a subducting seamount beneath the nankai accretionary prism off shikoku southwestern japan a multi channel seismic mcs reflection survey was conducted to study the structure of the nankai convergent margin off shikoku island in july 1997 based on reflection characteristics we could identify three major seismic reflection units i e units a b and c the mcs data as well as swath bathymetric data reveal a buried circular seamount subducting beneath the nankai accretionary prism the subducting seamount is responsible for deformation of the accretionary wedge resulting in a compressed uplifted sediment knoll and many steep escarpments with ne sw strike a possible thrust fault was identified on the seaward flank of the seamount indicating compressive deformation a tectonic model for subduction of the seamount is presented based on the interpretation of the mcs data

three dimensional splay fault geometry and implications for tsunami generation megasplay faults very long thrust faults that rise from the subduction plate boundary megathrust and intersect the sea floor at the landward edge of the accretionary prism are thought to play a role in tsunami genesis we imaged a megasplay thrust system along the nankai trough in three dimensions which allowed us to map the splay fault geometry and its lateral continuity the megasplay is continuous from the main plate interface fault upwards to the sea floor where it cuts older thrust slices of the frontal accretionary prism the thrust geometry and evidence of large scale slumping of surficial sediments show that the fault is active and that the activity has evolved toward the landward direction with time contrary to the usual seaward progression of accretionary thrusts the megasplay fault has progressively steepened substantially increasing the potential for vertical uplift of the sea floor with slip we conclude that slip on the megasplay fault most likely contributed to generating devastating historic tsunamis such as the 1944 moment magnitude 8 1 tonankai event and it is this geometry that makes this margin and others like it particularly prone to tsunami genesis

moisture sources of the chinese loess plateau during 1979 2009 abstract the chinese loess plateau clp is one of the regions in china with the most serious water scarcity and precipitation is a key issue in the clp climate to investigate the main sources of moisture for the clp we apply a lagrangian particle dispersion model known as the flexible particle dispersion model flexpart to perform a 31 year transient simulation 1979 2009 with backtracking particles residing over the clp for 10 days in this study further a moisture source attribution method the areal source receptor attribution method is employed to quantify the contributions of the moisture source regions to the clp precipitation the results show that in summer june july august the moisture transported into the clp originates mainly from central eastern china northwestern china eastern central asia the south china sea the east china sea the bay of bengal and the arabian sea in contrast in winter december january february the main moisture source regions are northwestern china eastern central asia central eastern china the sea around the arabian peninsula the southern tibetan plateau northwestern indo china peninsula and the mediterranean sea because of significant losses en route such as those caused by terrain obstruction and the long path to the continental interior the eventual moisture release to the clp is much less than the original uptake of the above mentioned sources the continental moisture sources near the clp are the main contributors to precipitation over the clp in both summer and winter of these a high proportion is contributed by central eastern china and northwestern china eastern central asia while the oceanic moisture sources make a very small contribution the reasons for this phenomenon may include the great distance between the related oceans and the clp thus the direct oceanic water vapor transport into the clp encounters obstacles on conventional backtracking days furthermore the indirect oceanic moisture sources of the clp are also preliminarily discussed herein

basin centered asperities in great subduction zone earthquakes a link between slip subsidence and subduction erosion 1 published areas of high coseismic slip or asperities for 29 of the largest circum pacific megathrust earthquakes are compared to forearc structure revealed by satellite free air gravity bathymetry and seismic profiling on average 71 of an earthquake s seismic moment and 79 of its asperity area occur beneath the prominent gravity low outlining the deep sea terrace 57 of an earthquake s asperity area on average occurs beneath the forearc basins that lie within the deep sea terrace in sw japan slip in the 1923 1944 1946 and 1968 earthquakes was largely centered beneath five forearc basins whose landward edge overlies the 350 c isotherm on the plate boundary the inferred downdip limit of the locked zone basin centered coseismic slip also occurred along the aleutian mexico peru and chile subduction zones but was ambiguous for the great 1964 alaska earthquake beneath intrabasin structural highs seismic slip tends to be lower possibly due to higher temperatures and fluid pressures kilometers of late cenozoic subsidence and crustal thinning above some of the source zones are indicated by seismic profiling and drilling and are thought to be caused by basal subduction erosion the deep sea terraces and basins may evolve not just by growth of the outer arc high but also by interseismic subsidence not recovered during earthquakes basin centered asperities could indicate a link between subsidence subduction erosion and seismogenesis whatever the cause forearc basins may be useful indicators of long term seismic moment release the source zone for cascadia s 1700 a d earthquake contains five large basin centered gravity lows that may indicate potential asperities at depth the gravity gradient marking the inferred downdip limit to large coseismic slip lies offshore except in northwestern washington where the low extends landward beneath the coast transverse gravity highs between the basins suggest that the margin is seismically segmented and could produce a variety of large earthquakes

forearc basins types geometries and relationships to subduction zone dynamics forearc basins are one of the fundamental sedimentary basins and they preserve various geological phenomena arising from dynamic interactions between subducting and overriding plates along convergent plate boundaries previous studies have proposed descriptive classification schemes for such basins and performed analog experiments and numerical simulations examining basin formation and evolution however the great variability of forearc basins means that a comprehensive understanding of these features remains elusive this paper proposes a new classification scheme for forearc basins from the viewpoints of material transfer between the two plates accretionary or nonaccretionary and the long term strain field in the basin compressional or extensional by using characteristics of 41 modern forearc basins geometric characteristics were also analyzed in order to reveal the relationship to subduction zone dynamics including material transfer at the plate boundary accretion erosion plate convergence configurations and kinematics and the mechanics of strain and its partitioning compressional accretionary type basins which are characterized by landward tilting strata and the landward migration of depocenters show a constant width thickness ratio indicating self similar growth of the basin as the outer wedge builds up extensional accretionary type basins develop normal faults in the basin probably due to a gravitational instability of the outer wedge an increase of sediment flux at the trench may grow the accretionary prism and thicken the subduction channel faster leading to a reduction of basal and internal shear strengths of the outer wedge subsidence caused by crustal thinning characterizes the nonaccretionary type basins which have thin trench fill sediments large forearc slopes steady widths of the outer wedge and variable width thickness ratios of the basins from these results the sediment flux at the trench and the plate boundary can be considered as a dominant factor that determines accretion or erosion of the outer wedge and uplift or subsidence of the outer arc high and that is also responsible for the style of deposition in the forearc basin an increase or decrease of the sediment flux may change the type geometry and deformation manner of the forearc basin the results of this study provide important clues to further our understanding of dynamic interactions between forearc basins and subduction zone processes

erratum to splay fault slip in a subduction margin a new model of evolution earth planet sci lett 341 344 2012 170 175

when seamounts subduct data from an array of seismometers on the sea floor show the complex pattern of earthquakes around subducted seamounts

deformation of the nankai trough inner accretionary prism the role of inherited structures funding for this research was provided by the national science foundation grant oce 0451790 and a u s science support program post expedition award

