a physical basis for the frictional melting of some rock forming minerals abstract under conditions of coseismic slip 0 1 2 0 m s 1 within the upper crust differences in the shear yield strengths fracture toughnesses melting points and to a lesser extent thermal conductivities of rock forming minerals result in a hierarchy of friction melting susceptibilities in many so called pseudotachylytes it is observed that phyllosilicates and amphiboles are preferentially consumed to form the melt phase whilst quartz and feldspar tend to survive as clasts this can be explained in terms of the disequilibrium flash melting of those mineral phases possessing the lower shear yield strengths fracture toughnesses and thermal conductivities rather than by eutectic melting that would otherwise generate equilibrium minimum melts indicative of igneous processes at high strain rates typical of coseismic faulting the mechanical behaviour of minerals is primarily controlled by their fracture toughnesses with progressive comminution during slip leading to fusion the friction melting dependence on the mechanical properties of minerals indicates that there may be an important relationship between wall rock lithology and earthquake magnitude if frictional melting results in fault lubrication

a continuum of stress strength and slip in the cascadia subduction zone movement of the down going oceanic plate in subduction zones is accommodated by earthquakes slow slip and free slip with increasing depth analysis of accompanying tremor reveals a continuum of slow slip events in the cascadia subduction zone which suggests that deep free slip of the subducted plate may cause stress to be gradually transferred up the plate interface towards the seismogenic zone

magnetotelluric imaging of fluid processes at the subduction interface of the hikurangi margin new zealand 1 the subduction interface beneath the northern part of the hikurangi subduction margin on the east coast of new zealand s north island is exceptionally shallow and cool compared with other margins where slow slip has been observed here we use magnetotelluric data to show that a marked decrease in the conductivity of the fore arc sediments coincides with the onset of seismicity at 10 km depth below the sediments a dipping band of seismicity and intermediate conductivity at the subduction interface connects to a deeper more conductive zone above the down going plate this deeper conductive zone is interpreted to be under plated sediments these results and results from previous seismic tomography in the area suggest that the intermediate resistivity zone represents a region of upward fluid transport near the plate interface followed by fluid escape into the upper plate

geological evidence for past large earthquakes and tsunamis along the hikurangi subduction margin new zealand abstract the hikurangi subduction margin new zealand has not produced large subduction earthquakes within the short written historic period 180 years and the potential of the plate interface to host large m 7 to great m 8 earthquakes and tsunamis is poorly constrained the geological record of past subduction earthquakes offers a method for assessing the location frequency and approximate magnitude of subduction earthquakes to underpin seismic and tsunami hazard assessments we review evidence of holocene coseismic coastal deformation and tsunamis at 22 locations along the margin a consistent approach to radiocarbon age modelling is used and earthquake and tsunami evidence is ranked using a systematic assessment of the quality of age control and the certainty that the event in question is an earthquake to identify possible subduction earthquakes we use temporal correlation of earthquakes combined with the type of earthquake evidence likely primary fault source and the earthquake certainty ranking we identify 10 past possible subduction earthquakes over the past 7000 years along the hikurangi margin the last subduction earthquake occurred at 520 470 years bp in the southern hikurangi margin and the strongest evidence for a full margin rupture is at 870 815 years bp there are no apparent persistent rupture patches suggesting segmentation of the margin is not strong in the southern margin the type of geological deformation preserved generally matches that expected due to rupture of the interseismically locked portion of the subduction interface but the southern termination of past subduction ruptures remains unresolved the pattern of geological deformation on the central margin suggests that the region of the interface that currently hosts slow slip events also undergoes rupture in large earthquakes demonstrating different modes of slip behaviour occur on the central hikurangi margin evidence for subduction earthquakes on the northern margin has not been identified because deformation signals from upper plate faults dominate the geological record large uncertainties remain in regard to evidence of past subduction earthquakes on the hikurangi margin with the greatest challenges presented by temporal correlation of earthquake evidence when working within the uncertainties of radiocarbon ages and the presence of upper plate faults capable of producing deformation and tsunamis similar to that expected for subduction earthquakes however areas of priority research such as improving the paleotsunami record and integration of submarine turbidite records should produce significant advances in the future

fluid and melt pathways in the central chilean subduction zone near the 2010 maule earthquake 35 36 s as inferred from magnetotelluric data

continental crust composition constrained by measurements of crustal poisson s ratio deciphering the geological evolution of the earth s continental crust requires knowledge of its bulk composition and global variability the main uncertainties are associated with the composition of the lower crust seismic measurements probe the elastic properties of the crust at depth from which composition can be inferred of particular note is poisson s ratio this elastic parameter can be determined uniquely from the ratio of p to s wave seismic velocity and provides a better diagnostic of crustal composition than either p or s wave velocity alone1 previous attempts to measure have been limited by difficulties in obtaining coincident p and s wave data sampling the entire crust2 here we report 76 new estimates of crustal spanning all of the continents except antarctica we find that on average increases with the age of the crust our results strongly support the presence of a mafic lower crust beneath cratons and suggest either a uniformitarian craton formation process involving delamination of the lower crust during continental collisions followed by magmatic underplating or a model in which crust formation processes have changed since the precambrian era

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

imaging surface contacts power law contact distributions and contact stresses in quartz calcite glass and acrylic plastic a procedure has been developed to obtain microscope images of regions of contact between roughened surfaces of transparent materials while the surfaces are subjected to static loads or undergoing frictional slip static loading experiments with quartz calcite soda lime glass and acrylic plastic at normal stresses to 30 mpa yield power law distributions of contact areas from the smallest contacts that can be resolved 3 5 m2 up to a limiting size that correlates with the grain size of the abrasive grit used to roughen the surfaces in each material increasing normal stress results in a roughly linear increase of the real area of contact mechanisms of contact area increase are by growth of existing contacts coalescence of contacts and appearance of new contacts mean contacts stresses are consistent with the indentation strength of each material contact size distributions are insensitive to normal stress indicating that the increase of contact area is approximately self similar the contact images and contact distributions are modeled using simulations of surfaces with random fractal topographies the contact process for model fractal surfaces is represented by the simple expedient of removing material at regions where surface irregularities overlap synthetic contact images created by this approach reproduce observed characteristics of the contacts and demonstrate that the exponent in the power law distributions depends on the scaling exponent used to generate the surface topography

geometry of the hikurangi subduction thrust and upper plate north island new zealand 1 we use 2800 line km of seismic reflection data to map the offshore character and geometry of the hikurangi subduction thrust and outer forearc wedge to depths of 15 km for 200 km along strike south of hawke bay the subduction thrust is relatively smooth dips less than 8 and the wedge is characterized by accretion of young sediment and topographic slopes of less than 3 in hawke bay and north for 200 km a kink in the subduction thrust is apparent with a downdip increase in dip to angles greater than 8 at depths of 10 15 km there is a corresponding steepening of the topographic slope to greater than 3 outboard of the kink and the wedge is characterized by lithified sedimentary rock and slope failure the kink in the subduction thrust is a locus of inherent weakness in the subducting slab we suggest its occurrence relates to a northward increase in subduction rate that controls initial slab dehydration and fluid release rates and hence intraslab deformation patterns the subduction thrust geometry in combination with a northward increase in subducting plate roughness and decrease in the amount of sediment accreted causes the observed spatial change in character of the subduction thrust and forearc wedge

the role of metasomatic alteration on frictional properties of subduction thrusts an example from a serpentinite body in the franciscan complex california abstract fluid rock interaction within accretionary prisms drastically changes the frictional strength and slip stability of the fault zone in order to understand the effect of ultramafic components on the degree of interplate coupling we present data on frictional properties of a reaction zone between serpentinite and argillite in a tectonic melange of the franciscan complex central california field and petrographic observations indicate that the argillite and serpentinite along the lithological boundary are metasomatized to tremolite and talc schists respectively forming a ductile shear zone simulated gouges made from fault and wall rock samples collected are sheared at effective normal stresses n eff of 60 180 mpa and temperatures t of 20 400 c using a hydrothermal ring shear machine the serpentinite is frictionally strong steady state friction coefficient ss 0 5 0 6 and exhibits both velocity weakening and velocity strengthening behavior the ss of the argillite and tremolite schist increases up to 0 7 and 0 6 respectively with increasing n eff and t while a b transitions from positive to negative with increasing t at a given experimental condition and a b values are lower and higher for the tremolite schist than for the argillite in contrast the talc schist exhibits very low friction ss 0 1 0 2 and velocity strengthening behavior at all experimental conditions tested our results demonstrate that si metasomatism along the argillite serpentinite contact results in reduced friction and stabilized slip forming frictionally weak velocity strengthening regions in the megathrust zone

