lithology and internal structure of the san andreas fault at depth based on characterization of phase 3 whole rock core in the san andreas fault observatory at depth safod borehole abstract we characterize the lithology and structure of the spot core obtained in 2007 during phase 3 drilling of the san andreas fault observatory at depth safod in order to determine the composition structure and deformation processes of the fault zone at 3 km depth where creep and microseismicity occur a total of approximately 41 m of spot core was taken from three separate sections of the borehole the core samples consist of fractured arkosic sandstones and shale west of the saf zone pacific plate and sheared fine grained sedimentary rocks ultrafine black fault related rocks and phyllosilicate rich fault gouge within the fault zone north american plate the fault zone at safod consists of a broad zone of variably damaged rock containing localized zones of highly concentrated shear that often juxtapose distinct protoliths two zones of serpentinite bearing clay gouge each meters thick occur at the two locations of aseismic creep identified in the borehole on the basis of casing deformation the gouge primarily is comprised of mg rich clays serpentinite lizardite chrysotile with notable increases in magnetite and ni cr oxides hydroxides relative to the surrounding host rock the rocks surrounding the two creeping gouge zones display a range of deformation including fractured protolith block in matrix and foliated cataclasite structure the blocks and clasts predominately consist of sandstone and siltstone embedded in a clay rich matrix that displays a penetrative scaly fabric mineral alteration veins and fracture surface coatings are present throughout the core and reflect a long history of syn deformation fluid rock reaction that contributes to the low strength and creep in the meters thick gouge zones

centroid depth and mechanism of trench outer rise earthquakes trench outer rise earthquakes occur by reactivation or creation of normal faults caused as the oceanic lithosphere approaches a subduction zone and bends into the deep sea trench these faults may cut deep enough into the mantle to allow sea water to penetrate into the lithosphere causing serpentinization the amount of water carried into the mantle is linked to the maximum depth that the tensional faults cut into the lithosphere which in turn is directly linked to the maximum focal depths of outer rise normal faulting earthquakes we analysed teleseismic p and s waves of seven earthquakes from the trench outer rise offshore of central america using teleseismic waveform inversion of broad band data for the computation of green s functions for waveform inversion probabilistic earthquake locations were calculated to study the rupture process earthquake centroid depths and focal mechanisms for a sequence of subevents were calculated both hypocentral depths from the relocation process and the estimated centroid depths from the waveform inversion show that all events occur at shallow depths 30 km furthermore the locations of the subevents relative to each other suggest that fault planes for mw 6 are in the order of 50 km in length and only 5 10 km in width rupture generally propagates downdip and the focal mechanisms change for most events from normal faulting to strike slip or oblique thrusting with time the depth at which this mechanism change is observed may represent the depth of the nodal plane between tensional and compressional regions in the incoming plate

inversion of complex body waves we have developed a method that inverts seismic body waves to determine the mechanism and rupture pattern of earthquakes the rupture pattern is represented as a sequence of subevents distributed on the fault plane this method is an extension of our earlier method in which the subevent mechanisms were fixed in the new method the subevent mechanisms are determined from the data and are allowed to vary during the sequence when subevent mechanisms are allowed to vary however the inversion often becomes unstable because of the complex trade offs between the mechanism the timing and the location of the subevents many different subevent sequences can explain the same data equally well and it is important to determine the range of allowable solutions some constraints must be imposed on the solution to stabilize the inversion we have developed a procedure to explore the range of allowable solutions and appropriate constraints in this procedure a network of grid points is constructed on the i plane where and i are respectively the onset time and the distance from the epicenter of a subevent the best fit subevent is determined at all grid points then the correlation is computed between the synthetic waveform for each subevent and the observed waveform the correlation as a function of and i and the best fit mechanisms computed at each i grid point depict the character of allowable solutions and facilitate a decision on the appropriate constraints to be imposed on the solution the method is illustrated using the data for the 1976 guatemala earthquake

tsunamigenic potential of outer rise normal faults at the middle america trench in central america the middle america trench is formed by the subduction of the cocos and rivera plates under the caribbean and north american plates the subduction interface presents low coupling in central america showing its seismicity a high frequency of outer rise normal fault earthquakes these outer rise earthquakes are generated on the inherited structures of the seafloor spreading fabric during the subducting plate bending we analyze focal mechanism data in combination with the available structural data of the outer rise normal faults in order to constrain the rupture characteristics of the outer rise normal earthquakes a new empirical scaling relationship is developed to define earthquake magnitudes from normal fault dimensions in the outer rise context we numerically model the tsunami wave propagation due to the worst outer rise tsunamigenic source in the area to estimate its damaging potential wave elevations higher than 2 m are common at the coast in front of the fault extent with maximum wave elevations of 8 m the capability of these faults to generate ocean wide tsunamis is low however they can produce significant tsunamis locally

a slice and view fib sem study of clay gouge from the safod creeping section of the san andreas fault at 2 7 km depth abstract the san andreas fault is one of the most studied earthquake generating structures on earth but the reason that some sections are anomalously weak and creep without apparent seismicity remains poorly understood here we present results from nanoscale fib sem 3d microstructural observations of weak friction coefficient of 0 095 safod clay fault gouge containing serpentinite clasts recovered from the active central deforming zone at 2 7 km vertical depth our nanoscale observations confirm that frictional slip and extreme weakness occur via deformation of smectite clay that forms a shear fabric within the fault zone we infer that creep initiates by fracture controlled substrate growth of oriented mg smectite on r p and y shears followed by clay smearing and ductile flow of an evolving and expanding clay matrix at the crystal scale pervasive sliding occurs along hydrated smectite interlayers and surfaces occupied by exchangeable mg and ca ions with slip typically spaced at 3 5 lattice layers apart we conclude that the strength and seismic behaviour of major tectonic faults at shallow crustal levels evolves as clay fabric develops with accumulated fault slip

inversion of complex body waves ii abstract after our previous paper we developed a method to deconvolve body waves into a multiple shock sequence we first consider a general case where a rupture process consists of subevents with arbitrary source parameters and next a special case where a source process consists of subevents with two different fault mechanisms a simple criterion to check the significance of solution is given by a statistical consideration on the estimated values the method is applied to two large earthquakes the 1976 tangshan earthquake and the 1976 east turkey earthquake in both cases the earthquake sources consist of major strike slip events and minor thrust events the total seismic moments of the major events are comparable to the values obtained by surface wave analyses the 1968 borrego mountain earthquake is also analysed by the same method for comparison a single mechanism solution results in a good waveform match and no significant secondary mechanism is required for this event

mantle wedge hydration in nicaragua from local earthquake tomography the continental margin of nicaragua and costa rica is characterized by significant lateral changes from north to south such as a decreasing dip of the slab a decreasing magma production and a shift in the volcanic front to investigate this transition a joint on and offshore local earthquake tomography was performed low p wave velocities and high vp vs ratios indicative for hydration were found in the upper mantle and lowermost crust beneath the sandino basin the mantle wedge hydration can be estimated to 2 5 wt per cent beneath south nicaragua in contrast the mantle wedge beneath north costa rica is weakly or not hydrated the hydration leads to a local gap in the seismicity in nicaragua the lateral transition between the hydrated and non hydrated areas occurs within a distance of about 10 km this transition coincides with a change in the crustal thickness in the order of 5 10 km thickening to the south and in the tectonic regimes the change in the tectonic regimes towards a stronger extension along the margin of nicaragua could be the key for understanding the observations the extension may support the opening of pathways for a wide zone of fluid migration and hydration through the overriding plate which are identified with areas of low vp high vp vs and low seismicity

seismic structure of the northeastern japan convergent margin a synthesis many studies recently made on the basis of seismic observations have revealed a detailed structure of the crust and the upper mantle beneath the northeastern japan arc and its relationship to seismic and volcanic activity spatial distributions of the depths to the conrad and the moho discontinuities estimated from shallow earthquake data and seismic explosion data show that both discontinuities are deep in the middle of the land area and shallow toward the coastlines of the japan sea and the pacific ocean the pn velocity has a lateral variation it is as low as 7 5 km s beneath the land area while that beneath the japan sea and the pacific ocean is 8 0 8 2 km s it changes abruptly at the transition zones which are located along the coastlines precise structure and location of the subducted pacific plate beneath the land area is inferred from converted or reflected seismic waves at the top or bottom of the plate the pacific plate is composed of a thin 5 km low velocity upper layer and a thick high velocity lower layer its total thickness being 80 90 km the upper plane seismicity of the double seismic zone is confined to the thin low velocity upper layer which probably corresponds to the subducted former oceanic crust the lower plane seismicity lies at the middle of the high velocity lower layer and the lower half of the plate below it is incapable of generating earthquakes the shallower portion of the upper surface of the plate beneath the pacific ocean along which major seismicity with low angle thrust faultings is actually occurring is also located by seismic observations on land and in the sea the pacific plate subducts at an extremely low angle of 5 for the first 25 km depth and then the dip steepens rather abruptly to 30 normal fault type events at the top of the plate have not been detected in the portion where the downward bending is the largest but have been ditected near the trench axis where it is rather small tomographic inversions for seismic velocity structure clearly delineate the inclined high velocity pacific plate with a thickness of 80 90 km and low velocity zones in the crust and the mantle wedge beneath active volcanoes seismic attenuation tomography also shows similar zones of low q value beneath active volcanoes although its spatial resolution is much lower the low velocity zones with 2 6 velocity lows are continuously distributed from the upper crust just beneath active volcanoes to a depth of 100 150 km in the mantle wedge and are approximately parallel to the dip of the underlying pacific plate these low velocity zones probably reflect the pathway of magma ascent from a depth in the mantle wedge to the earth s surface corresponding to a portion of the subduction induced secondary mantle wedge flow

