seismic evidence for an earthquake nucleation phase near source observations show that earthquakes initiate with a distinctive seismic nucleation phase that is characterized by a low rate of moment release relative to the rest of the event this phase was observed for the 30 earthquakes having moment magnitudes 2 6 to 8 1 and the size and duration of this phase scale with the eventual size of the earthquake during the nucleation phase moment release was irregular and appears to have been confined to a limited region of the fault it was characteristically followed by quadratic growth in the moment rate as rupture began to propagate away from the nucleation zone these observations suggest that the nucleation process exerts a strong influence on the size of the eventual earthquake

lack of enhanced preservation of organic matter in sediments under the oxygen minimum on the oman margin the impingement of oxygen minima on continental margins is widely thought to promote the accumulation of sedimentary facies enriched in well preserved organic matter it is shown here however that such a relationship does not clearly apply to the productive oman margin in the arabian sea which hosts one of the most severe oxygen minima in the oceans measurements made on the 0 1 cm depth interval from fourteen box cores collected from the outer shelf upper continental slope area off oman show that 1 1 deposited organic matter is overwhelmingly of marine origin 2 2 there is no significant correlation between the abundance of sedimentary organic carbon corg and the bottom water o2 concentration 3 3 there is no relation between the sedimentary corg n ratio and bottom water o2 4 4 there is no correlation between the hydrogen index hi of the organic matter and bottom water oxygen there are however significant correlations between the corg n ratio and the i corg cr al and zr al ratios as well as between the corg n ratio and the hydrogen index these relationships are interpreted as being the result of winnowing and attendant reworking which produce a progressive increase in the corg n ratio and a decrease in the hi and the i corg ratio as organic matter becomes progressively degraded commensurate rises in the cr al and zr al ratios result from the increased proportion of heavy minerals associated with winnowed lag deposits overall these data suggest that the bottom water oxygen concentration has little effect in governing either the distribution or the degree of preservation of organic matter on this margin thus the generally high but spatially variable corg content of the sediments on the oman margin may not reflect the occurrence of an oxygen minimum but instead be the result of a high settling flux of organic matter supported by monsoon driven upwelling and post depositional redistribution of the organic material by hydrodynamic influences

rates of formation of modern phosphorite off western mexico abstract phosphorite deposition represents a significant but poorly constrained sink in the marine p cycle this study establishes conclusively that phosphorite forms today throughout a widespread region along the upper continental slope off southern baja california mexico in this region o2 depleted 5 50 m bottomwater overlies the seafloor and a narrow range of suboxic to anoxic redox conditions prevail in the modern 0 18 000 yr old near surface 0 50 cm organic c rich 2 10 wt sediment interstitial gradients of total alkalinity ta and inferred effects of organic matter decay correlate directly with sedimentation rates calculated from 14c dating other porewater and solid phase data indicate that francolite currently precipitates authigenically at each of five study sites moderately high concentrations 20 100 m of dissolved inorganic phosphate dip produced in the upper 5 cm of sediment sustain francolite growth at slightly greater depths while the necessary f diffuses into the sediment from above neither organic matter decay nor fish bone dissolution seem capable of producing the steep near surface gradients of dip measured in this study at some sites dip production coincides spatially with the reversible production of dissolved fe2 but thorough evaluation of the possible link between these processes requires further study the mechanism responsible for the near surface enrichment of interstitial dip thus remains unidentified box cores from each site contain an unreworked sequence of up to six thin 1 3 cm discrete francolite rich horizons several of which qualify as true phosphorite 50 wt francolite estimated diffusional burial fluxes of inorganic p and f range respectively from 2 40 g cm 2 y 1 and 4 12 g cm 2 y 1 or the equivalent in francolite of 16 260 g cm 2 y 1 these present day accumulation rates lie within the range of those determined for several ancient phosphorite deposits and for the extensively studied region of modern phosphorite deposition off peru

lack of evidence for enhanced preservation of sedimentary organic matter in the oxygen minimum of the gulf of california a test of the hypothesis that hydrogen rich organic matter accumulates preferentially in sediments under oxygen minima has been carried out by examining the composition of laminated and homogeneous diatomaceous sediments of the guaymas basin in the gulf of california the laminated sediments occur where an intense oxygen minimum intersects the slope around the margins of the basin above and below the depth of the oxygen minimum the sediments are homogenized by a burrowing infauna the two sediment types have very similar bulk chemical and mineral compositions and the organic matter is overwhelmingly marine the total organic carbon contents and rock eval parameters hydrogen and oxygen indices of the two sediment types are not significantly different whereas a road organic carbon maximum in the homogeneous sediments is centered at roughly 1200 m depth on the slope of the basin this maximum lies deeper than the core of the oxygen minimum thus both the concentration and the nature of the organic matter in the sediments of this highly productive marginal marine basin are not related to the concentration of dissolved oxygen in the bottom waters

the role of episodic overturn in generating the surface geology and heat flow on enceladus saturn s satellite enceladus shows higher heat loss than expected and a wide range of surface ages numerical simulations indicate that occasional catastrophic overturn events could be responsible for both observations by recycling portions of the icy lid to the interior which would cause transiently enhanced heat loss

plate motion on europa and nonrigid behavior of the icy lithosphere the castalia macula region we have developed a generalized quantitative technique for determining the finite pole of rotation between two rigid plates and use it to critically examine differing reconstructions of a region surrounding a prominent dark spot on europa castalia macula this region is located near the equator of europa s trailing hemisphere and has been suggested as a site where crustal convergence may have occurred previous reconstructions of the region have indicated that a ridge set and or a band like complex that define a collection of tectonic plates in the region accommodated surface contraction however a critical examination of the differences between these reconstructions has been complicated by the lack of a finite pole of rotation for the plates involved in either reconstruction we have applied our modeling technique coupled with a detailed examination of the morphology and cross cutting relationships involving this ridge set and band like complex to determine if a unique reconstruction exists for several tectonic plates in this region the cross cutting relationships involving the ridge set also allow us to test the general assumption that plates behave rigidly on europa assuming rigid behavior our results suggest that a unique reconstruction does exist indicating the ridge set accommodated surface contraction however analysis performed to test the assumption of plate rigidity indicates that one or more of the plates in the region did not behave rigidly this does not rule out surface contraction along the ridge set but does indicate that a component of nonrigid behavior must be considered

