the stable isotopic composition of modern soil carbonate and its relationship to climate the oxygen isotopic composition of modern soil carbonate is well correlated with the isotopic composition of local meteoric water the carbon isotopic cycle for co 2 in soils can be described in terms of the proportion of biomass using the c 4 photosynthetic pathway and the co 2 respiration rate of the soil at low soil respiration rates significant atmospheric co 2 mixing can occur in general the carbon isotopic composition of soil carbonate is related to the proportion of c 4 biomass present in soil but soils that freeze to the depth of carbonate formation often have a significant atmospheric component this suggests that freezing of the soil solution should be considered as another important mechanism for soil carbonate formation because of these relationships the isotopic composition of soil carbonate may be a paleoclimatic and paleoecologic indicator in cases in which diagenetic alteration has not occurred

stress field constraints on intraplate seismicity in eastern north america focal mechanisms of 32 north american midplate earthquakes mo 3 8 6 5 were evaluated to determine if slip is compatible with a broad scale regional stress field derived from plate driving forces and if so under what conditions stress regime pore pressure and frictional coefficient using independent information on in situ stress orientations from well bore breakout and hydraulic fracturing data and assuming that the regional principal stresses are in approximately horizontal and vertical planes 10o the constraint that the slip vector represents the direction of maximum resolved shear stress on the fault plane was used to calculate relative stress magnitudes defined by the parameterb s2 s3 s s3 from the fault stress geometry as long as the focal mechanism has a component of oblique slip i e the b axis does not coincide with the intermediate principal stress direction this calculation identifies which of the two nodal planes is a geometrically possible slip plane gephart 1985 slip in a majority of the earthquakes 25 of 32 was found to be geometrically compatible with reactivation of favorably oriented preexisting fault planes in response to the broad scale uniform regional stress field slip in five events was clearly inconsistent with the regional stress field and appears to require a localized stress anomaly to explain the seismicity significantly all five of these events occurred prior to 1970 when many regional networks were installed and their focal mechanisms are inconsistent with more recent solutions of nearby smaller events the frictional likelihood of the geometrically possible slip on the selected fault planes was evaluated in the context of conventional frictional faulting theory the ratio of shear to normal stress on the fault planes at hypocentral depth was calculated relative to an assumed regional stress field regional stress magnitudes were determined from 1 s s3 ratios based on the frictional strength of optimally oriented faults the basis for the linear brittle portion of lithospheric strength profiles 2 the computed relative stress magnitude b values and 3 a vertical principal stress assumed equal to the lithostat two end member possibilities were examined to explain the observed slip in these less than optimally oriented fault planes first the frictional coefficient was held constant on all faults hydrostatic pore pressure was assumed regionally and the fault zone pore pressure was determined since pore pressure is a measurable quantity with real limits in the crust p0 s3 this end member case was used to determine which of the geometrically possible slip planes were frictionally likely slip planes alternately pore pressure was fixed at hydrostatic everywhere and the required relative lowered frictional coefficient of the fault zone was computed slip in 23 of the 25 geometrically compatible earthquakes was determined to also be frictionally likely in response to an approximately horizontal and vertical regional stress field derived from plate driving forces whose magnitudes are constrained by the frictional strength of optimally oriented faults assuming hydrostatic pore pressure regionally the conditions for slip on these 23 relatively well oriented earthquake faults were determined relative to this regional crustal strength model and require only moderate increases in pore pressure between about 0 4 0 8 of lithostatic hydrostatic is about 0 37 of lithostatic or alternately moderate lowering 50 of the frictional coefficient on the faults which slipped superlithostatic pore pressures are not required focal mechanisms for the two other earthquakes with slip vectors geometrically consistent with the regional stress field however did require pore pressures far exceeding the least principal stress or extremely low coefficients of friction these events may reflect either local stress rotations undetected with current sampling or poorly constrained focal mechanisms the analysis also confirmed a roughly north to south contrast in stress regime between the central eastern united states and southeastern canada previously inferred from a contrast in focal mecha nisms between the two areas most central eastern united states earthquakes occur in response to a strike slip stress regime whereas the southeastern canadian events require a thrust faulting stress regime this contrast in stress regime with a constant maximum horizontal stress orientation determined by far field plate driving forces requires a systematic lateral variation in relative stress magnitudes superposition of stresses due to simple flexural models of glacial rebound stresses are of the correct sense to explain the observed lateral variation but maximum computed rebound related stress magnitude changes are quite small about 10 mpa and do not appear large enough to account for the stress regime change if commonly assumed stress magnitudes determined from frictional strength apply to the crust at seismogenic depths

morphology of the distorted subducted pacific slab beneath the hokkaido corner japan abstract the intersection of the japan and kurile arcs is expressed as a cuspate feature at the trench a bend in the japanese islands and a complex lithospheric structure and is known as the hokkaido corner the pacific plate is subducting beneath the two arcs in the northwest pacific at different velocities which has resulted in an arc arc collision and distortion of the subducting oceanic lithosphere using p and s wave tomographic images and seismicity the distorted shape of the subducted pacific plate beneath the hokkaido corner can be interpreted in three dimensions the pacific plate is imaged as continuous along the japan kurile arc with dip angle increasing from south to north but at the arc arc junction the geometry is complex and appears crumpled beneath the hokkaido corner there are two features in the tomography that are clearly imaged a region of low velocity at approximately 50 150 km depth between 142 5 145 e and 41 43 n and a distorted buckled lower slab boundary these two unusual characteristics in the slab morphology are likely to be related to deformation of the subducted pacific plate at the arc arc junction

development of magnitude bound relations for paleoliquefaction analyses new zealand case study abstract magnitude bound relations are often used to estimate paleoearthquake magnitudes from paleoliquefaction data this study proposes new zealand based magnitude bound curves that are developed using a liquefaction field observations and b a newly proposed back calculation approach that combines the simplified liquefaction evaluation procedure with a regionally appropriate ground motion prediction equation for b both deterministic and probabilistic frameworks are proposed the magnitude bound curves back calculated using either the deterministic or probabilistic frameworks are advantageous in that they can be used to predict the spatial distribution of liquefaction in regions where historical liquefaction field observations are limited or poorly documented and because soil and site specific conditions can be incorporated into magnitude bound analyses moreover curves developed using the probabilistic framework allow for the range of possible causative earthquake magnitudes to be better understood and quantified to demonstrate the use of the proposed relations paleoliquefaction features discovered in eastern christchurch nz are analyzed the 1869 mw4 8 christchurch earthquake and or 1717 mw8 1 alpine fault earthquake are found to be the most likely candidates amongst known historical and paleoearthquakes for triggering liquefaction over the permissible time range ca 1660 to 1905 a d this study demonstrates the potential of the proposed magnitude bound curves to provide insight in to past present and future hazards proving their utility even in cases of limited evidence the approach of developing and applying magnitude bound curves proposed herein is not limited to parts of new zealand but rather can be applied worldwide

seismotectonic implications of sand blows in the southern mississippi embayment we explore seismically induced sand blows from the southern mississippi embayment and their implications in resolving the question of near or distal epicentral source region this was accomplished using aerial photography field excavations and cone penetration tests our analysis shows that three sand blow fields exhibit a distinct chronology of strong ground motion for the southern embayment 1 the ashley county arkansas sand blow field near the arkansas louisiana state border experienced four holocene sand venting episodes 2 to the north the desha county field experienced at least three episodes of liquefaction and 3 the lincoln jefferson counties field experienced at least one episode cone penetration tests cpt conducted in and between the sand blow fields suggest that the fields may not be distal liquefaction associated with new madrid seismic zone earthquakes but rather are likely associated with strong earthquakes on local faults this conclusion is consistent with the differences in timing of the southern embayment sand venting episodes and those in the new madrid seismic zone these results suggest that active tectonism and strong seismicity in intraplate north america may not be localized at isolated weak spots but rather widespread on fault systems that are favorably oriented for slip in the contemporary stress field

explosive craters and soil liquefaction curious dry craters formed in the aftermath of a disastrous earthquake are explained the bhuj earthquake in january 2001 with a moment magnitude of 7 7 was one of the most disastrous in india s recorded history although its fault plane did not rupture the surface the earthquake caused ground failure including the formation of sand blow craters and sand fissures indicative of liquefaction of subsurface sediment over an area of about 15 000 square kilometres1 here we investigate the two large dry craters that formed during the earthquake and estimate the likely subsurface pressure at the time from a trajectory analysis of the ejected clasts we conclude that the craters were created by explosive deformation of the ground that probably resulted from the delayed effects of soil liquefaction

neotectonics of the southeastern reelfoot rift zone margin central united states and implications for regional strain accommodation the northeast striking new madrid fault system of central north america has been described as a right lateral strike slip system with a left stepping restraining bend thrust the fault system has one of the highest rates of seismic energy release in an intraplate setting and it has been regarded as a zone of significant earthquake hazard the new madrid fault system is the central part of the wider reelfoot rift fault system a northeast striking basement fault zone although component faults of the new madrid system are well defined by microseismicity recent geodetic surveys suggest that little if any strain is accumulating on the principal southern strike slip arm of the fault system seismicity and geologic data show that the restraining bend thrust continues past the southern strike slip arm to the southeastern reelfoot rift margin thus we suggest that earthquakes defining the southern arm of the new madrid fault system are primarily aftershocks of an earthquake on that arm during the last sequence of great earthquakes a d 1811 1812 and it is the southeastern rift margin fault system that is currently accommodating right lateral strain along the boundary of the thrust block this interpretation is consistent with recent geodetic results the southeastern rift margin coincides with a 150 km long linear topographic scarp from near memphis to the tennessee kentucky line and s wave reflection profiles auger data and a trench excavation reveal late wisconsin early holocene surface faulting and late holocene liquefaction associated with this fault line scarp variation in sense of throw along strike and flower structure geometry suggest that this is a strike slip fault recognition of this rift margin as an important element of active tectonism in the mississippi embayment has broad implications for assessment of the seismic hazard of this and similar intraplate settings temporal shifts in strain accommodation may give rise to short term seismicity patterns and or geodetic velocities that do not reveal long term tectonic patterns

