the subducting crust estimated temperature of the landward limit of the seismogenic zone is 350 500 c when the frictional stress is considered the temperature becomes 100 c higher than that of the onset of plasticity of granitic rocks 350 c the brittle plastic transition of gabbroic rocks might control the depth extent of the seismogenic zone in lower crustal depths flow velocity at the bottom of the prism is estimated to be 0 01m yr indicating that 25 of the conversion has been consumed by the deformation of the accretionary prism in this subduction zone

evolution of 3 d subduction induced mantle flow around lateral slab edges in analogue models of free subduction analysed by stereoscopic particle image velocimetry technique abstract we present analogue models of free subduction in which we investigate the three dimensional 3 d subduction induced mantle flow focusing around the slab edges we use a stereoscopic particle image velocimetry spiv technique to map the 3 d mantle flow on 4 vertical cross sections for one experiment and on 3 horizontal depth sections for another experiment on each section the in plane components are mapped as well as the out of plane component for several experimental times the results indicate that four types of maximum upwelling are produced by the subduction induced mantle flow the first two are associated with the poloidal circulation occurring in the mantle wedge and in the sub slab domain a third type is produced by horizontal motion and deformation of the frontal part of the slab lying on the 660 km discontinuity the fourth type results from quasi toroidal return flow around the lateral slab edges which produces a maximum upwelling located slightly laterally away from the sub slab domain and can have another maximum upwelling located laterally away from the mantle wedge these upwellings occur during the whole subduction process in contrast the poloidal circulation in the mantle wedge produces a zone of upwelling that is vigorous during the free falling phase of the slab sinking but that decreases in intensity when reaching the steady state phase the position of the maximum upward component and horizontal components of the mantle flow velocity field has been tracked through time their time evolving magnitude is well correlated to the trench retreat rate the maximum upwelling velocity located laterally away from the subducting plate is 18 24 of the trench retreat rate during the steady state subduction phase it is observed in the mid upper mantle but upwellings are produced throughout the whole upper mantle thickness potentially promoting decompression melting it could thereby provide a source for intraplate volcanism such as mount etna in the mediterranean the chiveluch group of volcanoes in kamchatka and the samoan hotspot near tonga

curvature of oceanic arcs oceanic arcs and deep sea trenches are the surface expressions of oceanic plates subducting into the earth s mantle we use a new numerical technique for simulating the dynamical evolution of the lithosphere mantle interaction in order to assess the causes of arc curvature we group the possible causes into two classes external feedback between the migrating lithosphere and the secondary induced mantle flow and internal heterogeneities within the lithosphere e g owing to differences in cooling ages of the plate at the trench we statistically assess that almost all arcs on the earth can be described by these hypotheses the method is also directly applied to the tonga and aleutian arcs bringing new insights on the origin of their shapes

slow slip rate and excitation efficiency of deep low frequency tremors beneath southwest japan abstract we estimated the long term average slip rate on the plate interface across the nankai subduction zone during 2002 2013 using deep low frequency tremors as a proxy for short term slow slip events based on empirical relations between the seismic moment of short term slow slip events and tremor activities the slip rate in each region is likely to compensate for differences between the convergence rate and the slip deficit rate of the subducting philippine sea plate estimated geodetically although the uncertainty is large this implies that the strain because of the subduction of the plate is partially stored as the slip deficit and partially released by slow slip events during the interseismic period the excitation efficiency of the tremors for the slow slip events differs among regions it is high in the northern kii region some events in the western shikoku region show a somewhat large value antigorite serpentinite of two types exists in the mantle wedge beneath southwest japan slips with more effective excitation of tremors presumably occur in high temperature conditions in the antigorite olivine stability field other slip events with low excitation efficiency are distributed in the antigorite brucite stability field considering the formation reactions of these minerals and their characteristic structures events with high excitation efficiency can be correlated with a high pore fluid pressure condition this result suggests that variation in pore fluid pressure on the plate interface affects the magnitude of tremors excited by slow slip events

heat flow distribution and thermal structure of the nankai subduction zone off the kii peninsula detailed heat flow surveys were carried out in the central part of the nankai trough southeast of the kii peninsula off kumano for investigation of the thermal structure of the subducting plate interface at stations in the kumano trough forearc basin and its vicinity long term monitoring of temperature profiles in surface sediments was conducted because bottom water temperature variations btv significantly disturb subbottom sediment temperatures heat flow values were successfully determined at seven stations by removing the influence of btv from temperature records for 300 to 400 days the surface heat flow data were combined with estimates from depths of methane hydrate bottom simulating reflectors to construct a heat flow profile across the subduction zone heat flow decreases from 90 110 mw m2 on the floor of the nankai trough to 50 60 mw m2 at around 30 km from the deformation front while it is rather uniform 40 60 mw m2 in the kumano trough the values measured on the nankai trough floor are concordant with the value estimated from the age of the subducting philippine sea plate about 20 m y taking into account the effect of sedimentation the obtained heat flow profile was used to constrain thermal models of the subduction zone the subsurface thermal structure was calculated using a two dimensional steady state model in which the frictional heating along the plate interface and the radioactive heat production are treated as unknown parameters comparison of the calculated surface heat flow in the kumano trough with the observed data indicates that the effective coefficient of friction is small about 0 1 or less and thus the shear stress on the plate interface is very low in this subduction zone

effect of three dimensional slab geometry on deformation in the mantle wedge implications for shear wave anisotropy shear wave splitting observations from many subduction zones show complex patterns of seismic anisotropy that commonly have trench parallel fast directions three dimensional flow may give rise to trench parallel stretching and provide an explanation for these patterns of seismic anisotropy along strike variations in slab geometry produce trench parallel pressure gradients and are therefore a possible mechanism for three dimensional flow in this study we quantify the effects of variable slab dip curved slabs oblique subduction and slab edges on flow geometry and finite strain in the mantle wedge of subduction zones temperature dynamic pressure velocity and strain are calculated with high resolution three dimensional finite element models these models include temperature and stress dependent rheology and parameterized slab and trench geometry thick layers 20 60 km with strong trench parallel stretching are observed in the mantle wedge when slab geometry involves a transition to slab dip less than 15 or strong curvature in the slab in these cases strong trench parallel stretching develops when flow lines have an oblique to trench normal orientation this suggests that trench parallel seismically fast directions may not indicate trench parallel flow lines in systems with large along strike variations an oblique component of stretching is confined to a 20 30 km layer above the slab in systems with oblique subduction the effects of slab edges include strong toroidal flow and focusing in the mantle near slab edges and trench parallel flow that extends 50 100 km into the core of the mantle wedge

volcano tectonic interactions during rapid plate boundary evolution in the kyushu region sw japan evolution of the local plate tectonic and volcanic system relationship at kyushu island is defined by major changes in tectonics and volcanic style at ca 15 10 6 and 2 ma plate reconstructions presented here suggest that prior to 15 ma the pacific plate subduction dominated kyushu tectonics from 15 to 6 ma the evolving relative plate motions shifted the triple junction between the pacific plate philippine sea plate and southwest japan northwards so that the philippine sea plate was subducted beneath kyushu we suggest that a lack of subduction related volcanism from 10 to 6 ma is due to shallow subduction of the young shikoku basin lithosphere by 6 5 ma changes in the philippine sea plate motion led to more rapid nearly trench normal subduction of the eocene west philippine basin crust beneath kyushu this model is supported by an increase in arc like geochemistry of lavas since ca 6 5 ma subduction of fluid rich features such as the kyushu palau ridge introduced large volumes of fluids into the kyushu arc system leading to voluminous volcanism across kyushu focused particularly in areas where the ridge subduction occurs in tandem with local extensional tectonics key issues such as the timing of izu arc collision with central japan and the history of motion of the philippine sea plate are reassessed here resulting in a model that favors izu arc central japan collision at ca 8 6 ma rather than the more widely accepted date of ca 15 ma

trench migration net rotation and slab mantle coupling article i nfo laboratory models have been conducted to improve our understanding of the role that the resistance of the slab to bending and its coupling to the ambient mantle play in subduction dynamics over geological time scales our models are set up with a viscous plate of silicone lithosphere subducting under negative buoyancy in a viscous layer of glucose syrup mantle for our study the lithosphere upper mantle viscosity contrast has been systematically varied from 10 to 10 5 in order to explore the parameter space between weak and strong slab dynamics we found that subduction is characterized by a retreating mode for viscosity ratios n10 4 by the coexistence of a retreating mode and an advancing mode for viscosity ratios between 10 4

