propagating rift pseudofaults zones of crustal underplating imaged by multichannel seismic reflection data two multichannel seismic reflection lines recorded over the northern juan de fuca plate off western canada provide images of the crustal structure of three pseudofaults the traces of propagating rifts anomalous reflectivity including both subhorizontal and dipping events at times below the normal crust mantle transition is prevalent in the region of the pseudofaults although the crustal thickness elsewhere on the lines varies by only about 10 crust associated with the pseudofaults is as much as 30 thicker than average this may be due to an increase in thickness of the interpreted oceanic crustal layer 3b or crustal underplating the results indicate that magma supply at propagating rifts may at times be large perhaps due to the effects of nearby hotspots or convective partial melting associated with the initiation of rifting

the cleft revealed geologic magnetic and morphologic evidence for construction of upper oceanic crust along the southern juan de fuca ridge 1 the geology and structure of the cleft segment of the southern juan de fuca ridge jdfr have been examined using high resolution mapping systems observations by remotely operated vehicle rov rov mounted magnetometer and the geochemical analysis of recovered lavas bathymetric mapping using multibeam em300 coupled with in situ observations that focused on near axis and flank regions provides a detailed picture of 0 to 400 ka upper crust created at the southern terminus of the jdfr a total of 53 rock cores and 276 precisely located rock or glass samples were collected during three cruises that included sixteen rov dives our observations of the seafloor during these dives suggest that many of the unfaulted and extensive lava flows that comprise and or cap the prominent ridges that flank the axial valley emanate from ridge parallel faults and fissures that formed in the highly tectonized zone that forms the walls of the axial valley the geochemically evolved and heterogeneous nature of these near axis and flank eruptions is consistent with an origin within the cooler distal edges of a crustal magma chamber or mush zone in contrast the most recent axial eruptions are more primitive higher mgo chemically homogeneous lobate sheet and massive flows that generate a distinct magnetic high over the axial valley we suggest that the syntectonic capping volcanics observed off axis were erupted from near axis and flank fissures and created a thickened extrusive layer as suggested by the magnetic and seismic data this model suggests that many of the lavas that comprise the elevated ridges that bound the axial valley of the cleft segment were erupted during the collapse of a magmatic cycle not during the robust phase that established a new magmatic cycle

basin centered asperities in great subduction zone earthquakes a link between slip subsidence and subduction erosion 1 published areas of high coseismic slip or asperities for 29 of the largest circum pacific megathrust earthquakes are compared to forearc structure revealed by satellite free air gravity bathymetry and seismic profiling on average 71 of an earthquake s seismic moment and 79 of its asperity area occur beneath the prominent gravity low outlining the deep sea terrace 57 of an earthquake s asperity area on average occurs beneath the forearc basins that lie within the deep sea terrace in sw japan slip in the 1923 1944 1946 and 1968 earthquakes was largely centered beneath five forearc basins whose landward edge overlies the 350 c isotherm on the plate boundary the inferred downdip limit of the locked zone basin centered coseismic slip also occurred along the aleutian mexico peru and chile subduction zones but was ambiguous for the great 1964 alaska earthquake beneath intrabasin structural highs seismic slip tends to be lower possibly due to higher temperatures and fluid pressures kilometers of late cenozoic subsidence and crustal thinning above some of the source zones are indicated by seismic profiling and drilling and are thought to be caused by basal subduction erosion the deep sea terraces and basins may evolve not just by growth of the outer arc high but also by interseismic subsidence not recovered during earthquakes basin centered asperities could indicate a link between subsidence subduction erosion and seismogenesis whatever the cause forearc basins may be useful indicators of long term seismic moment release the source zone for cascadia s 1700 a d earthquake contains five large basin centered gravity lows that may indicate potential asperities at depth the gravity gradient marking the inferred downdip limit to large coseismic slip lies offshore except in northwestern washington where the low extends landward beneath the coast transverse gravity highs between the basins suggest that the margin is seismically segmented and could produce a variety of large earthquakes

