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

slow slip near the trench at the hikurangi subduction zone new zealand the range of fault slip behaviors near the trench at subduction plate boundaries is critical to know as this is where the world s largest most damaging tsunamis are generated our knowledge of these behaviors has remained largely incomplete partially due to the challenging nature of crustal deformation measurements at offshore plate boundaries here we present detailed seafloor deformation observations made during an offshore slow slip event sse in september and october 2014 using a network of absolute pressure gauges deployed at the hikurangi subduction margin offshore new zealand these data show the distribution of vertical seafloor deformation during the sse and reveal direct evidence for sses occurring close to the trench within 2 kilometers of the seafloor where very low temperatures and pressures exist

the frictional hydrologic metamorphic and thermal habitat of shallow slow earthquakes slow earthquakes have recently been discovered on the shallow parts of some subduction zones a review of the conditions under which these quakes form reveals that they could be common in most subduction zones globally

recurring and triggered slow slip events near the trench at the nankai trough subduction megathrust the discovery of slow earthquakes has revolutionized the field of earthquake seismology defining the locations of these events and the conditions that favor their occurrence provides important insights into the slip behavior of tectonic faults we report on a family of recurring slow slip events sses on the plate interface immediately seaward of repeated historical moment magnitude mw 8 earthquake rupture areas offshore of japan the sses continue for days to several weeks include both spontaneous and triggered slip recur every 8 to 15 months and are accompanied by swarms of low frequency tremors we can explain the sses with 1 to 4 centimeters of slip along the megathrust centered 25 to 35 kilometers km from the trench 4 to 10 km depth the sses accommodate 30 to 55 of the plate motion indicating frequent release of accumulated strain near the trench

global ice volumes at the last glacial maximum and early lateglacial observations of sea levels during the last glacial maximum lgm at localities far from the former ice margins constrain the global change in ice volume from the lgm to the present at about 52u10 6 km 3 regional studies of sealevel change from observations near and within the margins of the former ice sheets constrain ice volumes of the individual ice sheets and have led to an imbalance between the global estimate of ice volume and the sum of the individual ice sheet volumes the latter estimates are reliable only when the observational record from localities close to the ice mass extends well into lateglacial times which is generally not the case for the major ice sheets ice volumes during the lgm and earliest part of the lateglacial period can therefore be substantially increased without affecting the predictions of lateglacial and postglacial sea level in a significant manner provided that a rapid reduction in ice volume occurred in early lateglacial time new far field data for lgm and lateglacial sea level change indicates that a rapid rise in sea level of about 15 m occurred at about 16 500 16 000 14 c or 19 200 18 700 calibrated years ago this leads to the inference that during the lgm the ice sheet volumes of the major ice sheets were greater than inferred from regional rebound analyses and that rapid reductions in volume occurred at the termination of the lgm the timing of this occurrence does not coincide with any recognised heinrich event although pulses of ice rafted debris originating from both northern ice sheets do occur in some high latitude cores an antarctic contribution to the post lgm melting event also cannot be ruled out the timing coinciding with evidence for the onset of warming in southern latitudes and the addition of meltwater into the southern ocean fl 2000 elsevier science b v all rights reserved

virtual shear box experiments of stress and slip cycling within a subduction interface melange abstract what role does the progressive geometric evolution of subduction related melange shear zones play in the development of strain transients we use a virtual shear box experiment based on outcrop scale observations from an ancient exhumed subduction interface the chrystalls beach complex cbc new zealand to constrain numerical models of slip processes within a meters thick shear zone the cbc is dominated by large competent clasts surrounded by interconnected weak matrix under constant slip velocity boundary conditions models of the cbc produce stress cycling behavior accompanied by mixed brittle viscous deformation this occurs as a consequence of the reorganization of competent clasts and the progressive development and breakdown of stress bridges as clasts mutually obstruct one another under constant shear stress boundary conditions the models show periods of relative inactivity punctuated by aseismic episodic slip at rapid rates meters per year such a process may contribute to the development of strain transients such as slow slip

