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

feldspar dissolution rates in the topopah spring tuff yucca mountain nevada abstract two different field based methods are used here to calculate feldspar dissolution rates in the topopah spring tuff the host rock for the proposed nuclear waste repository at yucca mountain nevada the center of the tuff is a high silica rhyolite consisting largely of alkali feldspar 60 wt and quartz polymorphs 35 wt that formed by devitrification of rhyolitic glass as the tuff cooled first the abundance of secondary aluminosilicates is used to estimate the cumulative amount of feldspar dissolution over the history of the tuff and an ambient dissolution rate is calculated by using the estimated thermal history second the feldspar dissolution rate is calculated by using measured sr isotope compositions for the pore water and rock pore waters display systematic changes in sr isotopic composition with depth that are caused by feldspar dissolution the range in dissolution rates determined from secondary mineral abundances varies from 10 16 to 10 17 mol s 1 kg tuff 1 with the largest uncertainty being the effect of the early thermal history of the tuff dissolution rates based on pore water sr isotopic data were calculated by treating percolation flux parametrically and vary from 10 15 to 10 16 mol s 1 kg tuff 1 for percolation fluxes of 15 mm a 1 and 1 mm a 1 respectively reconciling the rates from the two methods requires that percolation fluxes at the sampled locations be a few mm a 1 or less the calculated feldspar dissolution rates are low relative to other measured field based feldspar dissolution rates possibly due to the age 12 8 ma of the unsaturated system at yucca mountain because oxidizing and organic poor conditions limit biological activity and or because elevated silica concentrations in the pore waters 50 mg l 1 may inhibit feldspar dissolution

shear zone thickness and the seismicity of chilean and marianas type subduction zones chilean type convergent margins have many large m 7 6 earthquakes whereas marianas type ones do not this dichotomy is enigmatic if the plate interface is viewed as a thin frictional decollement whereas it becomes understandable if it is viewed as a relatively thick sediment filled shear zone which thins or thickens arcward depending on subduction speed and sediment supply chilean type margins have thick trench fills and their shear zones generally thin arcward from inlets as much as several thousand metres high the most pronounced thinning being located near backstops tall up to several kilometres seamounts are subducted essentially intact to relatively great depths and confining pressures before jamming into the roof of the channel and becoming seismogenic asperities their near basal ruptures can generate large thrust type earthquakes mainly concentrated in seismic fronts near backstops marianas type margins in contrast have thin trench fills and their shear zones generally thicken arcward from inlets that can be as little as 300 m high seamounts are truncated near the inlet at low confining pressures and generate only small earthquakes after passing the inlet they do not touch the roof and therefore cannot generate large earthquakes a similar mechanism may explain seismic gaps at sediment poor regions of subduction zones

subduction of the daiichi kashima seamount in the japan trench lallemand s culotta r and von huene r 1989 subduction of the daiichi kashimi seamount in the japan trench in j p cadet and s uyeda editors subduction zones the kaiko project tectonophysics 160 231 247 in 1984 1985 the kaiko consortium collected seabeam single channel seismic and submersible sampling data in the vicinity of the daiichi kashima seamount and the southern japan trench we performed a prestack migration of a shell multichannel seismic profile that crosses this area and examined it in the light of this unusually diverse kaiko dataset unlike the frontal structure of the northern japan trench where mass wasting appears to be the dominant tectonic process the margin in front of the daiiclr kashima shows indentation imbrication uplift and erosion emplacement of the front one third of the seamount beneath the margin front occurs without accretion we conclude that the daiichi kashima seamount exemplifies an intermediate stage between the initial collision and subduction of a seamount at a continental margin

