microstructure measurements along a quasi meridional transect in the northeastern atlantic ocean 1 this study presents vertical profiles of turbulence parameters obtained in the upper 100 m of the northeastern atlantic ocean along a transect from tropical permanently stratified waters to subpolar seasonally stratified waters in july august 2009 the focus is to fully characterize the vertical mixing along this transect for further studies related to phytoplankton and nutrient distributions derived values of temperature eddy diffusivity kt of temperature variance dissipation rate t and of turbulent kinetic energy dissipation rate e indicate a northward increase of turbulent mixing below the mixed layer in the northern stations where the wind stress is sufficiently high the vertical distributions of e in the mixed layer follow the wind stress scaling the related low wind scaling factors and the low values of e encountered at this cruise are in agreement with a strongly stratified upper water column at midday in summer

deep structure of northern mississippi embayment in september 1980 the u s geological survey conducted a seismic refraction investigation of the northern mississippi embayment during the investigation 34 shots from nine shotpoints were recorded along a series of profiles the profiles were parallel to and across an inferred precambrian rift zone which is outlined by a series of magnetic anomalies and covers an area at least 200 km 125 mi long and 70 km 45 mi wide along the northeast southwest trending axis of the rift a 0 7 to 1 1 km 2 297 to 3 609 ft thick 1 8 km sec layer representing the unconsolidated tertiary and cretaceous sediments overlies a 2 km 6 562 ft thick 5 95 km sec layer representing a paleozoic carbonate sequence beneath the carbonates is a 3 km 9 843 ft thick low velocity layer which probably consists of late precambrian clastics the presence of this low velocity layer is indicated on the seismic profiles by an abrupt cutoff of the 5 95 km sec arrivals at about 20 km 12 mi from the shotpoints this cutoff is followed by a 30 to 40 km 20 to 25 mi long zone of no arrivals beyond which are seen delayed arrivals from a 6 2 km sec crystalline basement the 11 km 6 8 mi thick 6 2 km sec basement overlies a lower crus composed of a 6 6 km sec layer whose thickness decreases to the northeast owing to its replacement by an anomalous high velocity 7 3 km sec basal layer the base of the crust ranges from a 39 km 24 mi depth at the southern end of the profile to a 46 km 29 mi depth at the northern end where the 7 3 km sec layer is 20 km 12 mi thick below the base of the crust moho the upper mantle has a velocity of 8 0 km sec the velocity structure beneath the west flank of the embayment is simpler than that of the axis 5 95 6 2 6 6 7 3 and 8 0 km sec the low velocity layer is absent and the anomalous 7 3 km sec layer is thinner than along the axial profile the profiles perpendicular to the rift show that the low velocity layer is restricted to the axial zone and that the anomalous 7 3 km sec basal crustal layer reaches maximum thickness there the tectonic model proposed to explain the origin of this embayment velocity structure includes a late precambrian mantle plume that intruded the lower crust of the northern embayment causing uplift and or crustal stretching and subsequent rifting of the axial area this was followed by erosion subsidence and subsequent deposition of sediments in the resulting trough

long lived deformation in the southern mississippi embayment revealed by high resolution seismic reflection and sub bottom profiler data three high resolution seismic reflection profiles and two sub bottom profiler sections acquired along the mississippi river in southern central u s image deformation in post paleozoic sediments the northernmost profile images two faults offsetting cretaceous through at least eocene cane river reflectors interpreted to strike northwest and to be part of the arkansas river fault zone the central profile shows a down to the north fault displacing cretaceous and paleocene midway group reflectors by 210 m and 160 m respectively the fault is interpreted as the northern edge fault of the monroe uplift a late cretaceous uplift associated with igneous intrusions the southernmost profile displays a down to the south fault offsetting cretaceous and paleocene eocene wilcox group reflectors by 125 m and 32 m respectively tilted reflectors in the first 80 m indicate eocene oligocene activity of the fault although quaternary activity cannot be ruled out quaternary tectonic activity is proposed for a series of faults that offset shallow 40 m depth eocene sequences and the base of the quaternary alluvium as imaged on two sub bottom profiler sections these shallow faults are imaged in the vicinity of holocene earthquake induced liquefaction fields corroborating the evidence for recent tectonic activity in the area the spatial coincidence of the imaged faults with the inferred location of the alabama oklahoma transform strongly argues toward a long lived influence of this precambrian continental margin in focusing tectonic activity in the southern u s by controlling the reactivation of triassic jurassic syn rift basement structures and guiding the emplacement of late cretaceous igneous intrusions and the location of cenozoic deformation

tectonic stress within the new madrid seismic zone refraction data indicate a significant high density rift pillow beneath the new madrid seismic zone we present results of linear and nonlinear viscoelastic finite element modeling to determine whether support of the rift pillow may contribute significantly to the total present day stress field and we consider the implications for intraplate seismicity these models were run for a loading time of 100 m y to account for relaxation and transfer of stress since the last reactivation of the rift in the mid mesozoic results indicate that the nonlinear viscoelastic model with rheological stratification based on composition and temperature agrees well with the observed deformation within the seismic zone and with estimates of regional stress magnitudes the model predicts a maximum compression of 30 40 mpa above the rift pillow in the center of the rift axis if the magnitude of local compression predicted by the nonlinear model produces the inferred clockwise rotation of the order of 10 30 in the direction of shmax maximum horizontal compression near the rift axis the magnitude of regional compression is a factor of 1 to 2 times the magnitude of local compression and consistent with an origin due to ridge push forces the addition of the local stress associated with the rift pillow however results in an approximately 30 reduction in the resolved maximum horizontal shear stress thus while the stress associated with the rift pillow can rotate the stress field into an orientation favorable for failure reduction in the resolved shear stress requires a separate mechanism for strength reduction results of the modeling indicate that stresses from the load of the rift pillow may still be present in the upper crust even after 100 m y and may still play a role in present day deformation and seismicity of the new madrid seismic zone local stress fields of significant tectonic magnitudes may also occur around other ancient rift pillows and help explain the observed correlation between intraplate seismicity and failed rift zones

long term behaviour of australian stable continental region scr faults abstract australia boasts arguably the richest late neogene to quaternary faulting record in stable continental region scr crust anywhere in the world variation in fault scarp length vertical displacement proximity to other faults and relationship to topography permits division of the continent according to fault character six onshore neotectonic domains are recognised with an additional offshore domain proposed by analogy with the eastern united states each domain relates to a distinct underlying crustal type and architecture broadly considered to represent cratonic non cratonic and extended environments in general greater topographic expression associated with faults occurring in extended crust relative to non extended crust suggests a higher rate of seismic activity in the former setting consistent with observations worldwide using the same reasoning non cratonic crust might be expected to have a higher rate of seismic activity than cratonic crust this distinction together with the variation in fault character between domains should be recognised in attempts to identify analogous systems worldwide a common characteristic of morphogenic earthquake occurrence in australia appears to be temporal clustering periods of earthquake activity comprising a finite number of large events are separated by much longer periods of seismic quiescence at the scale of a single fault and of proximal faults in several instances there is evidence for deformation at scales of several hundred kilometres switching on and off over the last several million years what is not clear from the limited palaeoseismological data available is whether successive active periods are comparable in terms of slip number of events magnitude of events etc regardless this apparent bimodal recurrence behaviour poses challenges for probabilistic seismic hazard assessment psha these rely on the simplifying assumption that large earthquake recurrence for long return periods is not random i e poissonian presently our ability to incorporate such time dependent models is limited by available data

