a sea floor spreading event captured by seismometers two thirds of earth s surface is formed at mid ocean ridges yet sea floor spreading events are poorly understood because they occur far beneath the ocean surface at 9 50 n on the east pacific rise ocean bottom seismometers recently recorded the microearthquake character of a mid ocean ridge eruption including precursory activity a gradual ramp up in activity rates since seismic monitoring began at this site in october 2003 suggests that eruptions may be forecast in the fast spreading environment the pattern culminates in an intense but brief 6 hour inferred diking event on 22 january 2006 followed by rapid tapering to markedly decreased levels of seismicity

the january 1998 earthquake swarm at axial volcano juan de fuca ridge hydroacoustic evidence of seafloor volcanic activity on january 25 1998 at 11 33z an intense swarm of earthquakes began within the summit caldera of axial volcano central juan de fuca ridge jdfr the earthquake swarm was detected using the u s navy hydrophones in the northeast pacific ocean the earthquake swarm lasted 11 days and produced 8247 detected 1037 located earthquakes during the first two days of the swarm earthquake activity migrated from the summit caldera 50 km along the south rift zone of axial volcano at rates of 0 92 0 13 and 0 23 0 09 m s 1 earthquake epicenter migration is characteristic of a lateral magma dike injection the largest three swarm earthquakes occurred within the caldera their timing and mechanisms are consistent with adjustment of the caldera floor as magma is removed from beneath the summit earthquakes with short t wave rise times shallow focal depths cluster at two spots along the summit and south rift zone and are considered possible seafloor eruption sites in situ ground deformation and hydrothermal plume monitors and later shipboard observations confirmed the occurrence of a seafloor volcanic eruption at the volcano s summit

the role of magma injection in localizing black smoker activity the mechanisms for localization of black smoker systems at mid ocean ridges remain to be fully understood seismic data for a segment of the juan de fuca ridge with long lived black smoker vents reveal ongoing magma recharge into the crustal magma chamber thereby providing an explanation for the localization

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

fine scale segmentation of the crustal magma reservoir beneath the east pacific rise mid ocean ridges are composed of segmented faults and magma reservoirs seismic images from the east pacific rise show that the magma reservoirs are segmented on the same fine scale as the surface faults and that distinct lava eruptions are sourced from largely isolated magma lenses

long term tilt and acceleration data from the logatchev hydrothermal vent field mid atlantic ridge measured by the bremen ocean bottom tiltmeter 1 long term seafloor deformations in the logatchev hydrothermal vent field lhf at the mid atlantic ridge are largely unexplored and unknown even though the lhf has been the focus of international research for many years as seafloor tilt and vertical acceleration provide key information about seafloor deformations the bremen ocean bottom tiltmeter obt was deployed in may 2005 at position 14 45 11 7 n 44 58 47 0 w 3035 m water depth in the lhf the obt recorded 384 days and was recovered in january 2007 strong tilt steps and strong gradual tilt changes over less than a minute to days in the range of some 10 mrad and aligned mostly with the topography possibly indicate nearby mass movements like avalanches of bulk material due to local uplift or subsidence or may show tectonic activity a vertically aligned high resolution microelectromechanical systems mems accelerometer of type servo k beam in the sensor package seems to be helpful to distinguish between tilt signals caused by a true rotation and fake tilt related to a transient translational motion of the obt in a horizontal direction hodographs show elliptic motion patterns with about 1 mrad total tilt amplitude and distinct orientations of tilt toward hydrothermal vents it is up to speculation whether the latter signals are related to hydrothermal fluid circulation the amplitude spectra of these tilt signals and acceleration show discrete lines mostly between 0 1 and 50 mhz the spectra show the periodic character of those signals and also proof that tides or bottom currents which are known to show lower signal frequencies or tremor which generally has higher frequencies are most likely not the reason compared with studies onshore and offshore the lhf is most likely an area of strong and highly variable seafloor deformations

electrical image of passive mantle upwelling beneath the northern east pacific rise sea floor magnetotelluric soundings along a portion of the east pacific rise reveal a symmetric high conductivity triangular region at depths of 20 90 kilometres that is consistent with partial melting of passively upwelling mantle the east pacific rise is a fast spreading mid ocean ridge where sea floor volcanoes and hydrothermal vents result from a robust mantle melt supply to explore how this magma is supplied to the surface kerry key and colleagues use sea floor magnetotelluric soundings to image mantle electrical structure to a depth of about 160 km along the northern east pacific rise the images reveal a symmetrical high conductivity anomaly beneath the ridge consistent with partial melting of passively upwelling mantle they explain a deeper conductor observed east of the ridge by asymmetric upwelling due to viscous coupling across two nearby transform faults these findings suggest that plate driven passive upwelling dominates this ridge segment while dynamic forces are negligible melt generated by mantle upwelling is fundamental to the production of new oceanic crust at mid ocean ridges yet the forces controlling this process are debated1 2 passive flow models predict symmetric upwelling due to viscous drag from the diverging tectonic plates but have been challenged by geophysical observations of asymmetric upwelling3 4 5 that suggest anomalous mantle pressure and temperature gradients2 6 7 and by observations of concentrated upwelling centres8 consistent with active models where buoyancy forces give rise to focused convective flow2 here we use sea floor magnetotelluric soundings at the fast spreading northern east pacific rise to image mantle electrical structure to a depth of about 160 kilometres our data reveal a symmetric high conductivity region at depths of 20 90 kilometres that is consistent with partial melting of passively upwelling mantle9 10 11 the triangular region of conductive partial melt matches passive flow predictions suggesting that melt focusing to the ridge occurs in the porous melting region rather than along the shallower base of the thermal lithosphere a deeper conductor observed east of the ridge at a depth of more than 100 kilometres is explained by asymmetric upwelling due to viscous coupling across two nearby transform faults significant electrical anisotropy occurs only in the shallowest mantle east of the ridge axis where high vertical conductivity at depths of 10 20 kilometres indicates localized porous conduits this suggests that a coincident seismic velocity anomaly12 is evidence of shallow magma transport channels13 14 rather than deeper off axis upwelling we interpret the mantle electrical structure as evidence that plate driven passive upwelling dominates this ridge segment with dynamic forces being negligible

axial summit trough of the east pacific rice 9 10 n geological characteristics and evolution of the axial zone on fast spreading mid ocean ridge the nature and morphological characteristics of axial summit troughs on fast 90 130 mm yr 1 full spreading rate and superfast spreading 130 mm yr 1 mid ocean ridge crests reflect the time integrated effects of long term magmatic cycles short term volcanic episodicity and the tensional stress regime imposed on young ocean crust two principal types of axial trough morphology have been identified and associated with distinct volcanic and tectonic processes occurring at fast and superfast spreading mid ocean ridge crests 1 narrow axial troughs 300 2000 m wide and 30 100 m deep on the east pacific rise crest are classified as axial summit graben the dimensions of axial summit graben as well as the morphological and structural character of their walls and floors suggest a primary tectonic origin an axial summit graben may contain a nested axial summit collapse trough implying that processes responsible for these endemic features may be linked near bottom side looking sonar and observational data collected using the towed vehicle argo i and submersible alvin have been used to characterize the axial summit trough of the fast spreading east pacific rise between 9 and 10 n a four stage model is presented for the evolution of this axial summit collapse trough as well as for other well studied portions of the east pacific rise crest from 21 n to 20 s we propose that the transition from a narrow surface collapse dominated axial trough to a broader fault bounded graben is controlled by the relative importance of diking volcanism hydrothermal cooling and tectonism along a ridge segment over time periods 104 years

