seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing water and hydrous minerals play a key role in geodynamic processes at subduction zones however there are few seismological constraints on the presence of water within the subducting plate audet et al model converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the cascadia continental margin to its intersection with the forearc mantle their observations suggest that water is pervasively present in fluid form at high pore pressures indicating that the megathrust is a low permeability boundary these results may hold important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip this study models converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the cascadia continental margin to its intersection with the forearc mantle the observations suggest that water is pervasively present in fluid form at high pore pressures indicating that the megathrust is a low permeability boundary these results may hold important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip water and hydrous minerals play a key part in geodynamic processes at subduction zones1 2 3 by weakening the plate boundary aiding slip and permitting subduction and indeed plate tectonics to occur4 the seismological signature of water within the forearc mantle wedge is evident in anomalies with low seismic shear velocity marking serpentinization5 6 7 however seismological observations bearing on the presence of water within the subducting plate itself are less well documented here we use converted teleseismic waves to obtain observations of anomalously high poisson s ratios within the subducted oceanic crust from the cascadia continental margin to its intersection with forearc mantle on the basis of pressure temperature and compositional considerations the elevated poisson s ratios indicate that water is pervasively present in fluid form at pore pressures near lithostatic values combined with observations of a strong negative velocity contrast at the top of the oceanic crust our results imply that the megathrust is a low permeability boundary the transition from a low to high permeability plate interface downdip into the mantle wedge is explained by hydrofracturing of the seal by volume changes across the interface caused by the onset of crustal eclogitization and mantle serpentinization these results may have important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip

modern carbonate marine sands in estuaries of southwest england large deposits of marine sand occur in the taw camel and hayle estuaries of north devon and cornwall total carbonate concentrations reach 67 in the hayle 62 in the camel and 40 in the taw well mixed the sands contain modern skeletal debris of consistent composition which persists up to 18 km and 12 km landward from the mouths of the taw and camel estuaries although primarily a molluscan sand remains of barnacles bryozoans echinoids foraminifera sponge spicules decapods and coralline algae are common carbonate mineralogy provided by these components on a 100 carbonate basis consists of 45 aragonite 38 low mg calcite and 17 high mg calcite two estuaries of the southeast coast of devon the exe and teign contain smaller carbonate concentrations of up to 9 and 14 values are low there owing to dilution by readily eroded cliff talus from local exposures of new red sandstone these permo triassic sediments provide carbonate lithoclasts however which supplement the modern biogenic supply strontium values have been used to estimate the extent of carbonate lithoclasts supplied to the exe and teign the present source of marine sand of estuaries in southwest england is considered to be mostly near shore detritus supplemented by eroded cliff talus derived largely from local outcrops once this material enters the estuarine regime it is available as a supply of sediment for further up estuary transport

stratigraphic evolution of the late holocene ganges brahmaputra lower delta plain sediment cores from the ganges brahmaputra delta in bangladesh were examined for sedimentological character clay mineralogy elemental trends c n s and 14c geochronology to develop a model for the sedimentary sequence resulting from lower delta plain progradation in the late holocene a widespread facies succession from muddy sand to interbedded mud records progradation of shoal island complexes and the transition from subtidal to intertidal conditions mangrove vegetated islands and peninsulas represent the final phase of progradation a mottled mud that is deposited by penetration of turbid coastal water into the mangroves during high water events organic matter preservation is generally low 1 toc in most of these well drained deposits that are characterized by a permeable silt dominated granulometry clay mineralogy in the cores records the relative influence of smectite and kaolinite rich ganges sediments and illite and chlorite rich brahmaputra material the lower delta plain west of the modern river mouths was deposited as a ganges dominated delta in three phases since 5000 cal years bp with brahmaputra influence confined to the meghna estuary area and to the supratidal section of western delta deposits evolution of the lower delta plain in the late holocene was influenced by regional subsidence patterns in the tectonically active bengal basin which controlled distributary channel avulsion and migration and the creation of accommodation space

alaska megathrust 1 seismicity 43 years after the great 1964 alaska megathrust earthquake 1 the largest moment release during the 1964 mw 9 2 alaska earthquake was on the portion of the megathrust under the eastern kenai peninsula and the prince william sound the area is currently locked geodetically and corresponds to where the yakutat terrane is subducting in 2006 2009 a seismic array consisting of 34 broadband seismometers was deployed in the region an automatic algorithm was used to detect 12 563 local earthquakes using 13 months of data from the experiment of these 9427 good quality earthquakes could be relocated in a joint inversion for hypocenters and velocity structure they were then relocated by double difference to generate a final catalog of 8308 hypocenters these microearthquakes delineate a deeper steeply dipping wadati benioff zone contiguous with an 11 km wide seismic zone dipping at 3 between 20 km and 40 km depth along the 1964 earthquake s rupture zone focal mechanisms do not show interplate thrust faulting events but mostly normal faulting events with t axes generally parallel the slab dip direction indicating them to be intraslab seismicity the shallow narrow band of seismicity lies within the subducting yakutat terrane and right below the thrust zone possibly thrust faulting has not yet resumed yet in this early phase of the earthquake cycle thick subducted sediments overlying the yakutat terrane could also form a large strong contact zone on a relatively smooth plate boundary which does not favor seismic sliding on small patches but ruptures homogenously in great earthquakes

shumagin seismic gap alaska peninsula history of great earthquakes tectonic setting and evidence for high seismic potential the shumagin seismic gap a segment of the plate boundary along the eastern aleutian arc has not ruptured during a great earthquake since at least 1899 1903 because at least 77 years have elapsed since the shumagin gap last ruptured in a great earthquake and repeat times for the 1938 rupture zone and part of the shumagin gap are estimated to be 50 to 90 years a high probability exists for a great earthquake to occur within the shumagin gap during the next one to two decades reconsideration of the rupture zones of the aleutian earthquakes of 1938 1946 and 1948 suggests that those events did not break the interplate boundary beneath the shumagin islands thus the shumagin seismic gap extends from the western end of the 1938 rupture zone to the eastern end of that of 1946 these boundaries also coincide with transverse structural features at least the eastern half of the shumagin gap broke in great earthquakes in 1788 and 1847 and possibly in 1898 1903 the shumagin gap is probably not the result of aseismic slip rather plate motion is accommodated there seismically and episodically and can be expected to produce large earthquakes in the future although there is no definitive evidence of long term precursors of a possible future earthquake several observations suggest that the shumagin gap is in an advanced stage of the earthquake cycle both teleseismic and local network data indicate a near absence of seismic activity m 2 above a depth of 30 km along the main thrust zone within the gap this is in strong contrast to adjacent portions of the arc where seismic activity is scattered across most of the main thrust zone two earthquakes with high stress drops 600 900 bars which occurred at the base of the main thrust zone may indicate the accumulation of a considerable amount of strain energy within the gap a possible seismic gap at the eastern end of the aftershock zone of the aleutian earthquake of 1957 has been identified near unalaska island an earthquake that nucleates in the shumagin gap could also rupture the possible unalaska gap to the west the 1938 aftershock zone to the east or both with resultant magnitude up to mw 9 0 alternatively the shumagin gap alone or in one of the above combinations could rupture in a series of very large earthquakes instead of a single great shock past alaska aleutian earthquakes including those of 1788 1938 1946 1957 1964 and 1965 have generated very large tsunamis future large earthquakes in the shumagin islands region could generate wave heights of several tens of meters along shorelines near the rupture areas the shumagin gap is one of two major gaps along the united states portion of the alaska aleutian plate boundary and is one of the few areas in the united states where processes leading to a great earthquake are likely to be observed within a reasonable span of time

control of seafloor roughness on earthquake rupture behavior earthquake rupture complexity is described for three recent large underthrusting earthquakes along the costa rican subduction zone the 1983 osa 1990 nicoya gulf and 1999 quepos events these earthquakes occurred in regions characterized by distinctly different morphologic features on the subducting plate the 1990 and 1999 events occurred along linear projections of subducting seamount chains and had fairly simple earthquake rupture histories both events are interpreted as failure of the basal contact of closely spaced isolated seamounts acting as asperities in contrast the 1983 event occurred along the subducting cocos ridge and had a complex rupture history comparison of rupture characteristics of these large underthrusting earthquakes with size and location of subducting features provides evidence that seamounts can be subducted to seismogenic depths and that variations in seafloor bathymetry of the subducting plate strongly influence the earthquake rupture process

deltaic morphodynamics and stratigraphic evolution of middle barataria bay and middle breton sound regions louisiana usa implications for river sediment diversions abstract river sediment diversions have been recognized as a key strategy for offsetting land loss in coastal louisiana recently much attention has been focused on sediment capture from the main stem river and conveyance of that material through diversion outflows yet the performance and long term feasibility of diversions as land building agents also hinges upon the subsurface stratigraphic architecture of receiving basins a concept that has largely been unexplored a major core collection and analysis program was undertaken to study the geological properties of substrates in the middle barataria bay and middle breton sound diversion receiving basins louisiana usa over a region of 200 square kilometers in both basins 50 vibracores up to 5 5 m in length were collected in spring and summer of 2015 cores were logged for bulk density and imaged with a geotek multi sensor core logger split cores were then subsampled for granulometry organic content and 14c dating both receiving basins are characterized by 1 2 m of organic rich surficial strata underlain by 2 m of mineral rich sand and silt beds that display a greater bulk density additionally some cores contain deeper peaty strata intercalated within muds and sands 14c dates of these buried peats range between 1910 and 3203 calendar years before present demonstrating that relatively old deltaic strata exist at shallow depths age comparisons of stratigraphically similar peat beds from this study and previous delta lobe chronological models indicate that the st bernard lobe prograded eastward at a time averaged rate of 75 m yr from a geotechnical standpoint the surficial uncompacted

sources of tsunami and tsunamigenic earthquakes in subduction zones we classified tsunamigenic earthquakes in subduction zones into three types earth quakes at the plate interface typical interplate events earthquakes at the outer rise within the subducting slab or overlying crust intraplate events and tsunami earthquakes that generate considerably larger tsunamis than expected from seismic waves the depth range of a typical interplate earthquake source is 10 40km controlled by temperature and other geological parameters the slip distribution varies both with depth and along strike recent examples show very different temporal change of slip distribution in the aleutians and the japan trench the tsunamigenic coseismic slip of the 1957 aleutian earthquake was concentrated on an asperity located in the western half of an aftershock zone 1200km long this asperity ruptured again in the 1986 andreanof islands and 1996 delarof islands earthquakes by contrast the source of the 1994 sanriku oki earthquake corresponds to the low slip region of the previous interplate event the 1968 tokachi oki earthquake tsunamis from intraplate earthquakes within the subducting slab can be at least as large as those from interplate earthquakes tsunami hazard assessments must include such events similarity in macroseismic data from two southern kuril earthquakes illustrates difficulty in distinguishing interplate and slab events on the basis of historical data such as felt reports and tsunami heights most moment release of tsunami earthquakes occurs in a narrow region near the trench and the concentrated slip is responsible for the large tsunami numerical modeling of the 1996 peru earthquake confirms this model which has been proposed for other tsunami earthquakes including 1896 sanriku 1946 aleutian and 1992 nicaragua