decollement processes at the nankai accretionary margin southeast japan propagation deformation and dewatering the decollement zone expressed on seismic profiles and observed in drill cores from the nankai accretionary margin off the southeast coast of japan reveals several unique characteristics which appear to distinguish it from thrust faults identified in the same setting physical manifestations of these include evidence for the asymmetric distribution of deformation structures about the decollement the extension of this fault zone well in front of the tectonic deformation front and the absence of features indicative of precursory shear for example folded sediments shear bands and penetrative mineral fabrics these characteristics suggest that the mode of formation and evolution of this decollement zone may be unique from that of most thrust faults we propose that the decollement zone propagates not as a shear fracture controlled by tectonic stress conditions but rather as a subhorizontal tension fracture propagating under high pore pressures to test this possibility physical property measurements and clay mineral fabrics were obtained for several samples from the nankai decollement zone using computed tomography and x ray texture methods our findings suggest that deformation within the decollement zone is partitioned into a volumetric component preserved as reduced porosities within coherent fragments and a localized shear component evidenced by mineral preferred orientations along discrete slip surfaces we suggest that the reduced porosities result from the destruction of cementation in the sediments during the early stages of deformation and may arise from cyclic fatiguing of the sediment induced by fluctuating pore pressures the nonpenetrative shear fabrics probably develop as the tectonic deformation front migrates seaward and the weakened protodecollement subsequently accommodates shear displacements along discrete fractures

the thickness of subduction plate boundary faults from the seafloor into the seismogenic zone the thickness of an active plate boundary fault is an important parameter for understanding the strength and spatial heterogeneity of fault behavior we have compiled direct measurements of the thickness of subduction thrust faults from active and ancient examples observed by ocean drilling and fi eld studies in accretionary wedges we describe a general geometric model for subduction thrust decollements which includes multiple simultaneously active anastomosing fault strands tens of meters thick the total thickness encompassing all simultaneously active strands increases to 100 350 m at 1 2 km below seafl oor and this thickness is maintained down to a depth of 15 km thin sharp faults representing earthquake slip surfaces or other discrete slip events are found within and along the edges of the tens ofmeters thick fault strands although fl attening primary inherited chaotic fabrics and fault migration through subducting sediments or the frontal prism may build melange sections that are much thicker to several kilometers this thickness does not describe the active fault at any depth these observations suggest that models should treat the subduction thrust plate boundary fault as 1 20 cm thick during earthquakes with a concentration of postseismic and interseismic creep in single to several strands 5 35 m thick with lesser distributed interseismic deformation in stratally disrupted rocks surrounding the fault strands

mixed deformation styles observed on a shallow subduction thrust hikurangi margin new zealand geophysical observations show spatial and temporal variations in fault slip style on shallow subduction thrust faults but geological signatures and underlying deformation processes remain poorly understood international ocean discovery program iodp expeditions 372 and 375 investigated new zealand s hikurangi margin in a region that has experienced both tsunami earthquakes and repeated slow slip events we report direct observations from cores that sampled the active p paku splay fault at 304 m below the seafloor this fault roots into the plate interface and comprises an 18 m thick main fault underlain by 30 m of less intensely deformed footwall and an 10 m thick subsidiary fault above undeformed footwall fault zone structures include breccias folds and asymmetric clasts within transposed and or dismembered relatively homogeneous silty hemipelagic sediments the data demonstrate that the fault has experienced both ductile and brittle deformation this structural variation indicates that a range of local slip speeds can occur along shallow faults and they are controlled by temporal potentially far field changes in strain rate or effective stress

diverse slow slip behavior at the hikurangi subduction margin new zealand 1 the installation of a continuous gps cgps network in new zealand since 2002 has enabled the discovery of 15 slow slip events at the hikurangi subduction margin our analysis and interpretation of the cgps data reveal a marked diversity in characteristics of slow slip events sses in the north island with durations varying from 6 days to 1 5 years equivalent moment release between mw 6 3 7 2 and recurrence intervals of repeating sses on the order of 2 years to more than 5 years the duration and magnitude characteristics of hikurangi sses appear to be related to the depth where they occur the deepest longest duration and largest sses occur at the southern hikurangi margin near the downdip limit of deep down to 40 km depth strong interseismic coupling the shallowest shortest duration smallest and most frequent sses occur at the northern and central hikurangi margin near the downdip limit of unusually shallow 10 15 km depth interseismic coupling the direction of slip on the interface in the hikurangi sses is consistent with slip partitioning at the hikurangi subduction margin we also show that moment accumulation rates on the interface for the time periods between sses are 40 higher than for the period of more than a decade averaging through the sses indicating that sses compose a major portion of the overall moment release budget of the hikurangi subduction interface