role of large scale tectonic forces in intraplate earthquakes of central and eastern north america central and eastern united states ceus have experienced large intraplate earthquakes yet at present there is no comprehensive model to explain stresses strain and seismicity in this intraplate setting models to explain the intraplate stresses in ceus include glacio isostatic adjustment ridge push effects local stresses along preexisting fracture zones and large scale convection in this paper we present a self consistent model of the dynamics of ceus that explains the stress field responsible for these intraplate earthquakes the earthquakes represent slow ongoing deformation associated with forces arising from a combination of lithosphere topography and structure together with the effects of density driven mantle flow using gps data we calculate strain rates that are likely to arise from tectonic effects and conclude that intraplate strain rates associated with tectonic effects are unlikely to exceed 1 x 10 9 year 1 we test several models of lateral viscosity variations by comparing model stress orientation output with earthquake moment tensors shmax directions from stress inversion and p axes of earthquakes a model that satisfies stress and earthquake constraints and also strain rate magnitude constraints requires high viscosity 1025 pa s craton and old oceanic lithosphere of the western atlantic block and weaker 5 x 1024 pa s accreted appalachian terrane other strength contrasts within the lithosphere are likely present incorporation of these into future models using constraints from seismology along with refined geodetic measurements and improved estimates of crust and upper mantle densities is needed to further refine long term dynamic models and better evaluate the hazards associated with this very slow ongoing permanent deformation within the eastern and central united states

on the heterogeneous electrical conductivity structure of the earth s mantle with implications for transition zone water content 1 we have investigated the lateral variations in mantle electrical conductivity structure using electromagnetic sounding data for this purpose we used very long time series up to 51 years of geomagnetic observatory data at six locations encompassing different geological settings to compute response functions that cover the broadest possible frequency range 3 9 to 95 2 days furstenfeldbruck fur europe hermanus her south africa langzhou lzh china alice springs asp australia tucson tuc united states north america and honolulu hon united states north pacific we inverted the response functions beneath each observatory for a local radial conductivity profile using a stochastic sampling algorithm specifically we found significant lateral variations in conductivity throughout the mantle with resolution limited to the depth range 500 1200 km at 600 km depth conductivity varies between 0 1 and 0 4 s m and increases to 1 3 2 0 s m at 800 km depth beneath all stations except her 0 5 s m at 900 km depth conductivity increases further to 1 4 2 4 s m with her hon and asp being most conductive this trend persists to a depth of 1200 km comparison with conductivity profiles constructed from laboratory measurements of mantle mineral conductivities and models of earth s mantle composition and thermal state reveal that significant thermochemical variations are at the origin of the observed heterogeneities in mantle conductivity found here because of the somewhat large error bounds on sampled conductivity profiles and the reduced sensitivity of the electromagnetic sounding data above 500 km depth constraints on transition zone water content are less conclusive although h2o contents 0 5 wt in the midtransition zone appear less likely

observations of stratified turbulent mixing in an estuary neap to spring variations during high river flow abstract observations of turbulent mixing stratification and currents were made with a free falling microstructure profiler and a 600 khz adcp in the hudson river off manhattan in may 1994 the ship was anchored in a relatively straight and uniform channel about 15 m deep over 4 days of intense observations from 19 to 24 may 1994 the progression from neap tides to spring tides caused a severe reduction in stratification from an initial top to bottom salinity difference of 18 to 3 6 while low gradient richardson numbers ri occurred only in the weakly stratified bottom layer on flood during neap tides low ri spanned the water column on the second half of ebb during spring tides turbulent dissipation rates varied roughly inversely to ri during neap conditions turbulent mixing was intense only in the bottom layer on flood where eddy diffusivities k reached more than 10 2 m 2 s 1 while mixing was weak in the centre of the halocline near 5 m during spring tides strong mixing occurred throughout the water column during the latter part of ebb with typical k 5 10 3 m 2 s 1 and typical vertical salt flux of 0 0015 kg m 2 s 1 if it existed in isolation this flux would eliminate the haline stratification on a time scale of 4 h ebb time bands of low ri and intense mixing showed increasing phase lag with increasing distance from the bottom the spring tide ebb time internal turbulent stress was relatively uniform with depth with a typical magnitude of 0 4 pa 4 dyne cm 2 turbulent overturning thorpe scales ranged from below 1 cm in the neap tide halocline to a few metres in the weakly stratified bottom layer on flood the strong spring tide ebb time mixing had thorpe scales of typically 0 5 m throughout the water column ozmidov scales tracked thorpe scales except in the weakly stratified layer above the bottom

seismic velocity model of the central united states version 1 description and simulation of the 18 april 2008 mt carmel illinois earthquake abstract we have developed a new three dimensional seismic velocity model of the central united states cusvm that includes the new madrid seismic zone nmsz and covers parts of arkansas mississippi alabama illinois missouri kentucky and tennessee the model represents a compilation of decades of crustal research consisting of seismic aeromagnetic and gravity profiles geologic mapping geophysical and geological borehole logs and inversions of the regional seismic properties the density p and s wave velocities are synthesized in a stand alone spatial database that can be queried to generate the required input for numerical seismic wave propagation simulations we test and calibrate the cusvm by simulating ground motions of the 18 april 2008 m w 5 4 mt carmel illinois earthquake and comparing the results with observed records within the model area the selected stations in the comparisons reflect different geological site conditions and cover distances ranging from 10 to 430 km from the epicenter the results based on a qualitative and quantitative goodness of fit gof characterization indicate that both within and outside the mississippi embayment the cusvm reasonably reproduces 1 the body and surface wave arrival times and 2 the observed regional variations in ground motion amplitude cumulative energy duration and frequency content up to a frequency of 1 0 hz in addition we discuss the probable structural causes for the ground motion patterns in the central united states that we observed in the recorded motions of the 18 april mt carmel earthquake online material simulated and observed waveforms and response spectral acceleration ground motion prediction equation comparison

strain accumulation in the new madrid and wabash valley seismic zones from 14 years of continuous gps observation the mechanical behavior and hence earthquake potential of faults in continental interiors is an issue of critical importance for the resultant seismic hazard but no consensus has yet been reached on this controversial topic the debate has focused on the central and eastern united states in particular the new madrid seismic zone struck by four magnitude 7 or greater earthquakes in 1811 1812 and to a lesser extent the wabash valley seismic zone just to the north a key aspect of this issue is the rate at which strain is currently accruing on those plate interior faults a quantity that remains debated here we address this issue with an analysis of up to 14 6 years of continuous gps data from a network of 200 sites in the central united states centered on the new madrid and wabash valley seismic zones we find that the high quality sites in these regions show motions that are consistently within the 95 confidence limit of zero deformation these results place an upper bound on strain accrual on faults of 0 2 mm yr and 0 6 mm yr in the new madrid and wabash valley seismic zones respectively for the new madrid region where a paleoseismic record is available for the past 5000 years we argue that strain accrual if any does not permit the 500 900 year repeat time of paleo earthquakes observed in the upper mississippi embayment these results together with increasing evidence for temporal clustering and spatial migration of earthquake sequences in continental interiors indicate that either tectonic loading rates or fault properties vary with time in the new madrid seismic zone and possibly plate wide