detailed tectonics near the tip of the galapagos 95 5 w propagator how the lithosphere tears and a spreading axis develops a model for propagation of an active spreading axis into preexisting lithosphere has been developed based on intensive detailed studies using seamarc ii sea beam deep tow and alvin near the tip of the propagator growing westward at 52 km m y along the east west trending galapagos spreading axis near 95 5 w initial lithospheric rifting appears to be accommodated along reactivated roughly east west abyssal hill faults at the tectonic tip extension leads to downdropping of a keystone block which is pervasively cut by normal faults and fissures the initial volcanism initial volcanic tip occurs about 6 5 km behind the tectonic tip as basalt erupts through these fissures covering most of the keystone block the neovolcanic axial tip occurs another 4 5 km back at the western termination of a well defined axial pillow ridge along which volcanism is localized spreading along the propagator axis accelerates to the full rate 29 mm yr half rate at the full rate tip interpreted to be 10 km behind the neovolcanic axial tip or 21 km behind the tectonic tip a complex pattern of strain in the tip area results from the interaction of plate spreading transform shear stress crack propagation dynamic depression of the western end of the axial valley and the transform zone and variations in the strength thickness heterogeneity and anisotropy of the lithosphere southwesterly curvature of the neovolcanic axis near its tip is inferred to result primarily from the response of young thin weak relatively homogeneous lithosphere to counterclockwise rotation of the maximum tensional stress direction relative to the spreading direction due to the influence of shear stress from the transform zone similar stress interactions likely occur at the tectonic tip but are inferred to be accommodated in this older thick strong heterogeneous and anisotropic lithosphere by oblique slip along a set of left stepping reactivated preexisting east west abyssal hill structures although on a time scale of 106 years the overall process of propagation is mostly continuous rifting at the tectonic tip appears discontinuous at a scale of 1500 m or 30 000 years based on the east west spacing of grabens formed as rifting begins the scale of discontinuity of propagation of the neovolcanic axis is 3 5 km or 50 000 100 000 years based on the distribution of northeast trending features interpreted as beheaded western terminations of paleoneovolcanic axes the extension involved in rifting is 10 20 based on summing the horizontal normal separation of all normal faults and fissures integrated and averaged over the zone of rifting these values are approximately doubled if simple shear listric faulting is dominant rather than pure shear normal faulting

a left lateral strike slip fault seaward of the oregon convergent margin we have mapped a recently active left lateral strike slip fault the wecoma fault on the floor of cascadia basin west of the oregon convergent margin using seamarc i sidescan sonar seabeam bathymetry and multichannel seismic and magnetic data the fault intersects the base of the continental slope at 45 10 n and extends northwest 293 for at least 18 5 km the fault s western terminus was not identified and the eastern end of the fault splays apart and disrupts the lower continental slope the fault extends to the base of the 3 5 km thick sedimentary section and overlies a basement discontinuity that may be related to movement along the wecoma fault prominent seafloor features crosscut by the fault individually display between 120 and 2500 m of left lateral separation allowing the general history of fault motion to be evaluated the fault s average slip rate since 10 24 ka is inferred to be 5 12 mm yr based on the age of an offset submarine channel surficial structural relationships in conjunction with the maximum inferred slip rate indicate that fault movement initiated at least 210 ka and that the fault has been active during the holocene

iceberg drift and ocean circulation in the northwestern weddell sea antarctica abstract icebergs are distinctive features in the southern ocean and can influence the physicochemical properties of the ocean environment around them their trajectories are influenced by ocean currents sea ice bathymetry and wind action here twenty five icebergs were identified and tracked by applying image classification methods to a series of 142 advanced synthetic aperture radar asar images obtained during 2008 and 2009 tracking these icebergs made it possible to associate the patterns of iceberg drift with the main ocean currents and front systems in this region including the antarctic coastal current the antarctic slope front and the weddell front a case study investigated here revealed the recirculation of a weddell sea iceberg within the bransfield strait in the northern antarctic peninsula as well as its corresponding mass loss starting in the weddell sea and extending throughout the monitoring period the icebergs studied experienced an average disintegration percentage of 21 15 and an average drift speed of 0 22 0 11 km h 1 the icebergs that drifted at least 500 km had an average meltwater injection rate of 62 76 38 27 m3 s 1 in the ocean around the northern antarctic peninsula

slip vectors and stretching of the sumatran fore arc slip vectors from thrust earthquakes at the java trench southwest of sumatra demonstrate that the sumatran fore arc is not a single rigid plate that is translated to the northwest by oblique plate convergence instead they indicate arc parallel stretching of the fore arc at a uniform strain rate of 3 4 x 10 8 yr the northwestward motion of the fore arc relative to the upper plate southeast asia increases from near zero at the sunda strait to 45 60 mm yr in northwest sumatra and should result in variable slip rates on the sumatran fault