insar observations of post rifting deformation around the dabbahu rift segment afar ethiopia summary increased displacement rates have been observed following manylarge earthquakes and mag matic events although an order of magnitude smaller than the displacements associated with the main event the post seismic or post rifting deformation may continue for years to decades after the initial earthquake or dyke intrusion due to the rare occurrence of subaerial rifting events thereareveryfewobservationstoconstrainmodelsofpost riftingdeformation in2005 september a 60 km long dyke was intruded along the dabbahu segment of the nubia arabia plate boundary afar ethiopia marking the beginning of an ongoing rifting episode con tinued activity has been monitored using satellite radar interferometry and data from global positioning system instruments deployed around the rift in response to the initial intrusion using multiple satellite passes we are able to separate the rift perpendicular and vertical displacement fields around the dabbahu segment rift perpendicular and vertical rates of up to 180 and 240 mm yr 1 respectively here we show that models of viscoelastic relaxation alone are insufficient to reproduce the observed deformation field and that a large portion of the observed signal is related to the movement of magma within the rift segment our models suggest upper mantle viscosities of 10 18 19 pas overlain by an elastic crust of between 15 and 30km to fit the observations inflation and deflation of magma chambers in the centre of the rift and to the south east of the rift axis is required at rates of 0 13 and 0 08km 3 yr 1

crustal thickness and moho character of the fast spreading east pacific rise from 9 42 n to 9 57 n from poststack migrated 3 d mcs data we computed crustal thickness 5740 270 m and mapped moho reflection character using 3 d seismic data covering 658 km2 of the fast spreading east pacific rise epr from 9 42 n to 9 57 n moho reflections are imaged within 87 of the study area average crustal thickness varies little between large sections of the study area suggesting regionally uniform crustal production in the last 180 ka however individual crustal thickness measurements differ by as much as 1 75 km indicating that the mantle melt delivery has not been uniform third order but not fourth order ridge discontinuities are associated with changes in the moho reflection character and or near axis crustal thickness this suggests that the third order segmentation is governed by melt distribution processes within the uppermost mantle while the fourth order ridge segmentation arises from midcrustal to upper crustal processes in this light we assign fourth order ridge discontinuity status to the debated ridge segment boundary at 9 45 n and third order status at 9 51 5 n to the ridge segment boundary previously interpreted as a fourth order discontinuity our seismic results also suggest that the mechanism of lower crustal accretion varies along the investigated section of the epr but that the volume of melt delivered to the crust is mostly uniform more efficient mantle melt extraction is inferred within the southern half of our survey area with greater proportion of the lower crust accreted from the axial magma lens than that for the northern half this south to north variation in the crustal accretion style may be caused by interaction between the melt sources for the ridge and the lamont seamounts

shallow seismicity and tectonics of the central and northern lau basin abstract the lau basin is an active backarc comprising several spreading centers and microplates rapidly evolving in time as such crustal earthquakes within the lau basin derive from intrabasin tectonic volcanic and hydrothermal sources because of high mantle attenuation small earthquakes from the basin are seldom recorded on land seismographs and it has been difficult to study the pattern of crustal seismicity however the 1994 labatts ocean bottom seismograph experiment in the lau basin recorded more than 100 000 local t and body phases allowing for a detailed examination of basin seismotectonics nearly 1000 events are located within the basin proper with another 2000 associated with the forearc and the aseismic front we identify two previously undocumented tectonic features a triple junction in the northwestern basin and a nanoplate in the central lau basin the single most seismically active feature within the lau basin is the lau extensional transform zone letz just north of the central lau spreading center significant seismicity also occurs near the northern extension of the eastern lau spreading center and along a southeast extension of the letz delineating the boundary of a previously unknown nanoplate swarm activity is common in the backarc basin and dominates regions associated with actively reorganizing tectonics we see no evidence of deformation along the southern boundary of the niuafo ou microplate where 4 5 cm yr of strain is predicted it is possible that the plate boundary extends eastward from the fonualei rift fr tip towards the trench creating a niuatoputapu tonga plate division rather than westward implied by the currently accepted niuafo ou tonga plate system but more data are necessary from the fr to test that hypothesis instead we suggest that the niuafo ou tonga pole is 1 5 further north than previously proposed reducing the predicted strain in this region to

seismic character of volcanic activity at the ultraslow spreading gakkel ridge never before has a volcanic eruption on a slow or ultraslow spreading mid ocean ridge been both observed seismically and confirmed on the seafloor during the first half of 1999 a long lived volcanic spreading event occurred on the ultraslow spreading gakkel ridge in the arctic ocean the seismicity associated with this event was unprecedented in duration and magnitude for a seafloor eruption sonar images from the u s s hawkbill which passed over the area within four months of the start of activity are consistent with the presence of a large recently erupted flow and a volcanic peak directly in the area of seismic activity seismic activity began in mid january and continued vigorously for three months a reduced rate of activity persisted for an additional four months or more in total 252 events were large enough to be recorded on global seismic networks although a limited number of volcanic spreading events have been observed globally the duration and magnitude of the gakkel ridge swarm when compared with volcanic seismicity at ridges spreading at intermediate and fast spreading rates suggest that a negative power law relationship may exist between these parameters and spreading rate fault activation in response to magmatic emplacement appears to have occurred over a broad region suggesting that magma may have been tapped from mantle depths the slow migration of the largest magnitude events along the axis of the rift valley suggests multiple magmatic pulses at depth in combination with bathymetric setting and sidescan sonar confirmation the seismic data for this event have provided a unique look at the scale and character of eruption processes at ultraslow spreading rates

210po 210pb dating of recent volcanic eruptions on the sea floor the globe encircling mid ocean ridge system accounts for most of the earth s volcanism and influences its heat budget morphology and the composition of the oceans it is therefore important to constrain the size frequency and compositional variability of ridge eruptions but this is made difficult by the remoteness of much of the sea floor and the paucity of applicable radioactive chronometers remote monitoring has provided indirect evidence for active ridge volcanism1 3 which in some cases has been strengthened by submersible observations4 5 but it has not been possible to date these eruptions directly here we present a new chronometer based on 210po 210pb radioactive disequilibrium which allows us to date glassy eruption products within a few years of their eruption our first results on lavas from 9 50 n on the east pacific rise confirm that the ridge erupted only months before sample collection and indicate that the eruptions extended over at least one year the chronometer begins with po volatilization during eruption all glasses analysed were 75 100 degassed a crystalline pillow interior was 20 degassed however implying that the flow surface may control po degassing during deep submarine eruptions although the 210po 210pb chronometer is useful for only a short time frame it should prove valuable for dating recent but unobserved submarine eruptions and determining details of eruptive sequences

sustained volcanically hosted venting at ultraslow ridges piccard hydrothermal field mid cayman rise at slow spreading mid ocean ridges sustained submarine venting and the deposition of large seafloor massive sulfide deposits have previously been ascribed to tectonically controlled hydrothermal circulation unrelated to young volcanic activity here by contrast we show that the piccard hydrothermal field phf on the ultraslow spreading mid cayman rise represents a site of sustained fluid flow and sulfide formation hosted in a neovolcanic setting the lateral extent and apparent longevity associated with the phf are comparable to some of the largest tectonically hosted vent sites known along the slow spreading mid atlantic ridge if such systems recur along all ultraslow ridges which comprise 20 of the 55 000 km global ridge crest potential implications would include i a higher probability of locating large economically valuable mineral deposits along ultraslow ridges together with ii larger fluxes than previously anticipated of chemicals released from high temperature venting entering the oceans along the atlantic indian ocean sectors of the deep ocean thermohaline conveyor