slow deformation and lower seismic hazard at the new madrid seismic zone global positioning system gps measurements across the new madrid seismic zone nmsz in the central united states show little if any motion these data are consistent with platewide continuous gps data away from the nmsz which show no motion within uncertainties both these data and the frequency magnitude relation for seismicity imply that had the largest shocks in the series of earthquakes that occurred in 1811 and 1812 been magnitude 8 their recurrence interval should well exceed 2500 years longer than has been assumed alternatively the largest 1811 and 1812 earthquakes and those in the paleoseismic record may have been much smaller than typically assumed hence the hazard posed by great earthquakes in the nmsz appears to be overestimated

seismically invisible water in earth s transition zone abstract ringwoodite the dominant mineral at depths between 520 km and 660 km can store up to 2 3 wt of water in its crystal structure making the earth s transition zone a plausible water reservoir that plays a central role in earth s deep water cycle experiments show that hydration of ringwoodite significantly reduces elastic wave velocities at room pressure but the effect of pressure remains poorly constrained here a novel experimental setup enables a direct quantification of the effect of hydration on ringwoodite single crystal elasticity and density at pressures of the earth s transition zone and high temperatures our data show that the hydration induced reduction of seismic velocities almost vanishes at conditions of the transition zone seismic data thus agree with a wide range of water contents in the transition zone

predicting plate velocities with mantle circulation models 1 we predict plate motions from a comprehensive inversion of theoretical estimates of tectonic forces in order to evaluate the relative importance of these and the uncertainties of such models plate driving forces from the mantle are calculated using global flow models that are driven by tomography and subduction derived density fields observed and predicted plate velocities agree well for a variety of models leading to varied conclusions about the relative importance of forces the dominance of the subduction related density pattern in the mantle is confirmed it appears that p wave models do not satisfactorily image all of the slab associated anomalies in the upper mantle furthermore lower mantle structure always improves the plate motion fit with respect to models that are based on upper mantle anomalies and lithospheric thickening only we show that the average torques from the lower mantle scale with the radial flow through the 660 km phase transition the amplitude of the lower mantle torques will be significant for a range of models if there is mass flux through 660 km we also evaluate parameterized edge forces and find that the additional inclusion of such torques does not significantly improve the model fit the main reason for the nonuniqueness of the inversions is plate boundary geometry since all plate motions are dominated by the trench ridge system and plates move from ridges to trenches

seismic structure of the central us crust and shallow upper mantle uniqueness of the reelfoot rift abstract using seismic surface waves recorded with earthscope s transportable array we apply surface wave imaging to determine 3d seismic velocity in the crust and uppermost mantle our images span several proterozoic and early cambrian rift zones mid continent rift rough creek graben rome trough birmingham trough southern oklahoma aulacogen and reelfoot rift while ancient rifts are generally associated with low crustal velocity because of the presence of thick sedimentary sequences the reelfoot rift is unique in its association with low mantle seismic velocity its mantle low velocity zone lvz is exceptionally pronounced and extends down to at least 200 km depth this lvz is of variable width being relatively narrow 50 km wide within the northern reelfoot rift which hosts the new madrid seismic zone nmsz we hypothesize that this mantle volume is weaker than its surroundings and that the reelfoot rift consequently has relatively low elastic plate thickness which would tend to concentrate tectonic stress within this zone no other intraplate ancient rift zone is known to be associated with such a deep mantle low velocity anomaly which suggests that the nmsz is more susceptible to external stress perturbations than other ancient rift zones

commerce geophysical lineament its source geometry and relation to the reelfoot rift and new madrid seismic zone the commerce geophysical lineament is a northeast trending magnetic and gravity feature that extends from central arkansas to southern illinois over a distance of 400 km it is parallel to the trend of the reelfoot graben but offset 40 km to the northwest of the western margin of the rift floor modeling indicates that the source of the aeromagnetic and gravity anomalies is probably a mafic dike swarm the age of the source of the commerce geophysical lineament is not known but the linearity and trend of the anomalies suggest a relationship with the reelfoot rift which has undergone episodic igneous activity the commerce geophysical lineament coincides with several topographic lineaments movement on associated faults at least as young as quaternary and intrusions of various ages several earthquakes m b 3 coincide with the commerce geophysical lineament but the diversity of associated focal mechanisms and the variety of surface structural features along the length of the commerce geophysical lineament obscure its relation to the release of present day strain with the available seismicity data it is difficult to attribute individual earthquakes to a specific structural lineament such as the commerce geophysical lineament however the close correspondence between quaternary faulting and present day seismicity along the commerce geophysical lineament is intriguing and warrants further study

tectonic history and setting of a seismogenic intraplate fault system that lacks microseismicity the saline river fault system southern united states abstract although the northwest striking saline river fault system of southeastern arkansas is not defined by microseismicity it is associated with sand blows and shows evidence of pleistocene and holocene surface ruptures suggesting a significant seismogenic potential this fault system is within the northern gulf of mexico interior coastal plain a region only recently recognized as containing seismogenic faults to better characterize this active fault system we reconstructed its post paleozoic history using petroleum and coal industry wire line well log and seismic reflection subsurface data the saline river fault system initiated as a series of northwest striking grabens during triassic jurassic uplift and incipient gulf of mexico rifting along the basement alabama oklahoma transform margin of the north american proterozoic craton during post rift subsidence these grabens were buried by gulf sediments until mid cretaceous uplift and igneous activity resulted in minor extensional reactivation of graben faults faulting style changed from extension to transpression during the late cretaceous due to compression of eastern north america as the north atlantic rapidly widened and due to thermal weakening of the alabama oklahoma transform lithospheric discontinuity as it obliquely crossed a mantle hot spot in the late cretaceous graben faults experienced contractional reactivation and steep deeply rooted transpressional faults developed within and parallel to the graben system these transpressional faults locally displace eocene pleistocene and holocene sediments fault activity continues on the saline river fault system due to thin crust along the alabama oklahoma transform and to high heat flow which act together to weaken the crust and promote seismogenic tectonism the fault system may lack appreciable microseismicity because the aftershock sequence of the last large earthquake has had time to dissipate

sinking mafic body in a reactivated lower crust a mechanism for stress concentration at the new madrid seismic zone we propose a geodynamic model for stress concentration in the new madrid seismic zone nmsz the model postulates that a high density mafic body situated in the deep crust directly beneath the most seismically active part of the nmsz began sinking several thousands of years ago when the lower crust was suddenly weakened based on the fact that deformation rates in the nmsz have accelerated over the past 9 k y we envision the source of this perturbation to be related to the last north american deglaciation excess mass of the mafic body exerts a downward pull on the elastic upper crust leading to a cycle of primary thrust faulting with secondary strike slip faulting after which continued sinking of the mafic body reloads the upper crust and renews the process this model is consistent with the youth of activity the generation of a sequence of earthquakes and the velocity evolution during interseismic periods which depend upon the density contrast of the mafic body with respect to the surrounding crust its volume and the viscosity of the lower crust

