application of pair analysis statistics to aftershocks of the 1984 morgan hill california earthquake we apply statistical analysis based on pairs of earthquakes to study spatial and temporal patterns of earthquake distribution in the aftershock zone of the 1984 morgan hill california earthquake m 6 2 we examine 6 years of preshock and 23 months of aftershock data using frequency distributions of earthquake pairs the nonrandomness in the observed spatial and temporal distribution of pairs is studied by comparison with simulated expected distributions the significance of departures from the expected distributions is appraised by using tolerance limits that account for the scatter of the single trial expected frequencies we find that the eathquake spatial distribution is nonuniform in two major ways 1 short distance clustering distances less than 7 km occurs in both the preshock and aftershock period it is more pronounced and covers larger zones during the aftershock sequence 2 a long distance nonuniformity is also observed distances greater than 11 km caused by concentration of events in two areas separated by a gap in earthquake activity this feature is present in the preshock and early aftershock period but disappears in the late aftershock sequence the distribution of time intervals between preshocks does not indicate any substantial nonrandom features in contrast the temporal distribution of aftershocks although dominated by the classic power law decay in time suggests the possible existence of characteristic time intervals that increase with magnitude and time after the mainshock

3 d inhomogeneities in the upper mantle abstract the method developed by aki christofferson and husebye for determining a 3 d seismic image of the earth s interior from the travel time data obtained at a 2 d array has been applied to seventeen arrays around the world by various authors the most outstanding results from these works are that small scale lateral velocity anomalies exist in the lower crust and upper mantle everywhere studied the reality of these deep seated anomalies is supported by 1 1 significant reduction of residual variance 2 2 resolution analysis for decoupling from shallower anomalies and 3 3 error analysis for the significance of anomalies in this paper we try to strengthen the credibility of these deep seated anomalies by drawing attention to those found where they were expected namely low velocity bodies under several geothermal areas and high velocity anomalies associated with the subducting plate

upper ordovician mudrock facies and trace fossils in the northern holy cross mountains poland and their relation to oxygen and sea level dynamics abstract the bulk of the ordovician sedimentary record in the northern holy cross mts poland is composed of dark claystones which record the long term dysoxic conditions lasting throughout the entire caradoc however a relatively short term oxygenation event corresponding to the middle caradoc sea level lowstand interrupts this monotonous succession termination of the low oxygen environment was triggered by a major change in oceanic circulation due to onset of polar cooling it resulted in the progressive benthic oxygenation and increase of colonization of the highly enriched sediment by soft bodied burrowers as can be envisaged from ichnofabric of the lower to middle ashgill sedimentary record the rapid but gradual upward progradation of the upper ordovician mudrock facies into the coarse grained deposits appears to be closely related to the late ashgill glacio eustatic regressive event

turbulence transport vorticity dynamics and solute mixing under plunging breaking waves in surf zone plunging breaking waves generate turbulence and vorticity which are of great importance for the solute and sediment transport in surf zone in this paper the complex breaking processes are simulated by using an accurate numerical model that solves the reynolds equations for the mean flow and modified k e equations for the turbulence field a solute transport model is employed to investigate the solute mixing under plunging waves after validation of the numerical model by comparing numerical results with available experimental data the numerical model is further utilized to study the detailed mechanisms of turbulence transport and vorticity dynamics the differences between spilling and plunging breaking waves are discussed the impact of the wave breaking on solute mixing in the surf zone is also examined

stochastic synthesis of earthquake catalogs a model of earthquake occurrence is proposed that is based on results of statistical studies of earthquake catalogs we assume that each earthquake generates additional a probability that depends on time as t 1 this assumption together with one regarding the independence of branching events on adjacent branches of the event tree is sufficient to permit the generation of complete catalogs of earthquakes that have the same time magnitude statistical properties as real earthquake catalogs if is about 0 5 the process generates sequences that have statistical properties similar to those for shallow earthquakes many well known relations are reproduced including the magnitude frequency law omori s law of the rate of aftershock and foreshock occurrence the duration of a recorded seismic event versus its magnitude the self similarity or lack of scale of rate of earthquake occurrence in different magnitude ranges etc a value of closer to 0 8 or 0 9 seems to simulate the statistical properties of intermediate and deep shocks a formula for seismic risk prediction is proposed and the implications of the model for risk evaluation are outlined the possibilities of the determination of long term risk from real or synthetic catalogs that have the property of self similarity are dim

palaeogeographic significance of mid silurian palaeomagnetic results from southern britain major revision of the apparent polar wander path for eastern avalonia summary wenlock c 430 ma lavas from the east mendips inlier southern briain are characterized by either single component magnetizations type 1 or a more complex system of three magnetization components type 2 during stepwise thermal demagnetization the type of demagnetization behaviour is related to changes in magnetic petrology within the lava pile the lower and intermediate unblocking temperature components of type 2 represent magnetic overprints of tertiary recent and hercynian mid carboniferous origin respectively the primary nature of the high unblocking temperature magnetization component types 1 and 2 dec 095 c inc 24 ag5 8 8 is demonstrated by positive agglomerate tests conversely an overlying upper old red sandstone sequence did not provide primary magnetizations the magnetic signature is governed by syn tectonic hercynian remagnetizations the primary silurian data demonstrate that the east mendips inlier experienced considerable clockwise rotation probably of the order of 80 prior to partial remagnetization during hercynian deformation and folding the silurian palaeopole vgp 13 n 271 e dp dm 5 9 cannot therefore be used for apparent polar wander path construction nevertheless the palaeolatitude estimate 13 f 5s suggests that southern britain eastern avalonia was at comparable mid silurian latitudes to baltica and northern britain 10 20 s both the iapetus ocean and tornquist sea therefore which separated southern britain from laurentia and baltica respectively in ordovician times were closed by wenlock time as ordovician and silurian palaeomagnetic data from baltica and eastern avalonia now record a comparable northwards translation from high southerly to near equatorial latitudes during this time we postulate pre wenlock collision of these blocks to form balonia baltica eastern avalonia a new apparent polar wander path for eastern avalonia constructed on the basis of this palaeogeographic model is presented

