frequency magnitude distribution of microearthquakes beneath the 9 50 n region of the east pacific rise october 2003 through april 2004 1 relocated hypocentral data from a 7 month deployment october 2003 to april 2004 of ocean bottom seismometers provide an opportunity to map microearthquake frequency magnitude distributions fmds along the 9 49 52 n region on the east pacific rise these analyses which incorporate more than 9000 earthquakes represent the first investigation of the 3 d spatial and temporal patterns of fmds along any mid ocean ridge spreading center the data are described well by a gutenberg richter model indicating a power law or fractal relationship between earthquake size and frequency the scaling exponent or b value shows significant spatial variability exceeding a value of 2 0 at the shallowest depths on axis and dropping below 1 0 away from the axial trough this spatial pattern is consistent with an inverse relationship between b value and ambient stress conditions with the lowest stress levels at shallow depths and relatively high stress levels or low pore pressures observed away from the axial zone intermediate b values are observed on axis above the ridge system s melt lens however within this region there also exists significant spatial variability this indicates that stress conditions and or structural heterogeneity may vary at subkilometer scales within the hydrothermal circulation cell although the observational period is characterized by increasing seismicity rates building toward an eruptive episode in january 2006 the first order spatial pattern of b values is sustained with no overall temporal trend as a byproduct of this b value analysis the detection capabilities of the array are assessed empirically

microearthquake characteristics and crustal velocity structure at 29 n on the mid atlantic ridge the architecture of a slow spreading segment we report the results of a microearthquake and seismic tomography experiment conducted along the southern half of the mid atlantic ridge segment at 29 n and aimed at investigating the relationship of earthquake and seismic structural characteristics to spreading processes the seismic velocity structure is obtained from two dimensional 2 d and three dimensional 3 d tomographic inversions of travel times from shots along an axial refraction line inversion solutions indicate that the velocity structure in the lower crust is heterogeneous with higher velocities and relatively thin crust near the segment end and lower velocities and a thicker layer 3 near the central bathymetrie high the thickness of the lower crust at the segment end is asymmetric across axis with thinner crust beneath the inside corner the indicated variations in crustal thickness are consistent with those inferred from mantle bouguer gravity anomalies the microearthquakes located along axis during the 41 day recording period cluster in three separate along axis regions 1 the southern segment end near 28 55 n 2 the central along axis topographic high at 29 11 n near and north of the broken spur hydrothermal vent field and 3 a region midway between beneath a volcano near 29 02 n the greatest level of microearthquake activity was in a diffuse zone off axis beneath the inside corner of a nontransform offset this pattern of off axis microearthquake activity and the cross axis asymmetry in crustal thickness at the segment end support tectonic models in which normal faulting and consequent crustal thinning occur preferentially at inside corner regions anomalous focal mechanisms for microearthquakes beneath the along axis volcano and the significant seismicity beneath the axial volcanic ridge at the segment center in contrast may be the result of volcanic and hydrothermal processes such as magma movement or thermal stresses generated near cooling plutons a comparison of microearthquake characteristics with residual gravity data and velocity structure leads to the hypothesis that microearthquakes associated with areas of thin crust near the segment end and inside corner are dominantly tectonic in nature whereas microearthquakes associated with volcanic and hydrothermal processes are more likely to occur toward the segment center in areas of greater rates of magma supply and thicker crust along axis well resolved focal depths determined with a 3 d velocity model range from 3 to 6 km beneath the seafloor and do not shoal toward the segment center these observations indicate that the thermal structure of the crust along this slow spreading ridge segment is not in steady state

earthquake scaling laws for creeping and non creeping faults the dependence of the summed moment release on the fault dimension differs between non creeping faults and creeping faults or volcanic regions this can be attributed to different earthquake scaling laws the number n of events with fault length l scales for non creeping faults as n l l 2 and for creeping faults as n l l 3 this difference offers a means for mapping the distribution of creep in the seismogenic crust

microearthquakes beneath median valley of mid atlantic ridge near 23 n tomography and tectonics data from a microearthquake experiment in the median valley of the mid atlantic ridge near 23 n in 1982 are used to measure earthquake source parameters to determine the laterally heterogeneous seismic velocity structure across the inner floor and to develop a kinematic tectonic model for this portion of the median valley fifty three microearthquakes occurred over a 10 day period beneath the median valley inner floor and eastern rift mountains twenty of 23 well located inner floor epicenters define a line of activity about 17 km long having a strike of n25 e and located near an along axis depression some 300 400 m deeper than surrounding regions earthquakes with well resolved hypocenters generally have focal depths of 4 8 km beneath the seafloor of both the inner floor and the rift mountains the hypocentral locations are robust with respect to plausible lateral variations in seismic velocity structure composite fault plane solutions for inner floor events indicate normal faulting on planes dipping at angles near 45 normal faulting mechanisms although poorly constrained are also indicated for the rift mountain microearthquakes seismic moments approximate fault dimensions and average stress drops for the largest events recorded are 1019 1020 dyn cm 200 400 m and 1 70 bars respectively a twodimensional tomographic inversion of p wave travel time residuals from microearthquakes and local shots indicates a well resolved lateral heterogeneity in crustal velocity structure across the median valley inner floor p wave velocities at 1 5 km depth within a zone less than 10 km wide beneath the central inner floor are lower by several percent than in surrounding regions the most likely explanation for the low velocities is that the region is the site of the most recent local magmatic injection and remains pervasively fractured as a result of rapid hydrothermal quenching of the newly emplaced crustal column by this view the seismic velocity structure at the ridge axis evolves probably by the sealing of cracks and pores within the first few hundred thousand years of crustal accretion consideration of the detailed sea beam bathymetry in this region of the inner floor the characteristics of large earthquakes that the region has experienced during the past 25 years and the results of the microearthquake and tomography analysis suggests that this section of the median valley has been undergoing continued horizontal extension and block faulting without significant crustal injection of magma for at least the past 104 years

