transport and entrainment in the red sea outflow plume the red sea outflow exhibits strong seasonal variability in outflow transport due to effects of monsoon winds and seasonal fluctuations in buoyancy forcing as it descends the continental slope in the western gulf of aden it entrains significantly less dense near surface water which itself varies on seasonal time scales high resolution hydrographic and direct velocity data collected during the 2001 red sea outflow experiment redsox are used herein to characterize and quantify the pathways of the red sea outflow water rsow and the associated entrainment of gulf of aden water the outflow transport exhibits a maximum in winter of about 0 29 sv sv 10 6 m 3 s 1 at the exit of the bab el mandeb and approximately doubles to 0 56 sv as it descends into the gulf of aden and entrains ambient water in summer the outflow is much weaker reaching about 0 06 sv at the strait and about 0 18 sv downstream the outflow plume divides into three distinct branches in winter consisting of descending branches along two bathymetrically confined channels the northern and southern channels respectively and an adjusted intrusion layer at shallower depths in the water column estimates of transport of pure red sea outflow water through salt flux conservation show the general partitioning of the outflow between the individual plumes where the northern channel nc accounts for 52 of red sea outflow water the southern channel sc carries 31 and the intrusion layer il the remaining 17 the results also indicate that the transport of red sea outflow water is subject to considerable synoptic temporal variability that is unresolved by the present study

empirical relationships among seismic velocity effective pressure porosity and clay content in sandstone we use a multivariate analysis to investigate the influence of effective pressure pe porosity and clay content c on the compressional velocity vp and shear velocity vs of sandstones laboratory measurements on water saturated samples of 64 different sandstones provide a large data set that was analyzed statistically for each sample relationships between effective pressure and vp and vs have been determined all samples were well fit by relationships that have an exponential increase in velocity at low pe tapering to a linear increase with pe for pe greater than 0 2 kbar there are differences in the pressure dependences of velocity for different rocks particularly at very low pressures however the differences cannot be attributed to or c for the combined set of measurements from all samples the best fitting formulations are vp 5 77 6 94 1 73c 0 446 pe e 16 7pe and vs 3 70 4 94 1 57c 0 361 pe e 16 7pe while this is admittedly a very simplified parameterization it is remarkable how well

observations on the characteristics of tides in the bab el mandab strait 1 while well known for its multilayered exchange flow the bab el mandab strait is also of interest as a transitional region between two distinctly different tidal regimes the gulf of aden where tides are mixed and have a range in excess of 2 m and the red sea where tidal fluctuations are principally semidiurnal and their range is generally less than 1 m observations collected in the strait indicate that dominant constituents for both elevations and currents are the k1 and m2 the vertical structure of the tidal currents differs between semidiurnal and diurnal constituents and depends on the location and stratification the major part of the tidal signal is barotropic but energetic baroclinic currents mainly of the diurnal period are observed near perim narrows and the hanish sill during winter months the relationship between the barotropic currents and surface tides near perim is fairly well explained by a simple momentum balance to first order a balance exists between local acceleration and the elevation gradient in the along strait direction while in the cross strait direction a balance between local acceleration coriolis term and elevation gradient is observed at the same location the barotropic transport fluctuations reach amplitudes of 1 6 sv the k1 and m2 barotropic energy fluxes are small and are directed into the strait at perim narrows and the hanish sill for the m2 constituent and into the strait near perim and out of the strait near hanish for the k1 constituent

extension of continental crust by anelastic deformation during the 2011 tohoku oki earthquake the role of extensional faulting in the generation of a great tsunami abstract observations of seafloor morphologies and environments made before and after the 2011 tohoku oki earthquake reveal open fissures generated during the earthquake where the fault trace is interpreted on seismic profiles to intersect the seafloor anomalously high heat flow was observed at a landward dipping normal fault in august 2011 five months after the earthquake but by august 2012 heat flow measured at the same station had decreased to close to the background value which suggests that the normal fault ruptured during the 2011 earthquake these seafloor observations and measurements demonstrate deformation that was both extensional and anelastic within the overriding continental plate during the 2011 earthquake seismic profiles as well as seafloor bathymetry data in the tsunami source area further demonstrate that landward dipping normal faults extensional faults collapse the continental framework and detach the seaward frontal crust from the landward crust at far landward from the trench the extensional and anelastic deformation i e normal faulting observed in both seafloor observations and seismic profiles allows the smooth seaward movement of the continental crust seaward extension of the continental crust close to the trench axis in response to normal faulting is a characteristic structure of tsunami source areas as similar landward dipping normal faults have been observed at other convergent plate margins where tsunamigenic earthquakes have occurred we propose that the existence of a normal fault that moves the continental crust close to the trench can be considered one indicator of a source area for a huge tsunami

exchange through the bab el mandab during the last 10 years there have been a number of important developments in the understanding of the exchange flow in the bab el mandab new and improved estimates of the fluxes of mass heat and salt through the strait have been made directly from oceanographic measurements and indirectly from air sea flux data hydraulic theory has been applied to explain the seasonal variations in the exchange flow and in particular the three layer summer circulation the effects of wind stress friction and seasonal sea level variability on hydraulic control are addressed in this paper analyses of oceanographic data have failed to provide direct evidence of hydraulic control but they have highlighted the importance of tidal scale variability and differences between real continuously stratified fluids and idealised layer flows often used in hydraulic theory other studies have investigated the coupling between the flow in the strait and the circulation within the red sea

elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the nankai trough subduction megathrust 1 recent seismic reflection and ocean bottom seismometer obs studies reveal broad regions of low seismic velocity along the nankai subduction plate boundary megathrust offshore sw japan these low velocity zones lvz s extend 55 km landward from the trench corresponding to depths of 10 km below sea floor here we estimate the in situ pore pressure and stress state within these lvz s by combining p wave velocities obtained from the geophysical surveys with new well constrained empirical relations between p wave velocity porosity and effective mean stress defined by laboratory deformation tests on drill core samples of the incoming oceanic sediment we document excess pore pressures of 17 87 mpa that extend 55 km into the subduction zone indicating that trapped pore fluids support 45 91 of the overburden stress along the base of the upper plate and surrounding major fault zones the resulting effective stresses in the lvz are limited to 1 3 of the values expected for non overpressured conditions these low effective stresses should lead to a mechanically weak and predominantly aseismic plate boundary the region of lowest effective stress coincides with precisely located very low frequency earthquakes providing the first quantitative evidence linking these anomalous slip events to low stress and high pore pressure

direct observations of seasonal exchange through the bab el mandab strait the exchange flow between the red sea and the gulf of aden indian ocean through the bab el mandab strait was measured continuously for 10 months june 1995 march 1996 adcp and temperature salinity chain moorings allow an unprecedented look at the magnitude and seasonal evolution of the inflow layer from the gulf of aden and the high salinity outflow layer from the red sea the timing structure and evolution of the summer season mid depth intrusion of cold low salinity water into the red sea from the gulf of aden is measured for the complete intrusion cycle of 1995 we unexpectedly find the deep outflow still strong in june 1995 with speeds of 0 6 m sec and transport of 0 4 sv 1 sv 106 m sec from july to mid september the deep outflow persists but is attenuated to speeds of 0 2 m sec and transport of 0 05 sv the dominant summer feature the cold low salinity intermediate layer intrusion persists for 3 months occupies 70 of the water column in the strait and carries approximately 1 7 1012 m of cold nutrient rich water into the red sea the winter regime begins in mid september is fully developed by early november and continues to the end of our first observation interval in march 1996 speeds in the lower layer are 0 8 1 0 m sec and 0 4 0 6 m sec in the upper layer at maximum exchange in mid february outflow transport reaches 0 7 sv ubiquitous oscillations in current and salinity at synoptic and intraseasonal periods appear closely related to fluctuations in the along channel wind forcing and perhaps to coastally trapped waves

effective pressure or what is the effect of pressure using time lapse seismics as a reservoir monitoring tool geophysics can help distinguish different reservoir production scenarios for example eiken et al 2000 successfully detected fluid saturation changes after co2 injection using time lapse seismics at sleipner field over the cycle of a reservoir life oil saturation usually decreases reservoir pressure declines and gas breakout may occur these changes cause rock property changes that are detectible in time lapse seismics therefore it is important to understand the effects of pressure and saturation changes on rock properties while the effects of saturation changes are often well described by gassmann 1951 brown and korringa 1975 and mavko 1975 the effects of pressure changes are less understood here we focus on understanding the effects of fluid pressure on velocities