210pb derived sediment accumulation and mixing rates for the washington continental slope abstract total sediment accumulation rates g yr in four canyons indenting the washington continental shelf are about twice those on the nearby open slope even though canyons comprise less than 40 of the depositional area of the entire slope this is because 210 pb derived sediment accumulation rates g cm 2 yr for astoria and quinault canyons are 3 4 times greater than those for willapa and grays canyons and intermediate open slope areas supporting the hypothesis that columbia river derived solids are preferentially transported to these two canyons quinault canyon currently appears to be accumulating solids at rates much greater than its low long term accumulation rate which is attributed to turbidity currents transporting solids down this canyon roughly every 500 years the following four different approaches all show that mixing may cause 210 pb derived sediment accumulation rates for slope and canyon sediments to be 2 3 times higher than true accumulation rates 1 comparison between accumulation rates calculated from 210 pb profiles with and without surface zones in which mixing has obviously affected 210 pb distributions 2 comparison between 210 pb derived and independently determined long term accumulation rates 3 comparison between 210 pb derived accumulation and modern columbia river discharge rates 4 comparison between two and three layer steady state one dimensional diffusion advection decay models mixing coefficients for surface sediments calculated using a one dimensional two layer vertical eddy diffusion model show large differences within the region with most in the 3 30 cm 2 yr range mixing rates are several times larger in canyon sediments from 0 2 1 1 km water depth probably due to physical reworking processes erroneously neglected mixing at depth of no more than 5 of that calculated for the surface mixed layer can cause the 2 3 fold calculated overestimation of true sediment accumulation rates and 2 4 fold calculated underestimation of true mixing coefficients

can earth like plate tectonics occur in ocean world ice shells abstract the outer h2o ice shell of europa a galilean satellite of jupiter thought to exhibit a global interior water ocean shows evidence for plate like surface motions along tectonic boundaries in this study we investigate whether forces similar to those that drive plate tectonics on earth may drive plate motions on icy satellites we focus on whether the forces of slab pull and ridge push driven by thermal and compositional buoyancy can overcome the mechanical forces resisting plate motions to sustain interaction between surface ice and the interior ocean we find that earth like plate tectonics is implausible within the outer ice shell of icy satellites because the forces resisting self sustaining plate cycling are of significantly greater order than the driving forces in the case of ganymede and triton if it has a thick shell plate tectonics would require warm low viscosity slabs to remain intact for timescales that are orders of magnitude longer than those of interior convection

a new method of quickly estimating epicentral distance and magnitude from a single seismic record rapid estimation of the epicentral distance and magnitude is of fundamental importance for early earthquake detection and warning systems we present a novel method of estimating the epicentral distance from a single seismic record in a short amount of time in order to quantitatively evaluate the difference in observed seismic waveforms we introduced a simple function with the form of bt exp at and determined a and b in terms of the least squares method by fitting this function to the initial part of the waveform envelope we have found that log b is inversely proportional to log where is the epicentral distance this relation holds true regardless of the earthquake magnitude by using this relation we can roughly estimate the epicentral distance nearly immediately after the p wave arrival then we can readily estimate the magnitude from the maximum amplitude observed within a given short time interval after the p wave arrival by using an empirical magnitude amplitude relation that includes the epicentral distance as a parameter manuscript received 11 january 2002

an automatic processing system for broadcasting earthquake alarms in japan most of the damage caused by large earthquakes is concentrated within areas less than about 100 km from the focal regions we have developed an earthquake alarm system that determines earthquake parameters within a few seconds of the p wave s arrival at the closest station and then transmits the earthquake information before the s wave arrival in areas of possible serious earthquake damage since an earthquake alarm system requires the determination of reliable earthquake parameters as quickly as possible it is unreasonable to wait until waveform data from numerous stations have been collected for analysis for this reason we have developed a novel method of determining the hypocentral location by using the arrival times for only a few stations as well as the lack of p wave arrivals at other stations at a given time the use of not yet arrived data makes it possible not only to determine reliable hypocenter parameters within a few seconds but also to detect erroneous arrival time readings and remove them automatically since the available waveform data increases with time the system was designed to redetermine earthquake parameters every second our method was deployed in a real time system starting in july 2002 the real time system locates 10 20 events per day which includes a few felt earthquakes occurring in and around japan it was shown from the waveform data for about 100 felt earthquakes that almost all the events except those far from the network could be located within a few seconds when most of seismic energy has not even arrived at some stations close by we recently started widespread broadcasting of earthquake information in real time by using a satellite transmission system

sedimentary organic matter preservation a test for selective degradation under oxic conditions the authors report here a test of the hypothesis that the extent of organic matter preservation in continental margin sediments is controlled by the average period accumulating particles reside in oxic porewater immediately beneath the water sediment interface oxygen penetration depths organic element compositions and mineral surface areas were determined for 16 sediment cores collected along an offshore transect across the washington continental shelf slope and adjacent cascadia basin individual amino acid sugar and pollen distributions were analyzed for a 11 to 12 cm horizon from each core and sup 14 c based sediment accumulation rates and stable carbon isotope compositions were determined from depth profiles within a subset of six cores from representative sites sediment accumulation rates decreased and dissolved o sub 2 penetration depths increased offshore along the sampling transect as a result oxygen exposure times oet increased seaward from decades mid shelf and upper slope to more than a thousand years outer cascadia basin organic contents and compositions were essentially constant within individual sediment cores but varied consistently with location in particular organic carbon surface area ratios decreased progressively offshore and with increasing oet three independent compositional parameters demonstrated that the remnant organic matter in farther offshore sediments is more degraded both concentrationmore and compositional patterns indicated that sedimentary organic matter exhibits a distinct and reproducible oxic effect oet helps integrate and explain organic matter preservation in accumulating continental margin sediments and hence provides a useful tool for assessing transfer of organic matter from the biosphere to the geosphere less