cycling of fallout pu 241am 137cs and natural u th 210pb radionuclides in washington continental slope sediments abstract we report activity profiles of pu isotopes 241am 137cs 210pb u and th isotopes in washington continental slope sediments from 332 1415 m water identify major processes responsible for them and compare results with those previously reported for washington shelf sediments specific activities of all nuclides but 137cs are higher in slope surface sediments than shelf surface sediments these higher specific activities which are due to more efficient scavenging by finer grained particles and higher concentrations of dissolved nuclides in slope waters do not cause proportionally higher slope sediment radionuclide inventories because they are counterbalanced by 5 10 fold lower sediment accumulation rates on the slope profiles of natural u and th series isotopes reveal that fallout nuclide profiles are not markedly affected by greater exchange of pore and overlying waters but they are dependent upon rate and depth of solid phase mixing 239 240pu activities inventories in slope sediments correlate with those of excess 210pb as in shelf sediments but with higher ratios of 210 pb pu in slope deposits 241 am 239 240 pu activity and inventory ratios in slope sediments average twice ratios in shelf sediments due primarily to greater particle affinity and preferential downward transport of 241am am pu activity ratios change little with depth in slope sediments suggesting insignificant separation of the two nuclides during diagenesis 239 240 pu 137 cs ratios are 2 3 times greater in slope than shelf sediments due to a combination of preferential scavenging of pu and greater 137cs loss by decay and desorption pu cs activity ratios decrease with depth in more porous slope sediments due to downward migration of 137cs 137cs is found in slope sediments from all water depths even though it is barely detectable below about 700 m in north pacific waters

microearthquakes at intermediate spreading rate ridges the cleft segment megaplume site on the juan de fuca ridge we discuss models for earthquake generation at intermediate spreading rate ridges and present results from an ocean bottom seismograph obs deployment on the cleft segment of the juan de fuca ridge jdfr although the cleft segment is spreading at approximately 6 cm yr an intermediate spreading rate it has no seismic activity detectable by land networks which provide a detection threshold of ml 4 or by northeast pacific underwater hydrophone arrays with a detection threshold of ml 2 to monitor seismicity four obss were deployed at the cleft segment for 11 days during august 1990 these obss detected 34 regional earthquakes associated with adjacent transform faults in addition 10 local microearthquakes ml 0 to 1 were observed over a 10 km segment of the ridge crest four local events were detected sufficiently well by multiple instruments to allow their hypocenters to be determined all these events occurred within the rift valley of the cleft segment spreading axis three events lie directly beneath a linear surface zone of hydrothermal venting this zone has been identified as the site of a seafloor volcanic eruption dike injection and large scale hydrothermal venting or megaplume event that occurred during 1986 and 1987 these earthquakes were located at depths coincident with the dike injection zone a possible mechanism for earthquake production might be tectonic release of stress acquired during the dike injection either along the dike zone or along the bounding faults of the rift valley an alternative mechanism may be stress release associated with hydrothermal activity and cooling of the dike a fourth event was located southeast of the linear hydrothermal venting and dike injection zone at a depth coincident with the spreading center crustal melt zone these observations suggest that the cleft segment may primarily experience seismic events correlated with periods of volcanic intrusion

the radiocarbon age of organic carbon in marine surface sediments abstract long term carbon cycling and climate change are strongly dependent on organic carbon oc burial in marine sediments radiocarbon 14 c has been widely used to constrain the sources sinks and processing of sedimentary oc to elucidate the dominant controls on the radiocarbon content of total organic carbon 14 c toc accumulating in surface sediments we construct a box model that predicts 14 c toc in the sediment mixed layer measured as fraction modern fm our model defines three distinct oc pools degradable semi labile and refractory and assumes that 14 c toc flux to sediments is exclusively derived from surface ocean primary productivity and hence follows a generic surface ocean dissolved inorganic carbon dic bomb curve model predictions are compared to a set of 75 surface sediment samples which span a wide geographic range and reflect diverse water column and depositional conditions and for which sedimentation rate and mixed layer depth are well characterized our model overestimates the fm value for a majority 65 of these sites especially at shallow water depths and for sites characterized by depleted 13 c toc values the model is most sensitive to sedimentation rate and mixed layer depth therefore slight changes to these parameters can lead to a match between modeled and measured fm values at many sites yet in some cases we find that measured fm values cannot be simulated without large and unrealistic changes to sedimentation rate and mixed layer depth these results point to sources of pre aged oc to surface sediments and implicate soil derived terrestrial oc reworked marine oc and or anthropogenic carbon as important components of the organic matter present in surface sediments this approach provides a valuable framework within which to explore controls on sedimentary organic matter composition and carbon burial over a range of spatial and temporal scales

deposition of bomb 14c in continental slope sediments of the mid atlantic bight assessing organic matter sources and burial rates abstract as part of the ocean margins program omp organic carbon 14c measurements have been made on benthic fauna and kasten core sediments from the north carolina continental slope these analyses are used to evaluate the nature and burial flux of organic matter in the omp study area off cape hatteras despite the fact that surface sediment 14c contents ranged from 41 to 215 per mil the benthic fauna primarily polychaetes all contained significant amounts of bomb 14c body tissue 14c contents ranging from 20 to 82 per mil bomb 14c clearly is reaching the seabed on the north carolina slope and the labile planktonic material carrying this signal is a primary source of nutrition to the benthic ecosystem the enrichment of 14c in benthic faunal tissue relative to the 14c content of bulk surface sediment organic matter a difference of 150 per mil is attributed to a combination of particle selection and selective digestive processes organic carbon burial rates from 12 stations on the north carolina slope varied from 0 02 to 1 7 mol of c m 2 yr 1 with a mean value of 0 7 mol of c m 2 yr 1 the accumulation of organic matter on the upper slope accounts for