determination of heat capacity of yucca mountain stratigraphic layers the heat generated from the radioactive waste to be placed in the proposed geologic repository at yucca mountain nevada will affect the thermal hydrology of the yucca mountain stratigraphic layers in order to assess the effect of the movement of repository heat into the fractured rocks accurate determination of thermodynamic and hydraulic properties is important heat capacity is one of the properties that are required to evaluate energy storage in the fractured rock rock grain heat capacity the subject of this study is the heat capacity of the solid part of the rock yucca mountain consists of alternating lithostratigraphic units of welded and non welded ash flow tuff mainly rhyolitic in composition and displaying varying degrees of vitrification and alteration a number of methods exist that can be used to evaluate heat capacity of the stratigraphic layers that consist of different compositions in this study the mineral summation method has been used to quantify the heat capacity of the stratigraphic layers based on kopp s rule the mineral summation method is an addition of the weighted heat capacity of each mineral found in a specific layer for this study the weighting was done based on the mass percentage of each mineral in the layer the method utilized a mineralogic map of the rocks at the yucca mountain repository site the calico hills formation and adjacent bedded tuff layers display a bimodal mineral distribution of vitric and zeolitic zones with differing mineralogies based on this bimodal distribution in zeolite abundance the boundary between the vitric and zeolitic zones was selected to be 15 zeolitic abundance thus based on the zeolite abundance subdivisions have been introduced to these layers into vitric and zeolitic zones heat capacity values have been calculated for these layers both as layer average and zone average the heat capacity determination method presented in this report did not account for spatial variability in the horizontal direction within each layer

erratum to geochemistry of rock units at the potential repository level yucca mountain nevada applied geochemistry 17 6 2002 683 698

in situ erosion measurements on fine grained sediments from the bay of fundy amos c l daborn g r christian h a atkinson a and robertson a 1992 in situ erosion measurements on finegrained sediments from the bay of fundy mar geol 108 175 196 in situ measurements of the erodibility of fine grained sediments have been made in two regions of the bay of fundy using the benthic annular flume sea carousel the regions were markedly different in benthic biology atmospheric exposure ice loading and wave effects nevertheless remarkable similarities were found in erosion character three patterns of erosion were detected type ia erosion is characterized by an asymptotically decreasing erosion rate with time it is a surface phenomenon associated with the erosion of 1 mm organic pellets formed by the browsing amphipod corophium volutator and was found in regions of high benthic activity only onset of this erosion type is equated with z0 of otsubo and muraoka 1988 type ib erosion also showed asymptotically decreasing erosion with time but erosion generated rip up clasts and aggregates up to 7 mm in diameter type ib erosion occurred at all locations at imposed bed stresses between 1 0 and 4 4 pa peak erosion lasted only 1 3 minutes at any imposed stress level and thereafter ceased this pattern of erosion is the result of a linearly increasing sediment shear strength with depth with an internal friction angle circa 70 onset of this erosion is equated with to2 of otsubo and muraoka 1988 type i erosion occurred only in the uppermost 0 5 mm of the bed below type ii erosion was dominant type ii erosion was constant with time and over the time scales of our experiments was independent of changes in bulk bed shear strength with depth it was found at all locations and was produced by bed shear stresses between 2 and 10 pa type ii erosion succeeded type ib erosion video observations show that type ii erosion is characterized by failure along fine planes of weakness within the sediment probably related to the sediment microfabric generated after deposition a relationship r 2 0 65 n 72 was found between peak erosion rate e of all types and excess bed shear stress of the form proposed by parchure and mehta 1985 ln e e r z zs z 5 kg m s where ef 5 1 x 10 s kg m2 s and t 1 62 m n s this relationship held for all data irrespective of exposure sediment texture or benthic activity notwithstanding similar erosion rates changes in erosion type will have a profound impact on net erosion given its relationship to microfabric erosion type is not predictable using standard laboratory measurements