temporal trends in vent fluid iron and sulfide chemistry following the 2005 2006 eruption at east pacific rise 9 50 n the chemistry of vent fluids that emanate to the seafloor undergoes dramatic changes after volcanic eruptions data on these changes are still limited but the best studied example is the east pacific rise epr at 9 50 n where the temporal evolution of the vent fluid chemistry after the 1991 1992 eruption was documented the area underwent another eruption sequence during late 2005 early 2006 and here we show that a similar evolution is recurring in the iron and sulfide contents of the high temperature fluids sampled in june 2006 january 2007 and june 2008 the vents have had increasing dissolved iron and decreasing acid volatile sulfide free sulfide plus fes concentrations with 1 order of magnitude variation in addition chromium reducible sulfide mainly pyrite also had fivefold decreasing concentrations over the 3 years our results confirm a pattern that was noted only once before for 9 50 n epr and emphasize the dramatic yearly variability in the concentrations of iron sulfur species emanating from vents

pulse of the seafloor tidal triggering of microearthquakes at 9 50 n east pacific rise 1 unequivocal evidence of tidal triggering is observed for microearthquakes 0 4 to 2 0 ml recorded between october 2003 to april 2004 near 9 50 n on the east pacific rise epr although semidiurnal tidal stress changes are small 99 9 nonrandom temporal distribution with events occurring preferentially near times of peak extension due to the proximity of this site to an ocean tidal node where changes in sea surface height are minimal periodic stress changes are dominated by the solid earth tide in contrast previous studies on the juan de fuca ridge have shown microearthquake triggering to be a response to seafloor unloading during times of low ocean tide the modulation of 9 50 n microearthquakes by small amplitude periodic stresses is consistent with earthquake nucleation within a high stressing rate environment that is maintained near a critical state of failure by on axis magmatic and hydrothermal processes

length and timescales of rift faulting and magma intrusion the afar rifting cycle from 2005 to present although fault and magmatic processes have achieved plate spreading at mid ocean ridges throughout earth s history discrete rifting episodes have rarely been observed this paper synthesizes ongoing seismic structural space based geodetic and petrologic studies from the subaerial red sea rift in ethiopia where a major rifting episode commenced in september 2005 our aims are to determine the length and timescales of magmatism and faulting the partitioning of strain between faulting and magmatism and their implications for the maintenance of along axis segmentation most of the magma for the initial and subsequent 12 intrusions was sourced from the center of the dabbahu manda hararo rift segment strain is accommodated primarily by axial dike intrusions fed from mid segment magma chamber s these findings show that episodic approximate century interval rapid opening of discrete rift segments is the primary mechanism of plate boundary deformation the scale 65 km 8 km and intensity of crustal

holocene environmental changes in the horqin desert revealed by osl dating and δ 13 c analyses of paleosols abstract the horqin desert in northeastern china is located on the climatically sensitive margin of the east asian summer monsoon front and the paleosols in the desert potentially provide a record of environmental changes in the region during the last two decades several paleoenvironmental studies have increased our knowledge of past shifts in the desert grassland border in the deserts of northeastern china however studies of holocene vegetation changes with high quality age data are scarce here we present the results of optically stimulated luminescence dating and analyses of magnetic susceptibility total organic carbon and the stable carbon isotope composition of organic matter and inferred c4 biomass of the paleosol sequences of the tuquan and mutougou sections in the horqin desert and utilize them to document holocene environmental changes in the region the results reveal that the east asian summer monsoon precipitation gradually increased and previously unstable dunes were semi stabilized from 11 to 9 ka the semiarid conditions were then replaced by the more humid environment of the holocene climatic optimum from 9 to 5 ka during the holocene climatic optimum the paleosols in the tuquan and mutougou sections developed and the percentage of c4 plants increased and the dunes in the horqin desert were extensively stabilized subsequently alternation of eolian sand mobilization and paleosol development occurred indicating that the environment became cold and dry with temporary wet and warm stages estimates of c4 biomass from the two sections demonstrate that the favorable growing conditions for c4 plants extended to at least 45 n in north china during the holocene we conclude that holocene environmental changes in the horqin desert were controlled by movements of the east asian summer monsoon rainfall belt on a millennial time scale and that human activity also played a significant role in environmental change during the late holocene

mantle upwelling magmatic differentiation and the meaning of axial depth at fast spreading ridges since the fi rst systematic mapping of the east pacifi c rise its axial depth profi le has been used to infer variability in magma supply or mantle temperature here however we show that segment scale rise parallel undulations of both on and off axis seafl oor depth result pri marily from variations in the bulk density of oceanic crust using seismic images of crustal and upper mantle structure combined with gravity data and lava chemistry we demonstrate that segment scale variations in crustal density are caused by magmatic differentiation rise parallel changes in magmatic differentiation are attributed to a skew between the axes of mantle upwelling and plate spreading we conclude that segmentation of axial depth along fast spreading ridges is controlled primarily by the geometry of mantle upwelling

new insights into mid ocean ridge volcanic processes from the 2005 2006 eruption of the east pacific rise 9 46 n 9 56 n digital seafloor imagery collected on 37 camera tows and alvin dives in which we identify 186 contacts between new and old lava are used to create the most detailed map of a mid ocean ridge mor eruption to date lava flows erupted in 2005 2006 at the east pacific rise epr covered an area of 14 6 km2 along 18 km of the epr crest between 9 46 and 9 56 n the 2005 2006 lava is characterized by inflated lobate and sheet morphologies in the flow interiors and pillow forms at terminal flow fronts numerous lava channels 10 50 m wide and 1 5 m deep trending approximately east west served as distributory pathways eruptions were sourced from fissures within the epr axial summit trough as well as fissures located on an off axis fissure mound 600 m east of the epr axis between 9 52 and 9 56 n portions of the lava flow reached as far as 2 km east of the axis near 9 51 2 n using a conservative estimate of 1 5 m for the average flow thickness implies that the 2005 2006 eruptions produced 22 106 m3 of lava 4 5 times larger than estimated volumes of 1991 1992 epr lava flows estimated lava volume for the 2005 2006 eruptions represents 15 of the magma available in the axial magma chamber

monitoring pacific ocean seismicity from an autonomous hydrophone array since may 1996 an array of autonomous hydrophone moorings has been continuously deployed in the eastern equatorial pacific to provide long term monitoring of seismic activity including low level volcanic signals along the east pacific rise between 20 n and 20 s and the galapagos ridge the instruments and moorings were designed to continuously record low frequency acoustic energy in the sofar channel for extended periods and produce results comparable to those previously derived by using the u s navy sound surveillance system sosus in the northeast pacific the technology and methodology developed for this experiment including instrument design mooring configuration analysis software location algorithms with an analysis of errors and a predicted error field are described in detail volcanic activity is observed throughout the pacific along with seismicity along transform faults subduction zones and intraplate regions comparison data sets indicate detection thresholds and accuracy better than the land networks for open ocean areas and results comparable to or better than sosus volcanic seismicity along the fast spreading east pacific rise appears similar to documented examples in the northeast pacific but with much shorter durations one example from the intermediate spreading galapagos ridge is comparable to northeast pacific examples and several episodes of activity were observed in the wilkes transform fault zone a site of continuing off axis seismicity is located near 18 s and 116 w isolated intraplate earthquakes are observed throughout the study area earthquake information from this experiment and future observations will be provided through the world wide web and earthquake data centers