modern and ancient seismogenic out of sequence thrusts in the nankai accretionary prism comparison of laboratory derived physical properties and seismic reflection data 1 to investigate characteristics of a seismogenic out ofsequence thrust oost imaged as a strong reflection on seismic profiles in the nankai accretionary prism we determined acoustic properties of discrete samples from an fossil nobeoka oost outcrop under confining pressures and compared the acoustic properties with those of an active oost in the nankai accretionary prism we observed anisotropy of velocity and attenuation in the hanging wall of nobeoka oost attributed to foliation of pelitic phyllite in contrast the footwall is composed of brittlely deformed chaotic shales and fine sandstones and velocities in the footwall are lower than those in the hanging wall amplitude variation with offset avo modeling utilizing contrasts in p and s wave velocities and densities between the hanging wall and footwall of the nobeoka oost indicates that fractures filled with overpressured fluid likely account for angle dependent reflection amplitudes of the active oost in the nankai trough citation tsuji t g kimura s okamoto f kono h mochinaga t saeki and h tokuyama 2006 modern and ancient seismogenic out ofsequence thrusts in the nankai accretionary prism comparison of laboratory derived physical properties and seismic reflection data geophys res lett 33 l18309 doi 10 1029 2006gl027025

runaway slip to the trench due to rupture of highly pressurized megathrust beneath the middle trench slope the tsunamigenesis of the 2011 tohoku earthquake off the east coast of northern japan abstract the gigantic 2011 march 11 mw 9 tohoku earthquake is examined from the viewpoint of the pre seismic forearc structure the seismic reflection properties of a megathrust around the usual up dip limit of the seismogenic zone the thermal state of a shallow subduction zone and the dehydration of underthrust sediments at the japan trench the pacific plate is subducting westward beneath northeast japan at a dip angle of 4 6 the middle and lower slopes of the landward side dip eastward at angles of 2 5 and 8 0 respectively the forearc prism beneath the middle and lower slopes is inferred to be in extensionally and compressively critical states respectively based on the presence of clear internal deformation features and on the occurrence of aftershock earthquakes the rapid uplift of the forearc that caused the 2011 tohoku tsunami may have been associated with this internal deformation of the prism the critical state of the prism indicates that the effective basal friction b of the plate boundary megathrust is 0 08 for the lower prism the megathrust especially under the middle slope is characterized by a prominent reflector with negative polarity one of multiple possible explanations is that the megathrust hosts highly pressurized fluids underthrust sediments in this part of the japan trench are dominated by pelagic and siliceous vitric diatomaceous silt with clay the dehydration kinetics of opal a to quartz the clay transformation of smectite illite and the thermal structure of the japan trench suggest that maximum dehydration of the sediments would take place at 50 60 km horizontally from the deformation front where the temperature along the megathrust is 100 120 c the zone of maximum dehydration coincides with the prominent seismic reflector that has negative polarity we hypothesize a possible free slip along this portion of the megathrust during the 2011 tohoku earthquake caused by anomalously high fluid pressure resulting from fluid accumulation over centuries

mixing and entrainment in the red sea outflow plume part i plume structure when the salty and heavy water of the red sea exits from the strait of bab el mandeb it continues downslope into the gulf of aden mainly along two channels the 130 km long northern channel nc is topographically confined and is typically only 5 km wide in it the red sea plume shows unanticipated patterns of vertical structure turbulent mixing and entrainment above the seafloor a 25 120 m thick weakly stratified layer shows little dilution along the channel hence this bottom layer undergoes only weak entrainment in contrast a 35 285 m thick interfacial layer shows stronger entrainment and is shown in a companion paper to undergo vigorous turbulent mixing it is thus the interface that exhibits the bulk of entrainment of the red sea plume in the nc the interfacial layer also carries most of the overall plume transport increasingly so with downstream distance the southern channel sc is wider than the nc and is accessed from the latter by a sill about 33 m above the floor of the nc entrainment into the bottom layer of the sc is diagnosed to be strong near the entry into the sc such that the near bottom density and salinity are smaller in the sc than in the nc at the same distance from bab el mandeb in comparison with winter conditions the authors encountered weaker outflow with shallower equilibration depths during the summer cruise bulk froude numbers computed for the whole plume varied within the range 0 2 1 local maxima occurred in relatively steep channel sections and coincided with locations of significant entrainment

spreading of red sea overflow waters in the indian ocean as a result of its remarkably high salinity and despite its small volume input remnants of red sea water rsw have been identified in the agulhas current over 6000 km distant of their source this provided the motivation to investigate the long term mean spreading of rsw throughout the indian ocean using a comprehensive set of observations taken from the national oceanographic data center archives and from the world ocean circulation experiment hydrographic program for the indian ocean after emerging from the gulf of aden into the arabian sea rsw spreads predominantly southwestward along the african continental slope as indicated by strongly inclined isohalines across the arabian sea there is some monsoon variability so that during the winter monsoon there is more rsw present in the gulf of aden and an intensification of southward spreading along the western boundary between 12 and 5 n elsewhere the intermediate depth salinity field of the indian ocean appears relatively stationary between 5 and 10 s in the region of the south equatorial current isohalines of the rsw layer become quasi zonal across the width of the indian ocean only dipping southward toward the western boundary west of 50 e south of here there is a strong tongue of rsw spreading southward through the mozambique channel and into the agulhas current these conclusions concur with previous localized investigations of intermediate water properties using a simple mixing model the percentage of rsw throughout the indian ocean was quantified it was found that the flux of salt into the gulf of aden from the red sea is similar to that estimated to cross 32 s in the agulhas current this result implies that all the rsw which is mixed into the interior of the indian ocean may eventually be exported at the western boundary furthermore it implies that rsw is the dominant component of the salt budget for the intermediate layer and that input from the indonesian seas and via diapycnic processes are small

process modeling studies of physical mechanisms of the formation of an anticyclonic eddy in the central red sea surface drifters released in the central red sea during april 2010 detected a well defined anticyclonic eddy around 23 n this eddy was 45 60 km in radius with a swirl speed up to 0 5 m s the eddy feature was also evident in monthly averaged sea surface height fields and in current profiles measured on a cross isobath shipboard ctd adcp survey around that region the unstructured grid finite volume community ocean model fvcom was configured for the red sea and process studies were conducted to establish the conditions necessary for the eddy to form and to establish its robustness the model was capable of reproducing the observed anticyclonic eddy with the same location and size diagnosis of model results suggests that the eddy can be formed in a red sea that is subject to seasonally varying buoyancy forcing with no wind but that its location and structure are significantly altered by wind forcing initial distribution of water stratification and southward coastal flow from the upstream area momentum analysis indicates that the flow field of the eddy was in geostrophic balance with the baroclinic pressure gradient forcing about the same order of magnitude as the surface pressure gradient forcing

pore pressure distribution of a mega splay fault system in the nankai trough subduction zone insight into up dip extent of the seismogenic zone we use the pore pressure distribution predicted from a waveform tomography wt velocity model to interpret the evolution of the mega splay fault system in the nankai trough off kumano japan to map pore pressure around the mega splay fault and plate boundary decollement we integrate the high resolution wt velocities with laboratory data and borehole well log data using rock physics theory the predicted pore pressure distribution shows that high pore pressures close to lithostatic pressure along the footwall of the mega splay fault extend seaward to the trough region and the normalized pore pressure ratio is nearly constant over that extent this continuity of the overpressured zone indicates that a coseismic rupture can potentially propagate nearly to the trough axis we interpret a high pressure belt within an accretionary wedge on the landward side of the present mega splay fault as evidence of the ancient mega splay fault because the ancient mega splay fault soles into the active mega splay fault the active mega splay fault may function as a basal detachment fault and is directly connected to the seaward plate boundary decollement

circulation characteristics in three eddy permitting models of the north atlantic a systematic intercomparison of three realistic eddy permitting models of the north atlantic circulation has been performed the models use different concepts for the discretization of the vertical coordinate namely geopotential levels isopycnal layers terrain following sigma coordinates respectively although these models were integrated under nearly identical conditions the resulting large scale model circulations show substantial differences the results demonstrate that the large scale thermohaline circulation is very sensitive to the model representation of certain localised processes in particular to the amount and water mass properties of the overflow across the greenland scotland region to the amount of mixing within a few hundred kilometers south of the sills and to several other processes at small or sub grid scales the different behaviour of the three models can to a large extent be explained as a consequence of the different model representation of these processes

a low velocity zone with weak reflectivity along the nankai subduction zone three dimensional seismic reflection data reveal the presence of a low seismic velocity zone lvz with weak reflectivity character along the nankai accretionary prism this lvz is intercalated between an upper offscraped layer and a lower underthrusting layer in the outer accretionary wedge wide angle ocean bottom seismograph data also support the presence of the lvz which is estimated to be a maximum of 2 km thick 15 km wide and 120 km long the lvz could be an underthrust package underplated in response to the lateral growth of the nankai accretionary prism underplating of the underthrusting layer beneath the overlying offscraped layer would maintain a critical taper of the accretionary prism so that the offscraped layer can continue to grow seaward the lvz could have elevated fluid pressure leading to rigidity reduction of the entire outer accretionary wedge the rigidity lowered outer wedge containing the lvz may be more easily uplifted and thus eventually foster tsunami generation during a nankai megathrust earthquake if the fluid rich lvz supplies a significant amount of the fluid to the megasplay fault zone at depth it may affect stick slip behavior of the fault