evidence for mechanical decoupling of the upper plate at the nankai subduction zone constraints from core scale faults at nantroseize sites c0001 and c0002 1 detailed analyses of cores from two drill sites landward of the nankai trough provide constraints on the evolution of stress across the sw japan subduction zone core scale faults at sites c0001 and c0002 suggest changing stress geometries that shed light on mechanical coupling across this plate boundary normal faults dominate the record of brittle strain in both the sedimentary cover and the upper accretionary prism and these cut older shear zones and vein structures based on cross cutting relations normal faults postdate thrust faults with the latter interpreted to be inactive the normal faults in contrast are consistent with contemporary stress proxies such as borehole breakout and anelastic strain recovery data stress inversion for faults that are restored to original orientations using paleomagnetic data reveal that thrusting has accommodated trench subperpendicular shortening associated with interplate coupling this phase of deformation is overprinted by normal faulting attributed to mechanical decoupling of the upper plate from the lower plate in response to the underplating of anomalously weak sediments between an out of sequence thrust and the decollement these observations suggest that core scale faults record a stress history that is to date not apparent from in situ stress proxies such as borehole breakout data or anelastic strain recovery data we conclude that the brittle faults provide a unique record of changing stress conditions that can be recognized only through coring

tectonic evolution of the japanese island arc system abstract growth of the japanese arc system which has mainly taken place along the continental margin of asia since the permian is the result of subduction of the ancient pacific ocean floor backarc basin formation in the tertiary shaped the present day arc configuration the neotectonic regime which is characterized by strong east west compression has been triggered by the eastward motion of the amur plate in the quaternary the tectonic evolution of the japanese arc system includes formation of rock assemblages common in most orogenic belts because the origin and present day tectonics of these assemblages are better defined in the case of the japanese arc system study of the system provides useful insight into orogenesis and continental crust evolution

formation of forearc basins and their influence on subduction zone earthquakes recent observations of an association between forearc basins and slip during subduction thrust earthquakes suggest a link between processes controlling upper plate structure and seismic coupling on the subduction zone thrust fault we present a mechanism for the formation of these basins where sedimentation occurs on landward dipping segments of the subduction wedge which itself is actively growing through the accretion of material from the subducting plate our numerical simulations demonstrate that sedimentation stabilizes the underlying wedge preventing internal deformation beneath the basin maximum slip during great thrust earthquakes tends to occur where sedimentary basins stabilize the overlaying wedge the lack of deformation in these stable regions increases the likelihood of thermal pressurization of the subduction thrust allows the fault to load faster and allows greater healing of the fault between rupture events these effects link deformation of the subduction wedge to the seismic coupling of the subduction thrust

in situ stress and pore pressure in the deep interior of the nankai accretionary prism integrated ocean drilling program site c0002 strength and slip behaviors of subduction megathrusts are controlled by the physical properties and stress state not only of the fault zones themselves but also the adjacent wall rocks in overriding and subducting plates in the nankai trough a 3 km deep riser borehole at integrated ocean drilling program site c0002 was drilled into the kumano fore arc basin and the underlying accretionary prism we quantify the full in situ stress tensor and pore pressure at site c0002 by integrating drilling data geophysical logging and downhole measurements using empirical relations that describe sediment compaction behavior as a function of loading path the kumano basin is loaded in uniaxial vertical strain conditions whereas the prism below is loaded with greater differential stress has a modestly elevated pore pressure and lies in a strike slip faulting regime this can be reconciled with slip on the megathrust fault if the horizontal stresses vary with time or depth and or if shear stress along the megathrust is low

cross section restoration of salt related deformation best practices and potential pitfalls abstract cross section restoration typically assumes plane strain deformation and area conservation constraints that are usually invalid for salt because of its characteristic three dimensional flow and possible dissolution thus restoration of salt related deformation provides added challenges and uncertainty in this review paper we summarize the historical development of ideas methods and applications of restoration in salt basins while most published restorations do not maintain salt area constraints on its variation range from arbitrary assumptions to quantitatively incorporating isostatic calculations we illustrate several scenarios in which the presence of salt adds ambiguity to restoration primarily because it can hide deformation diapirs can widen during extension and narrow during shortening translating overburden can move into salt and drive allochthonous advance secondary minibasin subsidence can be accommodated at both shallow and deep salt levels and allochthonous salt can record evacuation of deeper salt although we caution against using restoration to test and validate small scale details of interpretations we emphasize that sequential restoration remains an essential tool in structural and basin analyses however because of the uncertainties a regional three dimensional approach and sound geological reasoning are critical for deriving meaningful and useful results from cross section restoration of salt structures

fault relays bends and branch lines abstract branch lines between normal faults and their sub parallel splays mapped from 3d seismic reflection data show a range of forms from straight lines to closed loops the different geometries are interpreted as representing stages in the failure of relay zones and in the progressive replacement of fault tip lines with fault branch lines the geometries of these normal fault branch lines are similar to those for thrusts previously inferred from limited two dimensional data the orientation of the axis of a relay and its associated bends relative to a fault slip direction is identified as an important control on the structures developed within the relay neutral restraining and releasing bends can each occur on any fault type normal reverse and strike slip but data bias is a major factor in determining which bend geometry is most often observed with each fault type on normal faults the initial relay zone geometry controls the dominant branch line orientation and the same control is likely on branch lines associated with the other modes of faulting a review of the relay geometries and strains occurring with all three modes of faulting highlights the role of the orientation of the mechanical anisotropy of a bedded sequence relative to the orientations of fault surface and slip directions this relative orientation determines how the relay strain is accommodated and hence the degree of hard linkage and development of branch lines

a model for the structure and development of fault zones cross sections of fault zones commonly show two sub parallel slip surfaces which bound more or less deformed rock this characteristic geometry can be explained by a combination of 9tip line9 and 9asperity9 bifurcation processes fault tip line bifurcation is due to irregular tip line propagation resulting in leading edge segmentation fault overlap and eventual duplex formation asperity bifurcation removes a fault surface irregularity by the generation of a new slip surface which by passes the original slip surface repetitions of either or both bifurcation processes give rise to a fault zone with a complex and unpredictable internal geometry these processes are described mainly on the basis of observation of normal faults hut can be applied to other fault types fault zone thickening by bifurcation is intermittent rather than progressive and localised fault zone thinning may occur

the role of mud volcanism and deep seated dewatering processes in the nankai trough accretionary prism and kumano basin japan circulation of water at moderate depths in subduction zones is dominantly driven by clay mineral dehydration over distinct pressure and temperature gradients the signature of these dehydration reactions is found in mud volcano pore waters however it is largely unknown how much of the deep seated fluids are emitted at mud volcanoes to unravel this relation for the region off the kii peninsula japan we calculated the water volume that is subducted in the nankai trough using input data from iodp holes c0011 and c0012 and the correspondent water volume released from the subducted plate under the kumano basin in an area where 13 mud volcanoes are located according to our model water released at depth in the mud volcano area is derived almost entirely from basaltic saponite and sedimentary smectite transformation up to 96 nonetheless the mud volcanoes themselves expel 1 of the total volume to test the contribution of the accreted strata and the kumano basin fill to the water budget we run a second model water loss due to compaction of sediments and smectite illite transition below the basin floor have been calculated the results were compared with salinity measurements on background cores scattered in the study area to extrapolate the volume of water loss at depth the comparison of the two methods yielded similar results and led us to conclude that the bulk part of the deep seated fluid re enters the hydrosphere via the basin floor a mechanism rarely taken into account in fluid budgets in the literature