oblique strike slip faulting of the central cascadia submarine forearc at least nine wnw trending left lateral strike slip faults have been mapped on the oregon washington continental margin using sidescan sonar seismic reflection and bathymetric data augmented by submersible observations the faults range in length from 33 to 115 km and cross much of the continental slope five faults offset both the juan de fuca plate and north american plates and cross the plate boundary with little or no offset by the frontal thrust left lateral separation of channels folds and holocene sediments indicate active slip during the holocene and late pleistocene offset of surficial features ranges from 120 to 900 m and displaced subsurface piercing points at the seaward ends of the faults indicate a minimum of 2 2 to 5 5 km of total slip near their western tips fault ages range from 300 ka to 650 ka yielding late pleistocene holocene slip rates of 5 5 2 to 8 5 2 mm yr the geometry and slip direction of these faults implies clockwise rotation of fault bounded blocks about vertical axes within the cascadia forearc structural relationships indicate that some of the faults probably originate in the juan de fuca plate and propagate into the overlying forearc the basement involved faults may originate as shears antithetic to a dextral shear couple within the slab as plate coupling forces are probably insufficient to rupture the oceanic lithosphere the set of sinistral faults is consistent with a model of regional deformation of the submarine forearc defined to include the deforming slab by right simple shear driven by oblique subduction of the juan de fuca plate

interseismic coupling segmentation and mechanical behavior of the central chile subduction zone global positioning system gps measurements carried out in chile over the last two decades showed that an entire portion of the nazca south america subduction zone 38 s 24 s was locked over this period of time the induced accumulation of elastic deformation in the upper plate was not released until the recent maule earthquake of 27 february 2010 mw 8 8 that ruptured the southern part of this section locking or coupling between the two plates varies both with depth and along strike here we use our own gps data an updated solution of our extended network in central chile combined with other published data sets to quantify the spatial variations of the coupling that prevailed before the maule earthquake using a simple elastic model based on the back slip assumption we show that coupling variations on the subduction plane are sufficient to explain the observed surface deformation with no need of a sliver in central chile we identify four segments characterized by higher coupling and separated by narrow areas of lower coupling this segmentation is in good agreement with historical and recent seismicity in chile in particular the narrow zones of lower coupling seem to have stopped most large seismic ruptures including maule s these zones are often associated with irregular bathymetric or coastal features fracture zones or peninsulas finally coseismic and early post seismic slip distribution of the maule earthquake occurring either in previously highly or weakly coupled zones map a complex distribution of velocity weakening and velocity strengthening patches on the subduction interface

the structure and kinematics of the central taiwan mountain belt derived from geological and seismicity data 1 the structure of the taiwan mountain belt is thought to be that of an imbricate thrust and fold belt developed in a forward breaking sequence above a shallowly dipping basal detachment in recent years however a growing amount of seismicity data from the internal part of the mountain belt indicates the existence of widespread fault activity in the middle and lower crust suggesting that deeper levels of the crust must be involved in the deformation than predicted by the shallow detachment imbricate thrust belt model to address this issue we present new geological mapping together with earthquake focal mechanism and seismic energy release data from the central part of taiwan we concur with the interpretation that the foreland basin part of the western foothills comprises an imbricate thrust system that is developing as a forward breaking sequence that is structurally and kinematically linked to a basal detachment at between 7 and 10 km depth to the east of the foreland basin however in the hsuehshan and central ranges our data show the presence of two fault systems an earlier inactive thrust system with a well developed cleavage is cut by a system of steeply dipping active faults that penetrate to a depth of 25 to 30 km or more in the hsuehshan range the second fault system is best represented by a structural and kinematic model in which this part of the mountain belt forms a zone of transpression with a structural architecture similar to that of a crustal scale positive flower structure eastward in the central range mesozoic basement rocks are over thrusting strongly folded and cleaved deep water sediments of the first now inactive thrust system the involvement of deep crustal levels and mesozoic basement in the second fault system is suggestive of the reactivation of preexisting basin bounding faults that were located on the eurasian continental margin

oceanic rift propagation a cause of crustal underplating and seamount volcanism we present the first seismic reflection data across a pseudofault zone the trace of a propagating rift away from a spreading axis strong reflections from the crust mantle transition are discontinuous across the pseudofault deeper reflections which originate near the base of the crust formed at the propagator tip dip beneath the older oceanic crust and become subhorizontal they are interpreted to represent the lower limit of an underplated subcrustal plutonic complex an anomalously smooth basement surface indicative of massive lava flows and a 1200 m high seamount are above the underplated zone the sill complex extends tens of kilometres ahead of and off axis from the former location of the propagating rift indicating that the magmatic supply to the propagator tip was unusually vigorous other seamounts in the northeast pacific lie close to pseudofaults and may have formed as a result of rift propagation