structure and composition of the plate boundary slip zone for the 2011 tohoku oki earthquake the mechanics of great subduction earthquakes are influenced by the frictional properties structure and composition of the plate boundary fault we present observations of the structure and composition of the shallow source fault of the 2011 tohoku oki earthquake and tsunami from boreholes drilled by the integrated ocean drilling program expedition 343 and 343t logging while drilling and core sample observations show a single major plate boundary fault accommodated the large slip of the tohoku oki earthquake rupture as well as nearly all the cumulative interplate motion at the drill site the localization of deformation onto a limited thickness less than 5 meters of pelagic clay is the defining characteristic of the shallow earthquake fault suggesting that the pelagic clay may be a regionally important control on tsunamigenic earthquakes

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

dilatant strengthening as a mechanism for slow slip events 1 the mechanics of slow slip events sse in subduction zones remain unresolved we suggest that sse nucleate in areas of unstable friction under drained conditions but as slip accelerates dilatancy reduces pore pressure p quenching instability competition between dilatant strengthening and thermal pressurization may control whether slip is slow or fast we model sse with 2 d elasticity rate state friction and a dilatancy law where porosity evolves toward steady state ss over distance dc and ss 0 ln v v0 v is slip speed we consider two diffusion models membrane diffusion md is approximated by p p tf where p and p are shear zone and remote pore pressure and tf is a characteristic diffusion time homogeneous diffusion hd accurately models fault normal flow with diffusivity chyd for md linearized analysis defines a boundary 1 a b between slow and fast slip where f0e b p f0 a and b are friction parameters and is compressibility when 1 a b slip speeds remain quasi static for hd ep eh p defines dilatancy efficiency where h is shear zone thickness and v is plate velocity sse are favored by large eh and low effective stress the ratio ep to thermal pressurization efficiency scales with 1 p so high p favors sse consistent with seismic observations for ep 10 3 transient slip rates repeat times average slip and stress drops are comparable to field observations model updip propagation speeds are comparable to those observed along strike many simulations exhibit slow phases driven by steady downdip slip and faster phases that relax the accumulated stress model sse accommodate only a fraction of plate motion the remaining deficit must be accommodated during coseismic or postseismic slip

thickening of fault zones a mechanism of melange formation in accreting sediments sediments accreted at subduction zones undergo stratal disruption and form a type of melange the thickness of the disrupted zones grows with progressive deformation this suggests that initial fault surfaces are abandoned and deformation propagates into adjacent undeformed sediment factors causing the abandonment of fault surfaces during continuing deformation include 1 strengthening owing to porosity loss during consolidation 2 localized drops in fluid pressure on fault surfaces that act as dewaterinig conduits and 3 reorientation of fault surfaces the disruptive processes occurring in accretionary prisms result principally from the deformation of a consolidating sediment mass

geologic heterogeneity can produce aseismic slip transients 1 slow slip and tectonic tremor in subduction zones take place at depths where there is abundant evidence for distributed shear over broad zones 10 103 m composed of rocks with marked differences in mechanical properties here we model quasi dynamic rupture along faults composed of material mixtures characterized by different rate and state dependent frictional properties to determine the parameter regime capable of producing slow slip in an idealized subduction zone setting keeping other parameters fixed the relative proportions of velocity weakening vw and velocity strengthening vs materials control the sliding character stable slow or dynamic along the fault the stability boundary between slow and dynamic is accurately described by linear analysis of a double spring slider system with vw and vs blocks our results place bounds on the volume fractions of vw material present in heterogeneous geological assemblages that host slow slip and tremor in subduction zones