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

the 1946 nankai earthquake and segmentation of the nankai trough abstract we perform a simple subevent analysis of seismic p waveforms from historical records of the 1946 nankai earthquake the results of this analysis establish that the earthquake rupture consisted of two main subevents one lying within the 1 day aftershock area off the kii peninsula while the other lies much farther to the west in the area of pronounced crustal deformation of western shikoku these conform to two segments of the nankai trough which have long been referred to in studies of historical earthquakes and geology of the nankai trough but to our knowledge have not previously been established as having distinct megathrust rupture behavior we show that these two subevents are bounded by pronounced features in the geometry of the subducting philippine sea plate psp a tear in the plate to the east beneath the kii peninsula and a subducting seamount farther to the west we speculate that these features may be associated with low strength parts of the upper and or lower plates that may be susceptible to permanent anelastic deformation which prevents the accumulation of the elastic strain energy necessary to sustain seismic rupture

high frequency turbulence and suspended sediment concentration measurements in the garonne river tidal bore the study details new sediment concentration measurements associated with some turbulence characterisation conducted at high frequency in the undular tidal bore of the garonne river france acoustic doppler velocimetry was used and the suspended sediment concentration was deduced from the acoustic backscatter intensity the field data set demonstrated some unique flow features of the tidal bore including some large and rapid turbulent velocity fluctuations during and after the bore passage some unusually high suspended sediment concentration was observed about 100 s after the tidal bore front lasting for more than 10 min it is thought that the tidal bore passage scoured the bed and convected upwards the bed material reaching the free surface after the bore passage behind the tidal bore the net sediment flux magnitude was 30 times larger than the ebb tide net flux and directed upstream a striking feature of the data set was the intense mixing and suspended sediment motion during the tidal bore and following flood tide this feature has been rarely documented

coulomb theory applied to accretionary and nonaccretionary wedges possible causes for tectonic erosion and or frontal accretion based on observations from both modem convergent margins and sandbox modeling we examine the possible conditions favoring frontal accretion and or frontal and basal tectonic erosion mean characteristic parameters t t and are used to discuss the mechanical stability of 28 transects across the frontal part of convergent margins where the coulomb theory is applicable natural observations reveal that typical accretionary wedges are characterized by low tapers with smooth surface slope and subducting plate low convergence rates and thick trench sediment while nonaccretionary wedges display large tapers with irregular surface slopes and rough subducting plate high convergence rates and almost no trench fill sandbox experiments were performed to illustrate the effects of seamounts ridges in the subduction zone on the deformation of an accretionary wedge these experiments show that a wedge of sand is first trapped and pushed in front of the seamount which acts as a moving bulldozer this is followed by a tunnelling effect of the subducting seamount through the frontal wedge material which results in considerable sand reworking at an advanced subduction stage the d6collement jumps back from a high level in the wedge to its former basal position we conclude that a high trench sedimentation rate relative to the convergence rate leads to frontal accretion in contrast several conditions may favor tectonic erosion of the upper plate first oceanic features such as grabens seamounts or ridges may trap upper plate material during their subduction process second destabilization of the upper plate material by internal fluid overpressuring causing hydrofracturing is probably another important mechanism

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

deformation of accretionary wedges in response to seamount subduction insights from sandbox experiments sandbox experiments using a two dimensional and a three dimensional approach are used to study the deformation of margins in response to seamount subduction successive mechanisms of deformation are activated during the subduction of conical seamounts first reactivation of the frontal thrusts and compaction of the accretionary wedge is observed then back thrusting and conjugate strike slip faulting develops above the leading slope of the subducted seamount the basal d6collement is deflected upward in the wake of the subducting high and a large shadow zone develops behind the seamount trailing slope consequently frontal accretion is inhibited and part of the frontal margin is dragged into the subduction zone when the main d6collement returns to its basal level in the wake of the seamount the margin records a rapid subsidence and a new accretionary wedge develops closing the margin reentrant the sediments underthrusted in the wake of the seamount into the shadow zone are underplated beneath the rear part of the accretionary wedge substantial shortening and thickening of the deformable seaward termination of the upper plate basement associated with basal erosion is observed seamount subduction induces significant material transfer within the accretionary wedge favors large tectonic erosion of the frontal margin and thickening of the rear part of the margin the subduction and underplating of relatively undeformed water ladden sediments associated with fluid expulsion along the fractures affecting the margin could modify the fluid pressure along the basal ddcollement consequently significant variations of the effective basal friction and local mechanical coupling between the two plates could be expected around the subducting seamount

