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

upper crustal seismic velocity structure and microearthquake depths at the endeavour segment juan de fuca ridge 1 we present the results of a study to invert microearthquake and explosive shot data from the endeavour segment of the intermediate spreading juan de fuca ridge the average isotropic p wave velocity structure derived from the shot data in the uppermost 1 5 km of the oceanic crust is characterized by an increase with age of 8 from the axis to at least 0 5 ma that is attributed to the sealing of layer 2a porosity by hydrothermal processes superimposed on this variation are axis parallel 2 km wide alternating bands of high and low velocity with a peak to peak variation of 5 12 high and low velocities away from the axis correspond to bathymetric trenches and ridges respectively and are likely due to variations in layer 2a thickness p wave azimuthal anisotropy is present in the data that is best fit with a model of 9 anisotropy at 750 m depth decreasing to 1 at 3 km depth and is likely due to the preferential alignment of vertical cracks and fissures in the along axis direction anisotropy and velocity heterogeneity are coupled anisotropy alone may explain the form but not the magnitude of the axis parallel bands there are strong trade offs between the hypocentral depths of microearthquakes and the p and s wave velocity structures changing the mean hypocentral depth by up to 0 5 km leads to only modest increases in the travel time rms but the resulting velocity models appear more feasible when the earthquakes are forced deeper than when they are forced shallower

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

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

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

seismic evidence for variations in axial magma chamber properties along the southern juan de fuca ridge abstract multichannel seismic data collected along the cleft segment on the southern juan de fuca ridge show that this intermediate spreading center is underlain by a mid crustal reflector interpreted as the top of an axial magma chamber amc the amc reflection is present along most of the segment and deepens gently from 2 0 km near the southern end of the segment beneath the ridge cleft observatory site to 2 3 km at the northern end beneath the site of the mid 1980s submarine eruption we analyzed the one dimensional seismic structure of the amc at two locations with contrasting lava chemistry beneath two different hydrothermal vent fields at the northern site waveform modeling in the time intercept slowness p domain indicates that the amc is 100 m thick and it is characterized by a decrease in p wave velocity from 6 km s to 3 7 km s in contrast the p wave velocity within the shallower 100 m thick amc at the southern site is higher 5 0 km s the decrease in seismic velocity within the amc indicates that it is partially molten and that it is not a cracking front as previously suggested for other intermediate spreading segments the data show a coherent seismic phase interpreted as the p to s wave conversion at the amc pamcs stacking of this event shows that the pamcs is only present along the northern part of the segment our results thus suggest along axis variations in the crystallinity of the amc the amc along cleft varies from a high crystal content

p s velocity and vp vs ratio beneath the toba caldera complex northern sumatra from local earthquake tomography summary in this paper we investigate the crustal and uppermost mantle structure beneath toba caldera which is known as the location of one of the largest cenozoic eruptions on earth the most recent event occurred 74 000 yr bp and had a significant global impact on climate and the biosphere in this study we revise data on local seismicity in the toba area recorded by a temporary passcal network in 1995 we applied the newest version of the lotos 07 algorithm which includes absolute source location optimization of the starting 1 d velocity model and iterative tomographic inversion for 3 d seismicp s or thevp vs ratio and source parameters special attention is paid to verification of the obtained results beneath the toba caldera and other volcanoes of the arc we observe relatively moderate for volcanic areas negative p and s velocity anomalies that reach 18 per cent in the uppermost layer 10 12 per cent in the lower crust and about 7 per cent in the uppermost mantle much stronger contrasts are observed for the vp vs ratio that is a possible indicator of dominant effect of melting in origin of seismic anomalies at a depth of 5 km beneath active volcanoes we observe small patterns 7 15 km size with a high vp vs ratio that might be an image of actual magmatic chambers filled with partially molten material feeding the volcanoes in the mantle wedge we observe a vertical anomaly with low p and s velocities and a high vp vs ratio that link the cluster of events at 120 140 km depth with toba caldera this may be an image of ascending fluids and melts released from the subducted slab due to phase transitions however taking into account poor vertical resolution these results should be interpreted with prudence although the results show clear signatures that are quite typical for volcanic areas low velocity and high vp vs ratio beneath volcanoes we do not observe any specific features in seismic structure that could characterize toba as a super volcano