evolution of the hourglass structures in the laminaria high timor sea implications for hydrocarbon traps abstract in cross section an hourglass structure can be visualized as an older horst block and superimposed younger graben bounding faults of the horst and graben blocks represent separate conjugate fault systems formed by two distinct episodes of extension in the timor sea during late jurassic early cretaceous 1st phase and middle miocene pliocene 2nd phase with an 120 my hiatus of limited or no fault activity in between horst blocks were formed by the 1st phase of extension and buried post deformation with the onset of the 2nd phase of extension the hourglass geometry began to form by nucleation of the graben bounding faults in the shallow sedimentary section in isolation from the horst bounding faults location of the graben is biased by the buried horst block and graben bounding faults grew down dip from the shallow locus of nucleation toward the underlying horst block on which only minor reactivation occurred detachment of the two systems in this way was predominantly controlled by the first order mechanical layering a thick shale rich ductile layer separates the horst and graben bounding fault systems and acts as a barrier to vertical fault propagation confinement of the graben bounding faults into the shallow section was also facilitated by outer arc style extension due to lithospheric flexure controlling the 2nd phase strain in the region the complex evolution history and the composite nature of the hourglass structures resulted in systematic along dip variation of displacement this variation predominantly relates to syn kinematic deposition and location of fault tips that are controlled by the ductile layer the presented evolution model of the hourglass structures concentrates fault tips and related stress perturbation onto the top seal and is likely to be detrimental to top seal integrity

geometry and development of relay ramps in normal fault systems relay ramps occur between normal fault segments that overstep in map view the geometry and evolution of exposure scale relay ramps are described from the somerset coast england and are compared with larger scale ramps from elsewhere relay ramps can be classified into four groups based on the degree of interaction and linkage between the overstepping segments these groups are interpreted as being evolutionary stages in stage 1 the segments do not interact stage 2 involves the reorientation of bedding between two interacting faults to produce a relay ramp in stage 3 connecting fractures start to break the relay ramp stage 4 is when the relay ramp is destroyed to produce a single fault that has an along strike bend these evolutionary stages can develop through ti e but they can also be seen spatially a branch line between normal faults or an along strike bend may represent a stage 4 relay with progressively earlier stages occurring updip or downdip characteristic variability in displacement distance profiles for fault segments and linked faults accompanies the interaction and linkage processes displacement transfer by relay ramps is accompanied by steep displacement gradients along fault segments at oversteps relay ramps often contribute to a minimum in total fault displacement at a linkage point

changes in paleostress state along a subduction zone preserved in an on land accretionary complex the yokonami melange in the cretaceous shimanto belt kochi southwest japan a change in paleostress along a subduction zone plate interface in the shallow portion of a seismogenic zone was detected in an on land accretionary complex the yokonami melange in southwest japan using the microfault inversion method microfaults were classified into two groups based on location those occurring throughout the yokonami melange and those occurring in the goshikinohama fault zone which is considered to be a fossil seismogenic fault and is located at the northern end of the yokonami melange stresses obtained for these classified microfaults indicate that two different stress states exist for each deformation feature one is subhorizontal 1 and subvertical 3 with a smaller stress ratio and the other is subvertical 1 and subhorizontal 3 with a larger stress ratio the difference between these stress states could be related to the horizontal stress change by stress drop after large earthquakes that were recently observed after the tohoku oki great earthquake

three dimensional distinct element modelling of relay growth and breaching along normal faults three dimensional numerical models of neutral i e slip parallel relay zones on normal faults that cut massive sandstone host rocks have been constructed using the distinct element method code particle flow code in 3 d pfc3d the models successfully reproduce the geometries displacement profiles and strains observed in natural relay zones in contrast to boundary element method simulations the modelled relay ramps dip towards the hanging wall consistent with observations of most natural relay zones the modelling shows that relay zones are stable structures that grow by progressive rotation of an approximately planar relay ramp without significant propagation of the relay bounding faults prior to breaching stable growth is terminated when a breaching fault propagates across the top or bottom of the relay ramp breaching fault propagation is not instantaneous and the ramp continues to rotate and therefore transfer displacement between the relay bounding faults until they become fully hard linked following hard linkage displacement is accommodated by slip on the through going fault surface the modelling results confirm previous conceptual models of relay growth and breaching based on geometric and kinematic analysis of natural relay zones

subducted seamount imaged in the rupture zone of the 1946 nankaido earthquake the nankai trough is a vigorous subduction zone where large earthquakes have been recorded since the seventh century with a recurrence time of 100 to 200 years the 1946 nankaido earthquake was unusual with a rupture zone estimated from long period geodetic data that was more than twice as large as that derived from shorter period seismic data in the center of this earthquake rupture zone we used densely deployed ocean bottom seismographs to detect a subducted seamount 13 kilometers thick by 50 kilometers wide at a depth of 10 kilometers we propose that this seamount might work as a barrier inhibiting brittle seismogenic rupture

rheological dependence of extension in wedge models of convergent orogens abstract although several mechanical models of extension in contractional orogens have been proposed in many cases these models have been based on viscous constitutive laws lack of more complicated rheological models has primarily been due to the technical difficulty of solving for deformation with complex rheological behavior and boundary conditions a finite element model is presented that allows for solution of the wedge mechanical problem with viscous non linear viscous and coulomb plastic rheologies this model is used to investigate extension in a doubly vergent wedge model in which deformation is primarily contractional driven by a reversal of basal traction proposed models showing extension in the crust at shallow crustal levels in spite of overall contraction and contraction at depth are confirmed but only for high argand number low crustal viscosity at low argand number little or no extension is observed non linear stress dependence power law flow also suppresses extension but not entirely some extension is observed at high effective argand number however with a coulomb plastic rheology no extension is observed for a wide range of parameter values no extension is observed for models with viscous deformation at depth and plastic deformation near the surface this suggests that the extension is a function of the rheologic model not the effective strength of the crust coeval extension and contraction in convergent orogens with steady external forcing therefore requires viscous deformation to very shallow depths