condensation processes in shallow platforms abstract sequence stratigraphy allows elucidation of the time space relationships of deposits in sedimentary basins however the relative distribution of unconformities and condensed sections can neither be used as diagnostic criteria for particular palaeogeographic settings nor as indicators of maximum regional transgressions condensation processes also occur in the shallowest portions of platforms and during maximum regression episodes three separate processes of condensation are here distinguished stratigraphic sedimentary and taphonomic these three categories of condensation can be used to analyze patterns of sediment distribution and also to estimate lateral and vertical variations in sedimentation rates contrary to the usual interpretation the maximum values of rate of sedimentation and degree of accommodation of sediments as well as the highest relative sea level occur in the lowest levels of the shallowing upward sequences the distinction between these condensation processes allows one to predict that the degree of sedimentary and stratigraphic condensation will be higher towards the distal portions of the platforms whereas the stratigraphic condensation processes without sedimentary condensation will show the maximum intensity and frequency in the shallowest portions of the platforms

changes in static stress on southern california faults after the 1992 landers earthquake the magnitude 7 5 landers earthquake of 28 june 1992 was the largest earthquake to strike california in 40 years the slip that occurs in such an earthquake would be expected to induce large changes in the static stress on neighbouring faults these changes in stress should in turn affect the likelihood of future earthquakes stress changes that load faults towards failure have been cited as the cause of small1 5 moderate6 and large7 earthquakes conversely those that relax neighbouring faults have been related to a decrease in seismicity5 here we use an elastic half space model8 to estimate the stress changes produced by the landers earthquake on selected southern california faults including the san andreas we find that the estimated stress changes are consistent with the triggering of four out of the five aftershocks with magnitude greater than 4 5 and that the largest changes 1 10 bar occurring on part of the san bernardino segment of the san andreas fault may have decreased the time to the next magnitude 8 earthquake by about 14 years

petrology and major element geochemistry of peru margin phosphorites and associated diagenetic minerals authigenesis in modern organic rich sediments abstract new petrographic and major element geochemical data from modern peru margin upper slope outer shelf phosphorites are presented which provide insight into their origin and paragenetic relationship with other authigenic minerals glauconite pyrite and dolomite occurring in organic rich sediments glauconites are precipitated relatively early following the partial reduction of ferric iron and following this process phosphate pyrite and then dolomite precipitation take place at progressively deeper levels in the sediment in association with microbial reduction of sulfate as in many ancient economic phosphorite deposits the phosphatic facies here consist of nodules crusts coatings and strata composed of phosphatic pelletal grains ooids structureless grains intraclasts clumps and biogenic grains in association with organic rich biosiliceous sediments all are considered to have formed within a few centimeters or within a few tens of centimeters below the sediment water interface important factors that influence which morphology will tend to develop include the amount of available pore space the presence of suitable nucleation sites the amount and size of siliciclastic detritus incorporated as inclusions and the relative solution chemistries of the precipitating solutions bacterial mediation may play an important but as yet unspecified role in the precipitation process textural data and factor analysis of chemical data suggest that structureless pellets are relatively inclusion free na f mg co3 substituted pore water precipitates whereas ooids are inclusion rich pore water precipitates poor in lattice substituted components variations in nodular cement birefringence and crystallinity are suggested to have been produced by similar lattice substitutions that directly reflect pore water carbonate ion concentrations and thus relative degrees of organic matter degradation phosphate and dolomite are intimately mixed yet mineralogically distinct phases in phosphatized dolomicrites depth stratified threshold carbonate ion concentrations may control the lower limit at which phosphatic minerals may precipitate below depths of a few centimeters excessive carbonate ion concentrations and diminished reactive iron and sulfate concentrations favor the development of dolomite while precluding further development of phosphatic minerals and pyrite periodic sediment reorganization bioturbation current winnowing and erosion mass wasting etc plays an important role in both concentrating pelletal grains and maintaining nodules and crusts at critical depth levels in the sediment as well as mixing ordered mineral parageneses into complicated sequences

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

diagenetic alteration of chamositic clay minerals to ferric oxide in oolitic ironstone abstract in the clinton oolitic ironstone silurian of pennsylvania ooids that were initially composed of chamositic clay minerals were diagenetically altered to ferric oxide evidence for this alteration includes the preservation of intermediate stages in a variety of partly altered ooids finely laminated chamositic ooids in marginal zones of ferric oxide storm beds bed parallel transitions of ooid compositions in bioturbated oolite and spastolithic shapes of ferric oxide ooids electron microprobe analysis demonstrates that this oxic alteration did not leave significant abundances of residual silicon and aluminum based on textural characteristics and on a permian age of remagnetization the most likely time for the alteration was during early phases of the alleghany orogeny recognitio that the ooids were initially chamositic clay minerals resolves at least part of the oxidation reduction paradox maynard 1986 for ferrous phyllosilicate could presumably accumulate in the low oxygen conditions associated with the medial silurian oceanic anoxic event

influence of static stress changes on earthquake locations in southern california earthquakes induce changes in static stress on neighbouring faults that may delay hasten or even trigger subsequent earthquakes1 10 the length of time over which such effects persist has a bearing on the potential contribution of stress analyses to earthquake hazard assessment but is presently unknown here we use an elastic half space model11 to estimate the static stress changes generated by damaging magnitude m 5 earthquakes in southern california over the past 26 years and to investigate the influence of these changes on subsequent earthquake activity we find that in the 1 5 year period following a m 5 earthquake any subsequent nearby m 5 earthquake almost always ruptures a fault that is loaded towards failure by the first earthquake after this period damaging earthquakes are equally likely to rupture loaded and relaxed faults our results suggest that there is a short period of time following a damaging earthquake in southern california in which simple coulomb failure stress models could be used to identify regions of increased seismic hazard

sea level changes an integrated approach sea level changes an integrated approach in october 1985 sepm sponsored a four day conference entitled sea level changes an integrated approach the purpose of the conference was to provide a forum for an interdisciplinary exchange of ideas on sea level changes and to provide an opportunity for integrating various types of evidence in approaching unresolved issues the conference was successful in bringing together scientists from industry academia and government representing all of the major geosciences disciplines presentations of many new papers plus significant releases of data that were previously held proprietary provided fertile ground for discussion this much cited volume represents the best of the material presented at the conference includes the early vail chart