investigation of microearthquake activity following an intraplate teleseismic swarm on the west flank of the southern east pacific rise between february 1991 and may 1992 33 intraplate earthquakes having body wave magnitudes between 4 3 and 6 0 were located on the west flank of the southern east pacific rise by the international seismological center seven months after the last teleseismic event we deployed four ocean bottom seismometers at the site of the teleseismic swarm one hundred and ninety two microearthquakes were located using p and s travel times of events recorded by three or more instruments during the 16 day deployment most of the microearthquakes were in a band about 30 km long and 6 km wide between and parallel to seamount chains in addition several events were distributed along a line perpendicular to the main seismicity band and parallel to the ridge axis the focal depths of the microearthquakes range from 1 to 15 km and most are between 5 and 12 km similar to the depth range of the teleseismic events hung and forsyth 1996 the composite p wave polarities indicate that the microearthquakes had a variety of focal mechanisms we developed a new grid search inversion technique that utilizes the p wave polarities and the empirically corrected ratios of p and s wave amplitudes to find the focal mechanisms of individual events within the acceptable travel time and amplitude misfits focal solutions are fairly stable normal faulting is found in the ridge parallel seismicity line the thrust and strike slip faulting in the main seismicity band is distinctly different from the exclusively normal faulting mechanisms of the teleseismic events there is no apparent depth dependence of fault types none of the existing models of the sources of stress ridge push thermoelastic stresses loading by local topographic features caldera collapse and north south extension of the pacific plate provides a satisfactory explanation for both the teleseismic swarm and microearthquakes we propose a new tectonic scenario in this scenario the lithosphere is prestressed by the cooling of the plate magma rising from the deeper mantle induces normal faulting ahead of the dike tips in the lower lithosphere which is already under extensional thermal stress producing the larger teleseismically detected events once the dikes propagate into the lithosphere the region surrounding the dikes behind the tips is compressed by the overpressure of magma depending on the geometry of the dikes the local orientations of the minimum principal stress and the local weaknesses in the lithosphere thrust or strike slip faulting the microearthquakes may occur

evolution of the raman spectrum of carbonaceous material in low grade metasediments of the glarus alps switzerland terra nova 22 354 360 2010 abstract the raman spectrum of carbonaceous material cm from advanced diagenesis 200 c to low grade metamorphism 320 c is documented in the helvetic flysch of the glarus alps switzerland the spectrum is complex with several defect bands at 1200 d4 1350 d1 1500 d3 and 1620 d2 cm 1 we document the evolution of these bands relative to the graphite g band with increasing metamorphic grade and we show that this qualitative evolution may be used as a proxy for temperature in the glarus alps we develop a robust peak fitting method and propose quantitative parameters that may be used as proxies for thermal metamorphism in this region further work in other geological contexts is needed to assess whether the spectral evolution observed in the glarus alps may be generalized and to critically assess the potential for calibrating a general empirical and quantitative thermometer based on the raman spectrum of cm in low grade rocks

multiple palaeoproterozoic carbon burial episodes and excursions organic rich rocks averaging 2 5 total organic carbon and positive carbonate carbon isotope excursions view the mathml source and locally much higher i e the lomagundi jatuli event are hallmark features of palaeoproterozoic successions and are assumed to archive a global event of unique environmental conditions following the c 2 3 ga great oxidation event here we combine new and published geochronology that shows that the main palaeoproterozoic carbon burial episodes cbes preserved in russia gabon and australia were temporally discrete depositional events between c 2 10 and 1 85 ga in northwest russia we can also show that timing of the termination of the lomagundi jatuli event may have differed by up to 50 ma between localities and that ni mineralisation occurred at c 1920 ma further cbes have traits in common with mesozoic oceanic anoxic events oaes both are exceptionally organic rich relative to encasing strata associated with contemporaneous igneous activity and marked by organic carbon isotope profiles that exhibit a stepped decrease followed by a stabilisation period and recovery although cbe strata are thicker and of greater duration than oaes 100 s of metres versus metres 106 years versus 105 years their shared characteristics hint at a commonality of cause s and feedbacks this suggests that cbes represent processes that can be either basin specific or global in nature and a combination of circumstances that are not unique to the palaeoproterozoic our findings urge circumspection and re consideration of models that assume cbes are a deep time singularity

travel time residuals of teleseismic p waves at the rodriguez triple junction in the indian ocean using ocean bottom seismometers we conducted three weeks of seismic observations at the rodriguez triple junction rtj in the indian ocean using 18 ocean bottom seismometers over an area of 90km 90km we identified six teleseismic events and obtained significant anomalies in the relative travel time residuals of the teleseismic p waves the residuals are positive at the rtj the northern part of the southeast indian ridge seir and the eastern side of the central indian ridge cir and are negative at and around the southwest indian ridge swir it is suggested that there is a relatively hotter mantle under the triple junction and the northern part of the seir segment along axis variations in mantle temperature along the seir segment and cooler mantle under the swir segment the cir segment has an asymmetrical distribution of relative residuals

ocean bottom seismograph observations on the charlie gibbs fracture zone summary two networks of four ocean bottom seismographs were deployed successfully in 1982 september on spreading centre sites in the region of the charlie gibbs fracture zone activity was observed beneath both networks and hypocentres for over 100 events have been determined the observations confirm the existence of seismic activity along a suspected short spreading centre near longitude 31 75 w linking the north and south transform valleys of the fracture zone a relatively thick 7 9 km seismogenic zone is seen beneath the axis which is in agreement with earlier microearthquake observations on the mid atlantic ridge