initial rupture of earthquakes in the 1995 ridgecrest california sequence close examination of the p waves from earthquakes ranging in size across several orders of magnitude shows that the shape of the initiation of the velocity waveforms is independent of the magnitude of the earthquake a model in which earthquakes of all sizes have similar rupture initiation can explain the data this suggests that it is difficult to estimate the eventual size of an earthquake from the initial portion of the waveform previously reported curvature seen in the beginning of some velocity waveforms can be largely explained as the effect of anelastic attenuation thus there is little evidence for a departure from models of simple rupture initiation that grow dynamically from a small region the results of this study indicate that any precursory radiation at seismic frequencies must emanate from a source region no larger than the equivalent of a m0 5 event i e a characteristic length of 10 m the size of the nucleation region for magnitude 0 to 5 earthquakes thus is not resolvable with the standard seismic instrumentation deployed in california

friction of ice on ice 1 new measurements have been made of the friction coefficient of freshwater polycrystalline ice sliding slowly 5 10 8 to 1 10 3 m s 1 upon itself at temperatures from 98 to 263 k under low normal stresses 98 kpa sliding obeys coulomb s law the shear stress is directly proportional to the normal stress across the interface while cohesion offers little contribution to frictional resistance the coefficient of kinetic friction of smooth surfaces varies from k 0 15 to 0 76 and at elevated temperatures 223 k exhibits both velocity strengthening at lower velocities 10 5 to 10 4 m s 1 and velocity weakening at higher velocities strengthening and weakening are attributed to creep deformation of asperities and localized melting respectively at intermediate temperatures of 173 and 133 k the kinetic coefficient appears to not exhibit significant dependence upon velocity however at the low temperature of 98 k the coefficient of kinetic friction exhibits moderate velocity strengthening at both the lowest and the highest velocities but velocity independence over the range of intermediate velocities no effect was detected of either grain size or texture over the range of roughness 0 4 10 6 m ra 12 10 6 m a moderate effect was detected where k ra0 08 slide hold slide experiments revealed that the coefficient of static friction increases by an amount that scales logarithmically with holding time implications of the results are discussed in relation to shearing across tiger stripe faults within the icy crust of saturn s enceladus sliding of the arctic sea ice cover and brittle compressive failure of cold ice

friction of ice the frictional strength of ice seems to be well below that for all other rocks triaxial testing of cylinders of pure water ice containing a 45 inclined sawcut at temperatures of 77 t 115 k and confining pressures 0 1 p 250 mpa reveals the frictional laws 0 20 n 8 3 mpa for p 10 mpa and 0 55 n 1 0 mpa for p 5 mpa where and n are the shear and normal stresses on the sawcut surface friction is independent of t and over the one order of magnitude tested of average sliding velocity the sliding behavior is invariably stick slip examination of sliding surfaces shows only minor gouge development a curious arrangement of densely packed short vertical fractures occurs primarily in samples with anomalously low strength these results apply to models of near surface tectonic activity on jupiter s three large icy moons europa callisto and ganymede they indicate for example that a global expansion on ganymede of 3 linear percent will cause extensional movement on preexisting faults at depths to 7 3 km

active formation of chaos terrain over shallow subsurface water on europa the galileo spacecraft revealed a number of chaos regions on jupiter s moon europa where the surface terrain appears to have been disrupted from below in many places the surface contains sharp edged blocks or rafts of ice that have at some point been flipped or rotated some characteristics of these regions have been hard to explain such as the fact that the archetypal conamara chaos stands above its surroundings and contains matrix domes schmidt et al apply lessons learned from analogous processes within earth s subglacial volcanoes and ice shelves to an analysis of archival data that suggests chaos terrain forms above liquid water lenses that are perched only 3 kilometres deep within the ice shell the data suggest that ice water interactions and freeze out give rise to the varied morphology of chaos terrains implying that more water is involved than has been previously appreciated for instance the sunken topography of thera macula a large chaos area may indicate that europa is actively resurfacing over a lens comparable in volume to north america s great lakes europa the innermost icy satellite of jupiter has a tortured young surface1 2 3 4 and sustains a liquid water ocean1 2 3 4 5 6 below an ice shell of highly debated thickness1 2 3 4 5 7 8 9 10 quasi circular areas of ice disruption called chaos terrains are unique to europa and both their formation and the ice shell thickness depend on europa s thermal state1 2 3 4 5 7 8 9 10 11 12 13 14 15 16 17 no model so far has been able to explain why features such as conamara chaos stand above surrounding terrain and contain matrix domes10 18 melt through of a thin few kilometre shell3 7 8 is thermodynamically improbable and cannot raise the ice10 18 the buoyancy of material rising as either plumes of warm pure ice called diapirs1 9 10 11 12 13 14 15 or convective cells16 17 in a thick 10 kilometres shell is insufficient to produce the observed chaos heights and no single plume can create matrix domes10 18 here we report an analysis of archival data from europa guided by processes observed within earth s subglacial volcanoes and ice shelves the data suggest that chaos terrains form above liquid water lenses perched within the ice shell as shallow as 3 kilometres our results suggest that ice water interactions and freeze out give rise to the diverse morphologies and topography of chaos terrains the sunken topography of thera macula indicates that europa is actively resurfacing over a lens comparable in volume to the great lakes in north america