deep lithospheric structures along the southern central chile margin from wide angle p wave modelling p crustal and upper mantle structures of the subduction zone in south central chile between 42 degrees s and 46 degrees s are determined from seismic wide angle reflection and refraction data using the seismic ray tracing method to calculate minimum parameter models three profiles along differently aged segments of the subducting nazca plate were analysed in order to study subduction zone structure dependencies related to the age that is thermal state of the incoming plate the age of the oceanic crust at the trench ranges from 3 ma on the southernmost profile immediately north of the chile triple junction to 6 5 ma old about 100 km to the north and to 14 5 ma old another 200 km further north off the island of chiloe remarkable similarities appear in the structures of both the incoming as well as the overriding plate the oceanic nazca plate is around 5 km thick with a slightly increasing thickness northward reflecting temperature changes at the time of crustal generation the trench basin is about 2 km thick except in the south where the chile ridge is close to the deformation front and only a small 800 m thick trench infill could develop in south central chile typically three quarters 1 5 km of the trench sediments subduct below the decollement in the subduction channel to the north and south of the study area only about one quarter to one third of the sediments subducts the rest is accreted above similarities in the overriding plate are the width of the active accretionary prism 35 50 km and a strong lateral crustal velocity gradient zone about 75 80 km landward from the deformation front where landward upper crustal velocities of over 5 0 5 4 km s su 1 su decrease seaward to around 4 5 km s su 1 su within about 10 km which possibly represents a palaeo backstop this zone is also accompanied by strong intraplate seismicity differences in the subduction zone structures exist in the outer rise region where the northern profile exhibits a clear bulge of uplifted oceanic lithosphere prior to subduction whereas the younger structures have a less developed outer rise this plate bending is accompanied by strongly reduced rock velocities on the northern profile due to fracturing and possible hydration of the crust and upper mantle the southern profiles do not exhibit such a strong alteration of the lithosphere although this effect may be counteracted by plate cooling effects which are reflected in increasing rock velocities away from the spreading centre overall there appears little influence of incoming plate age on the subduction zone structure which may explain why the m w 9 5 great chile earthquake from 1960 ruptured through all these differing age segments the rupture area however appears to coincide with a relatively thick subduction channel

thirty five year creep rates for the creeping segment of the san andreas fault and the effects of the 2004 parkfield earthquake constraints from alignment arrays continuous global positioning system and creepmeters we present results from differential global positioning system gps surveys of seven alignment arrays and four continuous gps sites along the creeping segment of the san andreas fault surveys of four alignment arrays from the central creeping segment yield 33 to 36 year average minimum slip rates of 21 26 mm yr these rates are consistent with previous alignment array surveys spanning a 10 year period and with rates determined by creepmeters indicating approximate steady state creep along the central creeping segment for at least 35 years motion between continuous gps sites that span the central creeping segment is 28 2 0 5 mm yr for two sites that are 1 km apart and 33 6 1 mm yr for two sites that are 70 km apart slip rates therefore increase with distance from the creeping segment of the san andreas fault all rates reported here are significantly slower than the 39 2 mm yr rate predicted for motion between the sierra nevada great valley block and the pacific plate repeat surveys of three alignment arrays following the 2004 parkfield earthquake demonstrate that its coseismic and short term postseismic offsets decrease rapidly with distance from the epicenter from 150 mm to 15 mm to 5 mm at respective distances of 9 36 and 54 km to the northwest continuous gps data confirm that little coseismic and postseismic slip occurred along the central creeping segment geodetic and geologic slip rates are compared and different models for the accommodation of transcurrent deformation across the creeping segment are discussed

upper lithospheric structure of the subduction zone offshore of southern arauco peninsula chile at 38 s velocities 8 4 km s 1 in the trench outer rise area the top of incoming oceanic plate is pervasively fractured and likely hydrated as shown by extensional faults horst and graben structures and a reduction of both crustal and mantle velocities these slow velocities are interpreted in terms of extensional bending related faulting leading to fracturing and hydration in the upper part of the oceanic lithosphere the incoming mocha fz coincides with an area of even slower velocities and thinning of the oceanic crust 10 15 thinning suggesting that the incoming fracture zone may enhance the flux of chemically bound water into the subduction zone slow mantle velocities occur down to a maximum depth of 6 8 km into the upper mantle where mantle temperatures are estimated to be 400 430c in the overriding plate the tomographic model reveals two prominent velocity transition zones characterized by steep lateral velocity gradients resulting in a seismic segmentation of the marine fore arc the margin is composed of three main domains 1 a 20 km wide frontal prism below the continental slope with vp 3 5 km s 1 2 a 50 km area with vp 4 5 5 5 km s 1 interpreted as a paleoaccretionary complex and 3 the seaward edge of the paleozoic continental framework with vp 6 0 km s 1 frontal prism velocities are noticeably lower than those found in the northern erosional chile margin confirming recent accretionary processes in south central chile

determinations of focal depths of earthquakes associated with the bending of oceanic plates at trenches abstract the earthquakes examined in this paper are all within the oceanic lithosphere and are associated with the bending of plates before subduction accurate determinations of the depth of these earthquakes are needed to study the stress pattern within a bending plate routinely determined depths of shallow sub oceanic earthquakes published in bulletins are unreliable the depths can be accurately determined to within a few kilometers if the original seismograms from these events are studied in some cases the reflected phases pp and pwp can be clearly identified there exists the possibility that the wave reflected at the water air interface pwp may be misidentified as pp leading to erroneous estimates of depth additional methods of analysis such as surface wave radiation patterns or the apparent frequency dependence of reflection at the crust water interface can remove this possible source of confusion one of the most powerful techniques for depth analysis is the modelling of long period waveforms the pattern of stresses within the bending oceanic lithosphere revealed by the depths and focal mechanisms of these intraplate earthquakes is one of horizontal deviatoric tension down to a depth of about 25 km with horizontal compression at greater depths

seismic evidence of tectonic control on the depth of water influx into incoming oceanic plates at subduction trenches water transported by slabs into the mantle at subduction zones plays key roles in tectonics magmatism fluid and volatiles fluxes and most likely in the chemical evolution of the earth s oceans and mantle yet incorporation of water into oceanic plates before subduction is a poorly understood process several studies suggest that plates may acquire most water at subduction trenches because the ocean crust and uppermost mantle there are intensely faulted caused by bending and or slab pull and display anomalously low seismic velocities the low velocities are interpreted to arise from a combination of fluid filled fractures associated to normal faulting and mineral transformation by hydration mantle hydration by transformation of nominally dry peridotite to water rich serpentinite could potentially create the largest fluid reservoir in slabs and is therefore the most relevant for the transport of water in the deep mantle the depth of fracturing by normal fault earthquakes is usually not well constrained but could potentially create deep percolation paths for water that might hydrate up to tens of kilometers into the mantle restrained only by serpentine stability yet interpretation of deep intraplate mineral alteration remains speculative because active source seismic experiments have sampled only the uppermost few kilometers of mantle leaving the depth extent of anomalous velocities and their relation to faulting unconstrained here we use a joint inversion of active source seismic data and both local and regional earthquakes to map the three dimensional distribution of anomalous velocities under a seismic network deployed at the trench seafloor we found that anomalous velocities are restrained to the depth of normal fault micro earthquake activity recorded in the network and are considerably shallower than either the rupture depth of teleseismic normal fault earthquakes or the limit of serpentine stability extensional micro earthquakes indicate that each fault in the region slips every 2 3 months which may facilitate regular water percolation deeper teleseismic earthquakes are comparatively infrequent and possibly do not cause significant fracturing that remains open long enough to promote alteration detectable with our seismic study our results show that the stability field of serpentine does not constrain the depth of potential mantle hydration

microstructures and their implications for faulting processes insights from dglab core samples from the gulf of corinth abstract we have examined microstructures mineralogical composition geochemical alteration and texture of four selected fault rock samples from the deep geodynamical laboratory dglab gulf of corinth project using optical microscopy cathodoluminescence microscopy cl scanning electron microscopy sem transmission electron microscopy tem and synchrotron x ray diffraction measurements the fault core is composed of red and gray clayey gouge material and surrounded by a damage zone of brecciated limestones pressure solution features calcite veins and calcite clasts in the breccia and gouge material attest the presence of paleo fluids and fluid driven mass transfer during deformation differences in cl colors between the matrix and calcite vein cement and inside the vein cement suggest repeated infiltration of fluids with different composition from various sources formation water and meteoric water twin lamellae densities estimated in calcite veins are used as paleo piezometer the deduced differential stress is 140 70 mpa for the older vein generation and appears to be higher than stress for the youngest veins 45 23 mpa in spite of the relatively small clay content in both samples newly formed clay minerals have been observed in gray as well as red clayey gouge material differences between gray and red clay gouge material are found in fault rock composition porosity and clay fabric the proportion of chlorite in the red gouge is significantly less than that in the gray gouge whereas the initial porosity is significantly higher than in the gray gouge material the detection of a well oriented clay fabric in red clay gouge samples is unique in comparison to other major fault zones