plutonium and210pb distributions in northeast atlantic sediments subsurface anomalies caused by non local mixing abstract the depth distributions of 210 pb and 239 240 pu measured in a suite of box cores collected from water depths of 4000 5000 m in the northeast atlantic ocean exhibit pronounced subsurface maxima caused by sediment reworking by benthic infauna small scale spatial heterogeneity in bioturbation rates is indicated by large differences in tracer profiles from duplicate cores separated only by a few centimeters 210 pb and 239 240 pu activity distributions from each subcore exhibit a high degree of correlation and most tracer profiles exhibit one or more subsurface maxima one dimensional biodiffusion analogue models do not adequately simulate the principal features of this data set however an inverse conveyer belt mixing model which simulates subsurface egestion or a functionally equivalent process of surficial material which is enriched both in organic debris and radioactive tracers can reproduce the subsurface tracer maxima single event and continuous subsurface egestion models have been formulated and solved for different feeding rates and background biodiffusive fields the single event model provides a better fit to the data and in particular ensures the observed high degree of correlation between the 210 pb and 239 240 pu activity profiles regardless of the different tracer input functions the most likely candidate responsible for subsurface tracer egestion is a large infaunal worm of the phylum sipunculida which dominates the biomass below a depth of 3 cm

variations in the frequency magnitude distribution with depth in two volcanic areas mount st helens washington and mt spurr alaska the frequency magnitude distribution of earthquakes characterized using the b value is examined as a function of space beneath mount st helens 1988 1996 and mt spurr 1991 1995 at mount st helens two volumes of anomalously high b b 1 3 can be observed at depths of 2 6 3 6 km below the crater floor and below 6 4 km these anomalies coincide with 1 the depth of vesiculation of ascending magma and 2 the suggested location of a magma chamber at mount st helens study of mt spurr reveals an area of high b value b 1 3 at a depth of about 2 3 4 5 km below the crater floor of the active vent crater peak we propose that the higher material heterogeneity in the vicinity of a magma chamber or conduit due to vesiculation of the ascending magma is the main cause of the increased b value at shallow depths alternatively interaction of magma with groundwater may have increased pore pressure and lowered the effective stress the deeper anomaly at mount st helens is likely caused by high thermal stress gradients in the vicinity of the magma chamber our results indicate that detailed mapping of the frequency magnitude distribution can be used as a tool to trace vesiculation and locate active magma chambers

temporal and three dimensional spatial analyses of the frequency magnitude distribution near long valley caldera california summary the 3 d distribution of the b value of the frequency magnitude distribution is analysed in the seismically active parts of the crust near long valley caldera california the seismicity is sampled in spherical volumes containing n 150 earthquakes and centred at nodes of a grid separated by 0 3 km significant variations in the b value are detected with b ranging from b 0 6 to b 2 0 high b value volumes are located near the resurgent dome and at depths below 5 km at mammoth mountain b values are found to be much lower south of the long valley caldera we interpret this to indicate that an active magma body has advanced from depths below 8 km to depths of 4 to 5 km beneath mammoth mountain in 1989 and that anomalous crust either highly fractured or containing unusually high pore pressure such as is the case in the vicinity of active magma bodies exists north of the seismically active area beneath the resurgent dome at all depths we also investigate the spatial distribution of temporal variations of the frequency magnitude distribution by introducing diverential b value maps b values increased from b 0 8 to b 1 5 underneath mammoth mountain at the onset of the 1989 earthquake swarm and remained high thereafter this suggests that an intrusion permanently altered the average distribution of cracks at 5 10 km depth or that the pore pressure permanently increased we propose that high b values are a necessary but not suycient condition near a magmatic body and therefore spatial b value mapping can be used to aid in the identification of active magma bodies

δ14c of the organic matter in sediments from the antarctic polar front origin and dynamics of sedimentary organic carbon abstract 14 c of sedimentary organic matter was measured on one sediment core from the deep sea below the subantarctic front of the indian ocean using acceleration mass spectrometry in the mixed layer values decrease from the sediment water interface 263 to 10 cm depth 446 with a sharp gradient in the first 2 cm this indicates the loss of organic younger fractions due to mineralization of labile organic matter during bioturbation and burial in the upper sediment layer a previously published model limnol oceanogr 43 1998 420 combined with suppositions concerning the 14 c content of each organic carbon fraction produces a good agreement with the radiocarbon data our data set supports a strong biogeochemical coupling between the pelagic zone and the sediment in this region the low 14 c values found at depth in the sediment point to a particular source of the refractory organic matter it could originate from adsorption of old deep sea dissolved organic carbon on falling particles nature 347 1990 172 and contribute largely to the burial of organic carbon in this type of sediment

particle mixing rates in sediments of the eastern equatorial pacific evidence from 210pb 239 240pu and 137cs distributions at manop sites abstract particle mixing rates db calculated from excess 210pb gradients in sediments of the east equatorial pacific range from 0 04 to 0 5 cm2 y with variation of a factor of 3 4 at a single site diffusion of the 236ra daughter 222rn may affect 210pb distributions under conditions of slow mixing and low 210pb flux to the seafloor as shown by a siliceous ooze clay core which contained the fallout radionuclides 239 240pu and 137cs but no excess 210pb relative to 226ra there is no clear relationship between 210pbderived mixing rates and sediment type accumulation rate or organic carbon flux to the sediments comparison of 210pb mixing rates with those calculated from 239 240pu and 137cs distributions reveals better agreement for a pulse input of the fallout radionuclides db 0 03 0 4 cm2 y than for continuous input at a constant rate db 0 1 1 6 cm2 y although the pu and 137cs data are better fit by the latter model the agreement may be fortuitous because 239 240pu and 137cs appear significantly deeper than 210pb in at least one core tracer separation could be caused by particle size selective mixing by the benthic fauna or by chemical mobilization if the fallout radionuclides are scavenged from surface waters by large organic rich particles such as fecal pellets their release and migration may result from decomposition of the carrier in surface sediments either a relatively unreactive form of pu e g oxidized pu has been released by this process or a one dimensional model is inadequate to explain its observed penetration into the sediments activity ratios of 239 240 pu 137 cs in the sediments decrease with increasing north latitude and the trend reflects higher fluxes of 239 240pu near the weapons test site at christmas island 2 n the 239 240 pu 137 cs ratios and fluxes to the sediment assuming constant input at the siliceous ooze red clay site are consistent with published sediment trap data from a nearby site thus if fallout radionuclide fluxes to the sea floor were higher in the past both 239 240pu and 137cs have been released from sinking particles