early diagenetic processes fluxes and reaction rates in sediments of the south atlantic abstract porewaters recovered from sediment cores gravity corers box corers and multicorers from various subrogions of the south atlantic amazon river mouth equatorial upwelling congo river mouth benguela coastal upwelling area and angola basin were investigated geochemically objectives included determination of eh ph oxygen nitrate sulfate alkalinity phosphate ammonium fluoride sulfide cal mg sr fe mn and si in order to quantify organic matter diagenesis and related mineral precipitation and dissolution processes porewater profiles from the eastern upwelling areas of the south atlantic suggest that sulfate reduction in the deeper parts of the sediment may be attributed mainly to methane oxidation whereas organic matter degradation by sulfate reduction is restricted to the near surface sediments further a prominent concentration gradient change of sulfate and related mineralization products occurred typically in the upwelling sediments at a depth of 4 to 8 m far below the zone of bioturbation or bioirrigation because other sedimentological reasons seem to fail as explanations an early diagenetic sulfide oxidation to sulfate within the anoxic environment is discussed porewater profiles from the sediments of the amazon fan area are mainly influenced by reactions with fe iii phases the remarkable linearity of the concentration gradients of sulfate supports the idea of distinct reaction layers in these sediments in contrast to the upwelling sediments the sulfate gradient develops from the sediment surface to a sulfate reduction zone at a depth of 5 3 m probably because a reoxidation of sulfide is prevented by the reaction with iron oxides and the formation of iron sulfide minerals a comparison of organic matter degradation rates from the different areas of the south atlantic show the expected relationship to primary productivity oxygen is the dominating oxidant whereas organic matter degradation by nitrate mn iv and probably also fe iii reduction is several orders of magnitude lower sulfate reduction is quantitatively of similar importance as oxygen respiration in the organic rich sediments but may also include methane oxidation reoxidation of ammonium and mn2 by oxygen or nitrate do not alter significantly the estimation of organic matter degradation in the oxic zone but may contribute to the nitrate reduction in suboxic layers

dynamical weakening of faults by acoustic fluidization when a fault slips seismically some of the energy released may excite strong short wavelength vibrations near the fault core such vibrations can temporarily reduce the normal stress on the fault allowing it to slip at lower shear stresses than predicted by laboratory coefficients of friction this phenomenon of acoustic fluidization provides an alternative to theories that invoke pressurized fluids as an explanation for why some faults appear to be so weak

plate force due to bending at subduction zones 1 bending of the lithosphere at subduction zones generates stresses that contribute to the force balance on plates we determine the net horizontal force on subducting plates by imposing a simple kinematic description of strain in the equations that govern the mechanical equilibrium of a thin viscous sheet the magnitude of the force depends on the velocity thickness and curvature of the subducting plate using representative values for old oceanic lithosphere we obtain an effective horizontal force that is nearly 40 of the buoyancy force due to slabs in the upper mantle

porosity and salt content determine if subduction can occur in europa s ice shell motivated by recent evidence for subduction in europa s ice shell we explore the geophysical feasibility of this process here we construct a simple model to track the evolution of porosity and temperature within a slab that is forced to subduct we also vary the initial salt content in europa s ice shell and determine the buoyancy of our simulated subducting slab we find that porosity and salt content play a dominant role in determining whether the slab is nonbuoyant and subduction in europa s ice shell is actually possible generally we find that initially low porosities and high salt contents within the conductive lid are more conducive to subduction if salt contents are laterally homogenous and europa has a reasonable surface porosity of 0 0 1 the conductive portion of europa s shell must have salt contents exceeding 22 for subduction to occur however if salt contents are laterally heterogeneous with salt contents varying by a few percent subduction may occur for a surface porosity of 0 0 1 and overall salt contents of 5 thus we argue that under plausible conditions subduction in europa s ice shell is possible moreover assuming that subduction is actively occurring or has occurred in europa s recent past provides important constraints on the structure and composition of the ice shell

evidence for separation across a gray band on europa abstract thynia linea a gray band on europa is found to be a 25 km wide and 900 km long region of lithospheric separation which has been infilled by relatively dark material roughly a dozen older features and the cuspate segments that form its outline appear to have been displaced across its width displacement azimuths indicate a best fit pole of opening near 79 s 200 w however displacement magnitude decreases toward either end of the gray band indicating that it is more akin to a tear in a nonrigid europan lithosphere opening was in response to nw se directed tensile stress in accord with the stress predictions of nonsynchronous rotation observations of thynia linea are consistent with a laterally mobile brittle lithosphere decoupled from ductile or liquid material below as previously suggested to account for opening of wedge shaped bands in europa s antijovian region lithospheric separation and contemporaneous emplacement of new material offers a possible volcano tectonic scenario for resurfacing europa if this process is ongoing resurfacing can be accomplished on a time scale consistent with the satellite s surface age if one gray band or zone of wedge shaped bands forms every 10 3 10 4 yr and becomes unrecognizable with age this would imply that features on europa brighten with age as through continuous deposition of frost onto the surface

pathways of organic carbon oxidation in three continental margin sediments we have combined several different methodologies to quantify rates of organic carbon mineralization by the various electron acceptors in sediments from the coast of denmark and norway rates of nh4 and sigma co2 liberation sediment incubations were used with o2 penetration depths to conclude that o2 respiration accounted for only between 3 6 17 4 of the total organic carbon oxidation dentrification was limited to a narrow zone just below the depth of o2 penetration and was not a major carbon oxidation pathway the processes of fe reduction mn reduction and sulfate reduction dominated organic carbon mineralization but their relative significance varied depending on the sediment where high concentrations of mn oxide were found 3 4 wt mn only mn reduction occurred with lower mn oxide concentrations more typical of coastal sediments fe reduction and sulfate reduction were most important and of a similar magnitude overall most of the measured o2 flux into the sediment was used to oxidized reduced inorganic species and not organic carbon we suspect that the importance of o2 respiration in many coastal sediments has been overestimated whereas metal oxide reduction both fe and mn reduction has probably been well underestimated