accretion and recent deformation of sediments along the northern cascadia subduction zone new multichannel seismic reflection profiles acoustic imagery and swath bathymetric data from the vancouver island continental margin reveal the surface morphology cross sectional geometry and deformation style of this part of the cascadia accretionary prism the incoming 3 to 5 km thick sediment section fails most commonly along landward dipping thrusts that penetrate to or very close to the top of the underlying oceanic crust the faults are listric with approximately circular cross sections and hanging wall sediments are back tilted in a planar manner high near lithostatic pore pressures in the deeper part of the section are inferred from the flattening of the faults near the decollement and from the long distances over which the decollement has propagated seaward the faults dip more steeply at 40 45 near the surface where pleistocene turbidites are displaced similar folds have developed where the turbidites are particularly thick 4 km and near hydrostatic pore pressures are inferred to occur in the turbidite part of the section an increase in pore pressure with depth is expected as a consequence of sedimentation and tectonic loading particularly because the deeper hemipelagic sediments are probably far less permeable than are the overlying turbidites the top of the downgoing juan de fuca plate which defines the base of the accreted sedimentary wedge beneath the margin is well imaged in the new seismic reflection profiles and is further defined by land reflection data seismic refraction and earthquake hypocenters the landward dipping continental backstop to the prism is also well located if these constraints are used for the geometry of the accretionary prism a general balance is found between the present prism volume and the estimated total sediment supply over the approximately 42 m y accretion history this result along with the observation that deformation front faults penetrate most or all of the sediment section suggests that little or no sediment has been subducted since initiation of the present phase of subduction in the eocene application of the critically tapered wedge model of davis and others to the observed prism geometry suggests that high average pore pressures occur in the prism particularly the seawardmost part beneath the lower slope beneath the continental shelf the dip of the subducting slab increases to greater than 11 a dip that according to the critically tapered wedge model is too steep to allow upward growth of an accretionary prism this provides a simple explanation for ongoing subsidence of the tofino sedimentary basin over the older part of the accretionary complex to the southeast the juan de fuca plate dips much more gently beneath the olympic peninsula of northern washington allowing the accretionary prism to grow upward well above sea level to form the olympic mountains

central cascadia subduction zone creep central cascadia between 43 n and 46 n has reduced interseismic uplift observed in geodetic data and coseismic subsidence seen in multiple thrust earthquakes suggesting elevated persistent fault creep in this section of the subduction zone we estimate subduction thrust decade scale locking and crustal block rotations from three component continuous global positioning system gps time series from 1997 to 2013 as well as 80 year tide gauge and leveling derived uplift rates geodetic observations indicate coastal central oregon is rising at a slower rate than coastal washington southern oregon and northern california modeled locking distributions suggest a wide locking transition zone that extends inland under central oregon paleoseismic records of multiple great earthquakes along cascadia indicate less subsidence in central oregon the cascade thrust under central oregon may be partially creeping for at least 6500 years the length of the paleoseismic record reducing interseismic uplift and resulting in reduced coseismic subsidence large accretions of eocene age basalt siletzia terrane between 43 n and 46 n may be less permeable compared to surrounding terranes potentially increasing pore fluid pressures along the fault interface resulting in a wide zone of persistent fault creep in a separate inversion three component gps time series from 1 july 2005 to 1 january 2011 are used to estimate upper plate deformation locking between slow slip events sses slip from 16 sses and an earthquake mechanism cumulative sses and tectonic tremor are weakest between 43 n and 46 n where partial fault creep is increased and siletzia terrane is thick suggesting that surrounding rock properties may influence the mode of slip

aseismic zone and earthquake segmentation associated with a deep subducted seamount in sumatra the subduction of topographic features has been linked both to earthquake generation and the segmentation of faults seismic imaging reveals a seamount subducted to 40 km depth below sumatra that is associated with an aseismic zone suggesting that at this location the seamount reduces coupling of the slab and overriding plate

seismic evidence for a magma chamber beneath the slow spreading mid atlantic ridge seismic reflections from magma chambers have been observed along the fast spreading east pacific rise1 2 and the intermediate spreading valu fa ridge3 4 sub axial reflections also exist beneath the intermediate spreading juan de fuca ridge5 but no magma chambers have been identified beneath the slow spreading mid atlantic ridge suggesting that here magma chambers lie unusually deep or are transient features6 11 seismic reflection profiles acquired in 1989 over the snake pit hydrothermal area in the rift valley of the mid atlantic ridge 25 km south of the kane fracture zone showed no evidence of magmatic activity12 although geochemical analyses of hydrothermal vent fluids suggest the existence of magma at depths as shallow as 1a 2 km13 14 by suppressing in these data high amplitude coherent noise generated at the sea floor i have obtained images in an otherwise non reflective crust of seismic reflections beneath and just south of the snake pit hydrothermal area these reflections define a small 4 km wide dome whose apex is 1 200 m beneath the sea floor as bright reflections from the upper flanks of this dome occur in the depth range suggested by the vent fluid geochemistry i interpret the dome to be the seismic expression of a small magma chamber