soft sediment deformation produced by tides in a meizoseismic area turnagain arm alaska turnagain arm is a semidiurnal hypertidal estuary in southeastern alaska with a recorded tidal range of 9 m contorted bedding and flow rolls preserved in tidal sediments within the estuary have previously been interpreted as resulting from the mw 9 2 great alaskan earthquake of 1964 horizons of flow rolls between undeformed beds in sediments and rock strata have been used to infer ancient earthquakes in other areas although many types of soft sediment deformation structures can be formed by earthquakes observations of sedimentation on tidal flats in the inner parts of turnagain arm in the summers of 2003 and 2004 show that a wide range of soft sediment deformation structures similar to those inferred to have been formed by earthquakes can form in macrotidal estuaries in the absence of seismic shock during sedimentation rate measurements in 2004 soft sediment deformation structures were recorded that formed during one day s tide either in response to overpressurization of tidal flats during rapid tidal drawdown or by shear stress exerted on the bed by the passage of a 1 8 m tidal bore structures consisted of flow rolls dish structures flames and small dewatering pipes in a bed 17 cm thick in the future if the flow rolls in turnagain arm were found in isolated outcrops across an area 11 km in length in an estuary known to have been influenced by large magnitude earthquakes would they be interpreted as seismites these examples show that caution is needed when using horizons of flow rolls to infer paleoseismicity in estuarine deposits because many of the mechanisms tidal flux tidal bores slumping flooding that can cause deformation in rapidly deposited unconsolidated silts and sands are orders of magnitude more common than great earthquakes

the effect of seamount subduction on seismic coupling it has long been speculated that the subduction of seamounts may have profound mechanical consequences particularly on the generation of large subduction zone earthquakes we consider this problem as a special case of the seismic coupling model of scholz and campos the model predicts that the subduction of a large seamount will increase the normal stress across the subduction interface and hence will enhance seismic coupling in the case of coupled arcs this increased coupling will result in a large increase in the recurrence intervals of earthquakes such events will therefore appear rare in the instrumental record as is observed for decoupled arcs the subduction of a large seamount should cause them to become locally coupled three of the latter localities have been found with satellite altimetry data two on the tonga kermadec trench and one on the izu bonin trench the diagnostic feature is a shallowing or obstruction of the trench axis adjacent to a chain or group of large seamounts on the subducting plate in each case the small to moderate plate interface seismicity typical of decoupled seismic zones is almost completely absent in a 100 200 km arc segment indicating that the subduction zone is locally locked as predicted by the model the occurrence of large interplate earthquakes at both tonga sites confirms that they are seismically coupled

inception of bed load motion beneath a bore abstract a tidal bore is a series of waves propagating upstream in the river mouth as the tidal flow turns to rising in macro tidal conditions some related geophysical processes include the tsunami induced bores and uprushing bores on beaches in the present study the inception of sediment motion beneath tidal bores was investigated physically no sediment motion was observed in the initially steady flow and beneath undular bores a transient sheet flow motion was observed beneath breaking bores and the onset of sediment motion was closely linked with the passage of the roller toe the forces acting on the movable gravel bed particles were estimated from the physical measurements the results showed that the longitudinal pressure gradient force was the dominant contribution de stabilising the particles and inducing the onset of sediment motion the drag force added a sizeable contribution to maintain the upstream particle motion although the entire sheet flow motion was brief