correlation of clayey gouge in a surface exposure of serpentinite in the san andreas fault with gouge from the san andreas fault observatory at depth safod abstract magnesium rich clayey gouge similar to that comprising the two actively creeping strands of the san andreas fault in drill core from the san andreas fault observatory at depth safod has been identified in a nearby outcrop of serpentinite within the fault zone at nelson creek each occurrence of the gouge consists of porphyroclasts of serpentinite and sedimentary rocks dispersed in a fine grained foliated matrix of mg rich smectitic clays the clay minerals in all three gouges are interpreted to be the product of fluid assisted shear enhanced reactions between quartzofeldspathic wall rocks and serpentinite that was tectonically entrained in the fault from a source in the coast range ophiolite we infer that the gouge at nelson creek connects to one or both of the gouge zones in the safod core and that similar gouge may occur at depths in between the special significance of the outcrop is that it preserves the early stages of mineral reactions that are greatly advanced at depth and it confirms the involvement of serpentinite and the mg rich phyllosilicate minerals that replace it in promoting creep along the central san andreas fault

nanocoatings of clay and creep of the san andreas fault at parkfield california mudrock samples were investigated from two fault zones at 3066 m and 3296 m measured depth md located outside and within the main damage zone of the san andreas fault observatory at depth safod drillhole at parkfield california all studied fault rocks show features typical of those reported across creep zones with variably spaced and interconnected networks of polished displacement surfaces coated by abundant polished films and occasional striations electron microscopy and x ray diffraction study of the surfaces reveal the occurrence of neocrystallized thin film clay coatings containing illite smectite i s and chlorite smectite c s minerals 40 ar 39 ar dating of the illitic mix layered coatings demonstrated miocene to pliocene crystallization and revealed an older fault strand 8 1 3 ma at 3066 m md and a probably younger fault strand 4 4 9 ma at 3296 m md today the younger strand is the site of active creep behavior reflecting a possible re activation of these clay weakened zones we propose that the majority of slow fault creep is controlled by the high density of thin

patterns of mineral transformations in clay gouge with examples from low angle normal fault rocks in the western usa abstract neoformed minerals in shallow fault rocks are increasingly recognized as key to the behavior of faults in the elasto frictional regime but neither the conditions nor the processes which wall rock is transformed into clay minerals are well understood yet understanding of these mineral transformations is required to predict the mechanical and seismogenic behavior of faults we therefore present a systematic study of clay gouge mineralogy from 30 outcrops of 17 low angle normal faults lanf s in the american cordillera to demonstrate the range and type of clay transformations in natural fault gouges the sampled faults juxtapose a wide and representative range of wall rock types including sedimentary metamorphic and igneous rocks under shallow crustal conditions clay mineral transformations were observed in all but one of 28 faults one fault contains only mechanically derived clay rich gouge which formed entirely by cataclasis clay mineral transformations observed in gouges show four general patterns 1 growth of authigenic 1m d illite either by transformation of fragmental 2m 1 illite or muscovite or growth after the dissolution of k feldspar illitization of fragmental illite smectite is observed in lanf gouges but is less common than reported from faults with sedimentary wall rocks 2 retrograde diagenesis of an early mechanically derived chlorite rich gouge to authigenic chlorite smectite and saponite mg rich tri octahedral smectite 3 reaction of mechanically derived chlorite rich gouges with mg rich fluids at low temperatures 50 150 c to produce localized lenses of one of two assemblages sepiolite saponite talc lizardite or palygorskite chlorite quartz and 4 growth of authigenic di octahedral smectite from alteration of acidic volcanic wall rocks these transformation groups are consistent with patterns observed in fault rocks elsewhere the main controls for the type of neoformed clay in gouge appear to be wall rock chemistry and fluid chemistry and temperatures in the range of 60 180 c

outer rise earthquakes and seismic coupling outer rise earthquakes can be separated into two basic categories based on focal mechanism compressional and tensional events reflecting compressional and tensional outer rise stress regimes we propose a model that connects the stress regime in the outer rise to the behavior of large subduction earthquakes and explains the focal mechanisms and both the temporal and spatial occurrence of outer rise earthquakes we find that compressional outer rise events tend to occur oceanward of seismic gaps in seismically coupled subduction zones or equivalently just prior to large subduction zone earthquakes the compressional stress regime is probably caused by slip in adjacent subduction zone segments there are two modes of occurrence for tensional outer rise earthquakes depending on whether they occur in coupled or uncoupled subduction zones in coupled subduction zones tensional events tend to occur after large subduction events as tensional stress from slab pull is temporarily transmitted to the outer rise in subduction zones that are inherently uncoupled tensional events can occur at any time therefore in uncoupled subduction zones the outer rise is always in tension and only tensional outer rise events occur there while in coupled subduction zones the outer rise is in a compressional state prior to large subduction earthquakes and a tensional state afterwards due to the large displacements

the large normal faulting mariana earthquake of april 5 1990 in uncoupled subduction zone a large ms 7 5 shallow earthquake occurred beneath the mariana trench on april 5 1990 from the relocated aftershock distribution the fault area is estimated to be 70 40 km2 a tsunami observed on the japanese islands verifies that the depth of the main shock is shallow for waveform analysis we use long period surface waves and body waves recorded at global networks of gdsn iris geoscope and erios the centroid moment tensor cmt solution from surface waves indicates normal faulting on a fault whose strike is parallel to the local axis of the mariana trench with the tension axis perpendicular to it the seismic moment is 1 4 1020 nm 1027 dyn cm which gives mw 7 3 far field p and sh waves from 13 stations are used to determine the source time function since the sea around the epicentral region is about 5 km deep body waveforms are contaminated with water reverberations the inversion results in a source time function with a predominantly single event with a duration of 10 sec a seismic moment of 2 1 1020 nm and a focal mechanism given by strike 198 dip 48 slip 90 the short duration indicates a small area of the rupture the location of the main shock with respect to the aftershock area suggests that the nodal plane dipping to the west is preferred for the fault plane the local stress drop of the single subevent is estimated to be 150 mpa 1 5 kbars the mariana earthquake is considered to have occurred in an uncoupled region in response to the gravitational pull caused by the downgoing pacific plate

impact of bending related faulting on the seismic properties of the incoming oceanic plate offshore of nicaragua impact of bending related faulting on the seismic properties of the incoming oceanic plate offshore of nicaragua

the mackenzie delta sedimentary processes and facies of a high latitude fine grained delta the mackenzie delta is a large fine grained delta deposited in a cold arctic setting the delta has been constructed upon a flooding surface developed on a previous shelf phase delta there are three principal depositional zones the subaerial delta plain the distributary channel mouth region and the subaqeous delta the subaerial delta plain is characterized by an anastomosing system of high sinuosity channels and extensive thermokarst lake development this region is greatly influenced by the annual cycle of seasonal processes including winter freezing of sediments and channels ice jamming and flooding in the early spring and declining river stage during the summer and autumn deposition occurs on channel levees and in thermokarst lakes during flood events and is commonly rhythmic in nature with discrete annual beds being distinguishable in the channel mouth environment deposition is dominated by landward accretion and aggradation of mouth bars during river and storm surge induced flood events the subaqeous delta is characterized by a shallow water platform and a gentle offshore slope sediment bypassing of the shallow water platform is efficient as a result of the presence of incised submarine channels and the predominance of suspension transport of fine grained sediments facies of the shallow platform include silty sand with climbing ripple lamination offshore facies are dominated by seaward fining fine sand to silt tempestites sea ice scouring and sediment deformation are common beyond 10 m water depth where bioturbated muds are the predominant facies the low angle profile of the shallow water platform is interpreted to be the combined response of a fine grained delta to 1 storm sediment dispersal 2 autoretreat as a result of the increasing subaerial and subaqeous area of deposition as the delta progrades out of its glacial valley 3 limited water depth above the underlying flooding surface and 4 efficient nearshore bypassing of sediment through subice channels at the peak of spring discharge several indicators of the cold climate can be used as criteria for the interpretation of ancient successions including thermokarst lake development submarine channel scours freeze thaw deformation and ice scour deformation structures permafrost inhibits compaction subsidence and together with the shallow water setting also limits autocyclic lobe switching the cold climate can thus influence stratal architecture by favouring the development of regional scale clinoform sets rather than multiple smaller scale lobes separated by autocyclic flooding surfaces

microstructures in landslides in northwest china implications for creeping displacements abstract microstructures mineralogical composition and texture of selected landslide samples from three landslides in the southern part of the gansu province china were examined with optical microscopy transmission electron microscopy tem x ray diffraction xrd and synchrotron x ray diffraction measurements common sheet silicates are chlorite illite muscovite kaolinite pyrophyllite and dickite other minerals are quartz calcite dolomite and albite in one sample graphite and amorphous carbon were detected by tem edx analyses and tem high angle annular dark field images the occurrence of graphite and pyrophyllite with very low friction coefficients in the gouge material of the suoertou and xieliupo landslides is particularly significant for reducing the frictional strength of the landslides it is proposed that the landslides underwent comparable deformation processes as fault zones the low friction coefficients provide strong evidence that slow moving landsliding is controlled by the presence of weak minerals in addition tem observations document that grain size reduction in clayey slip zone material was produced mainly by mechanical abrasion for calcite and quartz grain size reduction was attributed to both pressure solution and cataclasis therefore besides landslide composition the occurrence of ultrafine grained slip zone material may also contribute to weakening processes of landslides tem images of slip zone samples show both locally aligned clay particles as well as kinked and folded sheet silicates which are widely disseminated in the whole matrix small newly formed clay particles have random orientations based on synchrotron x ray diffraction measurements the degree of preferred orientation of constituent sheet silicates in local shear zones of the suoertou and duang he ba landslide is strong this work is the first reported observation of well oriented clay fabrics in landslides