subducted seamounts and recent earthquakes beneath the central cascadia forearc bathymetry and magnetic anomalies indicate that a seamount on the juan de fuca plate has been subducted beneath the central cascadia accretionary complex and is now located 45 km landward of the deformation front passage of this seamount through the accretionary complex has resulted in a pattern of uplift followed by subsidence that has had a profound influence on slope morphology gas hydrate stability and sedimentation based on potential field data and a new three dimensional seismic velocity model we infer that this is the most recent of several seamounts subducted over the past several million years beneath this segment of cascadia more deeply subducted seamounts may be responsible for recent earthquake activity on the plate boundary in this region and for along strike variations in the thickness of the subduction channel which may affect coupling across the plate boundary

stable creeping fault segments can become destructive as a result of dynamic weakening faults in earth s crust accommodate slow relative motion between tectonic plates through either similarly slow slip or fast seismic wave producing rupture events perceived as earthquakes these types of behaviour are often assumed to be separated in space and to occur on two different types of fault segment one with stable rate strengthening friction and the other with rate weakening friction that leads to stick slip the 2011 tohoku oki earthquake with moment magnitude m w 9 0 challenged such assumptions by accumulating its largest seismic slip in the area that had been assumed to be creeping here we propose a model in which stable rate strengthening behaviour at low slip rates is combined with coseismic weakening due to rapid shear heating of pore fluids allowing unstable slip to occur in segments that can creep between events the model parameters are based on laboratory measurements on samples from the fault of the m w 7 6 1999 chi chi earthquake the long term slip behaviour of the model which we examine using a unique numerical approach that includes all wave effects reproduces and explains a number of both long term and coseismic observations some of them seemingly contradictory about the faults at which the tohoku oki and chi chi earthquakes occurred including there being more high frequency radiation from areas of lower slip the largest seismic slip in the tohoku oki earthquake having occurred in a potentially creeping segment the overall pattern of previous events in the area and the complexity of the tohoku oki rupture the implication that earthquake rupture may break through large portions of creeping segments which are at present considered to be barriers requires a re evaluation of seismic hazard in many areas

the structure of cretaceous oceanic crust of the nw pacific constraints on processes at fast spreading centers a 725 km long transect along a flow line in the nw pacific provides new images of the internal structure of cretaceous oceanic crust formed at fast spreading rates reflections from the moho transition zone document changes in crustal travel time along the profile variations with a wavelength of 100 150 km may representing fluctuations in the magma supply with a 2 3 m y periodicity to the south the crust may be several kilometers thicker than normal in places possibly resulting from increased magma supply due to the distal effects of the shatsky hotspot at about the depth expected for the boundary between seismic layers 2 and 3 sub horizontal reflections are imaged over much of the profile the clearest are positive polarity and of an amplitude consistent with a reflection coefficient of 0 1 implying a sharp velocity and density discontinuity northward ridgeward dipping reflections in the lower crust are imaged over much of the profile particularly toward the north away from the region possibly affected by the hotspot these reflections stop at the moho reflection but die out perhaps 1 s beneath top basement they probably represent lithological layering as predicted by models of ductile flow accompanying passive upwelling during plate separation although they could represent secondary shears generated by basal drag during active mantle upwelling beneath the spreading center the discrete character and in places regular spacing every 8 km of these features hint at a cyclicity in the layering and thus at some episodicity in magmatic processes over a timescale of 0 1 0 2 m y

automated detection and location of tectonic tremor along the entire cascadia margin from 2005 to 2011 abstract we have constructed an automated routine to identify prominent bursts of tectonic tremor and locate their source region during time periods of raised amplitude in the tremor passband this approach characterizes 62 episodes of tectonic tremor between 2005 and 2011 with tremor epicenters forming a narrow band spanning the entire length of the cascadia subduction zone we find a range of along strike lengths in individual episodes but the length appears proportional to both duration and geodetic moment consistent with proposed scaling laws for slow earthquake phenomena examination of individual episodes in detail reveals intriguing updip downdip migration patterns including slow updip migration during initiation and repetitive downdip migration between different episodes the broader catalog of tremor episodes refines the inferences from earlier work that episodic tremor and slip are segmented along strike and correlated with apparent seismogenic zone segmentation in most cases the overall band of tremor is offset 50 km from the downdip edge of interseismic coupling along the central and northern parts of the subduction zone along the southern part of the subduction zone it is adjacent to this boundary suggesting that the locked and transition zones may be more closely linked in southern cascadia