deformation and fluid flow of a major out of sequence thrust located at seismogenic depth in an accretionary complex nobeoka thrust in the shimanto belt kyushu japan 1 nobeoka thrust in kyushu southwest japan was investigated to understand the relationship between the seismogenic out of sequence thrust ost and fluid flow in accretionary prisms the nobeoka thrust is a fossilized ost being active at seismogenic depth the hanging wall exhibits a penetrative plastic deformation while a brittle cataclastic melange like occurrence characterizes the footwall although both of them have same shale and sandstone dominant protolith vitrinite reflectance analyses indicate that the maximum temperatures of the hanging wall and footwall are approximately 320 and 250 c respectively this thermal gap across the thrust corresponds to 8 6 14 4 km of displacement assuming a 28 47 c km geothermal gradient the brittle damage zone of the thrust is asymmetric only 2 m for hanging wall side and 100 m for footwall three types of mineral veins quartz and carbonate are well developed especially in the damaged footwall the tension crack filling vein the fault filling vein and postmelange one the first is harmonious with fabric perpendicular to the p surface fluid inclusion geothermobarometry indicates the p t of fluid in the intensively damaged zone of the footwall is 300 c 230 250 mpa higher than that from vitrinite reflectance which suggests that hydrothermal fluid flow is associated with deformation the same type vein in the lowest portion of the footwall damaged zone includes a much lower p t fluid this difference suggests that continuous underplating caused the damaged zone to propagate downward with cooling and shallowing which differs from faults characterized by shear localization and might be unique to aquiferous ost in accretionary complexes

a fault model of the 1946 nankaido earthquake derived from tsunami data abstract a fault model of the 1946 nankaido earthquake m 8 2 is determined by the use of tsunami records of uwajima shimotsu and hososhima which were located within or near the area of major coseismic crustal deformation synthetic tsunamis computed for various fault models are matched with the observed tsunamis to determine the fault parameters a low angle thrust model slightly revised from a previous model by ando is consistent with the observed tsunamis the duration of faulting is constrained as less than 10 min based upon the tsunami the fault is divided into an eastern and a western segment corresponding to areas associated with and without aftershocks respectively the fault area and dislocation for the western segment are 150 70 km2 and 6 m and those for the eastern segment are 150 70 km2 and 3 m respectively the total seismic moment is 4 7 1028 dyn cm significantly smaller than that obtained from a geodetic model by fitch and scholz but still larger than that of the seismic model by kanamori the discrepancy in seismic moment between the seismic and the present models ran2 could be interpreted in terms of a slow dislocation on the fault but this interpretation does not match the seismic intensity distribution and damage pattern and the slow slip model for the nankaido earthquake is rejected the discrepancy between the two seismic moments is considered insignificant within error involved in data and modeling assumptions if the revised geodetic model ran2 is modified the seismic moment required to explain the observed tsunamis would be reduced further by 30 if we consider the uncertainties involved in the fault model of kanamori and the fault finiteness effect affecting the amplitude of seismic waves the seismic moment required to interpret the seismic wave data could be increased possibly being more than twice that of kanamori thus the two seismic moments from the different data sets could be close to each other within allowable tolerance this implies that the rise time of the nankaido earthquake was short enough to generate short period seismic waves from both the western and the eastern fault segments

elevated fluid pressure and extreme mechanical weakness of a plate boundary thrust nankai trough subduction zone pore fluid pressure is a key parameter governing both the shear strength of faults and the conditions for seismic and tsunamigenic slip on subduction zone megathrusts however quantitative constraints on this parameter based on in situ data in these or any faults have proven elusive using seismic interval velocities derived from a three dimensional seismic reflection survey we compute porosity fluid pressure and effective stress at the plate interface in the nankai trough subduction zone offshore southwestern japan to 20 km down the fault dip using well constrained locally calibrated empirical relationships between velocity porosity and effective stress we show that the fault and immediately subjacent footwall are nearly undrained implying that the subduction megathrust slips under a shear stress of

mechanism of the 1992 nicaragua tsunami earthquake the 1992 nicaragua earthquake generated larger tsunamis than expected from its surface wave magnitude ms 7 2 and is known as a tsunami earthquake seismological studies showed that the duration was very long for its size about 100 s other studies have shown that the seismic moment estimated from tsunamis is an order of magnitude larger than that from seismic waves even after the long duration is accounted for numerical computations of tsunamis from various fault models are made to reconcile this discrepancy comparison of calculated waveforms with tide gauge records shows that the fault width is 40 km much narrower than the aftershock area and extends only into the upper 10 km of the ocean bottom slip amount on the fault is estimated to be 3 m from amplitude comparisons the fault length is estimated to be 250 km slightly longer than the aftershock area from comparison of the tsunami height distribution the rigidity around the shallow fault may be smaller than that of a standard underthrust fault and the seismic moment is estimated as 3 1020 nm consistent with the seismic observations a slow rupture on the shallow fault presumably in the subducted sediments is responsible to the unusually large tsunami excitation

fluid accumulation and channeling along the northern barbados ridge decollement thrust a volume of three dimensional seismic reflection data acquired in 1992 imaged the decollement beneath the northern barbados ridge accretionary prism revealing reflection amplitude and waveform variations attributed to fluid accumulations along the plate boundary fault we model the seismic reflection by inversion for seismic impedance the product of velocity and density throughout the 5 x 25 km survey area and thus map physical property variations in 1997 ocean drilling program leg 171a penetrated the protodecollement and decollement at five sites with a logging while drilling lwd tool to log density and other physical properties of the decollement we construct a regional map of density and inferred porosity within the decollement from seismic models calibrated with lwd density data in the sediments out in front of the trench the protodecollement forms in a radiolarian rich miocene mudstone with an anomalously high porosity 70 75 that appears as a pervasive inherent characteristic of this interval seaward of the deformation front in the decollement beneath the wedge a consolidation trend of decreasing porosity runs perpendicular to the deformation front with porosity decreasing from 70 at the wedge toe to 50 4 km from the wedge toe a second distinct trend also forms along a 10 km long 1 to 2 km wide ne sw zone in which porosity is 70 as high as it is in the protodecollement this zone can be explained as an area of the decollement where fluid accumulations develop by maintaining high fluid content we postulate that high fluid content is maintained by continuous recharge flowing into and along this channel this porosity distribution within the decollement also strongly influences fluid migration into the overlying accretionary wedge and is directly associated with fluid charging of ramps and out of sequence thrusts above the decollement

effective stress and pore pressure in the nankai accretionary prism off the muroto peninsula southwestern japan 1 we developed a theoretical method for predicting effective stress and pore pressure based on rock physics model we applied the method to reveal the pore pressure distribution within the nankai accretionary prism off southwestern japan and to investigate variations in pore pressure associated with evolution of the plate boundary decollement from the crack aspect ratio spectrum estimated from laboratory and well log data we calculated a theoretical relationship between acoustic velocity and mean effective stress by using differential effective medium theory by iteratively fitting the theoretically calculated velocity to the seismic velocities derived from 3d tomographic inversion we estimated in situ mean effective stress within the accretionary prism pore pressure is then the difference between the effective stress and the confining stress when we calculated pore pressure we considered compressive state of stress in the accretionary prism our results confirm that pore fluid pressure is high within the subducting sedimentary sequence below the decollement we determined a normalized pore pressure ratio of 0 4 0 7 abnormal pore pressures develop in the under thrust sequence as a result of the increase in overburden load because of the thickened overlying prism and a low permeability barrier across the decollement overpressuring within the accreted sequence is initiated at the deformation front and proceeds landward the increase in horizontal compaction within the accreted sequence may raise pore pressures within the accreted sequence and the pore pressure mean effective stress contrast at the decollement becomes smaller landward of the deformation front

seismic velocity anisotropy and fluid pressure in the barbados accretionary wedge from an offset vertical seismic profile with seabed sources 1 the state of compaction and fluid pressure in the barbados accretionary wedge near its toe at ocean drilling program site 949 were investigated by modeling travel times of seismic waves from ocean bottom shots to a borehole geophone array the model constrained by a three dimensional seismic survey and well logs shows 1 a velocity gradient of about 1 1 25 s 1 in the uppermost 180 230 m of the wedge 2 a zone of variable but no net change in velocity between 230 and 350 m depth 3 a low velocity zone 40 50 m thick just above the decollement at 391 m and 4 a displacement of the low velocity zone by thrust faults pore fluid pressure sections derived from p wave velocity show that the upper half of the wedge is normally pressured while the lower half is overpressured the 160 m thick underconsolidated basal zone shows anisotropy which increases downward the lowest 40 50 m has velocity varying 1 azimuthally 3 being fastest in the direction of plate convergence and 2 in the vertical plane 2 5 horizontal faster than vertical after correction for the effect of anisotropy in the derivation of effective stress from seismic velocity the calculated pore fluid pressure ratio does not exceed 0 9 and in the lowest 40 50 m of the basal zone is between 0 71 and 0 82 with fluid pressure hydrostatic lithostatic pressure hydrostatic between 0 5 and 0 65 in accordance with in situ measurements of fluid pressure in the decollement zone beneath these indicate that the accretionary wedge is stronger and less overpressured than was previously supposed