fine scale segmentation of volcanic hydrothermal systems along fast spreading ridge crests abstract fine scale 3rd and 4th order segments of the mid ocean ridge mor are morphologically defined units of crustal accretion with enigmatic origins at fast spreading centers 3rd order segments are bounded by discontinuities in the structure of the axial high most 3rd order segments are 20 10 km long and terminate at ridge axis discontinuities rads exhibiting 0 2 3 km lateral offsets and or 20 m axial depth increases nested within 3rd order segments are shorter 4th order segments bounded by smaller rads along the fast spreading east pacific rise epr at 10 02 9 08 n and 17 11 42 s spatial distributions of active high temperature hydrothermal vents biological communities and lava flow morphology are known accurately from extensive near bottom surveys and submersible dives along a total of seven 3rd order segments and twenty four 4th order segments the cumulative northern and southern epr data show that 3rd order segments but not 4th order segments consistently exhibit apparently higher rates of lava effusion near segment middles in comparison to segment ends supporting the hypothesis that each 3rd order segment is a discrete centrally fed volcanic system or volcanic segment spatial analyses of the cumulative hydrothermal data reveal a striking increase in abundance of hydrothermal features within the middle portions of 3rd order segments approximately 60 of ongoing high temperature focused flow is concentrated within the middle 40 of 3rd order segments and twice as many biological communities are found at mid segment in comparison to segment ends the two independent data sets volcanic and hydrothermal both indicate that magmatic heat sources are focused beneath the central portions of 3rd order segments and that magmatic hydrothermal systems are spatially and temporally segmented at 3rd order length and time scales we propose that 3rd order segmentation arises from processes controlling melt supply to the middle to lower crust and that 4th order segmentation largely arises from more rapid frequent upper crust processes of crack formation and dike intrusion future work is needed to test the applicability of our results along portions of the fast spreading epr notched by fault bounded axial troughs and in areas farther south where spreading rate reaches a maximum

primitive melt replenishment and crystal mush disaggregation in the weeks preceding the 2005 2006 eruption 9 50 n epr abstract the 2005 2006 eruption at 9 50 n east pacific rise provides an exceptional opportunity to investigate the magma plumbing system beneath a well studied ridge system the eruption was preceded by two years of increasingly intense seismicity and occurred from the same location as a previous and well characterized eruption in 1991 1992 here we use the crystal cargo of samples from this eruption to investigate magma reservoir processes in the lead up to the eruption as well as their temporal relationship to the seismicity that preceded it compositional zoning in some plagioclase crystals indicates primitive melt replenishment occurred roughly six weeks or less before the eruption this replenishing event is seen only in the crystals from the central region of the eruption 9 50 9 52 n this is also the area where the most primitive lava compositions are observed and together these observations support models of replenishment being spatially focused the short time between the input of a more primitive melt and eruption onto the seafloor suggests replenishment likely contributed to triggering the eruption rare resorbed plagioclase crystals and glomerocrysts of plagioclase and olivine some of which have rims far from equilibrium with their host melt suggest that disaggregation of a crystal mush occurred within a few days prior to eruption interstitial melt from within this mush zone must have been mixed back into the erupted lava a form of in situ crystallization thus the erupted magmas evolved in a replenished tapped magma reservoir in which at least a part of the crystallization occurred in situ

rapid ascent and emplacement of basaltic lava during the 2005 06 eruption of the east pacific rise at ca 9 51 n as inferred from co2 contents abstract eruption rates at the mid ocean ridges mors are believed to strongly control the morphology and length of lava flows emplaced along the ridge axis and thus the structure and porosity of the upper oceanic crust eruption rate also represents one of the few tools to gain insight into the driving pressures within sub ridge magmatic systems as eruption rate is inferred to vary systematically along the global mid ocean ridge system understanding of how to assess eruption rate in submarine systems and how it maps to observable features of the ridge axis would provide a powerful tool to understand earth s largest magmatic system eruption rates at mors are poorly constrained however because of a lack of direct observations preventing the duration of an eruption to be quantified this study uses decompression experiments of morb samples and numerical modeling of co 2 degassing to reconstruct the timescales for magma ascent and lava emplacement during the 2005 06 eruption of the east pacific rise at ca 9 51 n samples collected from the lava flow are all supersaturated in dissolved co 2 contents but co 2 decreases with distance from the vent presumably as a consequence of progressive co 2 diffusion into growing bubbles samples collected at the vent contain 10 5 vesicles per cm 3 pieces of these samples were experimentally heated to 1225 c at high pressure and then decompressed at controlled rates results plus those from numerical modeling of diffusive bubble growth indicate that magma rose from the axial magma chamber to the seafloor in 1 h and at a rate of 2 3 km h 1 our modeling as validated by experimental decompression of morb samples with 10 6 vesicles cm 3 also suggests that co 2 degassed from the melt within 10 100 min as the vesicular lava traveled 1 7 km along the seafloor implying a volumetric flow rate on order of 10 3 4 m 3 s 1 given an ascent rate of 0 2 m s 1 the width of a rectangular dike feeding the lava would have been 1 2 m wide morb samples from the pacific ridge are generally more supersaturated in dissolved co 2 than those from slower spreading atlantic and indian ridges our results suggest that pacific morbs ascend to the seafloor faster than atlantic or indian morbs

comparison of seafloor tectonic fabric at intermediate fast and super fast spreading ridges influence of spreading rate plate motions and ridge segmentation on fault patterns we have conducted a comparative study of the tectonic morphology of young seafloor using seamarc ii side scan sonar surveys of the intermediate spreading ecuador rift the fast spreading east pacific rise epr 8 30 10 n and the super fast spreading epr 18 19 s we find that characteristics of fault populations are not only a function of spreading rate but also vary along axis within individual ridge segments i e with proximity to large and short offset discontinuities we also find that fault azimuths can be used to examine plate kinematics on a finer scale than can be obtained using magnetic data alone most of the variation in fault populations with spreading rate can be explained by an inverse relationship between spreading rate and thickness of the brittle layer for example regions of super fast spreading are characterized by the largest numbers of short faults the smallest average fault spacing and throw and the highest fault density in addition clusters of short closely spaced antithetic faults subsidiary to long master inward dipping faults are common within the super fast spreading area presumably the result of a thinner weaker brittle layer faults facing away from the ridge axis occur in increasing numbers with increasing spreading rate such that few outward facing faults are found at slow to intermediate rates and approximately equal numbers of inward and outward facing faults are observed at the fastest rates rapid thickening of the brittle layer with distance from the ridge may account for the predominance of inward facing faults at slower spreading rates outward facing faults at all spreading rates have shorter mean lengths and lower vertical offsets these differences may reflect the shorter time outward facing faults are active owing to increasing strength of the lithosphere with distance from the ridge fault lengths and spacings in all areas approximate exponential distributions the extensional strain represented by fault populations is calculated from the displacement and length distributions of faults and strain estimates of 4 are obtained for each area assuming that fault spacing reflects fracture depth extent where faults initiate we infer a brittle layer thickness of 1 km when faulting begins fault populations are examined for ridge segment scale variations in amagmatic extension we see evidence for greater amagmatic extension associated with long term reduced magma supply along the eastern third of the ecuador rift evidence for local increased brittle extension is also found within 15 km of transform faults discordant zones left by overlapping spreading centers oscs are characterized by low fault abundances at oscs discrete events of ridge tip propagation may accommodate extension taken up elsewhere along the ridge by normal faulting fault azimuths do appear to be useful indicators of plate motion within the epr 8 30 10 n area fault trends record a recent change in pacific cocos plate motion 3 6 at 1 m y consistent with magnetic anomaly and fault lineation data from elsewhere along the northern epr within the ecuador rift fault azimuths scatter within 3 of predicted trends and are consistent with constant spreading about one pole for the past 1 5 m y