source parameters of large historical 1918 1962 earthquakes south island new zealand we have studied 18 earthquakes of m w 5 5 within the north island new zealand region to determine fault parameters for earthquake hazards and regional tectonic studies since most large m w 6 5 earthquakes in the north island occurred prior to 1961 these events provide important information to supplement studies of more recent seismicity our results indicate that no large plate interface earthquakes have occurred in the central and southern north island for the past 80 years although six strike slip earthquakes of m w 6 8 occurred within the australian upper plate during an extremely active 25 year period that began in 1917 only the m w 7 2 strike slip events were associated with surface faulting indicating the difficulties that may arise in attempting to identify active faults within this region two earthquakes m w 6 9 7 1 off the northeastern north island in 1947 appear to have occurred along the plate interface and were associated with local tsunamis having runup heights of up to 10 m two m w 6 8 events also occurred within the pacific lower plate highlighting the hazards related to intraslab events slip vectors for the earthquakes studied suggest that the majority of transcurrent motion along the plate margin is accommodated within the australian plate similar to the results obtained from studies of more recent smaller earthquakes pure thrusting occurs along the plate interface and t axes of intraslab events indicate downdip tension in the pacific plate

the brittle ductile transition in porous sedimentary rocks geological implications for accretionary wedge aseismicity abstract thrusting earthquakes in subduction zones generally occur along only part of the plate boundary with motion along the shallowest part of the plate boundary occurring ascismically the maximum size of subduction boundary thrust earthquakes depends strongly upon the down dip width of the seismogenic zone the single most uncertain factor in determining that width is the location of the up dip limit of the zone the seismic front which depends upon the mechanical state of the sedimentary rocks in the plate boundary zone in order to come to a better understanding of the seismic potential of sediments in a subduction zone we carried out a series of triaxial experiments on berea and kayenta sandstones based on our experimental data a brittle ductile transition map was constructed showing that both porosity and effective pressure are important factors controlling the transition from brittle to macroscopically ductile behavior in porous rocks in the brittle field a sample fails by shear localization on one slip plane accompanied by strain softening and dilatancy whereas in the ductile field a sample deforms homogeneously with a constant yield stress or slight hardening by comparing such a map with the estimated porosity profile of an accretionary wedge the likely nature and rough location of the boundary between brittle and ductile behavior can be inferred if the sediments along a plate boundary are too young and undercompacted to be capable of brittle shear localization then their deformation is likely to be aseismic in this way it may be possible for even a very broad fore arcs to produce no great earthquakes however great earthquakes are to be expected at margins that have large zones of plate contact along which many sediments are compacted and well lithified such rocks are expected to be capable of shear localization and brittle failure with the potential for stick slip behavior

slow and delayed deformation and uplift of the outermost subduction prism following ets and seismogenic slip events beneath nicoya peninsula costa rica abstract two odp cork ocean drilling program circulation obviation retrofit kit borehole hydrologic observatory sites deployed in 2002 at the toe of the subduction prism off nicoya peninsula costa rica were visited in december 2013 the five years of seafloor and formation fluid pressure data collected since the previous visit include clear signals associated with an episodic tremor and slip ets event off the coast of nicoya peninsula in 2009 and a mw 7 6 subduction thrust earthquake beneath the peninsula in 2012 formation pressure anomalies associated with the ets event are similar to ones observed following ets events observed previously here as well as ones following very low frequency earthquake swarms within the nankai accretionary prism off southwestern japan positive and negative impulsive transients in the hanging wall and foot wall of the subduction thrust respectively suggest contractional and dilatational strain generated by local slip propagating up the thrust fault beneath the outermost prism in the case of the 2009 event the transients occurred roughly two weeks after the initiation of slip observed at gps sites along the adjacent coast at the same time a decrease in seafloor pressure at the prism site relative to the subducting plate was observed indicating concurrent uplift of the prism of 1 2 cm other events at the prism toe following ets events closer to the coast are seen in 2006 2007 2008 2010 and 2011 the time between the initiation of ets slip constrained by gps and the onset of the prism toe transients suggest up dip rupture propagation along the seaward part of the subduction thrust at rates of a few km day in the case of the 2009 event the slip at the prism toe c 11 cm estimated from the 1 2 cm uplift and the local dip on the decollement 6 is roughly a factor of 5 greater than the slip further landward estimated from gps data by dixon et al in press in other cases slip at the toe is less or unresolvable similar uplift and formation fluid pressure transients occurred two days after the 2012 nicoya thrust earthquake then episodically unrelated to aftershocks every 1 3 weeks over the next few months slip rates at the prism toe estimated from the magnitude and duration of these and ets related uplift events are roughly 5 cm day such rates are much too slow for tsunamigenesis or seismogenesis total slip of the outermost prism after the 2012 earthquake determined from the uplift accumulated until the end of the observation period and the estimated local fault dip is roughly 75 cm a significant fraction of the seismogenic slip of the mw 7 6 earthquake itself a maximum of over 3 m negative steps in formation pressure are observed at both the prism and subducting plate sites simultaneous with the 2012 thrust earthquake the sign of these steps is inconsistent with the contractional deformation predicted by an elastic fault dislocation model with slip limited to the seismogenic zone highly compliant low shear modulus sediment below the decollement or very low stress deformation along splay faults may be responsible for the co seismic dilatation observed in the outermost prism and incoming plate