magmatic and tectonic extension at mid ocean ridges 1 controls on fault characteristics we use 2 d numerical models to explore the thermal and mechanical effects of magma intrusion on fault initiation and growth at slow and intermediate spreading ridges magma intrusion is simulated by widening a vertical column of model elements located within the lithosphere at a rate equal to a fraction m of the total spreading rate i e m 1 for fully magmatic spreading heat is added in proportion to the rate of intrusion to simulate the thermal effects of magma crystallization and the injection of hot magma into the crust we examine a range of intrusion rates and axial thermal structures by varying m spreading rate and the efficiency of crustal cooling by conduction and hydrothermal circulation fault development proceeds in a sequential manner with deformation focused on a single active normal fault whose location alternates between the two sides of the ridge axis fault spacing and heave are primarily sensitive to m and secondarily sensitive to axial lithosphere thickness and the rate that the lithosphere thickens with distance from the axis contrary to what is often cited in the literature but consistent with prior results of mechanical modeling we find that thicker axial lithosphere tends to reduce fault spacing and heave in addition fault spacing and heave are predicted to increase with decreasing rates of off axis lithospheric thickening the combination of low m particularly when m approaches 0 5 as well as a reduced rate of off axis lithospheric thickening produces long lived large offset faults similar to oceanic core complexes such long lived faults produce a highly asymmetric axial thermal structure with thinner lithosphere on the side with the active fault this across axis variation in thermal structure may tend to stabilize the active fault for longer periods of time and could concentrate hydrothermal circulation in the footwall of oceanic core complexes

chemical evidence of differential particle dispersal in the southern washington coastal environment aliphatic hydrocarbons cupric oxide oxidation products of lignin and polycyclic aromatic hydrocarbons pah were analyzed by capillary gas chromatography in sediments from the southern washington continental shelf and slope the concentration of diploptene relative to plantwax n alkanes increased systematically in surface sediments with distance offshore along east west transects of the study area and remained constant in surface sediments along the midshelf silt deposit analogous trends were also observed for the concentration of cinnamyl phenols relative to vanillyl phenols and total methylphenanthrenes relative to phenanthrene these changes in sedimentary composition are evidence that diploptene from some terrestrial source lignin characteristic of non woody vascular plant tissue and a fossil organic material contained within weathered rock debris disperse across the washington continental shelf and slope in geographic patterns distinct from that for other river derived chemically related materials the compositional variations are explained by the particulate associations of the land derived chemicals and differential hydraulic dispersion of their respective carrier particles after discharge at the mouth of the columbia river

numerical simulations of convection in europa s ice shell implications for surface features 1 europa s icy surface displays numerous small 5 to 30 km diameter pits spots and uplifts that have been suggested to result from convection in the ice shell to test this hypothesis we present numerical simulations of convection in europa s ice shell including temperature dependent viscosity and tidal heating ice shells 15 and 50 km thick are considered consistent with several estimates of the shell thickness on europa the convection produces deep pits consistent with some of the observed features when the lithospheric viscosity is 103 105 times greater than that of the underlying ice but greater viscosity contrasts lead to topography insufficient to explain the observed pits if ductile creep is the only deformation mechanism these results imply that convection cannot produce the observed pits and uplifts because in that case the predicted surface viscosity exceeds that of the warm underlying ice by at least a factor of 1010 however the strength of europa s surface may be low enough for plastic deformation to play a role in the convection opening the possibility that convection could produce some of europa s pits for plausible viscosities 1013 pa s at the melting temperature the pits are 100 300 m deep and 10 20 km in diameter greater or lesser viscosities lead to wider or narrower pits respectively none of our simulations produced isolated uplifts of any diameter however so these probably formed by another mechanism the convection can induce surface stresses 1 bar which exceeds the inferred strength of europa s crust and indicates the likelihood of surface disruption the maximum tidal heat fluxes that can be transported by convection at realistic ice viscosities are 0 05 0 07 w m 2

stresses generated in cooling viscoelastic ice shells application to europa 1 a cooling viscoelastic ice shell overlying an ocean develops stresses due to two effects thermal contraction of the ice due to cooling and the expansion of the shell due to the ice water volume change the former effect generates near surface compression and deeper extension the second effect generates extension only in both cases stresses are smaller at depth due to viscous creep the resulting combined stresses are extensional except at shallow 1 km depths in thin ice shells for ice shells thicker than 45 km stresses are extensional throughout the extensional stresses exceed 10 mpa for shells thicker than 20 km and thus dominate all other likely sources of stress as long as shell cooling occurs the dominantly extensional nature of the stresses may help to explain the puzzling lack of compression observed on europa and other large icy satellites however after 100 myr of conductive cooling the maximum theoretical elastic strains for europa are 0 35 which are probably insufficient to explain the total amount of observed extension

gulf of california biogeographic regions based on coastal zone color scanner imagery topographically the gulf of california is divided into a series of basins and trenches that deepen to the south maximum depth at the mouth is greater than 3000 m most of the northern gulf is less than 200 m deep the gulf has hydrographic features conducive to high primary productivity upwelling events have been described on the basis of temperature distributions at the eastern coast during winter and spring and at the western coast during summer tidal amplitude may be as high as 9 m in the upper gulf on the basis of discrete phytoplankton sampling the gulf was previously divided into four geographic regions this division took into consideration only the space distribution taxonomic composition and abundance of microphytopla nkton with the availability of the coastal zone color scanner czcs imagery we were able to in clude the time variability of pigments to make a more detailed biogeographic division of the gulf with weekly composites of the imagery we generated time series of pigment concentrations for 33 locations throughout the gulf and for the whole life span of the czcs the time series show a clear seasonal variation with maxima in winter and spring and minima in summer the effect of upwelling at the eastern coast is clearly evident with high pigment concentrations the effect of the summer upwelling off the baja california coast is not evident in these time series time series from locations on the western side of the gulf also show maxima in winter and spring that are due to the eddy circulation that brings upwelled water from the eastern side principal component analysis was applied to define 14 regions ballenas channel between angel de la guarda and baja california and the upper gulf always appeared as very distinct regions some of these 14 regions relate to the geographic distributions of important faunal groups including the benthos or their life cycles for example the upper gulf is a place for reproduction and the nursery of many fish species marine mammals and birds are specially abundant in ballenas channel sardine spawning mostly occurs in the central gulf in spring and shrimp are abundant off mainland mexico