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

shear and compressional wave structure of the east pacific rise 9 10 n we use shear and compressional arrivals recorded by 45 ocean bottom seismograph receivers to model regional structure of the 9 10 n region of the east pacific rise modeling indicates that shear conversion occurs at the base of layer 2a producing ps energy that travels through oceanic crust as p and converts to s at the base of layer 2a on upgoing path and ps energy that travels as p through layer 2a and converts to s at the base of layer 2a on downgoing path arrivals the travel times of these arrivals require a shear wave velocity within layer 2a of 0 4 0 8 km s poisson s ratio of 0 46 0 49 waveform inversion was used to model the poisson s ratio structure of layers 2b and 3 for nine instruments with good shear arrivals poisson s ratio within layer 2b was highly variable with values as low as 0 24 and a mean value of 0 263 some of this variability might be due to lateral variability in layer 2a structure the mean poisson s ratio of layer 3 was 0 271 according to the cracking model of shaw 1994 the low poisson s ratios within the upper portion of layer 2b indicate that thick cracks aspect ratio 0 1 extend to depths of 1 5 1 7 km in this region two dimensional travel time modeling indicates that the southwestern portion of our study area is associated with anomalously low seismic velocities with layer 2b and layer 3 velocities reduced by up to 11 from the regional values cracking theory suggests that these low velocities could be caused by porosities of 0 3 5 5 depending on the crack aspect ratio a maximum porosity of 1 5 is predicted from our one poisson s ratio measurement in this area the anomalous velocities are located near the western discordant zone left by the 9 03 n overlapping spreading center we suggest that shearing associated with on axis rotation of the overlap basins is responsible for the low seismic velocities the pattern of anomalies indicates that faulting extends to distances of 10 15 km from the basins

variable crustal structure along the juan de fuca ridge influence of on axis hot spots and absolute plate motions multichannel seismic and bathymetric data from the juan de fuca ridge jdfr provide constraints on axial and ridge flank structure for the past 4 8 ma within three spreading corridors crossing cleft northern symmetric and endeavour segments along axis data reveal south to north gradients in seafloor relief and presence and depth of the crustal magma lens which indicate a warmer axial regime to the south both on a regional scale and within individual segments for young crust cross axis lines reveal differences between segments in moho two way traveltimes of 200 300 ms which indicate 0 5 1 km thicker crust at endeavour and cleft compared to northern symmetric moho traveltime anomalies extend beyond the 5 15 km wide axial high and coincide with distinct plateaus 32 and 40 km wide and 200 400 m high found at both segments on older crust moho traveltimes are similar for all three segments 2100 100 ms indicating little difference in average crustal production prior to 0 6 and 0 7 ma the presence of broad axis centered bathymetric plateau with thickened crust at cleft and endeavour segments is attributed to recent initiation of ridge axis centered melt anomalies associated with the cobb hot spot and the heckle melt anomaly increased melt supply at cleft segment upon initiation of axial volcano and southward propagation of endeavour segment during the brunhes point to rapid southward directed along axis channeling of melt anomalies linked to these hot spots preferential southward flow of the cobb and heckle melt anomalies and the regional scale south to north gradients in ridge structure along the jdfr may reflect influence of the northwesterly absolute motion of the ridge axis on subaxial melt distribution

three dimensional seismic tomography from p wave and s wave microearthquake travel times and rock physics characterization of the campi flegrei caldera 1 the campi flegrei cf caldera experiences dramatic ground deformations unsurpassed anywhere in the world the source responsible for this phenomenon is still debated with the aim of exploring the structure of the caldera as well as the role of hydrothermal fluids on velocity changes a multidisciplinary approach dealing with three dimensional delay time tomography and rock physics characterization has been followed selected seismic data were modeled by using a tomographic method based on an accurate finite difference travel time computation which simultaneously inverts p wave and s wave first arrival times for both velocity model parameters and hypocenter locations the retrieved p wave and s wave velocity images as well as the deduced v p v s images were interpreted by using experimental measurements of rock physical properties on cf samples to take into account steam water phase transition mechanisms affecting p wave and s wave velocities also modeling of petrophysical properties for site relevant rocks constrains the role of overpressured fluids on velocity a flat and low v p v s anomaly lies at 4 km depth under the city of pozzuoli earthquakes are located at the top of this anomaly this anomaly implies the presence of fractured overpressured gas bearing formations and excludes the presence of melted rocks at shallow depth a high v p v s anomaly located at 1 km suggests the presence of rocks containing fluids in the liquid phase finally maps of the v p v s product show a high v p v s horseshoe shaped anomaly located at 2 km depth it is consistent with gravity data and well data and might constitute the on land remainder of the caldera rim detected below sea level by tomography using active source seismic data