spontaneous and triggered aseismic deformation transients in a subduction fault model 1 aseismic deformation transients can emerge as a natural outcome of the rate and state friction processes revealed in laboratory fault sliding experiments when that constitutive formulation is applied to model subduction earthquake sequences transients can arise spontaneously for certain effective stress variations with depth we show that if interstitial fluids are present and pore pressure is near lithostatic around and downdip from the frictional stability transition transients with recurrence intervals of 1 year are predicted on the basis of laboratory friction parameters and their temperature hence depth variations the recurrence interval decreases with and reaches 14 months when is 2 3 mpa dimensional analysis and numerical studies show that the fault response primarily depends on a parameter w h here the high pore pressure zone extends distance w updip from the stability transition and h is the stable patch size for steady sliding evidence that such fluid conditions may actually be present is independently provided by the occurrence of nonvolcanic tremors as apparent responses to extremely small stress changes and by petrological constraints on expected regions of dehydration for the shallow dipping subduction zones where transients are observed transient sequences can also be triggered by a modest one time step like interseismic stress perturbation on the subduction fault due to nearby earthquakes or to pore pressure changes e g during episodes of metamorphic fluid release properties of triggered transients and future thrust earthquakes depend on the interseismic time when the perturbation is introduced its relative location along the subduction fault and its magnitude

hikurangi margin tsunami earthquake generated by slow seismic rupture over a subducted seamount abstract tsunami earthquakes generate much larger tsunami than their surface wave magnitude would suggest and are a problem for tsunami warning systems they are often not accompanied by intense or even strong ground shaking and hence do not provide a natural warning for self evacuation the lesser known 1947 offshore poverty bay and tolaga bay earthquakes along the east coast of the north island new zealand share many characteristics with other well known tsunami earthquakes including low amplitude shaking long durations and anomalously large tsunami however these two new zealand events are rare in that their source area has been imaged directly by long offset 2d seismic reflection profiles in this contribution we propose a source model for the 1947 offshore poverty bay tsunami earthquake recognising that the hypocentre occurs in a region where seismic reflection and magnetic data support the existence of a shallow m 7 earthquakes with complex low velocity rupture scenarios that enhance tsunami waves and their role in seismic hazard should not be under estimated

interactions between deformation and fluids in the frontal thrust region of the nantroseize transect offshore the kii peninsula japan results from iodp expedition 316 sites c0006 and c0007 integrated ocean drilling program iodp expedition 316 sites c0006 and c0007 examined the deformation front of the nankai accretionary prism offshore the kii peninsula japan in the drilling area the frontal thrust shows unusual behavior as compared to other regions of the nankai trough drilling results integrated with observations from seismic reflection profiles suggest that the frontal thrust has been active since 0 78 0 436 ma and accommodated 13 to 34 of the estimated plate convergence during that time the remainder has likely been distributed among out of sequence thrusts further landward and or accommodated through diffuse shortening unlike results of previous drilling on the nankai margin porosity data provide no indication of undercompaction beneath thrust faults furthermore pore water geochemistry data lack clear indicators of fluid flow from depth these differences may be related to coarser material with higher permeability or more complex patterns of faulting that could potentially provide more avenues for fluid escape in turn fluid pressures may affect deformation well drained sand rich material under the frontal thrust could have increased fault strength and helped to maintain a large taper angle near the toe recent resumption of normal frontal imbrication is inferred from seismic reflection data associated decollement propagation into weaker sediments at depth may help explain evidence for recent slope failures within the frontal thrust region this evidence consists of seafloor bathymetry normal faults documented in cores and low porosities in near surface sediments that suggest removal of overlying material overall results provide insight into the complex interactions between incoming materials deformation and fluids in the frontal thrust region