subduction of transitional crust at the manila trench and its geophysical implications abstract the subducting south china sea crust at the northern manila trench has been categorized as the transitional crust which is a rifted thinned continental crust we tested the consequence of the subduction of transitional crust with a numerical dynamic model our numerical model indicates that with subducting transitional crust normal fault earthquakes underneath the accretionary prism would occur deeper and probably with larger magnitude normal fault earthquakes near the trench outer rise are usually shallower than 30 km depth around the world a notable exception is at the northern manila trench where normal fault earthquakes around 40 km depth are common which can be explained by our model our result indicates that the stress on the northern segment of the manila trench is relaxed which is less likely to generate great megathrust earthquakes

geophysical evidence for hydration of the crust and mantle of the nazca plate during bending at the north chile trench water transported in subducting oceanic plates plays a key role in a number of phenomena including intraslab seismicity and arc magmatism however the locus of plate hydration and water distribution in crust and mantle of plates entering subduction zones is debated we present evidence for anomalously low seismic velocities and densities of the crust and upper mantle of the nazca plate at the north chile trench crustal seismic velocities at the trench are lower than velocities of mature fast spreading crust and even lower than velocities of highly extended slow spreading crust in addition the nazca plate at the north chile trench may contain an 20 km thick upper mantle layer with 17 serpentine which implies 2 5 wt water these results document pervasive rock alteration by water percolation linked to bending related extensional faulting

frictional properties of saponite rich gouge from a serpentinite bearing fault zone along the gokasho arashima tectonic line central japan abstract we studied a serpentinite bearing fault zone in gokasho arashima tectonic line mie prefecture central japan characterizing its internal structures mineral assemblage permeability and frictional properties the fault core situated between the serpentinite breccia and the adjacent sedimentary rocks is characterized by a zone locally altered to saponite the clayey gouge layer separates fault rocks of serpentinite origin containing talc and tremolite from fault rocks of sedimentary origin containing chlorite but no quartz the minerals that formed within the fault are the products of metasomatic reaction between the serpentinite and the siliceous rocks permeability measurements show that serpentinite breccia and fault gouge have permeability of 10 14 10 17 m 2 and 10 15 10 18 m 2 respectively at 5 120 mpa confining pressure frictional coefficient of the saponite rich clayey fault gouge ranged between 0 20 and 0 35 under room dry condition but was reduced to 0 06 0 12 when saturated with water the velocity dependence of friction was strongly positive mostly ranging between 0 005 and 0 006 in terms of a b values the governing friction law is not constrained yet but we find that the saponite rich gouge possesses an evolutional behavior in the opposite direction to that suggested by the rate and state friction law in addition to its direct velocity dependence

new kinematic models for pacific north america motion from 3 ma to present i evidence for steady motion and biases in the nuvel 1a model we use velocities derived from 2 4 5 years of continuous gps observations at 21 sites on the pacific and north american plates along with a subset of the nuvel 1a data to examine the steadiness of pacific north america motion since 3 16 ma the transfer of baja california to the pacific plate and the magnitude of biases in the nuvel 1a estimate of pacific north america motion we find that pacific north america motion has remained steady since 3 16 ma but at rates significantly faster than predicted by nuvel 1a in the vicinity of baja california our gps derived model and recent seafloor spreading rates in the southern gulf of california both indicate that the nuvel 1a model underestimates pacific north america rates by 4 2 mm yr 1 steady pacific north america motion since 3 16 myr and increasing seafloor spreading rates since 3 58 myr in the gulf of california imply that pacific north america motion was partitioned between seafloor spreading in the gulf of california and decelerating slip along faults in or offshore from the baja peninsula

postseismic pressure solution creep evidence and time dependent change from dynamic indenting experiments active faults in the earth s upper crust can slide either steadily by aseismic creep or abruptly causing earthquakes seismic and aseismic processes are closely related earthquakes are often followed by transient afterslip creep postseismic displacement rates progressively decrease with time over a period of years or decades so seismic fracturing activates the creep rate and various healing processes progressively reduce it this article presents pressure solution indenter experiments on halite calcite and plaster that show how fracturing and comminution processes induced by dynamic stress loading applied by dropping steel balls drastically accelerate the displacement rates accommodated by pressure solution creep by decreasing the dissolution contact area and the diffusive mass transfer distance along this contact however as fractures progressively heal and dissolution contacts flatten these effects disappear and the displacement rates slow down the time dependent change in indenter displacement after dynamic stress loading has been measured and is best fitted by power laws with exponents that change with time from 0 3 to 1 when healing is achieved natural postseismic afterslip displacement time relationships have been analyzed and also show a power law change with a power law exponent in the range of 0 25 0 4 it is proposed that the variation in power law exponent with time is related to the change in morphology of the dissolution contact that is fractured or comminuted during the dynamic event and is then progressively healed and smoothed in natural faults monitoring the power law parameters could give access to the characteristic healing time in the fault

intraplate seismicity and related mantle hydration at the nicaraguan trench outer rise summary we examine micro earthquake records from a dense temporary array of ocean bottom seismometers obs and hydrophones that has been installed from september to november 2005 at the trench outer rise offshore nicaragua approximately 1 5 locatable earthquakes per day within the array of 110 120 km show the high seismic activity in this region seismicity is restricted to the upper 15 km of the mantle and hence where temperatures reach 350 400 c which is smaller than values observed for large mantle intraplate events 650 c determination of moment tensor solutions suggest a change of the stress region from tensional in the upper layers of the oceanic plate to compressional beneath the neutral plane between both regimes is located at 6 9 km beneath moho and thus very shallow fluids which are thought to travel through the tensional fault system into the upper mantle may not be able to penetrate any deeper the earthquake catalogue which seems to be complete for magnitudes above mw 1 6 1 8 suggests a strong change of the lithospheric rheology when approaching the trench and b factors that is the ratio between small and large earthquakes increase significantly in the closest 20 km to the trench axis implying that the crust and upper mantle is massively weakened and hence ruptures more frequently but under less release of stress we explain this with a partly serpentinized upper mantle

comparative mineral chemistry and textures of safod fault gouge and damage zone rocks abstract creep in the san andreas fault observatory at depth safod drillhole is localized to two foliated gouges the central deforming zone cdz and southwest deforming zone sdz the gouges consist of porphyroclasts of serpentinite and sedimentary rock dispersed in a foliated matrix of mg smectite clays that formed as a result of shearing enhanced reactions between the serpentinite and quartzofeldspathic rocks the cdz takes up most of the creep and exhibits differences in mineralogy and texture from the sdz that are attributable to its higher shearing rate in addition a 0 2 m wide sector of the cdz at its northeastern margin ne cdz is identical to the sdz and may represent a gradient in creep rate across the cdz the sdz and ne cdz have lower clay contents and larger porphyroclasts than most of the cdz and they contain veinlets and strain fringes of calcite in the gouge matrix not seen elsewhere in the cdz matrix clays in the sdz and ne cdz are saponite and corrensite whereas the rest of the cdz lacks corrensite saponite is younger than corrensite reflecting clay crystallization under declining temperatures and clays in the more actively deforming portions of the cdz have better equilibrated to the lower temperature conditions

rollovers in salt tectonics the inadequacy of the listric fault model abstract rollover anticlines are syn sedimentary extensional structures common in salt tectonics that are classically considered to result from layer bending during hangingwall displacement above a listric normal fault however theoretical considerations and seismic images of natural examples put in question the validity of the listric fault model from which rollover growth is steady state by nature and controlled by the relative displacement of two blocks we used laboratory experiments of gravity driven deformation of brittle ductile models simulating salt tectonics processes to study in which circumstances rollovers are obtained both in time and space it is argued that rollover growth is a nonsteady process that results from a mechanical instability whose critical lifetime can be divided in three characteristic stages birth growth and decay in kinematic terms the development of a synthetic or antithetic rollover requires a three block system an upslope block and a downslope block that move downslope and that separate from each other on top of an extending decollement layer lying above a basement block synthetic and antithetic rollovers developed with upward concave shapes whatever the sense of shear in the salt layer and can pass laterally to each other without transform fault this points out that the concave shape of a rollover base is not an argument in favour of listric faulting the concave shape simply results from the connection between a steeply dipping normal fault and a flat lying or gently dipping decollement