confidence intervals for motion and deformation of the juan de fuca plate finite rotation parameters for 17 isochrons from anomalies 1 4a for the juan de fuca pacific plate pair are reported comparison of confidence intervals for plate rotations derived by different techniques indicates that a single plate misfit criterion significantly underestimates the confidence region defining the confidence interval by a threshold level of misfit determined by an f statistic gives similar results to the technique of bootstrap resampling for internally consistent data bootstrap resampling offers far greater robustness in the case of inconsistent data precision of about 1 km in the direction normal to isochrons is available for the younger anomalies this precision allows confident recognition of 2 5 km of eastward deformation of the pacific plate 50 100 km south of the active sovanco transform slight deformation of the northern juan de fuca plate adjacent to the nootka fault zone is also apparent east of the northern gorda ridge younger 3 ma anomalies do not show measurable deformation near the blanco fracture zone but older anomalies are up to 30 km east of their position predicted by rigid behavior of the juan de fuca plate reconstruction of the overlap geometry of past propagators shows no evidence that the extent of overlap has ever exceeded 70 km even with offset of 75 km in one case and 140 km in another new techniques for testing for intervals of constant plate motion require at least two changes in instantaneous motion at 99 confidence a clockwise change in motion direction at about 5 ma and an increase in the gradient of spreading rate since 2 ma two other changes in gradient since 5 ma are required at 95 confidence

probable low angle thrust earthquakes on the juan de fuca north america plate boundary in 2004 two clusters of earthquakes occurred in the central part of the cascadia forearc which displays several characteristics indicative of along strike and downdip variations in plate coupling moment tensor analysis for the main shock in each cluster indicates that both events which had magnitudes of 4 9 and 4 8 were compatible with low angle thrust motion on fault planes dipping 6 15 to the east consistent with the plate boundary dip of 12 by tracing rays through a high resolution two dimensional crustal velocity model to match arrival times of secondary arrivals pp and pmp we estimate that the source depth was 9 11 km for the m4 9 event and 15 17 km for the m4 8 event we conclude that these earthquakes probably represent seismogenic thrust motion on the nominally locked or transitional part of the cascadia megathrust updip of where episodic tremor and slip ets have been documented continued high resolution observations of seismicity and improved crustal models are needed to confirm whether apparent temporal correlations between ets and continued seismic activity in these clusters indicate stress transfer along the megathrust

gravity and seismic study of crustal structure along the juan de fuca ridge axis and across pseudofaults on the ridge flanks variations in topography and seismic structure are observed along the juan de fuca jdf ridge axis and in the vicinity of pseudofaults on the ridge flanks left by former episodes of ridge propagation here we analyze gravity data coregistered with multichannel seismic data from the jdf ridge and flanks in order to better understand the origin of crustal structure variations in this area the data were collected along the ridge axis and along three ridge perpendicular transects at the endeavor northern symmetric and cleft segments negative mantle bouguer anomalies of 21 to 28 mgal are observed at the axis of the three segments thicker crust at the endeavor and cleft segments is inferred from seismic data and can account for the small differences in axial gravity anomalies 3 7 mgal additional low densities elevated temperatures within and or below the axial crust are required to explain the remaining axial mba low at all segments gravity models indicate that the region of low densities is wider beneath the cleft segment gravity models for pseudofaults crossed along the three transects support the presence of thinner and denser crust within the pseudofault zones that we attribute to iron enriched crust on the young crust side of the pseudofaults a 10 20 km wide zone of thicker crust is found reflection events interpreted as subcrustal sills underlie the zones of thicker crust and are the presumed source for the iron enrichment