fore arc deformation controls frequency size distribution of megathrust earthquakes in subduction zones 1 seismotectonic deformation in subduction zones seems to follow rather simple spatiotemporal patterns with fore arc basins overlying the areas of large slip during quasiperiodic megathrust earthquakes to study the possible coupling between long term deformation and earthquake behavior we use compressive granular wedges overlying a rate and state dependent frictional interface as analogue models of subduction zone fore arcs overlying a seismogenic megathrust for different seismogenic zone geometries we analyze deformation time series with respect to the accumulation of permanent strain and frequency size distributions of episodic slip events equivalent to great m 8 earthquakes we observe that permanent deformation in the wedges localizes at the periphery of unstable slip at depth over tens of simulated seismic cycles for updip limited seismogenic zone models this leads to structural wedge segmentation characterized by an elastic domain overlying the zone of unstable basal slip along with the evolution of segmentation the frequency size distributions of episodic slip events develop from more random gutenberg richter like events b value 0 6 toward more periodic characteristic events b value 0 1 corresponding coefficients of variation cv of recurrence intervals decrease from cv 0 6 in deforming wedges to cv 0 3 in segmented wedges from the experiments we thus infer a positive feedback between fore arc tectonics and megathrust seismogenesis which brings the system from a stochastic to a more deterministic state our experimental observations imply that the quasiperiodic recurrence of great subduction earthquakes evident from existing earthquake records is a long term feature intrinsically related to the seismotectonic segmentation of the fore arc wedges

the age and significance of the ngadarunga granite in proterozoic central australia abstract the emplacement of the ngadarunga granite in the arunta region of central australia directly constrains the timing of the first high grade metamorphic tectonic and magmatic event in the arunta region and the timing of the geographically extensive unconformity that separates the earliest regional basin phase from younger overlying sedimentary successions previous dating had correlated this event with the 1880 1840 ma barramundi orogeny interpreted in far north australia linking the arunta region to the barramundi model of widespread synchronous cratonisation in the period 1880 1840 ma new data revise this granite emplacement to 1803 6 ma placing it in the younger 1810 1800 ma stafford event of the arunta region the provenance of detrital zircons in early arunta region sedimentary rocks was dominated by crystalline rocks formed in the period 1840 1880 ma thus the earliest rocks in the arunta region were deposited after ca 1840 ma in a basin marginal to 1880 1840 ma continental crust and their subsequent tectonism began with the stafford event

3 d seismic attributes using a semblance based coherency algorithm seismic coherency is a measure of lateral changes in the seismic response caused by variation in structure stratigraphy lithology porosity and the presence of hydrocarbons unlike shaded relief maps that allow 3 d visualization of faults and channels from horizon picks seismic coherency operates on the seismic data itself and is therefore unencumbered by interpreter or automatic picker biases we present a more robust multitrace semblance based coherency algorithm that allows us to analyze data of lesser quality than our original three trace cross correlation based algorithm this second generation semblance based coherency algorithm provides improved vertical resolution over our original zero mean crosscorrelation algorithm resulting in reduced mixing of overlying or underlying stratigraphic features in general we analyze stratigraphic features using as narrow a temporal analysis window as possible typically determined by the highest usable frequency in the input seismic data in the limit one may confidently apply our new semblance based algorithm to a one sample thick seismic volume extracted along a conventionally picked stratigraphic horizon corresponding to a peak or trough whose amplitudes lie sufficiently above the ambient seismic noise in contrast near vertical structural features such as faults are better enhanced when using a longer temporal analysis window corresponding to the lowest usable frequency in the input data the calculation of reflector dip azimuth throughout the data volume allows us to generalize the calculation of conventional complex trace attributes including envelope phase frequency and bandwidth to the calculation of complex reflector attributes generated by slant stacking the input data along the reflector dip within the coherency analysis window these more robust complex reflector attribute cubes can be combined with coherency and dip azimuth cubes using conventional geostatistical clustering and segmentation algorithms to provide an integrated multiattribute analysis

three dimensional geometry and growth of conjugate normal faults abstract good quality 2 d and 3 d seismic reflection data from the timor sea are used to determine the threedimensional geometry displacement patterns and development of intersecting conjugate normal faults these data are supplemented by data from previous physical modelling studies conjugate structures which comprise two intersecting opposed dipping normal faults or fault sets form synchronously on a geological time scale and develop due to the incidental intersection of the faults factors which affect both the formation and imaging of these structures include the fault density the spatial distribution of opposed dipping faults the seismic resolution and the vertical extent of the imaged fault data large conjugate structures grow from smaller ones larger conjugates are associated with more numerous and larger faults than small structures on the scale of the seismic data fault throws range from ca 10 400 m synchronous fault movements are accommodated by a reduction of displacements on discrete fault surfaces towards the fault intersection zone and a corresponding increase in ductile strain of this region high strains in the volume proximal to the fault intersection zone are expressed as thinning of stratigraphic units between the conjugate faults and are believed to be accommodated by numerous small sub seismic faults intersection of two opposed dipping faults does not prevent their continued synchronous movement and does not result in mechanical locking of the system

accretionary prisms in subduction earthquake cycles the theory of dynamic coulomb wedge 1 we expand the theory of critically tapered coulomb wedge for accretionary prisms by considering stress changes in subduction earthquake cycles building on the coulomb plasticity of the classical theory we assume an elastic perfectly coulomb plastic rheology and derive exact stress solutions for stable and critical wedges the new theory postulates that the actively deforming most seaward part of an accretionary prism the outer wedge overlies the updip velocity strengthening part of the subduction fault and the less deformed inner wedge overlies the velocity weakening part the seismogenic zone during great earthquakes the outer wedge is pushed into a compressively critical state with an increase in basal and internal stresses and pore fluid pressure after the earthquake the outer wedge returns to a stable state the outer wedge geometry is controlled by the peak stress of the updip velocity strengthening part of the subduction fault achieved in largest earthquakes the inner wedge generally stays in the stable regime throughout earthquake cycles acting as an apparent backstop and providing a stable environment for the formation of forearc basins the new theory has important implications for the studies of the updip limit of the seismogenic zone the evolution of accretionary prisms and forearc basins activation of splay faults and tsunami generation evolution of the fluid regime and mechanics of frontal prisms at margins dominated by tectonic erosion

evolution of tectono sedimentary systems in the kumano basin nankai trough forearc abstract sedimentary deposits in the distal kumano forearc basin of the nankai accretionary margin off kii peninsula japan have been imaged using three dimensional 3d seismic data the seismic data along with logging and core data from the integrated ocean drilling program iodp show that the unconformity between the accretionary prism and overlying forearc sediments is time transgressive the unconformity at site c0002 separates 5 ma prism rocks from 3 65 ma basin deposits at site c0009 it separates 5 6 ma prism from 3 8 ma basin sediments acoustic reflections in the basal deposits are sub parallel to the underlying accretionary prism the acoustic facies varies in thickness from 50 to 750 m the mudstone deposits and laterally equivalent turbidites are interpreted as lower trench slope deposits the condensed slope sediment ss section decreases in age from 3 5 to 1 5 ma at site c0002 to 1 5 0 9 ma at c0009 acoustic sequences within the lower forearc basin lfb contain higher proportions of silt and sand turbidites and progressively onlap the ss unit along a low angle discontinuity kl in a landward direction because of the landward onlap of the lfb unit the oldest lfb strata at c0002 are older than 1 67 ma whereas those at c0009 are younger than 0 9 ma thus the amount of time missing or characterized by condensed sedimentation across the kl unconformity decreases in duration in the landward direction the landward onlapping sequences tilt progressively landward in response to regional uplift along an out of sequence thrust oost mega splay fault regional tilting shifted the basin s depocenter progressively landward expanding that part of the basin from 10 km in width to 30 km the onset of sand silt turbidite deposition in the distal basin began after more accommodation space was created by the uplift of the outer ridge along the splay fault at 1 9 ma conversely turbidites of the upper forearc basin ufb progressively onlap lfb in a seaward direction furthermore the respective thicknesses of the lfb and ufb units switch from the seaward side of the basin c0002 farther landward c0009 the lfb unit is 800 m thick in the seaward region whereas it is only 200 300 m thick in the landward region the ufb unit is