microearthquake characteristics and crustal vp vs structure at the mid atlantic ridge 35 n we report on the results of a microearthquake experiment conducted at the along axis bathymetric high of the slow spreading ridge segment near 35 n on the mid atlantic ridge mar of a total of 255 microearthquakes recorded during the 43 day experiment 31 were located near the oceanographer transform at the northern end of the segment 79 occurred at the nontransform offset at the southern end of the segment and 145 were at the segment center at the segment center earthquake epicenters lay within the median valley inner floor and formed a 12 km long trend paralleling a steep scarp bounding the western wall of the inner valley focal depths were 3 4 km below the seafloor most 80 of the focal mechanisms for 32 segment center earthquakes are consistent with normal faulting on faults approximately parallel to the axial trend from a joint inversion for hypocenters and p and s wave velocity structures we determined a horizontally averaged v p v s ratio that decreases from 2 9 in the shallowmost 300 m to 1 7 at 2 km depth and we interpret this decrease as indicating a decreasing contribution of thin cracks to fracture porosity with depth the maximum depth of seismicity 4 km is anomalously shallow compared with other mar segments at which microearthquake experiments have been carried out cross axis relief is also anomalously low for this segment s center and on the basis of this and other mar microearthquake experiments there appears to be a correlation between cross axis relief and maximum depth of seismicity from the correlation of cross axis relief and inferred crustal thickness we suggest a relationship between thick crust high crustal temperatures and low cross axis relief in qualitative agreement with thermomechanical models for the depth of the axial valley

controls on extrusion at mid ocean ridges a magma lens can erupt to form extrusives only if it is under greater pressure than the static pressure in a column of magma reaching from the lens to the surface the excess pressure results partly from overburden pressure caused by the presence of high and low density rocks dikes and extrusives respectively above the lens the thicker the pile of low density extrusives the lower the average overburden density thus extrusion above a lens should be self regulating in that thickening the extrusive layer reduces the driving pressure for subsequent eruptions flexural stresses may affect extrusion by altering the pressure on a magma chamber for ridges lacking an axial valley we predict that deeper magma lenses should correlate with thicker extrusive layers consistent with recent observations

multi channel seismic imaging of a crustal magma chamber along the east pacific rise a reflection observed on multi channel seismic profiles along and across the east pacific rise between 8 50 n and 13 30 n is interpreted to arise from the top of a crustal magma chamber located 1 2 2 4 km below the sea floor the magma chamber is quite narrow 4 6 km wide but can be traced as a nearly continuous feature for tens of kilometres along the rise axis

the influence of porosity and crack morphology on seismic velocity and permeability in the upper oceanic crust 1 the purpose of this study is to explore the relationships between porosity crack morphology seismic velocity and permeability in the upper oceanic crust in a theoretical model velocity is a quadratic function of porosity and the coefficients are functions of the fractional area of asperity contact af across the cracks a measure of crack morphology thus v v af fits to data sets from the lava and dike sections in holes 504b and 1256d yield af 0 096 8 and 0 045 3 with rms errors of 1 7 and 0 3 respectively lower values of af account for the high porosities and low velocities observed in layer 2a after 0 2 ma the porosities of both layer 2a and the deeper part of the lava pile remain in the range 8 4 the observed increase of layer 2a velocities after 0 2 ma is caused largely by changing crack morphology as opposed to decreasing porosity at 8 porosity an increase of af from 0 003 to 0 1 accounts for an increase of velocity from 3 to 5 km s an empirical log linear relationship between permeability and v suggests a relationship between and af as well this relationship between and v can be explained if the permeability coefficient o is a closely constrained quadratic function of the area of asperity contact af af is thus the primary variable affecting both seismic velocities and the permeability of the uppermost oceanic crust

seismic precursors and magma ascent before the april 2011 eruption at axial seamount for volcanoes at submarine rift zones a direct link between seismicity seafloor deformation and magma intrusion has not been demonstrated recordings from ocean bottom hydrophones and bottom pressure recorders map an increasing rate of seismicity at axial seamount northeast pacific over several years before its eruption in april 2011