reflection signature of seismic and aseismic slip on the northern cascadia subduction interface at the northern cascadia margin the juan de fuca plate is underthrusting north america at about 45 mm yr 1 ref 1 resulting in the potential for destructive great earthquakes2 3 the downdip extent of coupling between the two plates is difficult to determine because the most recent such earthquake thought to have been in 1700 4 occurred before instrumental recording thermal and deformation studies5 indicate that off southern vancouver island the interplate interface is presently fully locked for a distance of 60 km downdip from the deformation front great thrust earthquakes on this section of the interface with magnitudes of up to 9 4 5 have been estimated to occur at an average interval of about 590 yr ref 3 further downdip there is a transition from fully locked behaviour to aseismic sliding where high temperatures allow ductile deformation with the deep aseismic zone exhibiting slow slip thrust events6 here we show that there is a change in the reflection character on seismic images from a thin sharp reflection where the subduction thrust is inferred to be locked to a broad reflection band at greater depth where aseismic slip is thought to be occurring this change in reflection character may provide a new technique to map the landward extent of rupture in great earthquakes and improve the characterization of seismic hazards in subduction zones

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

juan de fuca slab geometry and its relation to wadati benioff zone seismicity 1 a new model of the subducted juan de fuca plate beneath western north america allows first order correlations between the occurrence of wadati benioff zone earthquakes and slab geometry temperature and hydration state the geo referenced 3d model constructed from weighted control points integrates depth information from earthquake locations and regional seismic velocity studies we use the model to separate earthquakes that occur in the cascadia forearc from those that occur within the underlying juan de fuca plate and thereby reveal previously obscured details regarding the spatial distribution of earthquakes seismicity within the slab is most prevalent where the slab is warped beneath northwestern california and western washington suggesting that slab flexure in addition to expected metamorphic dehydration processes promotes earthquake occurrence within the subducted oceanic plate earthquake patterns beneath western vancouver island are consistent with slab dehydration processes conversely the lack of slab earthquakes beneath western oregon is consistent with an anhydrous slab double differenced relocated seismicity resolves a double seismic zone within the slab beneath northwestern california that strongly constrains the location of the plate interface and delineates a cluster of seismicity 10 km above the surface that includes the 1992 m7 1 mendocino earthquake we infer that this earthquake ruptured a surface within the cascadia accretionary margin above the juan de fuca plate we further speculate that this earthquake is associated with a detached fragment of former farallon plate other subsurface tectonic elements within the forearc may have the potential to generate similar damaging earthquakes

segmentation and holocene surface faulting on the median tectonic line southwest japan the median tectonic line mtl active fault system is a 300 km long arc parallel right lateral strike slip fault related to oblique subduction of the philippine sea plate beneath the eurasian plate at the nankai trough southwest japan the fault on shikoku island has a slip rate of 5 10 mm yr one of the highest in onshore japan we identified 12 geometric segments along the mtl separated by discontinuities such as en echelon steps bends changes in strike and gaps in the surface trace a chronology of latest holocene surface faulting on the mtl was constructed based on previously published and new work including trenching geomorphic mapping and acoustic profiling and piston coring of the submarine extension of the fault zone the mtl probably does not rupture along its entire length in a single earthquake but instead consists of multiple earthquake segments that rupture independently of one another the recurrence interval and surficial offset for surface rupturing earthquakes at four individual sites on the mtl in shikoku island are 1000 3000 years and 5 8 m respectively the relatively long recurrence interval and large surficial offset per earthquake suggest large seismic moment release during surface rupturing earthquakes on the mtl as commonly observed for japanese moderate to large intraplate earthquakes because the predicted rupture lengths based on empirical relations between surface displacement and rupture length are considerably larger than those of individual segments large earthquakes on the mtl probably rupture multiple geometrically defined segments the fault was previously thought to be unruptured at least in the past 1000 years based on the absence of historical evidence for destructive earthquakes however trench excavations show that part of the mtl ruptured most recently during or after the 16th century a d with at least 6 9 0 7 m of slip along the main fault trace this rupture may be correlated to the 1596 keicho kinki earthquake but further geological and historical investigations are needed to confirm this hypothesis

a kinematic model for the formation of the siletz crescent forearc terrane by capture of coherent fragments of the farallon and resurrection plates 1 the volcanic basement of the oregon and washington coast ranges has been proposed to represent a pair of tracks of the yellowstone hotspot formed at a mid ocean ridge during the early cenozoic this interpretation has been questioned on many grounds especially that the range of ages does not match the offshore spreading rates and that the presence of continental coarse clastic sediments is difficult to reconcile with fast convergence rates between the oceanic plates and north america updates to basement geochronology and plate motion history reveal that these objections are much less serious than when they were first raised forward plate kinematic modeling reveals that predicted basement ages can be consistent with the observed range of about 55 49 ma and that the entire basement terrane can form within about 300 km of continental sources for clastic sediments this kinematic model indicates that there is no firm reason to reject the near ridge hotspot hypothesis on the basis of plate motions a novel element of the model is the resurrection plate previously proposed to exist between the farallon and kula plates by including the defunct resurrection plate in our reconstruction we are able to model the farallon hotspot track as docking against the oregon subduction margin starting about 53 ma followed by docking of the resurrection track to the north starting about 48 ma accretion of the farallon plate fragment and partial subduction of the resurrection fragment complicates the three dimensional structure of the modern cascadia forearc we interpret the so called e layer beneath vancouver island to be part of the resurrection fragment our new kinematic model of mobile terranes within the paleogene north american plate boundary allows reinterpretation of the three dimensional structure of the cascadia forearc and its relationship to ongoing seismotectonic processes