variations of p wave speeds in the mantle transition zone beneath the northern philippine sea using waveforms and travel times from deep earthquakes we constructed 16 seismic profiles each of which constrains the radial variation in vp over a small area beneath the northern philippine sea taken together the azimuthal coverage of these profiles also places tight bounds on the lateral extent of a region of anomalously high vp up to 3 faster than average earth models originally suggested by travel time tomography unlike travel time tomography which relies heavily on arrival times of the direct p phase we utilize the waveforms and move out of later arrivals that mainly sample the mantle transition zone of interest our results identify three important characteristics of the northern philippine sea anomaly that are distinct from previous results first being approximately a subhorizontal laterally uniform feature the anomaly is localized beneath the northwestern corner of the philippine sea within a region of approximately 500 500 km2 immediately east of the ryukyu arc second the anomaly is well constrained to occur in the lower portion of the transition zone extending all the way down to the 660 km discontinuity third the presence of such a distinct anomaly reduces the contrast in vp across the 660 km discontinuity from approximately 6 to 3 such a configuration is consistent with the interpretation that the anomaly is caused by a remnant of subducted slab as negative buoyancy should rest the slab just above the 660 km discontinuity where resistance to subduction is expected from a negative clapeyron slope during the spinel mg fe perovskite transition

shear wave velocity structure beneath europe the northeastern atlantic and western asia from waveform inversions including surface wave mode coupling summary waveforms of 449 seismograms have been inverted for s wave velocity structures beneath europe the northeastern atlantic and western asia down to a depth of 670 km recorded waveforms were used in the time window starting at the s wave arrival and ending after the fundamental mode rayleigh wave arrival the inversion method is based on the partitioned waveform inversion nolet 1990 with the difference that synthetic seismograms are calculated taking surface wave mode coupling into account in order to model body waves in laterally heterogeneous media more accurately the partitioning of the inversion procedure makes non linear optimization feasible even for inversions including surface wave mode coupling the non linear inversion of the waveforms resulted in linear constraints on the 3 d velocity structure in a second step these constraints were used in a linear inversion for the 3 d shear wave velocity model beneath europe the northeastern atlantic and western asia the eur sc 95 model is presented which is characterized by a wide range of length scales of the velocity structures in central europe where the ray density is highest small scale structures are recovered such as the presence of high velocities associated with the hellenic subduction zone on the edges of the inversion model where the ray density and therefore also the resolution is poorer we find larger scale features such as the relatively laterally homogeneous high velocity structure beneath the russian platform to a depth of 300 km in this paper we discuss the inversion method data processing parametrization difficulties due to the introduction of surface wave mode coupling spatial resolution of the model and structures in the eur sc 95 model

the s velocity structure of the east asian mantle from inversion of shear and surface waveforms summary the structure of the mantle beneath east asia down to 800 km depth is investigated using full waveforms of seismic shear and surface waves epicentral distances are limited to less than 40 in contrast with previous waveform inversions we avoid ray theoretical or path integral approaches instead we use 1 exact 3 d waveform sensitivity kernels that correctly reflect off path sensitivity and the existence of fresnel zones we apply 2 an accurate 3 d forward modelling technique based on a coupled mode multiple forward scattering approach allowing us 3 to iterate the inversion procedure through several 3 d models and 4 to evaluate the true misfit between the data and the synthetics for the 3 d model average lateral resolution of the model in regions with good path coverage is 400 km throughout the upper mantle in the depth range from 100 to 250 km the lateral resolution even approaches 200 km since wave front healing is taken into account the amplitudes of velocity perturbations are larger than in other tomographic models moreover the waveform sensitivity kernels provide an intrinsic physical smoothing of the model stabilizing the inversion finally owing to the use of exact sensitivities better resolution can be achieved with a given set of seismograms major features of the model are high velocity subducting slabs along the west pacific subduction zones stagnating at the 660 km discontinuity a strong low velocity zone between 80 and 250 km depth in the west pacific backarc regions a plume like low velocity feature beneath the southern tip of the baikal rift zone extending into the transition zone a low velocity region under the tien shan with connection into the transition zone and thick crust under tibet reaching its maximum thickness of approximately 80 km close to the 35th parallel the lithospheric mantle underneath southern tibet is very fast indicating underthrusting of the indian lithosphere in contrast the upper mantle beneath northern tibet exhibits average to slow values from the moho down to the 660 km discontinuity very high s velocities are again observed beneath the tarim basin the high velocity mantle lithospheres under southern tibet and the tarim join beneath the karakorum range at the western tip of the tibet plateau

toward quantifying uncertainty in travel time tomography using the null space shuttle 1 the solution of large linear tomographic inverse problems is fundamentally non unique we suggest to explore the non uniqueness explicitly by examining the null space of the forward operator we show that with the null space shuttle it is possible to assess robustness in tomographic models and we illustrate the concept for the global p wave model mit p08 we found a broad range of acceptable solutions compatible with the travel time data the root mean square rms velocity perturbations vary from 0 2 to 0 6 in the lowermost mantle and from 0 3 to 1 3 in the upper mantle such large variations in average amplitudes prohibit meaningful inferences on temperature or chemical variations in the earth from tomographic models alone on a global scale much short wavelength structure resides in the null space of the forward operator suggesting that the data do not everywhere resolve structure on the smallest length scale 200 km allowed by the block parameterization used in mit p08 and similar models this indicates that great care should be taken when interpreting such structure as a practical measure we suggest that only those structures for which the wave speed perturbations do not change sign within the range of models permitted by the data should be considered robust with this criterion the model null space analysis shows that the high velocity anomalies in the lower mantle which are often interpreted as remnants of slabs of subducted lithosphere are required by the seismic data low velocity anomalies underneath for instance hawaii iceland and africa show varying degrees of robustness

channel trenching and climatic change in the southern u s great plains fifteen alluvial sequences in texas and oklahoma exhibit the same late holocene record of channel trenching at 1 ka the erosion was preceded by slow alluvial sedimentation in most stream valleys resulting in the formation of a cumulic organic rich flood plain soil previously named the copan soil the soil formed during a period of regionally moister climate as indicated by pollen spectra molluscan faunas vertebrate faunas sedimentary structures and high alluvial water tables at 1 ka the regional climate changed from moist to dry coinciding with an episode of channel incision of valley floors throughout the southern great plains channel trenching occurred simultaneously in both small and large streams in drainage basins of the arkansas red trinity brazos and colorado rivers the sequences are the first documented example of widespread holocene incision accompanied by firm evidence for a synchronous change in regional climate

detached deep earthquakes are they really abstract we use primarily the generation of acoustic t waves into the ocean by deep seismic sources to investigate the propagation of high frequency seismic energy from the bottom of subduction zones to the shoreline at the earth s surface conversion from shear waves to oceanic acoustic waves can be used as a proxy for the existence of a continuous slab featuring low anelastic attenuation with the help of other techniques such as the estimation of q from s to p spectral amplitude ratios we examine systematically a number of regions where earthquakes have been described as detached we establish the mechanical continuity of the slab to the hypocenters of the 1990 sakhalin and 1982 bonin events which occurred several hundred kilometers in front of the mainstream seismic zone the study of the 1989 paraguay shock is inconclusive probably due to its much smaller size the vertical continuity of the south american slab through its aseismic depth range is verified and a similar situation probably exists in java attenuation data suggests that the deep spanish earthquakes occur within a vertically large segment of colder material and a similar situation may exist in colombia the only clearly detached deep events with no mechanical connection to the surface make up the vityaz cluster under the north fiji basin based on a variety of geophysical evidence the small deep earthquakes under new zealand are likely to take place in a detached blob at least 350 km below the termination of mainstream seismicity these results support a model integrating buoyancy forces over a long continuous slab as the source of the down dip compressional stresses observed in large earthquakes at the bottom of the transition zone

alluvial landform and the occurrence of paleosols in a humid subtropical climate have an effect on long term soil organic carbon storage abstract soil organic carbon soc storage in alluvial depositional zones is important as it serves as a centennial to millennial timescale sink of c this study examines soc variability in alluvial bars floodplains and terraces in a forested humid subtropical setting along the clarks river in the mississippi river basin mrb using replicated soil sampling characterization and modeling soil and depositional profiles range in age from a minimum of 8000 years ago to modern normalized by depth mean soc stocks kg m 2 of the surface soil and buried layers i e buried soils and layers underlying lithologic discontinuities are highest in the terraces normalized mean surface soc stocks decrease from terraces to floodplains and then to bars whereas normalized mean buried layer soc stocks decrease from terraces to bars and then floodplains the depth distribution of stocks showed that buried layers accounted for 50 of the total soc pool regardless of landform type a regression tree analysis shows that soil horizon ph landform and magnetic susceptibility were significant predictors of mean soc content notably the regression tree shows that alkaline ph 7 9 observed only in buried layers in the terrace is an important predictor in higher mean soc these alkaline soil ph values were associated with paleosol calcite where the 13c and 14c from the calcite suggests it formed under c3 vegetation during the drier than modern mid holocene hypsithermal 8000 years ago buried soil and sediment contain the majority of the soc in this humid subtropical valley bottom while landform position and past buried soil forming environments paleosols played an important role in the storage of that soc this work reconfirms that there is a long record of a passive buried soc pool found deep in our river valley bottoms that holds clues to the past biogeochemical cycling of c on land prior to intensive agriculture and industrialization the legacy of past holocene climates and paleosols are directly linked to our current understanding of long term storage of soc