why the new madrid earthquakes are m 7 8 and the charleston earthquake is m 7 estimates of magnitudes of large historical earthquakes are an essential input to and can seriously affect seismic hazard estimates the earthquake intensity observations modified mercalli intensities mmi and assigned magnitudes m of the 1811 1812 new madrid events have been reinterpreted several times in the last decade and have been a source of controversy in making seismic hazard estimates in the central united states observations support the concept that the larger the earthquake the greater the maximum felt distance for the same crustal attenuation and local soil conditions magnitude should be the main influence on intensity values at large distances we apply this concept by comparing the mean mmi at distances of 600 1200 km for each of the four largest new madrid 1811 1812 earthquakes the 1886 charleston south carolina earthquake the 1929 m 7 2 grand banks earthquake and the 2001 m 7 6 bhuj india earthquake we fit the intensity observations using the form mmi a c dist 0 8 log dist to better define intensity attenuation in eastern north america ena the intensity attenuation in cratonic india differs from ena and is corrected to ena using both the above estimate and published intensity relations we evaluate source marine geophysical q and stress drop information as well as a 1929 milne shaw record at chicago to confirm that the 1929 grand banks earthquake occurred in ena crust our direct comparison of mean intensities beyond 600 km suggests m 7 5 7 3 7 7 and 6 9 for the three new madrid 1811 1812 mainshocks and the largest aftershock and m 7 0 for the 1886 charleston south carolina earthquake with an estimated uncertainty of 0 3 units at the 95 confidence level based on a monte carlo analysis our mean new madrid and charleston mainshock magnitudes are similar to those of bakun and hopper 2004 and are much higher than those of hough and page 2011 for new madrid online material tables of mean modified mercalli intensity for 800 1200 and 600 1000 km distance ranges and figures of least squares fit for all intensity measures used and for frequency dependent q in easternmost canada

broadband acoustic quantification of stratified turbulence high frequency broadband acoustic scattering techniques have enabled the remote high resolution imaging and quantification of highly salt stratified turbulence in an estuary turbulent salinity spectra in the stratified shear layer have been measured acoustically and by in situ turbulence sensors the acoustic frequencies used span 120 600 khz which for the highly stratified and dynamic estuarine environment correspond to wavenumbers in the viscous convective subrange 500 2500 m 1 the acoustically measured spectral levels are in close agreement with spectral levels measured with closely co located micro conductivity probes the acoustically measured spectral shapes allow discrimination between scattering dominated by turbulent salinity microstructure and suspended sediments or swim bladdered fish the two primary sources of scattering observed in the estuary in addition to turbulent salinity microstructure the direct comparison of salinity spectra inferred acoustically and by the in situ turbulence sensors provides a test of both the acoustic scattering model and the quantitative skill of acoustical remote sensing of turbulence dissipation in a strongly sheared and salt stratified estuary

the dynamics of western north america stress magnitudes and the relative role of gravitational potential energy plate interaction at the boundary and basal tractions summary we investigate the forces involved in driving long term large scale continental deformation in western north america and quantify the vertically averaged deviatoric stress field arising from internal buoyancy forces and the accommodation of relative plate motions in addition we investigate the ability of regional models to resolve the level of tractions acting at the base of the lithosphere we directly solve force balance equations for vertically averaged deviatoric stresses associated with differences in values of 1 lithospheric thickness times the gravitational potential energy per unit area gpe the gpe values are inferred using both the etopo5 topographic data set and the crust2 0 crustal thickness model deviatoric stresses associated with basal tractions are calculated globally with inputs determined from an isoviscous upper mantle 10 21 pa s 3 d large scale convection model in which mantle density variations were inferred from tomographic data and the history of subduction in a 211parameter iterative inversion we then solve for a stress field boundary condition by fitting stress field indicators i e the directions and relative magnitudes of the principal axes of kinematic strain rates magnitudes of the total vertically averaged deviatoric stress field sum of gpe solution with the boundary condition solution range from 5 to 10 mpa within a 100 km thick lithosphere these magnitudes are calibrated by the gpe differences along with the spatial variation in deformation style there is a trade off between the scaling of the basal traction deviatoric stress field and the boundary condition solution however the combined boundary conditions plus basal traction solution is robust in both magnitude and style and when added to the contribution from gpe differences provides a global minimum of misfit between the total deviatoric stress solution and the stress field indicators gpe variations account for 50 per cent of the deviatoric stress magnitudes driving deformation while boundary condition stresses account for the remaining 50 per cent of deviatoric stress magnitude by comparing possible end member strength profiles with our vertically averaged deviatoric stresses we infer that the bulk of the strength within the lithosphere in western north america lies within the brittle seismogenic layer

an updated stress map of the continental united states reveals heterogeneous intraplate stress knowledge of the state of stress in earth s crust is key to understanding the forces and processes responsible for earthquakes historically low rates of natural seismicity in the central and eastern united states have complicated efforts to understand intraplate stress but recent improvements in seismic networks and the spread of human induced seismicity have greatly improved data coverage here we compile a nationwide stress map based on formal inversions of focal mechanisms that challenges the idea that deformation in continental interiors is driven primarily by broad uniform stress fields derived from distant plate boundaries despite plate boundary compression extension dominates roughly half of the continent and second order forces related to lithospheric structure appear to control extension directions we also show that the states of stress in several active eastern united states seismic zones differ significantly from those of surrounding areas and that these anomalies cannot be explained by transient processes suggesting that earthquakes are focused by persistent locally derived sources of stress such spatially variable intraplate stress appears to justify the current spatially variable estimates of seismic hazard future work to quantify sources of stress stressing rate magnitudes and their relationship with strain and earthquake rates could allow prospective mapping of intraplate hazard

necessary conditions for intraplate seismic zones in north america the cause of intraplate seismic zones persists as an important scientific and societal question most intraplate earthquakes are concentrated in specific seismic zones along or adjacent to large scale basement structures e g rifts or sutures at ancient plate boundaries within continental crust the major intraplate seismic zones are limited to specific segments and are not distributed along the lengths of the ancient structures we present a new hypothesis that major intraplate seismic zones are restricted to places where concentrated crustal deformation ccd is overprinted on large scale basement structures examples where ccd affects the stability of specific parts of large scale structures in response to present day stress conditions include the most active seismic zones in central and eastern north america charlevoix eastern tennessee and new madrid our hypothesis has important implications for the assessment of seismic hazards

joint inversion of satellite detected tidal and magnetospheric signals constrains electrical conductivity and water content of the upper mantle and transition zone we present a new global electrical conductivity model of earth s mantle the model was derived by using a novel methodology which is based on inverting satellite magnetic field measurements from different sources simultaneously specifically we estimated responses of magnetospheric origin and ocean tidal magnetic signals from the most recent swarm and champ data the challenging task of properly accounting for the ocean effect in the data was addressed through full three dimensional solution of maxwell s equations we show that simultaneous inversion of magnetospheric and tidal magnetic signals results in a model with much improved resolution comparison with laboratory based conductivity profiles shows that obtained models are compatible with a pyrolytic composition and a water content of 0 01 wt and 0 1 wt in the upper mantle and transition zone respectively

constraints on 3 d stress in the crust from support of mid ocean ridge topography 1 the direction of crustal stresses acting at mid ocean ridges is well characterized but the magnitude of these stresses is poorly constrained we present a method by which the absolute magnitude of these stresses may be constrained using seafloor topography and gravity the topography is divided into a short wavelength portion created by rifting magmatism and transform faulting and a long wavelength portion associated with the cooling and subsidence of the oceanic lithosphere the short wavelength surface and moho topography are used to calculate the spatially varying 3 d stress tensor in the crust by assuming that in creating this topography the deviatoric stress reached the elastic plastic limiting stress the moho topography is constrained by short wavelength gravity variations under these assumptions an incompressible elastic material gives the smallest plastic failure stress associated with this topography this short wavelength topographic stress generally predicts the wrong style of earthquake focal mechanisms at ridges and transform faults however the addition of an in plane regional stress field is able to reconcile the combined crustal stress with both the ridge and transform focal mechanisms by adjusting the magnitude of the regional stress we determine a lower bound for in situ ridge perpendicular extension of 25 40 mpa along the slow spreading mid atlantic ridge 40 50 mpa along the ultra slow spreading ridges in the western indian ocean and 10 30 mpa along the fast spreading ridges of the southeastern indian and pacific oceans furthermore we constrain the magnitude of ridge parallel extension to be between 4 and 8 mpa in the atlantic ocean between 1 and 7 mpa in the western indian ocean and between 1 and 3 mpa in the southeastern indian and pacific oceans these observations suggest that a deep transform valley is an essential feature of the ridge transform spreading center