the tornquist sea and baltica avalonia docking early ordovician late arenig limestones from the sw margin of baltica scania bomholm have multicomponent magnetic signatures but high unblocking components predating folding and the corresponding palaeomagnetic pole latitude 19degreesn longitude 051degreese compares well with arenig reference poles from baltica collectively the arenig poles demonstrate a midsoutherly latitudinal position for baltica then separated from avalonia by the tomquist sea tornquist sea closure and the baltica avalonia convergence history are evidenced from faunal mixing and increased resemblance in palaeomagnetically determined palaeolatitudes for avalonia and baltica during the mid late ordovician by the caradoc avalonia had drifted to palaeolatitudes compatible with those of sw baltica and subduction beneath eastern avalonia was taking place we propose that explosive vents associated with this subduction and related to andean type magmatism in avalonia were the source for the gigantic mid caradoc c 455 ma ash fall in baltica i e the kinnekulle bentonite avalonia was located south of the subtropical high during most of the ordovician and this would have provided an optimum palaeoposition to supply baltica with large ash falls governed by westerly winds in scama we observe a persistent palaeomagnetic overprint of late ordovician ashgill age pole latitude 4degreess longitude 012degreese the remagnetisation was probably spurred by tectonic derived fluids since burial alone is inadequate to explain this remagnetisation event this is the first record of a late ordovician event in scania but it is comparable with the shelveian event in avalonia low grade metamorphism in the north sea basement of ne germany 440 450 ma and sheds new light on the baltica avalonia docking c 2002 elsevier science b v all rights reserved less

phosphogenesis carbon isotope stratigraphy and carbonate platform evolution along the lower cretaceous northern tethyan margin we distinguish three different stages in the evolution of the tithonian late jurassic to aptian early cretaceous northern tethyan carbonate platform 1 carbonate production in the coral oolite mode late tithonian to early valanginian early barremian to early aptian 2 carbonate production in the crinoid bryozoan mode early valanginian haute rivian late aptian and 3 platform retrogradation and destruction condensation and phosphogenesis that is platform drowning early valanginian to early hauterivian middle hauterivian late hauterivian to early barremian early to early late aptian and latest aptian to earliest albian transitions from the coral oolite mode to the crinoid bryozoan mode and consequently to platform drowning may have been driven by increases in nutrient levels on the shelf phases of relative sea level rise in times of carbonate production in the coral oolite mode are named constructive because of the observed platform regeneration following phases of widespread emersion during late sea level highstands in contrast phases of relative sea level rise in times of platform drowning are termed destructive the 13 c stratigraphies obtained from valanginian hauterivian and aptian albian hemipelagic successions beyond the platform correlate well with the early cretaceous pelagic 13 c record positive excursions in the pelagic 13 c record correspond in time to episodes of platform drowning this suggests the existence of a coupling mechanism between changes in the global carbon cycle and platform drowning in our view the early cretaceous crises in carbonate platform growth were the consequence of reinforced greenhouse conditions which may have been triggered by episodes of extensive flood basalt volcanism strong greenhouse conditions may have induced the following chain of feedback mechanisms enabling the biosphere to return to normal conditions climate warming sea level rise accelerated water cycle intensified weathering nutrient mobilization platform destruction increased productivity increased phosphogenesis and carbon burial weakened greenhouse conditions

sedimentary and microbial record of the middle late ordovician phosphogenetic episode in the northern holy cross mountains poland abstract numerous phosphorite beds and phosphatic nodules occur in the upper middle and lower upper ordovician carbonate shale succession of the bukowiany formation outcropping in the northern holy cross mountains central poland the vertical stacking pattern of this succession indicates that phosphatic and accompanying strata reflect a conformable sedimentary succession accumulated during a rise of relative sea level the base of the bukowiany formation is marked by a conspicuous phosphorite horizon revealing a low net sediment accumulation rate reflecting a switch into a mesotrophic ecological system this horizon was produced by reworking and redeposition of pristine phosphate sediment e g by currents activity during the late darriwilian transgression the overlying sedimentary record appears to reflect nucleation of the phosphate phase in the sediment water interface and its subsequent burial by the accompanying sediment the phosphatized tiny stromatolites and nodules preserved within the bukowiany formation indicate that benthic microbial communities played an important role in redistribution and concentration of phosphate during deposition of this succession

distribution texture and composition of modern phosphate pellets in peru shelf muds abstract textural analysis including estimates of concentrations of authigenic phosphate pellets were made for eight sediment cores from the peru continental margin phosphatic pellets separated from these modern organic rich sediments are black spherical ovoidal in shape and in thin section often display a concentric structure around a nucleus consisting of inorganic mineral grains some pellets have a gray white exterior coating which appears to be secondary diagenetic calcite phosphatic pellets account for upwards of 80 of the sediment mass in some cores pellets concentrate in specific size classes generally between 125 and 500 m in diameter and occur within a poorly sorted sediment

delayed triggering of the 1999 hector mine earthquake by viscoelastic stress transfer stress changes in the crust due to an earthquake can hasten the failure of neighbouring faults and induce earthquake sequences in some cases1 2 3 4 5 the 1999 hector mine earthquake in southern california magnitude 7 1 occurred only 20 km from and 7 years after the 1992 landers earthquake magnitude 7 3 this suggests that the hector mine earthquake was triggered in some fashion by the earlier event but uncertainties in the slip distribution and rock friction properties associated with the landers earthquake have led to widely varying estimates of both the magnitude and sign of the resulting stress change that would be induced at the location of the hector mine hypocentre with estimates varying from 1 4 bar ref 6 to 0 5 bar ref 7 more importantly coseismic stress changes alone cannot satisfactorily explain the delay of 7 years between the two events here we present the results of a three dimensional viscoelastic model that simulates stress transfer from the ductile lower crust and upper mantle to the brittle upper crust in the 7 years following the landers earthquake using viscoelastic parameters that can reproduce the observed horizontal surface deformation following the landers earthquake our calculations suggest that lower crustal or upper mantle flow can lead to postseismic stress increases of up to 1 2 bar at the location of the hector mine hypocentre during this time period contributing to the eventual occurrence of the 1999 hector mine earthquake these results attest to the importance of considering viscoelastic processes in the assessment of seismic hazard8 9 10 11