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

present day stress states underneath the kumano basin to 2 km below seafloor based on borehole wall failures at iodp site c0002 nankai accretionary wedge we constrain the state of stress to 2 km below seafloor in the nankai accretionary prism at the integrated ocean drilling program iodp site c0002f southwest japan based on borehole wall failures and rock strengths the logging while drilling resistivity images from 872 5 to 2005 5 meters below seafloor show that drilling mud control in riser drilling worked properly to minimize borehole wall failures available breakouts indicate a consistent maximum compression orientation subparallel to the subducting plate margin breakout analysis with drill logs suggests that breakouts occurred only when borehole pressure was slightly lowered and time lag between hole cutting and image logging was several hours this indicates that the observed breakouts are not immediate stress induced failure but brought up into shape gradually with time due to other mechanisms laboratory investigations on deformation and failure of the cores suggest that the time delayed breakout might be a result of progressive rock spall out in borehole wall damage zones that occur at a stress level close to failure condition we constrain stress magnitudes assuming that the stress state is sufficient to bring about the damage zones at the borehole wall an integrated method utilizing breakouts drilling induced tensile fractures and a leak off test suggests that the stress states are on the boundary between strike slip faulting and normal faulting stress regimes and somewhat variable depending on depth the stress magnitudes in the accretionary wedge appear to be controlled by frictional strength of the rock such that the differential stresses are constrained by the laboratory determined frictional coefficients this article is protected by copyright all rights reserved

analysis of normal fault populations in the kumano forearc basin nankai trough japan 1 multiple orientations and generations of faults from 3 d coherency mapping 1 analyses of normal faults in the kumano forearc basin of the nankai trough reveal multiple normal fault populations in a region generally thought to be under compression most faults have offsets of less than 20 m and dips of 60 70 and show no growth structures indicating that the faults were active for short periods of time the oldest generation of faults is older than 0 9 ma and strikes 50 60 the next oldest faults strike 160 170 are older than 0 44 ma and are related to local uplift along the western edge of the region the youngest faults cut the seafloor shallow faults near the se margin of the basin curve from 100 in the middle of the survey area to 145 at the se corner of the area the pattern of the two youngest fault populations is consistent with the regional stress pattern maximum horizontal stress subparallel to the trench orientations of older fault populations are caused by uplift of the underlying accretionary prism implying that the forearc basin region is not as stable as previously thought reconstruction of displacements on the youngest faults shows that the overall horizontal extension is less than 2 concentrated near the seaward edge of the basin the active normal faults distributed throughout the basin support the idea that the horizontal stress parallel to the plate convergence direction does not reach the critical stress to activate or form thrust faults and produce horizontal shortening within the shallow portion of the inner wedge

in situ stress state in the nankai accretionary wedge estimated from borehole wall failures we constrain the orientations and magnitudes of in situ stress tensors using borehole wall failures borehole breakouts and drilling induced tensile fractures detected in four vertical boreholes c0002 c0001 c0004 and c0006 from nw to se drilled in the nankai accretionary wedge the directions of the maximum horizontal principal stress shmax indicated by the azimuths of borehole wall failures are consistent in individual holes but those in c0002 margin parallel shmax are nearly perpendicular to those in all other holes margin normal shmax constrained stress magnitudes in c0001 and c0002 using logged breakout widths combined with empirical rock strength derived from sonic velocity as well as the presence of the drilling induced tensile fractures suggest that the stress state in the shallow portion of the wedge fore arc basin and slope sediment formations is predominantly in favor of normal faulting and that the stress state in the deeper accretionary prism is in favor of probable strike slip faulting or possible reverse faulting thus the stress regime appears to be divided with depth by the major geological boundaries such as unconformities or thrust faults the margin perpendicular tectonic stress components in the two adjacent sites c0001 and c0002 are different suggesting that tectonic force driven by the plate pushing of the philippine sea plate does not uniformly propagate rather the stress field is inferred to be influenced by additional factors such as local deformation caused by gravitation driven extension in the fore arc and thrusting and bending within individual geologic domains

structure of the nankai trough accretionary zone from multichannel seismic reflection data new multichannel seismic reflection data collected over the nankai trough image the accretionary complex in two areas the international program of ocean drilling leg 87 transect area western area and the region of upcoming ocean drilling program leg 131 eastern area the incoming shikoku basin sedimentary section consists of hemipelagic clays and thin terrigenous turbidites the basin section is overlain by a trench wedge that is 12 16 km wide and 350 750 m thick at the thrust front accretionary deformation begins in a protothrust zone that is characterized by thickening and seaward tilting of the trench wedge the zone in the western area is 4 5 km wide and is characterized by kink folds the zone in the eastern area is only 2 5 km wide and does not exhibit such folds the frontal thrusts in each area are imaged as fault plane reflections and ramp upward from within the basin hemipelagic section the overthrusting sediments form fault bend folds over these ramps thrust spacing at the toe of the slope is 1 5 2 5 km the second thrust cuts up from an inferred decollement within the shikoku basin sedimentary section in the eastern area a reflection marking the top of the basin pelagic sediment section changes from normal to reversed polarity about 6 3 km seaward of the thrust front and underlies the entire protothrust zone this reflector continues with reversed polarity under the accretionary complex and is at the level of the basal decollement the underlying basin pelagic section is apparently thrust undisturbed beneath the accretionary prism the reversal of polarity indicates a change in reflection coefficient that is due to a combination of decreasing seismic velocity and density across the interface this decrease in velocity and density may indicate that the decollement is a zone of high porosity due to fluid expulsion from deeper within the accretionary prism the reflections from the first and second thrusts are also reversed polarity possibly indicating that they also are pathways of fluid expulsion the critical wedge taper of the western area is greater than that of the eastern area an observation that is consistent with the existence of an overpressured decollement in the eastern area

large trench parallel gravity variations predict seismogenic behavior in subduction zones we demonstrate that great earthquakes occur predominantly in regions with a strongly negative trench parallel gravity anomaly tpga whereas regions with strongly positive tpga are relatively aseismic these observations suggest that over time scales up to at least 1 million years spatial variations of seismogenic behavior within a given subduction zone are stationary and linked to the geological structure of the fore arc the correlations we observe are consistent with a model in which spatial variations in frictional properties on the plate interface control trench parellel variations in fore arc topography gravity and seismogenic behavior