fluid rock interactions in seismic faults implications from the structures and mineralogical and geochemical compositions of drilling cores from the rupture of the 2008 wenchuan earthquake china we describe the structural features and mineralogical and geochemical compositions of the fault rocks recovered from boreholes at the golden river site on the yingxiu beichuan fault which activated and slipped along a 240 km long main surface rupture zone during the 2008 wenchuan earthquake the fault which accommodated co seismic slip cuts granitic rocks from the pengguan complex in which this earthquake most likely nucleated fault rocks including cohesive cataclasite unconsolidated breccia and three fault gouges with distinct colors were identified from the drilling cores on going uplift and erosion in the area means that the fault rocks formed at different depth were exhumed to the shallow surface during the uplift history of the longmenshan fault zone a clear change from fracturing and comminution in the cataclasites and breccia to more pervasive shear formation of fine grained materials in the gouges has been observed the gouges are distinct and have accommodated significant displacement in multiple increments of shear furthermore fault rocks recovered from the boreholes display numerous features indicative of fluid infiltration and fluid rock interaction toward the fault core clay minerals have replaced feldspars the element enrichment depletion patterns of the fault rocks show general fluid infiltration trends such as 1 mobile elements are generally depleted in the fault rocks 2 the microstructural mineralogical and geochemical results of the fault rocks consistently indicate that pervasive fluid infiltration and fluid rock interactions altered feldspars and mafic minerals to clay minerals the fluid was mg2 and fe2 rich facilitating formation of chlorite isocon analyses further reveal that a large rock volume has been lost which is attributed to the removal of mobile elements associated with fluid infiltration and perhaps enhanced by pressure solution these results reflect the accumulated effects of cataclasis and fluid infiltration fluid rock interaction during the long evolution history of the longmenshan fault zone fluid rock interactions in seismic faults implications from the structures and mineralogical and geochemical compositions of drilling cores from the rupture of the 2008 wenchuan earthquake china

talc friction in the temperature range 25 400 c relevance for fault zone weakening abstract talc is one of the weakest minerals that is associated with fault zones triaxial friction experiments conducted on water saturated talc gouge at room temperature yield values of the coefficient of friction shear stress effective normal stress n in the range 0 16 0 23 and increases with increasing n talc gouge heated to temperatures of 100 400 c is consistently weaker than at room temperature and talc has a temperature pressure range of stability that extends from surficial to eclogite facies conditions making it of potential significance in a variety of faulting environments talc has been identified in exhumed subduction zone thrusts in fault gouge collected from oceanic transform and detachment faults associated with rift systems and recently in serpentinite from the central creeping section of the san andreas fault typically talc crystallized in the active fault zones as a result of the reaction of ultramafic rocks with silica saturated hydrothermal fluids this mode of formation of talc is a prime example of a fault zone weakening process because of its velocity strengthening behavior talc may play a role in stabilizing slip at depth in subduction zones and in the creeping faults of central and northern california that are associated with ophiolitic rocks

talc bearing serpentinite and the creeping section of the san andreas fault the section of the san andreas fault located between cholame valley and san juan bautista in central california creeps at a rate as high as 28 mm yr 1 ref 1 and it is also the segment that yields the best evidence for being a weak fault embedded in a strong crust serpentinized ultramafic rocks have been associated with creeping faults in central and northern california and serpentinite is commonly invoked as the cause of the creep and the low strength of this section of the san andreas fault however the frictional strengths of serpentine minerals are too high to satisfy the limitations on fault strength and these minerals also have the potential for unstable slip under some conditions here we report the discovery of talc in cuttings of serpentinite collected from the probable active trace of the san andreas fault that was intersected during drilling of the san andreas fault observatory at depth safod main hole in 2005 we infer that the talc is forming as a result of the reaction of serpentine minerals with silica saturated hydrothermal fluids that migrate up the fault zone and the talc commonly occurs in sheared serpentinite this discovery is significant as the frictional strength of talc at elevated temperatures is sufficiently low to meet the constraints on the shear strength of the fault and its inherently stable sliding behaviour is consistent with fault creep talc may therefore provide the connection between serpentinite and creep in the san andreas fault if shear at depth can become localized along a talc rich principal slip surface within serpentinite entrained in the fault zone

faulting processes in active faults evidences from tcdp and safod drill core samples abstract the microstructures mineralogy and chemistry of representative samples collected from the cores of the san andreas fault drill hole safod and the taiwan chelungpu fault drilling project tcdp have been studied using optical microscopy tem sem xrd and xrf analyses safod samples provide a transect across undeformed host rock the fault damage zone and currently active deforming zones of the san andreas fault tcdp samples are retrieved from the principal slip zone psz and from the surrounding damage zone of the chelungpu fault substantial differences exist in the clay mineralogy of safod and tcdp fault gouge samples amorphous material has been observed in safod as well as tcdp samples in line with previous publications we propose that melt observed in tcdp black gouge samples was produced by seismic slip melt origin whereas amorphous material in safod samples was formed by comminution of grains crush origin rather than by melting dauphine twins in quartz grains of safod and tcdp samples may indicate high seismic stress the differences in the crystallographic preferred orientation of calcite between safod and tcdp samples are significant microstructures resulting from dissolution precipitation processes were observed in both faults but are more frequently found in safod samples than in tcdp fault rocks as already described for many other fault zones clay gouge fabrics are quite weak in safod and tcdp samples clay clast aggregates ccas proposed to indicate frictional heating and thermal pressurization occur in material taken from the psz of the chelungpu fault as well as within and outside of the safod deforming zones indicating that these microstructures were formed over a wide range of slip rates

sequential deformation from serpentinite mylonite to metasomatic rocks along the sashu fault sw japan the deformation of serpentinites along the sashu fault saganoseki peninsula sw japan involved both mylonitization and brecciation the brecciation was accompanied by metasomatism and the metasomatized rocks record additional ductile deformation serpentinite mylonite is composed mainly of antigorite with minor magnetite and cr spinel foliation and lineation in this rock type are defined by the shape preferred orientation of antigorite and the alignment of fine grained magnetite and carbonate minerals the antigorite also shows a lattice preferred orientation lpo as measured with a universal stage with c axes oriented near perpendicular to the foliation and b axes oriented parallel to the lineation brecciation is concentrated in the reaction zone along the boundary between serpentinites and surrounding country rocks talc and chlorite schists developed in this zone as a result of associated metasomatism and deformation consequently they contain composite planar fabrics and drag folds the breccias themselves are composed of serpentinite fragments cut by veins of calcite and talc analyses of fluid inclusions in calcite from the breccia indicate that brecciation occurred at 200 300 c at depths of 3 8 11 8 km metasomatic reactions particularly the formation of talc contributed to weakening of this serpentinite bearing fault zone

constraints on paleofluid sources using the clumped isotope thermometry of carbonate veins from the safod san andreas fault observatory at depth borehole abstract the san andreas fault observatory at depth safod near parkfield california is a borehole drilled through two active deforming zones of the san andreas fault the southwest deforming zone sdz and the central deforming zone cdz these zones accommodate displacement by seismic slip and aseismic creep elevated fluid pressures and fluid rock interactions have been proposed to explain the low apparent strength and aseismic creep observed but the origin of the fluids and existence of high fluid pressures remains uncertain we use clumped isotope thermometry and 18o 13c compositions of calcite in veins to constrain the origin of paleofluids and compare these results to the isotopic composition of modern day pore fluids from the safod borehole and nearby areas we observe that 1 calcite vein temperatures vary from 81 to 134 c which overlaps the current ambient borehole temperatures of 110 115 c at sampled depths 2 vein calcite is not in carbon isotope equilibrium with modern day pore fluids 3 the 18o values of paleofluids close to the sdz and cdz calculated from vein 18o and temperature data are not in equilibrium with local modern day pore waters but approach equilibrium with modern pore waters far from these zones and 4 syntectonic vein calcite is only in c and o isotopic equilibrium with their host rocks within the sdz and cdz spatial patterns of 18o and 13c show little evidence for across fault fluid flow clumped isotope temperatures are consistent with locally derived fluid sources but not with continuous or episodic replenishment of fluids from shallow sedimentary brines or deep fluid sources our findings are compatible with flow of meteoric fluids from the southwestern damage zone into the sdz and cdz which would have favored the formation of weak phyllosilicates and contributed to the present day weakness of the two actively deforming zones

elevated time dependent strengthening rates observed in san andreas fault drilling samples abstract the central san andreas fault in california is known as a creeping fault however recent studies have shown that it may be accumulating a slip deficit and thus its seismogenic potential should be seriously considered we conducted laboratory friction experiments measuring time dependent frictional strengthening healing on fault zone and wall rock samples recovered during drilling at the san andreas fault observatory at depth safod located near the southern edge of the creeping section and in the direct vicinity of three repeating microearthquake clusters we find that for hold times of up to 3000 s frictional healing follows a log linear dependence on hold time and that the healing rate is very low for a sample of the actively shearing fault core consistent with previous results however considering longer hold times up to 350 000 s the healing rate accelerates such that the data for all samples are better described by a power law relation in general samples having a higher content of phyllosilicate minerals exhibit low log linear healing rates and the notably clay rich fault zone sample also exhibits strong power law healing when longer hold times are included our data suggest that weak faults such as the creeping section of the san andreas fault can accumulate interseismic shear stress more rapidly than expected from previous friction data using the power law dependence of frictional healing on hold time calculations of recurrence interval and stress drop based on our data accurately match observations of discrete creep events and repeating m w 2 earthquakes on the san andreas fault