linking hematite u th he dating with the microtextural record of seismicity in the wasatch fault damage zone utah usa techniques directly dating fault slip are few limiting the ability to interpret the rock record of seismicity hematite is commonly found in fault zones amenable to u th he dating and slip surface hematite may be reset by shear heating events and or recrystallization glossy hematite coated fault surfaces in the wasatch fault footwall damage zone utah usa exhibit evidence of hematite cataclasis and preserve pliocene hematite u th he dates apatite u th he and fission track data from the host gneiss indicate footwall unroofing through 2 km by ca 4 5 ma internally reproducible but disparate hematite u th he dates 4 5 ma and younger from isolated locations on a single fault surface do not reflect ambient cooling we hypothesize that these dates and associated iridescence and annealed crystal texture document rapid cooling from friction generated heat during small seismic slip events between ca 4 5 and 2 5 ma thus hematite u th he dating offers the potential to decipher thermal anomalies in the rock record associated with slip on 10 5 10 6 yr time scales

a comment on the form of the geometrical spreading equations with some numerical examples of seismic ray tracing in inhomogeneous anisotropic media summary we point out that while the equations of seismic ray geometrical spreading given recently by norris do differ as stated from equations given by cerveny it does not imply that the latter equations are wrong the two sets of equations differ only in form and in a way which in part at least can be ascribed to different choices of hamiltonian by the two authors quantitatively though the two sets of equations are entirely equivalent we also present some numerical results of ray tracing in anisotropic models simulating a continential rift a spreading ridge and a subduction zone these three structures span a range of geological mechanisms for seismic anisotropy though definitive conclusions cannot be easily drawn when there is both anisotropy and inhomogeneity the results do indicate the magnitude of ray path travel time and amplitude variations to be expected for p waves when anisotropy is introduced

mapping fluids to subduction megathrust locking and slip behavior in subduction zones high fluid content and pore pressure are thought to promote aseismic creep whereas well drained conditions are thought to promote locking and failure in earthquakes however observations directly linking fluid content and seismic coupling remain elusive heise et al 2017 use a magnetotelluric survey to image the electrical resistivity structure of the northern hikurangi subduction thrust to 30 km depth as an indicator of interconnected fluid content the authors document a clear correlation between high resistivity and a distinct geodetically locked patch and between conductive areas and weak coupling their study together with other recent geophysical investigations provides new evidence for the role of fluids in governing subduction thrust locking

seismic evidence for a fossil mantle plume beneath south america and implications for plate driving forces a teleseismic travel time study reveals the presence of a fossil plume in the deep upper mantle beneath brazil which has apparently remained geographically fixed with respect to the overly ing continent despite thousands of kilometres of plate motion this result implies that the upper mantle and lithosphere beneath south america have remained coupled since the breakup of gondwanaland and may provide an answer to the long standing question of what forces drive continental plates

the influence of ambient fault temperature on flash heating phenomena 1 recent high velocity friction experiments on room temperature rocks reveal a dramatic weakening caused by extreme frictional heating and thermal degradation of the strength of microscopic asperity contacts on a slip surface i e flash heating here we explore the effects of elevated ambient fault temperature up to that at the base of the earth s seismogenic zone on the flash heating behavior of quartzite westerly granite and india gabbro the experiments demonstrate that elevated ambient fault temperature causes 1 an increase in the characteristic weakening velocity for flash heating and 2 an increase in the friction coefficient at a given velocity above this characteristic weakening velocity these unexpected results are reconciled with flash heating theory when temperature specific values of various material parameters are incorporated in existing constitutive descriptions of flash heating our results suggest that constitutive equations for flash heating can be extrapolated with confidence to the elevated ambient temperatures of the seismogenic zone

late cretaceous india madagascar fit and timing of break up related magmatism a u pb zircon age of 91 2 0 2 myr from western india st mary islands confidently links india with the late cretaceous magmatic province in madagascar 84 92 ma and the u pb age is within analytical error of the u pb age of the analalava gabbro pluton 91 6 0 3 myr in northeastern madagascar palaeomagnetic data from india and madagascar allow us to postulate a new india madagascar fit euler latitude 14 24 longitude 38 8 and rotation angle 69 2 this fit is applicable to the late cretaceous directly prior to and during the early phase of madagascar india separation in our late cretaceous reconstruction south west india runs roughly subparallel with the first known break up related magnetic anomaly a34 it maintains a close connection between mada gascar and india but places india slightly rotated compared to the eastern margin of madagascar and more northerly compared with some reconstructions st mary magmatism is linked to the initial break up between india and madagascar and magmatism probably resulted from rift related extensional processes initially induced by the marion hotspot underlying southern madagascar during the late cretaceous

late glacial to post glacial sea levels in the western indian ocean late glacial to post glacial sea level changes provide direct evidence of the progress of melting of large ice sheets during the last deglaciation but although the correlation between ice and ocean volumes is incontrovertible the causal link is commonly obscured local effects including tectonics isostatic and hydroisostatic responses and equatorial ocean syphoning impose additional signals that hide the true picture a detailed regional study of the western indian ocean based on the analysis of drill cores carried out through modern reefs in combination with observations and sampling of reef foreslopes and investigations of outcrops provides a comprehensive data base sites from a range of tectonic settings include the microcontinental margins of madagascar the granitic seychelles and the isolated volcanic islands of reunion mauritius and the comoros in which the effects of subsidence can be shown to be small these cover a range of latitudes and comparisons with adjacent sites on continental margins allow the construction of sea level curves that closely reflect the eustatic response and disengage this from the effects of other mechanisms the mayotte foreslope in the comoro islands provides the first coral reef record of sea level change during the early deglaciation in the indian ocean 110 115 m below present sea level between 18 000 and 17 000 yr bp two distinctive reef terraces at 90 and 60 m water depth are dated at 13 600 yr bp and partly attributed to the younger dryas period 12 700 11 600 cal yr bp reef drowning at around 13 500 yr bp may correspond to meltwater pulse 1a and although there were surges in the rate of sea level rise most notably between 11 950 and 11 350 yr bp there is little evidence to support a well defined meltwater pulse 1b reconstructed holocene sea level curves are in good agreement and reflect a rapid sea level rise of about 6 mm yr 1 between 10 000 and 7500 yr bp followed by a clear inflection around 7500 yr bp when the rate fell to 1 1 mm yr 1 modern reefs started to grow 8000 9000 years ago in the post glacial period the rate of sea level rise was 1 1 5 mm yr 1 before stabilization at its present level 3000 2500 years ago curves for the 10 000 6000 yr 1 bp interval correspond closely with those predicted by theoretical models but lie below these in the subsequent period in particular and with the exception of the margins of the madagascar microcontinent influenced by hydroisostatic processes they do not reflect predicted higher sea level stands during the late holocene

three dimensional velocity structure of the northern hikurangi margin raukumara new zealand implications for the growth of continental crust by subduction erosion and tectonic underplating traveltimes between shots from nine marine seismic reflection lines and nine onshore recorders were used to construct a 3 d p wave velocity model of the northern hikurangi subduction margin new zealand from north to south between raukumara basin and raukumara peninsula the moho of the overriding plate increases in depth from 17 to similar to 35 km low seismic p wave velocities of 3 5 5 0 km s are localized within a similar to 10 km thick prism in the lower crust of the overriding plate immediately updip of the intersection between the subduction thrust and moho and beneath the topographic crest of east cape ridge and the raukumara range southward this region of low seismic velocities and surface uplift increases in distance from the trench as the thickness of the crust in the overriding plate increases we interpret this low velocity volume to be underplated sedimentary rocks and crustal materials that were tectonically eroded by subduction beneath the trench slope the buoyancy and low strength of these subducted materials are proposed to assist the escape from a subduction channel near the base of the crust and drive local rock uplift in the middle crust our observations of very low velocity suggest high fluid filled porosities of 12 18 and the implied buoyancy anomaly may enhance underplating at greater depths the process is driven by the contrast between upper crustal quartz feldspar mineralogy and the denser diabase or olivine rich lithologies of the lower crust and mantle we estimate a rate of lower crustal underplating at the northern hikurangi margin of 20 7 km 3 ma 1 km 1 since 22 ma we suggest that underplating provides an efficient means of accreting subducted sediment and tectonically eroded material to the lower crust and that the flux of forearc crustal rocks into the mantle at subduction zones may be systematically overestimated