dynamics of free subduction from 3 d boundary element modeling 1 in order better to understand the physical mechanisms underlying free subduction we perform three dimensional boundary element numerical simulations of a dense fluid sheet with thickness h and viscosity 2 sinking in an ambient mantle with viscosity 1 the mantle layer is bounded above by a traction free surface and is either 1 infinitely deep or 2 underlain by a rigid boundary at a finite depth h d similar to the typical geometry used in laboratory experiments instantaneous solutions in configuration 1 show that the sheet s dimensionless stiffness s determines whether the slab s sinking speed is controlled by the viscosity of the ambient mantle s 10 time dependent solutions with tracers in configuration 2 demonstrate a partial return flow around the leading edge of a retreating slab and return flow around its sides the extra edge drag exerted by the flow around the sides causes transverse deformation of the slab and makes the sinking speed of a 3 d slab up to 40 less than that of a 2 d slab a systematic investigation of the slab s interaction with the bottom boundary as a function of 2 1 and h h delineates a rich regime diagram of different subduction modes trench retreating slab folding trench advancing and reveals a new advancing folding mode in which slab folding is preceded by advancing trench motion the solutions demonstrate that mode selection is controlled by the dip of the leading edge of the slab at the time when it first encounters the bottom boundary

reconciling surface plate motions with rapid three dimensional mantle flow around a slab edge a central axiom of the theory of plate tectonics is that the surface plates are coupled to and carried along by convection in the earth s mantle the situation is rather different at subduction zones however margarete jadamec and magali billen now present calculations that quantify this variance they use three dimensional numerical models of buoyancy driven deformation with realistic slab geometry for the alaska subduction transform system to determine the surrounding mantle flow field they show that near a subduction zone edge mantle flow velocities can have magnitudes of greater than ten times the surface plate motions and that the complex mantle flow field is consistent with observations from seismic anisotropy these results imply local rapid transport of geochemical signatures through subduction zones the direction of tectonic plate motion at the earth s surface and the flow field of the mantle inferred from seismic anisotropy are well correlated globally implying large scale coupling between the mantle and surface plates but the fit is poor at subduction zones here three dimensional numerical models of buoyancy driven deformation for the alaska subduction transform system are used to find the origin of the regional decoupling of flow the results imply local rapid transport of geochemical signatures through subduction zones the direction of tectonic plate motion at the earth s surface and the flow field of the mantle inferred from seismic anisotropy are well correlated globally suggesting large scale coupling between the mantle and the surface plates1 2 the fit is typically poor at subduction zones however where regional observations of seismic anisotropy suggest that the direction of mantle flow is not parallel to3 4 5 6 7 and may be several times faster than6 plate motions here we present three dimensional numerical models of buoyancy driven deformation with realistic slab geometry for the alaska subduction transform system and use them to determine the origin of this regional decoupling of flow we find that near a subduction zone edge mantle flow velocities can have magnitudes of more than ten times the surface plate motions whereas surface plate velocities are consistent with plate motions8 and the complex mantle flow field is consistent with observations from seismic anisotropy5 the seismic anisotropy observations constrain the shape of the eastern slab edge and require non newtonian mantle rheology the incorporation of the non newtonian viscosity9 10 results in mantle viscosities of 1017 to 1018 pa s in regions of high strain rate 10 12 s 1 and this low viscosity enables the mantle flow field to decouple partially from the motion of the surface plates these results imply local rapid transport of geochemical signatures through subduction zones and that the internal deformation of slabs decreases the slab pull force available to drive subducting plates

mantle wedge flow pattern and thermal structure in northeast japan effects of oblique subduction and 3 d slab geometry abstract we develop a 3 d thermal model for the northeast japan subduction margin using a realistic slab geometry for the subducting pacific plate and investigate the effects of oblique subduction and 3 d slab geometry on the mantle wedge flow pattern and the thermal structure in the tohoku region the mantle wedge flow pattern is nearly two dimensional resulting in a thermal structure similar to those obtained by a 2 d model owing to the simple slab geometry and subduction nearly perpendicular to the margin however in hokkaido oblique subduction leads to 3 d mantle wedge flow with northerly inflow and west northwestward outflow and also results in lower temperatures in the shallow part of the mantle wedge than in tohoku due to lower sinking rate of the slab between hokkaido and tohoku the slab has a hinge like shape due to a relatively sharp change in the dip direction in this hinge zone northerly mantle inflow from hokkaido and westerly mantle inflow from tohoku converge discouraging inflow from northwest and resulting in a cooler mantle wedge the model predicted mantle wedge flow patterns are consistent with observed seismic anisotropy and may explain the orientations of volcanic cross chains the predicted 3 d thermal structure correlates well with the along arc variations in the location of the frontal arc volcanoes and help to provide new insights into the surface heat flow pattern and the down dip extent of interplate earthquakes

along arc variation in the 3 d thermal structure around the junction between the japan and kurile arcs the thermal structure in subduction zones has a strong influence on seismogenesis and arc volcanism traditional 2 d models have been used to provide reasonable agreement between models and observations but in a number of cases clear 3 d effects are present one such case is in the northern japan subduction system at the junction between japan and kurile arcs surface heat flow and the occurrence of intermediate depth seismicity are different than in the tohoku and hokkaido regions we investigate the effects of 3 d slab geometry and a local deepening of slab mantle decoupling depth on the thermal structure in this region based on 3 d finite element approach we find that both effects produce the along arc variation of slab surface temperature which could reach 100 c the warmer region arises through 3 d effects of thermal conduction and the colder region arises through localized slow incoming flow in the case where 3 d slab geometry is taken into account 3 d flow arises where a local deepening of slab mantle decoupling depth is assumed which leads to both warmer and colder regions the effects on surface heat flow are small while intermediate depth seismicity in the subducted crust is suggested to be controlled by temperature dependent phase transitions the predicted changes in thermal structure are not sufficient to cause the observed deepening of seismicity this suggests that the thermal structure of this subduction zone may be more strongly influenced by time dependent deformation of the overriding crust and slab

effects of oblique subduction on the 3 d pattern of small scale convection within the mantle wedge 1 we have developed a 3 d numerical model of obliquely subducting slab and mantle flow above using this model we explore the effects of oblique subduction on the small scale convection within the mantle wedge we find that the spatial pattern and time dependent characteristics of oblique subduction are similar to those of non oblique cases whose subduction speed is the same as the subduction velocity component normal to the strike of subducting slab this also implies that the axes of rolls of small scale convection are nearly normal to the plate boundary assuming that the plate boundary represents the strike of subducting plate only when the back arc lithosphere overlying the small scale convection moves along the strike of subduction a significant change of the orientation of roll axes is observed

thermal constraints on the seismogenic portion of the southwestern japan subduction thrust for coastal cities an important factor in earthquake hazard from subduction zone earthquakes is the landward extent of the seismogenic portion of the subduction thrust fault in this study we test the hypothesis that the maximum downdip extent is defined by a critical temperature we have developed a transient thermal model for the nankai subduction zone of southwest japan to allow comparison of the thermally estimated downdip extent of the seismogenic zone with that from 1 seismicity and tsunami data for two great subduction earthquakes 2 the coseismic faulting extent of these events estimated from geodetic deformation data and 3 the interseismic locked zone determined from interseismic geodetic data the nankai margin has extensive heat flow and heat production data to control thermal models and thus crustal temperatures it has earthquake tsunami and geodetic data that constrain the coseismic rupture portion of the subduction thrust fault for past great earthquakes and the portion of the thrust fault that is locked and storing interseismic elastic strain on the nankaido margin off shikoku island the thermal model indicates that a temperature of 350 c taken to be the limit for seismic initiation from laboratory and field data is reached on the subduction thrust fault 150 km from the trench a transition zone into which rupture may extend with decreasing offset taken to be 450 c extends an additional 45 km downdip the thermal model results are in excellent agreement with the maximum downdip extent of coseismic displacement for the 1946 nankaido ms 8 2 earthquake off shikoku island and with the downdip extent of the present locked zone in the region of the 1944 tonankai ms 8 2 earthquake to the northeast the subduction angle is much steeper and the thermal models indicate a narrower downdip seismogenic extent the seismogenic locked zone from earthquake and geodetic data is also narrower thus our analysis of the southwest japan margin indicates that all three constraints on the downdip extent of the seismogenic zone thermal coseismic and interseismic geodetic data are in general agreement the study also supports the hypothesis that the seismogenic portion of subduction thrust faults is limited primarily by temperature the thermal control implies that subduction thrust faults with shallow dip have wider seismogenic zones compared to those with steep dip the subducting plate age and thus heat flow and the thickness of the insulating sediments on the incoming plate are also very important to the thermal regime and thus to the seismogenic width the relation of the maximum seismic rupture area to the interseismic locked zone is particularly important for earthquake hazard estimation on subduction margins such as cascadia where there have been no great historical events