observation of undertow and turbulence in a laboratory surf zone abstract undertow and turbulence in the surf zone have been studied in a wave flume for a spilling breaker and a plunging breaker fluid velocities across a 1 on 35 sloped false bottom were measured using a fiber optic laser doppler anemometer and wave decay and set up were measured using a capacitance wave gage the characteristics of mean flow and turbulence in spilling versus plunging breakers were studied the mean flow is the organized wave induced flow defined as the phase average of the instantaneous velocity while the turbulence is taken as the deviations from the phase average it was found that under the plunging breaker turbulence levels are much higher and vertical variations of undertow and turbulence intensity are much smaller in comparison with the spilling breaker it was also found that turbulent kinetic energy is transported seaward under the spilling breaker and landward under the plunging breaker by the mean flow the study indicates that there are fundamental differences in the dynamics of turbulence between spilling and plunging breakers which can be related to the processes of wave breaking and turbulence production it is suggested that the types of beach profile produced by storm and swell waves may be the results of different relationships between mean flow and turbulence in these waves

small earthquakes tectonic forces earthquake scaling and frequency of occurrence relations require that small earthquakes be just as important as larger ones in redistributing the forces that drive relative displacements across active faults of any dimension including plate boundaries

sediment production on sediment starved continental margins the interrelationship between hardbottoms sedimentological and benthic community processes and storm dynamics abstract modern sediment starved continental shelves represent developing condensed sections analogous to those considered key stratigraphic markers in many stratigraphic models condensed sections and their associated hardbottoms on the modern high energy north carolina continental margin provide important benthic habitats that are modified on time scales of days to centuries by interrelated sedimentological biological and physical processes outcropping upper cenozoic strata of varying lithologies form distinct hardbottom morphologies that through differential bioerosion contribute significant volumes of new sediment to the surficial sand regime of the continental shelf 1 vertical and sloped mudstone to muddy sandstone hardbottoms of the miocene pungo river formation are dominated by the endolithic fauna jouanettia quillingi bivalve and upogebia sp shrimp 2 vertical and sloped hardbottoms consisting of harder miocene and pleistocene limestone are dominated by the endolithic bivalves lithophaga bisulcata gastrochaena stimpsoni and g ovata 3 the highly lithified flat hardbottoms of plio pleistocene limestone are dominated by the epifloral macroalgal species dictyopteris hoytii zonaria tournefortii and sargassum filipendula these three groups of bioeroders physically and or chemically degrade their respective lithologies develop relief on hardbottom surfaces produce large scale morphological features on the shelf and recycle ancient sediment into the modern surficial sediment system the rate of sediment production resulting from bioerosion varies from 5 5 kg m2 yr on the vertical and sloped miocene mudstone hardbottoms to 0 4 kg m2 yr on vertical and sloped pleistocene limestone to 0 03 kg m2 yr on the flat highly lithified plio pleistocene limestone hardbottoms depending on lithology and associated bioerosional processes bioeroders excavate exposed hardbottom surfaces and develop relief ranging from millimeters to meters whereas differential rates of bioerosion between different lithologic units results in relief ranging from meters to tens of meters recession rates measured on miocene mudstones at the chapel site range from 2 to 4 cm per year for the outcrop exposure which is 132 m long this would produce a ten meter overhang of the overlying pleistocene limestone in 250 500 years by removing 13 400 metric tons of eroded sediment 25 fine sand that would be contributed to the surficial sediments the overhang would ultimately break off during a storm to produce the next row of the limestone rubble blocks that form a ramp in front of the receding mudstone scarp these rates of sediment production are rapid enough to bury the hardbottoms producing the sediment however surface sediment is generally not accumulating on the shelf it is present only as thin 0 1 m highly variable and ephemeral sand bodies major storms modify the abundance and distribution of surface sediment on hardbottom habitats and routinely export large volumes of these sediments from the shelf system depositing them as fine sand clinoforms off the prograding shelf edge exposed hardbottom habitats free of sand are dominated by highly diverse communities of endolithic fauna and epilithic fauna and flora those habitats with 2 6 cm of sand are generally dominated by scattered epilithic fauna with small growths of epilithic flora irregularly distributed on topographic highs and those habitats with 6 cm of sand are generally dominated by softbottom benthic communities storms modify the distribution of bottom sediments which either exposes or buries additional hardbottom surfaces and controls the expansion or contraction of hardbottom benthic communities thus the intensity frequency and character of individual storms and the seasonal storm pattern determine the amount and location of sand accumulation which controls the benthic community structure in turn the benthic community determines the type rate and volume of hardbottom bioerosion and resulting sediment production

viscoelastic stress triggering of the 1999 hector mine earthquake by the 1992 landers earthquake the 1999 mw 7 1 hector mine earthquake occurred only 20 km from the 1992 mw 7 3 landers earthquake fault the close spacing between the two earthquakes suggests that the landers earthquake trigger the hector mine event based on an elastic half space model scientists from the usgs scec and cdmg 2000 have found a negative coulomb stress change at the hector mine hypocenter due to landers this negative stress change is inconsistent with the hypothesis of static stress triggering in this paper i show evidence of stress triggering of the hector mine earthquake by landers due to a process governed by viscoelastic flow in the lower crust this visoelastic flow has produced broad scale postseismic rebound observed by gps and insar measurements the result of this study is that viscoelastic flow has significantly modified the regional stress field in the mojave desert after the landers earthquake the evolving stress field including viscoelastic flow has gradually moved the coulomb stress change at the hector mine hypocenter to a positive level the increase in coulomb stress exceeded 1 bar right before the hector mine earthquake bringing the hector mine ruptures to the proximity of catastrophic failure

condensation and phosphatization of the middle and upper ordovician limestones on the malopolska block poland response to paleoceanographic conditions abstract the growth fault activity in the late middle ordovician resulted in the establishment of a horst like submarine paleohigh in the northern malopolska block this topographic submarine swell was the site of carbonate deposition characterized by a low overall net sediment accumulation rate the sedimentary and ecological conditions on this paleohigh were controlled by a paleocurrent that flowed along the northern margin of the malopolska block however seasonal intense storms associated with the mid latitude location of this terrane cannot be excluded the paleocurrent induced seasonal upwelling of nutrient rich waters from deeper parts of the current system that shifted onto the paleohigh and caused phosphatization of the sediment the winnowing and reworking of sediment together with slow carbonate production by a temperate water biota and relatively long periods of omission resulted in low accumulation rates and sediment condensation the cessation of the paleocurrent and associated phosphatization coincides with changes in the oceanic circulation system due to the late ordovician collision of avalonia with baltica and gradual closure of the tornquist sea