slickenline orientations as a record of fault rock rheology abstract contact of geometric asperities across rough faults causes perturbations to the shear traction resolved on a fault surface that could potentially deflect the local slip direction slickenlines which record the relative displacement of opposing sides of a fault may therefore be sensitive to fault surface geometry to investigate the relationship between fault geometry shear traction and slip we use ground based lidar to measure the orientations of slickenlines defined by centimeter amplitude corrugations on three fault surfaces slickenline rakes measured in the mean plane of each fault rarely vary by more than a few degrees deviations from the mean rake do not correlate with the fault surface topography at scales up to a few meters but a weak correlation may be present at larger scales slip directions are therefore insensitive to any shear traction perturbations from the contact of geometric asperities at small scales this observation is consistent with scale dependent deformation we show that the roughness of fault surfaces implies that short wavelength asperities fail inelastically because of flattening during fault slip a crossover to elastic deformation occurs at lengths of 10 2 to 10 0 m which defines the minimum dimension of a strength asperity that influences the displacement field the roughness at the crossover length scale corresponds to the typical thickness of fault rocks in the fault core suggesting that fault rock thickness is related to the crossover length scale the data requires that multiple processes combine to produce the fault surface geometry we reconcile the variety of processes with the consistent surface roughness scaling by noting that all of the processes are governed in different ways by the elastic limit of rock

pseudotachylyte controversy fact or friction high speed slip experiments performed on westerly granite using friction welding apparatus reveal that comminution is an essential precursor to melting by friction observations of slip surfaces via analytical scanning electron microscopy sem document the following sequence of events occurring in 2 s with increasing velocity up to 2 m s fracture progressive comminution surface melting of mineral fragments fragment to fragment adhesion and finally production of a fragment laden melt supported suspension explosive dehydration and melting of the epidote group mineral allanite indicates that temperatures of at least 1000 c were realized at the interface this is corroborated by calculation of the temperature rise for the known operating conditions contrary to earlier proposals these results show that comminution and frictional melting are complementary and not mutually exclusive processes depending on the velocity shear stress displacement relations prevailing during frictional slip rocks produced in seismogenic zones can be predominantly comminuted wall rock or fragment melt mixes pseudotachylytes

evaluation of asperity scale temperature effects during seismic slip abstract for the common rock forming minerals the joule thompson effect produces cooling on compression and heating on decompression experimental rock friction studies and seismological evidence suggest that the strength of faults is controlled by high stress asperities that represent a small fraction of the total fault area during seismic rupture the area immediately beneath asperities is inferred to undergo adiabatic compressions and decompressions on the order of 1 2 8 gpa joule thompson coefficients are in the range 261 372 c gpa resulting in temperature changes of 336 880 c quartz asperities with a compressive strength of 2 8 gpa can produce a heating or cooling of 880 c immediately beneath an asperity by comparison frictional heating flash temperatures are less than 1000 c for asperity contact areas 2 the joule thompson effect is independent of asperity size and may augment or counteract frictional heating the paucity of pseudotachylytes in the geologic record and the absence of localized heat flow anomalies on active faults might be explained by the joule thompson cooling effect

the rapid drift of the indian tectonic plate the breakup of the supercontinent gondwanaland into africa antarctica australia and india about 140 million years ago and consequently the opening of the indian ocean is thought to have been caused by heating of the lithosphere from below by a large plume whose relicts are now the marion kerguelen and reunion plumes plate reconstructions based on palaeomagnetic data suggest that the indian plate attained a very high speed 18 20 cm yr 1 during the late cretaceous period subsequent to its breakup from gondwanaland and then slowed to approximately 5 cm yr 1 after the continental collision with asia approximately 50 myr ago the australian and african plates moved comparatively less distance and at much lower speeds of 2 4 cm yr 1 refs 3 5 antarctica remained almost stationary this mobility makes india unique among the fragments of gondwanaland here we propose that when the fragments of gondwanaland were separated by the plume the penetration of their lithospheric roots into the asthenosphere were important in determining their speed we estimated the thickness of the lithospheric plates of the different fragments of gondwanaland around the indian ocean by using the shear wave receiver function technique we found that the fragment of gondwanaland with clearly the thinnest lithosphere is india the lithospheric roots in south africa australia and antarctica are between 180 and 300 km deep whereas the indian lithosphere extends only about 100 km deep we infer that the plume that partitioned gondwanaland may have also melted the lower half of the indian lithosphere thus permitting faster motion due to ridge push or slab pull

textural equilibria of iron sulfide liquids in partly molten silicate aggregates and their relevance to core formation scenarios the influence of the coexistence of silicate and iron sulfide melts on the wetting of mantle olivine by iron sulfide melt is investigated the dihedral angle between iron alloy and olivine is unaffected by the presence of silicate melt at 1 5 gpa and 1370 1410 c and is much greater than the maximum allowable dihedral angle for melt connectivity at low melt fractions in experimental charges where silicate and sulfide melt occupy the same melt pocket the sulfide melt adheres to olivine grain faces rather than forming discrete droplets the relative interfacial energy between iron sulfide and basaltic silicate melt is lower than that of iron sulfide with olivine but the contrast is not sufficient for basaltic silicate melt to wet fes olivine grain boundaries this result implies that very large silicate melt fractions are necessary to mobilize iron sulfide melts in partially molten silicate phase assemblages therefore segregation of fe rich metallic melts by porous flow at low melt fraction is not a likely process for metal separation the experiments also investigated the influence of the presence of a thermal gradient on sulfide melt migration although the sulfide melt can not migrate through the olivine network by porous flow evidence in the experiments indicates mobility of sulfide through the interconnected silicate melt network the sulfide melt pools in contact with silicate melt at the hot top part of the experimental charges grow over time at the expense of the silicate melt pools in the cooler part of the charge this points to a diffusive transport mechanism for local sulfide redistribution we review literature data on metal olivine wetting angles and find that the metal composition has a dominant influence on the wetting behavior of a metallic liquid on an olivine matrix iron rich metallic liquid compositions are characterized by large dihedral angles 90 the metal olivine dihedral angle decreases with increasing amounts of dissolved light element s c o for light element contents in excess of 59 at the dihedral angle approaches 60 and the wetting behavior of a metallic liquid is similar to that of a silicate liquid

a three dimensional image of shallow subduction crustal structure of the raukumara peninsula new zealand summary earthquake arrival time data from a 36 station deployment of portable seismographs on the raukumara peninsula have been used to determine the 3 d vp and vp vs structure of this region of shallow subduction a series of inversions have been performed starting with an inversion for 1 d structure then 2 d and finally 3 d this procedure ensures a smooth regional model in places of low resolution the subducted plate is imaged as a northwest dipping feature with vp consistently greater than 8 5 km s 1 in the uppermost mantle of the plate structure in the overlying plate changes significantly along strike in the northeast there is an extensive low velocity zone in the lower crust underlying the most rapidly rising part of the raukumara range it is bounded on its arcward side by an upwarp of high velocity a viable explanation for the low velocity zone is that it represents an accumulation of underplated subducted sediment while serpentinization of the uppermost mantle may be responsible for the adjacent high velocity region the low velocity zone decreases and the adjacent high velocity region is less extensive in the southwest this change is interpreted to be related to a change in the thickness of the crust of the overlying plate in the northeast the crust is thinner and subducted sediment ponds against relatively strong uppermost mantle while in the southwest the crust is thicker and the relatively weak lower crust allows sediment subduction to greater depths a narrow zone of high vp vs parallels the shallow part of the plate interface this suggests elevated fluid pressures with the distribution of earthquakes about this zone further suggesting that these pressures may be close to lithostatic the plate interface at 20 km depth beneath the raukumara peninsula may thus be a closed system for fluid flow similar to that

hot faults iridescent slip surfaces with metallic luster document high temperature ancient seismicity in the wasatch fault zone utah usa and localized iridescence in the footwall dam age zone of the wasatch fault utah usa and document elevated temperatures on these faults we propose that the formation of these iridescent slip surfaces requires frictional heat generated at geometric asperities at seismic slip rates wasatch fault zone abstract we document new geological indicators of ancient seismicity in the form of highly reflec tive iridescent hematite coated fault surfaces small faults that cut the paleoproterozoic farmington canyon complex in the footwall damage zone of the brigham city segment of the wasatch fault utah usa are smooth to striated surfaces tens of square centimeters to 30 m 2 in area the dull rusty to high metallic luster and moderate to high gloss surfaces exhibit multicolored elliptical iridescent patches 0 5 3 cm across preexisting hematite crystals were deformed during slip on 1 200 mm thick slip surfaces textural observations x ray diffraction x ray photoelectron spectroscopy electron backscattered diffraction analysis surface metrology and similarity to experimentally formed iridescent spots in rocks and metals indicate that iridescence is associated with high temperature 300 c reduction of iron fe 3 to fe 2 and associated conversion of hematite to magnetite we pro pose that the iridescent slip surfaces in the wasatch fault damage zone are the result of seismic slip and flash heating at asperities along the small faults the thousands of these sur faces represent coseismic or aftershock deformation down to magnitude 3 in the exhumed footwall damage zone of the wasatch fault