dynamics of plate bending at the trench and slab plate coupling 1 the bending strength of subducting lithosphere plays a critical role in the earth s plate tectonics and mantle convection modulating the amount of slab pull transmitted to the surface and setting the boundary conditions under which plates move and deform however it is the subject of a lively debate how much of the potential energy of the downgoing plate is consumed in bending the plate and how the lithospheric strength is defined during this process we model the subduction of a viscoelastic lithosphere driven solely by the downgoing plate s buoyancy freely sinking in a passive mantle represented by drag forces to investigate the dynamics of bending 1 we vary the density and the viscosity profile within the plate from isoviscous where strength is distributed to strongly layered where strength is concentrated in a thin core and 2 we map the stress strain and dissipation along the downgoing plate the effective plate strength during bending is not a simple function of average plate viscosity but is affected by rheological layering and plate thinning earth like layered plates allow for the transmission of large fractions of slab pull 75 80 through the bend and yield a net slab pull of fspnet 1 to 6 1012 n m 1 which varies with the buoyancy of plates in all models only a minor portion of the energy is dissipated in the bending surprisingly bending dissipation hardly varies with lithospheric viscosity because in our dynamic system the plates minimize overall dissipation rate by adjusting their bending curvature as a result bending dissipation b is mainly controlled by the bending moment work rate exerted by slab pull we propose a new analytical formulation that includes a viscosity dependent bending radius which allows for assessment of the relative bending dissipation in the earth s subduction zones using parameters from a recent global compilation this yields estimates of b tot 20 these results suggest that plates on earth weakly resist bending yet are able to propagate a large amount of slab pull

thermal regime of the southwest japan subduction zone effects of age history of the subducting plate abstract during a mid miocene at about 15 ma tectonic reorganization an ocean spreading ridge perpendicular to the strike of the nankai trough subduction zone stopped spreading since that time there has been subduction of the cooling fossil spreading ridge in the present study we have developed a time dependent thermal subduction model for this region using the finite element method there are good seismological and geological constraints for the plate geometry and for the subduction history of the past 15 ma boundary conditions are specified so that the oceanic plate subducting at the nankai trough becomes cooler as it gets older the model results agree with the present heat flow trend that decreases landward and explain the paleothermal regime of high mid miocene temperatures inferred from land geological studies the tectonics of the region prior to 15 ma has some uncertainties assuming subduction of an active spreading ridge or an 80 ma old lithosphere prior to 15 ma give the same results for the present thermal regime of the seaward portion of the forearc but different results for the most landward region 250 km the thermal history of the forearc since 15 ma can be summarized as a rapid warming period as a consequence of ridge subduction followed by a cooling trend to the present as a result of the aging of the subducting plate the results illustrate the thermal consequences of one type of ridge subduction they also demonstrate that the thermal regime of a subduction zone depends critically on the age history of the subducting oceanic lithosphere especially if it is young as well as such parameters as the subducting plate dip angle and thickness of insulating sediments on the incoming oceanic crust this dependence is especially important when the thermal regime is used to constrain the seismogenic behaviour of the subduction thrust fault

self consistent generation of tectonic plates in time dependent three dimensional mantle convection simulations 1 presented here are self consistent three dimensional simulations of mantle convection some of which display an approximation of plate tectonic behavior that is continuous in space and time plate behavior arises through a reasonable material description of silicate deformation with a simple yield stress being sufficient to give first order plate like behavior however the required yield strength or fault frictional coefficient is much less than experimentally determined values toroidal poloidal ratios are within geologically observed limits the sensitivity of the system to yield strength and the form of strength envelope is systematically investigated optimum plate character is obtained in a narrow range of yield strength below which diffuse boundaries and above which episodic behavior and eventually a rigid lid are observed models with mobile lids develop very long wavelength horizontal structure the longest wavelength possible in the domain two dimensional models display much greater time dependence than three dimensional models

three dimensionality of slab detachment due to ridge trench collision laterally simultaneous boudinage versus tear propagation the detachment of subducted tectonic plates is a process that has been increasingly associated with collisional scenarios and the end of subduction in various locations worldwide in particular the propagation of slab detachment tearing of a subducting plate has been described in conceptual models as a cause for spatially and temporally progressing surface effects such as slab gap volcanism and uplift however there is little understanding of the causes and dynamics associated with three dimensional 3 d slab tearing especially in the case of ridge trench collision here we show using fully dynamic 3 d numerical models that the process of detachment due to ridge trench collision depends on the geometry of the ridge segments approaching the trench for a finite laterally symmetric slab the 3 d detachment process occurs nearly simultaneously along strike by way of boudinage type necking and opening of holes central to the slab for a case involving the approach of two offset ridge segments to the trench slab tearing occurs in the form of 1 a vertical propagating separation along the age offset boundary within the slab that was previously weakened by a transform weak zone and 2 horizontal propagating detachment controlled by lateral transfer of slab pull to adjacent surface plate segments however lateral decoupling between offset adjacent plate segments and the propagating nature of the vertical and horizontal tearing are dependent upon fracture zones remaining weak through the subduction zone whether detachment occurs simultaneously along strike or propagates laterally the process is controlled by plastic yielding of the slab interior when young lithosphere entering the trench can no longer support slab pull

a simple semi dynamic model of the subduction zone effects of a moving plate boundary on the small scale convection under the island arc summary a simple semi dynamic numerical model of the subduction zone which incorporates the plate velocity and a moving plate boundary as a priori constraints has been developed this model may be useful for studying specific subduction zones where a history of kinematic movements of plates is well constrained as an application of this model the effects of a moving plate boundary on the nature of the small scale convection in the mantle wedge were studied under the restriction of given relative velocity between two converging plates the absolute velocity of each plate was changed and i found that the characteristics of small scale convection are not significantly affected by the movement of the plate boundary at least within the parameter range used in this study i also found a small systematic change which may be related to the difference of the heat supply to the mantle wedge caused by the moving plate boundary thus the model with a fixed continental plate and moving oceanic plate with a given relative velocity may be sufficient for studying the present subject this model can be extended into a simple 3d subduction zone model

the bending mechanics in a dynamic subduction system constraints from numerical modelling and global compilation analysis abstract lithospheric plates bend at subduction zones where the vertical motions of the slabs are converted to surface plate motions to understand the mechanics of plate bending we derive scaling laws for the deflection at the margin i e radius and dip from numerical models of a subducting viscoelastic plate in such dynamic system we find that the buoyancy and the stiffness of the plates control the radius and the dip as well as the plate motions toward the trench this mechanical model successfully predicts the curvature of published three dimensional laboratory and numerical models for a thorough comparison with the observable we have also implemented forces additional to the slab pull such as the suction force and far field stresses by increasing or resisting the torque applied at the trench by the slab these forces can largely rearrange the dip and the radius of slabs and the inherent plate motions although they do not alter the observed anticorrelation between radius and dip similar inverse correlation relationship and dip radius ranges are shown by most of the subduction zones analysed from a global compilation radii in the range of 100 350 km and dips of 30 70 for slabs that extends to the bottom of the upper mantle are compatible with the models and allow estimating an average lithospheric viscosity contrast of 200 in the bending with respect to the ambient mantle radii and dips outside of this range are in good agreement with the trends and the magnitudes of models that include suction and far field forces in all these subduction zones the correlation between dip radius and plate velocity is found to be compatible with that of the models showing how relevant bending is for the dynamics of earth