a reassessment of spectral te estimation in continental interiors the case of north america 1 conventional spectral te studies use the real part of the admittance between gravity anomalies and topography or alternatively the square of the magnitude of the coherency i e coherence here we show the utility of treating both the admittance and coherency as complex quantities inverting the real parts to estimate te we use the imaginary parts to tell if the inversion is biased by noise one method inverts the square of the real coherency with the internal to total load ratio f derived as a function of wave number directly from the gravity and topography the other method inverts the real part of the admittance assuming that f is wave number independent we test the methods using synthetic elastic plate models loaded at the surface and moho in such a way that the final relief is the actual north american topography in some of the models we add gravity noise generated by a model having both surface and internal loads such that the final topography is zero and find that both methods are susceptible to noise application of the two methods to north america gives te maps showing substantial agreement except in regions affected by noise but these are not a dominant part of the total area given the suggested mechanisms by which noise might arise it is not surprising that it is not a more widespread feature of the north american craton importantly both methods show that large parts of the canadian shield are characterized by te 100 km

estimating earthquake magnitudes from reported intensities in the central and eastern united states a new macroseismic intensity prediction equation is derived for the central and eastern united states and is used to estimate the magnitudes of the 1811 1812 new madrid missouri and 1886 charleston south carolina earthquakes this work improves upon previous derivations of intensity prediction equations by including additional intensity data correcting magnitudes in the intensity datasets to moment magnitude and accounting for the spatial and temporal population distributions the new relation leads to moment magnitude estimates for the new madrid earthquakes that are toward the lower range of previous studies depending on the intensity dataset to which the new macroseismic intensity prediction equation is applied mean estimates for the 16 december 1811 23 january 1812 and 7 february 1812 mainshocks and 16 december 1811 dawn aftershock range from 6 9 to 7 1 6 8 to 7 1 7 3 to 7 6 and 6 3 to 6 5 respectively one sigma uncertainties on any given estimate could be as high as 0 3 0 4 magnitude units we also estimate a magnitude of 6 9 0 3 for the 1886 charleston south carolina earthquake we find a greater range of magnitude estimates when also accounting for multiple macroseismic intensity prediction equations the inability to accurately and precisely ascertain magnitude from intensities increases the uncertainty of the central united states earthquake hazard by nearly a factor of two relative to the 2008 national seismic hazard maps our range of possible 1811 1812 new madrid earthquake magnitudes increases the coefficient of variation of seismic hazard estimates for memphis tennessee by 35 42 for ground motions expected to be exceeded with a 2 probability in 50 years and by 27 35 for ground motions expected to be exceeded with a 10 probability in 50 years

significant motions between gps sites in the new madrid region implications for seismic hazard position time series from global positioning system gps stations in the new madrid region were differenced to determine the relative motions between sta tions uncertainties in rates were estimated using a three component noise model con sisting of white flicker and random walk noise following the methodology of langbein 2004 significant motions of 0 37 0 07 one standard error mm yr were found between sites ptgvand stle for which the baseline crosses the inferred deep portion of the reelfoot fault baselines between stle and three other sites also show significant motion site mcty adjacent to stle also exhibits significant motion with respect to ptgv these motions are consistent with a model of interseismic slip of about 4 mm yr on the reelfoot fault at depths between 12 and 20 km if constant over time this rate of slip produces sufficient slip for an m 7 3 earthquake on the shallow portion of the reelfoot fault using the geologically derived recurrence time of 500 years this model assumes that the shallow portion of the fault has been pre viously loaded by the intraplate stress a gps site near little rock arkansas shows significant southward motion of 0 3 0 4 mm yr 0 08 mm yr relative to three sites to the north indicating strain consistent with focal mechanisms of earthquake swarms in northern arkansas

dense lower crust elevates long term earthquake rates in the new madrid seismic zone knowledge of the local state of stress is critical in appraising intraplate seismic hazard inverting earthquake moment tensors we demonstrate that principal stress directions in the new madrid seismic zone nmsz differ significantly from those in the surrounding region faults in the nmsz that are incompatible with slip in the regional stress field are favorably oriented relative to local stress we jointly analyze seismic velocity gravity and topography to develop a 3 d crustal and upper mantle density model revealing uniquely dense lower crust beneath the nmsz finite element simulations then estimate the stress tensor due to gravitational body forces which sums with regional stress the anomalous lower crust both elevates gravity derived stress at seismogenic depths in the nmsz and rotates it to interfere more constructively with far field stress producing a regionally maximal deviatoric stress coincident with the highest concentration of modern seismicity moreover predicted principal stress directions mirror variations observed independently in moment tensors at the nmsz and across the region

quaternary deformation along the meeman shelby fault near memphis tennessee imaged by high resolution marine and land seismic reflection profiles 1 a series of high resolution seismic reflection surveys was carried out in 2008 2010 and 2011 providing a total of five new seismic profiles constraining the location and character of the meeman shelby fault msf about 9 km west of memphis tennessee in the central u s the msf is the best documented fault closest to memphis yet discovered and shows a recurrent fault history the fault as imaged by the reflection profiles is 45 km long strikes n25 e and dips west northwest 83 exhibiting an up to the west sense of motion with a possible right lateral strike slip component the data show that on average the msf offsets the paleozoic unit 77 m and folds the top of the cretaceous unit and the paleocene eocene wilcox group 44 and 25 m respectively one seismic profile acquired along the mississippi river images the bottom of the quaternary alluvium warped up 28 m indicating recent activity of the msf calculated vertical slip rates of the msf during the deposition of the upper cretaceous paleocene eocene and quaternary sediments are 0 0022 0 0010 0 0004 and 0 2154 mm yr respectively suggesting an increase in fault activity during the quaternary consistent with the present stress field and the deformation of the new madrid seismic zone fault system we interpret the msf as a p shear fault in the context of a left stepping right lateral constraining strike slip fault system under a nearly east west oriented compressional stress field source scaling estimates indicate that the msf is capable of generating a m6 9 earthquake if rupturing in one event