the formation of sedimentary strata in an allochthonous shelf environment the washington continental shelf nittrouer c a and sternberg r w 1981 the formation of sedimentary strata in an allochthonous shelf environment the washington continental shelf mar geol 42 201 232 models of fine scale 120 m shelf regions the inner shelf sand and mid shelf sandy silt are the predominant accretionary deposits on the shelf important sedimento logical observations obtained by boxcoring the upper 1 2 m of these deposits are a progressive decrease in grain size with distance from the columbia b downward coarsening within the seabed c loss of distinct sedimentary structure homogenization of sediment with distance from the columbia non dimensional parameters based on sediment transport and benthic biological studies can be used to relate rates of sediment mixing re working to rates of accumulation these parameters have the potential for quantitative prediction of strata formation from measurements of active processes physical and biological mixing can be assumed to remain relatively constant in an alongshelf direction on the washington shelf and accumulation rate is known from pb 210 geochronology to decrease away from the columbia together these factors tend to increase the ratio of mixing to accumulation with distance from the columbia and predict the sedimentological observations listed above

on the origins of band topography europa we use stereo derived topography of extensional bands on europa to show that these features can be elevated by 100 150 m with respect to the surroundings and that the positive topography sometimes extends beyond the band margins lateral variations in shell thickness cannot maintain the observed topography for timescales greater than 0 1 myr lateral density variations can maintain the observed topography indefinitely mean density contrasts of 5 and 50 kg m 3 are required for shell thicknesses of 20 and 2 km respectively density variations caused by temperature contrasts require either present day heating or that bands are young features 1 myr old stratigraphic analyses suggest that these mechanisms are unlikely the observation that bands form from ridges may be explained by an episode of shear heating on ridges weakening the ridge area and leading to strain localization during extension fracture porosity is likely to persist over myr timescales in the top one third to one quarter of the conductive part of the ice shell lateral variations in this porosity of order 20 are the most likely mechanism for producing band topography if the ice shell is thin 2 km porosity variations of 2 or less are required if the shell is thicker 20 km if the ice shell is thick lateral variations in salt content are a more likely mechanism warm ice will tend to lose dense low melting temperature phases and be buoyant relative to colder salt rich ice thus lateral density variations will arise naturally if bands have been the sites of either localized heating or upwelling of warm ice during extension

quick and reliable determination of magnitude for seismic early warning this article reports efforts toward using real time earthquake monitoring by the taiwan central weather bureau to meet the needs of seismic early warning twenty three sets of strong motion data from moderate earthquakes m 5 0 in the taiwan area are used to demonstrate the feasibility of this goal for earthquakes larger than ml 5 epicenters can be reliably determined in about 15 sec after the arrival of the p wave at the nearest station the earthquake magnitude m l cannot be determined in the same time frame due to incomplete recording of shear waves at some stations however the magnitude based on the first 10 sec of signal ml10 can be related to ml as follows ml 1 28 ml10 0 85 0 13 our results show that the real time strong motion system routinely used by the cen tral weather bureau can be used to determine epicenters and magnitudes in about 30 sec after occurrence of earthquakes in taiwan such information hopefully can be used to reduce damage to society

size dependent microearthquake initiation inferred from high gain and low noise observations at nikko district japan we deployed a temporary seismic station at a low noise site in the nikko district japan in november 1993 and analyzed velocity waveforms of 17 microearthquakes 0 3 m 2 1 with small focal distances 11 km we compared the initial parts of the p wave velocity seismograms on the same absolute scale after correcting for the effects of geometrical spreading and radiation pattern we find that the p wave radiated from an earthquake of a larger final magnitude has a greater amplitude than that from a smaller earthquake even at a very early stage when the p wave from the smallest event is still rising with an increasing slope this observation does not necessarily mean that the initiation of the rupture is dependent on the eventual size because anelastically attenuated waveforms of earthquakes with short time duration could show such an apparent trend hence we computed synthetic seismograms to test a group of source scaling models in which source time function grows independently of its eventual size taking into account anelastic attenuation and instrument response we find that such scaling models do not easily reproduce simultaneously the size dependence of the initial rise and the pulse duration of the observed waveforms though poor data coverage above m1 5 prevents us from deriving a firm conclusion a model with size dependent growth rate is able to match our observations

finite extension and low angle normal faulting andersonian theory for infinitesimal strains predicts that normal faults will form at high angles to the surface in extensional terranes in contrast low angle normal faults are sometimes observed on which tens of kilometers of slip has occurred and moderate angle faults with slip of 5 to 15 km are common a simple extension of the theory to account for the stresses required to drive finite deformation shows that low angles are preferred for large extension on individual faults the formation of low angle faults may require inhomogeneities in stress or material properties but once slip begins on a low angle fault it is likely to continue whereas slip on high angle faults is likely to end after modest slip to be replaced by the formation of new high angle faults

folds on europa implications for crustal cycling and accommodation of extension regional scale undulations with associated small scale secondary structures are inferred to be folds on jupiter s moon europa formation is consistent with stresses from tidal deformation potentially triggering compressional instability of a region of locally high thermal gradient folds may compensate for extension elsewhere on europa and then relax away over time

geophysical controls of chemical disequilibria in europa the ocean in jupiter s moon europa may have redox balance similar to earth s on earth low temperature hydration of crustal olivine produces substantial hydrogen comparable to any potential flux from volcanic activity here we compare hydrogen and oxygen production rates of the earth system with fluxes to europa s ocean even without volcanic hydrothermal activity water rock alteration in europa causes hydrogen fluxes 10 times smaller than earth s europa s ocean may have become reducing for a brief epoch for example after a thermal orbital resonance 2 gyr after accretion estimated oxidant flux to europa s ocean is comparable to estimated hydrogen fluxes europa s ice delivers oxidants to its ocean at the upper end of these estimates if its ice is geologically active as evidence of geologic activity and subduction implies