quaternary faulting in the southern mississippi embayment and implications for tectonics and seismicity in an intraplate setting the recently recognized saline river fault zone in the southwestern mississippi embayment strike 135 is characterized by moderate historic seismicity we document paleoseismicity possibly strong on the saline river fault zone that is outside the known region of neotectonism in the northern embayment six surface faults were excavated within the saline river fault zone and all faults displace marine eocene units and fluvial pliocene pleistocene units five show post wisconsin loess movement thermoluminescence tl age 23 6 3 ka and three deform middle to late holocene eolian silt tl age 5 1 0 6 ka and 3 6 0 5 ka as fault tip flexures along the principal excavated fault fault plane braiding and plunging drag folding suggest a strong component of left slip and subsidiary faults show both normal and reverse displacement 1m of pliocene pleistocene wisconsin and holocene units we interpret these faults as components of a left slip flower structure a shallow seismic reflection profile acquired across the principal fault and a subsidiary fault shows eocene stratigraphy separated in a reverse sense below 120 m depth and separated in a normal sense above 120 m supporting a strike slip interpretation in addition the only focal plane mechanism available in the area indicates left slip with a small reverse component we conclude that the saline river fault zone is a paleozoic mesozoic basement fault zone reactivated as a left slip system in a quaternary east west compressive stress field field evidence suggests that earthquakes with several meters of strike slip may be characteristic and the width of a liquefaction field on the saline river fault zone suggests an event of magnitude 5 5 6 considering a similarity of strike and sense of holocene movements of the meers fault of southern oklahoma and of the saline river fault zone there may be additional active fault zones that pose seismic hazards concealed by sediments in southern north america

high resolution chronologies for loess comparing ams 14c and optical dating results the nuxloch loess section in south west germany is famous in central europe for its thick deposits from the last glacial maximum it has therefore been intensively studied during the past few years and offers an excellent opportunity to compare the performance of different dating techniques covering the period 15 45 ka here we present results fromoptical and ams 14 c dating the silt sized polymineral fraction of the clastic sediments and a multiple aliquot approach was used for ir osl dating 14 c dating was carried out on organic macro remnants and humin fractions extracted fromthe sedim ents with the exception of samples taken from the uppermost 1 5 m of the section ir osl and calibrated 14 c ams ages are consistent over the entire period the inconsistencies in the upper meters are thought to be due to disturbances during soil formation in the holocene the good agreement obtained on the remaining sequence demonstrates the high accuracy of both methods when studying loess sections for the ir osl ages no significant age underestimations are obvious the results clearly document that accurate chronologies can be developed for such continental sedimentary sequences this gives access to archives that can now be studied with a high temporal resolution and allows the establishment of new paleoclimatic proxies for the study of terrestrial responses to past climatic changes r 2003 elsevier science ltd all rights reserved

seismic studies of the upper mantle and transition zone in the last few years seismic studies of the upper mantle and transition zone have achieved significant new results in several areas progess has been driven largely by the increasing size quality and accessibility of digital seismic datasets from global and regional networks and from experiments using portable instruments this paper focuses on a few of the most interesting and important developments these include 1 measurements of upper mantle anisotropy obtained using a variety of techniques which can reveal details of the flow regimes and tectonic history of the upper mantle 2 improvements in models of upper mantle velocity and attenuation structure with some controversy arising regarding how deep slow velocity anomalies extend beneath oceanic ridges 3 direct seismic observations of topography on the 410 and 660 km discontinuities in the upper mantle providing constraints on mantle temperatures and dynamics and 4 observations suggesting secondary seismic discontinuities in some regions

short term uplift rates and surface deformation along the reelfoot fault new madrid seismic zone abstract this study of variability in the amount and rate of deformation along the intraplate reelfoot fault of the new madrid seismic zone nmsz utilizes diverse research methods including geophysics geomorphology and fluvial sedimentology the transpressional reelfoot fault deforms late holocene mississippi river sediment into the tiptonville dome and adjacent reelfoot basin because reelfoot lake submerges a portion of the basin subbottom acoustic profiling of the lake was used to quantify the relief on the uplift at the natural levee of the abandoned tiptonville meander the most appropriate landform along the scarp for measuring cumulative structural relief a maximum relative vertical uplift of approximately 11 3 m was measured to identify spatial and temporal variability in surface deformation using geologic methods we relate fluvial sedimentation to three documented earthquake events during the past 2300 yr tuttle et al 2002 2005 which is the approximate age of the oldest deformed floodplain sediment guccione et al 2002 radiocarbon dates from organic material in fluvial and lacustrine sediment indicate that most of the uplift occurred during the last two major seismic events a minimum of 1 6 3 9 m of uplift was associated with the seismic episode at a d 1450 150 and a maximum of 5 9 8 2 m of uplift was associated with the 1812 episode we estimate a mean short term uplift rate of 1 2 cm yr 0 4 2 1 cm yr with 80 probability or a mean short term slip rate of 1 3 cm yr 0 5 2 3 cm yr with 80 probability these values are approximately 2 5 times higher than the long term rates based on the total amount of uplift averaged over the past 2300 yr mueller et al 1999 van arsdale 2000 estimating deformation rates associated with individual earthquake recurrences provides information on fluctuating activity of the nmsz

p wave tomography of the mantle under the alpine mediterranean area 1 we study the upper mantle p wave velocity structure below the euro mediterranean area down to 1000 km depth by seismic travel time tomography we invert summary residuals constructed with both regional and teleseismic first arrival data reported by the international seismological centre isc 1964 1995 introducing some alternative strategies in the travel time tomographic approach and a new scheme to correct teleseismic data for global mantle structure our high resolution model pm0 5 is parameterized with three dimensional 3 d linear splines on a grid of nodes with 0 5 spacing in both horizontal directions and 50 km vertical spacing we obtain about 26 root mean square rms reduction of residuals by inversion in addition to roughly 31 reduction after summary rays formation and selection sensitivity analyses are performed through several test inversions to explore the resolution characteristics of the model at different spatial scales the distribution of large scale fast anomalies suggests that two different stages of a convection process presently coexist in very close regions the mantle dynamics of western central europe is dominated by blockage of subducted slabs at the 660 km discontinuity and ponding of seismically fast material in the transition zone contrarily in the eastern mediterranean fast velocity material sinks into the lower mantle suggesting that the flow of the cold downwelling here is not blocked by the 660 km discontinuity on a smaller scale the existence of tears in the subducted slab lithospheric detachment all along both margins of the adriatic plate as proposed by some authors is not supported by our tomographic images

bootheel lineament a possible coseismic fault of the great new madrid earthquakes a remote sensing examination of the new madrid seismic zone has revealed a feature the bootheel lineament that may be the surface expression of one of the coseismic faults of the great new madrid earthquakes of 1811 and 1812 the lineament extends about 135 km in a north northeast direction through northeastern arkansas and southeastern missouri the morphology and pattern of the lineament suggest that it reflects a fault with strike slip displacement field data indicate that liquefied sand was injected along the lineament probably in 1811 and 1812 the bootheel lineament does not coincide with any of the major arms of new madrid seismicity possibly indicating that the current seismicity does not precisely reflect the faults that ruptured in 1811 and 1812

the nature of the 660 kilometer upper mantle seismic discontinuity from precursors to the pp phase global seismic network data were used to image upper mantle seismic discontinuities stacks of phases that precede the pp phase thought to be underside reflections from the upper mantle discontinuities at depths of 410 and 660 kilometers show that the reflection from 410 kilometers is present but the reflection from 660 kilometers is not observed a continuous lame s constant lambda and seismic parameter at the 660 kilometer discontinuity explain the missing underside p reflections and lead to a p wave velocity jump of only 2 percent whereas the s wave velocity and density remain unchanged with respect to previous global models the model deemphasizes the role of lame s constant lambda with regard to the shear modulus and constrains the mineralogical composition across the discontinuity

late quaternary earthquake chronology from luminescence dating of colluvial and alluvial deposits of the arava valley israel abstract luminescence ages help provide a time scale for recurrent surface ruptures on a normal fault in the southern arava israel the dated samples come from alluvium and colluvium along a fault scarp north of elat the ages obtained mainly by infrared stimulation of sand size potassium feldspar grains accord with geomorphic age estimates from soils and degraded scarps four large earthquakes occurred between 37 000 and 14 000 yr ago five smaller earthquakes occurred thereafter continuing until recent times the mean recurrence interval for earthquakes of magnitudes greater than 6 is about 4000 yr

estimates of tidal energy dissipation and diapycnal diffusivity in the kuril straits using topex poseidon altimeter data the kuril straits separating the okhotsk sea from the north pacific ocean are representative regions of strong tidal mixing in the world s oceans in the present study we first carry out numerical simulation of the barotropic tidal elevation field in the okhotsk sea using a horizontally two dimensional primitive equation model it is found that to reproduce realistic tidal elevations in the okhotsk sea the energy lost by the incoming barotropic tides to internal waves within the kuril straits should be taken into account the numerical experiments show that the model predicted tidal elevations in the okhotsk sea best fit the topex poseidon altimeter data when we take into account the baroclinic energy conversion in the kuril straits 16 gw for the k 1 tidal constituent and 37 gw for the major four tidal constituents k 1 o 1 m 2 s 2 for this baroclinic energy conversion diapycnal diffusivity averaged over the whole area of the kuril straits is estimated to be 8 x 10 4 m 2 s 1 this value is about 1 order of magnitude less than assumed for the kuril straits in previous ocean general circulation models we offer this study as a warning against using diapycnal diffusivity just as a tuning parameter to reproduce large scale phenomena

thermoluminescence dating of fault scarp derived colluvium deciphering the timing of paleoearthquakes on the weber segment of the wasatch fault zone north central utah the timing of middle to late holocene faulting on the weber segment of the wasatch fault zone utah is constrained by thermoluminescence tl and radiocarbon age estimates on fine grained fault related colluvial sediments the stratigraphy in two trenches excavated across fault scarps is characterized by a stack of three colluvial wedges deposited in response to three separate faulting events the oldest of which buried a soil developed on a middle holocene debris flow thermoluminescence age estimates by the partial and total bleach methods and the regeneration method on fine grained colluvium from the trenches agree within 1 sigma and are concordant with the radiocarbon chronology a synthesis of the tl and 14c age estimates indicate that these three faulting events occurred sometime between 4500 and 3500 between 3200 and 2500 and between 1400 and 1000 years ago detailed investigation of a sequence of fine grained scarp derived distal colluvium shows that much of the sediment was deposited during 600 year intervals immediately after faulting the sedimentation rate of colluvium is inferred to increase shortly after faulting and tl dating of these sediments provides additional information to constrain the timing of faulting events