determination of the pore fluid pressure ratio at seismogenic megathrusts in subduction zones implications for strength of asperities and andean type mountain building 1 we construct the differential stress profile across the fore arc in a subduction zone from the force balance between the shear stress at seismogenic megathrust and the lithostatic pressure we assume that is written by 1 n where is the pore fluid pressure ratio is the coefficient of static friction and n is the normal stress given a density structure of the fore arc wedge we determine by comparing calculated fore arc stresses with observed ones as 0 95 0 98 in shikoku miyagi peru north chile and south chile and 0 90 0 93 in south vancouver island and washington the parameter averaged over the seismogenic megathrust is of the order of 10 mpa stress drops of great earthquakes in these zones occupy 14 87 and not a constant fraction of they on the other hand increase linearly with 1 we propose a simple fault model in which the area of asperities as a fraction of the total fault area is proportional to 1 variation of fractional area of asperities thus may explain the observed correlation and the regional variation of assuming that the differential stress at summit of the andean mountains is zero not at the coast as observed at present we determine to be 0 84 in north chile in the mountain building stage such a smaller value of along with 0 9 in subduction zones would suggest that variation of controls the tectonic style of the earth

dynamics of retreating slabs 1 insights from two dimensional numerical experiments from the effects of mantle flux part 2 therefore in this paper we apply forces to the slab using simple analytical functions related to buoyancy and viscous forces in order to isolate the role of rheology on slab dynamics we analyze parameters for simplified elastic viscous and nonlinear viscoelastoplastic single layer models of slabs and compare them with a stratified thermomechanical viscoelastoplastic slab embedded in a thermal solution the near surface behavior of slabs is summarized by assessing the amplitude and wavelength of forebulge uplift for each rheology in the complete thermomechanical solutions vastly contrasting styles of slab dynamics and force balance are observed at top and bottom bends however we find that slab subduction can be modeled using simplified rheologies characterized by a narrow range of selected benchmark parameters the best fit linear viscosity ranges between 5 10 22 pa s and 5 10 23 pa s the closeness of the numerical solution to nature can be characterized by a deborah number 0 5 indicating that elasticity is an important ingredient in subduction index terms 8120 tectonophysics dynamics of lithosphere and mantle general 8159 tectonophysics rheology crust and lithosphere 8160 tectonophysics rheology general keywords subduction numerical models lithospheric rheology

slip weakening as a mechanism for slow earthquakes slow earthquakes form part of a spectrum of fault behaviour between steady creep and fast rupture during a normal earthquake laboratory simulations of slow slip in rock samples taken from the nankai subduction zone japan reveal similar characteristics to fast earthquakes implying that some slow slip events could be prematurely arrested earthquakes

self consistent generation of tectonic plates in three dimensional mantle convection abstract despite the fundamental importance of plates in the earth s mantle convection plates have not generally been included in numerical convection models or analog laboratory experiments mainly because the physical properties which lead to plate tectonic behavior are not well understood strongly temperature dependent viscosity results in an immobile rigid lid so that plates where included at all in 3 d models have always been imposed by hand an important challenge is thus to develop a physically reasonable material description which allows plates to develop self consistently this paper focuses on the role of ductile shear localization in two dimensional geometry it is well established that strain rate softening non newtonian rheologies e g power law visco plastic cause weak zones and strain rate localization above up and down wellings resulting in a rudimentary approximation of plates three dimensional geometry however is fundamentally different due to the presence of transform plate boundaries with associated toroidal motion since power law and visco plastic rheologies do not have the property of producing shear localization it is not surprising that they do not produce good plate like behavior in three dimensional calculations here it is argued that a strain rate weakening rheology previously shown to produce plate like behavior in a two dimensional sheet representing the lithosphere is a reasonable generic description of various weakening processes observed in nature one and two dimensional models are used to show how this leads to shear localization and the formation of faults this rheology is then applied to the high viscosity lithosphere of 3 d mantle convection calculations and the velocity pressure viscosity solution for the entire 3 d domain lid and underlying mantle is solved self consistently it is found that the lithosphere divides into a number of very high viscosity plates separated by narrow sharply defined weak zones with a viscosity many orders of magnitude less than the plate interiors broad weak zones with dominant convergent divergent motion above up and down wellings are interconnected by a network of narrow weak zones with dominant strike slip motion passive spreading centers are formed in internally heated cases while the resulting plates are not fully realistic these results show that self consistent plate generation is a realizable goal in three dimensional mantle convection and provide a promising avenue for future research

maximum depth of seismicity and thermal parameter of the subducting slab general empirical relation and its application the maximum depth of seismicity dm in subduction zones is limited by some physical mechanisms related to the rheological strength of slab material metastability conditions failure strength etc the main factors controlling these properties are temperature and pressure within the lithosphere sinking into the mantle these p t conditions may be coarsely assessed from thermal models of slab subduction the observed relationship between dm the age of the subducted lithosphere a and the convergence rate v can be used to test the theoretical models and reveal inconsistencies which may indicate changes in failure mechanisms with depth we analyze the relation between dm and the thermal parameter of the descending slab which is a product of the age of the subducted lithosphere and the vertical component of convergence rate v seismicity profiles across the subduction zones of mexico chile kamchatka kuriles japan sumatra new hebrides aleutians tonga and marianas are used to estimate dm the quasi linear part of the dependence dm 240 km and 2000 km corresponds to relatively young and slowly descending slab and is in general agreement with the critical temperature models of deep earthquakes for 2000 km which corresponds to the relatively older lithosphere subducting at a higher rate the dm f dependence is nonlinear the flattening of the empirical dm f curve in the range of 2000 km 3500 km is found to be a direct indication of the influence of the equilibrium ol sp phase transition on dm models invoking the metastable ol sp phase transition as a mechanism which controls the deepest seismicity cannot be definitely constrained by the results of this study the general dependence dm f may be applied as a standard relation between dm a and v for the analysis of deep earthquakes and seismotectonic studies of the subduction zones

syndeformational antigorite dehydration produces stable fault slip we conducted deformation experiments in which temperature was increased above the thermal stability of antigorite to directly test the dehydration embrittlement hypothesis and its potential link to intermediate depth earthquakes the experiments were designed to maximize the possibility for instability pore pressure was undrained pre dehydration stress was high and deformation was localized prior to initiation of reaction however stick slip instabilities or unstable fault behavior did not develop rather we find that weakening occurs stably at a rate that depends on the ratio of the temperature ramp rate to the strain rate resulting in laboratory slow slip or stress relaxation events

heat sources and melting in subduction zones in order to understand the mechanism of melt generation and the origin of high heat flow in subduction zones a series of numerical models of the thermal and flow structures in the mantle have been tested in two dimensional boxes with an inclined subducting slab of constant velocity in contrast to previous models 1 large convecting cells are used without imposing a high temperature profile on the backarc boundary which enables us to discuss the global heat balance and to seek for the heat sources responsible for the melting and high heat flow observed in subduction zones and 2 various mechanical conditions e g coupling between the slab and overlying mantle wedge buoyancy associated with melting and heat sources e g heat flux from below internal heat generation viscous heating are tested in varying proportions in all the calculations steady state or near steady state with a small instability periodically occurring at the upper thermal boundary layer is achieved under these conditions the global heat balance can be described by a simple boundary layer argument the results show that in order to attain a high enough temperature for melting and a high heat flux a large amount of internal heating i e more than 2 5 10 7 w m3 is required if the convection is limited within the upper mantle the high internal heating required may be explained if the radioactive nuclides in fluids expelled from the subducting slab are added to the wedge and circulate in the convection cell for a sufficient time another possible explanation for melting and the high heat flux is that the hot material is supplied from the lower mantle based on the thermal and flow structures obtained melting regimes in subduction zones are discussed in which the following key processes take place 1 melting associated with pressure change of a rock packet with its movement especially compression melting in the downward flow of the mantle wedge along the slab and 2 melting due to compositional change of the rock packet associated with migration of h2o and melt to solve these problems in detail further studies on distribution and migration of the fluids will be required