seismic quality factors across a bottom simulating reflector in the makran accretionary prism arabian sea the hydrate bearing sediments above the bottom simulating reflector bsr are associated with low attenuation or high quality factor q whereas underlying gas bearing sediments exhibit high attenuation hence estimation of q can be important for qualifying whether a bsr is related to gas hydrates and free gas this property is also useful for identifying gas hydrates where detection of bsr is dubious here we calculate the interval q for three submarine sedimentary layers bounded by seafloor bsr one reflector above and another reflector below the bsr at three locations with moderate strong and no bsr along a seismic line in the makran accretionary prism arabian sea for studying attenuation q 1 characteristics of sediments interval q for hydrate bearing sediments layer b above the bsr are estimated as 191 11 223 12 and 117 5 whereas interval q for the underlying gas bearing sediments layer c are calculated as 112 7 107 8 and 124 11 at moderate strong and no bsr locations respectively the large variation in q is observed at strong bsr thus q can be used for ascertaining whether the observed bsr is due to gas hydrates and for identifying gas hydrates at places where detection of bsr is rather doubtful interval q of 98 4 108 5 and 102 5 respectively at moderate strong and no bsr locations for the layer immediately beneath the seafloor layer a show almost uniform attenuation

seismic reflection imaging of ultradeep roots beneath the eastern aleutian island arc seismic reflection data show that the eastern aleutian arc is characterized by reflectors that extend continuously from the lower arc crust to 50 km depth which is considerably deeper than the crustal thickness of 27 35 km previously inferred from coincident wide angle seismic surveys because the upper mantle is commonly homogeneous and therefore nonreflective relative to the overlying crust we interpret these reflectors to be gabbro garnet gabbro and pyroxenite intrusions within two 50 km wide roots that represent a 25 km thick heterogeneous transition from mafic lower crustal rocks to ultramafic mantle rocks we suggest that the reflectivity is linked to repeated differentiation and intrusion of mantle derived melts into the subarc lithosphere and that the depth of these roots shows that fractionation of arc crust can extend well below the seismically determined moho because these deep roots are not evident beneath the central aleutian arc either the roots form sporadically perhaps as a consequence of an elevated magmatic supply or such roots ultimately founder into the underlying mantle due to their relatively high mass density

structural style and kinematics of an underplated slate belt kodiak and adjacent islands alaska the kodiak formation composed of coherent maastrichtian turbidites is a slate belt whose dominant structures developed during underplating to an accretionary wedge in the latest cretaceous it consists of about 80 coherent landward dipping thrust packets zones of disrupted sandstone associated with a scaly argillite matrix constitute the remainder about half of these disrupted sandstone zones are related to pre accretion deformation and the rest formed along late stage strike slip faults that postdate development of slaty cleavage the formation is divided into three structural belts the landward and seaward belts include steeply dipping structures and the central belt contains shallowly dipping structures and rocks that have experienced the highest strain the central belt probably acted as a low angle southeast verging floor thrust zone beneath the landward belt the structural history of the kodiak formation includes 1 early soft sediment disruption 2 tectonic stratal disruption 3 thrust faulting slaty cleavage s 1 development and folding f 1 4 intrusion of granodioritic plutons dikes and sills and associated normal faulting 5 development of crenulations f 2 and crenulation cleavage s 2 6 thrust faulting and 7 development of right lateral strike slip faults event 3 produced the dominant northwest dipping structural grain and caused the greatest amount of shortening the timing of event 7 relative to events 5 and 6 is not certain the dominant structures of the kodiak formation are interpreted as developing during underthrusting underplating and intra wedge shortening during latest cretaceous time within an accretionary wedge the maximum time from deposition to emplacement was about 12 m y and the timing is constrained by the maximum depositional age 74 m y and the age of plutonic rocks 62 m y that crosscut the dominant fabric pre cleavage zones of stratal disruption reflect deformation on the lower plate during underthrusting slaty cleavage thrusts and f 1 folds developed during underplating that also resulted in the formation of duplexes crenulations are probably related to post underplating subhorizontal shortening of sediments within the accretionary wedge syn deformation dynamic recrystallization of quartz minimum syn accretion metamorphic temperatures between 205 and 250 c and pressures of at least 2 65 kb recorded during the earliest stages of deformation suggest underplating at 10 km

transition from locked to creeping subduction in the shumagin region alaska 1 gps velocities from the alaska peninsula are modeled to determine the extent of locking on the alaska aleutian subduction interface the observations which span from the semidi islands to sanak island encompass the 1938 mw 8 3 rupture zone and the transition into the shumagin gap model parameters are optimized using a simulated annealing method coupling variation along strike of the plate interface show a nearly fully locked 90 subduction zone at the semidi islands decreasing to about 30 locked at the shumagin islands and freely slipping to the west of the shumagins independent rupture of the shumagin segment could produce repeated mw 7 6 earthquakes unless a significant fraction of the slip on the interface occurs as afterslip following large earthquakes southwest directed velocities at most of the sites may be attributed to clockwise rotation of a bering block

relationship between shallow and intermediate depth seismicity in the eastern aleutian subduction zone the transition from shallow interplate thrusting to intermediate depth seismicity is often poorly observed but critical for understanding the fate of the downgoing slab in order to better examine the transition 1448 earthquakes are relocated from data recorded by a regional seismic network in the eastern aleutian arc using an improved three dimensional velocity model and accurate ray tracing single event first motion solutions are determined from these rays for 31 slab events the interplate thrust zone is a planar fault zone dipping 10 15 at 25 35 km depth and is no more than 5 10 km wide most intermediate depth earthquakes are localized to a plane no wider than 5 km near the top of the descending plate fault plane solution orientations for these events vary by several tens of degrees in orientation although 73 show t axes aligned within 45 of the slab dip a parallel seismic zone 20 25 km deeper into the slab also shows down dip plunges of t axes for 3 of 5 solutions the fault plane solutions are poorly explained by plate bending or unbending about a neutral fiber hypocenters show that intermediate depth events are confined near the subducted oceanic crust supporting compositional rather than pure thermal control of intermediate depth seismicity one explanation is that the upper plane events are an indirect consequence of phase changes in subducted crust perhaps similar processes are important in producing earthquakes in the lower parallel zone

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

petrography geochemistry and depositional setting of the san pedro and santo tomas coal zones anomalous algae rich coals in the middle part of the claiborne group eocene of webb county texas two coal zones the san pedro and the overlying santo tomas are present for nearly 35 km in outcrop surface and underground mines and shallow drill holes along the strike of the middle part of the claiborne group eocene in webb county texas a sandstone dominated interval of 25 to 35 m separates the two coal zones which range up to 3 m in thickness each coal zone contains carbonaceous shales thin 0 75 m impure coal beds and thin 0 85 m but commercially significant nonbanded coal beds the nonbanded coals are different from other tertiary coals of the gulf of mexico coastal plain unlike lignites that are typical of the older wilcox group paleocene eocene and younger jackson group eocene nonbanded coals of the claiborne group have high vitrinite reflectance values 0 53 rmax and high calorific yields average 6670 kcal kg or 12 000 btu dry basis the coals are weakly agglomerating free swelling index is 1 5 2 0 and have an apparent rank of high volatile bituminous the coal bearing portion of the middle claiborne group in the rio grande area represents a fining upward transition from sandstone dominated marine influenced lower delta plain depositional environments to more inland mudstone rich predominantly freshwater deltaic settings discontinuities within the san pedro coal zone are attributed mainly to the influence of contemporaneous deposition of distributary mouth bar sand bodies the less variable nature of the santo tomas coal zone reflects its origin in the upper part of an interlobe basin that received only minor clastic influx petrographic attributes of the nonbanded coals indicate that they formed subaqueously in fresh to possibly brackish waters a highly degraded groundmass composed of eugelinite is the main petrographic component approximately 71 mineral matter free basis an enriched liptinite fraction approximately 23 probably accounts for unusually high calorific values there is negligible inertinite petrographic study of polished blocks indicates that approximately 10 percent of the nonbanded coal from both coal zones is composed of green algae fructifications which also occur in clastic rocks of the coal bearing interval such algal material cannot be identified or quantified by conventional coal petrographic techniques that utilize particle pellets or by palynological analyses that include acid preparation

caesium from former dartmoor volcanism its incorporation in new red sediments of sw england high concentrations of caesium occur in the new red sandstone of devon england values are most abundant in breccias at the base of the permian and decrease upwards through the sandstones and marls caesium resides in the clay fraction extensive volcanic activity on the south eastern edge of the cornubian massif provided a supply of cs due to its low ionic potential this alkali was concentrated at source under extreme weathering in the hydrolysate fraction of red bed sediments the source area was largely exhausted by late permian reflected by a reduction in cs content of new red sandstone detritus

lithoprobe southern vancouver island cenozoic subduction complex imaged by deep seismic reflections reply the lithoprobe seismic reflection project on vancouver island was designed to study the large scale structure of several accreted terranes exposed on the island and to determine the geometry and structural characteristics of the subducting juan de fuca plate in this paper we interpret two lithoprobe profiles from southernmost vancouver island that were shot across three important terrane bounding faults leech river san juan and survey mountain to determine their subsurface geometry and relationship to deeper structures associated with modem subduction the structure beneath the island can be divided into an upper crustal region consisting of several accreted terranes and a deeper region that represents a landward extension of the modern offshore subduction complex in the upper region the survey mountain and leech river faults are imaged as northeast dipping thrusts that separate wrangellia a large mesozoic paleozoic terrane from two smaller accreted terranes the leech river schist mesozoic rock