evolution of the nankai trough decollement from the trench into the seismogenic zone inferences from three dimensional seismic reflection imaging we mapped the amplitude of the nankai trough subduction thrust seismic reflection from the trench into the seismogenic zone with three dimensional seismic reflection data the decollement thrust forms within the lithologically homogeneous lower shikoku basin facies along an initially nonreflective interface the reflection develops from a porosity contrast between accreted and underthrust sedimentary material because of accretionary wedge consolidation and rapid loading and delayed consolidation of the underthrust section a decollement amplitude map shows a significant decline from high amplitudes at the trench to barely detectable levels 25 30 km landward three other observations coincide with the amplitude decline 1 the decollement initially steps down to deeper stratigraphic levels 2 the wedge taper increases dramatically and 3 the thrust becomes seismogenic the amplitude decline and the coincident decollement and accretionary wedge tectonic and seismogenic behavior are attributed to the loss of fluids and potentially loss of excess fluid pressures downdip along the subduction thrust

updip limit of the seismogenic zone beneath the accretionary prism of southwest japan an effect of diagenetic to low grade metamorphic processes and increasing effective stress off southwest japan the seaward limit of coseismic displacement or updip limit of the seismogenic zone of the 1946 mw 8 3 thrust earthquake reaches to 4 km depth and 40 km landward of the trench this limit coincides with the estimated location of the 150 c isotherm and has been linked to changes in physical properties associated with the smectite to illite clay mineral transition here we show that this limit correlates with a suite of diagenetic to low grade metamorphic processes characterized by 1 declining fluid production and decreasing fluid pressure ratio and 2 active clay carbonate and zeolite cementation and the transition to pressure solution and quartz cementation these diagenetic to low grade metamorphic changes cause the onset of velocity weakening during thrust faulting an increase in effective stress and strengthening of the hanging wall which together combine to produce recordable earthquakes

successive impact of tidal bores on sedimentary processes arcins channel garonne river a tidal bore is a hydrodynamic shock propagating upstream as the tidal flow turns to rising with macro tidal conditions in a funnel shaped system with shallow waters the tidal bore of the garonne river was extensively investigated in the arcins channel between 2010 and 2013 typically over one to two days in 2015 new field measurements were repeated systematically at the same site on 29 august 1 september 2015 and on 27 october 2015 the nature of the observations was comprehensive encompassing hydrodynamics and turbulence as well as sediment properties and transport the tidal bore occurrence had a marked effect on the velocity and suspended sediment field including a rapid flow deceleration and flow reversal during the bore passage with very large suspended sediment concentrations sscs during the passage of the tidal bore front and early flood tide as well as very large suspended sediment flux during the very early flood tide the suspended sediment concentration ssc data indicated a gradual increase in initial mean ssc estimate prior to the bore from 29 august to 1 september 2015 a comparison between suspended sediment flux data showed very significant suspended sediment flux on the first day of tidal bore occurrence with a decreasing magnitude over the next three days the data suggested a two stage bed scour process at each tidal bore event surface erosion occurred initially in the form of stripping the first stage was followed by delayed mass erosion occurring about 5 15 min after the tidal bore

fluid flow and sediment entrainment in the garonne river bore and tidal bore collision a detailed field study was carried out on a tidal bore to document the turbulent processes and sediment entrainment which occurred the measured bore within the arcins channel of the garonne river france was undular in nature and was followed by well defined secondary wave motion due to the local river geometry a collision between the arcins channel tidal bore and the bore which formed within the main garonne river channel was observed about 800m upstream of the sampling site this bore collision generated a transient standing wave with a black water mixing zone following this collision the bore from the main garonne river channel propagated backward to the downstream end of the arcins channel velocity measurements with a fine temporal resolution were complemented by measurements of the sediment concentration and river level the instantaneous velocity data indicated large and rapid fluctuations of all velocity components during the tidal bore large reynolds shear stresses were observed during and after the tidal bore passage including during the backward bore propagation large suspended sediment concentration estimates were recorded and the suspended sediment flux data showed some substantial sediment motion consistent with the murky appearance of the flood tide waters copyright c 2015 john wiley sons ltd