simple relationship between seismic activity along philippine sea slab and geometry of oceanic moho beneath southwest japan summary many earthquakes with various types of focal mechanisms occur along the subducting philippine sea slab phs beneath southwest japan and the hypocentre distribution is quite inhomogeneous in this region distinct non volcanic deep low frequency tremors are often observed to clarify the cause of these apparently complicated and peculiar seismic activities we first construct a detailed configuration of the oceanic moho slab moho within the phs based on a receiver function rf analysis to convert the time domain rfs into their depth domain equivalents we adopt a fine scaled velocity model based on the traveltime tomography the depth contours of the oceanic moho exhibit complexities but the active region of the intraslab earthquakes corresponds well to the oceanic moho geometry in the western part of shikoku the intraslab seismicity is very high and the focal mechanisms mainly exhibit normal faults with east west extension axes since the phs is subducting to the west and the dip angle of the slab becomes steep in the western part of this region the stress field in this region can be attributed to the slab pull induced by the westwards slab subduction similar types of focal mechanisms are observed in the tokai district with a focal depth greater than 35 km since the oceanic moho exhibits a ridge shape in this region its stress field can be attributed to the slab pull and the effect of slab bending shallow earthquakes in the tokai and eastern shikoku regions have strike slip focal mechanisms and their extension axes are parallel to the strike of the oceanic moho these focal mechanisms may be influenced by both the oblique subduction of the slab and low slab dip angle earthquakes beneath the kii peninsula exhibit strike slip or downdip compression focal mechanisms with east west compression axes since the moho beneath this peninsula shows a valley shape compressional stress occurs around the moho moreover the slab convergence compresses this valley this also induces the east west compression field in this region we have newly confirmed the existence of an aseismic slab beneath the northern part of the kinki district central part of japan in this aseismic region the dip direction of the slab is parallel to that of the slab convergence direction this tendency is observed in the central part of shikoku where the slab seismicity is relatively low these features imply that the intraslab seismicity is constrained by the interaction between the slab geometry and slab motion we also revealed the possibility that the oceanic moho beneath not only ise bay but also the kii channel has a ridge shape in both these regions the non volcanic low frequency tremor activity becomes low more than 80 per cent of the tremor epicentres are distributed within contour depths of 38 3 km of the oceanic moho we find that the interaction between the moho geometry and the plate motion of the subducting phs plays an important role in determining the local stress field and the location of both seismicity and low frequency tremors

a community benchmark for subduction zone modeling we have developed a suite of benchmarks to facilitate the comparison of numerical models for the dynamics and thermal structure of subduction zones the benchmark cases are based on a thermomechanical approach in which the slab is prescribed kinematically and the wedge flow is computed dynamically we propose various cases to investigate the influence of boundary conditions and rheology on wedge flow and resulting thermal structure a comparison between the codes suggest that accurate modeling of the thermal field requires a good implementation of the velocity discontinuity along the seismogenic zone and high resolution in the thermal boundary layers a minor modification to the boundary conditions of the wedge flow is also necessary to avoid a pressure singularity that exists in analytical solutions of the cornerflow model

distribution of methane hydrate bsrs and its implication for the prism growth in the nankai trough abstract detailed mapping of a bottom simulating reflector bsr which marks the phase transition from the methane hydrate layer above the reflector to free gas below was conducted in the nankai accretionary prism off shikoku and tokai bsrs are widely distributed in the prism slope from the toe region to the forearc basin bsr positions provide us with the regional heat flow variations using pressure temperature conditions for methane hydrate stability estimated heat flows generally show constant values about 50 mw m2 shallower than the middle slope of the prism and gradually increase seaward in the lower prism slope occurrences of bsrs are regarded as accumulation of free gas beneath the base of a gas hydrate stability field bghs and or concentration of methane hydrate above the bghs these conditions can be accomplished by updip migration of methane gas because it is unlikely that such methane concentration is completed by in situ biogenic methanogenesis within sediments including low total organic carbon moreover sedimentation uplifting and sediment stacking by thrust faulting cause upward migration of the bghs and migration of methane from the dissociated hydrate to new bghs such recycling of methane gas may have actively occurred in accretionary prisms in contrast there are five regions of no bsrs the nankai trough floor prism toe slope basin steep slope and deep sea canyon the trough floor the prism toe and the slope basin are characterized by young sediments with low production of methane gas and sub horizontal strata unsuitable for migration of gases and fluids erosion at the steep slope and the canyon causes removal of hydrated sediments and downward movement of the bghs bsr distribution and thermal structure estimated from bsr positions offer information about active processes occurring in accretionary prisms

a synthesis of heat flow determinations and thermal modeling along the nankai trough japan 1 we review marine heat flow data along the nankai trough and show that observations 30 km seaward of the deformation front are 20 below conductive predictions 129 94 mw m 2 but consistent with the global heat flow average for oceanic crust of the same age 16 28 ma heat flow values 30 km seaward of the deformation front are generally 20 higher than conductive predictions this heat flow pattern is consistent with the advection of heat by fluid flow in the subducting oceanic crust and explains both the high heat flux in the vicinity of the trench 200 and 140 mw m 2 and steep landward declines to values of approximately 60 mw m 2 over distances of 65 and 50 km along the muroto and kumano transects respectively along the ashizuri transect the lack of heat flow data precludes a definitive interpretation we conclude that fluid flow in the subducting oceanic crust leads to temperatures that are generally 25 c higher near the toe of the margin wedge and 50 100 c lower near the downdip limit of the seismogenic zone than estimated by purely conductive models

reinitiation of subduction and magmatic responses in sw japan during neogene time southwestern japan during paleogene time was affected by subduction of the kula and pacifi c plates beneath the eastern margin of eurasia followed by a brief episode of transform faulting to allow the shikoku basin to open at 27 15 ma subsequent opening of the japan sea back arc basin in middle miocene time broke japan away from eurasia and sw japan rotated clockwise 45o as it drifted south 17 15 ma southward drift required subduction of the philippine sea plate beneath sw japan leading to the formation of a magmatic arc subduction of the hot shikoku basin lithosphere and spreading ridge resulted in distinctive volcanism in the sw japan forearc between 17 and 12 ma this volcanism included midoceanic ridge basalt morb like intrusions and oceanic island basalt oib type alkali basalts felsic plutons in the accretionary prism and high magnesium andesites along the setouchi forearc basin mafi c magmas in the outermost forearc originated from the subducted shikoku basin spreading ridge whereas setouchi high magnesium andesites hma may have resulted from the interaction of melts of the subducted philippine sea plate with the overlying mantle felsic magmas in the forearc resulted from melting of shimanto belt sediments caused by intrusion of hma magmas persistent rear arc volcanism was caused by upwelling asthenosphere associated with opening of the sea of japan rear arc volcanism began ca 25 ma as rift fi ll low alkali tholeiite volcanism and was replaced gradually after 12 ma by alkali basalt volcanism upwelling related alkali volcanism continues up to the present whereas forearc volcanism ceased at 12 ma the volcanic arc narrowed with time as the philippine sea slab descended and slowly cooled adakitic dacites erupted after 1 7 ma above the 100 km depth contour of the subducted philippine sea plate suggesting that melting resulted from interaction of the slab with upwelling asthenosphere interactions between upwelled rear arc asthenosphere and subduction of the hot philippine sea slab appear to have been the main controls of magmatism for the neogene sw japan arc

influence of trench width on subduction hinge retreat rates in 3 d models of slab rollback subduction of tectonic plates limited in lateral extent and with a free trailing tail i e free subduction is modeled in a three dimensional 3 d geometry the models use a nonlinear viscoplastic rheology for the subducting plate and exhibit a wide range of behaviors depending on such plate characteristics as strength width and thickness we investigate the time evolution of this progressive rollback subduction measure the accompanying return flow in the upper mantle and quantify the plate kinematics due to the 3 d geometry flow is allowed to accompany slab rollback around the lateral edges of the slab the toroidal component as opposed to 2 d geometry where material is forced to flow underneath the slab tip the poloidal component a simple force balance is provided which relates the speed of backward trench migration to the resistive forces of generating flow and weakening the plate our results indicate most of the gravitational energy of the system i e the negative buoyancy of the subducting slab is converted into a toroidal flow 69 a much smaller amount goes into weakening the plate 18 and the remaining amount goes into driving flow parallel to displacement of the slab 13 for the trench widths w we investigate 1500 km a maximum trench retreat rate occurs for trenches 600 km wide which is attributed to the interaction between a plate of finite width and the induced flow which has a lengthscale in the horizontal direction these numerical results quantitatively agree with comparable 3 d laboratory experiments using analogue models with a purely viscous plate material schellart 2004a 2004b including correlations between increasing retreat rate with increasing plate thickness trench width for maximum retreat rate 500 km and estimated amount of slab buoyancy used to drive rollback induced flow 70 several implications for plate tectonics on earth result from these models such as rollback subduction providing a physical mechanism for ephemeral slab graveyards situated above the more viscous lower mantle and endothermic phase transition prior to a flushing event into the lower mantle mantle avalanche