crustal heterogeneity and subduction processes 3 d vp vp vs and q in the southern north island new zealand summary along the southern hikurangi subduction zone in the southern north island new zealand the subduction zone changes from volcanic with moderately weak interseismic coupling to non volcanic with strong coupling in wellington to investigate how material heterogeneity relates to subduction processes we compute 3 d seismic properties using 342 earthquakes and 2 shots recorded by a 165 station seismic array the time base for some telemetered temporary stations was not stable which required revising the inversion code to allow an additional origin time related to the telemetered stations progressive inversions from 2 d to fine 3 d were carried out for vp vp vs and qp combined interpretation of these three seismic properties highlights features of the subduction zone the subducted slab is imaged as a high velocity high q feature in the overlying crust from southeast to northwest there is low velocity in the accretionary wedge high velocity in the axial ranges and low velocity in the wanganui basin high vp vs and low qp characterize the crust east of the north island dextral fault belt high vp vs is imaged above the subducted slab to 40 km depth in the northern half of the study area low qp is also prominent above the slab particularly trenchwards of the volcanic region in marked contrast to the high vp vs the low qp above the slab extends as a continuous feature up to the low qp in the shallow crust underlying the volcanic region the low qp and high vp vs are correlated with the region of weaker coupling and are consistent with significant subducted sediments with associated fluid release vp vs is particularly sensitive to overpressured fluid in the compressional forearc whereas qp also images the hydrostatically pressured fluid transport in the more permeable extensional volcanic region the rheology of the lower crust of the overlying plate is a factor in large scale subduction coupling as evidenced by the terrane boundaries and haast schist in the section near wellington the changing rheology of the lower crust of the overlying plate in the southern north island also provides a possible explanation for the southward cessation of volcanism

factors influencing magmatism during continental breakup new insights from a wide angle seismic experiment across the conjugate seychelles indian margins we present a model of the northern seychelles continental margin derived from controlled source wide angle seismic traveltime inversion and teleseismic receiver functions this margin has been widely cited as a classic example of rifting in association with a continental flood basalt province the deccan traps however we do not find the typical set of geophysical characteristics reported at other margins linked to continental flood basalts such as those of the north atlantic the oceanic crust formed immediately after breakup and throughout the first 3 ma of seafloor spreading is just 5 2 km thick less than half that typically seen at other volcanic margins the continent ocean transition zone is narrow and while two packages of seaward dipping reflectors are imaged within this transition they are weakly developed beneath the thinned continental crust there is an approximately 4 km thick layer of high velocity material 7 5 7 8 km s that we interpret as mafic material intruded and underplating the lower crust however we believe that this underplating most likely happened prior to the breakup overall the observations show that the rifting of india from the seychelles was characterized by modest magmatism the spatial extent of the deccan flood basalt province is therefore smaller than previously thought we speculate that either the lateral flow of deccan related hot material beneath the breakup region was hampered perhaps as the rifted margins did not intersect the center of the deccan source or there was incomplete melt extraction from the wide melting region that formed between the rapidly diverging plates if the latter explanation is correct then the rate of plate separation as indicated by the initial seafloor spreading rate is more important in controlling the volume of magmatism generated during continental rifting than has been previously recognized

dynamical weakening of faults by acoustic fluidization when a fault slips seismically some of the energy released may excite strong short wavelength vibrations near the fault core such vibrations can temporarily reduce the normal stress on the fault allowing it to slip at lower shear stresses than predicted by laboratory coefficients of friction this phenomenon of acoustic fluidization provides an alternative to theories that invoke pressurized fluids as an explanation for why some faults appear to be so weak

pseudotachylite from the main zone of the hidaka metamorphic belt hokkaido northern japan pseudotachylite veins have been found in the mylonite zone of the hidaka metamorphic belt hokkaido northern japan they are associated with faults with wnw ese to ene wsw or ne sw trends which make a conjugate set cutting foliations of the host mylonitic rocks with high obliquity the mylonitic rocks comprise greenschist facies to prehnite pumpellyite facies mineral assemblages the mode of occurrence of the pseudotachylite veins indicates that they were generated on surfaces of the faults and were intruded as injection veins along microfractures in the host rocks during brittle deformation in near surface environments an analysis of the deformational and metamorphic history of the hidaka main zone suggests that the ambient rock temperature was 200 300 c immediately before the formation of the hidaka pseudotachylite three textural types of veins are distinguished cryptocrystalline microcrystalline and glassy the cryptocrystalline or glassy type often occupies the marginal zones of the microcrystalline type veins the microcrystalline type is largely made up of quench microlites of orthopyroxene clinopyroxene biotite plagioclase and opaque minerals with small amounts of amphibole microlites the interstices of these microlites are occupied by glassy and or cryptocrystalline materials the presence of microlites and glasses in the pseudotachylite veins suggests that the pseudotachylites are the products of rapid cooling of silicate melts at depths of less than 5 km the bulk chemical composition of the pseudotachylite veins is characterized by low sio2 and a high water content and is very close to that of the host mylonitic rocks this indicates that the pseudotachylite was formed by virtual total melting of the host rocks with sufficient hydrous mineral phases local chemical variation in the glassy parts of the pseudotachylite veins may be due to either crystallization of quench microlites or the disequilibrium nature of melting of mineral fragments and incomplete mixing of the melts pyroxene microlites show a crystallization trend from hypersthene through pigeonite to subcalcic augite with unusually high al contents the presence of pigeonite and high al pyroxene microlites of hornblende and biotite microlites and rare plagioclase microlites may indicate the high temperature and high water content of the melt which formed the pseudotachylite veins the melt temperatures were estimated to be up to 1100 c using a two pyroxene geothermometer using published data relating water solubilities in high temperature andesitic magmas to pressure a depth estimate of about 4 km is inferred for the hidaka pseudotachylites evidence derived from pseudotachylites in the hidaka metamorphic belt supports the conclusion that pseudotachylite is formed by frictional melting along fault surfaces at shallow depths from rocks containing hydrous minerals

tectonic tremor along the northern hikurangi margin new zealand between 2010 and 2015 we present the first comprehensive tectonic tremor catalog for the northern hikurangi margin obtained from the systematic analysis of data from the new zealand seismic network between 2010 and 2015 the recent densification of this array along the northern hikurangi margin has allowed for the detection and location of tectonic tremor associated with 20 out of 26 identified transients in the continuous gps record along the east coast of the raukumara peninsula eighteen of these transients have never been reported in the literature and are likely to be slow slip events with magnitudes smaller than those previously studied we also investigate the spatiotemporal relationships between shallow offshore slow slip tectonic tremor and earthquakes for the duration of the study period

how subduction interface roughness influences the occurrence of large interplate earthquakes the role of seafloor roughness on the seismogenic behavior of subduction zones has been increasingly addressed over the past years although their exact relationship remains unclear do subducting features like seamounts fracture zones or submarine ridges act as barriers preventing ruptures from propagating or do they initiate megathrust earthquakes instead we address this question using a global approach taking into account all oceanic subduction zones and a 117 year time window of megathrust earthquake recording we first compile a global database subquake that provides the location of a rupture epicenter the overall rupture area and the region where the largest displacement occurs the seismic asperity for m w 7 5 subduction interplate earthquakes with these data we made a quantitative comparison with the seafloor roughness seaward of the trench which is assumed to be a reasonable proxy for the subduction interface roughness we compare the spatial occurrence of megathrust ruptures seismic asperities and epicenters with two roughness parameters the short wavelength roughness r sw 12 20 km and the long wavelength roughness r lw 80 100 km we observe that ruptures with m w 7 5 tend to occur preferentially on smooth subducting seafloor at long wavelengths which is especially clear for the m w 8 5 events at both short and long wavelengths seismic asperities show a more amplified relation with smooth seafloor than rupture segments in general for the epicenter correlation we see a slight difference in roughness signal which suggests that there might be a physical relationship between rupture nucleation and subduction interface roughness plain language summary subduction zones are regions on earth where an oceanic plate dives below another plate earthquakes that occur along the contact between plates in such regions are among the largest and most destructive on earth to better understand where these large earthquakes are most likely to occur we look at the effect of seafloor roughness a rough seafloor is often characterized by many topographic features such as seamounts or ridges while a smooth seafloor is generally more flat on a global scale we compared the roughness of the incoming seafloor of the downgoing plate with the occurrence of large earthquakes in each subduction zone we find that the seafloor in front of large earthquakes is generally smoother than in areas where no large earthquakes have occurred this is the clearest for very large earthquakes with magnitudes larger than 8 5 investigating which parameters play a role in the location of earthquakes helps us to understand where future earthquakes are more likely to occur