chemical controls on fault behavior weakening of serpentinite sheared against quartz bearing rocks and its significance for fault creep in the san andreas system 1 the serpentinized ultramafic rocks found in many plate tectonic settings commonly are juxtaposed against crustal rocks along faults and the chemical contrast between the rock types potentially could influence the mechanical behavior of such faults to investigate this possibility we conducted triaxial experiments under hydrothermal conditions 200 350 c shearing serpentinite gouge between forcing blocks of granite or quartzite in an ultramafic chemical environment the coefficient of friction of lizardite and antigorite serpentinite is 0 5 0 6 and increases with increasing temperature over the tested range however when either lizardite or antigorite serpentinite is sheared against granite or quartzite strength is reduced to 0 3 with the greatest strength reductions at the highest temperatures temperature weakening and slowest shearing rates velocity strengthening the weakening is attributed to a solution transfer process that is promoted by the enhanced solubility of serpentine in pore fluids whose chemistry has been modified by interaction with the quartzose wall rocks the operation of this process will promote aseismic slip creep along serpentinite bearing crustal faults at otherwise seismogenic depths during short term experiments serpentine minerals reprecipitate in low stress areas whereas in longer experiments new mg rich phyllosilicates crystallize in response to metasomatic exchanges across the serpentinite crustal rock contact long term shear of serpentinite against crustal rocks will cause the metasomatic mineral assemblages which may include extremely weak minerals such as saponite or talc to play an increasingly important role in the mechanical behavior of the fault our results may explain the distribution of creep on faults in the san andreas system

relations between earthquake magnitude and fault rupture dimensions how regionally variable are they data on subsurface rupture dimensions and seismic moments have been gathered for earthquakes in new zealand screened for data quality and analyzed along with the multiregion rupture area data of hanks and bakun 2002 regression analyses have been carried out for relations among magnitude and rupture length subsurface width area slip and aspect ratio statistical comparisons show that there are significant differences between the regressions of new zealand and multiregion data the new zealand relations among magnitude area and displacement are linear those among magnitude subsurface rupture length and width are bilinear whereas those between magnitude and rupture aspect ratio l w are trilinear the magnitudes predicted from rupture subsurface lengths widths or areas by multiregion relations are about 0 4 units smaller than those obtained from relations based on new zealand data the multiregion relations predict ruptures that are untenably large for new zealand on the basis of structural and other evidence separate regressions were carried out for the california china and japan subsets of the multiregion data and it was found that there were statistically significant differences between new zealand and california new zealand and japan new zealand and china and japan and california the respective contributions of rupture area and mean displacement to seismic moment are very different in new zealand from those in california japan and china suggesting that rupture styles differ with geological region in a manner yet to be appropriately defined thus multiregion relations are a poor approximation of the new zealand data and perhaps also of the data of other regions where geological conditions differ substantially from the multiregion average

a global earthquake discrimination scheme to optimize ground motion prediction equation selection we present a new automatic earthquake discrimination procedure to determine in near real time the tectonic regime and seismotectonic domain of an earth quake its most likely source type and the corresponding ground motion prediction equation gmpe class to be used in the u s geological survey usgs global shakemap system this method makes use of the flinn engdahl regionalization scheme seismotectonic information plate boundaries global geology seismicity cat alogs and regional and local studies and the source parameters available from the usgs national earthquake information center in the minutes following an earthquake to give the best estimation of the setting and mechanism of the event depending on the tectonic setting additional criteria based on hypocentral depth style of faulting and regional seismicity may be applied for subduction zones these criteria include the use of focal mechanism information and detailed interface models to discriminate among outer rise upper plate interface and intraslab seismicity the scheme is validated against a large database of recent historical earthquakes though developed to assess gmpe selection in global shakemap operations we anticipate a variety of uses for this strategy from real time processing systems to any analysis involving tectonic classification of sources from seismic catalogs

outer slope faulting associated with the western kuril and japan trenches summary elongated fault escarpments on the outer slopes of the western kuril and japan trenches have been investigated through detailed swath bathymetric mapping numerous horsts and grabens formed by these escarpments were identified distinct n70 e linear alignment of the escarpments parallel to the magnetic anomaly lineations was revealed on the outer slope of the western kuril trench in the japan trench north of 39 00aen most of the escarpments are parallel to the trench axis and oblique to the magnetic lineations a zig zag pattern of faulting exists south of 39 00aen each topographic profile was decomposed by computer analysis into two curves representing 1 the smoothed long wavelength slope of the subducting ocean crust surface and 2 the short wavelength 10 km roughness of plateaus and valleys edged by outwardand inward facing fault escarpments throughout the surveyed areas escarpment heights increase from the crest of the trench outer swell down to a depth of about 6000 m on the slope of the outer trench wall but with no distinct increase below that depth no significant diverence is recognized in fault throws towards and away from the trench it can be concluded that these elongated escarpments originate from normal faults on the upper layer of the oceanic crust under extensional stress in a direction perpendicular to the trench axis which is caused by downward bending of the subducting lithosphere the relationship of escarpment height to escarpment length is similar to that obtained from normal fault escarpments in the east pacific rise crest the maximum length and height of escarpments are small in the kuril trench compared with those in the japan trench implying a diverence in mechanical strength depending on the fault orientation the crust is weakest along the inherited spreading fabric second weakest probably along the non transform ovset direction and strongest in directions very oblique to these orientations seamounts appear to be more rigid than normal ocean crust with no particular weak orientations resulting in fewer but larger faults along the axis of plate bending as most clearly represented in the subducting daiichi kashima seamount

a study of natural and experimental metasomatic assemblages in an ultramafic quartzofeldspathic metasomatic system from the haast schist south island new zealand the mineralogy of a metasomatic sequence formed between ultramafic and quartzofeldspathic protoliths from the southern alps of new zealand consists of a forsterite antigorite core surrounded concentrically by zones of antigorite magnesite magnesite talc talc tremolite chlorite and muscovite with discontinuous pods of albite associated with the muscovite zone on the basis of trace element data the original ultramafite schist contact is positioned between the present tremolite and chlorite zones

depth extent of rupture of the 1981 chilean outer rise earthquake as inferred from long period body waves we have investigated how well the depth extent of rupture can be determined for a shallow dip slip earthquake from long period wwssn body waves the 16 october 1981 chilean outer rise earthquake mw 7 2 is analyzed as an example calculation of the resolution matrix for the vertical distribution of displacement rate on the fault plane shows that the depth extent of rupture cannot be resolved better than 20 km even when the data set consists of both p and s waves although the depth extent of rupture of the 1981 earthquake is too small to be resolved it is possible to infer some constraints from the point source depth in combination with the source duration while inversion for the depth extent of rupture can be formulated as a linear problem determination of the point source depth is nonlinear and a formal resolution study cannot be conducted the point source depth estimate for the 1981 earthquake is 28 to 32 km 80 confidence interval the source duration is 8 sec assuming a rupture velocity of 3 km sec we have explored several rupture models in a synthetic experiment to shed light on the relation between point source depth and centroid depth the experiments show that point source depths agree with the centroid depth given that there is poor resolution of the depth extent of rupture of the 1981 earthquake we conclude from the synthetic experiments that the earthquake may have ruptured updip to a depth as shallow as 17 km or downdip to a depth as deep as 43 km

source retrieval for deep local earthquakes with broadband records abstract body wave data recorded at a small network of broadband seismograph stations are analyzed to investigate local events with focal depths deeper than about 50 km for these events the initial portion of p wave displacement represents well the source time function with a scaler correction for the seismic moment the magnitudes of the analyzed earthquakes range from m w 3 1 to 6 5 it is shown that the seismic moment m 0 and the pulse width are well correlated as m 0 3 constant indicating that the stress drop is largely constant this dynamic similarity seems to be valid for a vast range of earthquake sizes m w 1 8 it is also shown that source complexity such as a multiple shock nature is not a characteristic of only large earthquakes but is often observed even for small earthquakes

strengths of serpentinite gouges at elevated temperatures serpentinite has been proposed as a cause of both low strength and aseismic creep of fault zones to test these hypotheses we have measured the strength of chrysotile lizardite and antigorite rich serpentinite gouges under hydrothermal conditions with emphasis on chrysotile which has thus far received little attention at 25 c the coefficient of friction of chrysotile gouge is roughly 0 2 whereas the lizardite and antigorite rich gouges are at least twice as strong the very low room temperature strength of chrysotile is a consequence of its unusually high adsorbed water content when the adsorbed water is removed chrysotile is as strong as pure antigorite gouge at room temperature heating to 200 c causes the frictional strengths of all three gouges to increase limited data suggest that different polytypes of a given serpentine mineral have similar strengths thus deformation induced changes in polytype should not affect fault strength at 25 c the chrysotile gouge has a transition from velocity strengthening at low velocities to velocity weakening at high velocities consistent with previous studies at temperatures up to 200 c however chrysotile strength is essentially independent of velocity at low velocities overall chrysotile has a restricted range of velocity strengthening behavior that migrates to higher velocities with increasing temperature less information on velocity dependence is available for the lizardite and antigorite gouges but their behavior is consistent with that outlined for chrysotile the marked changes in velocity dependence and strength of chrysotile with heating underscore the hazards of using room temperature data to predict fault behavior at depth the velocity behavior at elevated temperatures does not rule out serpentinite as a cause of aseismic slip but in the presence of a hydrostatic fluid pressure gradient all varieties of serpentine are too strong to explain the apparent weakness of faults such as the san andreas