seismic characterization of mantle flow in subduction systems can we resolve a hydrated mantle wedge this study provides new constraints on the resolvability of mantle flow in subduction zone settings as inferred by observations of seismic anisotropy we are motivated by the broad range of shear wave splitting observations in subduction systems that suggest complex flow geometries changes in the deformation state of mantle minerals or a combination of both while shear wave splitting fast polarization directions are typically interpreted as a proxy for flow or maximum finite extension experimental studies suggest that olivine slip systems change under higher stress and hydration states conditions likely appropriate for subduction systems in this study we predict shear wave splitting as a result of mantle silicate lattice preferred orientation development in steady state two dimensional mantle flow models using a textural development theory that incorporates the combined effects of intracrystalline slip and dynamic recrystallization we utilize the resulting textures to predict shear wave splitting for populations of seismic raypaths traversing the model within the subduction zone mantle wedge the results of our study demonstrate that combined observations of variations in fast polarization directions and splitting times make it possible to resolve a shift from anhydrous to hydrous mantle insubduction zone settings provided very good sampling of the mantle wedge our models are generally consistent with observed splitting variations for several subduction zones with dense data sampling including tonga japan and kamchatka the implications of our work suggest that provided adequate data sampling shear wave splitting measurements can provide the necessary information to evaluate potential competing effects between variations in mantle flow direction and changes in the stress and hydration states of subduction zone mantle wedges

slab stiffness control of trench motion insights from numerical models 1 subduction zones are not static features but trenches retreat roll back or advance here we investigate the dominant dynamic controls on trench migration by means of two and three dimensional numerical modeling of subduction this investigation has been carried out by systematically varying the geometrical and rheological model parameters our viscoplastic models illustrate that advancing style subduction is promoted by a thick plate a large viscosity ratio between plate and mantle and a small density contrast between plate and mantle or an intermediate width w 1300 km advancing slabs dissipate 45 to 50 of the energy in the system thin plates with relatively low viscosity or relatively high density or wide slabs w 2300 km on the other hand promote subduction in the retreating style i e slab roll back the energy dissipated by a retreating slab is 35 to 40 of the total dissipated energy most of the energy dissipation occurs in the mantle to accommodate the slab motion whereas the lithosphere dissipates the remaining part to bend and unbend with a simple scaling law we illustrate that this complex combination of model parameters influencing trench migration can be reduced to a single one plate stiffness stiffer slabs cause the trench to advance whereas more flexible slabs lead to trench retreat the reason for this is that all slabs will bend into the subduction zone because of their low plastic strength near the surface but stiff slabs have more difficulty unbending at depth when arriving at the 660 km discontinuity those bent slabs tend to cause the trench to advance in a similar way variation of the viscoplasticity parameters in the plate may change the style of subduction a low value of friction coefficient weakens the plate and results in a retreating style while higher values strengthen the plate and promote the advancing subduction style given the fact that also on earth the oldest and therefore probably stiffest plates have the fastest advancing trenches we hypothesize that the ability of slabs to unbend after subduction forms the dominant control on trench migration

temperature distribution of the upper surface of the subducted philippine sea plate along the nankai trough southwest japan from a three dimensional subduction model relation to large interplate and low frequency earthquakes summary the nankai subduction fault in southwest japan is a plate boundary along which large megathrust earthquakes have repeatedly occurred a previous 2 d thermal model suggested that the updip and downdip limits of the seismogenic zone are closely related to the temperature of the plate interface we estimated a new temperature distribution for the plate interface based on a 3 d thermal convection model that incorporates revised governing equations temporal changes in relative plate motion the shape of the subducted slab temperature variations related to basin evolution and more heat flow data and model regions than have been used in previous models the margin parallel spatial variation in the temperature of the plate interface was caused by spatio temporal changes associated with the cooling of the shikoku basin the updip limit of the thermally delineated seismogenic zone thermal seismogenic zone was approximately parallel to the trough axis due to a spreading rate that increased with distance from a fossil ridge although the margin parallel age dependency of the subducting plate was taken into account the downdip limit of the thermal seismogenic zone was determined by temperature except in eastern kyushu where it was determined by a boundary between the continental moho depth and the upper surface of the subducting plate the maximum coseismic slip associated with the 1944 tonankai m 7 9 and the 1946 nankai m 8 0 earthquakes and a strongly coupled region estimated from gps data inversion obtained by previous studies were both located in the thermal seismogenic zone on the plate interface low frequency earthquakes located at the downdip of the thermal seismogenic zone took place at temperatures of about 400 500 c this may be related to the phase transformation of hydrous minerals in oceanic crust from lawsonite blueschist jadeite to amphibole eclogite and the associated decreases in water content of approximately 2 4 wt the thermal seismogenic zone is narrowest off the kii peninsula where the hypocentres of the tonankai and nankai earthquakes were located the narrow seismogenic zone may have determined the rupture initiation points for these earthquakes as was previously demonstrated by numerical simulations of the earthquake cycle

a regime diagram for subduction styles from 3 d numerical models of free subduction abstract previous models of subduction both analogue and numerical have observed a number of distinct styles of subduction each with particular subduction motions partitioned between slab rollback and forward plate advance and associated slab morphologies we use 3 d numerical models to investigate subduction dynamics by varying the strength of slabs as well as the buoyancy and propose a new classification based on these parameters the slab strength is specified both through the ratio of viscosities between the subducting plate and upper mantle plate um as well as the plate thickness h plate only a very restrictive range of plates strong plates with smaller buoyancy tend to favor modes of subduction which are exclusively advancing plates which have greater negative buoyancy will eventually transition into a retreating style we find that the flexural strength and the buoyancy determine the subduction style as distinguished by a characteristic slab morphology and control several subduction characteristics including the partitioning between slab rollback and plate advance the trench curvature and the slab s radius of curvature plates that are 80 100 km thick with plate um 100 300 are classified here as weak and are the only plates to exhibit slab geometry with several recumbent folds atop the more viscous lower mantle this regime of weak plates with their associated slab morphologies predominant folding is argued to be most similar to slabs on earth based on the presence of folded slab piles in earth s upper mantle as interpreted from seismic tomography

the influence of trench migration on slab penetration into the lower mantle abstract a two dimensional numerical convection model in cartesian geometry is used to study the influence of trench migration on the ability of subducted slabs to penetrate an endothermic phase boundary at 660 km depth the transient subduction history of an oceanic plate is modelled by imposing plate and trench motion at the surface the viscosity depends on temperature and depth a variety of styles of slab behaviour is found depending predominantly on the trench velocity when trench retreat is faster than 2 4 cm a the descending slab flattens above the phase boundary at slower rates it penetrates straight into the lower mantle although flattening in the transition zone may occur later leading to a complex slab morphology the slab can buckle independent of whether it penetrates or not especially when there is a localised increase in viscosity at the phase boundary flattened slabs are only temporarily arrested in the transition zone and sink ultimately into the lower mantle the results offer a framework for understanding the variety in slab geometry revealed by seismic tomography

subduction of the western pacific plate underneath northeast china implications of numerical studies abstract the geodynamic process of the deep subduction of the western pacific plate underneath northeast china is critical for understanding the extensional events and volcanism in northeast china understanding of this process depends on 1 the initial time of the subduction 2 the trench retreat velocity during the subduction process 3 the contribution of indian eurasian collision and pacific eurasian subduction to extensional events in the northeast of china however information on these three issues is very limited we use several regional models to gain insight into these three issues each of the models includes temperature dependent viscosity structures and distinct velocity patterns at the surface our results show that the subduction of the pacific plate under the eurasian plate started most probably around 70 ma to be consistent with the tomography under northeast china trench retreat must be included in the models with a rate less than 45 km ma that has been estimated in the past we suggest that the extension events in the northeast china are attributed to indian eurasian collision and pacific eurasian subduction according to the velocity evolution in our models

strength of stick slip and creeping subduction megathrusts from heat flow observations subduction faults called megathrusts can generate large and hazardous earthquakes the mode of slip and seismicity of a megathrust is controlled by the structural complexity of the fault zone however the relative strength of a megathrust based on the mode of slip is far from clear the fault strength affects surface heat flow by frictional heating during slip we model heat flow data for a number of subduction zones to determine the fault strength we find that smooth megathrusts that produce great earthquakes tend to be weaker and therefore dissipate less heat than geometrically rough megathrusts that slip mainly by creeping