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

origin of dipping structures in fast spreading oceanic lower crust offshore alaska imaged by multichannel seismic data abstract multi channel seismic mcs reflection profiles across the pacific plate south of the alaska peninsula reveal the internal structure of mature oceanic crust 48 56 ma formed at fast to intermediate spreading rates during and after a major plate re organization oceanic crust formed at fast spreading rates half spreading rate 74 mm yr has smoother basement topography thinner sediment cover with less faulting and an igneous section that is at least 1 km thicker than crust formed at intermediate spreading rates half spreading rate 28 34 mm yr mcs data across fast spreading oceanic crust formed during plate re organization contain abundant bright reflections mostly confined to the lower crust above a highly reflective moho transition zone which has a reflection coefficient rc of 0 1 the lower crustal events dip predominantly toward the paleo ridge axis at 10 30 reflections are also imaged in the uppermost mantle which primarily dip away from the ridge at 10 25 the opposite direction to those observed in the lower crust dipping events in both the lower crust and upper mantle are absent on profiles acquired across the oceanic crust formed at intermediate spreading rates emplaced after plate re organization where a moho reflection is weak or absent our preferred interpretation is that the imaged lower crustal dipping reflections within the fast spread crust arise from shear zones that form near the spreading center in the region characterized by interstitial melt the abundance and reflection amplitude strength of these events rc 0 15 can be explained by a combination of solidified melt that was segregated within the shear structures mylonitization of the shear zones and crystal alignment all of which can result in anisotropy and constructive signal interference formation of shear zones with this geometry requires differential motion between the crust and upper mantle where the upper mantle moves away from the ridge faster than the crust active asthenospheric upwelling is one possible explanation for these conditions the other possible interpretation is that lower crustal reflections are caused by magmatic mafic ultramafic layering associated with accretion from a central mid crustal magma chamber considering that the lower crustal dipping events have only been imaged in regions that have experienced plate re organizations associated with ridge jumps or rift propagation we speculate that locally enhanced mantle flow associated with these settings may lead to differential motion between the crust and the uppermost mantle and therefore to shearing in the ductile lower crust or alternatively that plate reorganization could produce magmatic pulses which may lead to mafic ultramafic banding

a slow slip event in the south central alaska subduction zone and related seismicity anomaly 1 we detected a slow slip event in the south central alaska subduction zone by analyzing continuous gps data from the plate boundary observatory pbo network the slow slip event started in early 2010 at a depth of 35 km beneath the cook inlet near the down dip end of the locked zone and is ongoing as of november 2011 with an accumulated magnitude ofmw 6 9 analysis of the earthquake catalog in the same area using the stochastic epidemic type aftershock sequence model etas shows a small but detectable seismicity increase during the slow slip event we also find a change in seismicity rate around 1998 that may suggest an earlier slow slip event in the same region slow slip events in alaska appear more widespread than previously thought but have remained undetected due to their long durations the time intervals between them and the limited time records from the continuous gps

possible relationship between nonvolcanic tremor and the 1998 2001 slow slip event south central alaska 1 we document the occurrence of nonvolcanic tremor nvt in south central alaska for 3 months of each year between 1999 and 2001 to provide a first order attempt to understand the relationship between nvt and a large slow slip event that occurred over a 2 to 3 year period between 1998 and 2001 the majority of the nvt signals observed after the onset of the slow slip event are bursts lasting between 10 and 15 min with frequencies ranging from 1 to 6 hz by the summer of 2001 the quantity of nvt has decreased dramatically from the previous summers and episodes rarely last up to 10 min the decrease in nvt activity in the summer of 2001 is coincident with the end of the slow slip event there is a clear spatial correlation between the observed nvt and the slow slip event with most of the nvt events located on the downdip edge of the region of highest slip depth estimates range widely over the region with approximately half concentrated near the interpreted north america yakutat pacific plate interface and the remainder located above the plate interface however owing to inadequate station coverage and poor knowledge of the local structure determining accurate depths is difficult and thus the relationship between the nvt and the plate interface is problematic this is the first study giving evidence that the relationship between slow slip events and nvt seen in cascadia and southwest japan may also exist in alaska

edge deep seismic reflection transect of the eastern aleutian arc trench layered lower crust reveals underplating and continental growth an edge deep crustal seismic reflection transect of the eastern aleutian arc trench traces oceanic crust and moho more than 200 km beneath the accretionary prism to depths of more than 30 km these horizons project beneath a prominent sequence of layered reflectors that extends from about 9 to 35 km beneath the mesozoic core of the prism earthquake hypocenters imply continuity of the downgoing lithosphere from the base of the layered reflectors to beneath and beyond the active augustine volcano rapid lateral growth of the prism in eocene oligocene time coincided with uplift of the mesozoic core of the prism during lateral growth maintenance of critical taper requires thickening either by internal deformation or underplating because exposed rocks show only modest postemplacement shortening thickening most likely occurred by underplating probably of the layered reflectors the overall geometry of the layered reflectors is reminiscent of nappe structures and their emplacement may represent crustal scale duplexing associated with underplating the edge reflection data and borehole results indicate that the self edge is marked by an active out of sequence thrust that separates the paleogene and neogene prisms this thrust apparently developed in response to the prism s need to maintain critical taper and demonstrates that contrasts inmore lithology can result from mechanisms other than terrane emplacement less

variable rupture mode of the subduction zone along the ecuador colombia coast three large earthquakes occurred within the rupture zone of the 1906 colombia ecuador earthquake m w 8 8 in 1942 m s 7 9 1958 ms 7 8 and 1979 m s 7 7 we compared the size and mechanism of these earthquakes by using long period surface waves tsunami data and macroseismic data the 1979 event is a thrust event with a seismic moment of 2 9 10 28 dyne cm and represents subduction of the nazca plate beneath south america the rupture length and direction are 230 km and n40 e respectively examination of old seismograms indicates that the 1906 event is also a thrust event which ruptured in the northeast direction the seismic moment estimated from the tsunami data and the size of the rupture zone is 2 10 29 dyne cm the 1942 and 1958 events are much smaller about 1 5 to 1 10 of the 1979 event in the seismic moment than the 1979 event we conclude that the sum of the seismic moments of the 1942 1958 and 1979 events is only formula of that of the 1906 event despite the fact that the sequence of the 1942 1958 and 1979 events ruptured approximately the same segment as the 1906 event this difference could be explained by an asperity model in which the fault zone is held by a discrete distribution of asperities with weak zones in between the weak zone normally behaves aseismically but slips abruptly only when it is driven by failure of the asperities a small earthquake represents failure of one asperity and the rupture zone is pinned at both ends by adjacent asperities so that the effective width and the amount of slip are relatively small a great earthquake represents failure of more than one asperity and consequently involves much larger width and slip

the effect of seamount subduction on seismic coupling it has long been speculated that the subduction of seamounts may have profound mechanical consequences particularly on the generation of large subduction zone earthquakes we consider this problem as a special case of the seismic coupling model of scholz and campos the model predicts that the subduction of a large seamount will increase the normal stress across the subduction interface and hence will enhance seismic coupling in the case of coupled arcs this increased coupling will result in a large increase in the recurrence intervals of earthquakes such events will therefore appear rare in the instrumental record as is observed for decoupled arcs the subduction of a large seamount should cause them to become locally coupled three of the latter localities have been found with satellite altimetry data two on the tonga kermadec trench and one on the izu bonin trench the diagnostic feature is a shallowing or obstruction of the trench axis adjacent to a chain or group of large seamounts on the subducting plate in each case the small to moderate plate interface seismicity typical of decoupled seismic zones is almost completely absent in a 100 200 km arc segment indicating that the subduction zone is locally locked as predicted by the model the occurrence of large interplate earthquakes at both tonga sites confirms that they are seismically coupled

source scaling of earthquakes in the shumagin region alaska time domain inversions of regional waveforms the scaling of pulse duration to seismic moment is estimated for earthquakes along an interplate thrust zone from digital waveforms recorded by short period and broad band instruments of the east aleutian shumagin seismic network we measure pulse duration using an empirical green s function technique based on damped time domain deconvolution from several thousand events 22 earthquakes with magnitudes 3 0 7 0 and depths 23 56 km are found to give reliable estimates of pulse duration durations are also determined directly from one parameter nonlinear inversions for a variety of simple functional forms of source time functions symmetric source pulses boxcar or triangle shapes fit waveforms better than an asymmetric model t exp 2t d for most 62 per cent of the waveform pairs while the asymmetric model fits best for only 8 per cent of the data pulse duration increases with the size of events from 0 1 to 10s over the seismic moment m0 range of 1014 to 3 1019 nm when normalized by the cube root of seismic moment pulse durations show 8 x variation comparable static stress drop estimates range from 0 2 to 135 mpa contrary to predictions of some laboratory and theoretical studies earthquakes at the deepest part of the thrust zone do not show significantly higher stress drops than do shallower events rupture properties however show a strong dependence on earthquake size the three largest events m0 5 1018nm have the three longest normalized durations on average 3 8 times longer than those for smaller events the durations require smaller events to have 10 100 x larger static stress drops or 4 x faster rupture velocities or some combination of the two possibly the largest events rupture both strong and weak patches while smaller events just rupture strong patches on the fault surface the characteristic dimension that separates large from small events 3 15 km is comparable to characteristic wavelengths of pacific basin bathymetry and may reflect the influence of the subducted sea floor upon fault zone heterogeneity

quantifying large scale historical formation of accommodation in the mississippi delta large volumes of new accommodation have formed within the mississippi delta plain since the mid 1950s in association with rapid conversion of coastal wetlands to open water the three dimensional aspects and processes responsible for accommodation formation were quantified by comparing surface elevations water depths and vertical displacements of stratigraphic contacts that were correlated between short sediment cores integration of data from remotely sensed images sediment cores and water depth surveys at 10 geologically diverse areas in the delta plain provided a basis for estimating the total volume of accommodation formed by interior wetland subsidence and subsequent erosion results indicate that at most of the study areas subsidence was a greater contributor than erosion to the formation of accommodation associated with wetland loss tens of millions of cubic meters of accommodation formed rapidly at each of the large open water bodies that were formerly continuous interior delta plain marsh together the individual study areas account for more than 440 106 m3 of new accommodation that formed as holes in the mississippi river delta plain fabric between 1956 and 2004 this large volume provides an estimate of the new sediment that would be needed just at the study areas to restore the delta plain wetlands to their pre 1956 areal extent and elevations published 2010 this article is a us government work and is in the public domain in the usa

deep low frequency earthquakes in tectonic tremor along the alaska aleutian subduction zone 1 we characterize and locate tremor not associated with volcanoes along the alaska aleutian subduction zone using continuous seismic data recorded by the alaska volcano observatory and the alaska earthquake information center from 2005 to present visual inspection of waveform spectra and time series reveal dozens of 10 to 20 min bursts of tremor along the length of the alaska aleutian subduction zone we use autocorrelation to demonstrate that these tremor signals are composed of hundreds of repeating low frequency earthquakes lfes the tremor activity we characterize is localized in four segments from east to west kodiak island shumagin gap unalaska and andreanof islands although the geometry age thermal structure frictional and other relevant properties of the alaska aleutian subduction zone are poorly known these characteristics are likely to differ systematically from east to west locations near kodiak island are the most reliable because station coverage is more complete lfe hypocenters in this region are located on the plate interface near the down dip limit of the 1964 mw 9 2 alaska earthquake rupture area lfe hypocenters in the remaining areas along the arc are also located down dip of the most recent mw 8 megathrust earthquakes although these locations are less well constrained our results support the hypothesis that tremor activity marks the down dip rupture limit for great megathrust earthquakes in this subduction zone lastly there is no correlation between the presence of tremor and particular aspects of over riding or subducting plate geology or coupling it appears that lfes are a fundamental characteristic of the alaska aleutian subduction zone