spatial variability of seismicity parameters in aftershock zones the spatial variability of the b value of the frequency magnitude relationship and the decay rate of aftershocks as described by the p value of the modified omori law is investigated by using dense spatial grids we map out the distribution of b and p values within the landers northridge morgan hill and kobe aftershock sequences considerable spatial variability is found with b values of independent subvolumes ranging from 0 6 to 1 4 and p values ranging from 0 6 to 1 8 these systematic and statistically highly significant differences argue that it is an oversimplification to assign one single p and b value to an aftershock sequence that extends up to 100 km the spatial distribution of these two parameters is compared with the slip distribution during the mainshock suggesting that the areas of largest slip release correlate with high b value regions we hypothesize that the frictional heat created during the event may influence the p value distribution within an aftershock zone while applied shear stress crack density and pore pressure govern the frequency magnitude distribution by investigating the frequency magnitude distribution separately for preseismic and postseismic periods for the morgan hill mainshock we find that only the volume in the vicinity of the highest slip release shows a significant increase in the b value which decays to premainshock values within a year surrounding areas of the aftershock zone show an approximately constant b value with time because the aftershock hazard after a mainshock depends strongly on both the b and p value we propose that aftershock hazard assessment can be improved by taking into account the spatial distribution of the parameters

mapping active magma chambers by b values beneath the off ito volcano japan the b value of the frequency magnitude relation and thus the mean magnitude in the off ito volcanic area is not uniform based on detailed b value tomography using about 10 000 events in an area of 10 km radius and in the upper 15 km of the crust a high b value anomaly b 15 below 7 km depth and with a radius of about 2 km located below the coast southeast of ito contrasts with lower values of typically b 0 7 north of it and at shallower depths on the basis of surface deformations a tensile crack dislocation source was located above 7 km depth thus we surmise that the magma chamber in the off ito area was located below 7 km depth the correlation of a high b value anomaly with this model supports our hypothesis that active magma chambers may be mapped by high b value anomalies the magma body may be described as consisting of two parts a lower well established chamber below 11 km depth and a shallower part 11 7 km depth the b value in the volume surrounding the magma chamber increased from 0 7 during the 1980s to 1 5 during the earthquake swarms of the 1990s this may reflect a trend of increasing crack density due to the intrusion activity

three dimensional magnetic and gravity studies of the rodriguez triple junction in the indian ocean the rodriguez triple junction rtj is a ridge ridge ridge triple junction connecting three mid ocean ridges in the indian ocean the central indian ridge cir southeast indian ridge seir and southwest indian ridge swir we carried out three dimensional analyses of sea beam bathymetry magnetic and gravity anomaly data covering an area of about 200 km 140 km at the triple junction and obtained the magnetization distribution and the bouguer anomaly field in the magnetic inversion result some of the magnetic lineations of the cir and seir are recognized within the swir valley and the bouguer anomaly field shows little variation over the swir valley in spite of its large topographic relief these results indicate the tectonic rifting at the swir by crustal thinning without volcanic activity a 4 km wide graben in the western cir flank was found to have extremely high magnetization and lower bouguer anomaly than the eastern flank of the cir because the cir and seir have similar spreading directions and rates at the triple junction we consider the graben represents a fossil propagating rift of the seir which began to propagate toward the cir at about 1 ma and then became inactive when the swir grew to reach the cir seir spreading axis at about 0 5 ma we suggest the above process repeats several times and composes part of the triple junction evolution on a longer timescale we also suggest that the whole process is closely related to formation of the cir segments at the triple junction since 20 ma

constraining the maximum depth of brittle deformation at slow and ultraslow spreading ridges using microseismicity the depth of earthquakes along mid ocean ridges is restricted by the relatively thin brittle lithosphere that overlies a hot upwelling mantle with decreasing spreading rate earthquakes may occur deeper in the lithosphere accommodating strain within a thicker brittle layer new data from the ultraslow spreading mid cayman spreading center mcsc in the caribbean sea illustrate that earthquakes occur to 10 km depth below seafloor and hence occur deeper than along most other slow spreading ridges the mcsc spreads at 15 mm yr full rate while a similarly well studied obliquely opening portion of the southwest indian ridge swir spreads at an even slower rate of 8 mm yr if the obliquity of spreading is considered the swir has previously been proposed to have earthquakes occurring as deep as 32 km but no shallower than 5 km these characteristics have been attributed to the combined effect of stable deformation of serpentinized mantle and an extremely deep thermal boundary layer in the context of our mcsc results we reanalyze the swir data and find a maximum depth of seismicity of 17 km consistent with compilations of spreading rate dependence derived from slow and ultraslow spreading ridges together the new mcsc data and swir reanalysis presented here support the hypothesis that depth seismicity relationships at mid ocean ridges are a function of their thermal mechanical structure as reflected in their spreading rate