the frictional strength of talc gouge in high velocity shear experiments talc is present in several large scale fault zones world wide and is mineralogically stable at temperature of the upper crust it is therefore necessary to gain a better understanding of the frictional behavior of talc under a wide range of slip velocity conditions occurring during the seismic cycle we analyzed the frictional and structural characteristics of room dry and water saturated talc gouge by shear experiments on a confined gouge layer at slip velocity range of 0 002 0 66 m s and normal stress up to 4 1 mpa room dry talc showed a distinct slip strengthening with the initial friction coefficient of 0 4 increased systematically to 1 at slip distance d 1 m room dry talc also displayed velocity strengthening at slip distances shorter than 1 m the water saturated talc gouge displayed systematic low frictional strength of 0 1 0 3 for the entire experimental range with clear velocity strengthening behavior with positive a b values rate dependence parameter of rate state friction of 0 01 0 04 the microstructural analyses revealed distributed shear and systematic dilation up to 50 for the room dry talc in contrast to the extreme slip localization and strong shear compaction for water saturated talc we propose that talc frictional strength is controlled by lubrication along cleavage surfaces that is facilitated by adsorbed water room dry and surplus water water saturated this mechanism can explain our experimental observations of slip strengthening and velocity strengthening for both types of talc gouge as well as other clay minerals it is thus expected that talc presence in fault zones would enhance creep and inhibit unstable slip

the wet nicaraguan slab 1 nicaraguan volcanoes show globally high concentrations of geochemical tracers from dehydration of subducting crust which may reflect a slab with unusually high amounts of h2o to test this possibility we measure seismic velocities at the top of the subducted plate and compare them with predictions for hydrated mafic rocks regional seismic p waves for intraslab events at 100 150 km depth show a high frequency late arrival apparently trapped in a low velocity waveguide 2 5 6 km thick at the top of the downgoing plate 14 5 2 2 slower than surrounding mantle the velocities can be explained by 5 wt h2o in the subducted crust 2 3 times the hydration inferred for other slabs by similar methods this interpretation implies extensive hydration of the cocos plate off nicaragua perhaps enhanced by up dip fluid flow within the slab at 100 km depth

serpentinization in the trench outer rise region offshore of nicaragua constraints from seismic refraction and wide angle data summary recent seismic evidence suggested that most oceanic plate hydration is associated with trenchouter rise faulting prior to subduction hydration at trenches may have a significant impact on the subduction zone water cycle previous seismic experiments conducted to the northwest of nicoya peninsula northern costa rica have shown that the subducting cocos lithosphere is pervasively altered which was interpreted to be due to both hydration serpentinization and fracturing of the crustal and upper mantle rocks new seismic wide angle reflection and refraction data were collected along two profiles running parallel to the middle american trench axis offshore of central nicaragua revealing lateral changes of the seismic properties of the subducting lithosphere seismic structure along both profiles is characterized by low velocities both in the crust and upper mantle velocities in the uppermost mantle are found to be in the range 7 3 7 5 km s 1 thus are 8 10 per cent lower than velocities typical for unaltered peridotites and hence confirm the assumption that serpentinization is a common process at the trench outer rise area offshore of nicaragua in addition a prominent velocity anomaly occurred within the crust beneath two seamounts here velocity reduction may indicate increased porosity and perhaps permeability supporting the idea that seamounts serve as sites for water percolation and circulation

an analysis of the factors that control fault zone architecture and the importance of fault orientation relative to regional stress the moment magnitude 7 2 el mayor cucapah emc earthquake of 2010 in northern baja california mexico produced a cascading rupture that propagated through a geometrically diverse network of intersecting faults these faults have been exhumed from depths of 6 10 km since the late miocene based on low temperature thermochronology synkinematic alteration and deformational fabrics coseismic slip of 1 6 m of the emc event was accommodated by fault zones that displayed the full spectrum of architectural styles from simple narrow fault zones 100 m in width that have multiple anastomosing high strain cores as fault zone complexity and width increase the full spectrum of observed widths 20 200 m coseismic slip becomes more broadly distributed on a greater number of scarps that form wider arrays thus the infinitesimal slip of the surface rupture of a single earthquake strongly replicates many of the fabric elements that were developed during the long term history of slip on the faults at deeper levels of the seismogenic crust we find that factors such as protolith normal stress and displacement which control gouge production in laboratory experiments also affect the architectural complexity of natural faults fault zones developed in phyllosilicate rich metasedimentary gneiss are generally wider and more complex than those developed in quartzo feldspathic granitoid rocks we hypothesize that the overall weakness and low strength contrast of faults developed in phyllosilicate rich host rocks leads to strain hardening and formation of broad multi stranded fault zones fault orientation also strongly affects fault zone complexity which we find to increase with decreasing fault dip we attribute this to the higher resolved normal stresses on gently dipping faults assuming a uniform stress field compatible with this extensional tectonic setting the conditions that permit slip on misoriented surfaces with high normal stress should also produce failure of more optimally oriented slip systems in the fault zone promoting complex branching and development of multiple high strain cores overall we find that fault zone architecture need not be strongly affected by differences in the amount of cumulative slip and instead is more strongly controlled by protolith and relative normal stress

revised tectonic boundaries in the cocos plate off costa rica implications for the segmentation of the convergent margin and for plate tectonic models the oceanic cocos plate subducting beneath costa rica has a complex plate tectonic history resulting in segmentation new lines of magnetic data clearly define tectonic boundaries which separate lithosphere formed at the east pacific rise from lithosphere formed at the cocos nazca spreading center they also define two early phase cocos nazca spreading regimes and a major propagator in addition to these sharply defined tectonic boundaries are overprinted boundaries from volcanism during passage of cocos plate over the galapagos hot spot the subducted segment boundaries correspond with distinct changes in upper plate tectonic structure and features of the subducted slab newly identified seafloor spreading anomalies show oceanic lithosphere formed during initial breakup of the farallon plate at 22 7 ma and opening of the cocos nazca spreading center a revised regional compilation of magnetic anomalies allows refinement of plate tectonic models for the early history of the cocos nazca spreading center at 19 5 ma a major ridge jump reshaped its geometry and after 14 5 ma multiple southward ridge jumps led to a highly asymmetric accretion of lithosphere a suspected cause of ridge jumps is an interaction of the cocos nazca spreading center with the galapagos hot spot

the 1977 sumba earthquake series evidence for slab pull force acting at a subduction zone the great 1977 sumba earthquake occurred at the eastern sunda trench just west of the collision of australian continental lithosphere with the island arc the length of the aftershock zone of this normal faulting earthquake is about 200 km aftershocks are concentrated 65 115 km east of the main shock epicenter with very few aftershocks in a 50 km long segment that spans the main shock epicenter relocated hypocenters and focal mechanism data are consistent with normal faulting throughout the upper 28 km of the oceanic lithosphere there is no evidence for thrust faulting of the deeper aftershocks these data imply that the neutral bending surface must be at least 35 40 km deep a second aftershock zone about 180 km northwest of the main shock became active immediately following the main shock but events were concentrated during days 50 52 this zone is a 70 km long lineation that trends toward the main shock epicenter and reflects right lateral strike slip faulting within the subducted oceanic plate seismicity exists to a depth of about 650 km in the very old plate beneath the sunda banda arc and that plate s negative buoyancy causes very large slab pull forces great interface thrust earthquakes are absent at the sumba region and slab pull forces are inferred to have partially decoupled the subducted plate from the overriding plate this decoupling permits slab pull stresses to be guided updip to the region of the sumba main shock such shallow acting slab pull provides a bending moment at the trench and explains the deformation and timing observed for the entire sumba earthquake series in this model slab pull forces stretch the subducted plate until the increasing stresses at the shallow subduction zone lead to a subduction zone earthquake postseismically the released oceanic plate undergoes a pulse of downdip strain returning the plate to a less extended state the moment of this downdip plate motion could exceed the seismic moment of the main shock

deep focus earthquakes and recycling of water into the earth s mantle for more than 50 years observations of earthquakes to depths of 100 to 650 kilometers inside earth have been enigmatic at these depths rocks are expected to deform by ductile flow rather than brittle fracturing or frictional sliding on fault surfaces laboratory experiments and detailed calculations of the pressures and temperatures in seismically active subduction zones indicate that this deep focus seismicity could originate from dehydration and high pressure structural instabilities occurring in the hydrated part of the lithosphere that sinks into the upper mantle thus seismologists may be mapping the recirculation of water from the oceans back into the deep interior of our planet

micromechanisms of creep in clay rich gouge from the central deforming zone of the san andreas fault we report the strength and constitutive behavior of gouge sampled from the central deforming zone cdz of the san andreas fault layers of flaked cdz gouge were sheared in the triaxial saw cut configuration using the stress relaxation technique to measure the gouge strength over 4 orders of magnitude in shear strain rate and at rates as low as 5 10 10s 1 and within an order of magnitude of in situ rates deformation conditions correspond to the in situ effective normal stress 100 mpa and temperature 65 to 120 c at the sampling depth of 2 7 km gouge was sheared dry and with brine pore fluid at 25 mpa pore pressure dry gouge is stronger and more rate strengthening than brine saturated gouge brine saturated cdz gouge strengthens with increasing strain rate and decreasing temperature and the dependencies of strength on strain rate and temperature increase at rates below 5 10 9s 1 at strain rates greater than 5 10 9s 1 the rate dependence is consistent with previous studies on the cdz gouge conducted at even higher rates the increase in rate dependence below 5 10 9s 1 indicates a change in the rate controlling deformation mechanism the magnitude of the friction rate dependence parameter a and the temperature sensitivity of a are consistent with crystal plasticity of the phyllosilicates we hypothesize a micromechanical model for the cdz gouge whereby a transition from fracture and delamination accommodated frictional flow to crystal plasticity accommodated frictional flow occurs with decreasing strain rate