body wave and surface wave analysis of large and great earthquakes along the eastern aleutian arc 1923 1993 implications for future events the 500 km long alaska peninsula and shumagin islands segments of the aleutian arc have a moderate to high probability of rupturing in one or more large or great earthquakes in the next few decades to understand the likely modes of rupture in the next sequence of large and great events and to delineate the current geometry of the plate interface we determine focal mechanisms depths and source time functions from seismic records for the largest events since 1917 the great earthquake of november 10 1938 and seven events with surface wave magnitude ms of 6 9 to 7 5 teleseismic body waves and surface waves are used to estimate the seismic moment and gross rupture characteristics of the great earthquake of november 10 1938 along the alaska peninsula body wave inversion of five p and four sh waves gives a duration of about 110 s in which moment was released in two episodes each of about 50 s duration with the second being larger than the first the first source was located in the general vicinity of the epicenter of the mainshock and the second which occurred about 60 s later was centered about 180 km to the northeast the body wave derived seismic moment is 3 7 1021 n m mw 8 3 we corroborate the body wave results by calculating surface waves by normal mode summation and comparing them with data at periods greater than 50 s an adequate fit to observed seismograms is obtained for either a single point source or two point sources with one located about 180 km northeast of the mainshock epicenter rupture in 1938 appears to have been confined to the alaska peninsula segment uniform rupture into the shumagin region is not supported by the data the ms 6 9 earthquake of may 13 1993 ruptured a small portion of the shumagin gap the earthquake of may 14 1948 ms 7 5 occurred on a shallow dipping thrust fault with a depth of about 31 km not 60 km as originally suggested five ms 7 events in the 1938 rupture zone have locations depths and mechanisms that define a shallow dipping 16 19 plate interface which shallows to 8 under kodiak island subduction of seamounts of the gulf of alaska seamount province may explain the location moment focal mechanism and depth of the five ms 7 events the plate interface dips nearly uniformly between the alaska peninsula and shumagin segments indicating that segmentation if any is not controlled by the orientation of the plate interface a slight warp of the plate interface may form the boundary between rupture zones of great earthquakes along the kodiak and alaska peninsula segments of the arc and may explain why rupture zones of large to great earthquakes rarely cross this tectonic boundary

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

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

sedimentary underplating at the cascadia mantle wedge corner revealed by seismic imaging earth s largest earthquakes occur at the boundary between subducting oceanic crust and the overriding plate yet the position of the plate boundaries remains uncertain analysis of zones of low seismic wave velocities beneath the northern cascadia subduction zone implies that the plate boundary here may be deeper than previously thought

the seismic coupling of subduction zones revisited 1 the nature of seismic coupling for many of the world s subduction zones has been reevaluated geodetic estimates of seismic coupling obtained from gps measurements of upper plate deformation during the interseismic period are summarized we compared those with new estimates of seismic coupling obtained from seismological data the results show that with a few notable exceptions the two methods agree to within about 10 the seismological estimates have been greatly improved over those made 20 30 years ago because of an abundance of paleoseismological data that greatly extend the temporal record of great subduction earthquakes and by the occurrence in the intervening years of an unusual number of great and giant earthquakes that have filled in some of the most critical holes in the seismic record the data also again with a few notable exceptions support the frictional instability theory of seismic coupling and in particular the test of that theory made by scholz and campos 1995 overall the results support their prediction that high coupling occurs for subduction zones subjected to high normal forces with a switch to low coupling occurring fairly abruptly as the normal force decreases below a critical value there is also considerable variation of coupling within individual subduction zones earthquake asperities correlate with areas of high coupling and hence have a semblance of permanence but the rupture zones and asperity distributions of great earthquakes may differ greatly between seismic cycles because of differences in the phase of seismic flux accumulation

an equilibrium profile model for retreating marsh shorelines in southeast louisiana abstract louisiana s coastal marshes are experiencing the highest wetland loss rates in the u s in part due to subsidence driven relative sea level rise these marshes are also vulnerable to the erosive power of wave attack 1 on the marsh edge adjacent to open water bodies and 2 after the marsh platform is submerged marsh shorelines in barataria bay breton sound and the active balize delta of southeastern louisiana were examined in areas where the subaerial marsh platform had disappeared since 1932 vibracore transects of marsh and adjacent bay surface sediments to 2 m depth were analyzed using geotechnical stratigraphic and radiochemical 137 cs and 210 pb methods and the subaerial to subaqueous transition of the marsh was mapped for elevation using standard stadia rod transit and fathometer measurements results indicate that marsh edge erosion of the platform takes place subaqueously until water depths of 1 5 m are reached this is observed even in interior pond regions but the shoreface elevation profiles are a function of fetch exposed open bay sites display greater incision depth and rate of the marsh platform than protected interior bay or pond sites core stratigraphy reveals that the outer part of the subaqueous platform switches from erosional to depositional as retreat proceeds covering the incised marsh deposits unconformably with estuarine shelly muds 137 cs and excess 210 pb activity indicates that these muds are deposited within a few decades of subaerial marsh loss the consistency of the cross shore profile results suggests that a single profile of equilibrium can approximate the morphology of eroding marsh edges in southeast louisiana platform stratigraphy and resistance to erosion have a limited effect on profile shape this equilibrium profile and remote sensing images of shoreline change are used to estimate the sediment yield to adjacent estuarine areas by this process on average 1 5 m3 of sediment are yielded per m shoreline length annually from both barataria bay and breton sound due to the highly organic nature of the eroded sediment 30 this supply of organic rich material could significantly impact estuarine productivity and hypoxia on the louisiana continental shelf

seismic low velocity layer at the top of subducting slabs observations predictions and systematics abstract layered structure persists at the tops of subducting plates to depths in excess of 150 km a low velocity channel several km thick produces guided seismic guided waves which indicate that the top of the downgoing plate remains coherent and probably has not eclogitized to 150 km depth the surrounding medium has seismic velocities 3 6 faster than standard earth models as inferred from direct travel times presumably because the waveguide lies atop cold descending lithosphere however guided waves travel more slowly several tests with full waveform synthetics show that the guided waves can be observed only if excited by sources that lie near or within the low velocity channel thus the frequent observation of dispersed body waves indicates that intermediate depth earthquakes tend to lie in waveguides also receivers must be favorably placed observations for seven circum pacific subduction zones show such signals which can be explained as low velocity channels 2 8 km thick velocity anomalies within low velocity channels are largest at shallowest depths up to 14 slower than surroundings at depths less than 150 km diminishing to

on balanced and unbalanced accommodation peat accumulation ratios in the cretaceous coals from gates formation western canada and their sequence stratigraphic significance coal composition was investigated by means of photometric and maceral analyses on closely spaced lithotype based strip samples over the full thickness of several paralic coal seams from the cretaceous gates formation of the western canadian sedimentary basin the aim of this investigation was to test various methods of identifying accommodation trends in coal and use them to refine sequence stratigraphic interpretation of continental sediments conventional sequence stratigraphy derives its subdivisions and significant surfaces from the records left by relative sea level oscillations these records either do not project into the continental realm or are difficult to recognise in clastic non marine sediments paralic coal seams have been selected to study this problem because they are not entirely removed from marine influence and compared with most inorganic deposits coal has stored a greater wealth of information that can be analysed at a higher level of resolution the study has led to the identification of five new surfaces with chronostratigraphic potential in the sequence stratigraphic analysis of non marine sediments two of these surfaces called paludification surface pas and terrestrialisation surface tes occur at the bases of the investigated coal seams while two other surfaces referred to as non marine flooding surface nfs and give up transgressive surface guts form the tops of the coal the fifth and probably most important new surface called the accommodation reversal surface ars is independent of any particular facies and may either coincide with some of the other surfaces or occur separately the proportion of detrital minerals has been used as the chief discriminator between different mire types and accommodation trends other useful indicators of mire type and peat dispersal have been the proportions of sporinite and inertodetrinite as well as some derived maceral and or mineral ratios e g the groundwater influence index and the tissue preservation index isometamorphic variations of telovitrinite reflectance and fluorescence as well as their coefficients of variation were also found to contribute to the identification of cyclic shifts between balanced and unbalanced accommodation peat accumulation ratios some of these cycles which are backed up by clastic stratigraphy appear to correspond to the development of shallowing upward and deepening upward parasequences superimposed high frequency low amplitude perturbations in the coal cycles relate to smaller scale accommodation cycles of sub parasequence level not always recognised in non marine strata these sub parasequence coal cycles do not always continue the shallowing upward trend typical of conventional parasequences several coals were found to contain stacks of small scale cycles with upward increasing accommodation signatures either in their lower or upper halves or over the whole seam section

new constraints on subduction zone structure in northern cascadia summary a detailed passive seismic experiment was carried out across southwestern british columbia and northwestern washington to investigate the structure of the subducting juan de fuca plate and mantle wedge in cascadia and its relation to intraslab seismicity as part of the polaris project 31 three component broad band stations were deployed in an approximately linear array spanning southern vancouver island the gulf and san juan islands watcom county and the british columbia lower mainland p wave coda from 41 teleseismic events have been employed in formal inversions for fine scale shear velocity structure our results indicate a structure very similar to that identified across a comparable profile in central oregon the continental moho is evident at the eastern end of the profile near 35 km depth but disappears towards georgia strait puget sound a prominent low s velocity zone is clearly evident below southern vancouver island dipping eastwards through georgia strait puget sound and coincides with the e reflection zone originally identified in lithoprobe studies structure below the e layer is much less prominent and varies intermittently along the array based on the observations and interpretations of similar structures beneath oregon alaska and south america and its projection to mantle depths we suggest that the low velocity e layer represents the dehydrating oceanic crust of the subducting juan de fuca plate this interpretation is consistent with recent seismicity studies that place shallow wadati benioff events within the oceanic mantle and implies that the oceanic crust is 6 8 km shallower beneath vancouver island than previously assumed as in oregon we interpret the diminished signature of oceanic crust below a depth of 45 km to signal the presence of eclogitization which in turn supplies water to serpentinize the overlying forearc mantle

the shumagin islands alaska earthquake of 31 may 1917 abstract although macroseismic reports indicate that great earthquakes have occurred in the vicinity of the shumagin gap of the eastern aleutian arc the largest well documented event within the gap has been an m s 7 5 event that occurred in 1948 this earthquake apparently ruptured only a 75 km long segment of the 200 km long gap and as such is not considered a gap filling event relocations of turn of the century seismicity by boyd and lerner lam 1988 suggest however that another more poorly documented event nucleated near the shumagin islands on 31 may 1917 an m s of 7 9 determined from a single station bulletin has been assigned to this event although consistent with its apparent aftershock distribution macroseismic reports appear to be too small for such a large event we use surface waves to estimate the moment and rupture bounds and body waves to constrain the focal mechanism and depth of the 1917 event the average m s at 13 azimuthally well distributed stations is 7 4 0 3 much smaller than the previous estimate of 7 9 the spatial variation in individual magnitude estimates is azimuthally dependent 7 9 to the west and 6 8 to the east implying source directivity assuming a thrust fault mechanism body wave inversion yields a source depth of 29 km and m w of 7 4 in addition assuming rupture velocity and rupture propagation along arc to the west consistent with the aftershock distribution a rupture length of 75 km and m w of 7 5 are required to match long period surface waves recorded at uppsala sweden and gottingen germany these results suggest that the average slip of the 1917 event was only about 75 to 150 cm and was restricted to the easternmost portion of the gap these results when combined with rupture zones and recurrence intervals of other eastern aleutian events imply a temporally heterogeneous rupture style where the rupture bounds and consequently the sizes of earthquakes vary from cycle to cycle estimating the magnitudes or rupture extents of the next series of events based on the previous series appears to be unjustified