a laboratory investigation of effects of trench migration on the descent of subducted slabs a laboratory investigation of viscous slabs subducted from a migrating trench reveals a range of possible behaviours the slab dip in a uniform mantle is found to be steady or oscillatory depending on the rates of descent and trench migration in addition density and viscosity interfaces are used to model the increased resistance to sinking of stiff slabs through the seismic discontinuity at a depth of about 660 km in earth s mantle if the slab is denser than the lower layer and its dip in the upper layer is steady it can continue to descend through the lower layer in a tabular form and in either a steady or oscillatory manner or it can be laid horizontally on the interface for a distance before descending in a chain of diapirs resulting from gravitational instability at the interface if the slab dip in the upper layer is unstable the slab sinks into the lower layer in a chain of large diapirs at a spacing determined by the frequency of oscillations set by instability of the slab within the upper layer the style of penetration depends on the trench migration speed and the ratio of sinking velocities in the two layers estimates for mantle slabs indicate that they may range across the major regime transitions the experimental system provides a gross simplification of mantle conditions but the results are a testimony to the possibility of a range of complex behaviour of subducted lithosphere they also indicate that the relationship between the structural features in the lower mantle revealed by seismic imaging and present day tectonic processes at the surface may not be obvious tomographic images of western pacific subduction zones and data for the migration rates of associated trenches suggest a dependence of slab behaviour on migration rate similar to that seen in the experiments

global shear velocity heterogeneities in the d layer inversion from sd sks differential travel times a global map of shear velocity in the d layer results from the inversion of 340 differential travel times of diffracted s sh minus sks sv sd sks from long period records of global seismic networks the two phase design reduces contamination from upper mantle heterogeneities and errors in location and origin time of the events additional corrections are made for 1 azimuthal anisotropy at stations where shear wave splitting parameters are available and for 2 travel time perturbations due to lower mantle asphericity although both effects are minor compared with the observed residuals with respect to the preliminary reference earth model prem dziewonski and anderson 1981 the corrected residuals ranging from 16 to 18 s are attributed to anomalies in d sampled by both phases taking these residuals as data and assuming a constant 250 km thick d layer we invert for a lateral velocity variation model of d using spherical harmonics in parameterizing d velocities a high degree expansion l 14 avoids aliasing but only the reliably determined low degree components li l produce optimal models the optimal model li 6 achieves a variance reduction of 56 and mean error 0 019 km s and the velocity is insensitive to the thickness of the d layer assumed a baseline shift of 0 022 km s from prem is resolved the magnitude of variation 0 2 km s or 3 is nearly twice as large as that of tanimoto 1990 and su et al 1994 while comparable to that of liu and dziewonski 1994 which includes s sks data and li and romanowicz 1996 degree 2 variation dominates the d with positive anomalies beneath the asian australian and american continents and southernmost pacific and negative anomalies beneath the pacific western africa atlantic and the southern indian ocean this pattern is similar to that found in previous tomograpic models significant discrepancies in nondegree 2 features are discussed in terms of the constraints from sd the fast and slow regions of d correlate with the inferred paleoslab positions and the major hotspot distribution respectively both at the 99 confidence level implications of these correlations for mantle dynamics are suggested the global model agrees reasonably well with the velocities determined regionally from the sd slowness measurement and the scs s residuals in other studies the latest radial models of d constrained from s triplications at distances of 75 90 provide a completely independent benchmark models for eurasia alaska and the caribbean predict sd travel times corresponding to 0 03 0 08 km s anomalies consistent with our lateral variation model in respective regions these vertical structures are also consistent with this study in scs times indicating that they agree in a vertically averaged sense major inconsistency exists for the india model syll young and lay 1987b which predicts sd and scs arrivals much slower than those expected from the 0 1 km s fast anomaly we found suggesting either the presence of strong lateral heterogeneities or the need for further mapping in this area

subduction zones observations and geodynamic models abstract this review of subduction and geodynamic models is organized around three central questions 1 why is subduction asymmetric 2 are subducted slabs strong or weak 3 how do subducted slabs interact with phase transformations changes in mantle rheology and possibly chemical boundaries in the mantle based on laboratory measurements of the temperature dependence of olivine one would conclude that the core of a subducting slab is at least 10 000 times more viscous than ambient mantle however there are a number of complementary but independent observations that suggest that slabs are much weaker than this slabs undergo significant deformation in the upper mantle and may thicken to twice their original width by the time they reach the base of the transition zone the lack of a clear correlation between the observed dip angle of deep slabs and plate velocity rate of trench migration and slab age in modern subduction zones is consistent with hypothesis that subduction is a time dependent phenomenon both tank and numerical convection experiments with plates conclude that subduction is not a steady phenomenon but that slabs bend thicken stretch and change dip through time this is at odds with the assumptions used in steady state slab thermal models where slab deformation is not considered

neotectonics of the upper mississippi embayment abstract although the upper mississippi embayment is an area of low relief the region has been subjected to tectonic influence throughout its history and continues to be so today tectonic activity can be recognized through seismicity patterns and geological indicators of activity either those as a direct result of earthquakes or longer term geomorphic structural and sedimentological signatures the rate of seismic activity in the upper mississippi embayment is generally lower than at the margins of tectonic plates the embayment however is the most seismically active region east of the rocky mountains with activity concentrated in the new madrid seismic zone this zone produced the very large new madrid earthquakes of 1811 and 1812 geological and geophysical evidence of neotectonic activity in the upper mississippi embayment includes faulting in the benton hills and thebes gap in missouri paleoliquefaction in the western lowlands of missouri subsurface faulting beneath and tilting of crowley s ridge in northeastern arkansas and southeastern missouri subsurface faulting along the crittenden county fault zone near memphis tennessee faulting along the east flank of the tiptonville dome and numerous indicators of historic and prehistoric large earthquakes in the new madrid seismic zone paleoearthquake studies in the new madrid seismic zone have used trenching seismic reflection shallow coring pedology geomorphology archaeology and dendrochronology to identify and date faulting deposits of liquefied sand and areas of uplift and subsidence the cause of today s relatively high rate of tectonic activity in the mississippi embayment remains elusive it is also not clear whether this activity rate is a short term phenomenon or has been constant over millions of years ongoing geodetic and geological studies should provide more insight as to the precise manner in which crustal strain is accumulating and perhaps allow improved regional neotectonic models

constraints from seismic anisotropy on the nature of the lowermost mantle the d layer lies at the bottom of the earth s rocky mantle and separates it from the liquid metal alloy core this region extending from the core mantle boundary to a few hundred kilometres above fig 1 is geodynamically analogous to the more easily studied lithosphere at the top of the mantle the structure of d may reflect the style of lower mantle convection the nature of core mantle interaction and perhaps even the fate of subducting lithosphere1 observations of lithospheric seismic anisotropy have provided valuable insight into the nature of the upper mantle boundary layer but discussion of lower mantle seismic anisotropy has been somewhat contentious2 5 here we present evidence from seismic waves that have traversed the lowermost mantle beneath the caribbean region for a zone of seismic anisotropy below the d discontinuity which in this region lies 250 km above the core mantle boundary the anisotropy is most probably due to horizontal layering or aligned inclusions of a material with differing shear wave velocity if d is a graveyard for subducted lithosphere a plausible explanation of the anisotropy may be the contrast between cold lithospheric mantle and material that formerly constituted the oceanic crust which may have lower shear wave velocity owing to the presence of melt

travel time tomography of the european mediterranean mantle down to 1400 km abstract the 3 d p wave velocity structure of the mantle below europe the mediterranean region and a part of asia minor is investigated this study is a considerable extension of an earlier tomographic experiment that was limited to imaging upper mantle structure only here the earth s volume under study encompasses the mantle to a depth of 1400 km and we increase the number of international seismological centre isc data for inversion by a factor of four by taking more years of observation and by including data from teleseismic events the most important departure from the earlier study is that we do not use the jeffreys bullen model as a reference model but an improved radially symmetric velocity model the pm2 model which is appropriate for the european mediterranean mantle our inversion procedure consists of two steps first the radial model pm2 is determined from the isc delay times by a nonlinear trial and error inversion of the data as opposed to the jeffreys bullen model the new reference model has a high velocity lithosphere a low velocity zone and seismic discontinuities at depths of 400 and 670 km next the isc data are corrected for effects related to the change in reference model and inverted for 3 d heterogeneity relative to the pm2 model we follow this two step approach to attain a better linearizable tomographic problem in which ray paths computed in the pm2 model provide a better approximation of the actual ray paths than those computed from the jeffreys bullen model hence the two step scheme leads to a more credible application of fermat s principle in linearizing the tomographic equations inversion results for the 3 d heterogeneity are computed for both the uncorrected isc data and for the pm2 data the data fit obtained in the two step approach is slightly better than in the inversion of isc data using the jeffreys bullen reference model a comparison of the tomographic results demonstrates that the pm2 data inversion is to be preferred to assess the spatial resolution an analysis is given of hit count patterns sampling of the mantle by ray paths and results of sensitivity tests with 3 d synthetic velocity models the spatial resolution obtained varies with position in the mantle and is studied both in map view and in cross section in the well sampled regions of the mantle the spatial resolution for larger scale structure can qualitatively be denoted as reasonable to good and at least sufficient to allow interpretation of larger scale anomalies a comparison is made of the results of this study with independent models of s velocity heterogeneity obtained in a number of investigations and with a prediction of the seismic velocity structure of the mantle computed from tectonic reconstructions of the mediterranean region in the context of this comparison interpretations of large scale positive anomalies found in the mediterranean upper mantle in terms of subducted lithosphere are given specifically addressed are subduction below southern spain below the western mediterranean and italy and below the aegean in the last region a slab anomaly is mapped down to depths of 800 km