a pseudotachylyte geothermometer abstract pseudotachylytes invariably contain a conspicuous concentration of clasts and lithic fragments in their matrices which reflect brittle wear processes during frictional melting the ratio of clasts to matrix is interpreted as the ratio of wear to melt w m and measurements of this ratio show a range of 0 1 0 7 based on models of melting and wear processes it is shown that the ratio w m is independent of fault displacement stress fault area and mineralogy thermodynamic considerations indicate that w m corresponds to the thermodynamic efficiency of the conversion of work to heat and is defined as w q thigh tlow thigh where thigh refers to the melt temperature and tlow is the ambient country rock temperature this provides a simple new technique for estimating the ambient crustal temperature tcrust in degrees kelvin t crust 1 w m t melt assuming a reasonable value for the temperature of the melt estimates of country rock temperatures tcrust for pseudotachylytes from four different localities in the usa indicate a range of 123 387 c eclogite facies pseudotachylytes from western norway yield a mean country rock temperature of 658 c in good agreement with independent fe mg exchange geothermometry this new geothermometer appears to be applicable throughout the entire crust and may provide a better understanding of melting and wear processes during seismic faulting

pressure related feedback processes in the generation of pseudotachylytes pseudotachylyte formation is typically viewed as a self limiting process quantitative models have demonstrated that frictional melting is possible at seismic slip rates but once a thin film of melt forms on a seismic fault plane the dramatic decrease in fault friction is thought to suppress further melting volumes of melt generated pseudotachylyte observed in field studies however are in many cases substantially larger than the amounts of frictional melt predicted by theoretical models this suggests that previously unrecognized physical processes may enhance melting during seismic slip localized decompression at dilational jogs may be one such phenomenon transient unloading could play two important roles in the dynamics of pseudotachylyte generation first by setting up significant fluid pressure gradients it would lead to rapid migration of the melt to sites of low pressure thereby reestablishing frictional contact across the fault surface and favoring further melt generation second at significant depths sudden depressurization might lead to in situ decompression melting

high pore fluid pressure may cause silent slip in the nankai trough silent slip events have been detected at several subduction zones but the cause of these events is unknown using seismic imaging we detected a cause of the tokai silent slip which occurred at a presumed fault zone of a great earthquake the seismic image that we obtained shows a zone of high pore fluid pressure in the subducted oceanic crust located down dip of a subducted ridge we propose that these structures effectively extend a region of conditionally stable slips and consequently generate the silent slip

how strong is an asperity a recent study of repeating earthquakes on the san andreas fault in central california by nadeau and johnson 1998 found that the smallest events occurred on patches having a linear dimension of the order of 0 5 m displacements of about 2 cm and stress drops of the order of 2000 mpa roughly 10 times larger than rock strengths measured in the laboratory the stress drop for larger events was observed to decrease as a power law of the seismic moment reaching the commonly observed value of 10 mpa at about magnitude 6 these large strengths are shown here to be consistent with laboratory data if the preexisting microcracks are all healed a hierarchical fractal asperity model is presented which is based on recent laboratory observations of contact distributions in sliding friction experiments this cantor dust model is shown to be consistent with the observed power law decrease in stress drop and increase in displacement with increasing event size the spatial distribution of hypocenters in the parkfield area is shown to be consistent with this simple fractal model and with a hierarchical clustering of asperities having a fractal dimension of d 1 and discrete rescaling factor of about 20

reactivation of tectonics crustal underplating and uplift after 60 myr of passive subsidence raukumara basin hikurangi kermadec fore arc new zealand implications for global growth and recycling of continents we use seismic reflection and refraction data to determine crustal structure to map a fore arc basin containing 12 km of sediment and to image the subduction thrust at 35 km depth seismic reflection megasequences within the basin are correlated with onshore geology megasequence x late cretaceous and paleogene marine passive margin sediments megasequence y a similar to 10 000 km 3 submarine landslide emplaced during subduction initiation at 22 ma and megasequence z a neogene subduction margin megasequence the moho lies at 17 km beneath the basin center and at 35 km at the southern margin beneath the western basin margin we interpret reflective units as deformed gondwana fore arc sediment that was thrust in cretaceous time over oceanic crust 7 km thick raukumara basin has normal faults at its western margin and is uplifted along its eastern and southern margins raukumara basin represents a rigid fore arc block 150 km long which contrasts with widespread faulting and large neogene vertical axis rotations farther south taper of the western edge of allochthonous unit y and westward thickening and downlap of immediately overlying strata suggest westward or northwestward paleoslope and emplacement direction rather than southwestward as proposed for the correlative onshore allochthon spatial correlation between rock uplift of the eastern and southern basin margins with the intersection between moho and subduction thrust leads us to suggest that crustal underplating is modulated by fore arc crustal thickness the trench slope has many small extensional faults and lacks coherent internal reflections suggesting collapse of indurated rock rather than accretion of 1 km of sediment from the downgoing plate the lack of volcanic intrusion east of the active arc and stratigraphic evidence for the broadening of east cape ridge with time suggests net fore arc accretion since 22 ma we propose a cyclical fore arc kinematic rock moves down a subduction channel to near the base of the crust where underplating drives rock uplift oversteepens the trench slope and causes collapse toward the trench and subduction channel cyclical rock particle paths led to persistent trench slope subsidence during net accretion existing global estimates of fore arc loss are systematically too high because they assume vertical particle paths citation sutherland r et al 2009 reactivation of tectonics crustal underplating and uplift after 60 myr of passive subsidence raukumara basin hikurangi kermadec fore arc new zealand implications for global growth and recycling of continents tectonics 28 tc5017 doi 10 1029 2008tc002356

velocity shift in heterogeneous media with anisotropic spatial correlation summary converted waves analysis is used to map the anisotropy as a function of depth in the upper mantle stacking of sv a ndt component records of several seismic events well distributed in azimuth helps to detect these converted waves weighting coe cients are used to extract the anisotropic signal in the records synthetic examples show that a harmonic analysis helps to separate azimuthal anisotropy eiects from the eiects of lateral heterogeneity such as those resultingfrom dipping structures a good azimuthal coverage of the data set is important in order to separate the diierent eiects in the stack sections obtained for diierent azimuthal orders the procedure is applied to data observed at geoscope station can and gives a model with two anisotropic layers in the upper mantle the upper layer located between the crust and a depth of140 km has its fast axis in an east west direction the lower layer 40 km thick has its fast axis in a north south direction the fast axis directions are in agreement with results obtained from surface wave analysis and tectonic information

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

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

mesozoic tectono stratigraphic evolution of the seychelles microcontinent abstract the seychelles region developed as a unique continental sliver through the three phases of rift tectonism that since the late permian created the western indian ocean the history of rifting in seychelles can perhaps be traced back to 700 ma when the granites that form the central islands were emplaced the low 18 o of the high temperature minerals within these slightly alkaline granites on mahe indicate an intraplate origin and the fracturing of the lithosphere consequent to their emplacement created a locus of weakness for future rift tectonism to exploit thermally induced extensional rifting initiated during the carboniferous and rekindled in the late permian 225 ma eventually severed super gondwana in the middle jurassic 160 ma into east and west gondwana a proto indian ocean the somali basin which separated east and west gondwana ceased to expand in the early cretaceous 115 ma this rift drift event is recorded in seychelles by block faulted sequences of distal fluviatile through near shore to open marine deposits along its western margin and possibly at farquhar within east gondwana transform rifting then separated madagascar and seychelles india in the late cretaceous at 84 ma a spreading ridge developed between these blocks forming the mascarene basin as a new arm of the evolving indian ocean a thick progradational wedge of sediment initially alluvial followed by lacustro marine and eventually open marine resulted from this event along the southwest seychelles margin finally hotspot related thermally induced rifting causing block faulting and fluviatile to marine deposition eventually separated seychelles from india at the cretaceous tertiary boundary this separation coincided with the deccan volcanic event and the development of the carlsberg spreading ridge which replaced the mascarene spreading ridge and initiated the opening of the northwestern indian ocean basin sedimentation on the seychelles microcontinent throughout the tertiary produced predominantly shelf carbonates

mantle discontinuities beneath the deccan volcanic province abstract mantle discontinuities beneath the deccan volcanic province dvp are imaged using about 900 seismograms from a network of six broadband stations deployed in the dvp along a 350 km long north south profile paralleling the west coast of india teleseismic receiver function analysis of the data enabled distinct identification of the moho 410 and 660 km discontinuities presence of sub moho low velocity zones with velocity reductions in the range of 0 1 0 4 km s have been brought out through modeling of receiver function stacks at individual stations the arrival times of p410s and p660s are normal as predicted by the iasp91 model and the mantle transition zone thickness is 252 km which is close to the global average this study reveals that the upper mantle beneath dvp south of the narmada rift does not exhibit any anomalous signatures associated with a mantle plume a shield like lithosphere coupled with the presence of an upper mantle low velocity zone confined to shallow depths beneath dvp imposes constraints on the mechanisms feasible for deccan volcanism the present study does not favour the plume impact plume incubation models instead necessitates searching for clues offshore west coast of india and exploring alternate mechanisms