propagation as a mechanism of reorientation of the juan de fuca ridge we present a revised model of the tectonic evolution of the juan de fuca ridge by propagating rifting the new model has three different relative rotation poles covering the time intervals 17 0 8 5 ma 8 5 5 0 ma and 5 0 ma to the present the rotation pole shifts at 8 5 and 5 0 ma imply clockwise shifts in the direction of relative motion of 10 to 15 at each of these shifts the pattern of propagation reorganizes and the new ridges formed by propagation are at an orientation closer to orthogonal to the new direction of motion than the orientation of the preexisting ridges the model containing a total of seven propagation sequences shows excellent agreement with the isochrons inferred from the magnetic anomaly data except in areas complicated by the separate explorer and gorda plates the agreement between model and data near the explorer plate breaks down abruptly at an age of about 5 ma indicating that the probable cause of the rotation pole shift at that time was the separation of the explorer plate from the juan de fuca plate

anomalous lithospheric structure of northern juan de fuca plate a consequence of oceanic rift propagation abstract anomalous crustal and upper mantle structure of northern juan de fuca plate is revealed from wide angle seismic and gravity modelling a 2 d velocity model is produced for refraction line ii of the 1980 vancouver island seismic project visp80 the refraction data were recorded on three ocean bottom seismometers obss deployed at the ends and middle of a 110 km line oriented parallel to the north american continental margin the velocity model is constructed via ray tracing and conforms to first arrival amplitude observations and travel time picks of direct converted and reflected phases between sub sediment depths of 3 to 11 km depths normally associated with the lower crust and upper oceanic mantle the final model shows that compressional wave velocities decrease significantly from southeast to northwest along the profile at sub sediment depths of 11 km at the northwestern end of the profile p wave velocities are as low as 7 2 km s a complementary 2 d gravity model using the geometry of the velocity model and velocity density relationships characteristic of oceanic crust is produced the high densities required to match the gravity field indicate the presence of peridotites containing 25 30 serpentine by volume rather than excess gabbroic crust within the deep low velocity zone anomalous travel time delays and unusual reflection characteristics observed from proximal seismic refraction and reflection experiments suggest a broader zone of partially serpentinized peridotites coincident with the trace of a pseudofault we propose that partial serpentinization of the upper mantle is a consequence of slow spreading at the tip of a propagating rift

transform push oblique subduction resistance and intraplate stress of the juan de fuca plate the juan de fuca plate is a small oceanic plate between the pacific and north america plates in the southernmost region referred to as the gorda deformation zone the maximum compressive stress 1 constrained by earthquake focal mechanisms is n s off oregon and possibly off washington nw trending left lateral faults cutting the juan de fuca plate indicate a 1 in a ne sw to e w direction the magnitude of differential stress increases from north to south this is inferred from the plastic yielding and distribution of earthquakes throughout the gorda deformation zone to understand how tectonic forces determine the stress field of the juan de fuca plate we have modeled the intraplate stress using both elastic and elastic perfectly plastic plane stress finite element models we conclude that the right lateral shear motion of the pacific and north america plates is primarily responsible for the stress pattern of the juan de fuca plate the most important roles are played by a compressional force normal to the mendocino transform fault a result of the northward push by the pacific plate and a horizontal resistance operating against the northward or margin parallel component of oblique subduction margin parallel subduction resistance results in large n s compression in the gorda deformation zone because the force is integrated over the full length of the cascadia subduction zone the mendocino transform fault serves as a strong buttress that is very weak in shear but capable of transmitting large strike normal compressive stresses internal failure of the gorda deformation zone potentially places limits on the magnitude of the fault normal stresses being transmitted and correspondingly on the magnitude of strike parallel subduction resistance transform faults and oblique subduction zones in other parts of the world can be expected to transmit and create stresses in the same manner