origin of some conjugate or x fault structures cross cutting faults of the type illustrated by john castagna tle may 1995 are common in normal fault systems and can occur on a range of scales and in a variety of tectonic settings intersecting faults which produce the characteristic x in cross section are generally termed conjugate faults or hourglass structures john s article prompted several letters tle december 1995 explaining how such fault geometries may arise

a recent phase of accretion along the southern costa rican subduction zone abstract in 2011 we acquired a 3d seismic reflection volume across the costa rica margin nw of the osa peninsula to investigate the complex structure and the development of the seismogenic zone within the costa rican subduction zone in the vicinity of recent international ocean drilling program iodp drilling in contrast to previous interpretations these newly acquired seismic images show that the margin wedge is composed of a layered fabric that is consistent with clastic sediments similar to materials recovered from iodp drilling that have been thrust and thickened into thrust bounded folded sequences these structures are consistent with a balanced sequence that has been frontally accreted in the context of an accretionary model we interpret these sequences as sediment originally deposited on the subducting crust in a trench basin created by the southward migration of the cocos nazca caribbean triple junction and accreted during recent margin subduction that also accelerated with passage of the triple junction the margin is composed of relatively rapidly accreted sediment that was added to the margin during a phase of accretion within the last 5 ma that was probably preceded throughout the neogene by periods of non accretion or erosion

cross section restoration and balancing as aid to seismic interpretation in extensional terranes structural cross sections in extensional terranes can be improved significantly by restoration and balancing principles originally developed for fold and thrust belts in rift basins and passive continental margins use of restoration techniques in analysis of growth faults elongate salt swells and other extensional structures results in geologically valid seismic interpretations sections must be in depth as opposed to two way time and oriented in the direction of material transport and decompaction must accompany restoration hanging wall deformation is characterized by different processes depending on the mechanical properties of the rocks being extended and can be approximated by various geometric and kinematic models antithetic faulting is analogous to antithetic shear combined antithetic and synthetic faulting is approximated by vertical shear domino style extension corresponds to rigid body rotation and flexural slip flow is modeled by bed length balance methods selection of the appropriate algorithm is crucial for proper restoration and balancing applications of these techniques illustrated here using seismic data from the gulf coast and north sea include 1 evaluating and adjusting seismic interpretations 2 projecting listric fault trajectories to depth 3 delineating more accurately the geometry and extent of hydrocarbon reservoirs 4 identifying new exploration leads 5 determining the deformation history of an area and 6 constraining the timing and direction of hydrocarbon migration and entrapment

analysis of normal fault populations in the kumano forearc basin nankai trough japan 2 principal axes of stress and strain from inversion of fault orientations 1 we use a high resolution 3 d seismic survey to map a population of recent normal faults within the kumano basin of the nankai subduction zone in order to quantify patterns of strain and stress state over the last 0 44 myr we identify distinct fault populations that define three phases of extension phases 1 and 2 comprise nw se striking faults located along the western basin edge and in the northwestern portion of the study area respectively the ne sw striking faults of phase 3 comprise the largest population and extend 20 km landward from the basin s seaward edge phase 2 faults typically terminate within a few reflectors of the seafloor whereas most phase 3 faults form seafloor scarps inversion of the fault populations documents ne sw extension during phases 1 and 2 and nw se extension during phase 3 consistent with both core scale structures and horizontal stress orientations observed at integrated ocean drilling program iodp boreholes slip on phase 3 faults accommodates strain of up to 1 2 concentrated near the basin s seaward edge inversion for a best fit stress tensor yields a subvertical 1 and subhorizontal 2 and 3 for all faulting phases we find that during phase 3 in most portions of the basin 2 3 shmax shmin reflecting widely varying fault strikes this contrasts with distinct shmax and shmin magnitudes inferred from iodp borehole data these observations may be reconciled if the orientation of maximum horizontal stress fluctuates due to variation of subduction parallel compression through the seismic cycle

splay fault branching along the nankai subduction zone seismic reflection profiles reveal steeply landward dipping splay faults in the rupture area of the magnitude m 8 1 tonankai earthquake in the nankai subduction zone these splay faults branch upward from the plate boundary interface that is the subduction zone at a depth of approximately 10 kilometers approximately 50 to 55 kilometers landward of the trough axis breaking through the upper crustal plate slip on the active splay fault may be an important mechanism that accommodates the elastic strain caused by relative plate motion

present day principal horizontal stress orientations in the kumano forearc basin of the southwest japan subduction zone determined from iodp nantroseize drilling site c0009 a 1 6 km riser borehole was drilled at site c0009 of the nantroseize in the center of the kumano forearc basin as a landward extension of previous drilling in the southwest japan nankai subduction zone we determined principal horizontal stress orientations from analyses of borehole breakouts and drilling induced tensile fractures by using wireline logging formation microresistivity images and caliper data the maximum horizontal stress orientation at c0009 is approximately parallel to the convergence vector between the philippine sea plate and japan showing a slight difference with the stress orientation which is perpendicular to the plate boundary at previous nantroseize sites c0001 c0004 and c0006 but orthogonal to the stress orientation at site c0002 which is also in the kumano forearc basin these data show that horizontal stress orientations are not uniform in the forearc basin within the surveyed depth range and suggest that oblique plate motion is being partitioned into strike slip and thrusting in addition the stress orientations at site c0009 rotate clockwise from basin sediments into the underlying accretionary prism

distribution of stress state in the nankai subduction zone southwest japan and a comparison with japan trench to better understand the distribution of three dimensional stress states in the nankai subduction zone southwest japan we review various stress related investigations carried out in the first and second stage expeditions of the nankai trough seismogenic zone experiment nantroseize by the integrated ocean drilling program iodp and compile the stress data overall the maximum principal stress 1 in the shallower levels 1 km below seafloor or in underlying accretionary prism with 1 becoming horizontal is also suggested at all deeper drilling sites we also make a comparison of the stress state in the hanging wall of the frontal plate interface between site c0006 in the nankai and site c0019 in the japan trench subduction zone drilled after the 2011 mw 9 0 tohoku oki earthquake in the japan trench a comparison between stress state before and after the 2011 mega earthquake shows that the stress changed from compression before the earthquake to extension after the earthquake as a result of the comparison between the nankai trough and japan trench a similar current stress state with trench parallel extension was recognized at both c0006 and c0019 sites hypothetically this may indicate that in nankai trough it is still in an early stage of the interseismic cycle of a great earthquake which occurs on the decollement and propagates to the toe around site c0006

possible strain partitioning structure between the kumano fore arc basin and the slope of the nankai trough accretionary prism 1 a 12 km wide 56 km long three dimensional 3 d seismic volume acquired over the nankai trough offshore the kii peninsula japan images the accretionary prism fore arc basin and subducting philippine sea plate we have analyzed an unusual trench parallel depression a notch along the seaward edge of the fore arc kumano basin just landward of the megasplay fault system this bathymetric feature varies along strike from a single steep walled 3 5 km wide notch in the northeast to a broader 5 km wide zone with several shallower linear depressions in the southwest below the notch we found both vertical faults and faults which dip toward the central axis of the depression dipping faults appear to have normal offset consistent with the extension required to form a bathymetric low some of these dipping faults may join the central vertical fault s at depth creating apparent flower structures offset on the vertical faults is difficult to determine but the along strike geometry of these faults makes predominantly normal or thrust motion unlikely we conclude therefore that the notch feature is the bathymetric expression of a transtensional fault system by considering only the along strike variability of the megasplay fault we could not explain a transform feature at the scale of the notch strike slip faulting at the seaward edge of fore arc basins is also observed in sumatra and is there attributed to strain partitioning due to oblique convergence the wedge and decollement strength variations which control the location of the fore arc basins may therefore play a role in the position where an along strike component of strain is localized while the obliquity of convergence in the nankai trough is comparatively small 15 we believe it generated the kumano basin edge fault zone which has implications for interpreting local measured stress orientations and suggests potential locations for strain partitioning related deformation in other subduction zones