tidal bore dynamics in funnel shaped estuaries the formation and dynamics of tidal bores in funnel shaped estuaries is investigated from both a global tidal wave scaling analysis and new quantitative field observations we show that tidal bore occurrence in convergent estuaries can be estimated from a dimensionless scaling parameter characterizing the relative intensity of nonlinear friction versus local inertia in the momentum equation a detailed analysis of tidal bore formation and secondary wave structure is presented from a unique long term database observations of more than 200 tides acquired during four campaigns in the two main french tidal bore estuaries the seine and gironde garonne estuaries we describe the effect of freshwater discharge on the global tidal wave transformation at the estuarine scale and on local tidal bore occurrence in the upper estuary our field data suggest that the tidal bore intensity is mainly governed by the dimensionless tidal range which characterizes the local tidal wave nonlinearity we also show that the secondary wavefield associated with tidal bore propagating in natural estuaries differs significantly from those associated to undular bores in rectangular channels in particular we observe an abrupt decrease of the whelp steepness when the froude number goes below 1 1 this secondary field transition can explain why tidal bore occurrence in worldwide estuaries is certainly underestimated

structural factors controlling the rupture process of a megathrust earthquake at the nankai trough seismogenic zone summary the nankai trough is a vigorous subduction zone where large earthquakes have been recorded with a recurrence time of 100 200 yr the 1946 nankaido earthquake is well known as an unusual event among these earthquakes because the rupture zone estimated from long period geodetic data is more than twice as large as that derived from seismic wave data in the summer of 1999 an onshore offshore deep seismic survey was performed along a 355 km long profile in the western nankai trough seismogenic zone seismic signals both from an airgun array 207 l and land explosions maximum of 500 kg were recorded simultaneously by 98 oceanbottom seismographs and 93 land seismic stations conventional 2 d seismic reflection data were also acquired along part of the offshore profile from the wide angle seismic data we found a subducting seamount at the centre of the proposed rupture zone with dimensions of 13 km thick by 50 km wide at 10 km depth the seismic velocity image also shows that the seamount is now colliding with the japanese island arc crust from this significant structure this paper proposes that the subducted seamount functioned as a barrier at least during the 1946 earthquake i e the rupture of the 1946 earthquake extended over the entire locked zone to the east of the subducted seamount and then the rupture was deflected around the subducted seamount at the down dip end of the locked zone between cape muroto and cape ashizuri another significant structure a highly reflective layer is obtained beneath shikoku island a very slow p wave velocity 3 km s 1 is necessary in a thin layer at the base of the island arc crust in order to explain the observed high amplitude reflection phases an area of low resistivity obtained by a previous magnetotelluric study corresponds to the highly reflective layer this suggests a possible water layer at the base of the island arc crust the water may be generated by dehydration of the downgoing probably partially serpentinized mantle which is implied by a low p wave velocity 7 5 km s 1 beneath the subducted seamount a locally observed non slip region during the 1946 earthquake at the eastern part of shikoku island is interpreted as a result of weak coupling at the possible water layer