pore pressure estimation in reservoir rocks from seismic reflection data a method is used to obtain pore pressure in shaly sandstones based upon an acoustic model for seismic velocity versus clay content and effective pressure calibration of the model requires log data porosity clay content and sonic velocities to obtain the dry rock moduli and the effective stress coefficients as a function of depth and pore pressure the seismic p wave velocity derived from reflection tomography is fitted to the theoretical velocity by using pore pressure as the fitting parameter this approach based on a rock physics model is an improvement over existing pore pressure prediction methods which mainly rely on empirical relations between velocity and pressure the method is applied to the tune field in the viking graben sedimentary basin of the north sea we have obtained a high resolution velocity map that reveals the sensitivity to pore pressure and fluid saturation in the tarbert reservoir the velocity map of the tarbert reservoir and the inverted pressure distribution agree with the structural features of the tarbert formation and its known pressure compartments

effective stress coefficient in shales and its applicability to eaton s equation pore pressure within formations determines the mud weight required to build a balancing fluid pressure downhole an improper understanding of the subsurface geology and the formation pressures may result in fracturing the formation if the mud weight is too high in contrast if mud weight is too low then formation fluids can flow into the well potentially leading to well blowouts if not controlled complex geological settings make pore pressure prediction difficult and often inaccurate due to uncertainty in pressure generating mechanisms estimation of proper pore pressures is necessary for designing stable holes and an optimized casing program in exploration knowledge of pore pressures can assist in assessing seal effectiveness and in high grading reservoir sweet spots it also provides useful calibration information for basin modeling hence it becomes very important to accurately predict pore pressures

equilibration and circulation of red sea outflow water in the western gulf of aden hydrographic direct velocity and subsurface float observations from the 2001 red sea outflow experiment redsox are analyzed to investigate the gravitational and dynamical adjustment of the red sea outflow water rsow where it is injected into the open ocean in the western gulf of aden during the winter redsox cruise when outflow transport was large several intermediate depth salinity maxima product waters were formed from various bathymetrically confined branches of the outflow plume ranging in depth from 400 to 800 m and in potential density from 27 0 to 27 5 a result of different mixing intensity along each branch the outflow product waters were not dense enough to sink to the seafloor during either the summer or winter redsox cruises but analysis of previous hydrographic and mooring data and results from a one dimensional plume model suggest that they may be so during wintertime surges of strong outflow currents or about 20 of the time during winter once vertically equilibrated in the gulf of aden the shallowest rsow was strongly influenced by mesoscale eddies that swept it farther into the gulf the deeper rsow was initially more confined by the walls of the tadjura rift but eventually it escaped from the rift and was advected mainly southward along the continental slope there was no evidence of a continuous boundary undercurrent of rsow similar to the mediterranean undercurrent in the gulf of cadiz this is explained by considering 1 the variability in outflow transport and 2 several different criteria for separation of a jet at a sharp corner which indicate that the outflow currents should separate from the boundary where they are injected into the gulf

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

variation of physical properties beneath a fault observed by a two ship seismic survey off southwest japan 1 we conducted a two ship seismic reflection survey at kumano nada off the southwest japan arc in 2002 in order to clarify physical properties along a splay fault that branches from the plate boundary of the nankai trough subduction zone during the survey seismic reflection data were collected out to maximum offset of 20 km using multiple passes of a shooting and a receiving vessel separated by a different constant offset on each pass we conducted analyses on reflection polarity and amplitude variation with offset avo effects to examine physical properties of layers immediately beneath the splay fault reflections from the splay fault showed varying polarities along the fault in the present study we assumed that the poisson s ratio of the layer above the fault was 0 30 0 40 where polarity is reversed the poisson s ratio of the layer immediately below the fault was estimated to be 0 33 0 445 11 increase on average where polarity was normal the ratio was estimated to be 0 263 0 365 11 decrease on average if the reverse polarity indicates the existence of fluid as many authors suggest the polarity analysis demonstrates inhomogeneous distribution of fluid in the splay fault shear zone the results of avo analysis also support the possibility of the spatial variation of fluid distribution under some assumptions because the poisson s ratio changes with content of fluid in rocks the variation in reflection polarity and corresponding avo effects may signify the possibility of localized fluid migration in the splay fault shear zone

the 2011 tohoku oki earthquake displacement reaching the trench axis we detected and measured coseismic displacement caused by the 11 march 2011 tohoku oki earthquake moment magnitude mw 9 0 by using multibeam bathymetric surveys the difference between bathymetric data acquired before and after the earthquake revealed that the displacement extended out to the axis of the japan trench suggesting that the fault rupture reached the trench axis the sea floor on the outermost landward area moved about 50 meters horizontally east southeast and 10 meters upward the large horizontal displacement lifted the sea floor by up to 16 meters on the landward slope in addition to the vertical displacement

origin and evolution of a splay fault in the nankai accretionary wedge subduction zones are often characterized by wedge shaped sedimentary complexes called accretionary prisms that form when sediments are scraped off the subducting plate and added to the overriding plate large landward dipping thrust faults can cut through such a prism these faults known as megasplay faults 1 2 originate near the top of the subducting plate and terminate at the shallow landward edge of the prism1 3 4 5 6 megasplay faults have been the subject of numerous geological and geophysical studies4 5 6 7 8 9 10 11 12 13 14 15 but their initiation and evolution through time remains poorly constrained here we combine seismic reflection data from the nankai accretionary wedge with geological data collected by the integrated ocean drilling program iodp and find that the splay fault cutting this wedge initiated approx1 95 million years myr ago in the lower part of the prism as an out of sequence thrust oost after an initial phase of high activity the movement along the fault slowed down but uplift and reactivation of the fault resumed about 1 55 myr ago the alternating periods of high and low activity along the splay fault that we document hint at episodic changes in the mechanical stability of accretionary prisms

velocity porosity relationships for slope apron and accreted sediments in the nankai trough seismogenic zone experiment integrated ocean drilling program expedition 315 site c0001 in this study we focused on the porosity and compressional wave velocity of marine sediments to examine the physical properties of the slope apron and the accreted sediments this approach allows us to identify characteristic variations between sediments being deposited onto the active prism and those deposited on the oceanic plate and then carried into the prism during subduction for this purpose we conducted ultrasonic compressional wave velocity measurements on the obtained core samples with pore pressure control site c0001 in the nankai trough seismogenic zone experiment transect of the integrated ocean drilling program is located in the hanging wall of the midslope megasplay thrust fault in the nankai subduction zone offshore of the kii peninsula sw japan penetrating an unconformity at 200 m depth between slope apron sediments and the underlying accreted sediments we used samples from site c0001 compressional wave velocity from laboratory measurements ranges from 1 6 to 2 0 km s at hydrostatic pore pressure conditions estimated from sample depth the compressional wave velocity porosity relationship for the slope apron sediments shows a slope almost parallel to the slope for global empirical relationships in contrast the velocity porosity relationship for the accreted sediments shows a slightly steeper slope than that of the slope apron sediments at 0 55 of porosity this higher slope in the velocity porosity relationship is found to be characteristic of the accreted sediments textural analysis was also conducted to examine the relationship between microstructural texture and acoustic properties images from micro x ray ct indicated a homogeneous and well sorted distribution of small pores both in shallow and in deeper sections other mechanisms such as lithology clay fraction and abnormal fluid pressure were found to be insufficient to explain the higher velocity for accreted sediments the higher slope in velocity porosity relationship for accreted sediments can be explained by weak cementation critical porosity or differences in loading history

hydrogeology and mechanics of subduction zone forearcs fluid flow and pore pressure at subduction zones fluid flow pore pressure and tectonic processes are tightly interconnected excess pore pressure is driven by tectonic loading and fluids released by mineral dehydration and it has profound effects on fault and earthquake mechanics through its control on effective stress the egress of these overpressured fluids which is in part governed by the presence of permeable fault zones is a primary mechanism of volatile and solute transport to the oceans recent field measurements new constraints gained from laboratory studies and numerical modeling efforts have led to a greatly improved understanding of these coupled processes here we summarize the current state of knowledge of fluid flow and pore pressure in subduction forearcs and focus on recent advances that have quantified permeability architecture fluxes the nature and timing of transience and pressure distribution thus providing new insights into the connections between fluid metamorphic mechanical and fault slip proc