3 d seismic discontinuity for faults and stratigraphic features the coherence cube seismic data are usually acquired and processed for imaging reflections this paper describes a method of processing seismic data for imaging discontinuities e g faults and stratigraphic features one application of this nontraditional process is a 3 d volume or cube of coherence coefficients within which faults are revealed as numerically separated surfaces figure 1 compares a traditional 3 d reflection amplitude time slice with the results of the new method to our knowledge this is the first published method of revealing fault surfaces within a 3 d volume for which no fault reflections have been recorded

calcareous nannofossil biostratigraphy and geochronology of neogene trench slope cover sediments in the south boso peninsula central japan implications for the development of a shallow accretionary complex abstract the geological structure and calcareous nannofossil biostratigraphy of the middle to late miocene trench slope succession in the southern boso peninsula central japan were examined to obtain chronological constraints on the accretion and formation of the trench slope architecture as a result trench slope cover sediments kinone and amatsu formations are clearly distinguishable from the early miocene hota accretionary complex hota group the hota accretionary complex was deposited below the carbonate compensation depth ccd and was affected by intense shearing forming an east west trending and south verging fold and thrust belt in contrast the trench slope cover sediments basically have a homoclinal dip except at the northern rim where they are bounded by fault contact they contain many species of calcareous nannofossils and foraminifers which are indicative of their depositional environment above the ccd and they show shallowing upward sedimentary structures biostratigraphy revealed that the depositional age of the trench slope sediments is ca 15 5 5 ma suggesting that there is an approximately 2 myr hiatus beween the miura group and the underlying accretionary prism based on these results the age of accretion of the hota group is inferred to be between ca 17 15 ma and the group is covered by trench slope sediments overlain on it after ca 15 ma the timing of accretion and the age of the trench slope basin tend to be younger southward of the boso peninsula the accretionary system of the boso peninsula apparently developed in two stages in the middle miocene and in the late miocene to pliocene

strains at the intersections of synchronous conjugate normal faults abstract processes accommodating the mutual offsetting of intersecting conjugate faults are investigated by outcrop examination of three pairs of conjugate normal faults with maximum throws from 5 mm to 10 cm in two cases resolved fault throws decrease sharply towards the intersection zones which on the scale of observation are zones of ductile thinning of inter fault layers that accommodate the throw reductions strain within the intersection zone of conjugate faults in fluvio glacial sands is accommodated by inter grain slip and layer thinning is complemented by layer parallel extension at constant volume in a second example strain within the intersection zone of conjugate faults in inter layered chalk and marl is accommodated by volume reduction without layer extension in a third example also in fluvio glacial sands strain in the intersection zone is accommodated by small faults throws 1 mm when throws on all resolvable faults are aggregated there is no decrease in throw towards the conjugate intersection the presence or absence of a decrease in resolved throw towards conjugate intersections and of a complementary ductile zone accommodating the change in throw is a function not only of the resolution limit of the observations but also of the scale of the deformation process within the conjugate zone

strike slip deformation reflects complex partitioning of strain in the nankai accretionary prism se japan abstract previous studies have suggested predominant extensional tectonics acting at present on the nankai accretionary prism nap and following a parallel direction to the convergence vector between the philippine sea and amur plates however a complex set of thrusts pop up structures thrust anticlines and strike slip faults is observed on seismic data in the outer wedge of the nap hinting at a complex strain distribution across se japan three dimensional 3d seismic data reveal three main families of faults 1 ne trending thrusts and back thrusts 2 nnw to n trending left lateral strike slip faults and 3 wnw trending to e w right lateral strike slip faults such a fault pattern suggests that lateral slip together with thrusting are the two major styles of deformation operating in the outer wedge of the nap both styles of deformation reflect a transpressional tectonic regime in which the maximum horizontal stress is geometrically close to the convergence vector this work is relevant because it shows a progressive change from faults trending perpendicularly to the convergence vector to a broader partitioning of strain in the form of thrusts and conjugate strike slip faults we suggest that similar families of faults exist within the inner wedge of the nap below the kumano basin and control stress accumulation and strain accommodation in this latter region

crustal velocity field of southwest japan subduction and arc arc collision we investigate crustal deformation in southwest japan over a 3 year period revealed by a permanent dense global positioning system gps array southwest japan is a part of the amurian plate a microplate moving about 10 mm yr toward the east with respect to the eurasian plate it overrides the philippine sea plate at the nankai trough and collides with the northeast japan arc in the central part of japan in this paper we first derive gps site velocities relative to the stable part of the amurian plate in order to isolate signals of crustal deformation caused by the subduction and or the collision the velocity field has a conspicuous feature indicating the interseismic elastic loading by the philippine sea plate slab at the nankai trough characterized by the northwestward movements of points throughout the studied area their amplitudes are the largest at the pacific coast and decay toward the japan sea coast with a subtle systematic shift of azimuths a model assuming an elastic half space the convergence rate at the nankai trough based on a refined euler vector and the strength of the coupling inferred from a thermal model could explain the velocity field in the western part of the studied area to a large extent those in the eastern part systematically deviate from them and the residual components there show east west shortening and north south extension this may represent crustal thickening and trenchward extrusion of crustal blocks caused by the collision between southwest and northeast japan this suggests that the collision between southwest and northeast japan gives rise to not only crustal thickening but also trenchward extrusion of crustal block a velocity contrast was found across the median tectonic line the largest inland active fault in japan but the current permanent gps network is not dense enough for us to discuss its coupling depth