uplift and subsidence reveal a nonpersistent megathrust rupture boundary sitkinak island alaska we report stratigraphic evidence of land level change and tsunami inundation along the alaska aleutian megathrust during prehistoric and historical earthquakes west of kodiak island on sitkinak island cores and tidal outcrops fringing a lagoon reveal five sharp lithologic contacts that record coseismic land level change radiocarbon dates 137cs profiles computerized tomography scans and microfossil assemblages are consistent with rapid uplift circa 290 0 520 300 and 1050 790 cal yr b p and subsidence in a d 1964 and circa 640 510 cal yr b p radiocarbon 137cs and 210pb ages bracketing a sand bed traced 1 5 km inland and evidence for sudden uplift are consistent with russian accounts of an earthquake and tsunami in a d 1788 the mixed uplift and subsidence record suggests that sitkinak island sits above a nonpersistent boundary near the southwestern limit of the a d 1964 mw 9 2 megathrust rupture

a large slow slip event and the depth of the seismogenic zone in the south central alaska subduction zone abstract we modeled crustal deformation before during and after a slow slip event sse on the plate interface in southern alaska we divided the gps time series into three time periods before 1998 1998 to 2001 and 2001 2002 the first and last periods represent the steady deformation while the middle period represents the sse we found a stationary zone of 100 slip deficit in the shallow part of the seismogenic zone to depths of 25 30 km which we identify with the main asperity that ruptured in 1964 earthquake the sse occurs downdip of the asperity on a section that accumulated some slip deficit during the steady time periods during the sse 20 000 sq km of the plate interface slipped 10 cm for a cumulative moment magnitude of m w 7 2 the area of the sse did not have a significant slip in the 1964 earthquake compared to the slip of the main asperity we propose that between earthquakes this zone fails repeatedly in sses and remains at relatively low shear stress compared to the main asperity up dip of it

the reflection seismic survey of project tipteq the inventory of the chilean subduction zone at 38 2 s summary we describe results of an active source seismology experiment across the chilean subduction zone at 38 2 s the seismic sections clearly show the subducted nazca plate with varying reflectivity below the coast the plate interface occurs at 25 km depth as the sharp lower boundary of a 2 5 km thick highly reflective region which we interpret as the subduction channel that is a zone of subducted material with a velocity gradient with respect to the upper and lower plate further downdip along the seismogenic coupling zone the reflectivity decreases in the area of the presumed 1960 valdivia hypocentre the plate interface itself can be traced further down to depths of 50 60 km below the central valley we observe strong reflectivity at the plate interface as well as in the continental mantle wedge the sections also show a segmented forearc crust in the overriding south american plate major features in the accretionary wedge such as the lanalhue fault zone can be identified at the eastern end of the profile a bright west dipping reflector lies perpendicular to the plate interface and may be linked to the volcanic arc

nonlinear variations of the physical properties along the southern ecuador subduction channel results from depth migrated seismic data abstract we use two high quality pre stack depth migrated multichannel seismic profiles acquired to quantify physical properties variations of underthrust sediments along the first 30 km of subduction off the erosional southern ecuadorian margin seismic data show three zones along the subduction channel referred to as zones i ii and iii characterized by distinct velocity and velocity derived physical properties which are in agreement with values estimated from experimental results of deformation in granular media these three zones result from transformational changes of underthrust sediments governed by fundamentally different physical processes that control their mechanical behavior at increasing confining pressures based on our observations and its comparison with experimental results we argue that the transformations undergone by underthrust sediments as they dip into the subduction zone are the following within zone i progressively increasing velocity and decreasing velocity derived porosity indicates continuous sediment compaction which must be accompanied by effective fluid drainage along the decollement and or across the accretionary wedge the underthrust material is here unconsolidated from a mechanical point of view laboratory experiments indicate that the dominant processes at this range of pressures are grain rolling particle rotation and frictional slip at grain contacts within zone ii velocity and porosity remains constant for 16 km sis 72 and 12 km sis 18 this suggests undrained conditions resulting in growing fluid overpressure at the subduction channel grain deformation is similar to zone i within zone iii velocity increases and porosity falls rapidly indicating sediment compaction and subsequent release of over pressured fluids where grain deformation is likely to be elastic this might be the dominant process until the grains attain their crushing strength resulting in granular cataclasis and eventually in the collapse of the system we suggest that over pressured fluid release may induce hydrofracturation and it is likely to increase inter plate coupling down from zone iii

seismic and geodetic constraints on cascadia slow slip 1 automatically detected and located tremor epicenters from episodic tremor and slip ets episodes in northern cascadia provide a high resolution map of washington s slow slip region thousands of epicenters from the past four ets events from 2004 to 2008 provide detailed map view constraints that correlate with geodetic estimates of the simultaneous slow slip each of these ets events exhibits remarkable similarity in the timing and geographic distribution of tremor density and geodetically inferred slip analysis of the latest 15 month inter ets period also reveals ageodetic tremor activity similar both in duration and extent to ets tremor epicenters from both ets and inter ets tremor are bounded between the 30 and 45 km plate interface depth contours and locate approximately 75 km east of previous estimates of the locked portion of the subducting juan de fuca plate inter ets tremor overlaps but is generally downdip of ets tremor and does not yet correlate with geodetically observed slip but this is likely because the slip is below current gps detection levels based on the tremor and slip correlation and the tremor duration and slip magnitude relationship we suggest that the well resolved sharp updip edge of tremor epicenters reflects a change in plate interface coupling properties the region updip of this boundary may accumulate stress with the potential for coseismic shear failure during a megathrust earthquake alternatively plate convergence in this region could be accommodated by continuous slow slip with no detectable tremor or by slow slip events with sufficiently long recurrence intervals that none have been detected during the past 10 years of gps observations

deep high amplitude reflections from a major shear zone above the subducting juan de fuca plate seismic reflection data show that a regionally extensive band of landward dipping reflections exists above the subducting juan de fuca plate at the western canadian continental margin the reflections truncate at depth a major terrane boundary mapped near the surface and must have a structural origin many reflections are very strong having reflection coefficients locally as high as 10 20 these amplitudes are consistent with theoretical calculations of reflections from sharp poirosity contrasts if the pores are very thin however seismic data from the accreted sedimentary melange farther seaward from which much of the deep reflective zone is derived suggest that significant porosity contrasts do not occur alone because higher reflection amplitudes are found in regions of inferred high strain both geometric and amplitude constraints imply that the deep reflective layering represents intensely sheared rocks possibly originating close to the subduction decollement

updip limit of the seismogenic zone beneath the accretionary prism of southwest japan an effect of diagenetic to low grade metamorphic processes and increasing effective stress off southwest japan the seaward limit of coseismic displacement or updip limit of the seismogenic zone of the 1946 mw 8 3 thrust earthquake reaches to 4 km depth and 40 km landward of the trench this limit coincides with the estimated location of the 150 c isotherm and has been linked to changes in physical properties associated with the smectite to illite clay mineral transition here we show that this limit correlates with a suite of diagenetic to low grade metamorphic processes characterized by 1 declining fluid production and decreasing fluid pressure ratio and 2 active clay carbonate and zeolite cementation and the transition to pressure solution and quartz cementation these diagenetic to low grade metamorphic changes cause the onset of velocity weakening during thrust faulting an increase in effective stress and strengthening of the hanging wall which together combine to produce recordable earthquakes

crustal structure along the aleutian island arc new insights from receiver functions constrained by active source data 1 moho depth and vp vs estimates from stacking phases of receiver functions along the aleutian island arc give new constraints on its composition and structure they expand on the current understanding of island arcs and their relationship to continental crust production we also present an approach for including constraints from active source data in receiver function analysis in a region with sparse data coverage to complement this analysis moho depth averages 37 5 km with an average uncertainty of 2 5 km along the entire arc excluding the westernmost island of attu yields an average crustal thickness of 38 5 2 9 km the vp vs ratio decreases moving eastward along the arc with an average value of 1 80 in the western and central portion of the arc built on oceanic crust but 1 63 in the eastern section built on continental crust this may reflect tectonic and compositional changes along the arc however overall the arc appears more mafic than continental crust near constant crustal thickness despite significant compositional changes may indicate that nonmagmatic processes such as erosion and isostasy act to regulate arc thickness additionally strong conversions from an upper crustal magma chamber are observed beneath akutan island confirming and clarifying the geometry of the magma body inferred from other techniques they indicate a volcanic body much larger than the eruptive edifice a feature that must persist between eruptive cycles

link between plate fabric hydration and subduction zone seismicity in alaska subduction carries water into the earth where it can influence seismicity analysis of the structure of the alaskan subduction zone suggests fluid delivery is influenced by faults in the oceanic plate that formed at the mid ocean ridge

production preservation and prediction of source rock facies in deltaic settings abstract this article hypothesizes that production and preservation of source rock type facies in deltaic systems both landward and seaward of a coastline is mutually exclusive time successive and related to the dynamics of relative sea level sedimentologic research in the holocene mississippi and rhone delta complexes suggests that rheotrophic peats and associated organic rich beds preferably accumulate in the accommodation space created behind landward stepping shorelines in a transgressive systems tract tst such a setting also allows for a sufficient supply of recharging fresh nutrient rich groundwater into the peat forming mires kosters e c suter j r 1993 facies relationships and systems tracts in the late holocene mississippi delta plain journal of sedimentary petrology 63 4 727 733 independently carried out quantitative paleoecological studies in the same delta systems and in addition in the orinoco and po deltas suggest that in a progradational setting highstand systems tract hst seasonally discharged nutrient and sediment laden river waters on the shelf may give rise to anoxia or dysoxia subsequent overfertilization of the shelf leads to accumulation of organic rich mud belts on the shelf vanderzwaan g j jorissen f j 1991 biofacial patterns in river induced anoxia in tyson r v pearson t h eds modern and ancient continental shelf anoxia geological society of london special publication no 58 pp 65 82 thus production and preservation of source rock type facies landward of a shoreline as peats and related sediments is preferred in a tst when accommodation space and nutrient supply are landward of the shoreline vice versa production and preservation of such facies seaward of a shoreline as organic rich shelf muds occurs preferentially in a highstand systems tract in that situation accommodation space is on the shelf where river fed nutrients are supplied as well this hypothesis suggests further potential for application of sequence stratigraphic concepts for improved understanding of the occurrence of source rock type facies

splay fault branching along the nankai subduction zone seismic reflection profiles reveal steeply landward dipping splay faults in the rupture area of the magnitude m 8 1 tonankai earthquake in the nankai subduction zone these splay faults branch upward from the plate boundary interface that is the subduction zone at a depth of approximately 10 kilometers approximately 50 to 55 kilometers landward of the trough axis breaking through the upper crustal plate slip on the active splay fault may be an important mechanism that accommodates the elastic strain caused by relative plate motion