comparison of dike intrusions in an incipient seafloor spreading segment in afar ethiopia seismicity perspectives 1 oceanic crust is accreted through the emplacement of dikes at spreading ridges but the role of dike intrusion in plate boundary deformation during continental rupture remains poorly understood between 2005 and 2009 the 70 km long dabbahu manda hararo rift segment in ethiopia has experienced 14 large volume dike intrusions 9 of which were recorded on temporary seismic arrays a detailed comparison of the seismic characteristics of the seismically monitored dikes is presented with implications for dike intrusion processes and magmatic plumbing systems all of the migrating swarms of earthquakes started from a 80 of energy is released during the propagation phase with minimal seismic energy release after the dike propagation ceases we interpret that faulting and graben formation above the dikes occurs hours after the passage of the dike tip coincident with the onset of low frequency earthquakes dike lengths show no systematic reduction in length with time suggesting that topographic loading and stress barriers influence dike length as well as changes in tectonic stress the propagation velocities of all the dikes follow a decaying exponential northward propagating dikes had faster average velocities than those that propagated southward suggesting preconditioning by the 2005 megadike or ongoing heating from a subcrustal magma source north of the midsegment

seismic tremor at the 9 50 n east pacific rise eruption site ocean bottom seismic observations within the 9 50 n region of the east pacific rise indicate persistent low amplitude tremor activity throughout the october 2003 through february 2007 period of monitoring these signals exhibit either monochromatic or polychromatic spectral characteristics with a 6 hz fundamental frequency and up to two harmonics individual events cannot be correlated between nearby 1 km stations implying the presence of multiple small amplitude sources positioned within the shallow crust tremor exhibits a semidiurnal periodicity with some stations recording activity during times of increasing tidal extension and others detecting tremor signals during times of increasing compression the amplitude duration and rate of activity also correlate positively with fortnightly changes in the amplitude of the tides these spatiotemporal patterns are consistent with tremor generation in response to tidally modulated fluid flow within a network of shallow cracks tremor energy flux is spatially and temporally heterogeneous however there are extended periods of greater and lesser activity that can be tracked across portions of the array despite their shallow crustal origin changes in tremor amplitude and spectral character occur in the months prior to a major microearthquake swarm and inferred seafloor spreading event on 22 january 2006 with an increase in the degree of correlation between tremor activity and tidal strain in the weeks leading up to this event after the spreading event two eruption surviving stations near the axis continue to show high rates of tremor activity whereas these signals are suppressed at the single station recovered from the near axis flanks this off axis quiescence may result from the dike induced closing of cracks or perhaps from the emplacement of impermeable flows near the station

systematic along axis tidal triggering of microearthquakes observed at 9 50 n east pacific rise 1 hydrothermal fluid circulation at mid ocean ridges facilitates the exchange of heat and chemicals between the oceans and the solid earth and supports chemosynthetic microbial and macro faunal communities the structure and evolution of newly formed oceanic crust plays a dominant role in controlling the character and longevity of hydrothermal systems however direct measurements of subsurface processes remain technologically challenging to obtain previous studies have shown that tidally induced stresses within the subseafloor modulate both fluid flow and microearthquake origin times in this study we observe systematic along axis variations between peak microearthquake activity and maximum predicted tidal extension beneath the hydrothermal vent site at 9 50 n east pacific rise we interpret this systematic triggering to result from pore pressure perturbations propagating laterally through the hydrothermal system based on our observations and a one dimensional pore pressure perturbation model we estimate bulk permeability at 10 13 to 10 12 m2 within layer 2b over a calculated diffusive lengthscale of 2 0 km

distribution of magma beneath the east pacific rise between the clipperton transform and the 9 17 n deval from forward modeling of common depth point data we have reprocessed seven cross axis common depth point cdp profiles between the clipperton transform and the 9 17 n deval deviation in axial linearity on the east pacific rise epr to understand the relationship between axial magma chamber amc width and seafloor morphology forward modeling of cross axis cdp profiles suggests a segmented amc in which significant variations in width occur across minor rise axis discontinuities e g devals the modeled rise segment widths bounded by the 9 53 n 9 35 n devals the 9 35 n 9 17 n devals and south of the 9 17 n deval were 0 7 km 1 0 1 2 km and 4 15 km respectively transition in amc width across these discontinuities is unclear due to the sparseness of cross axis line spacing however a simple association of devals with decreased magma supply is doubtful the minimum 250 m and maximum 4150 m amc widths are found near the 9 35 n and 9 17 n devals respectively the reprocessing of cdp profiles included repicking stacking velocities to ensure a proper stack of the amc reflector and its associated diffractions imaging postcritical reflections from the base of layer 2a finite difference time migration ray theoretical depth migration and travel time modeling of amc diffraction patterns constraints on amc width were derived from forward modeling techniques based on analytic raytracing velocity models were constructed from seabeam bathymetry and modified expanding spread profile esp velocity depth functions esp velocity models were altered to compensate for off axis thickening of layer 2a as imaged in the cdp reflection data continuous two dimensional velocity models constructed from modified esp velocity depth functions and seabeam bathymetry should account for ray bending at the seafloor basalt interface and any lateral velocity gradients induced by a thickening layer 2a stacked data were time migrated using a finite difference algorithm and extrapolated to depth using ray theoretical depth migration reflector positions were input into our forward modeling scheme to produce a zero offset travel time response of our migrated solution the travel time response was then overlain on the stacked section to ensure an adequate match especially to diffractions generated at the amc edge forward modeling of amc diffraction patterns reveals that original amc width estimates were too large the under migration of the amc reflector resulted from the conversion of stacking to interval velocities without accounting for topographic effects on individual cmp gathers thus resulting in improperly collapsed diffraction hyperbolae ship wandering relative to the amc edge can account for variations in amc reflector amplitude and dropout on the along axis cdp line the continuity of the amc appears unbroken across several ridge axis discontinuities between the clipperton transform and the 9 17 n deval which suggests an amc whose along axis dimension exceeds 75 km reflectivity modeling of cmp gathers suggests that the available data are consistent with a magma chamber comprising only a thin layer of melt overlying a zone of partially solidified crystal mush such a thin layer of melt might inhibit along axis mixing of magmas and thereby account for variations in magma composition along the rise crest this melt lens model for the amc would also produce strong diffraction patterns as imaged in the cdp data

small scale spatial and temporal variations in mid ocean ridge crest magmatic processes data from a suite of closely spaced lava flows recovered within the axial summit caldera and on the crestal plateau of the east pacific rise around lat 9 31 n indicate that eruptions on this fast spreading part of the mid ocean ridge occur throughout the crestal region and are not restricted to the axis these eruptions contribute to a complex distribution of basalts of various ages and a significant thickening of seismic layer 2a away from the axis in our study area small scale 600 m diversity and nonsystematic distribution of lava types may reflect rapid changes in magma chemistry that occur during crystallization and replenishment in small magma lenses coupled with the effects of frequent low volume eruptions both within and outside of the axial summit caldera