splay fault slip in a subduction margin a new model of evolution in subduction zones major thrusts called splay faults are thought to slip coseismically during large earthquakes affecting the main plate interface we propose an analytical condition for the activation of a splay fault based on force balance calculations and suggest thrusting along the splay fault is generally conditioned by the growth of the accretionary wedge or by the erosion of the hanging wall in theory normal slip on the splay fault may occur when the decollement has a very low friction coefficient seaward such a low friction also implies an unstable extensional state within the outer wedge finite element elasto plastic calculations with a geometry based on the nankai kumano section were performed and confirm that this analytical condition is a valid approximation furthermore localized extension at a shallow level in the splay hanging wall is observed in models for a wide range of friction coefficients from 0 to the value of internal friction coefficient of the rock here equals to 0 4 the timing of slip established for the splay fault branch drilled on nankai kumano transect suggests a phase of concurrent splay and accretionary wedge growth 2 ma to 1 5 ma followed by a locking of the splay 1 3 ma active extension is observed in the hanging wall this evolution can be explained by the activation of a deeper and weaker decollement followed by an interruption of accretion activation of a splay as a normal fault as hypothesized in the case of the tohoku 2011 earthquake can be achieved only if the friction coefficient on the decollement drops to near zero we conclude that the tectonic stress state largely determines long term variations of tightly related splay fault and outer decollement activity and thus influences where and how coseismic rupture ends but that occurrence of normal slip on a splay fault requires coseismic friction reduction

anelastic strain recovery reveals extension across sw japan subduction zone 1 sediment dominated convergent margins typically record substantial horizontal shortening often associated with great earthquakes the convergent margin south of japan is arguably one of the most extensively investigated margins and previous studies have documented extensive evidence for accretion and horizontal shortening here we show results from anelastic strains recovered from three partially lithified sediment samples 40 porosities across the southwest japan accretionary prism and propose that the margin is dominated by horizontal extension rather than compression the anelastic strain results are also consistent with stress directions interpreted from two independent techniques bore hole breakout orientations and core scale fault data we interpret this unexpected result to reflect geologically recent underthrusting of a thick sediment package and concomitant weakening of the decollement

anatomy of the western java plate interface from depth migrated seismic images newly pre stack depth migrated seismic images resolve the structural details of the western java forearc and plate interface the structural segmentation of the forearc into discrete mechanical domains correlates with distinct deformation styles approximately 2 3 of the trench sediment fill is detached and incorporated into frontal prism imbricates while the floor sequence is underthrust beneath the decollement western java however differs markedly from margins such as nankai or barbados where a uniform continuous decollement reflector has been imaged in our study area the plate interface reveals a spatially irregular nonlinear pattern characterized by the morphological relief of subducted seamounts and thicker than average patches of underthrust sediment the underthrust sediment is associated with a low velocity zone as determined from wide angle data active underplating is not resolved but likely contributes to the uplift of the large bivergent wedge that constitutes the forearc high our profile is located 100 km west of the 2006 java tsunami earthquake the heterogeneous decollement zone regulates the friction behavior of the shallow subduction environment where the earthquake occurred the alternating pattern of enhanced frictional contact zones associated with oceanic basement relief and weak material patches of underthrust sediment influences seismic coupling and possibly contributed to the heterogeneous slip distribution our seismic images resolve a steeply dipping splay fault which originates at the decollement and terminates at the sea floor and which potentially contributes to tsunami generation during co seismic activity

broad weak regions of the nankai megathrust and implications for shallow coseismic slip abstract deep within the nankai trough subduction zone the plate boundary thrust slips along a well imaged megasplay fault system during the megathrust earthquakes that regularly strike southwest japan the routing of the active plate boundary thrust along an upward branching splay fault causes deep underthrusting of an unusually thick section of material attached to the subducting ocean crust here we present three dimensional seismic reflection data that shows this unusually thick section is fluid rich sediment that results in broad weakly coupled regions of the megathrust down into the updip end of the seismogenic zone the weakly coupled regions lie above an underthrust low seismic impedance presumed to be low velocity and low density sediment section that is between one and two kilometers thick and covers 3300 km2 at least one eighth of the total rupture area of the 1944 mw 8 1 tonankai earthquake this underthrust sediment section is broader and much deeper than inferred at other margins sediment underthrusting into the normally well coupled seismogenic zone likely releases fluids and elevates fluid pressure which reduces inter plate coupling along portions of the megasplay fault and allows coseismic rupture to propagate to unusually shallow depths and generate large tsunami as inferred for the 1944 tonankai event splay faults may be a common yet transient mechanism for developing weak subduction zone thrusts

in situ stress conditions at iodp site c0002 reflecting the tectonic evolution of the sedimentary system near the seaward edge of the kumano basin offshore from sw japan this paper presents a complete set of in situ stress calculations for depths of 200 1400 mbsf meters below seafloor at iodp site c0002 near the seaward margin of the kumano forearc basin offshore from southwest japan the vertical stress component was obtained by integrating bulk density calculations from moisture and density logging data and the two horizontal components were stochastically optimized by minimizing misfits between a probabilistic model and measured breakout widths for every 30 m vertical segment of the wellbore our stochastic optimization process reveals that the in situ stress regime is decoupled across an unconformity between an accretionary complex and the overlying kumano forearc basin the stress condition above the unconformity is close to the critical condition for normal faulting while below the unconformity the geologic system is stable in a normal to strike slip fault stress regime the critical state of stress demonstrates that the tectonic evolution of the sedimentary system has been achieved mainly by the regionally continuous action of a major out of sequence thrust fault during sedimentation in the forearc basin the stable stress condition in the accretionary prism is interpreted to have resulted from mechanical decoupling by the accommodation of large displacement along the megasplay fault

rapid forearc basin uplift and megasplay fault development from 3d seismic images of nankai margin off kii peninsula japan abstract offshore kii peninsula japan a large thrust within the overriding forearc the megasplay fault appears to move coseismically during great earthquakes 3d seismic images of the kumano forearc basin that overlies the megasplay correlated with iodp drilling data are a potential record of the history of large scale motion along this structure in the early quaternary uplift occurred in the southwest portion of the basin that may be a preliminary phase of motion along the megasplay more extensive landward tilting of the outer basin sediments across the seismic volume occurred over 300 kyr in the middle to late quaternary 1 3 1 ma this tilting event may represent the major period of motion along the megasplay that formed the modern fault geometry extensive normal faulting that cuts the forearc basin sediments clearly formed subsequent to the late quaternary tilting and in many cases offset the modern seafloor these faults may form either due to gravitational response to the uplift or as a by product of sediment underthusting these results suggest that the megasplay is a recently formed and transient structure and support the idea that out of sequence thrusts serving as the dominant structure for convergence driven shortening in a subduction zone may be short lived geologically but dominate a margin during these intervals

formation of segmented normal faults a 3 d perspective the interpretation of fault kinematics from geometric data is an essential step in developing an understanding of the growth of fault systems constraints on fault geometry are however often restricted to 2 d maps or cross sections in this article we consider the extent to which kinematic interpretations of faulting benefit from a 3 d rather than 2 d geometrical perspective concentrating on relay zones and segmented normal fault arrays we suggest that very different interpretations of their evolution arise from the recognition that the propagation directions of faults and fault segments will rarely be contained within the inspection plane of 2 d data a 3 d perspective favours an interpretation in which the segments of a fault array are kinematically interrelated from their initiation individual segments in such systems may link into a single fault surface out of the plane of inspection or may be unconnected in 3 d we argue that this interpretation which conflicts with the often suggested model of incidental overlap of originally isolated faults should be the preferred model for the generation and growth of segmented normal fault arrays