magnetic anomaly interpretation across the southern central andes 32 34 s the role of the juan fernandez ridge in the late tertiary evolution of the margin marine and terrestrial magnetic surveys have been integrated to study the tectonic structure of the convergent margin of chile between 32 34 s three magnetic domains have been identified oceanic continental margin and subaerial the oceanic domain has seafloor spreading anomalies 16 37 ma to 18 39 5 ma disturbed by anomalies of the juan fernandez hot spot chain in the continental margin the most prominent fabric are e w anomalies in the upper slope corresponding to onshore e w anomalies of large intrusive bodies onshore a n s lineament of short wavelength anomalies defines the roots of a cretaceous volcanic arc a resembling lineament offshore indicates a submerged older volcanic arc and that continental basement extends to 50 km landward of the trench absolute cenozoic plate motion for nazca and south american plates and dating of the juan fernandez chain provide a kinematic model of ridge continent collision the reconstruction indicates rapid southward migration of the collision point along 1400 km of the margin from 20 to 11 ma 20 cm yr 1 from 11 ma to present the collison point has migrated at a slower rate along 375 km of the margin 3 5 cm yr 1 the predicted location of the subducted portion of the juan fernandez chain coincides with the south edge of the southward migrating flat slab segment of the subducted lithosphere and with a cluster of deep earthquakes indicating a causal relationship in the last 10 myr the ridge has separated a sediment starved trench to the north where subduction erosion may dominate from a sediment filled trench to the south where recent sediment accretion dominates these observations indicate that subduction of the juan fernandez chain plays a major role in arc forearc tectonics

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

three dimensional architecture of the nankai accretionary prism s imbricate thrust zone off cape muroto japan prism reconstruction via en echelon thrust propagation 1 a 9 km wide 92 km long three dimensional 3 d seismic reflection volume acquired off shikoku island japan images the seaward portion of the subduction of the philippine sea plate at the nankai trough and nankai accretionary prism detailed interpretation of the imbricate thrust and protothrust zones the portions of the prism between the deformation front and the first out of sequence thrust shows a high degree of variability in the thrust faults that all parallel the frontal thrust but are arranged in en echelon patterns along strike and frequently include complications such as piggyback faults and fault splays interestingly the sinuous seafloor morphology of the prism does not accurately reflect the en echelon 3 d architecture of the primary prism thrusts seafloor morphology appears to average across several thrusts along strike and is further modified by near surface thrust splays and backthrusts suggesting that care must be taken in interpreting seafloor relief in terms of lateral continuity or thrust fault geometry subduction of the kinan seamounts 20 km northeast of the center of the muroto 3 d volume generated a scallop shaped embayment in the prism the rebuilding process appears to influence the northeastern portion of the 3 d volume where a 625 m landward step in the position of the frontal thrust and numerous changes in prism architecture are observed these observations imply that accretionary prisms may reattain equilibrium following seamount subduction by lateral en echelon fault propagation into damaged zones that facilitate an increase accretion rate until a laterally continuous deformation front is reestablished

undular tidal bore dynamics in the daly estuary northern australia measurements in the macro tidal daly estuary show that the presence of an undular tidal bore contributed negligibly to the dissipation of tidal energy no recirculation bubble was observed between a trough and the following wave crest in the lee waves following the undular bore this differs to stationary undular bores in laboratory experiments at larger froude numbers where a recirculation bubble exists secondary motions and the turbulence generated by the undular bore had no measurable influence on the sediment transport this situation contrasts with the intense sediment resuspension observed in breaking tidal bores the tidally averaged sediment budget in the daly estuary was controlled by the asymmetry of tidal currents the undular bore may widen the river by breaking along the banks that it undercuts leading to bank slippage a patch of river wide macro turbulence of 3 min duration occurred about 20 min after the passage of the bore during accelerating tidal currents