fractal analysis of three dimensional spatial distributions of earthquakes with a percolation interpretation although many studies have shown that faults and fractures are self similar over a large range of scales none have tested the fault structure for self similarity in three dimensions in this study earthquake hypocentral locations in central and southern california were used to illuminate three dimensional 3 d fault structures for which we measured the fractal capacity dimension d0 3 d hypocentral distributions from the joshua tree big bear and upland aftershock sequences as well as background seismicity at parkfield were found to be fractal where d0 3 d increased with increasing event density asymptotically approaching a stable value the joshua tree data set stabilized at d0 3 d 1 92 0 02 the parkfield data set asymptotically approached d0 3 d 1 82 and the big bear data set approached d0 3 d 2 01 as a test of the effects of location errors upon the measured value of d0 3 d the upland aftershock data were located with both the southern california hadley and kanamori 1977 h k velocity model and the more accurate hauksson and jones 1991 h j velocity model events located with the h k model asymptotically approached d0 3 d 2 07 and events located with the h j model approached d0 3 d 1 79 suggesting that improved hypocentral locations may decrease the measured fractal dimension one interpretation of our results of d0 3 d 2 0 for all of the hypocentral data is that earthquakes only occur on the percolation backbone of a fault network i e the active part of the network that accommodates finite strain deformation sahimi et al 1993 we show that a percolation model that allows for healing of previously broken bonds is consistent with this interpretation

earthquake hazard after a mainshock in california after a strong earthquake the possibility of the occurrence of either significant aftershocks or an even stronger mainshock is a continuing hazard that threatens the resumption of critical services and reoccupation of essential but partially damaged structures a stochastic parametric model allows determination of probabilities for aftershocks and larger mainshocks during intervals following the mainshock the probabilities depend strongly on the model parameters which are estimated with bayesian statistics from both the ongoing aftershock sequence and from a suite of historic california aftershock sequences probabilities for damaging aftershocks and greater mainshocks are typically well constrained after the first day of the sequence with accuracy increasing with time

the role of nutrient availability in bioerosion consequences to carbonate buildups abstract zooxanthellate organisms which are among the major carbonate producers on coral reefs are highly adapted to nutrient deficient conditions and tend to be outcompeted by filamentous or fleshy algae if nutrients are abundant reef dwelling bioeroding organisms on the other hand seem to increase in abundance with increasing availability of nutrient and food resources maximum rates of calcium carbonate production in a reef system are comparable in magnitude to maximum rates of bioerosion the dynamic interplay between accretion destruction of coral reefs is therefore likely to be strongly influenced by nutrient availability nutrient availability may also influence development of carbonate facies in reef systems that develop in oligotrophic waters skeletal sands and framework should dominate carbonate buildups in systems that develop in mesotrophic waters enhanced bioerosion should reduce much of the skeletal material to carbonate muds increasing the prevalence of mud rich facies eutrophic conditions should result in bioeroded hardgrounds and carbonate depositional hiatuses when carbonate accumulation rates fail to equal or exceed rates of local relative sea level rise reefs drown bored and lithified pavements with crusts of ferromanganese oxides phosphates or glauconite often separate shallow reef from overlying deeper water deposits nondeposition bioerosion and evidence for low redox potentials at the sediment water interface all implicate excess nutrient availability as a factor in reef drowning some carbonate depositional hiatuses and bioerosional surfaces are local phenomena evidence of responses to local increases in nutrient availability resulting from submergence of soils runoff or topographic upwelling others appear to reflect regional or worldwide events because many carbonate producing organisms have life history strategies specialized to nutrient deficient conditions they are unable to compete when nutrients become plentiful events that triggered ocean turnover or sharp increases in the rate of deep ocean circulation would have increased sea surface nutrient availability worldwide such eutrophication could have caused widespread extinctions in reef communities because eutrophication promotes bioerosion details of the fossil records of those events are probably lost in bioerosional hiatuses

fingerprinting gondwana versus baltica provenance nd and sr isotopes in lower paleozoic clastic rocks of the malopolska and łysogory terranes southern poland abstract neodymium and strontium isotope compositions of clastic material in lower paleozoic rocks in addition to detrital zircon and mica geochronology and faunal data are used to constrain the geotectonic provenance of the malopolska and ysogory terranes in southern poland being characterized by markedly different geological histories during early paleozoic times malopolska and ysogory were most often interpreted as exotic terranes but their identity provenance and accretionary history are still being discussed this study presents the first systematic nd and sr isotope survey of the lower paleozoic siliciclastic rocks cambrian clastic rocks of malopolska differ in their nd and sr isotope compositions from those of ysogory the former are characterized by a wide range of the end 505 ma values from 11 7 to 5 7 sr isotope ratios from 0 712 to 0 767 and model ages t2dm predominantly younger than 1 9 ga whereas the end t values of the latter vary within a fairly narrow range of the end 505 ma from 11 0 to 8 3 sr isotope ratios from 0 722 to 0 754 and model ages are older than 1 9 ga new nd data combined with detrital zircon ages provide evidence that malopolska must have been supplied with clastic material from amazonian sources during cambrian times being also progressively sourced from the baltic svecofennian crust in the middle cambrian detritus recognized in ysogory has mostly been derived from the svecofennian basement of fennoscandia but it has also a small addition from cadomian sources all provenance and biogeographic data suggest that malopolska was situated close to the amazonian craton near the western end of the cadomian belt during the early cambrian and it was not until the late middle cambrian that malopolska collided with the baltica margin during the late cambrian ysogory occupied a position between baltica and the cadomian belt but it was probably not yet accreted to the baltic crust at that time