dynamic deformation of the accretionary prism excites very low frequency earthquakes 1 we have detected anomalous very low frequency earthquakes within the accretionary prism along the nankai trough southwestern japan centroid moment tensor inversion analysis reveals that the earthquake hypocenters are distributed at 10 km depth above the upper surface of the subducting philippine sea plate and within 50 70 km landward of the trough axis the focal mechanisms indicate reverse faulting their hypocenters are distributed beneath a deformation zone of an accretionary prism in sea floor topography these observations suggest that the occurrence of very low frequency earthquakes is related to numerous reverse fault systems within the accretionary prism and that the earthquakes reflect the dynamics of deformation within this accretionary prism

seismic slip propagation to the updip end of plate boundary subduction interface faults vitrinite reflectance geothermometry on integrated ocean drilling program nantro seize cores seismic faulting along subduction type plate boundaries plays a fundamental role in tsunami genesis during the integrated ocean drilling program iodp nankai trough seismogenic zone experiment nantro seize stage 1 the updip ends of plate boundary subduction faults were drilled and cored in the nankai trough offshore japan where repeated large earthquakes and tsunamis have occurred including the a d 1944 tonankai mw 8 1 earthquake samples were obtained from the frontal thrust which connects the deep plate boundary to the seafloor at the toe of the accretionary wedge and from a megasplay fault that branches from the plate boundary decollement the toe of the accretionary wedge has classically been considered aseismic but vitrinite reflectance geothermometry reveals that the two examined fault zones underwent localized temperatures of more than 380 c this suggests that frictional heating occurred along these two fault zones and implies that coseismic slip must have propagated at least one time to the updip end of the megasplay fault and to the toe of the accretionary 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

overpressure detection from compressional and shear wave data by analyzing experimental data we show that in many room dry rocks the poisson s ratio pr decreases with decreasing differential pressure confining minus pore pressure this means that in gas saturated rocks pr decreases with increasing pore pressure we confirm the generality of the observed effect by theoretically reproducing it via effective medium modeling this effect can be used as a new tool for overpressure detection from surface seismic cross well sonic logs and measurements ahead of the drill bit

turbulent mixing in the red sea outflow plume from a high resolution nonhydrostatic model given the motivation that overflow processes which supply source waters for most of the deep and intermediate water masses in the ocean pose significant numerical and dynamical challenges for ocean general circulation models an intercomparison study is conducted between field data collected in the red sea overflow and a highresolution nonhydrostatic process model the investigation is focused on the part of the outflow that flows along a long narrow channel referred to as the northern channel that naturally restricts motion in the lateral direction such that the use of a two dimensional model provides a reasonable approximation to the dynamics this channel carries about two thirds of the total red sea overflow transport after the overflow splits into two branches in the western gulf of aden the evolution of the overflow in the numerical simulations can be characterized in two phases the first phase is highly time dependent during which the density front associated with the overflow propagates along the channel the second phase corresponds to that of a statistically steady state the primary accomplishment of this study is that the model adequately captures the general characteristics of the system i the gradual thickening of the overflow with downstream distance ii the advection of high salinity and temperature signals at the bottom along the channel with little dilution and iii ambient water masses sandwiched between the overflow and surface mixed layer to quantify mixing of the overflow with the ambient water masses an entrainment parameter is determined from the transport increase along the slope and is expressed explicitly as a function of mean slope angle bulk richardson numbers are estimated both from data and model and are related to the entrainment parameter the range of entrainment parameter and its functional dependence on bulk richardson number in this study are found to be in reasonable agreement with those reported from various laboratory experiments and that based on measurements of the mediterranean overflow the results reveal a complex dynamical interaction between shear induced mixing and internal waves and illustrate the high computational and modeling requirements for numerical simulation of overflows to capture at least in part turbulent transports explicitly

synoptic sections of the denmark strait overflow we report on a rapid high resolution survey of the denmark strait overflow dso as it crosses the sill the first such program to incorporate full water column velocity profiles in addition to conventional hydrographic measurements seven transects with expendable profilers over the course of one week are used to estimate volume transport as a function of density our observations reveal the presence of a strongly barotropic flow associated with the nearly vertical front dividing the arctic and atlantic waters the seven section mean transport of water denser than 27 8 is 2 7 0 6sv while the mean transport of water colder than 2 0 c is 3 8 0 8 sv although this is larger than the 2 9 sv of 2 c water measured by a 1973 current meter array we find that a sampling of our sections equivalent to the extent of that array also measures 2 9sv of cold water both the structure and magnitude of the measured flow are reproduced well by a high resolution numerical model of buoyancy driven exchange with realistic topography

character and dynamics of the red sea and persian gulf outflows historical hydrographic data and a numerical plume model are used to investigate the initial transformation dynamics and spreading pathways of red sea and persian gulf outflow waters where they enter the indian ocean the annual mean transport of these outflows is relatively small 0 4 sv but they have a major impact on the hydrographic properties of the indian ocean at the thermocline level because of their high salinity they are different from other outflows in that they flow over very shallow sills depth 200 m into a highly stratified upper ocean environment and they are located at relatively low latitudes 12 n and 26 n furthermore the red sea outflow exhibits strong seasonal variability in transport the four main results of this study are as follows first on the basis of observed temperature salinity t s characteristics of the outflow source and product waters we estimate that the red sea and persian gulf outflows are diluted by factors of 2 5 and 4 respectively as they descend from sill depth to their depth of neutral buoyancy the high dilution factor for the persian gulf outflow results from the combined effects of large initial density difference between the outflow source water and oceanic water and low outflow transport second the combination of low latitude and low outflow transport and associated low outflow thickness results in ekman numbers for both outflows that are o 1 this indicates that they should be thought of as frictional density currents modified by rotation rather than geostrophic density currents modified by friction third different mixing histories along the two channels that direct red sea outflow water into the open ocean result in product waters with significantly different densities which probably contributes to the multilayered structure of the red sea product waters in both outflows seasonal variations in source water and oceanic properties have some effect on the t s of the product waters but they have only a minor impact on equilibrium depth fourth product waters from both outflows are advected away from the sill region in narrow boundary currents at least during part of the year at other times the product water appears more in isolated patches

heterogeneous hydrofracture development and accretionary fault dynamics ocean drilling program observations at many accretionary wedges indicate that fluid flow in low permeability sediments is focused along faults rich in clay minerals recent ring shear experiments show that shear failure generally reduces permeability in muddy faults to values well below the surrounding sediments and that very little increase occurs in the fault permeability even when continued failure proceeds under steadily reduced effective loads it is thus unlikely that observed focused flow occurs through the matrix of muddy faults and we suggest that focused flow can only be explained by the development of open fracture systems the lithostatic fluid pressures necessary for hydrofracturing in weak sediments cannot however exist throughout the imbricate thrust system and basal decollement zone because a zero basal shear stress condition would result in regional instability and accretionary wedge collapse we propose that the regional hydrofracture networks must propagate heterogeneously leaving relatively strong asperity regions at sublithostatic fluid pressures to balance forces in the wedge in three dimensions

mediterranean outflow mixing and dynamics the mediterranean sea produces a salty dense outflow that is strongly modified by entrainment as it first begins to descend the continental slope in the eastern gulf of cadiz the current accelerates to 1 3 meters per second which raises the internal froude number above 1 and is intensely turbulent through its full thickness the outflow loses about half of its density anomaly and roughly doubles its volume transport as it entrains less saline north atlantic central water within 100 kilometers downstream the current is turned by the coriolis force until it flows nearly parallel to topography in a damped geostrophic balance the mixed mediterranean outflow continues westward slowly descending the continental slope until it becomes neutrally buoyant in the thermocline where it becomes an important water mass