degradation of the plaquemines sub delta and relative sea level in eastern mississippi deltaic coast during late holocene abstract the stratigraphic architecture of plaquemines sub delta lobe is identified from sedimentary cores and sub bottom seismic data collected at lower breton sound louisiana usa four lithofacies were identified in cores from bottom to top 1 organic poor mud 2 organic poor silty sand 3 massive mud 4 organic rich peat and root rich soft mud overall the lithofacies displays a typical progradational deltaic deposit at bottom with upward coarsening feature and then an upward fining deposit overlying on top stratigraphic position of the organic rich peat facies indicates that it is formed after the fluvial sediment source moves away from this region and it is recognized as a termination of progradation of the plaquemines sub delta dated peat samples show that sub delta progradation stops between 876 and 1384 yr ad during which a 20 70 cm relative sea level rise is detected the geological framework reported in this study provides critical information for future modeling on river diversions the stratigraphic architecture underlying lower breton sound implicates that top three facies of root rich soft mud organic rich peat and massive mud are likely to be eroded away before the development of new crevasse splay or sub delta when sediment diversion is in operation the deeper organic poor silty sand lithofacies however can serve as a firm foundation to build new land

uncovering the geodetic signature of silent slip through repeating earthquakes slow transient slip that releases stress along the deep roots of plate interfaces is most often observed on regional gps networks installed at the surface the detection of slow slip is not trivial if the dislocation along the fault at depth does not generate a geodetic signal greater than the observational noise level instead of the typical workflow of comparing independently gathered seismic and geodetic observations to study slow slip we use repeating low frequency earthquakes to reveal a previously unobserved slow slip event by aligning gps time series with episodes of low frequency earthquake activity and stacking we identify a repeating transient slip event that generates a displacement at the surface that is hidden under noise prior to stacking our results suggest that the geodetic investigation of transient slip guided by seismological information is essential in exploring the spectrum of fault slip

thrust wedges with decollement levels and syntectonic erosion a view from analog models abstract analog sandbox models have been set up to study the impact of syntectonic erosion on thrust wedges with one and two decollement levels different accretion mechanisms are activated depending on interactions between surface processes and wedge mechanics frontal accretion backthrusting underthrusting and underplating due to decollement induced duplex formation at depth these mechanisms may function simultaneously being located at different parts across the wedge for all the experiments a high friction is imposed at the base of models and the volume of eroded material remains equal to the volume of newly accreted material maintaining a constant surface slope during the shortening erosion limits the forward propagation of thrust wedges and favors the underthrusting of basal layers allowing duplex formation erosion promotes development of major backthrusts in the thrust wedges without or with one decollement but no backthrusts was formed in the wedges with two decollements slow erosion allows lower extent of basal underthrusting in comparison with regular rate erosion variations in the erosion taper lead to changes in duplex geometry and exhumation rate in thrust wedges with one or two decollements the 6 erosion taper promotes formation of antiformal stack at the rear part of thrust wedge high rate of basal underthrusting and high extent of erosional removal the cover layers are nearly completely eroded above the antiformal stack and form the synformal klippe in frontal part of thrust wedges the 8 erosion taper allows development of individual ramp anticlines and active forward thrusting of cover layers above the decollement and low rate of basal underplating below it with consequent low extent of erosional removal the results of our experiments support the observations on structural evolution and erosion in the alberta foothills of the canadian rockies

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

two dimensional full wavefield inversion of wide aperture marine seismic streamer data summary a 2 d full wavefield inversion method is presented for the processing of wide aperture data the diversity of information contained within such datasets may be handled in a complete manner by first matching the traveltimes of the main events and then progressing to waveform fitting of the data through explicit full wavefield modelling our wavefield inversion scheme is based upon a finite difference solution of the 2 d elastic wave equation in the time distance domain the strength of adopting such an approach is the ability to generate all possible wave types within a given 2 d model multiples converted waves etc and thus to simulate and accurately model complex seismic wavefields the aim of the inversion is to find the 2 d p wave velocity model that minimizes the least squared difference between the observed and synthetic data across the full range of offsets following extensive testing on synthetic data the wavefield inversion scheme has been applied to wide aperture real marine seismic streamer datasets we present results from the synthetic testing and the wavefield inversion of wide aperture real data out to 12 km offset that was recorded on a single streamer even though current computational restrictions allow only a small subsection of the data to be analysed these examples demonstrate the potential value of wide aperture 2 d full wavefield inversion

slow slip events and seismic tremor at circum pacific subduction zones 1 it has been known for a long time that slip accompanying earthquakes accounts for only a fraction of plate tectonic displacements however only recently has a fuller spectrum of strain release processes including normal slow and silent earthquakes or slow slip events and continuous and episodic slip been observed and generated by numerical simulations of the earthquake cycle despite a profusion of observations and modeling studies the physical mechanism of slow slip events remains elusive the concurrence of seismic tremor with slow slip episodes in cascadia and southwestern japan provides insight into the process of slow slip a perceived similarity between subduction zone and volcanic tremor has led to suggestions that slow slip involves fluid migration on or near the plate interface alternatively evidence is accumulating to support the notion that tremor results from shear failure during slow slip global observations of the location spatial extent magnitude duration slip rate and periodicity of these aseismic slip transients indicate significant variation that may be exploited to better understand their generation most slow slip events occur just downdip of the seismogenic zone consistent with rate and state dependent frictional modeling that requires unstable to stable transitional properties for slow slip generation at a few convergent margins the occurrence of slow slip events within the seismogenic zone makes it highly likely that transitions in frictional properties exist there and are the loci of slow slip nucleation slow slip events perturb the surrounding stress field and may either increase or relieve stress on a fault bringing it closer to or farther from earthquake failure respectively this paper presents a review of slow slip events and related seismic tremor observed at plate boundaries worldwide with a focus on circum pacific subduction zones trends in global observations of slow slip events suggest that 1 slow slip is a common phenomena observed at almost all subduction zones with instrumentation capable of recording it 2 different frictional properties likely control fast versus slow slip 3 the depth range of slow slip may be related to the thermal properties of the plate interface and 4 the equivalent seismic moment of slow slip events is proportional to their duration mo different from the mo 3 scaling observed for earthquakes

alkali metals in the permo triassic as geochemical indicators of surficial processes abstract techniques established as most successful in the examination of widely variable but mainly oxidised geological materials require separation and isolation of components prior to analysis these methods reduce the number of variables involved in the production of clastic debris and enable conclusions to be drawn from a practical reversal of rock manufacture alkali metals prove successful as indicators of the surficial processes operative during permo triassic times they demonstrate enrichment by igneous material during early breccia production and subsequent dilution by wider sedimentation in basins to the east of cornubia england alkalis in recent soils river estuarine and beach sediments of southwest england are concentrated in the fine grained m fraction soils developed over permo triassic bedrock modern river estuarine sediments and beach sands all show anomalous enrichment of trace alkali metals they reflect the abundance of li rb and cs in parent material geochemical analyses of clays from deep boreholes in the wessex and central somerset basins enable inter basin comparisons with the high alkali deposits of devon absolute trace alkali levels are broadly similar to those of correlated beds at outcrop concentrations of cs prove higher in the upper permian than in the triassic of the wessex basin whilst values show devonian old red sandstone sediments to be un enriched in li and cs enrichment of li over rb in triassic mercia mudstones of the south devon coast suggests some li capture from sabkha brines by clays during evaporite production

slab1 0 a three dimensional model of global subduction zone geometries 1 we describe and present a new model of global subduction zone geometries called slab1 0 an extension of previous efforts to constrain the two dimensional non planar geometry of subduction zones around the focus of large earthquakes slab1 0 describes the detailed non planar three dimensional geometry of approximately 85 of subduction zones worldwide while the model focuses on the detailed form of each slab from their trenches through the seismogenic zone where it combines data sets from active source and passive seismology it also continues to the limits of their seismic extent in the upper mid mantle providing a uniform approach to the definition of the entire seismically active slab geometry examples are shown for two well constrained global locations models for many other regions are available and can be freely downloaded in several formats from our new slab1 0 website http on doi gov d9arbs we describe improvements in our two dimensional geometry constraint inversion including the use of average active source seismic data profiles in the shallow trench regions where data are otherwise lacking derived from the interpolation between other active source seismic data along strike in the same subduction zone we include several analyses of the uncertainty and robustness of our three dimensional interpolation methods in addition we use the filtered subduction related earthquake data sets compiled to build slab1 0 in a reassessment of previous analyses of the deep limit of the thrust interface seismogenic zone for all subduction zones included in our global model thus far concluding that the width of these seismogenic zones is on average 30 larger than previous studies have suggested

rupture extent of the 1938 alaskan earthquake as inferred from tsunami waveforms using tsunami waveforms we estimate the source parameters of the 10 november 1938 alaskan earthquake the earthquake ruptured a 300 kin long segment of the alaskan arc which corresponds to the aftershock area the seismic moment is approximately 20x102o nm or mw 8 2 and the moment release was concentrated in the eastern end of the aftershock zone the tsunami and seismic evidence strongly suggest that the 1938 earthquake did not rupture into the shumagin islands gap

plant material deposition in the tropical burdekin river australia implications for ancient fluvial sediments abstract in the deposits of the burdekin river north queensland australia plant material character and directional fabric together with the sedimentary facies character are generally diagnostic of depositional environment plant material is selectively entrained transported and deposited in all flow conditions ranging from dry season minimal flow