nucleation process of magnitude 2 repeating earthquakes on the san andreas fault predicted by rate and state fault models with safod drill core data recent laboratory shear slip experiments conducted on a nominally flat frictional interface reported the intriguing details of a two phase nucleation of stick slip motion that precedes the dynamic rupture propagation this behavior was subsequently reproduced by a physics based model incorporating laboratory derived rate and state friction laws however applying the laboratory and theoretical results to the nucleation of crustal earthquakes remains challenging due to poorly constrained physical and friction properties of fault zone rocks at seismogenic depths here we apply the same physics based model to simulate the nucleation process of crustal earthquakes using unique data acquired during the san andreas fault observatory at depth safod experiment and new and existing measurements of friction properties of safod drill core samples using this well constrained model we predict what the nucleation phase will look like for magnitude 2 repeating earthquakes on segments of the san andreas fault at a 2 8 km depth we find that despite up to three orders of magnitude difference in the physical and friction parameters and stress conditions the behavior of the modeled nucleation is qualitatively similar to that of laboratory earthquakes with the nucleation consisting of two distinct phases our results further suggest that precursory slow slip associated with the earthquake nucleation phase may be observable in the hours before the occurrence of the magnitude 2 earthquakes by strain measurements close a few hundreds meters to the hypocenter in a position reached by the existing borehole

seismic coupling and outer rise earthquakes variation of interplate seismic coupling at subduction zones is a major factor controlling the size of the largest underthrusting events this variation also has a profound effect on the regional intraplate stresses in the vicinity of the subduction zone outer rise seismicity is strongly correlated with variations in interplate coupling reflecting the stress state of the interplate coupled zone over 200 outer rise earthquakes with known focal mechanisms are used to investigate the relationship between stresses in the outer rise and interplate seismic coupling these events occur within the downgoing i e oceanic plate near the bathymetric trench axes and generally fall into the categories of tensional normal or compressional thrust with their tensional or compressional stress axes oriented approximately horizontal and perpendicular to the trench in uncoupled subduction zones only tensional outer rise earthquakes occur which indicates that the outer rise is dominated by tensional stresses associated with plate bending and or slab pull forces in strongly coupled subduction zones both tensional and compressional outer rise events are found these events are related both spatially and temporally to the distribution of large underthrusting earthquakes and are thus an integral part of the earthquake cycle in the strongly coupled regions tensional outer rise events follow large underthrusting events as the outer rise is temporarily in tension due to the underthrusting motion compressional outer rise events take place as compressional stress slowly accumulates oceanward of locked sections of the interplate zone in four instances compressional outer rise earthquakes have been followed by large underthrusting events which have occurred 2 4 7 and 19 years after the associated outer rise event the remaining compressional outer rise events are located in regions that are either known seismic gaps or in regions where the seismic potential is unknown the occurrence of compressional outer rise earthquakes suggests that compressional stress is accumulating in the adjacent interplate region and that there is the potential for a future large underthrusting event in the region thirty compressional outer rise events have been located in trench segments of middle and south america the kurile islands the tonga and kermadec islands the new hebrides arc and the solomon islands regions in both the southern kamchatka and northern new hebrides regions the outer rise seismicity indicates that the stress regimes in the outer rise have changed with time from tensional following a previous large underthrusting event to compressional at present thus three stages of the cycle from underthrusting to tensional outer rise regime to compressional outer rise regime are present requiring only the occurrence of the next underthrusting event to complete the cycle the occurrence of compressional outer rise events is useful for assessing the seismic potential of a region on an intermediate time scale

co seismic and or a seismic microstructures of jfast 343 core samples from the japan trench abstract the microstructures mineralogy and chemistry of four representative samples collected from cores extracted from the japan trench during integrated ocean drilling project expedition 343 the japan trench fast drilling project jfast have been studied using optical microscopy tem sem xrf xrd and microprobe analyses the samples provide a transect from relatively undeformed marine sediments in the hanging wall to the undeformed footwall material crossing the thrust interface between the pacific and north american plate where the fault slipped during the march 2011 tohoku oki earthquake our preliminary results suggest that the low strength of jfast fault gouge material is caused by the high amount of clay minerals 60 smectite 14 illite clay minerals in the decollement gouge sample are partly replaced by newly formed manganese oxide which precipitated from hydrothermal fluids dauphine twins were found in quartz grains of the decollement sample suggesting local high stress possible during seismic loading other microstructures cannot be assigned unambiguously to co seismic or a seismic faulting processes the observed scaly clay fabric is consistent with observations in many other plate boundary fault zones significant grain size reduction was found in the fault decollement zone sample but a change in lithology of the fault material cannot be ruled out microstructures typical for a seismic deformation like dissolution precipitation features e g dissolved grain boundaries mineral alteration occur in all jfast core samples but more frequently in the decollement sample

detailed internal structure and along strike variability of the core of a plate boundary fault the highland boundary fault scotland the highland boundary fault near stonehaven ne scotland provides a rare opportunity to study the internal fault structure of a well exposed along strike section of an ancient plate boundary fault as in many plate boundaries serpentinite juxtaposes quartzo feldspathic crustal rocks of distinct terranes we report for the first time the complex internal structure of the highland boundary fault core comprised of four structurally and chemically distinct clay rich units that remain unmixed despite the evidence for internal strain relatively intact clasts of wall rock and microfossils are preserved within the clay the fault core clay minerology is consistent with a shallow low temperature authigenesis derived from shear enhanced chemical reactions between wall rocks of contrasting chemistry during sinistral strike slip the observed structure is comparable to those of other major weak cored plate boundaries e g the san andreas fault through detailed mapping we demonstrate that the internal structure of a plate boundary fault core can vary in thickness and composition along strike over centimetre to metre length scales earthquake rupture mechanics critically depend on the physical properties of fault rock assemblages therefore models that investigate rupture propagation at active plate boundaries should incorporate or else assess tolerance and sensitivity to variable fault core thickness and composition supplementary material further x ray diffraction xrd analytical data are available at https doi org 10 6084 m9 figshare c 4663640

passive and active seismological study of bending related faulting and mantle serpentinization at the middle america trench water transported within the subducting oceanic lithosphere into the earth s interior affects a wealth of subduction zone processes including intraslab earthquakes and arc magmatism in recent years growing evidence suggests that much of the hydration of oceanic plates occurs at the trench ocean slope right before subduction here normal faults are created while the rigid lithosphere bends into the trench offshore of middle america multi channel seismic reflection imaging suggests that bending related faults cut into the uppermost mantle providing a mechanism for hydration and transformation of mantle peridotites into serpentinites seismic wide angle reflection and refraction data were collected coincident with one of the seismic profiles where the faults have been imaged travel time inversion provides evidence that both crustal and uppermost mantle velocities are reduced with respect to the velocity structure found in mature oceanic crust away from deep sea trenches if mantle velocity reduction is solely produced by hydration velocities indicate 10 15 of serpentinization in the uppermost 3 km of the mantle where seismic data provide enough resolution a small network of ocean bottom hydrophones deployed for about a month detected 3 local micro earthquakes per day earthquake epicentres align with fault scarps at the seafloor and continuous earthquake activity might be an important process to facilitate the percolation of seawater into the upper mantle

surface fault slip associated with the 2004 parkfield california earthquake surface fracturing occurred along the san andreas fault the subparallel southwest fracture zone and six secondary faults in association with the 28 september 2004 m 6 0 parkfield earthquake fractures formed discontinuous breaks along a 32 km long stretch of the san andreas fault sense of slip was right lateral only locally was there a minor 1 11 mm vertical component of slip right lateral slip in the first few weeks after the event early in its afterslip period ranged from 1 to 44 mm our observations in the weeks following the earthquake indicated that the highest slip values are in the middle mountain area northwest of the mainshock epicenter creepmeter measurements indicate a similar distribution of slip surface slip along the san andreas fault developed soon after the mainshock field checks in the area near parkfield and about 5 km to the southeast indicated that surface slip developed more than 1 hr but generally less than 1 day after the event slip along the southwest fracture zone developed coseismically and extended about 8 km sense of slip was right lateral locally there was a minor to moderate 1 29 mm vertical component of slip right lateral slip ranged from 1 to 41 mm surface slip along secondary faults was right lateral the right lateral component of slip ranged from 3 to 5 mm surface slip in the 1966 and 2004 events occurred along both the san andreas fault and the southwest fracture zone in 1966 the length of ground breakage along the san andreas fault extended 5 km longer than that mapped in 2004 in contrast the length of ground breakage along the southwest fracture zone was the same in both events yet the surface fractures were more continuous in 2004 surface slip on secondary faults in 2004 indicated previously unmapped structural connections between the san andreas fault and the southwest fracture zone further revealing aspects of the structural setting and fault interactions in the parkfield area