microearthquake evidence for extension across the kane transform fault we report the results of a microearthquake experiment conducted in 1987 in the active transform portion of the kane fracture zone the kane is a slow slipping 25 mm yr large offset 150 km transform delineated by a pronounced transform valley the experiment site lies in the eastern half of the transform 40 km west of the eastern ridge transform intersection in a region of marked transform parallel topography the network consisted of 12 ocean bottom hydrophones and seismometers and extended for 25 km along the transform and 10 km in width spanning the transform valley and a narrow median ridge to the north hypocentral parameters were determined for 86 earthquakes epicenters within the network define a narrow transform parallel trend along the steep southern wall of the transform valley maximum focal depths increase from 6 to 9 km from east to west across the network significant activity was also detected outside the transform tectonized zone to the southeast this activity may be associated with the inside corner of the ridge transform intersection while to the north about 20 events occurred in 7 m y old crust in a region of ridge parallel bathymetry six focal mechanisms were obtained from p wave first motions for events within the transform valley the best constrained solutions for four earthquakes within the network show normal faulting on fault planes subparallel to the trend of the transform all the mechanisms indicate a tension axis perpendicular to the trend of the transform these results together with a significant historical record of large earthquakes near the experiment site lead us to conclude that the principal transform displacement zone was inactive during our experiment and that the activity we recorded is the result of extension in the adjacent lithosphere the observed focal mechanisms and the inference that the axis of least compressive stress is approximately perpendicular to the transform provide direct evidence that the transform fault is mechanically weak relative to the surrounding lithosphere potential sources of extension across the transform include thermal stress in the young oceanic lithosphere topographic loading a small component of plate divergence normal to the transform and northward motion of the asthenosphere relative to the surface plate

the geochemistry of fallout plutonium in the north atlantic ii ratios and their significance a systematic decrease in the 240pu239pu ratio in marine sediments is found with increasing water depth along a transect of cores between woods hole and bermuda the 240pu239pu atom ratios range from o 18 on the shelf to o 10 at 5000 m but do not change with depth in individual cores a model is presented which can account for the range of 240pu239pu ratios found in this and other similar studies noshkin and gatrousis 1974 scottet al 1983 we propose that there have been at least two distinct sources of fallout pu to this region the major source of pu is global stratospheric fallout characterized by a 240pu239pu ratio of 0 18 and a relatively long residence time in seawater the second source is characterized by a much lower 240pu239pu ratio and relative to global fallout it must have been much more efficiently removed from the water column to deep sea sediments we suggest that surface based low yield testing at the nevada test site is the only source of low ratio fallout pu which could account for the timing inventories and refractory characteristics of this second component of fallout pu inputs to the north atlantic

distributed extension at the indian ocean triple junction the indian ocean triple junction is a ridge ridge ridge type joining two medium spreading ridges with one slow spreading ridge gloria long range side scan sonar images show that while the axial valleys of the two medium spreading ridges are almost colinear apart from a small 5 km offset the valley of the slow spreading third axis does not meet the other two in a simple fashion the axis of this slow spreading southwest indian ridge swir beyond the rift valley walls of the other two ridges steadily deepens by over 1000 m away from the triple junction to reach 5000 m at 35 km the gloria images show large normal faults around the deep swir rift valley which increase in heave away from the triple junction and crosscut the abyssal hills formed at the faster spreading central indian ridge indicating that extension across the swir is distributed over a zone 10 km or more wide this zone also shows no evidence for the formation of new oceanic crust suggesting that the extension may be amagmatic near the triple junction the high relief of the swir rift flanks containing tilted seafloor of the other two ridges may be an isostatic response of the lithosphere to the deep valley produced by this rifting these observations which suggest the progressive development of a propagating swir rift by the extension of preexisting seafloor may have general implications for the dynamics of oceanic spreading centers in particular the deepening and widening of the valley away from the triple junction is attributed to the competing effects of tectonic thinning and lithospheric cooling thermal models of mid ocean ridges spreading at 30 mm yr e g lin and parmentier 1989 predict that the depth to the 700 c isotherm increases with distance from the spreading axis by a proportion similar to the observed doubling in the total width of the deformation with distance from 10 to 35 km away from the triple junction this rate of widening of the deformation zone may therefore support lithospheric necking models for oceanic rift valleys if the width and depth of the deforming lithosphere are related in a simple manner

bioturbation into a recently emplaced deep sea turbidite surface as revealed by210pbexcess 230thexcess and planktonic foraminifera distributions abstract this study concerns the surface of a single distal turbidite emplaced on the madeira abyssal plain a few hundred years ago the vertical and horizontal homogeneity of the unit and the low pelagic sediment accumulation rate enable a ready identification of the post emplacement signals of near surface bioturbation data have been gathered to define 210 pb excess 230 th excess and pelagic foraminifera profiles with depth these are used as constant input flux tracers and treated by models conventionally used to determine bioturbation parameters the mixed layer depths 7 11 cm bioturbation coefficients 0 08 0 18 cm 2 y 1 and inventory data 10 23 dpm cm 2 derived from the 210 pb excess data in five out of six cores are similar to those previously observed elsewhere with the same model in the deep sea under steady state sediment accumulation conditions although the mixed layer depths are similar for all three tracers the 210 pb excess bioturbation intensities are larger by a factor of 3 than those estimated from 230 th excess profiles and about an order of magnitude larger than those from the foraminifera profiles it is suggested that the conventional 210 pb excess model and those for other short lived tracers probably over estimate the intensity of biomixing of foraminifera for palaeontological and related geochemical purposes

microearthquakes in the black smoker hydrothermal field east pacific rise at 21 n in july and august 1980 an array of five ocean bottom seismographs was deployed within 3 km of the 350 c hydrothermal vents at the rivera submersible experiment rise site at 21 n on the east pacific rise two of these instruments were placed within 600 m of the vents using a transponder navigation network the array detected four basic types of events the first type consisted of local very small microearthquakes locations obtained for 11 of these events place three within 1 km of the vents with the others elsewhere along the rise crest they appear to originate either from movement on the faults in the area or from the hydrothermal system beneath this area a study of the s p times of this type indicates a maximum hypocentral depth of 2 3 km implying a similar limit to the depth of hydrothermal circulation and brittle fracturing in the vicinity of the vents the second type of event found consisted of emergent earthquakes that have many of the characteristics of volcanic harmonic tremor the frequency of these events falls in the 1 5 hz range and are similar in appearance to those seen at mount st helens prior to and during its may 1980 eruption they may be either hydrothermal or volcanic in origin the third type of event produced a very monochromatic high frequency seismogram with the energy concentrated at 20 hz these events also appear to have a local origin the particle motion of these earthquakes shows almost pure stonely wave propagation and they appear to originate from a regularly oscillating dilatational source the nature of the source of event types two and three is clearly not the usual sort of motion on a fault and is still not understood the fourth type of earthquake recordings were regional events apparently from the nearby rivera and tamayo fracture zones the low frequency acoustic noise in the water was 16 64 times higher within 300 m of the vents than it was 2 km away this implies that the black smoker vents may be generating a large acoustical disturbance as the 350 c water pours into the ocean the observations are among the first of seismicity on moderate to fast spreading ridges and many of the events appear to be related to the hydrothermal activity found there