folding of europa s icy lithosphere an analysis of viscous plastic buckling and subsequent topographic relaxation abstract potential contractional folds on jupiter s icy moon europa have been identified the best example is at the extensional band astypalaea linea where a series of subtle topographic undulations 25 km in wavelength possess parasitic tectonic features that support a folding origin a scenario has been qualitatively proposed whereby folds form via unstable contraction of the icy lithosphere compensate for extension elsewhere on europa and then subsequently relax here we quantitatively address this scenario applying a model for viscous plastic buckling of planetary lithospheres and finite element simulations of topographic relaxation our results suggest that the lithosphere of europa could indeed be unstable but the low required surface temperatures limit fold formation to higher latitudes and the high required driving stresses 9 10 mpa are difficult to achieve on the satellite the depth to the brittle ductile transition is well constrained and high thermal gradients are indicated implying heat flows near 100 mw m 2 in addition topographic relaxation progresses so slowly even at these heat flows that it is not a viable mechanism to eliminate such features over the age of europa s surface given the paucity of identified folds we conclude that the necessary conditions for their formation are rare and that lithospheric folding is a minor mechanism for compensating the large amounts of extension seen elsewhere on europa

formation of lenticulae on europa by saucer shaped sills abstract europa s surface contains numerous quasi elliptical features called pits domes spots and small chaos we propose that these features collectively referred to as lenticulae are the surface expression of saucer shaped sills of liquid water in europa s ice shell in particular the inclined sheets of water that surround a horizontal inner sill limit the lateral extent of intrusion setting the lateral dimension of lenticulae furthermore the inclined sheets disrupt the ice above the intrusion allowing the inner sill to thicken to produce the observed relief of lenticulae and to fracture the crust to form small chaos scaling relationships between sill depth and lateral extent imply that the hypothesized intrusions are or were 1 5 km below the surface liquid water is predicted to exist presently under pits and for a finite time under chaos and domes

role of a strong oxygen deficient zone in the preservation and degradation of organic matter a carbon budget for the continental margins of northwest mexico and washington state rates of organic carbon oxidation in marine sediments were determined for the continental margins of northwest mexico and washington state with the goal of assessing the role of oxygen in the preservation of organic matter on a margin with a strong oxygen deficient zone and on a typical western continental margin total carbon oxidation rates including rates for individual electron acceptors o2 no3 and so4 were determined at depths ranging from 100 to 3000 m on both margins carbon oxidation rates were generally higher on the washington margin than on the mexican margin the relative importance of the different electron acceptors varied across the two margins and was related primarily to the availability of o2 and no3 from the overlying water the relative contribution of o2 consumption increased in deeper sediments 2000 m as aerobic processes began to dominate the total carbon oxidation rate denitrification rates were highest in washington sediments however denitrification represented a larger fraction of the total carbon oxidation rate in the mexican sediments 40 for mexico vs 30 for washington sulfate reduction accounted for as much as 79 of the total carbon oxidation rate in shallow sediments and less than 20 in deep sediments on both margins the offshore trends in carbon oxidation rate appeared to be related to the organic carbon input rate pore water o2 and no3 penetration depths were shallowest in nearshore stations and increased offshore regeneration ratios of c n p reveal non redfield behavior on both margins carbon budgets for the two margins demonstrate that off mexico a much greater percentage of the organic matter produced in the surface ocean reached the sediments 15 vs 8 for mexico and washington respectively on the mexican margin 8 of the primary production escaped oxidation in the surface sediments to be permanently buried as compared with only 1 2 of the primary production on the washington margin this suggests that oxygen deficient conditions on mexican margin are linked to enhanced carbon preservation

quick estimation of the magnitude and epicentral distance using the p wave for earthquakes in iran abstract a reliable estimation of the earthquake magnitude and epicentral distance within a few seconds after the p wave arrival is of great importance for earthquake early warning systems this study aims to investigate the relations between the envelope of the initial part of the p wave with epicentral distance and magnitude of the earthquakes related to the tectonic conditions of iran to this end the accelerometer waveforms of 17 earthquakes with moment magnitudes less than 7 8 and epicentral distances less than 150 km are considered a simple function with the form of y bt exp at is fitted to the initial part of the waveform envelope on time windows of 2 3 4 and 5 s by the least squares method which is constructed simply by keeping the past maximum amplitude at every moment parameters a and b are obtained to scale magnitude and epicentral distance as well the results show that log is proportional to log b in which denotes the epicentral distance the distance scaling relations for all of the time windows indicate standard deviations between 0 16 and 0 17 km for log comparing to the adopted relation in the early warning system of the japan meteorological agency results illustrate a relatively distinct slope and intercept which arises from differences in tectonic structure and features of the earthquake sources in two areas the same process for records with epicentral distances less than 50 km is also repeated results indicate smaller standard deviations between 0 13 and 0 14 km for four time windows considering the arrangement of hazardous faults in many populated districts of iran including tehran utilization of the obtained relations is recommended for distances less than 50 km moreover the current study represents two distance dependent relations between magnitudes with p max and a p max within the initial 3 s of p wave arrivals with standard deviations of 0 42 and 0 46 magnitude units respectively

early magnitude and potential damage zone estimates for the great mw 9 tohoku oki earthquake the mw 9 0 2011 tohoku oki earthquake has reopened the discussion among the scientific community about the effectiveness of earthquake early warning for large events a well known problem with real time procedures is the parameter saturation which may lead to magnitude underestimation for large earthquakes here we measure the initial peak ground displacement and the predominant period by progressively expanding the time window and distance range to provide consistent magnitude estimates m 8 4 and a rapid prediction of the potential damage area this information would have been available 35 s after the first p wave detection and could have been refined in the successive 20 s using data from more distant stations we show the suitability of the existing regression relationships between early warning parameters and magnitude provided that an appropriate p wave time window is used for parameter estimation we interpret the magnitude under estimation as a combined effect of high pass filtering and frequency dependence of the main radiating source during the rupture process