seismic moment assessment of earthquakes in stable continental regions iii new madrid 1811 1812 charleston 1886 and lisbon 1755 summary the sizes of three major or great historical earthquakes are reassessed using the isoseismal area regression tools developed in parts i and ii of this study of stable continental region scr seismicity the earthquakes are 1811 new madrid central united states and its following sequence 1886 charleston coastal south carolina and 1755 lisbon oceanic intraplate off the continental shelf of portugal the analysis confirms the large size of these events and for the first time places constraints on the uncertainty of their seismic moment release because of the exceptionally low seismic wave attenuation of eastern north america ena a separate north american regression of seismic moment on isoseismal area was developed additionally the unknown western extents of the new madrid isoseismal areas were calibrated with the patterns of the m 6 3 6 6 1843 and 1895 earthquakes application of part ii analysis procedures with these corrections yields new madrid size estimates expressed as moment magnitude of m 8 1 0 31 for the 1811 december 16 m 7 8 0 33 for the 1812 january 23 and m 8 0 0 33 for the 1812 february 7 principal events the charleston earthquake s magnitude decreases from m 7 4 to m 7 3 0 26 after compensation for the effect of coastal plain sediments on its inner isoseismals intensity regressions for lisbon are calibrated against the isoseismal pattern of the nearly co located m 7 8 1969 st vincent earthquake which in this case increases the predicted size of lisbon from m 8 4 to m 8 7 0 39 these size estimates are supported by data from independent phenomena extent and severity of liquefaction the maximum distance of induced landslides and for lisbon tsunami wave amplitudes estimated source parameters are controlled by crustal or lithospheric temperature which governs the depth extent of brittle faulting using estimated continental and oceanic geotherms viable fault lengths are 30 80 km for charleston 120 180 km for 1811 new madrid and 180 280 km for lisbon for average displacements of 2 4 m 8 11 m and 10 14 m respectively and for average static stress and strain drops at the estimated seismic moments of this study the 1811 new madrid and the 1755 lisbon events are respectively the largest known scr and oceanic lithosphere earthquakes

pleistocene holocene transition in the central mississippi river valley abstract within the northern mississippi embayment the ancestral mississippi river flowed south through the western lowlands and the ancestral ohio river flowed through the eastern lowlands for most of the pleistocene previous investigators have mapped and dated the terraces of their respective braid belts this current research investigates the three dimensional aspect of the quaternary alluvium north of memphis tennessee through the interpretation of 3374 geologic well logs that are 91 4 m 300 ft deep the braid belts are capped by a thin silt clay horizon pleistocene loess that overlies gravelly sand which in turn overlies sandy gravel the base of the pleistocene alluvium beneath the ash hill 27 3 24 6 ka melville ridge 41 6 34 5 ka and dudley 63 5 50 1 ka terraces of the western lowland slope southerly by 0 275 m km and all have an average basal elevation of 38 m near beedeville arkansas the bases of these terraces descend 20 m across a northeast striking down to the southeast fault that coincides with the western margin of the cambrian reelfoot rift the maximum depth of flow lowest elevation of base of alluvium occurred in the eastern lowlands and appears to have been the downstream continuation of the ancestral ohio river cache valley course in southern illinois in traversing from west to east in the eastern lowlands the sikeston braid belt 19 7 17 8 ka has a basal elevation averaging 7 m the kennett braid belt 16 1 14 4 ka averages 13 m the morehouse 12 ka braid belt averages 24 m and the holocene 10 ka mississippi river floodplain has the highest average basal elevation at 37 m along this easterly traverse the base of the quaternary alluvium rises and the age of alluvium decreases the eastward thinning of the floodplain alluvium in the eastern lowlands appears to be caused by decreasing mississippi river discharge as it transitioned from the wisconsinan glacial maximum to the holocene the base of the holocene mississippi river floodplain averages 23 m higher in elevation than the pleistocene floodplain bases in the eastern lowlands this high suballuvial surface platform is bound by the tectonically uplifted joiner ridge blytheville arch charleston uplift and bluff line fault the spatial relationship and similar histories of the platform and bounding structures suggest that quaternary erosion and tectonics are related

ridge forces absolute plate motions and the intraplate stress field torque poles are calculated for a variety of possible forces acting on the plates including ridge push slab pull and collisional resistance these torque poles are then compared to the directions of absolute plate motions there is a strong correlation between ridge torque poles and the azimuth of absolute plate motions for the north american south american pacific cocos and eurasian plates simple slab pull torques correlate well with absolute motion azimuths for the pacific nazca and cocos plates and moderately well with the absolute motion azimuth of the indoaustralian plate collisional resistance torque poles correlate with the absolute motion azimuth of the eurasian plate only the correlations are presented as further evidence that the absolute reference frame for plate motion is determined by the surface plates themselves torque poles for various forces are also compared with several long wavelength features of the global intraplate stress field that also tend to be aligned with absolute motion directions in general ridge torque directions agree well with the orientations of maximum horizontal stresses for stable north america western europe and south america and provide an alternative explanation for the alignment in terms of ridge push forces rather than basal drag collisional resistance forces can also explain the alignment of stresses in western europe for the indoaustralian plate the torque pole for collisional resistance forces is consistent with the general pattern of stresses in at least the western half of the plate but is not a good predictor of the entire data set for the plate other processes in addition to collisional resistance must be important for the indo australian plate ridge push forces may account for a significant portion of the long wavelength features of the intraplate stress field especially away from continental collisions such a conclusion is consistent with negative buoyancy of the slab being an important component of the driving mechanism as previously suggested slab forces may be largely balanced within the subducted slab itself and thus have limited effect on deformation of the surface plates

high pressure single crystal elasticity of wadsleyite and the seismic signature of water in the shallow transition zone abstract earth s transition zone at depths between 410 km and 660 km plays a key role in earth s deep water cycle since large amounts of hydrogen can be stored in the nominally anhydrous minerals wadsleyite and ringwoodite mg fe 2 sio 4 previous mineral physics experiments on iron free wadsleyite proposed low seismic velocities as an indicative feature for hydration in the transition zone here we report simultaneous sound wave velocity and density measurements on iron bearing wadsleyite single crystals with 0 24 wt h 2 o by comparison with earlier studies we show that pressure suppresses the velocity reduction caused by higher degrees of hydration in iron bearing wadsleyite ultimately leading to a velocity cross over for both p waves and s waves modeling based on our experimental results shows that wave speed variations within the transition zone as well as velocity jumps at the 410 km seismic discontinuity both of which have been used in previous work to detect mantle hydration are poor water sensors instead the impedance contrast across the 410 km seismic discontinuity that is reduced in the presence of water can serve as a more robust indicator for hydrated parts of the transition zone

predicting the lithospheric stress field and plate motions by joint modeling of lithosphere and mantle dynamics the way in which basal tractions associated with mantle convection couples with the lithosphere is a fundamental problem in geodynamics a successful lithosphere mantle coupling model for the earth will satisfy observations of plate motions intraplate stresses and the plate boundary zone deformation we solve the depth integrated three dimensional force balance equations in a global finite element model that takes into account effects of both topography and shallow lithosphere structure as well as tractions originating from deeper mantle convection the contribution from topography and lithosphere structure is estimated by calculating gravitational potential energy differences the basal tractions are derived from a fully dynamic flow model with both radial and lateral viscosity variations we simultaneously fit stresses and plate motions in order to delineate a best fit lithosphere mantle coupling model we use both the world stress map and the global strain rate model to constrain the models we find that a strongly coupled model with a stiff lithosphere and 3 4 orders of lateral viscosity variations in the lithosphere are best able to match the observational constraints our predicted deviatoric stresses which are dominated by contribution from mantle tractions range between 20 70 mpa the best fitting coupled models predict strain rates that are consistent with observations that is the intraplate areas are nearly rigid whereas plate boundaries and some other continental deformation zones display high strain rates comparison of mantle tractions and surface velocities indicate that in most areas tractions are driving although in a few regions including western north america tractions are resistive citation ghosh a w e holt and l m wen 2013 predicting the lithospheric stress field and plate motions by joint modeling of lithosphere and mantle dynamics