vertical open burrows in deep sea sediments 2 m in length previous studies of the mixed layer in deep sea sediments suggest a maximum burrowing depth of about 10 30 cm ref 1 however we now report that two recently obtained cores from the north east atlantic s126 2 and s126 4 display open burrows to a depth of 214 cm within a distal pelagic turbidite an organic lining in one of the burrows indicates a modern origin many of the burrows are less than 0 5 mm across and are only visible on fractured surfaces their effects on sediment mixing are probably minimal but they provide a network of open vertical channels which considerably increases the permeability this is of crucial importance to studies involving pore water movement such as those associated with the radioactive waste disposal project the effect of the burrows changes the calculated permeability from the equivalent of a clay to that of a coarse sand the delicate structure of the burrows may result in their closure during handling thus requiring their effect on pore water fluxes to be studied in situ

hydrothermal circulation and oil migration at the root of the heterogeneous micro structure of carbonaceous material in the 2 0 ga zaonega formation onega basin russia abstract organic rich rocks of the 2 0 ga zaonega formation karelia russia have been studied extensively to gain understanding of the global carbon cycle and reconstruction of paleo environments directly after the great oxidation event goe this formation has a complex history of alteration involving pervasive hydrothermal circulation hydrocarbon generation migration and mineral authigenesis several previous studies have focused on the description of these secondary effects and the identification of primary geochemical signals in the carbonaceous phases migration and infiltration of organic rich fluids appear to have had only limited effect on the primary carbon isotope record 13corg however the structural variability of carbonaceous material cm appears to have been strongly affected with a range of reported structures including carbon onion shaped nanostructures and mineral templated graphite films here we present a systematic raman spectroscopy based study of the structural variability of cm in a drill core representing the middle and upper strata of the zaonega formation the raman spectra of cm show a systematic difference in structural order between the bulk carbonaceous matrix matrix cm and the cm occurring near mineral contacts contact cm indicating that mineral templating was an important process affecting structural order in the formation the templating effect was observed on the surface of a wide range of minerals the difference in structural order between matrix cm and contact cm can be traced throughout the ca 400 m stratigraphy the structural order varied with the degree of alteration and hydrothermal circulation from highly ordered structures directly above a large gabbro intrusion at the bottom of the stratigraphy to less ordered structures higher up in the sequence this trend directly correlates with the 18o trend of secondary calcite and can be attributed to the decreasing influence and temperature regime of hydrothermal circulation upward in the stratigraphy the results presented here suggest that organic rich hydrothermal fluids can locally strongly enhance graphitization of carbonaceous materials and cause sample scale heterogeneities in the structural order of organic materials this has implications for the interpretation of carbonaceous materials in other ancient rocks experiencing circulation of organic rich hydrothermal fluids

alkenones as paleoceanographic proxies 1 alkenone derived estimates of temperature and co2 are contributing to our understanding of earth s climate history in order to increase confidence in alkenone based climate proxies we recommend the following actions first the sedimentary component or fraction containing alkenones should be identified in order to assess the impact of horizontal advection and vertical mixing on alkenone derived temperature and co2 estimates second differential mixing rates of alkenone containing particles and sand sized foraminifera should be quantified by independent dating of the two phases until that is accomplished apparent temporal offsets of climate proxies in the two phases should be interpreted cautiously third the stability of the unsaturation ratio and carbon isotopic compositions of alkenones during all phases of diagenesis should be confirmed both field and laboratory observations are required fourth future alkenone investigations should be coupled with other paleoclimate proxy measurements at high deposition rate sites in a variety of oceanographic settings in upwelling regions and in the vicinity of river plumes salinity and nutrient proxies should be measured since changes in these parameters may affect alkenone biosynthesis

microearthquake seismicity and focal mechanisms at the rodriguez triple junction in the indian ocean using ocean bottom seismometers hypocenters and focal mechanisms of microearthquakes have been investigated at the rodriguez triple junction in the indian ocean little was known on microearthquake activity in this region we deployed 18 ocean bottom seismographs during the kh93 3 cruise of the r v hakuho maru ocean research institute university of tokyo from july 30 to august 20 1993 we obtained 579 well constrained hypocenters and 13 focal mechanisms microearthquakes were found to be active along all of the three ridges the central indian ridge the southeastern indian ridge and the southwestern indian ridge especially at the triple junction there was an earthquake swarm within narrow area of approximately 15 5 km2 all of the 13 focal mechanisms showed normal or strike slip faultings which means that the extensional stress field characterizes this region

microearthquakes on and near the east pacific rise 9 10 n records from a seismic network deployed as part of an active tomography experiment at 9 30 n on the east pacific rise epr provide an opportunity to characterize local microearthquake activity over an 8 day period with the exception of the region around the 9 03 overlapping spreading center osc no events were located on the rise axis one microearthquake located from p and s wave arrival times lies adjacent to the network 18 km to the west of the rise axis five more events located from p wave and t phase data are to the south of the network three cluster around the western arm of the 9 03 osc while two are located 25 30 km to the east of the osc the seismic moments of the microearthquakes range from 5 1019 to 4 1020 dyn cm several orders of magnitude larger than those reported in other studies of shallow events along the axis of the epr these results suggest that overlapping spreading centers may be the loci of substantial microearthquake activity and that at this location the normal faults that form on the young flanks of the epr are active off axis to distances of at least 20 km