temperate carbonates on a modern low energy isolated ramp the balearic platform spain abstract this paper presents a depositional model for a temperate low energy carbonate ramp based on descriptive studies of five areas around the balearic islands of mallorca and menorca this low energy ramp differs significantly from other present day temperate carbonate platforms that are primarily high energy open shelves it is characterized by the following lithofacies from shore to basin 1 lagoon 2 barrier island 3 shallow subtidal 4 inner ramp 5 middle ramp and 6 distal ramp subaqueous carbonate dunes are present near slope breaks off menorca and cabrera but they are not representative of the entire ramp balearic ramp sediments differ in composition texture biology and degree of cementation from those on modern low energy tropical ramps balearic ramp carbon tes lack ooids and peloids hermatypic coral buildups and calcareous green algae except in one restricted bay where halimeda is relatively common red algal sands and gravels extend to depths of up to 90 m and are coarser than their strandplain equivalent lime sands skeletal allochems consist of the bryomol rhodalgal association marine cementation is rare and the carbonate fraction of deep water muds is mainly mg calcitic and calcitic in composition aragonite is rare except for the red alga peyssonnelia which is composed of aragonite it is neither a dominant skeletal constituent nor common as a cement the mediterranean sea off mallorca and menorca is a low energy temperate oligotrophic clear water environment the depositional model for the region is an isolated platform configured as a homoclinal ramp ancient counterparts of the balearic ramp are present in the neogene of the mediterranean tethys and the paratethys and though the constituents of fossil assemblages vary with time the biota of the balearic ramp such as bryozoans red algae echinoderms and mollusks ranges from the paleozoic era to the present echinodermal bryomols passing to basinal muds on carbonate ramps are particularly characteristic of the early carboniferous in north america and europe this suggests that the balearic islands temperate ramp may be more representative of some ancient carbonate sequenc s than either temperate high energy open shelves or tropical ramps in the present oceans

late neoproterozoic to early palaeozoic palaoegeography of the holy cross mountains central europe an integrated approach study of geochemistry examination of isotope ages of detrital minerals palaeomagnetic analysis and a study of the trilobites were performed to provide constraints on the palaeogeographical position of the holy cross mountains in late ediacaran early palaeozoic time the geochemical results indicate an active continental margin or continental island arc provenance of the ediacaran sediments sediments from a passive continental margin were deposited here during the cambrian and ordovician the palaeomagnetic pole isolated from cambrian rocks of the malopolska region of the holy cross mountains corresponds to the cambrian segment of the baltic apparent polar wander path isotope age estimations indicate that cambrian sediments of the malopolska region contain detritus not only from a latest neoproterozoic source but also from sources with ages of c 0 8 0 9 ga 1 5 ga and 1 8 ga the malopolska brunosilesia dobrugea and moesia terranes which originally developed near the present southern edge of baltica and were partly involved in the cadomian orogen were dextrally relocated along its trans european suture zone margin the first stage of this movement took place as early as latest ediacaran time while baltica rotated anticlockwise anticlockwise rotation of baltica at the cambrian ordovician boundary implies further dextral movement of the malopolska block

ooidal ironstones from ordovician gondwana a review abstract ooidal ironstones occur widely in the ordovician shelf sequences of sw europe the western european platform the avalonian terranes and north africa they form the most important group of deposits of the earlier of the two major periods of phanerozoic ooidal ironstone generation ordovician devonian and jurassic paleogene individual ooidal ironstone horizons are generally thin less than 2 m but may have a very wide lateral extent the ooidal ironstones represent sediments formed under extremely low sedimentation rates and were most commonly developed as the initial deposit above a disconformity ooidal ironstone producing events occurred almost synchronously over wide areas of the gondwanan shelf reflecting the uniform sedimentary conditions across large areas of the shallow low gradient shelf clastic sediments deposited during periods of normal sedimentation show similar remarkable uniformity over large areas this geometry together with the abundant fauna both micro and macro in the ironstone facies means that these thin ironstone beds form important marker horizons for correlation within the region the stratigraphic distribution of the ironstones shows strong correlation with periods of high relative sea level sedimentological study of examples in sw europe indicate that they separate transgressive systems tracts with parasequences bounded by phosphorites from the early part of the following highstand systems tracts without significant parasequences and are therefore interpreted to lie on maximum flooding surfaces such maximum flooding surfaces occur in the early llavirn the late llanvirn the early caradoc and the early ashgill of sw europe outside sw europe the ooidal ironstones may have a different relationship to the sequence stratigraphy possibly reflecting the influence of absolute water depth

origin of a cool water oligo miocene deep shelf limestone eucla platform southern australia the abrakurrie limestone is an areally extensive bryozoan rich unit within the eucla platform a tertiary carbonate shelf which caps the central part of the southern australian continental margin the onshore portion the topic of this study has been exposed since middle miocene time and lies beneath the nullarbor plain the rocks are fine sand to granule sized calcarenites composed of bryozoans bivalves benthic foraminifera and echinoids with lesser numbers of brachiopods solitary corals and serpulids they conspicuously lack significant numbers of planktonic foraminifera and coralline algae most bryozoan remains are from delicate branching cyclostomes although delicate branching robust branching foliose fenestrate multilaminar encrusting and free living cheilostomes are variably abundant in specific units the poorly lithified sequence is punctuated by well cemented layers with erosional tops which are interpreted as hardgrounds the limestone is interpreted as a cool water deep shelf deposit which accumulated in water depths generally greater than 50 m on the inner part of the eucla platform a model which involves deposition and cementation on a carbonate shelf swept by open ocean swells is proposed to explain the style of sedimentation the shelf is envisaged as partitioned by the depth of the zone of wave abrasion sediments were produced throughout but accumulated only below this depth when the seafloor was above this depth it was an environment of cementation and erosion the vertical sequence correlates in a general way with the global sea level model for the mid cenozoic while accumulation rates for southern australian carbonates are similar to rates of cool water carbonate deposition elsewhere c 2 5 cm kyr 1 the rate of abrakurrie accumulation is much less suggesting that significant time periods are represented by the hardgrounds