holocene palaeoceanographic evolution off west greenland benthic foraminiferal assemblages from a core southwest of disko bugt provide a holocene perspective last 7 ka bp on ice sheet ocean interactions between the west greenland current wgc and the west greenland ice sheet changes in the fauna reveal significant variations in the water mass properties temperature and salinity of the wgc through time from 7 3 to 6 2 ka bp a relatively warm strong wgc influences ice sheet melt in disko bugt and causes enhanced meltwater production resulting in low surface water productivity the most favourable oceanographic conditions occur from 5 5 to 3 5 ka bp associated with thermal optimum like conditions encompassing minimum ice sheet extent in the disko bugt area these conditions are attributed to 1 reduced meltwater influence as the ice sheet is land based and 2 enhanced contribution of warm saline water masses from the irminger current to the wgc the transition into the late holocene last 3 5 ka bp is characterized by a cooling of oceanographic conditions caused by increased advection of cold low salinity water masses from the east greenland current a longer term late holocene cooling trend within the wgc is attributed to the onset of neoglacial cooling within the north atlantic region superimposed on this cooling trend multicentennial scale variability within the wgc matches reconstructions from a nearby coring site in disko bugt as follows 1 cooling at 2 5 ka bp linked to the 2 7 ka bp cooling event 2 a warm phase centred at 1 8 ka bp associated with the roman warm period 3 slight warming between 1 4 and 0 9 ka bp linked to the medieval climate anomaly 4 severe cooling of the wgc after 0 9 ka bp culminating at 0 3 ka bp during the little ice age we show that multicentennial scale palaeoceanography variability along the west greenland margin is driven by ocean forcing i e variations in the relative contribution of atlantic irminger current and polar east greenland current water masses to the wgc during the last 7 ka bp influencing ice sheet dynamics

mechanical decoupling and thermal structure at the east pacific rise axis 9 n constraints from axial magma chamber geometry and seafloor structures abstract we study the relationships between the seafloor structures and the axial magma chamber geometry in the 9 n overlapping spreading center osc area on the fast spreading east pacific rise epr our observations are based on a new high resolution bathymetric map of the 9 n osc area derived from picks of the seafloor arrival on 3d seismic data and on previously published data that constrain the presence and distribution of melt below the 9 n osc differences in the orientation of structures between the seafloor and the magma chamber indicate a sharp change in principal stress directions with depth suggesting that the brittle crust above the melt sill is decoupled from the melt sill itself and the ductile crust underlying it the stress field within the brittle upper crust results from a local interaction of the two overlapping spreading centers whereas the stress field in the crust below the melt sill corresponds to the regional stress field imposed by plate separation given this mechanical structure of the crust the melt sill shape and location appear to be controlled by the following factors the location of the deep melt source below the melt sill the ambient stress field at the depth of the melt sill and the stress field in the brittle upper crust above the melt sill which thermally shapes the roof of the melt sill through repeated eruptions

spatial distribution and temporal evolution of crustal melt distribution beneath the east pacific rise at 9 10 n inferred from 3 d seafloor compliance modeling determining the melt distribution in oceanic crust at mid ocean ridges is critical to understanding how magma is transported and emplaced in the crust seafloor compliance deformation under ocean wave forcing is primarily sensitive to regions of low shear velocity in the crust making it a useful tool to probe melt distribution analysis of compliance data collected at east pacific rise between 9 and 10 n through 3 d numerical modeling reveals strong along axis variations in the lower crustal shear velocities as well as temporal variation of crustal shear velocity near 9 48 n between measurements spanning 8 years compliance measured across the rise axis at 9 48 n and 9 33 n suggest a deep crustal low velocity zone beneath the ridge axis with a low vs vp ratio consistent with melt in low aspect ratio cracks or sills changes in compliance measured at 9 48 n between years 1999 and 2007 suggest that the melt fraction in the axial crust decreased during this interval perhaps following the 2005 2006 seafloor eruption this temporal variability provides direct evidence for short term variations of the magmatic system at a fast spreading ridge

termination of a 6 year ridge spreading event observed using a seafloor seismic network on the endeavour segment juan de fuca ridge 1 we present automatically determined epicenters and magnitudes for 36 523 earthquakes recorded along the endeavour segment between august 2003 and october 2006 using a local ocean bottom seismometer obs network the catalog is dominated by two swarm sequences in january and february 2005 in the vicinity of the endeavour overlapping spreading center which included earthquakes in west valley the northern portion of the endeavour segment southwest endeavour valley and the endeavour vent fields these swarms are attributed to volcanism including a dike intrusion on the northern endeavour in february 2005 and smaller diking events on the propagating tip of the west valley segment in both swarms the dike on the northern endeavour propagated to the south which is inconsistent with magma sourced from the axial magma chamber beneath the elevated central portion of the segment following the swarms seismic activity on the endeavour segment decreased on average to 15 of pre swarm values and almost ceased at the segment ends we infer that a 6 year non eruptive event that started with a swarm in 1999 and finished with the 2005 swarms ruptured the entire segment and relieved plate spreading stresses the inferred coupling between the 1999 and 2005 events the observation of extensive precursory activity prior to the 2005 swarms and the interaction between seismically active regions during the swarms is consistent with static triggering with delays influenced by viscoelastic relaxation hydraulic diffusion and magma withdrawal and replenishment

influence of fortnightly tides on earthquake triggering at the east pacific rise at 9 50 n the study of tidal forcing provides an opportunity to view the response of the solid earth to known changes in stress earthquakes triggered by diurnal and semidiurnal tidal forces have been well documented for the deep ocean and some terrestrial environments however few studies examine the influence of fortnightly tides on earthquake triggering and only in terrestrial environments we assess triggering of microearthquakes related to true fortnightly tides and fortnightly tidal modulations along the east pacific rise at 9 50 n where strong diurnal and semidiurnal triggering of earthquakes has been observed an ocean bottom seismograph experiment collected microearthquake data between october 2003 and january 2007 a time period that included a well documented submarine eruption culminating in january 2006 we examine how triggering changed through the build up to and aftermath of this seafloor spreading event results show that earthquakes occur preferentially during times of increasing peak volumetric stress leading up to the eruption with strong statistical significance schuster test p values up to 2 22e 31 however the correlation loses its strength immediately prior to and during the eruptive period p value as high as 0 2837 when competing tectonic and magmatic stresses may occur post eruption triggering reappears p value 1 45e 26 but with opposite phase statistical significance is also explored using monte carlo method highlighting limitations of swarm dominated time series for the schuster test nevertheless the observations are consistent with a shift in the dominating stress triggering the earthquakes from extensional tectonics to magmatic processes followed by a post eruption period of cooling relaxation and possibly magma chamber refilling

paving the seafloor volcanic emplacement processes during the 2005 2006 eruptions at the fast spreading east pacific rise 9 50 n 1 the 2005 2006 eruptions near 9 50 n at the east pacific rise epr marked the first observed repeat eruption at a mid ocean ridge and provided a unique opportunity to deduce the emplacement dynamics of submarine lava flows since these new flows were documented in april 2006 a total of 40 deep towed imaging surveys have been conducted with the woods hole oceanographic institution s whoi towcam system more than 60 000 digital color images and high resolution bathymetric profiles of the 2005 2006 flows from the towcam surveys were analyzed for lava flow morphology and for the presence of kipukas collapse features faults and fissures we use these data to quantify the spatial distributions of lava flow surface morphologies and to investigate how they relate to the physical characteristics of the ridge crest such as seafloor slope and inferred dynamics of flow emplacement we conclude that lava effusion rate was the dominant factor controlling the observed morphological variations in the 2005 2006 flows we also show that effusion rates were higher than in previously studied eruptions at this site and varied systematically along the length of the eruptive fissure this is the first well documented study in which variations in seafloor lava morphology can be directly related to a well documented ridge crest eruption where effusion rate varied significantly