tomographic inversion of p and pp data for aspherical mantle structure below the northwest pacific region summary to investigate the morphology of subducted slab in the mantle below northwest pacific island arcs we inverted traveltime residuals for aspherical variations in p wave propagation velocity relative to the radially symmetric iasp91 reference model the tomographic method used is based on a step wise linearization of the inversion problem first we relocated isc international seismological centre hypocentres with re identified p and pp phase data using the iasp91 traveltime tables the variance of p residuals relative to iasp91 traveltimes was 17 per cent less than the variance of p data reported by the isc relative to the jeffreys bullen j b traveltime tables second we performed a linearized lsqr inversion for earth structure and source relocation with the p and pp residuals obtained from the first step using iasp91 as the reference model for seismic velocities the incorporation of the depth phase pp in the tomographic inversions has two major advantages 1 the pp data provide constraints on focal depth and thus reduce the trade off between source relocation and structure and 2 the pp ray paths improve the sampling of earth structure in the shallow mantle and transition zone we used more than 2 times 106 and about 1 times 105p and pp wave traveltime residuals respectively from about 40 000 earthquakes with epicentres in the study region that were recorded at one or more of the 2300 globally distributed seismological stations considered in this study we assessed the spatial resolution in the tomographic images with checker board type sensitivity tests these tests reveal high resolution of upper mantle and transition zone structure particularly below the central part of our study region structure with wavelengths of the order of 100 km is resolved below japan whereas structure with wavelengths of the order of 300 km is well resolved below the kuril izu bonin and ryukyu arcs small scale structure is poorly resolved in depth below the northern part of the kuril kamchatka arc and below the izu bonin and mariana arcs this limits the interpretation of slab structure and mantle flow from tomographic images alone with this limitation in mind we conclude from the tomographic images that subducted slab deflects in the mantle transition zone below the geographical area encompassed by the kuril basin the japan sea and the northern part of the philippine sea this is in good agreement with the results of other recently published tomographic studies the occurrence of earthquakes several hundred kilometres off the inclined wadati benioff seismic zones and inferences about 660 km discontinuity topography in contrast slab like structures of high p wave velocity are imaged in the lower mantle below the deepest earthquakes of the northern kuril kamchatka and mariana seismic zones this is indicative of local slab penetration of the lower mantle from tomographic images we cannot discern between compositionally or thermally induced variations in seismic velocity however with regard to the nature of the boundary between upper and lower mantle our observations argue against either compositional mantle layering with large contrasts in intrinsic density or phase changes with steep clapeyron slopes

contrasts in seismic wave speeds and density across the 660 km discontinuity beneath the philippine and the japan seas 1 using triplicate waveforms we construct 34 seismic profiles east of taiwan to investigate lateral variations of p and s wave speeds vp and vs near the mantle transition zone data from broadband and densely spaced short period arrays are combined to achieve high resolution we find correspondingly high vs in the back arc of izu bonin where the northern philippine sea anomaly npsa was previously delineated in the lower portion of the transition zone by an average vp that is 1 5 0 5 higher than the surrounding regions high vs of the npsa reduces the contrast in vs across the 660 km discontinuity vs to about 3 9 consistent with the notion that npsa reflects a subhorizontal remnant of cold pacific slab resting immediately above the lower mantle the aseismic npsa contrasts the fiji tonga region where many outboard earthquakes low vp and vs and seismic anisotropy mark a large scale petrologic anomaly the contrasting pattern implies that the npsa is no longer cold enough to preserve materials such as metastable olivine or volatiles that can trigger earthquakes reduce vp and vs and cause anisotropy in regions surrounding the npsa vs is well constrained to be 5 9 0 5 this precise result and known impedance contrast 7 under the japan sea lead to a density contrast of about 8 circumventing the trade off between and vs the magnitude of is higher than recent estimates 5 without independent control on vs but close to longstanding global models implying considerable resistance to slab penetration into the lower mantle north of the npsa

signature of remnant slabs in the north pacific from p wave tomography summary a 3 d ray tracing technique was used in a global tomographic inversion in order to obtaintomographicimagesofthenorthpacific thedatareportedbythegeophysical surveyofrussia 1955 1997 wereusedtogetherwiththecataloguesoftheinternational seismologicalcenter 1964 1991 andtheusgeologicalsurveynationalearthquake information center 1991 1998 and the recompiled catalogue was reprocessed the final data set used for following the inversion contained 523430 summary ray paths the whole of the earth s mantle was parametrized by cells of 2 2 and 19 layers the large and sparse system of observation equations was solved using an iterative lsqr algorithm asubhorizontalhigh velocityanomalyisrevealedjustabovethe660kmdiscontinuity beneaththealeutiansubductionzone thishigh velocityfeatureisobservedatlatitudes of up to 70 n and is interpreted as a remnant of the subducted kula plate which disappeared through ridge subduction at about 48ma a further positive velocity perturbation feature can be identified beneath the chukotka peninsula and okhotsk sea extending from 300 to 660km depth and then either extending further down to 800km chukotka or deflecting along the 660km discontinuity okhotsk sea this high velocity anomaly is interpreted as a remnant slab of the okhotsk plate accreted to siberia at 55ma

philippine sea and east asian plate tectonics since 52 ma constrained by new subducted slab reconstruction methods we reconstructed philippine sea and east asian plate tectonics since 52 ma from 28 slabs mapped in 3 d from global tomography with a subducted area of 25 of present day global oceanic lithosphere slab constraints include subducted parts of existing pacific indian and philippine sea oceans plus wholly subducted proto south china sea and newly discovered east asian sea mapped slabs were unfolded and restored to the earth surface using three methodologies and input to globally consistent plate reconstructions important constraints include the following 1 the ryukyu slab is 1000 km n s too short to account for 20 philippine sea northward motion from paleolatitudes 2 the marianas pacific subduction zone was at its present location 200 km since 48 10 ma based on a 1000 km deep slab wall 3 the 8000 2500 km east asian sea existed between the pacific and indian oceans at 52 ma based on lower mantle flat slabs 4 the caroline back arc basin moved with the pacific based on the overlapping coeval caroline hot spot track these new constraints allow two classes of philippine sea plate models which we compared to paleomagnetic and geologic data our preferred model involves philippine sea nucleation above the manus plume 0 150 e near the pacific east asian sea plate boundary large philippine sea westward motion and post 40 ma maximum 80 clockwise rotation accompanied late eocene oligocene collision with the caroline pacific plate the philippine sea moved northward post 25 ma over the northern east asian sea forming a northern philippine sea arc that collided with the sw japan ryukyu margin in the miocene 20 14 ma

paleoseismic evidence for recurrence of earthquakes near charleston south carolina a destructive earthquake that occurred in 1886 near charleston south carolina was associated with widespread liquefaction of shallow sand structures and their extravasation to the surface several seismically induced paleoliquefaction structures preserved within the shallow sediments in the meizoseismal area of the 1886 event were identified field evidence and radiocarbon dates suggest that at least two earthquakes of magnitudes greater than 6 2 preceded the 1886 event in the past 3000 to 3700 years the evidence yielded an initial estimate of about 1500 to 1800 years for the maximum recurrence of destructive intraplate earthquakes in the charleston region

soil liquefaction during the arequipa mw 8 4 june 23 2001 earthquake southern coastal peru abstract the arequipa june 23 2001 earthquake with a moment magnitude of mw 8 4 struck southern peru northern chile and western bolivia this shallow 29 km deep interplate event occurring in the coupled zone of the nazca subduction next to the southeast of the subducting nazca ridge triggered very localized but widely outspread soil liquefaction although sand blows and lateral spreading of river banks and road bridge abutments were observed 390 km away from the epicenter in the southeast direction nearing the town of tacna close to the chile border liquefaction features were only observed in major river valleys and delta and coastal plains in the meizoseismal area this was strongly controlled by the aridity along the coastal strip of southern peru from the sand blow distribution along the coastal area a first relationship of isolated sand blow diameter versus epicentral distance for a single event is ever proposed the most significant outcome from this liquefaction field reconnaissance is that energy propagation during the main june 23 2001 event is further supported by the distribution and size of the isolated sand blows in the meizoseismal area the sand blows are larger to the southeast of the epicenter than its northwestern equivalents this can be stated in other words as well the area affected by liquefaction to the northwest is less spread out than to the southeast implications of these results in future paleoliquefaction investigations for earthquake magnitude and epicentral determinations are extremely important in cases of highly asymmetrical distribution of liquefaction features such as this one where rupture propagation tends to be mono directional it can be reliably determined an epicentral distance between earthquake and liquefaction evidence and an earthquake magnitude only if the largest sand blow is found therefore magnitude estimation using this uneven liquefaction occurrence will surely lead to underrating if only the shortest side of the meizoseismal area is unluckily studied which can eventually be the only part exhibiting liquefaction evidence depending on the earthquake location and the distribution of liquefaction prone environments

travel times from earthquakes near southern kuril and their implications for the fate of subducted lithosphere abstract analysis of travel time residuals from deep and intermediate focus earthquakes along the kuril kamchatka arc has been a major impetus in advancing the hypothesis of deep slab penetration the interpretation of travel times however has been controversial the quality of arrivals reported in bulletins has also been debated using both digital and analog seismograms we precisely measured approximately 400 arrival times of teleseismic p phases for seven earthquakes most of which occurred since 1987 near southern kuril travel times from many of these events have not been analyzed by other researchers including a unique pair of shallow and deep focus earthquakes beneath the island of sakhalin in general differences between our precisely measured p arrivals and those reported by the international seismological center isc are within 0 4 s although isolated large differences do exist at common stations large differences in travel time residuals up to 1 5 s are observed between one of the deepest earthquakes in northern kuril and a deep focus event beneath the island of sakhalin located some 150 km farther west of the well defined wadati benioff zone in southern kuril significantly faster travel times up to 1 2 s or more are also observed along north northeast and south southwest azimuths for the deep focus event in northern kuril than for a shallow focus earthquake west of the sakhalin island where the effect of subducted lithosphere should be minimal in both cases the observed differential residuals between earthquakes in northern and southern portions of the arc are too large to be caused by epicenter mislocation since differential travel time residuals largely reflect differences in near source structure seismic wave speeds below depths of approximately 650 km seem to vary along this arc the magnitude of slab like anomalies in the lower mantle beneath sakhalin is probably two to three times smaller than that in northern kuril kamchatka if the pattern of travel time anomalies observed for deep focus earthquakes in northern kuril kamchatka is caused by coherent subducted slab in the lower mantle wholesale slab penetration into the lower mantle probably did not occur beneath the sakhalin there also appears to be a correlation between the state of strain within subducted lithosphere in the upper mantle and the extent of slab penetration into the lower mantle along southern kuril a net down dip extensional strain is observed within the slab at intermediate depths and persists to an unusual depth of approximately 450 km on the other hand in the northern kuril kamchatka down dip compressional strain dominates at all depths in the subducted lithosphere