mesoscale eddies in the gulf of aden and their impact on the spreading of red sea outflow water abstract the gulf of aden goa in the northwestern indian ocean is the receiving basin for red sea outflow water rsow one of the world s few high salinity dense overflows but relatively little is known about spreading pathways and transformation of rsow through the gulf here we combine historical data satellite altimetry new synoptic hydrographic surveys and the first in situ direct observations of subsurface currents in the goa to identify the most important processes in the spreading of rsow the new in situ data sets were collected in 2001 2003 as part of the red sea outflow experiment redsox and consist of two ctd ladcp surveys and 49 one year trajectories from acoustically tracked floats released at the depth of rsow the results indicate that the prominent positive and negative sea level anomalies frequently observed in the goa with satellite altimetry are associated with anticyclonic and cyclonic eddies that often reach to at least 1000 m depth i e through the depth range of equilibrated rsow the eddies dominate rsow spreading pathways and help to rapidly mix the outflow water with the background eddies in the central and eastern gulf are basin scale 250 km diameter and have maximum azimuthal speeds of about 30 cm s at the rsow level in the western gulf smaller eddies not detectable with satellite altimetry appear to form as the larger westward propagating eddies impale themselves on the high ridges flanking the tadjura rift both the hydrographic and lagrangian observations show that eddies originating outside the gulf often transport a core of much cooler fresher water from the arabian sea all the way to the western end of the goa where the highest salinity outflow water is found this generates large vertical and horizontal gradients of temperature and salinity setting up favorable conditions for salt fingering and diffusive convection both of these mixing processes were observed to be active in the gulf two new annually appearing anticyclonic eddies are added to the previously identified gulf of aden eddy gae prasad and ikeda 2001 and somali current ring scr fratantoni et al 2006 these are the summer eddy se and the lee eddy le both of which form at the beginning of the summer monsoon when strong southwest winds blowing through socotra passage effectively split the gae into two smaller eddies the se strengthens as it propagates westward deeper in the goa while the lee eddy remains stationary in the lee of socotra island both eddies are strengthened or sustained by ekman convergence associated with negative wind stress curl patches caused by wind jets through or around high orography the annual cycle in the appearance propagation and demise of these new eddies and those described in earlier work is documented to provide a comprehensive view of the most energetic circulation features in the goa the observations contain little evidence of features that have been shown previously to be important in the spreading of mediterranean outflow water mow in the north atlantic namely a wall bounded subsurface jet the mediterranean undercurrent and submesoscale coherent lenses containing a core of mow meddies this is attributed to the fact that the rsow enters the open ocean on a western boundary high background eddy kinetic energy typical of western boundary regimes will tend to shear apart submesoscale eddies and boundary undercurrents even if a submesoscale lens of rsow did form in the goa westward self propagation would transport the eddy and its cargo of outflow water back toward rather than away from its source

widely distributed thrust and strike slip faults within subducting oceanic crust in the nankai trough off the kii peninsula japan abstract we identified widely distributed thrust and strike slip faults within subducting oceanic crust in the nankai trough southeast of the kii peninsula japan on the basis of 2d and 3d seismic reflection data the seafloor seaward of the trough axis is deformed by displacement on these intraoceanic reverse faults producing topographic highs part of kashinosaki knoll because the thrust faults extend to the moho and offset the moho reflection they may be related to serpentinization of the mantle due to seawater invasion these faults are seismically active given that their geometries are consistent with the focal mechanisms of intraplate earthquakes and microearthquakes the thrust faults appear to extend landward to a high density dome within the accretionary prism off the kii peninsula because the dome and the associated thick accretionary prism are expected to generate high friction at the plate interface due to their large vertical load the intraoceanic thrusts are likely to have grown with ongoing subduction furthermore because the geometry of the fault system we identified off the kii peninsula has characteristics similar to faults at zenisu ridge east of our study area the thrusts observed in the study area may be considered to be the westward continuation of those at zenisu ridge since the euler rotation pole of relative motion between the philippine sea plate and zenisu ridge is consistent with the high density dome off the kii peninsula we interpret the high density dome as well as kashinosaki knoll as a westward termination of the zenisu compression zone

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

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

intraoceanic thrusts in the nankai trough off the kii peninsula implications for intraplate earthquakes 1 we identified intraoceanic thrusts developed as imbricate structures within the subducting philippine sea plate off the kii peninsula in central japan manifesting as strong amplitude reflections observed in an industry standard three dimensional 3d seismic reflection data set these imbricate intraoceanic thrusts cut through the oceanic crust as a discontinuous thrust plane striking approximately parallel to the trench in our survey area large intraplate earthquakes with moment magnitudes mw over 7 occurred on 5 september 2004 causing strong ground motions on the islands of japan and tsunami waves the locations of the intraoceanic thrusts recognized in the seismic data are distributed around the estimated hypocenters of the mainshocks and aftershocks of the 2004 earthquakes furthermore their geometry extracted from the 3d seismic data could explain the kind of complex rupture pattern observed during the 2004 events therefore we propose that the intraoceanic thrusts are seismogenically active

velocity and inferred porosity model of the oregon accretionary prism from multichannel seismic reflection data implications on sediment dewatering and overpressure a two dimensional model of seismic velocity derived from multichannel seismic data collected off oregon in 1989 shows that as sediments are carried from cascadia basin into the accretionary prism there are measurable changes in velocity depth profiles in the seaward most area of the basin where no thrust faults are observed there is a landward and downward increase of velocity in the sedimentary section we attribute the velocity increase in the basin to a reduction of porosity resulting from consolidation and cementation accompanied by diffusive flow of pore water driven by lateral tectonic as well as gravitational stress near the base of the slope there is an area of incipient thrusting the protothrust zone where protothrusts sole out into a protodecollement synthetic seismogram modeling of the reverse polarity reflection from the protodecollement shows a 100 m thick layer with a slightly lower velocity relative to the sediments above it above the protodecollement velocity continues to increase landward we suggest that in this area the diffusive flow of pore water out of the sediment is augmented above the protodecollement by fault focused flow below the protodecollement a reversal in velocity may be due to an increase in porosity resulting from overpressuring of pore fluid trapped by reduction of the permeability of the sediment above the protodecollement farther landward where thrusting has formed a fault bend fold velocity values are lower in the accreted section of sediments relative to the velocity at a comparable subbottom depth in the protothrust zone the decrease in velocity is a result of microfracturing of the highly consolidated sediments accompanying uplift and folding and reflects the increasing role of fracturing and faulting in the control of dewatering of the sediments

mixing and entrainment in the red sea outflow plume part ii turbulence characteristics abstract south of the strait of bab el mandeb saline red sea water flows downslope into the gulf of aden mainly along the narrow 130 km long northern channel nc and the shorter and wider southern channel sc in the nc the red sea plume simultaneously exhibited weak entrainment into a 35 120 m thick weakly stratified bottom layer while a 35 285 m thick interfacial layer above showed signs of vigorous mixing overturns up to 30 m thick and extensive zones of gradient richardson numbers below 1 4 turbulent overturning scales or thorpe scales are extracted from regular ctd profiles and equated to ozmidov scales on this basis interfacial mixing is quantified in terms of estimated turbulent dissipation rates vertical turbulent salt flux and interfacial stress even though these estimates are subject to significant uncertainty they demonstrate the intensity of mixing during strong winter outflow in terms of eddy diffusivities k on the order of 10 2 m2 s 1 the large k occur in strong strati

acoustic and mechanical properties of nankai accretionary prism core samples we studied undeformed sediment and accreted strata recently recovered by ocean drilling program integrated ocean drilling program odp iodp drilling in nankai trough convergent margin to unravel the changes in physical properties from initial deposition to incipient deformation we have derived acoustic vp and mechanical uniaxial poroelastic compliance compaction amplitude properties of samples from various drill sites along the muroto odp 1173 and kii transects iodp c0001 c0002 c0006 and c0007 from isotropic loading tests where confining and pore pressure were independently applied we quantified the dependence of vp on both effective peff and confining pc pressure which can be used to correct atmospheric pressure measurements of vp experimental vp obtained on core samples extrapolated to in situ conditions are slightly higher than logging derived velocities which can be attributed either to velocity dispersion or to the effect of large scale faults and weak zones on waves with longer wavelength in the high porosity 30 60 tested sediments velocities are controlled at first order by porosity and not by lithology which is in agreement with our static measurements of drained framework incompressibility much smaller than fluid incompressibility rather than framework incompressibility shear modulus is probably the second order control on vp accounting for most of the difference between actual vp and the prediction by wood s 1941 suspension model we also quantified the mechanical state of nankai samples in terms of anisotropy diagenesis and consolidation both acoustic and mechanical parameters reveal similar values in vertical and horizontal directions attesting to the very low anisotropy of the tested material when considering the porous samples of the upper shikoku basin sediments site 1173 as examples of diagenetically cemented material several mechanical and acoustic attributes appeared as reliable experimental indicators of the presence of intergrain cementation we also detected incipient cementation in samples from iodp site c0001 accretionary prism unit in terms of consolidation we distinguished two classes of material response shallow deformable samples and deep hardly deformable ones based on the amount of compaction upon application of a peff large with respect to the inferred in situ value with a transition that might be related to a critical porosity

frontal wedge deformation near the source region of the 2011 tohoku oki earthquake 1 we report an uplift of 5 m with a horizontal displacement of more than 60 m due to the 2011 tohoku oki earthquake the uplift was measured by an ocean bottom pressure gauge installed before the earthquake on a frontal wedge which formed an uplift system near the japan trench horizontal displacements of the frontal wedge were measured using local benchmark displacements obtained by acoustic ranging before and after the earthquake the average displacements at the frontal wedge were 58 m east and 74 m east southeast these results strongly suggest a huge coseismic slip beneath the frontal wedge on the plate boundary the estimated magnitude of the slip along the main fault was 80 m near the trench our results suggest that the horizontal and vertical deformations of the frontal wedge due to the slip generated the tremendous tsunami that struck the coastal area of northeastern japan