assembly of the siberian craton constraints from paleoproterozoic granitoids abstract the paper presents a synthesis of geochronological geochemical and isotopic data on paleoproterozoic granitoids in the siberian craton and in some cases on volcanics related to these granitoids different evolution stages of the craton including its assembly are recorded in several major events in the history of paleoproterozoic granitic magmatism they are 2 52 2 40 ga and 2 15 2 04 ga granitoids of different tectonic settings 2 06 2 00 ga subduction related granitoids 2 00 1 87 ga collisional granitoids 1 88 1 84 postcollisional granitoids and 1 76 1 71 ga within plate anorogenic varieties granitoids with ages of 2 5 2 4 ga and 2 15 2 04 ga are distributed locally in separate blocks and terranes which later entered the craton structure these rocks are of different types and represent different tectonic settings of the respective blocks and terranes i e the siberian craton did not exist yet as a single unit at that time the 2 06 2 00 ga subduction related granitoids and volcanics are found in the southern and southeastern craton parts magmatism of that time interval probably was associated with subduction beneath the archean olekma aldan and anabar continental microplates and with the formation of their active margins granitoids of the 2 00 1 87 ga age interval represent a collisional stage of the craton evolution collisional granitoids that intruded between 2 00 and 1 95 ga record the first large scale stage of craton assembly with collisions of terranes that had built the core of the anabar aldan and olenek superterranes the intrusions of 1 95 1 90 ga granitoids mark the consolidation of the southeastern craton part collisional granitoids with the 1 90 1 87 ga ages which are especially abundant in the south and southwest of the craton but almost lack from the north led to general assembly of the craton the granitoid and volcanic magmatic activity of 1 88 1 84 ga in the craton south produced the south siberian postcollisional magmatic belt that had completed the assembly of the craton which in its turn may have been part of the paleoproterozoic supercontinent of columbia granitic intrusions at 1 76 1 71 ga are limited to the southwestern and southeastern craton parts and correspond to a setting of continental extension which never led to the breakup of the craton

structure inferred mechanical properties and implications for fluid transport in the decollement zone costa rica convergent margin faults in a variety of tectonic settings can act as both conduits for and barriers to fluid flow sometimes simultaneously documenting the interaction between hydrologic and tectonic processes in active faults in situ is the key to understanding their mechanical behavior and large scale fluid transport properties we present observations of the plate boundary decollement zone at the middle america trench off costa rica showing that it is structurally divisible into an upper brittle fracture dominated domain overlying a lower ductile domain pore water geochemical evidence shows that along fault flow is occurring specifically in the upper brittle domain but is hydrologically isolated from fluids in the underlying footwall sediments we propose a model for the mechanics of these contrasting domains in which differing stress paths coexist in the upper and lower parts of the decollement zone the data suggest a mechanically controlled permeability anisotropy at a scale of several meters to 10 m across the decollement zone this documentation of separate yet simultaneously active mechanical and hydrologic subregimes within a decollement provides a relatively simple explanation for enhanced along fault permeability coexisting with reduced cross fault permeability without requiring matrix scale permeability anisotropy

low frequency tremors associated with reverse faults in a shallow accretionary prism abstract the shallow part of a seismogenic subduction zone is thought to play an important role in tsunami genesis during large interplate thrust earthquakes near the updip portion of the seismogenic zone along the nankai trough splay faults which are major active reverse faults in the accretionary prism likely rupture during large interplate earthquakes such as the 1944 m w 8 2 tonankai earthquake off kii peninsula we observed low frequency tremors associated with reverse faults in a shallow accretionary prism off kii peninsula at close range by using ocean bottom seismographs the tremors were characterized by a dominant frequency range of 2 8 hz and a lack of energy in the frequency range above 10 hz their duration ranged from tens of seconds to a few minutes their source locations lay in three clusters on the landward slope of the nankai trough activity in each cluster continued for from one day to two weeks many of the low frequency tremors were located near the shallowest part of a major splay fault the episodic activity of low frequency tremors and of previously reported very low frequency earthquakes which indicate reverse faulting in the shallow accretionary prism suggests that the reverse faults in the accretionary prism are conditionally stable faults that can become unstable under sufficiently strong dynamic loading such as that caused by a large earthquake such reverse faults as typified by a splay fault can rupture during large interplate earthquakes and generate large tsunamis