eurasian fundamental mode surface wave phase velocities and their relationship with tectonic structures we automatically analyzed 32 000 fundamental mode love and rayleigh wave signals with earthquake station paths traversing eurasia and indonesia and obtained robust average phase velocity measurements between 20 s and 170 s periods along 4389 love and 4020 rayleigh paths these were inverted to give phase velocity maps at 14 fixed periods resolution tests suggest that features with diameter 750 km and 500 km are resolved over most of eurasia and central se asia respectively low period love waves image areas with thick sedimentary cover as low velocity zones and almost all periods image mountainous regions since these have thick crust and hence low average lithospheric shear velocity at long periods both love and rayleigh waves define high phase velocity zones across shield and cratonic areas reflecting their deep lithospheric roots we observe significant along strike heterogeneity in the zagros fold belt and tien shan altai system taking sections across eurasian phase velocity space allows us to make approximate interpretations in terms of shear velocity structure directly for example the red river and east vietnam boundary faults are traced on their eastern side by low velocities which extend at depth into indonesia we relate this to mantle upwelling associated with early eocene rotation of indochina and reversal of the sense of shear across the red river fault post 20 ma we observe dipping subduction of the mediterranean beneath the aegean of the philippine sea beneath indonesia and of the indian shield beneath tibet we also image a fossil subducted plate beneath ne borneo which we associate with subduction of the proto south china sea between 50 ma and 15 ma

refinement of thrust faulting models for the central new madrid seismic zone abstract we present the results of the joint relocation of events recorded during 1989 1992 by the panda network in the central new madrid seismic zone the near surface material in the study area is a gently dipping layer of poorly consolidated sediments with low p wave velocity and high v p v s estimated values 1 8 km s 1 and 3 the sediments are underlain by high velocity paleozoic rocks under the network the difference in sediment thickness is only 0 6 km but because of the low velocities the location of the events using layered models is affected by errors application of the joint hypocentral determination jhd technique to a subset of 580 events shows that the single event locations may be in error by as much as 1 km in depth depending on where the events are located analysis of synthetic data generated for a realistic 3 d velocity model supports the jhd results the analysis of synthetic data also suggests that a v p v s 2 3 is more appropriate for the post paleozoic mississippi embayment sediments based on the jhd locations we present a new interpretation of the seismicity with two en echelon sw dipping thrust faults connected by a west dipping thrust fault these faults appear associated with the reelfoot scarp and its northern extension the kentucky bend scarp

fast p and s wave velocities associated with the cold stagnant slab beneath the northern philippine sea abstract previous tomographic studies using short period p times have typically shown that the fast pacific slab is stagnant in the mantle transition zone beneath the northern philippine sea and eastern asia however there has been no consensus about the s wave velocity anomalies of the stagnant slab so that it has remained difficult to interpret the stagnant slab in terms of thermal and or compositional state here we analyze both p and s wave travel time anomalies associated with the stagnant slab in the izu bonin subduction zone based on broadband records from japan korea and china we found that p and s waves traveling through the izu bonin stagnant slab exhibit systematically travel time residuals of 3 to 4 s for p waves and 6 to 8 s for s waves with respect to the iasp91 model the observed p residuals are consistent well with an existing three dimensional p velocity model indicating that the stagnant slab beneath the studied region has fast p wave velocity anomalies of 1 4 the observed s residuals are consistent with an s wave velocity model constructed from the p wave velocity model with a constant scaling factor ln v s ln v p r of 1 4 agreement of the experimentally estimated r 1 38 and the seismologically obtained r 1 4 values suggests that both p and s velocity anomalies can be explained by a stagnant slab colder than the surrounding mantle by about 250 300 k

optical dating studies of quartz and feldspar sediment extracts abstract continuing developmental work into optical dating has led to some clarification of the optical behaviour of quartz and feldspar sediment extracts we have verified the key assumption intrinsic to the method that the optical signal is zeroed completely upon exposure to daylight we also show that the rate of zeroing in optical dating is much more rapid than it is in tl dating studies concerned with the extraction of the thermally stable signal from laboratory irradiated samples showed that problems may arise from medium to high temperature preheating of young samples optical alternatives to thermal stabilization were tested using dye and krypton lasers using a krypton laser we have succeeded in obtaining an equivalent dose of 0 0 0 7 gy for the quartz from a modern intertidal sand and an average dose of 120 22 gy for the feldspar extract of a 70 ka sand unit both were obtained using six wavelengths from the infrared to the violet and no dependence on wavelength was found we found that the photon energies of infrared and deep red light can stimulate luminescence in unirradiated quartz and feldspar extracts of sediments this was not expected and suggests that the process of trap emptying is more complex than we had envisioned but implies a far greater than expected degree of technical flexibility for the optical dating method

tectonic geomorphology of the mississippi valley between osceola arkansas and friars point mississippi abstract a geomorphic study of the mississippi river and its alluvial valley between osceola arkansas and friars point mississippi has identified anomalous surface features that can be linked to known geological structures for example relatively recent deformation along big creek fault zone white river fault zone bolivar mansfield tectonic zone blytheville arch crittenden county fault zone and reelfoot rift margins is suggested by river and topographic anomalies faults and plutons appear to affect drainage networks and the morphology of crowleys ridge suggests significant fault control the many anomalies probably reflect a fractured suballuvial surface although movement along these fractures will most likely occur in seismically active areas the probability of movement elsewhere

heterogeneity within the subducting pacific slab beneath the izu bonin mariana arc evidence from tomography using 3d ray tracing inversion techniques distinct zones of seismic heterogeneity along the izu bonin mariana arc have been investigated using complimenting regional bulk sound shear wave speed and p wave tomographic images the distribution of seismic anomalies and the inferred geometry of the subducting pacific plate have been modelled in unprecedented detail using joint tomography and new p wave model using a 3d inversion algorithm the use of 3d ray tracing techniques and smaller cell parameterizations have greatly enhanced the resolution of gradients therefore the models show much more detail about the structure and physical properties of the subduction zone the well defined features from the multiple wave speed images are used to elucidate the distinct morphology change between slab beneath the izu bonin horizontal slab and mariana vertical arcs and the distribution of physical properties in the mantle and the subducting oceanic lithosphere changes in physical properties within the slab tear at the southern end of the izu bonin arc identified as a bgap or thinningq in the tomographic images could be the result of the distortion of the pacific plate as its shape transforms between near horizontal to near vertical a decrease in the rigidity and strength of the lithosphere the subduction of the marcus necker ridge change in subduction velocity or a combination of all these factors strong slow anomalies exist both in the mantle wedge above and below the slab beneath the izu bonin arc but are not present beneath the mariana arc at the junction between the izu bonin and mariana arcs it appears that the there are two separate pieces of the pacific plate the torn slab north of 268 n and a buckled near vertically dipping slab south of the tear the transition between the two distinct slab morphologies coincides with the location of the ogasawara plateau and the trench d 2005 elsevier b v all rights reserved

analysis of drainage basin symmetry as a rapid technique to identify areas of possible quaternary tilt block tectonics an example from the mississippi embayment there is a coincidence of seismicity and major river valleys in the southwestern mississippi embayment that suggests control of the drainage pattern by northwest trending faults pleistocene river terraces reveal a strong southwestward preferred river migration in this region that may be a result of fault block tilting a rapid new technique of drainage basin analysis is herein presented that quantifies transverse topographic basin symmetry these data permit discrimination of random stream migrations and regionally preferred stream migrations and identify the direction of maximum migration if bedrock dip can be shown to be a negligible influence on stream migration then the direction of regionally preferred migration implies a period of ground tilting in that direction drainage basin asymmetry vectors bearing and degree of asymmetry calculated from transverse topographic profiles of 271 4 km basin segments in the study area suggest preferred stream migration in response to west southwest ward ground tilting bearing 259 the eastward regional dip of bed rock horizons only locally influences migration river terrace distributions reveal a chronology of stream migration episodes southwestward migration in the ouachita saline and arkansas river basins occurred during early to middle pleistocene time in the southwestern mississippi embayment a wisconsin early holocene southwestward migration episode is evidenced along southeast flowing reaches of the ouachita and arkansas rivers but not along the southeast flowing reach of the intervening saline river suggesting that these rivers overlie separate northwest trending tilt blocks although this technique does not provide direct evidence of ground tilting it provides a quick look method of identifying possible tilting elements in neotectonic regions using only topographic maps this technique is of greatest utility where active faults are concealed or poorly exposed such as in the mississippi embayment

magnitude estimates of two large aftershocks of the 16 december 1811 new madrid earthquake the three principal new madrid mainshocks of 1811 1812 were followed by extensive aftershock sequences that included numerous felt events although no instrumental data are available for either the mainshocks or the aftershocks available historical accounts do provide information that can be used to estimate magnitudes and locations for the large events in this article we investigate two of the largest aftershocks one near dawn following the first mainshock on 16 december 1811 and one near midday on 17 december 1811 we reinterpret original felt reports to obtain a set of 48 and 20 modified mercalli intensity values of the two aftershocks respectively for the dawn aftershock we infer a m w of approximately 7 0 based on a comparison of its intensities with those of the smallest new madrid mainshock based on a detailed account that appears to describe near field ground motions we further propose a new fault rupture scenario for the dawn aftershock we suggest that the aftershock had a thrust mechanism and occurred on a southeastern limb of the reelfoot fault for the 17 december 1811 aftershock we infer a m w of approximately 6 1 0 2 this value is determined using the method of bakun et al 2002 which is based on a new calibration of intensity versus distance for earthquakes in central and eastern north america the location of this event is not well constrained but the available accounts suggest an epicenter beyond the southern end of the new madrid seismic zone