the importance of rift history for volcanic margin formation rifting and magmatism are fundamental geological processes that shape the surface of our planet a relationship between the two is widely acknowledged but its precise nature has eluded geoscientists and remained controversial largely on the basis of detailed observations from the north atlantic ocean mantle temperature was identified as the primary factor controlling magmatic production with most authors seeking to explain observed variations in volcanic activity at rifted margins in terms of the mantle temperature at the time of break up however as more detailed observations have been made at other rifted margins worldwide the validity of this interpretation and the importance of other factors in controlling break up style have been much debated one such observation is from the northwest indian ocean where despite an unequivocal link between an onshore flood basalt province continental break up and a hot spot track leading to an active ocean island volcano the associated continental margins show little magmatism here we reconcile these observations by applying a numerical model that accounts explicitly for the effects of earlier episodes of extension our approach allows us to directly compare break up magmatism generated at different locations and so isolate the key controlling factors we show that the volume of rift related magmatism generated both in the northwest indian ocean and at the better known north atlantic margins depends not only on the mantle temperature but to a similar degree on the rift history the inherited extensional history can either suppress or enhance melt generation which can explain previously enigmatic observations

efficient calculation of differential seismograms for lithospheric receiver functions summary a new technique for computing differential seismograms for crustal and upper mantle response to a teleseismic wave receiver functions is developed using the matrix formalism of kennett the work was motivated by the difficulty of modelling teleseismic sv waves and has also proven useful for modelling teleseismic p waves this efficient method for calculating differential seismograms is based on three separate methods for computing synthetic seismograms two of the synthetic seismogram methods save intermediate results then the remaining synthetic seismogram algorithm uses the stored results in an efficient calculation of a new synthetic seismogram for a perturbed velocity model these developments have led to a faster for a 30 layer model a 90 per cent reduction in computation time and more accurate linearized inversion scheme for the determination of velocity models using teleseismic waves

seismological studies at parkfield vi moment release rates and estimates of source parameters for small repeating earthquakes abstract waveform data from a borehole network of broadband seismographic stations have been used to study microearthquakes along the parkfield segment of the san andreas fault saf analysis of almost 10 years of such data demonstrates that much of the seismicity in this region consists of repeating sequences quasiperiodic sequences of earthquakes that are essentially identical in terms of waveform size and location scalar seismic moments have been estimated for 53 of these repeating sequences and combined with equivalent estimates from 8 similar but larger event sequences from the stone canyon section of the fault and the main parkfield sequence these estimates show that seismic moment is being released as a function of time in a very regular manner measurements of the moment release rate combined with an assumed tectonic loading rate lead to estimates of the seismic parameters source area slip and recurrence interval such parameters exhibit a systematic dependence upon source size over a range of 10 10 in seismic moment that can be described by three simple scaling relationships several implications of these scaling relationships are explored including the repeat time of earthquakes average stress drop strength of the fault and heat generated by earthquakes what emerges from this analysis of moment release rates is a quantitative description of an earthquake process that is controlled by small strong asperities that occupy less than 1 of the fault area this means that the fault is highly heterogeneous with respect to stress strength and heat generation such heterogeneity helps to explain many of the apparent contradictions that are encountered in the study of earthquakes such as why faults appear weak why significant heat flow is not observed how significant high frequencies can be generated by large earthquakes and how various geologic features such as pseudotachylytes might form

dynamic weakening of smectite bearing faults at intermediate velocities implications for subduction zone earthquakes the hydrous clay mineral smectite which is pervasive in sediments on subducting oceanic plates is thought to weaken and stabilize subduction thrust faults however these frictional properties of smectite alone cannot explain the large coseismic slip in the vicinity of a trench here we performed friction experiments to demonstrate the rate dependence of friction at slip rates from 30 m s to 1 3 m s for water saturated smectite quartz mixtures with various smectite contents so as to shed light on the frictional response of smectite bearing faults at intermediate to high slip rates at slip rates of 30 to 150 m s the friction coefficients decreased gradually from 0 5 0 6 to 0 1 with an increase in smectite content from 20 to 50 wt in contrast at slip rates higher than 1 3 mm s friction exhibited marked slip weakening resulting in low friction coefficients of 0 1 0 05 even for low smectite contents roughly 30 wt drastic slip weakening occurred at smectite contents of 10 30 wt at slip rates of 10 mm s which is 1 to 2 orders of magnitude lower than the slip rate at which slip weakening was observed in previous experiments on various rock types the intermediate velocity weakening could be attributed to a rise in pore pressure caused by both shear enhanced compaction and microscopic thermal pressurization of pore fluids this process could weaken the fault even below seismic slip rates leading to an acceleration of fault motion and potentially facilitating large coseismic slip and a stress drop in the vicinity of a trench

exposed plate interface in the european alps reveals fabric styles and gradients related to an ancient seismogenic coupling zone 1 we present observations from a continuous exposure of an ancient plate interface in the depth range of its former seismogenic zone in the central alps of europe related to late cretaceous early tertiary subduction and accretion of the south penninic lower plate underneath the adriatic upper plate the material forming the exposed plate interface zone has experienced flow and fracturing over an extended period of time followed by syncollisional exhumation thus reflecting a multistage evolution fabric formation and metamorphism however chiefly record the deformation conditions of the precollisional setting along the plate interface we identify an unstable slip domain from pseudotachylytes occurring in the temperature range between 200 and 300 c this zone coincides with a domain of intense veining in the subduction melange with mineral growth into open cavities indicating fast possibly seismic rupture evidence for transient near lithostatic fluid pressure as well as brittle fractures competing with mylonitic shear zones continues into the region below the occurrence of pseudotachylytes possibly reflecting a zone of conditionally stable slip the zone above the unstable slip area is devoid of veins but displays ample evidence of fluid assisted processes similar to the deeper zone solution precipitation creep and dehydration reactions in the melange matrix hydration and sealing of the base of the upper plate seismic rupture here is possibly expressed by ubiquitous localized deformation zones we hypothesize that trenchward sealing of parts of the plate interface as well as reaction enhanced destruction of upper plate permeability is an important component localizing the unstable slip zone this relation may result from the competition of the pervasive presumably interseismic pressure solution creep destroying permeability and building elevated fluid pressure until the strength threshold is reached with seismic failure

the effect of normal stress on the real area of contact during frictional sliding in rocks the real area of contact during frictional sliding has been determined as a function of changing normal stress in triaxial experiments through the use of thermodyes utilizing the technique described by teufel and logan in 1978 with saw cut surfaces inclined 35 to the load axis determinations were made for monolithologic sliding of tennessee sandstone and indiana limestone and dilithologic sliding of the same rocks confining pressures to 200 mpa were investigated at a constant shortening rate of 10 2 mm sec and at room temperature direct measurements were made of single asperity areas and the asperity density the product of these measurements gives the percent area of real contact across the sliding surface single asperity area and density are found to remain relatively constant during the displacement single asperity areas are in the ranges of 0 4 to 6 10 2 mm2 for sandstone 0 8 to 2 10 2 mm2 for limestone and 0 2 to 24 10 2 mm2 for sandstone sliding against limestone these values are smaller than the grain size of either rock the values increase with increasing normal stress for both monolithologic and dilithologic sliding in sandstone the asperity density increases from about 0 8 to 2 75 contacts per square millimeter in a logarithmic fashion monolithologic limestone has values of about 0 9 contacts per square millimeter and does not show significant change with increasing normal stress the percent area of real contact increases in all cases with average maximum values of 16 of the apparent area at a normal stress of 374 mpa in sandstone 18 at 25 mpa in limestone and 22 at 123 mpa in the dilithologic specimens the normal stress recalculated for the real area of contact approaches the unconfined compressive strength for sandstone and limestone

do subducting seamounts generate or stop large earthquakes seamount subduction is a common process in subduction zone tectonics contradicting a widely held expectation that subducting seamounts generate large earthquakes seamounts subduct largely aseismically producing numerous small earthquakes on rare occasions when they do produce relatively large events the ruptures tend to be complex suggesting multiple rupture patches or faults we explain that the seismogenic behavior of these seamounts is controlled by the development and evolution of an adjacent fracture network during subduction and cannot be described using the frictional behavior of a single fault the complex structure and heterogeneous stresses of this network provide a favorable condition for aseismic creep and small earthquakes but an unfavorable condition for the generation and propagation of large ruptures

rupture mechanism and interface separation in foam rubber models of earthquakes a possible solution to the heat flow paradox and the paradox of large overthrusts abstract spontaneous stick slip along the interface between stressed foam rubber blocks is a simple analog of earthquake rupture and stick slip results from this model are used to elucidate the role of normal stress variations along the interface in the stick slip process observations indicate significant normal interface vibrations and separation during slip suggesting that dynamic changes in normal stress rather than a drop in the coefficient of friction may control stick slip as suggested for example by tolstoi oden and martins and brune and co workers observations of particle trajectories indicate that stick slip shear motion is associated with various degrees of fault separation for an asymmetric model the motion is consistent with slipping motion of the type suggested by schallamach and price for a symmetric model the motion is similar to that suggested by comninou and dundurs if interface waves of this type involving separation during slip occur in earthquakes they may be a solution to the heat flow paradox since a major part of the slip occurs during separation and during low normal stress thus frictional heat generation is reduced normal interface vibrations during stick slip may explain the high corner frequency of p wave spectra and the generally high levels of p wave spectra beyond the corner frequency schallamach comninou type waves are consistent with the partial stress drop abrupt locking self healing models of brune and heaton