ordovician conodonts and the tornquist lineament abstract the holy cross mts in southern poland are generally believed to be split by a tectonic dislocation into two separate parts a ne one being a part of the baltic craton and a sw part belonging to the malopolska terrane of a complex geotectonic history connected with the trans european suture zone tornquist lineament unexpectedly conodont assemblages of earliest middle ordovician early darriwilian age from pobroszyn in the northeastern ysogory region and from szumsko in the southwestern kielce region show virtually identical species composition one of the dominant species both in pobroszyn and szumsko trapezognathus pectinatus sp n characterized by denticulated m elements occurs elsewhere only on the northern margin of gondwana separation of the malopolska microcontinent from baltica continued after the disappearance of trapezognathus and an apparently allopatric speciation process was initiated by a population of baltoniodus also in this case denticulation developed in the m elements of the apparatus but the process of speciation of b norrlandicus denticulatus ssp n was truncated by re appearance of the baltic lineage of baltoniodus later conodont faunas from the region are of baltic affinities but remain distinct in showing a relatively high contribution from exotic species of sagittodontina phragmodus and complexodus the periodic north american midcontinent tropical influences observable in the baltic region did not reach the holy cross mts the ordovician conodont faunas from the holy cross mts can be fit into a transect extending from baltica to gondwana with several intermediate terranes this new evidence disproves the long held concept that the ysogory and kielce regions had separate early palaeozoic fates and that the malopolska terrane was unified with baltica already in the earliest ordovician the trans european suture zone truly marks the presence of a wide tornquist sea in the early palaeozoic

mechanism diversity of the loma prieta aftershocks and the mechanics of mainshock aftershock interaction the diverse aftershock sequence of the 1989 loma prieta earthquake is inconsistent with conventional models of mainshock aftershock interaction because the aftershocks do not accommodate mainshock induced stress changes instead the sense of slip of the aftershocks is consistent with failure in response to a nearly uniaxial stress field in which the maximum principal stress acts almost normal to the mainshock fault plane this orientation implies that i stress drop in the mainshock was nearly complete ii mainshock induced decreases of fault strength helped were important in controlling the occurrence of after shocks and iii mainshock rupture was limited to those sections of the fault with preexisting shear stress available to drive fault slip

ordovician metre scale cycles implications for climate and eustatic fluctuations in the central appalachians during a global greenhouse non glacial to glacial transition abstract metre scale shallowing upward cycles in ordovician carbonates of the central appalachian basin record climate and eustatic fluctuations during a transition from early ordovician global greenhouse to late ordovician icehouse conditions peritidal facies and shale abundance suggest a long term trend in this area from semi arid early ordovician to more humid middle to early late ordovician conditions with a return to semi arid conditions during the late ordovician the climatic fluctuations were most likely produced by tectonics uplift and erosion related to taconic orogenesis plate motion of north america and the areal extent of water covering the shelf peritidal cyclic facies indicate that high frequency relative sea level fluctuations were of small amplitude 20 m relative sea level fluctuations that decreased into the later ordovician if these sea level changes are eustatic then the increased amplitude may mark the early initiation of continental glaciation on gondwana in the late middle ordovician followed by waning of ice sheets prior to the latest ordovician glaciation

crustal stress field in southern california and its implications for fault mechanics we present a new high spatial resolution image of stress orientation in southern california based on the inversion of earthquake focal mechanisms we use this image to study the mechanics of faulting in the plate boundary region the stress field contains significant spatial heterogeneity which in some cases appears to be a result of the complexity of faulting and in other cases appears to be a cause temporal changes in the stress field are also observed primarily related to major earthquakes the observed 15 10 rotation of the stress axes due to the 1992 m7 3 landers mainshock implies that the deviatoric stress magnitude in the crust is low of the order of 10 mpa this suggests that active faults in southern california are weak the maximum principal stress axis near the san andreas fault is often at 50 to the fault strike indicating that the shear stress on the fault is comparable to the deviatoric stress the san andreas in southern california may therefore be a weak fault in a low strength crust

centroid moment tensor solutions for april june 1992 abstract centroid moment tensor solutions are presented for 245 earthquakes that occurred during the second quarter of 1992 the solutions are obtained using corrections for aspherical earth structure represented by a whole mantle shear velocity model sh8 u4l8 of dziewonski and woodward the solution for an earthquake under mindanao philippines on 9 may represents the 10 000th event in the cmt catalogue

entrapment and transformation of post bloom radiolarians in cyanobacterial mats as a factor enhancing the formation of black cherts in the early silurian sea

the conodont paroistodus horridus barnes and poplawski as a new biostratigraphical tool for the middle darriwilian ordovician abstract this contribution develops an analysis on the conodont paroistodus horridus barnes and poplawski from several argentine precordilleran sections from these collections we have established that p horridus occurs from the lenodus crassus zone to the amorphognathus tvaerensis zone darriwilian to early sandbian this study recognizes several primary morphological features of p horridus which vary through the darriwilian and three evolutionary morphological types can be identified early form middle form with two stages and late form the co occurrence of index conodonts with the successive stages of this evolutionary development allowed a significant biostratigraphical value to be assigned for each different morphological type of p horridus based on this information we propose a detailed biostratigraphical control and correlation with equivalent strata from canada china australia united states norway and western central asia

morphology sediments and depositional environments of a small carbonate platform serranilla bank nicaraguan rise southwest caribbean sea serranilla bank 16 degrees n 80 degrees w is located on the nicaraguan rise a major structural topographic high on the caribbean plate this bank is one of several small detached platforms upon which carbonate accretion is failing to keep pace with holocene sea level rise high resolution seismic profiling sediment sampling scuba diver observations and dredge hauls show that this relatively deep 10 to 40 m bank is bounded by steep escarpments lacks coral reefs and has few islands and minimal holocene sediment cover hardgrounds populated by brown algae and sponges predominate in the southeastern sector sediments are thin and coarse grained on the remainder of the bank halimeda molluscan dominated sediments are more common and finer grained sediment cover on about 80 of the banktop is too thin to resolve seismically 2 m though some topographic lows have slightly thicker accumulations that occur as large 3 8 m high bedforms indicating northwestward transport the largest field of bedforms is located near the northwestern margin of the platform platform margin sediment deposits are limited in thickness and areal extent muds are scarce and are winnowed off shelf by strong currents factors limiting holocene sediment accretion on serranilla bank include the platform s small size strong currents that promote off bank sediment transport rapid flooding during holocene sea level rise lack of active frame building benthic communities and ubiquitous bioerosion