switching between alternative responses of the lithosphere to continental collision summary we study possible responses to arc continent or continent continent collision using numerical models our short term integration models show that the initial stage of deformation following continental collision is governed by the competition between three potential weakness zones 1 mantle wedge 2 plate interface and 3 lower continental crust depending on which of these is the weakest zone in the system three different responses can be recognized 1 subduction polarity reversal 2 continuation of subduction and 3 delamination and back stepping subduction polarity reversal occurs if the mantle wedge is the weakest zone in the system this happens only if the viscosity of the mantle wedge is at least one order of magnitudelowerthantheaverageviscosityofthelithosphere incontinent continentcollision one additional condition needs to be satisfied for subduction polarity reversal to occur for the subducting lithosphere the ratio of the viscosity of the lower continental crust to the viscosity of the upper lithospheric mantle must be equal to or higher than 0 006 the time required for polarity reversal depends on several parameters the convergence rate the sinking velocity of the detached slab and the relative strength of the mantle wedge arc and backarc the response to collision is continued subduction if the plate interface is the weakest zone and is delamination and back stepping if the lower continental crust is the weakest area in the system our finding that a low viscosity wedge is a prerequisite for a reversal of subduction polarity agrees with inferences about regions for which subduction polarity reversal has been proposed

preliminary assessment of sand blows in the southern mississippi embayment trenching of sand blows in two liquefaction fields on late to mid holocene alluvium within the southern mississippi embayment was conducted to determine the origin of the sand blows and their possible relationship to new madrid seismic zone events trench stratigraphy and crosscutting relationships show evidence of multiple events of seismically induced sand venting these two fields of moderate to intense liquefaction are 175 km and 240 km southwest of the southern limit of similarly intense sand blows within the new madrid seismic zone nmsz liquefaction field the southernmost field ashley county arkansas is 25 km in diameter and sand blows are densely spaced sand blows are more sparsely spaced in the larger 30 km by 45 km northern field desha county arkansas various correlations of liquefaction events between these two fields and between each of these fields and the nmsz can be accommodated by our radiocarbon and infrared stimulated luminescence irsl ages of sediments predating and postdating vented sand deposits in our trenches these ages show a major liquefaction event at both the ashley and desha county fields between 4600 and 5500 years ago and a major event at the ashley county field approximately 700 years ago the early venting episode is older than the documented nmsz chronology and so a nmsz seismic source cannot be assessed the age of the later venting episode falls between two postulated nmsz events so it may reflect seismicity from a different source zone our trench logs show at least three other moderate to minor sand venting episodes at the ashley county field and two at the desha county field and one may be the same event at both fields about 2200 years ago if these sand blows formed during large nmsz events 175 km northeast the area of strong ground motion for this seismic source has been underestimated alternatively if these liquefaction fields formed due to local seismicity then additional significant seismic source zones are present in the mississippi embayment outside the nmsz each of the two fields sits astride seismicity alignments associated with the margins of a previously identified active fault block between the arkansas river fault zone that projects through the desha county field and the saline river fault zone that projects through the ashley county field

waveform inversion of ss precursors an investigation of the northwestern pacific subduction zones and intraplate volcanoes in china abstract the arrival time and amplitude of underside reflections from mantle seismic discontinuities ss precursors have made major contributions to the understanding of mantle composition and dynamics in this study we introduce a nonlinear waveform inversion technique to simultaneously constrain shear velocities and discontinuity depths beneath the northwestern pacific subduction system based exclusively on a large ss precursor waveform dataset we are able to clearly delineate the morphology of the descending pacific plate which flattens at the base of the upper mantle and extends westward by 1500 km toward northern central china our grid search yields the maximum correlation between shear velocity and transition zone thickness at an angle of 30 consistent with the reported average slab dip beneath the study region the strongly positive correlation suggests predominantly thermal rather than compositional variations along the descending pacific plate the joint depth velocity solution also shows a 5 10 km depression of the 410 km discontinuity and an average decrease of 1 2 in upper mantle shear velocity beneath the intraplate volcanic fields in northeastern china this anomaly which reaches the middle of the upper mantle transition zone beneath the changbai hotspot initiates at a significantly shallower 320 km depth beneath the wudalianchi region high amplitude reflections at depths greater than 410 km suggest a water poor melt layer in possible association with 1 decompression melting from passive upwelling and or 2 active upwelling through a slab window

spatial and temporal evolution of the subducting pacific plate structure along the western pacific margin 1 tomographic images of the subducting pacific plate beneath the izu bonin mariana arc illustrate a progression of geometries from shallow dipping to vertical from north to south along the arc recent advances in technology and inversion techniques have improved resolution of slab structure beneath the western pacific island arcs but reasons for the variation in geometry and morphology are still poorly understood by comparing high resolution tomographic images of the western pacific and updated paleogeographic reconstructions we are able to link the spatiotemporal evolution of the subducting pacific plate back to the mid miocene we have reconstructed tectonic motions along the kurile japan izu bonin mariana arc system and the ryukyu arc to provide an independent additional interpretation of the subducting pacific plate using the most current plate motion data we then investigate the plausibility of our model and three other proposed models based on the interpreted slab structure from tomographic images the new reconstruction agrees with the basic characteristics of former trench retreat models but illustrates the importance of the collision of the ogasawara plateau with the trench in the mid miocene and its subsequent effect on the slab structure at depth and the impact of other aseismic ridge collisions along the plate boundary the combination of evidence in changed physical properties imaged with tomography and the current interpreted slab morphology can be analyzed with the past plate motions to understand subduction zone processes

tomographic imaging of the lowermost mantle with differential times of refracted and diffracted core phases pkp pdiff the mapping of variations in p wave speed in the deep mantle is restricted by the uneven sampling of p waves in particular beneath the southern hemisphere to enhance data coverage we augmented the 1 6 million summary rays of p pp and pwp that we used in previous studies with differential travel times of diffracted and refracted core phases for the core refracted differential travel time residuals pkpab pkpdf and pkpab pkpbc we used 1383 cross correlated digital waveforms as well as 27 000 routinely processed bulletin data we used the waveform data to define quality criteria for the selection and reduction of the bulletin pkp data for pkpdf pdiff we only considered 543 records derived from waveform cross correlation no pcp data were used in this study we used optical ray theory to calculate the ray paths associated with the p pp pwp and pkp data which are measured at 1 hz however to account for the large fresnel zones of the low frequency 50 mhz pkpdf pdiff data we estimated the three dimensional shape of the frechet sensitivity kernels from kernels calculated by normal mode summation the use of these kernels allows us to properly distribute the sensitivity for a given seismic phase over a large mantle volume while allowing the high frequency data to constrain small scale structure the differential times are relatively insensitive to source mislocation and to structure in the shallow mantle beneath source and receivers and they have previously been interpreted exclusively in terms of lateral structure directly above the core mantle boundary cmb however images thus obtained can be contaminated by effects of small scale structure elsewhere in the mantle here we do not make a priori assumptions about the mantle source of anomalous time differentials from test inversions we conclude that both upper and lower mantle structures that are poorly resolved by p data can be mapped into the core along pkp paths but that the effect of outer core structures if any on the mantle model is small compared to the inversion of the p pp and pwp alone the inclusion of the pkpab pkpdf and pkpab pkpbc and pkpdf pdiff data improves the resolution of structure beneath 2200 km depth in particular the joint inversion puts better constraints on the long wavelength variations in the very deep mantle and yields an increase in the amplitude of velocity perturbations near the cmb that is in agreement with but still smaller than inferences from shear wave studies resolution tests indicate that in some regions the enhanced definition of structure is significant but in most regions the improvements are subtle and structure remains poorly resolved in large regions of the mantle

an example of neotectonism in a continental interior thebes gap midcontinent united states abstract some of the most intense neotectonic activity known in the continental interior of north america has been recently discovered on a fault zone in the thebes gap area missouri and illinois this faulting almost assuredly was accompanied by large earthquakes the zone is located approximately 30 km north of the new madrid seismic zone and consists of complex north northeast to northeast striking steeply dipping faults that have had a long lived history of reactivation throughout most of the phanerozoic geophysical studies by others suggest that the faults are rooted in the deeply buried late proterozoic and early cambrian reelfoot rift system quaternary deposits are cut by at least four episodes of faulting two of which occurred during the holocene the overall style of neotectonic deformation is interpreted as right lateral strike slip faulting at many locations however near surface displacements have stepped from one fault strand to another and produced normal and oblique slip faults in areas of transtension and high angle reverse faults thrust faults and folds in areas of transpression there is evidence of reactivation of some near surface fault segments during the great 1811 1812 new madrid earthquakes quaternary faulting at thebes gap demonstrates that there are additional seismic source zones in the midcontinent u s other than new madrid and that even in the absence of plate margin orogenesis intense neotectonic activity does occur over long time periods along crustal weaknesses in continental interiors

mapping mantle transition zone discontinuities beneath the central pacific with array processing of ss precursors we image mantle transition zone mtz discontinuities beneath the central pacific using 120 000 broadband ss waveforms with a wave packet based array processing technique curvelet transform we improve the signal to noise ratio of ss precursors and remove interfering phases so that precursors can be identified and measured over a larger distance range removal of interfering phases reveals possible phase shifts in the underside reflection at the 660 that is s 660 s which if ignored could lead to biased discontinuity depth estimates the combination of data quantity and improved quality allows improved imaging and uncertainty estimation time to depth conversions after corrections for bathymetry crustal thickness and tomographically inferred mantle heterogeneity show that the mean depths of 410 and 660 beneath the central pacific are 420 3 km and 659 4 km respectively the mean mtz thickness 239 2 km is close to global estimates and suggests an adiabatic mantle temperature of 1 400 c for the central pacific depth variations of the 410 and 660 appear to be relatively small with peak to peak amplitudes of the order of 10 15 km the 410 and 660 are weakly anticorrelated and mtz is thinner beneath hawaii and to the north and east of the hotspot and thicker southwest of it the relatively small discontinuity topography argues against the presence of large scale more than 5 wide thermal anomalies with excess temperatures over 200 k across the transition zone the data used cannot exclude stronger thermal anomalies that are of more limited lateral extent or that are not continuous across the mtz