variability of natural and bomb produced radionuclide distributions in abyssal red clay sediments the nature of sedimentation and mixing are examined in abyssal red clay sediments from the north central pacific using three types of indicators 23 th 232th organic 14c and 137cs and 239 24 pu 23 th 232th analysed revealed that the clay sedimentation rate in three box cores collected within a 50 km radius was less than 1 0 ram 10 3 yr however analyses of the organic carbon in thin layers of sediment revealed that radiocarbon was present much deeper in the cores down to 20 cm than was expected from the 23 th 232th distribution in addition both the stratigraphy and inventory of radiocarbon was significantly different between box cores the distributions and inventories of 137cs and 239 24 pu were similar to that found for radiocarbon further illustrating the spatial variability of radionuclides in oligotrophic north pacific red clays these data suggest that bioturbational processes are important for transporting organic carbon down into the sediment column

mapping the frequency magnitude distribution in asperities an improved technique to calculate recurrence times we hypothesize that highly stressed asperities may be defined by mapping anomalously low b values along the san andreas fault near parkfield the asperity under middle mountain with its b 0 46 can be distinguished from all other parts of the fault surface likewise along the calaveras fault the northern asperity of the morgan hill 1984 m6 2 rupture can be identified by its low b of 0 5 as a high stress patch along the fault we add further evidence to the observations that the b value of the frequency magnitude relationship of earthquakes is inversely proportional to stress by showing that it decreases with depth in the parkfield segment of the san andreas and along the calaveras fault in both of these areas b values above and below 5 km depth are 1 2 and 0 8 respectively we propose that probabilistic recurrence times tr based on the seismicity parameters a and b should be calculated from their values within asperities only instead of from the values of the entire rupture area of the maximum expected earthquake the strong patches on faults control the time of rupture because they are capable of accumulating larger stresses than the rest of the fault zone which slips along passively when an asperity breaks therefore no information on tr is contained in the passive fault segments only in the asperities at parkfield the probabilistic estimates of tr derived from the data in the whole rupture and in the asperity only are 72 18 24 and 23 12 18 years respectively compared to the historically observed repeat time of 22 years at morgan hill the tr estimates are 122 46 76 and 78 47 110 years respectively compared to the observed repeat time of 72 years

depth dependence of earthquake frequency magnitude distributions in california implications for rupture initiation statistics of earthquakes in california show linear frequency magnitude relationships in the range of m2 0 to m5 5 for various data sets assuming gutenberg richter distributions there is a systematic decrease in b value with increasing depth of earthquakes we find consistent results for various data sets from northern and southern california that both include and exclude the larger aftershock sequences we suggest that at shallow depth 0 to 6 km conditions with more heterogeneous material properties and lower lithospheric stress prevail rupture initiations are more likely to stop before growing into large earthquakes producing relatively more smaller earthquakes and consequently higher b values these ideas help to explain the depth dependent observations of foreshocks in the western united states the higher occurrence rate of foreshocks preceding shallow earthquakes can be interpreted in terms of rupture initiations that are stopped before growing into the mainshock at greater depth 9 15 km any rupture initiation is more likely to continue growing into a larger event so there are fewer foreshocks if one assumes that frequency magnitude statistics can be used to estimate probabilities of a small rupture initiation growing into a larger earthquake then a small m2 rupture initiation at 9 to 12 km depth is 18 times more likely to grow into a m5 5 or larger event compared to the same small rupture initiation at 0 to 3 km

fine scale distributions of porosity and particulate excess210pb organic carbon and caco3 in surface sediments of the deep equatorial pacific abstract sediment samples were recovered from the central equatorial pacific ocean sectioned at 1 mm intervals and analyzed for porosity organic carbon excess210pb and caco3 steep porosity gradients were measured in the upper 1 cm of the sediment column with extremely high values observed near the sediment surface similarly particulate organic carbon contents are highest at the sediment surface decrease sharply in the upper 1 cm and are relatively constant between 1 and 5 cm caco3 values on the other hand are lowest at the sediment surface and increase to a constant value below 5 10 mm depth at the carbonate ooze sites excess210pb is present throughout the upper 5 cm of the sediments suggesting relatively rapid particle mixing rates however extremely high excess210pb activities 100 dpm g are observed at the sediment surface with sharp gradients present in the upper 1 cm which would suggest slow rates of mixing this apparent contradiction along with the major features of the caco3 and particulate organic carbon profiles can be explained by a particle selective feeding mechanism in which organic carbon excess210pb enriched particles are preferentially maintained at the sediment surface via ingestion and defecation by benthic organisms

postrifting seismicity and a model for the 1993 diking event on the coaxial segment juan de fuca ridge microearthquake data from two deployments of ocean bottom seismographs made shortly after the june july 1993 rifting episode on the coaxial segment of the juan de fuca ridge constrain the propagation path of the magmatic dike and provide insight into the effect of the dike injection on the axial stress field the first deployment consisting of five instruments in a small array aperture of 2 km at the eruption flow site 46 30 n was made 3 months after the episode the second deployment consisting of two larger arrays apertures of 9 km of five instruments each one at the eruption site and one at a site of vigorous venting of bacterial flocculent material floe site 46 18 n was made 11 months after the eruption epicenters from nine posteruption earthquakes near the floe site at the south end of the segment are confined to a narrow zone on the rift axis epicenters from 61 earthquakes observed near the flow site are located throughout the rift valley and include an event cluster just south of the observed lava flows we combine these posteruption earthquakes with t wave data recorded by sosus hydrophones during the rifting episode to develop a model for the diking event in this model magma propagated at depth along the axial rift zone until reaching the eruption site near 46 31 n during the next few days the dike swelled as magma accumulated behind the arrested tip elevating local stress levels and generating a swarm of seismic activity about 3 days after the magma reached the north end of the segment the top of the dike penetrated the seafloor surface resulting in eruptions along the rift zone at the flow site