thick strong sediment subduction along south central chile and its role in great earthquakes abstract the south central chile margin experienced the largest and sixth largest earthquakes ever recorded the 1960 mw 9 5 valdivia and 2010 mw 8 8 maule megathrust earthquakes respectively in early 2017 we conducted a seismic survey along 1 000 km of south central chile to image these rupture zones using a 15 15 km long multi channel seismic streamer we processed these data using pre stack depth migration which provides the best look at the shallow part of the south central chile margin to date relative to other sediment dominated subduction zones where sediment is typically accreted at the toe an unusually large percentage of the thick trench sediments are consistently subducted beneath the slope with little thrust faulting or deformation analysis of the sediment p wave velocities and structure in the trench and outer wedge leads us to conclude that most of south central chile contains well drained strong sediments an exception in the vicinity of the subducting mocha fracture zone mfz has trench sediments that appear to experience localized delayed compaction thus lowering their strength and allowing the development of protothrusts similar to what is seen in other accretionary subduction zones the very shallow decollement along the south central chile allows more sediment to pass beneath the lower slope than almost all other subduction zone many of which have much thicker trench sections we conclude that subduction of the strong well drained thick sediment layer beneath the lower slope is typical for nearly all of the south central chile comparison to other thick sedimented subduction zones worldwide reveals that the subduction of such a large fraction of the trench sediment is particularly unusual this strong thick subducting sediment is likely a determining factor for developing a smooth plate interface located well above the subducted crust topography that ultimately becomes a broad megathrust with high homogeneous frictional properties and generates particularly large earthquakes along the south central chile margin

seamount subduction erosion in the nankai trough and its potential impact on the seismogenic zone seamount subduction along subduction zone plate boundary thrusts has long been implicated as a mechanism for abrasion and tectonic erosion of the base of the overriding plate however tectonic erosion processes have not been examined in detail with high quality three dimensional 3 d seismic reflection imaging in 1999 we acquired 3 d seismic reflection data from the nankai trough subduction zone to image the plate boundary fault and the overlying accretionary wedge structure fortuitously these data reveal a small to 1 km high basement ridge that has subducted to 7 km subseafloor updip from the basement ridge a 1 km thick sequence of sediment from the base of the accretionary wedge appears to be missing we interpret these data as evidence for tectonic erosion of the base of the accretionary wedge following the basement ridge subduction tectonic erosion has removed more than 25 km 3 from the updip edge of the seismogenic zone and carried it down into the seismogenic zone the tectonically eroded sediments are presumed to enhance fault zone fluid content potentially reducing fault zone effective stress and may temporarily inhibit earthquake rupture potential after the passage of the ridge the boundary fault returns to its former position in a period of enhanced underplating

thrust type subduction zone earthquakes and seamount asperities a physical model for seismic rupture a thrust type subduction zone earthquake of m sub w 7 6 ruptures an area of approximately 6 000 km sup 2 has a seismic slip of approximately 1 m and is nucleated by the rupture of an asperity approximately 25km across a model for thrust type subduction zone seismicity is proposed in which basaltic seamounts jammed against the base of the overriding plate act as strong asperities that rupture by stick slip faulting a m sub w 7 6 event would correspond to the near basal rupture of a approximately 2 km tall seamount the base of the seamount is surrounded by a low shear strength layer composed of subducting sediment that also deforms between seismic events by distributed strain viscous flow planar faults form in this layer as the seismic rupture propagates out of the seamount at speeds of kilometers per second the faults in the shear zone are disrupted after the event by aseismic slow viscous flow of the subducting sediment layer consequently the extent of fault rupture varies for different earthquakes nucleated at the same seamount asperity because new fault surfaces form in the surrounding subducting sediment layer during each fast seismic rupture

upper plate deformation associated with seamount subduction in many active margins severe deformation is observed at the front of the overriding plate where seamounts or aseismic ridges subduct such deformation appears to be a main tectonic feature of these areas which influences the morphology and the seismicity of the margin to better understand the different stages of seamount subduction we have performed sandbox experiments to study in detail the evolution of deformation both in space and time and thus complement seismic images and bathymetry interpretation we focus in this paper on the surface deformation directly comparable with seafloor morphology two types of subducting seamounts were modelled relatively small conical seamounts and larger flat topped seamounts the indentation of the margin by the seamount inhibits frontal accretion and produces a re entrant the margin uplift includes displacement along backthrusts which propagate from the base of the seamount and out of sequence forethrusts which define a shadow zone located on the landward flank of the seamount when the seamount is totally buried beneath the margin this landward shielded zone disappears and a larger one is created in the wake of the asperity due to