the earthquake potential of the new madrid seismic zone the fault system responsible for new madrid seismicity has generated temporally clustered very large earthquakes in a d 900 100 years and a d 1450 150 years as well as in 1811 1812 given the uncertainties in dating liquefaction features the time between the past three new madrid events may be as short as 200 years and as long as 800 years with an average of 500 years this advance in understanding the late holocene history of the new madrid seismic zone and thus the contemporary tectonic behavior of the associated fault system was made through studies of hundreds of earthquake induced liquefaction features at more than 250 sites across the new madrid region we have found evidence that prehistoric sand blows like those that formed during the 1811 1812 earthquakes are probably com pound structures resulting from multiple earthquakes closely clustered in time or earthquake sequences from the spatial distribution and size of sand blows and their sedimentary units we infer the source zones and estimate the magnitudes of earth quakes within each sequence and thereby characterize the detailed behavior of the fault system it appears that fault rupture was complex and that the central branch of the seismic zone produced very large earthquakes during the a d 900 and a d 1450 events as well as in 1811 1812 on the basis of a minimum recurrence rate of 200 years we are now entering the period during which the next 1811 1812 type event could occur

structure of the mantle transition zone beneath the north china craton abstract detailed information on deep structural features can improve current understanding of lithospheric thinning and destruction of the north china craton in this study we analyzed receiver functions rfs from teleseismic p waveforms recorded by 246 seismic stations beneath the north china craton ncc to determine the topography of the mantle transition zone mtz the results reveal specific features in the 410 and 660 km discontinuities and outlined mtz thickness beneath the study area the mtz structure shows two anomalously thickened regions beneath the ncc up to 30 40 km thicker than the global average which are mainly concentrated in the eastern and western sections of the eastern ncc encc the thickened regions are discontinuous and separated by a distance of about 400 km double 660 km discontinuities also appear in the southeastern encc combining these results with previous findings we propose a model in which mtz thickening is caused by discrete remnants of the subducted western pacific slab discontinuities resulting from the stagnant slabs may retain evidence of changes in the rate of pacific plate subduction as well as the phase transition of the subducted slabs within the mtz

structure of the upper mantle and transition zone beneath southeast asia from traveltime tomography 1 tomographic images of the mantle beneath east asia were obtained from the inversion of traveltime data from global and regional seismograph networks and from temporary arrays on and around the tibetan plateau our results are consistent with previous studies but the unprecedented resolution of mantle heterogeneity provides new insight into the large scale tectonic framework of the continental india asia collision in the western part of the study region and subduction of the oceanic lithosphere in the east in the realm of continental collision west of 100 e a relatively slow p wave speed characterizes the upper mantle beneath much of the tibetan plateau but the wave speed is high beneath cratonic india the southern and western part of the tibetan plateau hindu kush and the tian shan in the subduction realm east of 110 e the main structures are i pronounced low wave speed anomalies at a depth of between 100 and 400 km beneath asia s southeastern seaboard and the back arc regions of ongoing subduction ii narrow fast anomalies in the upper mantle beneath major subduction zones and iii widespread fast anomalies at a depth of 500 700 km beneath the sea of japan the northern part of the philippine sea plate and southeastern china if the latter anomalies represent stagnant slabs their fragmented nature and large lateral extent suggest that they are produced by different episodes of subduction beneath western pacific island arcs along the old se margin of asia or during the mesozoic collision of cratonic units in southeast asia attribution to ancient subduction systems implies that slab fragments can reside in the transition zone for at least several tens of millions of years shallow slow anomalies beneath the red river fault region connect to deep anomalies beneath the south china fold belt and south china sea suggesting a causal relationship between the evolution of the continental lithosphere of sw china and deeper mantle processes between the collision and the subduction realms tomography reveals high wave speed continental roots beneath the western part of the north china craton ordos block and the south china or yangtze craton sichuan basin to a depth of 300 km

metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere earth s deepest earthquakes occur as a population in subducting or previously subducted lithosphere at depths ranging from about 325 to 690 km this depth interval closely brackets the mantle transition zone characterized by rapid seismic velocity increases resulting from the transformation of upper mantle minerals to higher pressure phases deep earthquakes thus provide the primary direct evidence for subduction of the lithosphere to these depths and allow us to investigate the deep thermal thermodynamic and mechanical ferment inside slabs numerical simulations of reaction rates show that the olivine spinel transformation should be kinetically hindered in old cold slabs descending into the transition zone thus wedge shaped zones of metastable peridotite probably persist to depths of more than 600 km laboratory deformation experiments on some metastable minerals display a shear instability called transformational faulting this instability involves sudden failure by localized superplasticity in thin shear zones where the metastable host mineral transforms to a denser finer grained phase hence in cold slabs such faulting is expected for the polymorphic reactions in which olivine transforms to the spinel structure and clinoenstatite transforms to ilmenite it is thus natural to hypothesize that deep earthquakes result from transformational faulting in metastable peridotite wedges within cold slabs this consideration of the mineralogical states of slabs augments the traditional largely thermal view of slab processes and explains some previously enigmatic slab features it explains why deep seismicity occurs only in the approximate depth range of the mantle transition zone where minerals in downgoing slabs should transform to spinel and ilmenite structures the onset of deep shocks at about 325 km is consistent with the onset of metastability near the equilibrium phase boundary in the slab even if a slab penetrates into the lower mantle earthquakes should cease at depths near 700 km because the seismogenic phase transformations in the slab are completed or can no longer occur substantial metastability is expected only in old cold slabs consistent with the observed restriction of deep earthquakes to those settings earthquakes should be restricted to the cold cores of slabs as in any model in which the seismicity is temperature controlled via the distribution of metastability however the geometries of recent large deep earthquakes pose a challenge for any such models transformational faulting may give insight into why deep shocks lack appreciable aftershocks and why their source characteristics including focal mechanisms indicating localized shear failure rather than implosive deformation are so similar to those of shallow earthquakes finally metastable phase changes in slabs would produce an internal source of stress in addition to those due to the weight of the sinking slab such internal stresses may explain the occurrence of earthquakes in portions of lithosphere which have foundered to the bottom of the transition zone and or are detached from subducting slabs metastability in downgoing slabs could have considerable geodynamic significance metastable wedges would reduce the negative buoyancy of slabs decrease the driving force for subduction and influence the state of stress in slabs heat released by metastable phase changes would raise temperatures within slabs and facilitate the transformation of spinel to the lower mantle mineral assemblage causing slabs to equilibrate more rapidly with the ambient mantle and thus contribute to the cessation of deep seismicity because wedge formation should occur only for fast subducting slabs it may act as a parachute and contribute to regulating plate speeds wedge formation would also have consequences for mantle evolution because the density of a slab stagnated near the bottom of the transition zone would increase as it heats up and the wedge transforms to denser spinel favoring the subsequent sinking of the slab into the lower mantle

evolution of mantle structure beneath the northwest pacific evidence from seismic tomography and paleogeographic reconstructions 1 plate motions and subducting slab morphology are intricately connected and through the integration of seismicity tomographic images and relative plate motions the evolution of mantle structure can be interpreted tomographic images of p wave shear wave speed and bulk sound speed perturbations of the northwest pacific region have been interpreted to define the extent and geometry of the subducting pacific plate we have found that the subducted pacific plate beneath the japan and kurile arcs is coherent but is very complex at the junction of the two arcs near the hokkaido corner as the slab subduction angle decreases from north to south while the slab thickens the pacific slab beneath the japan arc is slightly thinner and subducting through the upper mantle at a less steep angle than the slab beneath the kurile arc as the slab reaches the 660 km discontinuity it becomes stagnant and lies in a horizontal position on top of the transition zone both beneath the izu and japan arcs in contrast the slab beneath the kurile arc is subducting at a steeper angle and penetrates through the transition zone into the lower mantle the difference in slab morphology appears to be due to a combination of change in dip of the subducting plate and the convergence velocity of the pacific plate along the margin a new paleogeographic reconstruction of northeast asia was assembled to evaluate the plate motions that could explain the foundation of the current slab morphology the plate reconstruction was based on euler pole motions of the pacific philippine amurian okhotsk and north american plates relative to stable eurasia plate the new tectonic model illustrates the collision of the japan and kurile arcs the opening of the kurile basin disparity in pacific plate velocities along the arc and different rates of trench retreat along the izu japan and kurile segments of the western pacific margin the plate motion model was interpreted in conjunction with the physical properties of the mantle imaged with the p wave and joint tomography to assess the evolution of the pacific plate morphology since the mid miocene and to provide constraints on the plausible plate motions in the region

relict nebkhas pimple mounds record prolonged late holocene drought in the forested region of south central united states the origin and significance of pimple mounds low elliptical to circular dune like features found across much of the south central united states have been debated for nearly two centuries we cored pimple mounds at four sites spanning the ozark plateau arkansas river valley and gulf of mexico coastal plain and found that these mounds have a regionally consistent textural asymmetry such that there is a significant excess of coarse grained sediment within their northwest flanks we interpret this asymmetry as evidence of an eolian depositional origin of these mounds and conclude they are relict nebkhas coppice dunes deposited during protracted middle to late holocene droughts these four mounds yield optically stimulated luminescence ages between 2400 and 700 yr that correlate with well documented periods of eolian activity and droughts on the southern great plains including the medieval climate anomaly we conclude vegetation loss during extended droughts led to local eolian deflation and pimple mound deposition these mounds reflect landscape response to multi decadal droughts for the south central u s the spatial extent of pimple mounds across this region further underscores the severity and duration of late holocene droughts which were significantly greater than historic droughts