seismically inferred dilatancy distribution northern barbados ridge decollement implications for fluid migration and fault strength a 5 x 25 km three dimensional seismic survey of the lower part of the northern barbados ridge accretionary prism creates a three dimensional image of a major active decollement fault the fault is usually a compound negative polarity reflection modeled as a low velocity high porosity zone less than 14 m thick this thickness is significantly less than that defined by drilling of a 40 m zone of deformation at ocean drilling program odp site 671b located within the surveyed area we infer that the seismically defined fault is a thin high porosity zone and is thus an undercompacted high fluid pressure dilatant section if these inferences are correct then map view variations in seismic reflection waveform and amplitude illustrate complex patterns of fault zone fluid content and fluid migration paths the amplitude map suggests kilometre wide channels of locally high porosity and thus focused fluid flow these paths are only subparallel to the expected minimum head as inferred from the shape of the overlying sediment wedge other factors must modify fluid concentrations and ultimately migration several areas of positive polarity fault reflections define square kilometre sized regions inferred to be lower porosity sections producing strong asperities in an otherwise weak fault one coincident with site 671b may explain the success of drilling through the fault here all other holes drilled in the area were within the negative polarity regions and were unsuccessful in penetrating through the entire fault zone possibly because of instability associated with high fluid pressures and a weak fault odp leg 156 planned for 1994 will test inferences related to fault permeability and fluid pressures

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

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

tsunamigenic potential of the shallow subduction plate boundary inferred from slow seismic slip at subduction zones slip along the shallowest parts of the plate boundary is generally thought to be aseismic observations of very low frequency earthquakes occurring at shallow levels on the plate boundary of the nankai trough subduction zone imply that slow but seismic slip can occur there and could potentially generate tsunamigenic earthquakes

detailed structural image around splay fault branching in the nankai subduction seismogenic zone results from a high density ocean bottom seismic survey 1 to investigate megathrust earthquake and tsunami generation in a subduction seismogenic zone it is important to know the detailed structure around the plate boundary and active splay fault system the nankai trough southwestern japan is among the best studied subduction zones with splay faults in the world this paper presents a detailed structural image around the splay fault in the coseismic rupture zone of the 1944 tonankai earthquake in the central nankai trough based on results from a wide angle high density ocean bottom seismograph obs survey our seismic image clearly shows for the first time the subduction structure along with the splay fault reflections imaged on a previously obtained seismic reflection profile the correspondence between the splay fault reflections imaged on the seismic reflection profile and our reflectivity image was confirmed by converting the time migrated section to a depth section by using a velocity model estimated from the wide angle obs data in this study the high resolution seismic image obtained by the high density obs survey demonstrates a zone of low velocity gradient just above and along the splay fault we focused specifically on the detailed structure around the splay fault a simple amplitude analysis of the wide angle reflections showed that it is likely that a thin low velocity layer with velocities 0 5 1 5 km s slower than those in the surrounding rock exists above the splay fault the presence of this layer suggests elevated fluid pressure in the fault zone which cut through the relatively homogeneous rock of the accretionary prism and generated past megathrust earthquakes

contrasts in physical properties between the hanging wall and footwall of an exhumed seismogenic megasplay fault in a subduction zone an example from the nobeoka thrust drilling project 1 we examined the physical properties of an exhumed and fossilized subduction zone megasplay fault by analyzing geophysical logging data obtained by the nobeoka thrust drilling project which provide a high resolution transect of properties across the main fault zone the footwall cataclasite exhibits higher averages of neutron porosity 7 6 and lower values of electric resistivity 232 m compared to the hanging wall phyllite 4 8 453 m this clear contrast between the hanging wall and footwall may account for the difference in maximum burial and structural variation despite the contrast observed between the hanging wall and footwall in macroscopic scale the resistivity and porosity data from both the hanging wall and footwall can be fit with a single curve using archie s law suggesting the similarities in microstructures and mineralogy in this low porosity range above the main fault core of the nobeoka thrust a brittle damage zone in the hanging wall contains pseudotachylyte as evidence of the seismogenic slip and does not follow archie s law damage zones in the hanging wall are also observed in the modern splay fault at shallow depth in the nankai trough but with much thicker width whereas the footwall damage zone is more extensive in the nobeoka thrust splay faults may exhibit strong deformation in the hanging wall in the early stage and as fault rocks get buried deeper and as displacement and physical property contrast increase across the fault the damage effect may eventually be enlarged in the footwall

seismic structure of western end of the nankai trough seismogenic zone 1 in 1998 a seismic experiment along a 320 km long profile using an air gun array and ocean bottom seismographs obss was collected in the on shore offshore forearc region of the western nankai trough seismogenic zone southwestern japan this area is outside the coseismic rupture area of the 1946 nankaido earthquake ms 8 2 the main objectives of this paper are to clarify the p wave and the poisson s ratio profile around the forearc region of southwestern japan and estimate differences between this area and the nankaido coseismic rupture area the p wave velocity model estimated by two dimensional 2 d ray tracing and travel time inversion indicates three units with distinctive velocity patterns the lens shaped unit has a maximum thickness of 5 km and velocity of 3 4 4 0 km s the underlying unit has velocity of 5 4 5 6 km s and its p wave velocity increases on land vp 6 2 6 4 km s the relative homogeneous unit with p wave velocity of 6 6 6 9 km s exists just beneath secondary unit with p velocity of 5 4 5 6 km s the angle of subduction is 7 we also obtained a poisson s ratio of the crustal structure of the forearc region the poisson s ratio of the southern part of secondary unit is larger than that of the northern part we conclude that these data indicate clay rich metamorphic rocks plus oceanic basaltic rocks rather than high porosity materials comprise the majority of the layer we suggest that the region of high poisson s ratio probably corresponds to underplating and the metamorphism of the clay rich sediments and possibly the oceanic basalt just below the decollement

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

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

full waveform tomography for lithospheric imaging results from a blind test in a realistic crustal model summary a comprehensive validation of 2 d frequency domain acoustic wave equation tomography was undertaken in a blind test using third party realistic elastic wave equation data the synthetic 2 d wide angle seismic data were provided prior to a recent workshop on the methods of controlled source seismology the true model was not revealed to the authors until after the presentation of our waveform tomography results the original model was specified on a detailed grid with variable p wave velocity s wave velocity density and viscoelastic q factor structure designed to simulate a section of continental crust 250 km long and 40 km deep synthetic vertical and horizontal component data were available for 51 shot locations spaced every 5 km recorded at 2779 receivers spaced every 90 m evenly spread along the surface of the model the data contained energy from 0 2 to 15 hz waveform tomography a combination of traveltime tomography and 2 d waveform inversion of the early arrivals of the seismic waveforms was used to recover crustal p velocity structure from the vertical component data using data from 51 sources 1390 receivers and frequencies between 0 8 and 7 0 hz the waveform tomography result contained apparent structure at wavelength scale resolution that was not evident on the traveltime tomography result the predicted acoustic waveforms in the final result matched the original elastic data to a high degree of accuracy during the workshop the exact model was revealed over much of the model the waveform tomography results provided a good correspondence with the true model from large to intermediate wavelength scales with a resolution limit on the order of 1 km a significant near surface low velocity zone invisible to traveltime methods was correctly recovered the results also provided a high resolution image of the complex structure of the entire crust and the depth and nature of the crust mantle transition some inaccuracies were observed near the edges of the images due to the limited ray coverage inherent to the footprint of the survey geometry several aspects of the waveform tomography strategy were critical to the success of the acoustic method with realistic synthetic viscoelastic data i the accuracy of the starting model from traveltime tomography ii implementation in the frequency domain iii the use of complex valued frequencies to effect time damping of the data residuals iv the selection of a suitable subset of data and data frequencies v progressive inversion of low to high frequency components of the data vi initial pre inversion matching of the amplitudes between observed and modelled data and vii sufficient preconditioning of both the data and the update images combined these strategies were effectively equivalent to a multiscale approach that mitigated the non linearity of the seismic inverse problem during the inversion we carried out repeated forward modelling to ensure our modelled waveforms matched the observed data as closely as possible in both frequency and time domains the synthetic data set used in this paper provides a benchmark for future testing of modelling inversion and imaging algorithms for wide angle lithospheric imaging

bottom layer turbulence in the red sea outflow plume abstract turbulence in the red sea outflow plume in the western gulf of aden was observed with an upward looking five beam 600 khz acoustic doppler current profiler adcp the bottom lander adcp was deployed on the seafloor in two narrow topographically confined outflow channels south of bab el mandeb for periods of 18 40 h at three locations at 376 496 and 772 m depths two deployments were taken during the winter season of maximum outflow from the red sea and two in the summer season of minimum outflow these short term observations exhibit red velocity spectra with high frequency fluctuations of typically a few centimeters per second rms velocity during strong plume flow as well as strong subtidal variations in one winter season event the plume flow was reduced by a factor of 4 over an 18 h time span in variance preserving form velocity spectra show a separation at frequencies of 0 3 3 cycles per hour between low frequency and high frequency signals the latter show significant coherence