contiguous rupture areas of two nankai trough earthquakes revealed by high resolution tsunami waveform inversion 1 we have developed a new method for inverting tsunami waveforms that reveals considerable detail in megathrust slip during subduction zone earthquakes previous methods have ensured compliance with the shallow water theory used to compute tsunami waveforms by using large subfaults that generate only long wavelength seafloor deformation we show that a better approach is to use small subfaults coupled with a smoothing criterion that ensures compliance with the shallow water approximation in an application of the method to historical earthquakes in the nankai trough southwestern japan we find that the areas with slip 1 m for the earthquakes of 1944 and 1946 which ruptured adjacent segments of the subduction zone are separated by a sharp non overlapping boundary this establishes that interseismic accumulation of strain energy extends very close to the boundary between rupture zones and strongly suggests that this boundary is associated with a physical barrier to rupture

subduction of the philippine sea plate beneath southwestern japan slab geometry and its relationship to arc magmatism 1 we carry out high resolution three dimensional seismic tomography of the crust and upper mantle beneath southwestern sw japan using arrival time data obtained from the nationwide seismograph network the tomographic images provide new insights into the configuration of the philippine sea slab and arc magmatism the results confirm the existence of an aseismic portion of the philippine sea slab at greater depths beyond the seismic portion the philippine sea slab is subducting aseismically down to at least a depth of 200 km in chubu 60 80 km in kinki and 60 km in chugoku and is subducting seismically to depths of 150 200 km in kyushu in the chubu district it is subducting seismically at a shallow dip and then bends downward beneath the volcanic area whereas in the kinki district it is subducting subhorizontally as far as the japan sea the differences in the slab geometry between adjacent regions correlate with the differences in volcanic activity suggesting the importance of the geometry of the philippine sea slab on arc magmatism our tomographic images further imply subduction of the philippine sea slab down to a depth of around 70 km northwest of the izu peninsula the philippine sea slab in that region might have split into western and eastern parts separated by a slab tear and collided with the pacific slab at depths of 150 200 km

imaging the subducting slabs and mantle upwelling under the japan islands summary a high resolution p wave tomography of the crust and mantle down to 700 km depth beneath the japan islands is determined using a large number of high quality arrival time data from local earthquakes and teleseismic events simultaneously the tomography shows that the philippine sea slab is subducting aseismically down to 430 km depth under southwest japan though the seismicity within the slab ends at 180 km depth a low velocity low v zone in the mantle wedge under tohoku and kyushu is found to extend westward from the volcanic front to the backarc under the japan sea and east china sea significant low v anomalies are revealed in the deep portion of the mantle wedge 400 500 km depth above the pacific slab under southwest japan which may reflect hot mantle upwelling associated with fluids from the deep dehydration of the pacific slab low v anomalies appear at 420 700 km depths beneath the pacific slab under eastern japan which may reflect hot mantle upwelling associated with the deep subduction of the pacific slab and its collapsing down to the lower mantle

scaling relationship for vulcanian explosions derived from broadband seismic signals 1 a proposed conceptual model of vulcanian explosions describes the volcanic explosion process as consisting of two stages the movement of the fragmented magma to the surface and a subsequent explosion and is constrained by four source parameters inferred from broadband seismic records the counter force of the eruption f acting in the conduit before an explosion and governing the fragmented magma movement to the surface the time d1 of the movement of fragmented magma in the conduit before an explosion the duration of the explosion in the conduit d2 and the energy e of the explosion our paper discusses within the framework of the proposed conceptual model the source scaling relationship inferred from the parameters of 135 vulcanian explosions occurring at two mexican andesitic volcanoes volcan de colima and popocatepetl during 1996 2006 two groups of explosions are discriminated small e 1011 j and large e 1011 j characterized by a different type of scaling and origin the scaling of the counter force f on the source duration of the single force is different for large and small explosions but is characterized by the same tendency of f 2 according to the dependence e versus d1 large explosions are generated within the narrow depth zone in the conduit corresponding to d1 from 3 5 to 7 5 s while small explosions are generated within a wide depth zone corresponding to d1 from 0 to 13 s the counter force of the eruption strongly governs the energy of explosions the duration of the explosion at the explosion surface within the conduit d2 has a tendency to negatively correlate with the energy of the explosion

revel a model for recent plate velocities from space geodesy 1 we present a new global model for recent plate velocities revel describing the relative velocities of 19 plates and continental blocks the model is derived from publicly available space geodetic primarily gps data for the period 1993 2000 we include an independent and rigorous estimate for gps velocity uncertainties to assess plate rigidity and propagate these uncertainties to the velocity estimates the velocity fields for north america eurasia and antarctica clearly show the effects of glacial isostatic adjustment and australia appears to depart from rigid plate behavior in a manner consistent with the mapped intraplate stress field two thirds of tested plate pairs agree with the nuvel 1a geologic 3 myr average velocities within uncertainties three plate pairs caribbean north america caribbean south america and north america pacific exhibit significant differences between the geodetic and geologic model that may reflect systematic errors in nuvel 1a due to the use of seafloor magnetic rate data that do not reflect the full plate rate because of tectonic complexities most other differences probably reflect real velocity changes over the last few million years several plate pairs arabia eurasia arabia nubia eurasia india move more slowly than the 3 myr nuvel 1a average perhaps reflecting long term deceleration associated with continental collision several other plate pairs including nazca pacific nazca south america and nubia south america are experiencing slowing that began 25 ma the beginning of the current phase of andean crustal shortening

three dimensional seismic velocity structure and configuration of the philippine sea slab in southwestern japan estimated by double difference tomography 1 three dimensional seismic velocity structure in and around the philippine sea plate subducting beneath southwestern sw japan is determined by applying double difference tomography method to arrival time data for earthquakes obtained by a dense nationwide seismic network in japan a region of low s wave velocity and high vp vs of several kilometers in thickness is recognized immediately above the region of intraslab seismicity in a wide area from tokai to kyushu this characteristic layer dips shallowly in the direction of slab subduction compared with the upper surface of the philippine sea slab based on seismic reflection and refraction surveys on seven survey lines we interpret that the low vs and high vp vs layer corresponds to the oceanic crust of the philippine sea slab on the basis of the position of the low vs and high vp vs layer and the precisely relocated hypocenter distribution of intraslab earthquakes the upper surface of the philippine sea slab is reliably determined for the entire area of sw japan nonvolcanic deep low frequency earthquakes that occurred associated with the subduction of the philippine sea slab are distributed along the isodepth contour of 30 km in sw japan except for the tokai district where the depth of deep low frequency earthquakes becomes gradually deeper toward northeast

precursors to intra eruptive reposes during the 1998 2019 lava dome building eruption at volcan de colima mexico the 1998 2019 lava dome building eruption at volcan de colima mexico involved two eruptive cycles of activity during which andesitic lava domes were extruded the first cycle of extrusion lasted from november 1998 to june 2011 and was followed by a period of intra eruptive repose from june 2011 to january 2013 the second cycle lasted from january 2013 to march 2017 the second intra eruptive repose began from march 2017 and continues through the time of writing april 2019 the reposes were preceded by interruptions in the endogenous regime of growth of the lava domes by extrusion of exogenous shear lobes before repose 1 and by collapse of the lava dome before repose 2 we conclude that interruptions in endogenous lava dome growth may herald the onset of intra eruptive repose periods at this volcano the nature of such precursors is potentially associated with the process of batch supply from deep magma storage into the shallow system feeding the eruption during the two cycles of eruption at volcan de colima the equal volumes of products about 75 106 m3 were erupted with a difference in the mean magma discharge rates approximately 5 106 m3 and 18 106 m3 per year for cycles 1 and 2 respectively it seems that a cycle was controlled by a volumetric limit of 75 106 m3 once this volume has been erupted then the cycle ends irrespective of discharge rate at the same time a difference in the mean magma discharge rates may be a reason for differences in the type of interruptions