seismic velocity structure of the crust and shallow mantle of the central and eastern united states by seismic surface wave imaging seismic surface waves from the transportable array of earthscope s usarray are used to estimate phase velocity structure of 18 to 125 s rayleigh waves then inverted to obtain three dimensional crust and upper mantle structure of the central and eastern united states ceus down to 200 km the obtained lithosphere structure confirms previously imaged ceus features e g the low seismic velocity signature of the cambrian reelfoot rift and the very low velocity at 150 km depth below an eocene volcanic center in northwestern virginia new features include high velocity mantle stretching from the archean superior craton well into the proterozoic terranes and deep low velocity zones in central texas associated with the late cretaceous travis and uvalde volcanic fields and beneath the south georgia rift which contains jurassic basalts hot spot tracks may be associated with several imaged low velocity zones particularly those close to the former rifted laurentia margin

the role of gravitational potential energy in active deformation in the southwestern united states in orogenic belts intrinsic buoyancy and extrinsic boundary forces act on the continental lithosphere determining the relative importance of these forces is fundamental to understanding the dynamics of continental deformation in the western united states estimates of gravitational potential energy show that given available constraints on rheology buoyancy forces are sufficient to produce the wide variety of rates and styles of observed deformation including absence of deformation this underscores the importance of gravitational potential energy for deformation in orogenic belts

on the loss of wind induced near inertial energy to turbulent mixing in the upper ocean wind induced near inertial energy has been believed to be an important source for generating the ocean mixing required to maintain the global meridional overturning circulation in the present study the near inertial energy budget in a realistic 1 12 degrees model of the north atlantic ocean driven by synoptically varying wind forcing is examined the authors find that nearly 70 of the wind induced near inertial energy at the sea surface is lost to turbulent mixing within the top 200 m and hence is not available to generate diapycnal mixing at greater depth assuming this result can be extended to the global ocean it is estimated that the wind induced near inertial energy available for ocean mixing at depth is at most 0 1 tw this confirms a recent suggestion that the role of wind induced near inertial energy in sustaining the global overturning circulation might have been overemphasized

displacement history and slip rate on the reelfoot fault of the new madrid seismic zone abstract a displacement history and slip rates were determined for the reelfoot fault in the new madrid seismic zone from a seismic reflection profile and trench data based on calculations from the seismic reflection line the average slip rate over the last 80 million years is 0 0009 mm year 1 slip rate during the late cretaceous was 0 0007 mm year 1 0 002 mm year 1 during the paleocene midway group 0 001 mm year 1 during paleocene eocene wilcox formation time 0 0003 mm year 1 during the post wilcox pre holocene period and a holocene slip rate of 1 8 mm year 1 based on trench data slip rate on the reelfoot fault has been 4 4 mm year 1 over the last 2400 years and a maximum of 6 2 mm year 1 during the two most recent earthquake cycles between ad 900 and ad 1812 the holocene slip rate is at least four orders of magnitude higher than the average late cretaceous and cenozoic slip rates for the reelfoot fault it would appear that there has been a quaternary change in the stress field in the central united states or the reelfoot fault is experiencing a short lived burst of seismic activity

toward a consistent model for strain accrual and release for the new madrid seismic zone central united states 1 at the heart of the conundrum of seismogenesis in the new madrid seismic zone is the apparently substantial discrepancy between low strain rate and high recent seismic moment release in this study we revisit the magnitudes of the four principal 1811 1812 earthquakes using intensity values determined from individual assessments from four experts using these values and the grid search method of bakun and wentworth 1997 we estimate magnitudes around 7 0 for all four events values that are significantly lower than previously published magnitude estimates based on macroseismic intensities we further show that the strain rate predicted from postglacial rebound is sufficient to produce a sequence with the moment release of one mmax6 8 every 500 years a rate that is much lower than previous estimates of late holocene moment release however mw6 8 is at the low end of the uncertainty range inferred from analysis of intensities for the largest 1811 1812 event we show that mw6 8 is also a reasonable value for the largest main shock given a plausible rupture scenario one can also construct a range of consistent models that permit a somewhat higher mmax with a longer average recurrence rate it is thus possible to reconcile predicted strain and seismic moment release rates with alternative models one in which 1811 1812 sequences occur every 500 years with the largest events being mmax 6 8 or one in which sequences occur on average less frequently with mmax of 7 0 both models predict that the late holocene rate of activity will continue for the next few to 10 thousand years

electromagnetic induction in the oceans and the anomalous behaviour of coastal c responses for periods up to 20 days 1 electromagnetic transfer functions at coastal sites are known to be strongly distorted by the conductivity of the seawater this ocean effect is generally considered to be small for periods greater than a few days we revise this statement by detailed and systematic model studies in the period range from 1 to 64 days with subsequent comparison of the modelled and observed electromagnetic responses the conductivity model consists of a radial symmetric 1 d section that is overlaid by a thin spherical surface shell the conductance of which is compiled using the noaa etopo topography bathymetry and map of sediment thicknesses the simulations were performed for spatial resolutions of the surface shell of 5 5 2 2 and 1 1 respectively and for two continental and oceanic underlying 1 d conductivity models the inducing source is described by the spherical harmonic p10 in dipole coordinates comparisons are made for the coastal geomagnetic observatories apia hermanus kakioka kanoya and simosato where an anomalous behaviour of the local response has been previously detected from the comparison of observed and modelled responses we conclude that peculiarities in the observed coastal responses in the period range from 1 to 20 days can be explained to a large amount by induction in the oceans we show that correction for the ocean effect results in responses that are much better interpretable by 1 d conductivity models compared to the uncorrected responses

crustal deformation in the new madrid seismic zone and the role of postseismic processes global navigation satellite system data across the new madrid seismic zone nmsz in the central united states over the period from 2000 through 2014 are analyzed and modeled with several deformation mechanisms including the following 1 creep on subsurface dislocations 2 postseismic frictional afterslip and viscoelastic relaxation from the 1811 1812 and 1450 earthquakes in the nmsz and 3 regional strain in agreement with previous studies a dislocation creeping at about 4 mm yr between 12 and 20 km depth along the downdip extension of the reelfoot fault reproduces the observations well we find that a dynamic model of postseismic frictional afterslip from the 1450 and february 1812 reelfoot fault events can explain this creep kinematic and dynamic models involving the cottonwood grove fault provide minimal predictive power this is likely due to the smaller size of the december 1811 event on the cottonwood grove fault and a distribution of stations better suited to constrain localized strain across the reelfoot fault regional compressive strain across the nmsz is found to be less than 3 10 9 yr if much of the present day surface deformation results from afterslip it is likely that many of the earthquakes we see today in the nmsz are aftershocks from the 1811 1812 new madrid earthquakes despite this conclusion our results are consistent with observations and models of intraplate earthquake clustering given this and the recent paleoseismic history of the region we suggest that seismic hazard is likely to remain significant

p and s wave tomography of the mantle beneath the united states mantle seismic structure beneath the united states spanning from the active western plate margin to the passive eastern margin was imaged with teleseismic p and s wave traveltime tomography including usarray data up to may 2014 to mitigate artifacts from crustal structure 5 40 s rayleigh wave phase velocities were used to create a 3 d starting model major features of the final p and s models include two distinct low velocity anomalies at depths of 60 300 km beneath the central and northern appalachians and passive margin the central appalachian low velocity anomaly coincides with eocene basaltic magmatism and the northern anomaly is located along the cretaceous track of the great meteor hot spot at depths of 300 700 km beneath the central and eastern u s large high velocity anomalies are inferred to be remnants of the farallon slab that subducted prior to 40 ma during the laramide orogeny