sensitivity of seafloor bathymetry to climate driven fluctuations in mid ocean ridge magma supply recent studies have proposed that the bathymetric fabric of the seafloor formed at mid ocean ridges records rapid 23 000 to 100 000 years fluctuations in ridge magma supply caused by sealevel changes that modulate melt production in the underlying mantle using quantitative models of faulting and magma emplacement we demonstrate that in fact seafloor shaping processes act as a low pass filter on variations in magma supply strongly damping fluctuations shorter than about 100 000 years we show that the systematic decrease in dominant seafloor wavelengths with increasing spreading rate is best explained by a model of fault growth and abandonment under a steady magma input this provides a robust framework for deciphering the footprint of mantle melting in the fabric of abyssal hills the most common topographic feature on earth

morphology of europan bands at high resolution a mid ocean ridge type rift mechanism 1 we utilize imaging data from the galileo spacecraft to investigate band formation on one of jupiter s moons europa bands are polygonal features first observed in voyager data close to europa s anti jovian point and represent areas where preexisting terrain has been pulled apart allowing new material to move up into the gap we examine the detailed morphology of several bands imaged at different resolutions and lighting geometries we identify several distinct morphological characteristics including central troughs hummocky textures and ridge and trough terrains some of which are common among the bands studied in many cases bands have initiated along segments of one or more preexisting double ridges ubiquitous within europa s ridged plains distinctive morphological features and high standing topography imply that the bands formed from compositionally or thermally buoyant ice rather than liquid water comparisons between europan band morphologies and features found on terrestrial mid ocean ridges reveal several similarities including axial troughs subcircular hummocks normal faults and indications of symmetrical spreading we conclude that terrestrial mid ocean ridge rifting is a good analogy for europan band formation if a terrestrial seafloor spreading model is applicable to europan bands we speculate that band morphologies might be related to the relative rate of spreading of each band bands may have contributed significantly to the resurfacing of europa europan bands we examine predate but do not postdate lenticulae and related features implying that the style of resurfacing on europa has changed over recent geological time in these regions

the effect of sediment mixing on pb 210 accumulation rates for the washington continental shelf abstract nine cores from the washington continental shelf were examined by radiochemical techniques in order to evaluate the effect of mixing on the calculation of sediment accumulation rates from pb 210 profiles th 234 profiles indicate mixing coefficients of 140 cm 2 yr 1 for the seabed offshore from the columbia river and 47 cm 2 yr 1 for the seabed of the mid shelf silt deposit 75 km north of the columbia river these large mixing coefficients demonstrate that particles can penetrate to the base of the intensely mixed surface layer 10 cm within one year after emplacement at the seabed surface observed depths of cs 137 penetration within the seabed are compared with depths predicted from the surface mixed layer thickness and the pb 210 accumulation rate the observed and predicted depths agree well at all but one station at this station a combination of th 234 co 60 pb 210 and cs 137 profiles suggests that active mixing occurs below the intensely mixed surface layer the agreement of pb 210 and cs 137 data at the other stations however indicates a general absence of deep mixing on the washington shelf thus the apparent accumulation rates calculated from pb 210 profiles below the intensely mixed surface layer on the washington shelf generally reflect the true rates of sediment accumulation

catastrophic initiation of subduction following forced convergence across fracture zones although the formation of subduction zones plays a central role in plate evolution the processes and geological settings that lead to the initiation of subduction are poorly understood using a visco elastoplastic model we show that a fracture zone could be converted into a self sustaining subduction zone after approximately 100 km of convergence modeled initiation is accompanied by rapid extension of the over riding plate and explains the inferred catastrophic boninitic volcanism associated with eocene initiation of the izu bonin mariana ibm subduction zone using global plate reconstructions we suggest that ibm nucleation was associated with a change in plate motion between 55 and 45 ma we estimate that the forces resisting ibm subduction initiation were substantially smaller than available driving forces

evidence for subduction in the ice shell of europa despite widespread evidence for extension there have been few signs of contraction on the icy surface of jupiter s europa evidence for a subduction like convergent boundary suggests that europa may have active plate tectonics

evidence for enhanced phosphorus regeneration from marine sediments overlain by oxygen depleted waters phosphorus regeneration and burial fluxes determined from in situ benthic flux chamber and solid phase measurements at sites on the californian continental margin peruvian continental slope north carolina continental slope and from the santa monica basin california are reported comparison of these sites indicates that o2 depleted bottomwaters enhance p regeneration from sediments diminishing overall phosphorus burial efficiency based on these observations a positive feedback linking ocean anoxia enhanced benthic phosphorus regeneration and marine productivity is proposed on shorter timescales these results also suggest that o2 depletion in coastal regions caused by eutrophication may enhance p regeneration from sediments thereby providing additional p necessary for increased biological productivity

effect of oxygen on degradation rate of refractory and labile organic matter in continental margin sediments in order to study the effect of oxygen on degradation rate of bulk sedimentary organic matter we developed a new incubation method in contrast to most previous experiments 1 all of the sediment undergoing decomposition was maintained under oxygenated or oxygen free conditions and 2 organic matter of varying lability was studied the production of co2 during incubations of sediment in glass test tubes corrected for dissolution precipitation of calcium carbonates was used as a measure of degradation rates the laboratory experiments using surficial and buried continental shelf and slope sediment from the open skagerrak northeastern north sea demonstrated that the effect of oxygen on the degradation rate of sedimentary organic matter is a function of the lability of the decomposing material fresh material was degraded with little difference in rates in the presence or absence of oxygen whereas old material was decomposed significantly up to 3 6 times faster with oxygen than without oxygen an implication of these findings is that bioturbation by exposing old buried material to oxygen may enhance integrated organic carbon oxidation in marine sediments this constitutes a previously unexplored mechanism by which faunal reworking may stimulate carbon degradation the anoxic decomposition rates of organic material buried at 20 cm depth in sediment were the lowest measured we found however that the extent of oxidation of this buried old sediment was considerably larger than that of surficial sediment under oxygenated conditions which indicated that the oxic anoxic oxic redox transitions deposition under oxic conditions burial under anoxia and reexposure to oxygen promoted degradation our results therefore also suggest that the extent of long term decomposition of sedimentary organic material is smaller under oxygenated or anoxic conditions alone than when the material is exposed to the repeated activities of both oxic and anoxic microorganisms