a model comparison study of large scale mantle lithosphere dynamics driven by subduction modelling subduction involves solving the dynamic interaction between a rigid solid yet deformable plate and the fluid easily deformable mantle previous approaches neglected the solid like behavior of the lithosphere by only considering a purely fluid description however over the past 5 years a more self consistent description of a mechanically differentiated subducting plate has emerged the key feature in this mechanical description is incorporation of a strong core which provides small resistance to plate bending at subduction zones while simultaneously providing adequate stretching resistance such that slab pull drives forward plate motion additionally the accompanying numerical approaches for simulating large scale lithospheric deformation processes coupled to the underlying viscous mantle flow have been become available here we put forward three fundamentally different numerical strategies each of which is capabable of treating the advection of mechanically distinct materials that describe the subducting plate we demonstrate their robustness by calculating the numerically challenging problem of subduction of a 6000 kin wide slab at high resolution in three dimensions the successfuly achievement of which only a few codes in the world can presently even attempt in spite of the differences of the approaches all three codes pass the simple qualitative test of developing an s bend trench curvature previously observed in similar models while reproducing this emergent feature validates that the lithosphere mantle interaction has been correctly modelled this is not a numerical benchmark in the traditional sense where the objective is for all codes to achieve exact agreement on a unique numerical solution however we do provide some quantitative comparisons such as trench and plate kinematics in addition to discussing the strength and weaknesses of the individual approaches consequently we believe these developed algorithms can now be applied to study the parameters involved in the dynamics of subduction and offer a toolbox to be used by the entire geoscience community c 2008 elsevier b v all rights reserved

two dimensional thermal modeling of subduction of the philippine sea plate beneath southwest japan abstract in this study we constructed a two dimensional thermal convection model associated with subduction of the philippine sea phs plate beneath southwest japan to estimate the thermal state to evaluate the validity of the calculated temperature distribution we compared the calculated heat flows with observed heat flow data such as the high sensitivity seismograph network japan hi net borehole land borehole marine probe and bottom simulating reflectors hi net heat flow data which have never been used for thermal modeling have spatially high resolutions on land and enable detailed estimation of the thermal state beneath the japanese islands we considered the spatio temporal change in the age of the phs plate change in its plate motion direction at 3 ma and the geometry of its upper surface calculated heat flows associated solely with subduction of the phs plate passing through central and eastern shikoku and the kii peninsula were not consistent with observations they tended to be lower on the seaward side of the corner of the mantle wedge and to have much longer spatial wavelengths on the continental side to explain heat flow data in southwest japan frictional heating at the plate interface on the seaward side of the corner of the mantle wedge and temperature changes due to surface erosion and sedimentation on land associated with crustal deformation during the quaternary must be incorporated into the model the most suitable pore pressure ratio to explain the heat flow data is estimated to be 0 95 temperatures on the upper surface of the phs plate obtained in this study are lower at shallow depths and higher at greater depths than those obtained by hyndman et al 1995 indicating that the seismogenic zone inferred from this study is narrower than that of their model microearthquakes within the subducting phs plate occur at temperatures below 700 800 c

evidence of lower mantle slab penetration phases in plate motions it is well accepted that subduction of the cold lithosphere is a crucial component of the earth s plate tectonic style of mantle convection but whether and how subducting plates penetrate into the lower mantle is the subject of continuing debate which has substantial implications for the chemical and thermal evolution of the mantle here we identify lower mantle slab penetration events by comparing cenozoic plate motions at the earth s main subduction zones with motions predicted by fully dynamic models of the upper mantle phase of subduction driven solely by downgoing plate density whereas subduction of older intrinsically denser lithosphere occurs at rates consistent with the model younger lithosphere of ages less than about 60 myr often subducts up to two times faster while trench motions are very low we conclude that the most likely explanation is that older lithosphere subducting under significant trench retreat tends to lie down flat above the transition to the high viscosity lower mantle whereas younger lithosphere which is less able to drive trench retreat and deforms more readily buckles and thickens slab thickening enhances buoyancy volume times density and thereby stokes sinking velocity thus facilitating fast lower mantle penetration such an interpretation is consistent with seismic images of the distribution of subducted material in upper and lower mantle thus we identify a direct expression of time dependent flow between the upper and lower mantle

chilean flat slab subduction controlled by overriding plate thickness and trench rollback how flat slab geometries are generated has been long debated it has been suggested that trenchward motion of thick cratons in some areas of south america and cenozoic north america progressively closed the asthenospheric wedge and induced flat subduction here we develop time dependent numerical experiments to explore how trenchward motion of thick cratons may result in flat subduction we find that as the craton approaches the trench and the wedge closes two opposite phenomena control slab geometry the suction between ocean and continent increases favoring slab flattening while the mantle confined within the closing wedge dynamically pushes the slab backward and steepens it when the slab retreats as in the peru and chile flat slabs the wedge closure rate and dynamic push are small and suction forces generate in some cases flat subduction we model the past 30 m y of subduction in the chilean flat slab area and demonstrate that trenchward motion of thick lithosphere 200 300 km currently 700 800 km away from the peru chile trench reproduces a slab geometry that fits the stress pattern seismicity distribution and temporal and spatial evolution of deformation and volcanism in the region we also suggest that varying trench kinematics may explain some differing slab geometries along south america when the trench is stationary or advances the mantle flow within the closing wedge strongly pushes the slab backward and steepens it possibly explaining the absence of flat subduction in the bolivian orocline

small scale convection under the back arc occurring in the low viscosity wedge abstract water released from subducting slabs through a dehydration reaction may lower the viscosity of the mantle significantly thus we may expect a low viscosity wedge lvw above the subducting slabs the lvw coupled with a large scale flow induced by the subducting slabs may allow the existence of roll like small scale convection whose axis is normal to the strike of the plate boundary such a roll structure may explain the origin of along arc variations of mantle temperature proposed recently in northeast japan we study this possibility using both 2d and 3d models with without pressure and temperature dependent viscosity 2d models without pressure and temperature dependence of viscosity show that with a reasonable geometry of the lvw and subduction speed small scale convection is likely to occur when the viscosity of the lvw is less than 10 19 pa s corresponding 3d model studies reveal that the wavelength of rolls depends on the depth of the lvw the inclusion of temperature dependent viscosity requires the existence of further low viscosity in the lvw since temperature dependence suppresses the instability of the cold thermal boundary layer pressure i e depth dependence coupled with temperature dependence of the viscosity promotes short wavelength instabilities the model which shows a relatively moderate viscosity decrease in the lvw most of the lvw viscosity is 10 18 10 19 pa s and a wavelength of roll 80 km has a rather small activation energy and volume 130 kj mol and 4 cm 3 mol of the viscosity this small activation energy and volume may be possible if we regard them as an effective viscosity of non linear rheology

slab dehydration and earthquake distribution beneath southwestern and central japan based on three dimensional thermal modeling we developed a 3 d thermal convection model to estimate the thermal regime water content distribution and slab dehydration beneath southwestern and central japan where deep tectonic tremors and short term slow slip events s sses are frequently observed on the plate interface extending from the western shikoku to the tokai district the results showed that the pressure temperature p t conditions for the s sses deep tectonic tremors and regular earthquakes indicate that they probably originated from slab dehydration in association with subduction resulting in a phase transformation from prehnite actinolite lawsonite blueschist to amphibolite with a large thermal gradient slab dehydration and the thermal gradient in the dip direction are considered key factors for controlling the seismogenesis of slow and regular earthquakes in the philippine sea plate beneath southwestern and central japan although the deep tectonic tremors are more complicated because they are likely favored by a comparatively low temperature gradient in southwestern japan

pollen pb 210 and opaque spherules an integrated approach to dating and sedimentation in the intertidal environment abstract a combination of techniques was applied to the problem of dating recent intertidal deposits from eastern long island careful separation of important indicator pollen types allowed identification of regional changes in vegetation comparison of pollen profiles and historic documents supplied three dated horizons 1 a 1640 1680 rise in agricultural indicator at the time of settlement 2 an 1800 decline in oak pollen with the initiation of the cordwood industry and 3 a 1920 decline in chestnut pollen following the chestnut blight pb 210 provided chronologies for the last 100 150 year sections of cores chronologies from each were compared and used to date a 1900 rise in industrially derived opaque spherules independently results showed that a combination of techniques could be sed to obtain detailed chronologies from intertidal sediments deposited within the last 300 350 years and to interpret salt marsh and barrier island dynamics although pb 210 may be restricted in its application due to erratic sedimentation pollen analysis can be used to date strata from high energy marshes where sediment accumulation rates fluctuate