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

seismic attenuation and mantle wedge temperatures in the alaska subduction zone 1 anelastic loss of seismic wave energy or seismic attenuation 1 q provides a proxy for temperature under certain conditions the q structure of the upper mantle beneath central alaska is imaged here at high resolution an active subduction zone where arc volcanism is absent to investigate mantle thermal structure the recent broadband experiment across the alaska range beaar provides the first dense broadband seismic coverage of this region the spectra of p and sh waves for regional earthquakes are inverted for path averaged attenuation operators between 0 5 and 20 hz along with earthquake source parameters these measurements fit waveforms significantly better when the frequency dependence of q is taken into account and in the mantle frequency dependence lies close to laboratory values inverting these measurements for spatial variations in q reveals a highly attenuating wedge with q 500 assuming frequency dependence comparison with p results shows that attenuation in bulk modulus is negligible within the low q wedge as expected for thermally activated attenuation mechanisms bulk attenuation is significant in the overlying crust and subducting plate indicating that q must be controlled by other processes the shallowest part of the wedge shows little attenuation as expected for a cold viscous nose that is not involved in wedge corner flow overall the spatial pattern of q beneath alaska is qualitatively similar to other subduction zones although the highest wedge attenuation is about a factor of 2 lower the q values imply that temperatures exceed 1200 c in the wedge on the basis of recent laboratory based calibrations for dry peridotite these temperatures are 100 150 c colder than we infer beneath japan or the andes possibly explaining the absence of arc volcanism in central alaska

the westphalian d fossil lepidodendrid forest at table head sydney basin nova scotia sedimentology paleoecology and floral response to changing edaphic conditions copyright c 1996 elsevier science b v all rights reserved strata of westphalian d age on the western coast of the sydney basin expose a fossil forest of approximately 30 lepidodendrid trees within one of several clastic splits of the harbour seam a mutidisciplinary approach was employed to interpret the origins of the coal bed the depositional history of the site and the response of the fossil forest to changing edaphic conditions the megaspore and miospore records indicate that the mire vegetation was dominated by arboreous lycopsids especially paralycopodites with subdominant tree ferns petrographic palynological and geochemical evidence suggest that the harbour coal bed at table head originated as a rheotrophic cf planarr mire eutric histosolr the mire forest is interpreted to have been engulfed by prograding distributary channel sediments sparse protist assemblages are suggestive of a freshwater delta plain lake environment occasionally in contact with brackish waters lepidodendrids persisted as site colonizers of clastic substrates even after burial of the rheotrophic peatland and influenced the morphology of deposited sediment but apparently were unable to colonize distributary channels equivocal taxonomic data compression fossilsr show the fossil forest to have been composed of both monocarpic lepidodendronr and polycarpic diaphorodendron paralycopodites sigillariar lycopsids genera recorded in the palynology of the uppermost ply of the underlying coal bed comparatively rare within the clastic beds of the fossil forest however is the stem compression of paralycopodites whose dispersed megapores and miospores dominate the underlying coal bed tree diameter data recorded equivalent to breast height indicate a forest of mixed age these data would appear to suggest that some lepidodendrids employing a polycarpic reproductive strategy were better able to cross the ecological barrier imposed between peat and clastic substrates foliar compressions indicate that an understory or stand of psaronius type tree ferns co existed with the lepidodendrids on clastic substrates which developed as incipient gleysol soils the entombment of the forest can be ascribed to its distributary coastal setting local subsidence and a seasonal climate that fostered wildfire and increased sedimentation

seismic evidence for fluids in fault zones on top of the subducting cocos plate beneath costa rica summary in the 2005 ticocava explosion seismology study in costa rica we observed crustal turning waves with a dominant frequency of 10 hz on a linear array of short period seismometers from the pacific ocean to the caribbean sea on one of the shot records from shot 21 in the backarc of the cordillera central we also observed two seismic phases with an unusually high dominant frequency 20 hz these two phases were recorded in the forearc region of central costa rica and arrived 7 s apart and 30 40 s after the detonation of shot 21 we considered the possibility that these secondary arrivals were produced by a local earthquake that may have happened during the active source seismic experiment such high frequency phases following shot 21 were not recorded after shots 22 23 and 24 all in the backarc of costa rica which might suggest that they were produced by some other source however earthquake dislocation models cannot produce seismic waves of such high frequency with significant amplitude in addition we would have expected to see more arrivals from such an earthquake on other seismic stations in central costa rica we therefore investigate whether the high frequency arrivals may be the result of a deep seismic reflection from the subducting cocos plate the timing of these phases is consistent with a shear wave from shot 21 that was reflected as a compressional s p and a shear s s wave at the top of the subducting cocos slab between 35 and 55 km depth the shift in dominant frequency from 10 hz in the downgoing seismic wave to 20 hz in the reflected waves requires a particular seismic structure at the interface between the subducting slab and the forearc mantle to produce a substantial increase in reflection coefficients with frequency the spectral amplitude characteristics of the s p and s s phases from shot 21 are consistent with a very high v p v s ratio of 6 in 5m thick slab parallel layers this result suggests that a system of thin shear zones near the plate interface beneath the forearc is occupied by hydrous fluids under near lithostatic conditions the overpressured shear zone probably takes up fluids from the downgoing slab and it may control the lower limit of the seismogenic zone

along strike structural changes controlled by dehydration related fluids within the philippine sea plate around the segment boundary of a megathrust earthquake beneath the kii peninsula southwest japan 1 active and passive seismic experiments in the kii peninsula southwest japan revealed prominent structural features around the segment boundary of a megathrust earthquake associated with the subduction of the philippine sea plate phs a distinct reflection band in the uppermost part of the phs shows significant lateral variation along its strike the thicker reflection band which corresponds to the deeper extension of the fault area of the 1944 tonankai earthquake is interpreted to be a zone of high pore fluid pressure causing a state of conditionally stable slip on the plate boundary by the reduction in effective normal stress a thinner reflective band corresponds to the deepest part of the rupture area of the 1946 nankai earthquake where plate coupling is stronger due to less effect of fluids this along strike structural variation controls the differences in frictional properties and the lateral limit of rupturing along the plate boundary

the updip and downdip limits to great subduction earthquakes thermal and structural models of cascadia south alaska sw japan and chile this study examines thermal and structural controls of the updip and downdip rupture limits of great subduction thrust earthquakes data on past great earthquake seismic limits have been compiled for four continental subduction zones cascadia sw japan south alaska and chile these limits have then been compared to the predictions of several models for what constrains great earthquake rupture temperatures on the subduction thrusts have been estimated by finite element numerical models the landward limits of the observed updip aseismic zones correspond to the position where the thrust temperature reaches about 100 c that is depths of about 2 to 10 km for the subduction zones studied this temperature agrees with the dehydration of stable sliding smectite clays to illite chlorite the temperatures in this region are controlled mainly by the thickness of sediment on the incoming crust and by the crustal age and thus heat flow the downdip limits correspond to the depth on the thrust where either 1 the temperature reaches about 350 c which corresponds to thermally activated stable sliding behavior for crustal rocks with a transition to 450 c or 2 about 40 km depth if 350 c is reached at greater depth depths of about 40 km approximately correspond to the intersection of the thrust with the continental forearc moho and this downdip limit may be a consequence of stable sliding serpentinite or talc and other hydrated forearc mantle rocks the primary temperature controls on the downdip region are the age of the subducting oceanic plate and the thrust dip profile secondary control comes from the thickness of incoming sediment the convergence rate and the radioactive heat generation in the overlying forearc the 100 c updip limit occurs near the trench for young subducting plates with a thick sediment section such as cascadia 6 8 ma and up to 80 km landward for older oceanic crust such as south alaska 50 ma the 350 c downdip thermal limit is applicable for young oceanic plates e g cascadia and nankai whereas the forearc mantle limit applies for older plates e g south alaska and chile except near the chile rise for the margins studied that have experienced great earthquakes there is generally good agreement between the postulated thermal and moho limits and the rupture or seismogenic zone as defined by the distribution of aftershocks and by waveform tsunami and dislocation modeling the downdip limit of the interseismic locked zone from dislocation modeling is also in agreement with these limits

three dimensional inversion of regional p and s arrival times in the east aleutians and sources of subduction zone gravity highs free air gravity highs over forearcs represent a large fraction of the power in the earth s anomalous field yet their origin remains uncertain seismic velocities as indicators of density are estimated here as a means to compare the relative importance of upper plate sources for the gravity high with sources in the downgoing plate p and s arrival times for local earthquakes recorded by a seismic network in the eastern aleutians are inverted for three dimensional velocity structure between the volcanic arc and the downgoing plate a three dimensional ray tracing scheme is used to invert the 7974 p and 6764 s arrivals for seismic velocities and hypocenters of 635 events one dimensional inversions show that station p residuals are systematically 0 25 0 5 s positive at stations 0 30 km north of the aleutian volcanic arc indicating slow material while residuals at stations 10 30 km south of the arc are 0 1 0 25 s negative both features are explained in three dimensional inversions by velocity variations at depths less than 25 35 km tests using a one dimensional or a two dimensional slab starting model show that below 100 km depth velocities are poorly determined and trade off almost completely with hypocenters for earthquakes at these depths the locations of forearc velocity highs in the crust of the upper plate correspond to the location of the gravity high between the trench and volcanic arc free air anomalies calculated from the three dimensional velocity inversion result match observed gravity for a linear density velocity relationship between 0 1 and 0 3 mg m 3 km s 1 when a 50 km thick slab is included with a density of 0 055 0 005 mg m 3 values outside these ranges do not match the observed gravity the slab alone contributes one third to one half of the total 75 150 mgal amplitude of the gravity high but predicts a high that is much broader than is observed the inclusion of upper plate velocity anomalies predicts the correct width of the anomaly 100 150 km where the anomaly is most positive because the forearc gravity high is continuous along the entire aleutian arc and is found in most arcs globally high upper plate forearc velocities are suspected to be a common feature of the upper plate of most subduction zones the forearc mass excesses appear to be sustained by upward regional flexure of the upper plate that partly balances the depression of the lower plate at the trench thus elevating high density and high velocity material thus a part of the downward flexure of the subducting plate is regionally compensated by shallow positive mass anomalies in the upper plate and the strength of the upper plate helps generate the forearc gravity highs

alaska megathrust 2 imaging the megathrust zone and yakutat pacific plate interface in the alaska subduction zone we image the slab underneath a 450 km long transect of the alaska subduction zone to investigate 1 the geometry and velocity structure of the downgoing plate and their relationship to slab seismicity and 2 the interplate coupled zone where the great 1964 earthquake mw 9 2 exhibited the largest amount of rupture the joint teleseismic migration of two array data sets based on receiver functions rfs reveals a prominent shallow dipping low velocity layer at 25 30 km depth in southern alaska modeling of rf amplitudes suggests the existence of a thin layer vs of 2 1 2 6 km s that is 20 40 slower than underlying oceanic crustal velocities and is sandwiched between the subducted slab and the overriding plate the observed megathrust layer with vp vs of 1 9 2 3 may be due to a thick sediment input from the trench in combination with elevated pore fluid pressure in the channel our image also includes an unusually thick low velocity crust subducting with a 20 dip down to 130 km depth at 200 km inland beneath central alaska the unusual nature of this subducted segment results from the subduction of the yakutat terrane crust our imaged western edge of the yakutat terrane aligns with the western end of a geodetically locked patch with high slip deficit and coincides with the boundary of aftershock events from the 1964 earthquake it appears that this sharp change in the nature of the downgoing plate could control the slip distribution of great earthquakes on this plate interface

seismic lamination and anisotropy of the lower continental crust abstract seismic lamination in the lower crust associated with marked anisotropy has been observed at various locations three of these locations were investigated by specially designed experiments in the near vertical and in the wide angle range that is the urach and the black forrest area both belonging to the moldanubian a collapsed variscan terrane in southern germany and in the donbas basin a rift inside the east european ukrainian craton in these three cases a firm relationship between lower crust seismic lamination and anisotropy is found there are more cases of lower crustal lamination and anisotropy e g from the basin and range province western us and from central tibet not revealed by seismic wide angle measurements but by teleseismic receiver function studies with a p s conversion at the moho other cases of lamination and anisotropy are from exhumed lower crustal rocks in calabria southern italy and val sesia and val strona ivrea area northern italy we demonstrate that rocks in the lower continental crust apart from differing in composition differ from the upper mantle both in terms of seismic lamination observed in the near vertical range and in the type of anisotropy compared to upper mantle rocks exhibiting mainly orthorhombic symmetry the symmetry of the rocks constituting the lower crust is either axial or orthorhombic and basically a result of preferred crystallographic orientation of major minerals biotite muscovite hornblende we argue that the generation of seismic lamination and anisotropy in the lower crust is a consequence of the same tectonic process that is ductile deformation in a warm and low viscosity lower crust this process takes place preferably in areas of extension heterogeneous rock units are formed that are generally felsic in composition but that contain intercalations of mafic intrusions the latter have acted as heat sources and provide the necessary seismic impedance contrasts the observed seismic anisotropy is attributed to lattice preferred orientation lpo of major minerals in particular of mica and hornblende but also of olivine a transversely isotropic symmetry system such as expected for sub horizontal layering is found in only half of the field studies azimuthal anisotropy is encountered in the rest of the cases this indicates differences in the horizontal components of tectonic strain which finally give rise to differences in the evolution of the rock fabric