age of seychelles india break up many continental flood basalt provinces are spatially and temporally linked with continental break up establishing the relative timing of the two events is a key step in determining their causal relationship here we investigate the example of the deccan traps and the separation of india and the seychelles whilst there has been a growing consensus as to the age of the main phase of the deccan emplacement 65 5 1 ma chron 29r the age of the rifting has remained unclear we resolve this issue through detailed seafloor magnetic anomaly modeling supported by wide angle and reflection seismic results of the north seychelles and conjugate laxmi ridge gop rift margins and geochemistry and 40ar 39ar geochronology of rocks from the north seychelles margin we show that syn rift volcanics offshore the seychelles islands in the form of seaward dipping reflectors were most likely erupted during chron 28n and the first organized seafloor spreading at the carlsberg ridge also initiated during this chron at 63 4 ma the severing of the seychelles occurred by a south eastward ridge propagation that was completed by the start of chron 27n 62 ma a brief pre 28r phase of seafloor spreading occurred in the gop rift possibly as early as 31r 32n 71 ma initial extension at the margin therefore preceded or was contemporaneous with the deccan emplacement and separation of the seychelles was achieved less than 3 5 ma afterwards this is the shortest time interval between flood basalt emplacement and break up yet reported for any continental flood basalt rifted margin pair a contributing factor to the apparently short interval in the deccan case may be that rifting occurred by a ridge jump into already thinned continental lithosphere however we conclude that external plate boundary forces rather than the impact of a mantle plume were largely responsible for the rifting of the seychelles from india

changes in electrical resistivity track changes in tectonic plate coupling 1 interplate coupling on the hikurangi subduction margin along the east coast of new zealand s north island changes north to south from almost uncoupled to locked clay rich sediments and aqueous fluids at the subduction interface have been invoked as key factors in the frictional processes that control interplate coupling here we use magnetotelluric data to show that the subduction interface in the weakly coupled region is electrically conductive but is resistive in the locked region these results indicate the presence of a layer of fluid and clay rich sediments in the weakly coupled region and support the idea that the presence of fluid and hydrated clays at the interface is a major factor controlling plate coupling

upper mantle anisotropy beneath the seychelles microcontinent 1 the seychelles plateau is a prime example of a microcontinent yet mechanisms for its creation and evolution are poorly understood recently acquired teleseismic data from a deployment of 26 stations on 18 islands in the seychelles are analyzed to study upper mantle seismic anisotropy using sks splitting results strong microseismic noise is attenuated using a polarization filter results show significant variation in time delays t 0 4 2 4 s and smooth variations in orientation 15 69 where is the polarization of the fast shear wave the splitting results cannot be explained by simple asthenospheric flow associated with absolute plate motions recent work has suggested that anisotropy measurements for oceanic islands surrounding africa can be explained by mantle flow due to plate motion in combination with density driven flow associated with the african superswell such a mechanism explains our results only if there are lateral variations in the viscosity of the mantle it has been suggested that the seychelles are underlain by a mantle plume predictions of flow induced anisotropy from plume lithosphere interaction can explain our results with a plume possibly impinging beneath the plateau finally we consider lithospheric anisotropy associated with rifting processes that formed the seychelles the large variation in the magnitude of shear wave splitting over short distances suggests a shallow source of anisotropy fast directions align parallel to an area of transform faulting in the amirantes farther from this area the orientation of anisotropy aligns in similar directions as plate motions this supports suggestions of transpressive deformation during the opening of the mascarene basin

strength of strained two phase mixtures application to rapid creep and stress amplification in subduction zone melange aseismic creep may occur by distributed deformation in melange shear zones comprising weak matrix and stronger clast materials slow slip events or steady tectonic displacement can be distributed over 100 m thick shear zones if weak matrix controls bulk shear zone deformation we use 2 d numerical models to quantify the rheology of moderately strained shear strain 1 75 melange for various volumetric proportions of competent clasts melange deformation with 50 clasts is matrix dominated and can accommodate steady creep at higher clast proportions melange viscosity increases more than tenfold after small strains because strong clasts interact and form force chains clast shear stress is amplified above the imposed shear stress by a factor of 14 where force chains develop slow slip events may occur due to a temporary absence of force chains while localized regions of high shear stress generate coincident fracturing and potentially tremor events

mineralogy and petrology of a mullite bearing pseudotachylyte constraints on the temperature of coseismic frictional fusion pseudotachylyte from the homestake shear zone central colorado contains acicular 5 10 x 100 m mullite crystals 1 5 m octahedra of magnetite and variably melted clasts of quartz in an originally molten matrix several lines of evidence support the interpretation that the pseudotachylyte represents the quenched equivalent of a molten phase derived from preferential melting of primarily biotite microcline and plagioclase and to a lesser extent quartz from the gneissic protolith the morphology of the mullite crystals which nucleate on and radiate from relic sillimanite clasts abundant magnetite octahedra scattered throughout the pseudotachylyte matrix and intergrown with mullite forming plumose quench structures partially melted relict quartz clasts the composition of the isotropic pseudotachylyte matrix and chemical gradients surrounding quartz clasts in pseudotachylyte matrix although the sample had been nearly completely molten except for 10 100 m relict quartz clasts hrtem imaging of the pseudotachylyte matrix reveals that it has devitrified the slightly birefringent matrix consists of intergrowths of submicrometer flakes of white mica microprobe analysis shows that the mullite is highly enriched in fe 8 10 wt fe 2 o 3 compared with other natural mullites hrtem and sem images reveal the presence of oriented platelets of an fe oxide phase dominantly hematite that appears to have exsolved from the mullite during cooling indicating that the limit of solid solution of fe 3 in mullite decreases with temperature the estimated melt temperature ranges from 1150 to 1200 c this interpretation is based on the fe 2 o 3 content of mullite the liquidus temperature for melts corresponding to the matrix compositions and liquidus temperatures for gneissic wall rocks which underwent complete to very high degrees of melting

plume scar in the mantle lithosphere beneath the seychelles revealed by seismic imaging abstract continental flood basalts are commonly thought to form by direct melting of lithospheric mantle either during rifting episodes or when a mantle plume impinges on the base of the lithosphere if correct then significant alteration of the chemical and physical properties of the lithospheric mantle should be observable beneath such provinces to date however this hypothesis remains unproved through lack of direct evidence here we present the results from a seismic experiment conducted in the seychelles a region that separated from india at the time of the eruption of the deccan traps the analysis shows a stratified upper mantle with exceptionally low s wave velocities at depths between 50 and 190 km it is difficult to explain these anomalies without invoking the presence of melt however the depth extent of the low velocities rules out the presence of silicate melt we argue that the most probable interpretation is that these layers contain sulphide melt trapped from earlier interactions of the lithosphere with episodes of continental flood basalt generation the presence of this melt over a depth interval of 130 km suggests that it is stagnant and has remained coupled with the seychelles lithosphere

stabilizing effect of high pore fluid pressure on slip behaviors of gouge bearing faults this work is supported by the department of energy office of science the office of basic energy sciences chemical sciences geosciences biosciences division under award no de fg 0207er15916 and by the national science foundation under grant no ear 1761912

palaeomagnetic and petrological reconstruction of the seychelles abstract the geology of mahe island and adjoining islets seychelles consists of three types of late precambrian granitic rock series 1 a gneissose granodiorite gneissose tonalite and amphibolite leucogranodiorite series 2 a porphyritic granite pink granite series and 3 a grey granite gneissose grey granite series all these granitic rocks are cut by dykes of doleritic compositions mostly with a wnw ese trend the gneissose granodiorite is a calcalkaline i type granite with normative ab or the porphyritic granite is a sub alkaline a type granite with normative ab or and the grey granite is an alkaline a type granite with normative ab or rb sr radiometric analyses yielded ages of 713 19 ma for the gneissose granodiorite 683 16 ma for the porphyritic granite and 570 5 ma for the grey granite petrological geological geochronological and geochemical studies suggest that the late precambrian granitic rocks of the seychelles are very similar to and are well correlated with those of the arabian nubian shield 408 late precambrian granite specimens were collected from the mahe island group seychelles progressive alternating field demagnetization and thermal demagnetization have been successfully performed to isolate stable remanence with well grouped direction from natural remanent magnetization of the granites the stable remanences of the granite fall around a northerly mean direction of which the declination is 348 5 and the inclination 48 7 the stable remanences of the mahe island group granite yield a palaeomagnetic pole at 54 0 n 38 2 e from the above data in conjunction with reported polar wander path and plate rotation poles it is concluded that the seychelles were originally located near the eastern end of the horn of africa and have drifted to the present position since continental break up