temporal and spatial patterns of biological community development at nascent deep sea hydrothermal vents 9 50 n east pacific rise the april 1991 discovery of newly formed hydrothermal vents in areas of recent volcanic eruption between 9 45 n and 9 52 n on the east pacific rise provided a unique opportunity to follow temporal changes in biological community structure from the birth of numerous deep sea hydrothermal vents in march l992 dsv alvin was used to deploy an on bottom observatory the biologic geologic transect to monitor faunal succession along a 1 37 km segment of the axial summit caldera between 9 49 61 n and 9 50 36 n depth 2520 m photo and videographic documentation of megafaunal colonization and chemical analyses of diffuse hydrothermal fluids associated with many of these developing communities within the transect were performed in march 1992 december 1993 october 1994 and november 1995 photographic and chemical time series analyses revealed the following sequence of events in low temperature venting areas 1 immediately following the 1991 eruption hydrogen sulfide and iron concentrations in diffuse fluids were extremely high 1 mmol kg 1 and microbially derived material blanketed active areas of venting in the form of thick microbial mats 2 mobile vent fauna e g amphipods copepods octopods and galatheid and brachyuran crabs and non vent fauna e g nematocarcinid shrimp proliferated in response to this increased biological production 3 within 1 yr of the eruption areal coverage of microbial mat was reduced by 60 and individuals of the vestimentiferan tube worm tevnia jerichonana settled gregariously in areas where diffuse flow was most intense 4 two years after the eruption maximum levels of h2s decreased by almost half from 1 90 to 0 97 mmol kg 1 and dense thickets of the vestimentiferan riftia pachyptila dominated vent openings previously inhabited by tevnia jerichonana 5 three years after the eruption maximum hydrogen sulfide levels declined further to 0 88 mmol kg 1 and mussels bathymodiolus thermophilus were observed on basaltic substrates 6 four years after the eruption galatheid crabs and serpulid polychaetes increased in abundance and were observed close to active vent openings as maximum hydrogen levels decreased to 0 72 mmol kg 1 also by this time mussels had colonized on to tubes of riftia pachyptila 7 between 3 and 5 yr after the eruption there was a 2 to 3 fold increase in the number of species in the faunal assemblages in the absence of additional volcanic tectonic disturbance we predict that mytilid and vesicomyid bivalves will gradually replace vestimentiferans as the dominant megafauna 5 10 yr following the eruption we also anticipate that the abundance of suspension feeders will decline during this period while the abundance of carnivores will increase we hypothesize that the above series of events 1 7 represents a general sequence of biological successional changes that will occur at newly formed low temperature deep sea hydrothermal vents along the northern east pacific rise and contiguous ridge axes megafaunal colonization at deep sea hydrothermal vents is considered to be the consequence of an intimate interaction of the life history strategies of individual species physical oceanographic processes and the dynamic hydrothermal environment our observations indicate that the successful sequential colonization of dominant megafaunal vent species from tevnia jerichonana to riftia pachyptila to bathymodiolus thermophilus also may be strongly influenced by temporal changes in geochemical conditions additional evidence demonstrating the close link between diffuse vent flux fluid geochemistry and faunal succession included the rapid death of several newly formed biological assemblages coincident with abrupt changes in the geochemical composition of the venting fluid and the local refocusing or cessation of vent flow these correlations suggest that future models of faunal succession at hydrothermal vents along intermediate to fast spreading mid ocean ridges should consider not only the interplay of species specific life history strategies community productivity and physical oceanographic processes but also the influence of changing geochemical conditions on the sequential colonization of megafaunal species

volcanic eruption of the mid ocean ridge along the east pacific rise crest at 9 45 52 n direct submersible observations of seafloor phenomena associated with an eruption event in april 1991 in april 1991 we witnessed from the submersible alvin a suite of previously undocumented seafloor phenomena accompanying an in progress eruption of the mid ocean ridge on the east pacific rise crest at 9 45 n 52 n the volume of the eruption could not be precisely determined although comparison of pre and post eruption seabeam bathymetry indicate that any changes in ridge crest morphology resulting from the eruption were 10 m high effects of the eruption included 1 increased abundance and redistribution of hydrothermal vents disappearance of numerous vent communities and changes in characteristics of vent fauna and mineral deposits within the eruption area since december 1989 2 murkiness of bottom waters up to tens of meters above the seafloor due to high densities of suspended mineral and biogenic particulates 3 destruction of a vent community by lava flows mass wasting and possible hydrovolcanic explosion at a site known as tubeworm barbecue in the axial summit caldera asc at 9 50 6 n 4 near critical temperatures of hydrothermal vent fluids ranging up to 403 c 5 temporal variations over a 2 week interval in both temperatures and chemical isotopic compositions of hydrothermal fluids 6 unusual compositions of end member vent fluids with ph values ranging to a record low of 2 5 salinities ranging as low as 0 3 wt nacl one twelfth that of seawater and dissolved gases reaching high concentrations 65 mmol l for both co2 and h2s 7 venting at temperatures above 380 c of visually detectable white vapor that transformed to plumes of gray smoke a few centimeters above vent orifices 8 disorganized venting of both high temperature fluids black and gray smoke and large volumes of cooler diffuse hydrothermal fluids directly from the basaltic seafloor rather than from hydrothermal mineral constructions 9 rapid and extensive growth of flocculent white bacterial mats species unknown on and under the seafloor in areas experiencing widespread venting of diffuse hydrothermal fluid and 10 subseafloor downslope migration of magma normal to the ridge axis in a network of small scale 1 5 m diameter lava tubes and channels to distances at least 100 200 m outside the asc we suggest that in april 1991 intrusion of dikes in the eruption area to 200 m beneath the asc floor resulted in phase separation of fluids near the tops of the dikes and a large flux of vapor rich hydrothermal fluids through the overlying rubbly cavernous lavas low salinities and gas rich compositions of hydrothermal fluids sampled in the eruption area are appropriate for a vapor phase in a seawater system undergoing subcritical liquid vapor phase separation boiling and phase segregation hydrothermal fluids streamed directly from fissures and pits that may have been loci of lava drainback and or hydrovolcanic explosions these fissures and pits were lined with white mats of a unique fast growing bacteria that was the only life associated with the brand new vents the prolific bacteria which covered thousands of square meters on the ridge crest and were also abundant in subseafloor voids may thrive on high levels of gases in the vapor rich hydrothermal fluids initially escaping the hydrothermal system white bacterial particulates swept from the seafloor by hydrothermal vents swirled in an unprecedented biogenic blizzard up to 50 m above the bottom the bacterial proliferation of april 1991 is likely to be a transient bloom that will be checked quickly either by decline of dissolved gas concentrations in the fluids as rapid heat loss brings about cessation of boiling and or by grazing as other organisms are re established in the biologically devastated area