distinguishing between natural and anthropogenic sources of metals entering the irish sea international agreements e g ospar on the release of hazardous substances into the marine environment and environmental assessments of shelf seas require that concentrations and bioavailability of metals from anthropogenic sources can be distinguished from those originating as a result of natural geological processes the development of a methodology for distinguishing between anthropogenic and natural sources of metals entering the irish sea through river inputs is described the geochemistry of stream river and estuarine sediments has been used to identify background geochemical signatures related to geology and modifications to these signatures by anthropogenic activities the british geological survey bgs geochemical database based on stream sediments from 1 to 2 km2 catchments was used to derive the background signatures where mining activity was present the impact on the signature was estimated by comparison with the geochemistry of sediments from a geologically similar but mining free area river sediment samples taken upstream and downstream of major towns were used respectively to test the validity of using stream sediments to estimate the chemistry of the major river sediment and to provide an indication of the anthropogenic impact related to urban and industrial development the geochemistry of estuarine sediments from surface samples and cores was then compared with river and offshore sediment chemistry to assess the importance of riverine inputs to the irish sea studies were undertaken in the solway ribble wyre and mersey estuaries the results verify that catchment averages of stream sediments and major river samples have comparable chemistry where anthropogenic influences are small major urban and industrial including mining development causes easily recognised departures from the natural multi element geochemical signature in river sediment samples downstream of the development and enhanced metal levels are observed in sediments from estuaries with industrial catchments stream sediment chemistry coupled with limited river and estuarine sampling provides a cost effective means of identifying anthropogenic metal inputs to the marine environment investigations of field and laboratory protocols to characterise biological impact bioaccumulation of metals in sediments of the irish sea and its estuaries show that useful assessments can be made by a combination of surveys with bioindicator species such as clams scrobicularia plana selective sediment measurements that mimic the biologically available fractions and laboratory mesocosm studies

organic clastic facies relationships and chronostratigraphy of the barataria interlobe basin mississippi delta plain abstract the lithofacies of barataria basin an interlobe basin of the mississippi delta plain were established from analyses of 115 boreholes five organic lithofacies are distinguished by organic matter content botanical origin and degree of decomposition organic rich facies accumulated in three stages a b and c during two depositional events stage a developed locally but stages b and c developed as blankets over abandoned delta lobes protected from marine influx by a shoreline stage b lasted from 2 300 1 000 yr bp and ended when clastic sediments from renewed deltaic sedimentation blanketed the peats expanding the basin about 50 km seaward peat accumulation of stage c lasted from 900 yr bp to the present and is currently being terminated by subsidence induced transgression high quality stage b peats accumulated beneath herbaceous freshwater floating and emergent marshes mainly above the footwall of a strike oriented reactivated growth fault above the footwall the basin was shallower than on the hanging wall where the deeper basin invited more overbank sedimentation which increased ash content and diluted peats thus both tectonic and sedimentary factors influenced organic facies the quality and quantity of barataria basin peats differ from those of many deltaic coals however these peats accumulated in only 2 300 yr immediately after a rapid sea level rise frequent delta lobe switching increased organic matter preservation potential but did not provide sufficient time for the accumulation of thick low ash peats the geometries of organic and clastic facies are similar to those of some coal bearing strata

downdip variations in seismic reflection character implications for fault structure and seismogenic behavior in the alaska subduction zone seismic reflection data collected offshore of alaska peninsula across the western edge of the semidi segment show distinctive variations in reflection characteristics of the megathrust fault with depth suggesting changes in structure that may relate to seismic behavior from the trench to 40 km landward two parallel reflections are observed which we interpret as the top and bottom of the subducted sediment section from 50 95 km from the trench the plate interface appears as a thin 400 ms reflection band deeper and farther landward the plate interface transitions to a thicker 1 1 5 s package of reflections where it appears to intersect the forearc mantle wedge based on our preferred interpretation of the continental moho synthetic waveform modeling suggests that the thin reflection band is best explained by a single 100 to 250 m thick low velocity zone whereas the thick reflection band requires a 3 to 5 km thick zone of thin layers the thin reflection band is located at the center of the 1938 mw 8 2 semidi earthquake rupture zone that now experiences little interplate seismicity the thick reflection band starts at the downdip edge of the rupture zone and correlates with a dipping band of seismicity and projects to the location of tremor at greater depth we interpret the thin reflection band as a compacted sediment layer and or localized shear zone the thick reflection band could be caused by a wide deformation zone with branching faults and or fluid rich layers representing a broad transition from stick slip sliding to slow slip and tremor

caesium in the up estuary transport of sediments abstract trace elements are used in the teign estuary of southwest england u k to identify the source of sediments barium present in clastic material derived from past mining activity characterises river input whilst strontium from skeletal debris marks the landward transport of marine derived sediments high caesium concentrations occurring in the estuary are traced to estuarine and coastal outcrops of permian sandstones significantly cs values reach a maximum 85 ppm in the middle and upper estuary where river and marine derived sediments mix caesium resides largely in both the clay fraction 40 230 ppm and argillaceous lithic material 60 180 ppm it proves a useful marker for sediment differentiation by indicating the status of up estuary sediment transport

local thickening of the cascadia forearc crust and the origin of seismic reflectors in the uppermost mantle seismic reflection profiles from three different surveys of the cascadia forearc are interpreted using p wave velocities and relocated hypocentres which were both derived from the first arrival travel time inversion of wide angle seismic data and local earthquakes the subduction decollement which is characterized beneath the continental shelf by a reflection of 0 5 s duration can be traced landward into a large duplex structure in the lower forearc crust near southern vancouver island beneath vancouver island the roof thrust of the duplex is revealed by a 5 12 km thick zone identified previously as the e reflectors and the floor thrust is defined by a short duration reflection from a b2 km thick interface at the top of the subducting plate we show that another zone of reflectors exists east of vancouver island that is approximately 8 km thick and identified as the d reflectors these overlie the e reflectors together the two zones define the landward part of the duplex the combined zones reach depths as great as 50 km the duplex structure extends for more than 120 km perpendicular to the margin has an along strike extent of 80 km and at depths between 30 km and 50 km the duplex structure correlates with a region of anomalously deep seismicity where velocities are less than 7000 m s 1 we suggest that these relatively low velocities indicate the presence of either crustal rocks from the oceanic plate that have been underplated to the continent or crustal rocks from the forearc that have been transported downward by subduction erosion the absence of seismicity from within the e reflectors implies that they are significantly weaker than the overlying crust and the reflectors may be a zone of active ductile shear in contrast seismicity in parts of the d reflectors can be interpreted to mean that ductile shearing no longer occurs in the landward part of the duplex merging of the d and e reflectors at 42 46 km depth creates reflectivity in the uppermost mantle with a vertical thickness of at least 15 km we suggest that pervasive reflectivity in the upper mantle elsewhere beneath puget sound and the strait of georgia arises from similar shear zones

subduction interface processes recorded by eclogite facies shear zones monviso w alps abstract the monviso ophiolite lago superiore unit constitutes a well preserved almost continuous upper fragment of oceanic lithosphere subducted at c 80 km depth thereby providing a unique opportunity to study mechanical coupling processes and meter scale fluid rock interactions occurring at such depths in present day subduction zones it is made of i a variably thick 50 500 m section of eclogitized basaltic crust associated with minor calcschist lenses overlying a 100 400 m thick metagabbroic body and of ii a c 1 km thick serpentinite sole we herein focus on the three major eclogite facies shear zones found at the top of the unit at the boundary between basalts and gabbros and between gabbros and serpentinites respectively strain localization occurred at lithological interfaces irrespective of material strength while ductile deformation dominates along the shear zones local brittle behavior is demonstrated by the existence of numerous eclogite breccias of fe ti metagabbros and widespread garnet fractures possibly linked with intermediate depth eclogite facies micro seismicity these m to hm sized fragments of fe ti metagabbros were later sheared and disseminated within serpentinite schists along the gabbro serpentinite boundary lower shear zone lsz pervasive and focused fluid flow is attested in the lsz by significant alteration of bulk rock compositions weakening of the rocks and widespread crystallization of hydrous parageneses by contrast the intermediate shear zone isz shows evidence for more restricted short range fluid flow the activity of both the isz and lsz ceased during early lawsonite eclogite facies exhumation when deformation localized deeper within the serpentinite sole allowing for the detachment and preservation of this large ophiolitic fragment

strong along arc variations in attenuation in the mantle wedge beneath costa rica and nicaragua attenuation structure in the central american subduction zone was imaged using local events recorded by the tomography under costa rica and nicaragua array a 20 month long deployment july 2004 until march 2006 of 48 seismometers that spanned the fore arc arc and back arc regions of nicaragua and costa rica p and s waveforms were inverted separately for the corner frequency and moment of each event and for the path averaged attenuation operator t of each event station pair assuming attenuation is slightly frequency dependent 0 27 then tomographic inversions were performed for s and p attenuation q s 1 and q p 1 since p wave amplitudes reflect both shear and the bulk moduli tomographic inversions were also performed to determine shear and bulk attenuation q s 1 and q 1 the loss of energy per cycle owing to shearing and uniform compression respectively damping and other inversion tomographic parameters were systematically varied as is typical in subduction zone attenuation studies a less attenuating slab upper plate and wedge corner and a more attenuating mantle wedge were imaged in addition first order differences between the mantles beneath nicaragua and costa rica were observed the slab in nicaragua is more attenuating than the slab in costa rica a larger zone of higher shear attenuation also characterizes the nicaraguan mantle wedge within the wedge maximum attenuation values at 1 hz correspond to qs 38 73 beneath nicaragua and qs 62 84 beneath costa rica and average values are qs 76 78 and qs 84 88 respectively attenuation variations correlate with along arc trends in geochemical indicators that suggest that melting beneath nicaragua occurs at more hydrated conditions and possibly to greater extents and depths relative to northern costa rica shear attenuation dominates over bulk attenuation in the well resolved regions of the wedge the more extensive zones of greater shear attenuation observed in the nicaraguan wedge could be explained by higher temperatures and or greater hydration but comparison with petrological data suggests that hydration variations play a larger role average wedge attenuation values are comparable to estimates for the andes and japan greater than those for alaska and less than those for tonga lau