simultaneous long term and short term slow slip events at the hikurangi subduction margin new zealand implications for processes that control slow slip event occurrence duration and migration 1 we document a sequence of simultaneous short term and long term slow slip events sses at the hikurangi subduction zone during the 2010 2011 period the sequence of short term events each 2 3 weeks in duration ruptured much of the shallow plate interface 15 km at central and northern hikurangi over a 6 month period was accompanied by microseismicity and involved patchy irregular migration of sse slip we suggest that the patchy migration of the short term sse is due to large scale 100 3500 km heterogeneities on the plate interface related to seamount subduction and sediment subduction and or underplating this is in contrast to a 2010 2011 long term sse at the central hikurangi margin which evolved steadily over 1 5 years and ruptured much of the plate interface between 20 and 70 km depth we suggest that the occurrence of long term versus short term sses at hikurangi is related to differences in effective normal stresses and relative heterogeneity of the subduction interface the long term sse sequence began 1 year before the short term sequence coulomb stress change models suggest that the long term sse may have triggered initiation of the subsequent short term sse sequence initiation of the short term sequence occurred in a region just updip of or within an interseismically locked portion of the plate interface and may be located within the updip transition from seismic to aseismic behavior alternatively it could be characteristic of a region undergoing partial interseismic coupling this is in contrast to sses observed elsewhere in the world that typically occur within the downdip transition from seismic to aseismic behavior

fault rock injections record paleo earthquakes fault rocks such as pseudotachylyte melt or granular gouge are sometimes injected into cracks off a fault surface the width to height ratio of the injection veins is a direct measure of the shear strain in the wall rock required to accommodate the injection so the aspect ratios preserve a record of the overpressure opening the cracks and injecting melt or fluidized granular material from the fault zone we measured the aspect ratios for 201 pseudotachylyte injections and 29 granular injections they have aspect ratios of 0 2 0 177 0 025 and 0 227 0 077 respectively within 99 confidence these are significantly proportionally wider than the aspect ratios of ordinary dikes from 10 4 to 10 3 the injection aspect ratios exceed the usual limit of elastic strain in the wallrock 1 so we infer that the injection veins were accommodated by permanent deformation by microcracking and slip on foliation surfaces in the host rock which is difficult to detect observationally the fully elastic model therefore offers an upper bound on the stress required to open these injection veins 10 8 210 10 pa with higher pressures in stiffer host rock lithologies these overpressures cannot be achieved by expansion of the fault rock fluids during melting or gouge fluidization mechanisms as the pressure along the fault is limited to 10 7

seismic slip record in carbonate bearing fault zones an insight from high velocity friction experiments on siderite gouge pseudotachylyte formed by frictional melting has been the only unequivocal evidence of past seismogenic fault slip we report from high velocity friction experiments on siderite bearing gouge that mineral decomposition due to frictional heating also can leave evidence of paleoseismic events along shallow crustal faults other than pseudotachylyte experiments were conducted room dry on simulated gouge composed of siderite or mixture of siderite calcite and quartz initially at room temperature under normal stresses of 0 6 1 3 mpa and at seismic slip rates of 1 3 2 0 m s in all cases gouge exhibited dramatic slip weakening and siderite was decomposed into nanocrystalline magnetite and co 2 gas as confirmed by co 2 measurement x ray diffraction analyses and transmission electron microscopy the weakening was caused by the low frictional strength of ultrafine decomposition products at seismic slip rates magnetite formation during shearing changed gouge color to black and increased magnetic susceptibility by a few orders of magnitude those changes can be recognized in natural fault zones and black gouge in the chelungpu fault zone in taiwan is perhaps such an example thus our results suggest that thermal decomposition in shallow crustal faults can be an important co seismic process not only for dynamic fault weakening but also for leaving seismic slip records

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

mapping subduction interface coupling using magnetotellurics hikurangi margin new zealand the observation of slow slip seismic tremor and low frequency earthquakes at subduction margins has provided new insight into the mechanisms by which stress accumulates between large subduction megathrust earthquakes however the relationship between the physical properties of the subduction interface and the nature of the controls on interplate seismic coupling is not fully understood using magnetotelluric data we show in situ that an electrically resistive patch on the hikurangi subduction interface corresponds with an area of increased coupling inferred from geodetic data this resistive patch must reflect a decrease in the fluid or sediment content of the interface shear zone together the magnetotelluric and geodetic data suggest that the frictional coupling of this part on the hikurangi margin may be controlled by the interface fluid and sediment content the resistive patch marking a fluid and sediment starved area with an increased density of small seismogenic asperities and therefore a greater likelihood of subduction earthquake nucleation

superheated friction induced melts in zoned pseudotachylytes within the adamello tonalites italian southern alps pseudotachylytes and cataclasites are present along a strike slip fault zone in the tonalites of the adamello intrusion italian southern alps ambient conditions during faulting were 0 25 0 3 gpa and 250 300 c pseudotachylyte veins thicker than 6 mm are zoned and consist of two symmetric microlitic domains towards the vein walls and a central spherulitic domain the thickness of the microlitic domains x increases linearly with the half total thickness a of the pseudotachylyte vein commonly 2 cm according to the relation x 0 29 0 12 a the spherulitic and microlitic domains have similar chemical composition but the microlitic domain has a lower amount of plagioclase clasts a numerical model explains zoning as the result of the different cooling rates and clast melt interactions at the center and periphery of thick veins zoning is compatible with the injection of a single pulse of superheated friction induced melt tmelt 1450 c melt temperatures estimated by the clast matrix ratio o hara 2001 are considerably lower 316 577 c it is suggested that the difference in the temperature estimates reflects a more complex slip history than a single seismic slip event along the fault during production of cataclasites pseudotachylytes

vitrinite reflectance and raman spectra of carbonaceous material as indicators of frictional heating on faults constraints from friction experiments abstract vitrinite reflectance r o and raman spectra of carbonaceous material rscm are both widely used as indicators of the maximum attained temperatures in sedimentary and metamorphic rocks however the potential of these methods to estimate temperature increases associated with fault slip has not been closely studied to examine this issue friction experiments were conducted on a mixture of powdered clay rich fault material and carbonaceous material cm at slip rates of 0 15 mm s and 1 3 m s in nitrogen n 2 gas with or without distilled water after the experiments we measured r o and rscm and compared to those in starting material the results indicate that when fault material suffers rapid heating at 500 c in 9 s at 1 3 m s r o and the intensity ratio of d1 and d2 raman bands of cm i d2 i d1 markedly increase comminution with very small temperature rise in 32 min at 0 15 mm s is responsible for very limited changes in r o and i d2 i d1 our results demonstrate that r o and rscm could be useful for the detection of frictional heating on faults when the power density is 0 52 mw m 2 however the conventionally used r o and rscm geothermometers are inadequate for the estimation of peak temperature during seismic fault slip the reaction kinetics incorporating the effects of rapid heating at high slip rates and studies of the original microtexture and composition of cm are required to establish a reliable thermometer for frictional heating on faults

coseismic dehydration of serpentinite evidence from high velocity friction experiments abstract high velocity friction hvf experiments on serpentinite under conditions equivalent to large amounts of earthquake slip produced large volumes of water vapor derived from the frictional heating induced dehydration of serpentinite fourier transform infrared ftir and thermogravimetry tg differential scanning calorimetry dsc microspectroscopic analyses show that the water in the slip zone and its bounding zones was released due to dehydration during the hvf experiments our experimental results demonstrate that i the run product consists of 10 wt molten materials with abundant vesicles and 90 wt fine grained clasts ii both serpentine and olivine minerals were melted during high velocity slip 1 1 1 3 m s iii rapid serpentine dehydration occurred in a zone of up to 3 mm wide including the slip zone and its bounding zones accompanying frictional melting during high velocity slip and iv no distinct dehydration or frictional melting occurred during low velocity slip

a silent slip event on the deeper cascadia subduction interface continuous global positioning system sites in southwestern british columbia canada and northwestern washington state usa have been moving landward as a result of the locked state of the cascadia subduction fault offshore in the summer of 1999 a cluster of seven sites briefly reversed their direction of motion no seismicity was associated with this event the sudden displacements are best explained by 2 centimeters of aseismic slip over a 50 kilometer by 300 kilometer area on the subduction interface downdip from the seismogenic zone a rupture equivalent to an earthquake of moment magnitude 6 7 this provides evidence that slip of the hotter plastic part of the subduction interface and hence stress loading of the megathrust earthquake zone can occur in discrete pulses