upper ordovician and silurian ash beds in the holy cross mountains poland preservation in mudrock facies and relation to atmospheric circulation in the southern hemisphere numerous volcanic ash beds k bentonite are recognized globally in ordovician to early devonian sedimentary successions and are important for regional stratigraphic correlations relatively little however is understood of the origin and transport of these ash deposits here we examine ash beds preserved in the upper ordovician and silurian dark shales and mudstones of the holy cross mountains poland and discuss their origin in the context of contemporaneous tectonic configurations the ordovician ash beds occur in the upper sandbian and katian mudrock succession whereas the silurian k bentonites form prominent beds in the upper wenlock and lower ludlow graptolite shales their various thickness and irregular distribution reflect fluctuation of volcanic activity and contribution of physical and biogenic factors in their preservation deposition under dysoxic or anoxic conditions and relatively rapid burial by muddy sediment seem to be dominant factors preventing physical and biological reworking and mixing of the volcanic ash the upper ordovician ash beds in the holy cross mountains as well as the baltoscandian k bentonites appear to be accumulated from pyroclastic material delivered by westerlies of the southern hemisphere from the avalonian volcanoes in the silurian the distribution of pyroclastic material was controlled by se trade winds and therefore the source area for the wenlock and ludlow k bentonites in the holy cross mountains appears to have been an active subduction zone developed close to the southeastern side of baltica however the delivery of volcanic ash to the central part of baltica might have been strongly affected by climatically driven latitudinal migration of the intertropical convergence zone

triggering of the 1999 mw 7 1 hector mine earthquake by aftershocks of the 1992 mw 7 3 landers earthquake 1 there is strong observational evidence that the 1999 mw 7 1 hector mine earthquake in the mojave desert california was triggered by the nearby 1992 mw 7 3 landers earthquake many authors have proposed that the landers earthquake directly stressed the hector mine fault our model of the landers aftershock sequence however suggests there is an 85 chance that the hector mine hypocenter was actually triggered by a chain of smaller earthquakes that was initiated by the landers main shock we perform our model simulations using the monte carlo method based on the gutenberg richter relationship omori s law bath s law and assumptions that all earthquakes including aftershocks are capable of producing aftershocks and that aftershocks produce their own aftershocks at the same rate that other earthquakes do in general our simulations show that if it has been more than several days since an m 7 main shock most new aftershocks will be the result of secondary triggering these secondary aftershocks are not physically constrained to occur where the original main shock increased stress this may explain the significant fraction of aftershocks that have been found to occur in main shock stress shadows in static coulomb stress triggering studies

stress transferred by the 1995 mw 6 9 kobe japan shock effect on aftershocks and future earthquake probabilities the kobe earthquake struck at the edge of the densely populated osaka kyoto corridor in southwest japan we investigate how the earthquake transferred stress to nearby faults altering their proximity to failure and thus changing earthquake probabilities we find that relative to the pre kobe seismicity kobe aftershocks were concentrated in regions of calculated coulomb stress increase and less common in regions of stress decrease we quantify this relationship by forming the spatial correlation between the seismicity rate change and the coulomb stress change the correlation is significant for stress changes greater than 0 2 1 0 bars 0 02 0 1 mpa and the nonlinear dependence of seismicity rate change on stress change is compatible with a state and rate dependent formulation for earthquake occurrence we extend this analysis to future mainshocks by resolving the stress changes on major faults within 100 km of kobe and calculating the change in probability caused by these stress changes transient effects of the stress changes are incorporated by the state dependent constitutive relation which amplifies the permanent stress changes during the aftershock period earthquake probability framed in this manner is highly time dependent much more so than is assumed in current practice because the probabilities depend on several poorly known parameters of the major faults we estimate uncertainties of the probabilities by monte carlo simulation this enables us to include uncertainties on the elapsed time since the last earthquake the repeat time and its variability and the period of aftershock decay we estimate that a calculated 3 bar 0 3 mpa stress increase on the eastern section of the arima takatsuki tectonic line attl near kyoto causes fivefold increase in the 30 year probability of a subsequent large earthquake near kyoto a 2 bar 0 2 mpa stress decrease on the western section of the attl results in a reduction in probability by a factor of 140 to 2000 the probability of a mw 6 9 earthquake within 50 km of osaka during 1997 2007 is estimated to have risen from 5 6 before the kobe earthquake to 7 11 afterward during 1997 2027 it is estimated to have risen from 14 16 before kobe to 16 22

rupture directivity and slip distribution of the m 4 3 foreshock to the 1992 joshua tree earthquake southern california details of the m 4 3 foreshock to the joshua tree earthquake were studied using p waves recorded on the southern california seismic network and the anza network deconvolution using an m 2 4 event as an empirical green s function corrected for complicated path and site effects in the seismograms and produced simple far field displacement pulses that were inverted for a slip distribution both possible fault planes north south and east west for the focal mechanism were tested by a least squares inversion procedure with a range of rupture velocities the results showed that the foreshock ruptured the north south plane similar to the mainshock the foreshock initiated a few hundred meters south of the mainshock and ruptured to the north toward the mainshock hypocenter the mainshock m 6 1 initiated near the northern edge of the foreshock rupture 2 hr later the foreshock had a high stress drop 320 to 800 bars and broke a small portion of the fault adjacent to the mainshock but was not able to immediately initiate the mainshock rupture

a record of late ordovician to silurian oceanographic events on the margin of baltica based on new carbon isotope data elemental geochemistry and biostratigraphy from two boreholes in central poland abstract new stable isotope data from organic carbon 13corg and inorganic elemental geochemistry data have been generated from upper ordovician to silurian strata in two boreholes in the lublin basin of poland grabowiec 6 and zwierzyniec 1 they have been integrated here with biostratigraphical data from graptolites acritarchs and chitinozoans faunal assemblages from grabowiec 6 indicate that it spans from the katian clingani graptolite biozone to the gorstian scanicus graptolite biozone 13corg values from this section record the sheinwoodian ireviken excursion the homerian mulde excursion and a minor positive shift associated with the lower ludfordian leintwardinensis biozone the second section zwierzyniec 1 spans the sandbian through gorstian nilssoni biozone 13corg values record the hirnantian carbon isotope excursion hice and the ireviken excursion as well elemental geochemistry data is used to recognize subtle changes in provenance and lithology significant increases in the abundance of v and mo are recognized in strata deposited above the ireviken excursion the enrichment of these redox sensitive elements suggests that persistent regional anoxia and euxinia may be associated with the aftermath of these oceanographic disturbances some of these same trace elements along with fe2o3 and pb are also abundant in strata coeval with or just below the ireviken and hice excursions this may have a causal link with malformed palynomorphs observed at these intervals by some workers which are thought to reflect toxic levels of dissolved heavy metals in the world s oceans