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

length scales for continental deformation in convergent divergent and strike slip environments analytical and approximate solutions for a thin viscous sheet model the deformation of a thin viscous layer that has a moving boundary is investigated for comparison with zones of deformation in the continental lithosphere exact analytical solutions for the case of a newtonian fluid and approximate solutions for the case of fluid with power law rheology show that when the imposed velocity vector is normal to the boundary congressional or extensional regime the deformation field decays away from the boundary with a characteristic length scale 1 3 to 1 10 the wavelength of the imposed boundary velocity distribution for n between 1 and 10 where n is the stress strain exponent in the rheology in contrast when the imposed velocity vector is parallel to the boundary transcurrent regime the length scale of the deformation field is approximately 4 times smaller in each case these length scales decrease approximately as n the difference in length scales arises even in the absence of any buoyancy forces acting on thickened or thinned crust such forces would modify the ratio of length scales but not sufficiently to affect this result

paleoseismology of the southeastern reelfoot rift in western tennessee and implications for intraplate fault zone evolution 1 the southeastern reelfoot rift margin is expressed as a series of collinear scarps across western tennessee and shallow s wave reflection profiles coring and trenches show late quaternary faulting along this lineament at the north end our seismic profile and coring suggest 1 5 m of wisconsin down to the nw faulting coring and trenches across the central lineament show 3 m of up to the nw and 8 15 m of right lateral late wisconsin holocene faulting averaged late quaternary right lateral slip has been 0 85 0 37 mm yr on one fault plane at this locality suggesting that total slip across all southeast rift margin fault planes may accommodate much of the regional strain our seismic profile at this site shows up to the west faulting of eocene strata folded down to the west indicating structural inversion seismic profiles electrical surveys coring and a trench across the southern lineament segment show holocene faulting trenching exposed a vertical fault with 0 5 m of late holocene down to the nw displacement nearby coring and electrical surveys show 0 5 m of late holocene up to the nw faulting we conclude the southeastern reelfoot rift margin is a right lateral system with high angle faults showing both up to the nw and down to the nw separations age constraints permit an earthquake circa 2500 to 2000 years b p on the southwestern central segment of this fault system that ruptured 80 km we suggest the northeastern segment of the southeastern rift margin turned off in holocene when the reelfoot step over thrust turned on

variations in apparent mixing efficiency in the north atlantic central water abstract microstructure data from the north atlantic tracer release experiment natre are presented providing detailed profiles of the thermal variance in the upper 360 m of the canary basin for the fall and spring seasons the osborn cox model is used to compute the diffusivity kt the diffusivity for the depth range 240 340 m is found to be 1 0 0 04 10 5 m2 s 1 in the fall and 2 2 0 1 10 5 m2 s 1 in the spring in good agreement with dye inferred diffusivities at similar depths measured turbulent kinetic energy tke dissipation rates were found to be contaminated by hydrodynamic noise so the osborn dissipation method was not used to compute k however data segments for which the tke dissipation rate e was large enough to be unaffected by noise were used to compute the apparent mixing efficiency d the computed d values are used to investigate variations in apparent mixing efficiency with respect to density ratio r and turbulence reynolds number e n2 in an attempt to eluci

crustal structure beneath the wabash valley seismic zone from the joint inversion of receiver functions and surface wave dispersion implications for continental rifts and intraplate seismicity

descent of the ancient farallon slab drives localized mantle flow below the new madrid seismic zone 1 the great earthquake sequence that occurred in the central mississippi river valley during the winter of 1811 1812 is unprecedented in the historical record of seismicity within stable continental plate interiors we show using viscous flow models based on high resolution seismic tomography that the descent of the ancient farallon slab into the deep mantle beneath central north america induces a highly localized flow directly below the new madrid seismic zone nmsz this localization arises because of structural variability in the farallon slab and the low viscosity of the sub lithospheric upper mantle and it represents a heretofore unrecognized and possibly significant driving mechanism for the enigmatic intraplate seismicity

mapping p wave azimuthal anisotropy of the new madrid seismic zone abstract we determine a detailed tomographic model of 3 d p wave azimuthal anisotropy in the crust and upper mantle beneath the new madrid seismic zone nmsz using a large number of high quality arrival time data of 877 local earthquakes and relative travel time data of 1511 teleseismic events recorded at 62 usarray stations the tomographic inversion is conducted by using a limited memory broyden fletcher goldfarb shanno algorithm for bound constrained optimization because damping regularization is not required in this inversion scheme the obtained results are less affected by human subjectivity a prominent low velocity zone with ne sw fast velocity directions of azimuthal anisotropy down to 160 km depth beneath the nmsz is imaged which may reflect upwelling mantle materials probably triggered by passage of the bermuda hotspot from the southwest of the mississippi embayment our results also reveal a subhorizontal high velocity anomaly around the moho discontinuity with a width of 150 200 km confirming the existence and defining the geometry of a lens shaped high density rift pillow near the moho which may result from the mantle upwelling nw se fast velocity directions of azimuthal anisotropy appear in the lower crust beneath the mississippi embayment this result may reflect recent weakening of a detachment fault above the rift pillow which drives recurrently sinking of the rift pillow and as a consequence causing large repeating intraplate earthquakes in the nmsz

reconciling short recurrence intervals with minor deformation in the new madrid seismic zone at least three great earthquakes occurred in the new madrid seismic zone in 1811 and 1812 estimates of present day strain rates suggest that such events may have a repeat time of 1000 years or less paleoseismological data also indicate that earthquakes large enough to cause soil liquefaction have occurred several times in the past 5000 years however pervasive crustal deformation expected from such a high frequency of large earthquakes is not observed this suggests that the seismic zone is a young feature possibly as young as several tens of thousands of years old and no more than a few million years old

shear and richardson number in a mode water eddy abstract measurements of stratification and shear were carried out as part of the eddies tracer release experiment in mode water eddy a4 during the summer of 2005 these measurements were accomplished using both shipboard instrumentation and a drifting mooring a strong relationship between shear intensity and distance from the center of the eddy a4 was observed with the shipboard adcp diapycnal diffusivity at the sf6 tracer isopycnal prior to and during the release was estimated from the drifting mooring to be 2 9 10 6 m2 s 1 diffusivity increased by an order of magnitude to 3 2 10 5 m2 s 1 during the period of the final tracer survey in early september which was similar to the value estimated from the tracer analysis for the whole experiment 3 5 10 5 m2 s 1 ledwell j r mcgillicuddy jr d j anderson l a 2008 nutrient flux into an intense deep chlorophyll layer in a mode water eddy deep sea research ii this issue doi 10 1016 j dsr2 2008 02 005

a basinwide estimate of vertical mixing in the upper pycnocline spreading of bomb tritium in the north pacific ocean abstract the vertical diffusivity kv in the upper half kilometer of the north pacific subtropical pycnocline is estimated from observations of the spreading rate of anthropogenic tritium the calculation is based on approximately 300 ocean tritium profiles made since tritium was introduced to the atmosphere via bomb testing in the 1960s the data coverage does not permit detailed mapping of tritium penetration especially in the sparsely sampled western pacific for this reason and to minimize advective effects the spreading rate is averaged within the closed streamlines of the subtropical gyre spanning 5 n to 40 n the result kv 1 5 0 7 10 5 m2 s 1 is consistent with inferences from microstructure and purposefully released tracer measurements in the north atlantic confirming that the spatially averaged rate of mixing in the upper pycnocline is substantially lower than the canonical munk estimate for the lower pycnocline