variations in axial magma lens properties along the east pacific rise 9 30 n 10 00 n from swath 3 d seismic imaging and 1 d waveform inversion we use three dimensional multistreamer seismic reflection data to investigate variations in axial magma lens aml physical properties along the east pacific rise between 9 30 n and 10 00 n using partial offset stacks of p and s converted waves reflecting off the top of the aml we image four 2 4 km long melt rich sections spaced 5 10 km from each other one dimensional waveform inversion indicates that the aml in a melt rich section is best modeled with a low vp 2 95 3 23 km s and vs 0 3 1 5 km s indicating 70 melt fraction in contrast the aml in a melt poor section requires higher vp 4 52 4 82 km s and vs 2 0 3 0 km s which indicates 40 melt fraction the thicknesses of the aml are constrained to be 8 32 m and 8 120 m at the melt rich and poor sites respectively based on the aml melt mush segmentation imaged in the area around the 2005 2006 eruption we infer that the main source of this eruption was a 5 km long section of the aml between 9 48 n and 51 n the eruption drained most of the melt in this section of the aml leaving behind a large fraction of connected crystals we estimate that during the 2005 2006 eruption a total magma volume of 9 83 106 m3 was extracted from the aml with a maximum of 71 106 m3 left unerupted in the crust as dikes from this we conclude that an eruption of similar dimensions to the 2005 2006 one would be needed with a frequency of years to decades in order to sustain the long term average seafloor spreading rate at this location

methane manganese and helium in hydrothermal plumes following volcanic eruptions on the east pacific rise near 9 50 n 1 as part of a rapid response cruise in may 2006 we surveyed water column hydrothermal plumes and bottom conditions on the east pacific rise between 9 46 0 n and 9 57 6 n where recent seafloor volcanic activity was suspected real time measurements included temperature light transmission and salinity samples of the plume waters were analyzed for methane manganese helium concentrations and the 13c of methane these data allow us to examine the effects of the 2005 2006 volcanic eruption s on plume chemistry methane and manganese are sensitive tracers of hydrothermal plumes and both were present in high concentrations methane reached 347 nm in upper plume samples 250 m above seafloor and exceeded 1085 nm in a near bottom sample mn reached 54 nm in the upper plume and 98 nm in near bottom samples the concentrations of methane and mn were higher than measurements made after a volcanic eruption in the same area in 1991 but the ratio of ch4 mn at 6 7 is slightly lower though still well above the ratios measured in chronic plumes high concentrations of methane in near bottom samples were associated with areas of microbial mats and diffuse venting documented in seafloor imagery the isotopic composition of the methane carbon shows evidence of active microbial oxidation however neither the fractionation factor nor the source of the eruption associated methane can be determined with any certainty considerable scatter in the isotopic data is due to diverse sources for the methane as well as fractionation as methane is consumed one sample at 21 versus peedee belemnite standard is among the most enriched methane carbon values reported in a hydrothermal plume to date

submeter bathymetric mapping of volcanic and hydrothermal features on the east pacific rise crest at 9 50 n 1 recent advances in underwater vehicle navigation and sonar technology now permit detailed mapping of complex seafloor bathymetry found at mid ocean ridge crests imagenex 881 675 khz scanning sonar data collected during low altitude 5 m surveys conducted with dsv alvin were used to produce submeter resolution bathymetric maps of five hydrothermal vent areas at the east pacific rise epr ridge2000 integrated study site 9 50 n bull s eye data were collected during 29 dives in 2004 and 2005 and were merged through a grid rectification technique to create high resolution 0 5 m grid composite maps these are the first submeter bathymetric maps generated with a scanning sonar mounted on alvin the composite maps can be used to quantify the dimensions of meter scale volcanic and hydrothermal features within the epr axial summit trough ast including hydrothermal vent structures lava pillars collapse areas the trough walls and primary volcanic fissures existing autonomous benthic explorer abe bathymetry data 675 khz scanning sonar collected at this site provide the broader geologic context necessary to interpret the meter scale features resolved in the composite maps the grid rectification technique we employed can be used to optimize vehicle time by permitting the creation of high resolution bathymetry maps from data collected during multiple coordinated short duration surveys after primary dive objectives are met this method can also be used to colocate future near bottom sonar data sets within the high resolution composite maps enabling quantification of bathymetric changes associated with active volcanic hydrothermal and tectonic processes

spectral analysis of dike induced earthquakes in afar ethiopia shallow dike intrusions may be accompanied by fault slip above the dikes a superposition which complicates seismic and geodetic data analyses the diverse volcano tectonic and low frequency local earthquakes accompanying the 2005 2010 large volume dike intrusions in the dabbahu manda hararo rift afar some with fault displacements of up to 3m at the surface provide an opportunity to examine the relations among the earthquakes dike intrusions and surface ruptures we apply the frequency index fi method to characterize the spectra of swarm earthquakes from six of the dikes these earthquakes often have broad spectra with multiple peaks making the usual peak frequency classification method unreliable our results show a general bimodal character with high fi earthquakes associated with deeper dikes top 3 km subsurface and low fi earthquakes associated with shallow dikes indicating that shallow dikes result in earthquakes with more low frequency content and larger amplitude surface waves low fi earthquakes are more common during dike emplacement suggesting that interactions between the dike and faults may lead to lower fi taken together likely source processes for low fi earthquakes are shallow hypocenters 3 km possibly with surface rupture slow rupture velocities and interactions with dike fluids strong site effects also heavily influence the earthquake spectral content additionally our results suggest a continuum of spectral responses implying either that impulsive volcano tectonic earthquakes and the unusual emergent earthquakes have similar source processes or that simple spectral analyses such as fi cannot distinguish different source processes

on the potential of recording earthquakes for global seismic tomography by low cost autonomous instruments in the oceans 1 we describe the development and testing of an autonomous device designed to revolutionize earth structure determination via global seismic tomography by detecting earthquakes at teleseismic distances in the oceans one prototype mermaid short for mobile earthquake recording in marine areas by independent divers was constructed and tested at sea the instrument combines two readily available relatively low cost but state of the art components a sounding oceanographic lagrangian observer or solo float and an off the shelf hydrophone with custom built data logging hardware we report on the development of efficient wavelet based algorithms for the detection and discrimination of seismic events and analyze three time series of acoustic pressure collected at a depth of 700 m in pilot experiments conducted offshore san diego ca in these tests over 120 hours of data were gathered and five earthquakes of which one was teleseismic were recorded and identified quantitative estimates based on these results suggest that instruments of the mermaid type may collect up to a hundred tomographically useful teleseismic events per year the final design will also incorporate a global positioning system receiver onboard signal processing software optimized for low power chips and high throughput satellite communication equipment for telemetered data transfer with these improvements we hope to realize our vision of a global array of autonomous floating sensors for whole earth seismic tomography

low frequency seismic signals recorded by obs at stromboli volcano southern tyrrhenian sea 1 five three component broadband ocean bottom seismometers obss were deployed on the seafloor around the aeolian islands southern tyrrhenian sea by comparing obss digital seismograms we found a low frequency seismicity recorded only at obs05 the nearest seafloor station to stromboli volcano this seismicity appears in the form of a continuous seismic signal tremor like signal as well as a considerable number of shock like events we focused on recordings from obs05 to verify their correlation with stromboli volcanic activity from the spectral analysis we observed low frequency events lp events superposed upon the continuous background noise tremor lp events and tremor showing similar energy fluctuations and frequency content appear to be produced by the same dynamic processes we interpret this low frequency seismicity as probably originating from a continuous uprising of gas bubbles from the deeper part of the stromboli magmatic column this could highlight the existence of a deeper source for low frequency seismicity