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

numerical simulation of the red sea outflow using hycom and comparison with redsox observations abstract the outflow of warm salty and dense water from the red sea into the western gulf of aden is numerically simulated using the hybrid coordinate ocean model hycom the pathways of the modeled overflow temperature salinity velocity profiles from stations and across sections and transport estimates are compared to those observed during the 2001 red sea outflow experiment as in nature the simulated outflow is funneled into two narrow channels along the seafloor the results from the three dimensional simulations show a favorable agreement with the observed temperature and salinity profiles along the channels the volume transport of the modeled overflow increases with the increasing distance from the southern exit of the bab el mandeb strait due to entrainment of ambient fluid such that the modeled transport shows a reasonable agreement with that estimated from the observations the initial propagation speed of the outflow is found to be smaller than the estimated interfacial wave speed the

an asperity deformation model for effective pressure abstract variations of the mechanical and transport properties of cracked and or porous rocks under isotropic stress depend on both the confining pressure p c and the pore fluid pressure p p to a first approximation these rock properties are functions of the differential pressure p d p c p p at least for low differential pressures however at higher differential pressures the properties depend in a more complicated way upon the two pressures the concept of effective pressure p e is used to denote this variation and it is defined as p e p c p p p c n p c p p p p if n 1 and therefore is independent of p c and p p the effective pressure is just the differential pressure we have used an asperity deformation model and a force balance equation to derive expressions for the effective pressure we equate the total external force in one direction f c to the total force on the asperities f a and the force of the fluid f p acting in that same direction the fluid force f p acts only on the parts of the crack or pore volume faces which are not in contact then the asperity pressure p a is the average force per unit area acting on the crack or grain contacts p a f a a f c a f p a p c 1 a c a p p where a is the total area over which f c acts and a c is the area of contact of the crack asperities or the grains thus the asperity pressure p a is greater than the differential pressure p d because p p acts on a smaller area a a c than the total area a for elastic asperities the area of contact a c and the strain e g crack and pore openings remain the same to a high degree of approximation at constant asperity pressure therefore transport properties such as permeability resistivity thermal conductivity etc are constant to the same degree of approximation at constant asperity pressure for these properties the asperity pressure is very accurately the effective pressure p c using this model we find that the dynamic undrained elastic modulus m cr of saturated cracks rocks at low effective pressure is given by m cr 1 p p a f m a 1 a f m f w dp c d w where m a is the dry matrix or crack asperity modulus m f is the fluid s modulus a f is the fractional area of contact a f a c a a f d a f d p a and w is a measure of the crack or pore openings this simple model accounts for the dependence of the rock modulus and elastic velocity on 1 the elastic properties of the fluid 2 the elastic properties of the dry rock and 3 the pore fluid and confining pressures explicit expressions depend upon the choice of the asperity or grain deformation models and their contact distribution functions the effective pressures for transport properties are different than the effective pressure for the mechanical properties calculated results based on the bed of nails model having power law or fractal asperity height distribution functions can be fitted quite well to experimental data with a minimum of fitting parameters

tsunami generation by horizontal displacement of ocean bottom tsunami generation by an earthquake is generally modeled by water surface displacement identical to the vertical deformation of ocean bottom due to faulting the effect of horizontal deformation is usually neglected however when the tsunami source is on a steep slope and the horizontal displacement is large relative to the vertical displacement the effect becomes significant we show this for two recent earthquakes which generated much larger tsunamis than expected from seismic waves in the case of the 1994 june 2 java indonesia earthquake the focal mechanism was a very shallow dipping thrust and the source was near a very steep trench slope in the case of the 1994 nov 14 mindoro philippines earthquake strike slip faulting extended from ocean to land perpendicular to the coast line in both cases we found that the horizontal motion of slope had an important contribution to the tsunami generation

fundamentals of seismic rock physics during the past 50 years or so tremendous progress has been made in studying physical properties of rocks and minerals in relation to seismic exploration and earthquake seismology during this period many theories have been developed and many experiments have been carried out some of these theories and experimental results have played important roles in advancing earth sciences and exploration technologies this tutorial paper attempts to summarize some of these results

on acoustic waveform tomography of wide angle obs data strategies for pre conditioning and inversion we successfully apply the acoustic laplace fourier waveform tomography method to delineate p wave velocity structures of the mega splay fault system in the central part of the seismogenic nankai subduction zone offshore japan using densely sampled wide angle ocean bottom seismograph obs data originally acquired in 2004 our success is due to new and carefully designed data preconditioning and inversion strategies to mitigate i the well known non linearity of waveform inversion ii the challenges arising from crustal scale survey designs e g undersampling of the obss and iii modelling errors due to the use of the acoustic assumption we identify a sixfold set of key components that together lead to the success of the high resolution waveform tomography image i availability of low frequency components starting at 2 25 hz reducing the non linearity and access to large offset data up to 55 km increasing the depth of illumination and the recovery of low wavenumber components ii a highly accurate traveltime tomography result with an rms error of approximately 60 ms that further mitigates the non linearity iii a hierarchical inversion approach in which phase spectra are inverted first to reduce artefacts from the acoustic assumption and amplitude information is only incorporated in the final stages iv a laplace fourier domain approach that facilitates a multiscale approach to mitigate non linearity by restricting the inversion to the low frequency components and early arrivals first and sequentially including higher frequencies and later arrivals v a pre conditioning strategy for eliminating undesirable high wavenumber components from the the gradient vi a strategy for source estimation that reduce the influence of the instrumental design in the obs case study used for illustration purposes laplace fourier waveform tomography retrieves velocity anomalies as small as 700 m horizontally and 350 m vertically above the top of the philippine sea plate the resulting velocity structures include low velocity zones and thrust structures which have not been previously identified clearly the velocity models are validated by scrutiny of synthetic and observed waveforms by evaluating the coherency of source estimates and by comparison with 3 d pre stack migrated presdm images chequerboard tests and point scatter tests demonstrate both the reliability and the limitations of the acoustic implementation

detailed coseismic slip distribution of the 1944 tonankai earthquake estimated from tsunami waveforms coseismic slip distribution on the fault plane of the 1944 tonankai earthquake is estimated from inversion of tsunami waveforms three improvements from a previous study satake 1993 are made these are 1 smaller subfaults are used to resolve detailed slip distribution 2 the sub faults fit better to the plate interface geometry and 3 finer and more accurate bathymetry data is used the inversion result shows that a maximum slip of about 3 m occurred on the plate interface off shima peninsula the total seismic moment is estimated to be 2 0 1021 nm mw 8 2 the result confirms that the 1944 tonankai earthquake did not rupture the plate interface beneath the tokai region and supports the existence of the seismic gap in the tokai region suggested by ishibashi 1981 the slip of about 1 5 m on the plate interface beneath atsumi peninsula northeast of the large slip area is necessary to explain the observed tsunami waveforms although no seismic moment release was estimated from strong motion data by kikuchi et al 1999 this may suggest that the rupture beneath atsumi peninsula was slow

vp vs ratio and shear wave splitting in the nankai trough seismogenic zone insights into effective stress pore pressure and sediment consolidation to estimate variation of stress state and sediment consolidation in the nankai plate subduction zone off southwest japan we measured the p wave to s wave velocity ratio vp vs and s wave splitting along the seismic line extending from the trench to the seismogenic zone for this purpose we used active source seismic data recorded by multicomponent ocean bottom seismometers obs because it is difficult to identify the ps converted reflection waveforms for each of the geological boundaries in this deep offshore region we focused on the more easily identified pps refracted waveforms that register the conversion of the up going p waves to s waves at the igneous crust surface we estimated the average vp vs ratio within the sedimentary section by using the time lag between the p refracted waves and pps converted waves this vp vs ratio changes abruptly at the trough axis i e the deformation front of the accretionary prism arguably because of compaction associated with the accretion process we observ

tsunami earthquakes as transient phenomena 1 recent modeling of tsunami waveforms caused by tsunami earthquakes indicates that fault slip occurred along the subduction boundary close to the trench axis on the other hand it is well known that seismicity is generally very low within ca 50 km from the trench axis and this has been attributed to the stable frictional sliding properties of the subducted unconsolidated sediments this makes tsunamigenic slip near the trench difficult to explain in order to solve this dilemma i propose that transition from stable sliding to nearly zero friction with a velocity weakening property occurs as a result of areal increase of zones of elevated fluid pressure which may make fairly rapid seismic slip possible following the breakage of asperities at normal seismogenic depths this transition can be identified as seismic reflections with negative polarities which may help to rate hazards for a coming large tsunami earthquake