understanding caldera structure and development an overview of analogue models compared to natural calderas abstract understanding the structure and development of calderas is crucial for predicting their behaviour during periods of unrest and to plan geothermal and ore exploitation geological data including that from analysis of deeply eroded examples allow the overall surface setting of calderas to be defined whereas deep drillings and geophysical investigations provide insights on their subsurface structure collation of this information from calderas worldwide has resulted in the recent literature in five main caldera types downsag piston funnel piecemeal trapdoor being viewed as end members despite its importance such a classification does not adequately examine a the structure of calderas particularly the nature of the caldera s bounding faults and b how this is achieved including the genetic relationships among the five caldera types various sets of analogue models specifically devoted to study caldera architecture and development have been recently performed under different conditions apparatus materials scaling parameters stress conditions the first part of this study reviews these experiments which induce collapse as a result of underpressure or overpressure within the chamber analogue the experiments simulating overpressure display consistent results but the experimental depressions require an exceptional amount of doming seldom observed in nature to form therefore these experiments are not appropriate to understand the structure and formation of most natural calderas the experiments simulating underpressure reveal a consistent scenario for caldera structure and development regardless of their different boundary conditions these show that complete collapse proceeds through four main stages proportional to the amount of subsidence progressively characterized by 1 downsag 2 reverse ring fault 3 peripheral downsag 4 peripheral normal ring fault the second part of this study verifies the possibility that these latter calderas constitute a suitable analogue to nature and consists of a comprehensive comparison of the underpressure experiments to natural calderas this shows that all the experimental structures as well as their progressive development are commonly observed at natural calderas highlighting a consistency between models and nature as the shallow structure of experimental calderas corresponds to a precise architecture at depth it provides a unique key to infer the deeper structure of natural calderas recognizing diagnostic surface features within a caldera will thus allow it to be categorized within a precise structural and evolutionary context the general relationship between the evolutionary stage of a caldera and its d s diameter subsidence ratio allows such a quantification with stage 1 calderas characterized by d s 40 stage 2 by 18 the four stages adequately explain the architecture and development of the established caldera end members along a continuum where one or more end members downsag piston funnel piecemeal trapdoor may correspond to a specific stage while such a continuum is controlled by progressive subsidence specific collapse geometries will result from secondary contributory factors roof aspect ratio collapse symmetry pre existing faults these considerations allow proposing an original classification of calderas incorporating their structural and genetic features

a multichannel seismic investigation of upper crustal structure at 9 n on the east pacific rise implications for crustal accretion reprocessed multichannel seismic profiles from the 9 n segment of the east pacific rise reveal prominent shallow subbasement events these events are identified as wide angle reflections from the base of seismic layer 2a based upon modeling of expanding spread profile data and velocity functions the layer 2a reflections typically increase from 0 15 s after the seafloor reflection at the rise axis to 0 3 0 45 s within 1 2 km of the axis corresponding to an increase in layer thickness of 200 600 m no further systematic increase in layer thickness is observed although lateral variability of the order of a few hundred meters in thickness is observed at greater offsets from the rise axis however the intermittent character of the imaged layer 2a reflection is attributed to focusing and defocusing of energy by the seafloor bathymetry rather than necessarily to intrinsic lateral variability at the base of the layer the base of layer 2a is interpreted as corresponding to the transition between the extrusive section pillow basalts and sheet flows and a sheeted dike complex the rapid thickening of the layer near the rise axis is attributed to successive lava flows burying the initially shallow top of the sheeted dike complex as the layer passes through the neovolcanic zone lateral variability of layer 2a can significantly affect the imaging of the underlying axial magma chambers as average velocities within layer 2a are approximately half that of layer 2b for an along axis profile apparent along axis variability in the depth of the axial magma chamber is traced to variability in the thickness of layer 2a caused by wandering of the profile relative to axis within the resolution of the data the time delay of the magma chamber reflection relative to the base of layer 2a is constant

a 2500 yr late holocene multi proxy record of vegetation and hydrologic changes from a cave guano clay sequence in sw romania we provide sedimentological geochemical mineral magnetic stable carbon isotope charcoal and pollen based evidence from a guano clay sequence in gaura cu musc cave sw romania from which we deduced that from 1230 bc to ad 1240 climate oscillated between wet and dry from 1230 bc to ad 1000 the climate was wetter than the present prompting flooding of the cave preventing bats from roosting and resulting in a slow rate of clay accumulation the second half of the medieval warm period mwp was generally drier the cave experienced occasional flash flooding in between which maternity bat roosts established in the cave one extremely wet event occurred around ad 1170 when fe mn and ti zr ratios show the highest values coincident with a substantial increase of sediment load in the underground stream the mineral magnetic characteristics for the second part of the mwp indicate the partial input of surface sourced sediments reflecting agricultural development and forest clearance in the area pollen and microcharcoal studies confirm that the overall vegetation cover and human land use have not changed much in this region since the medieval times

microearthquakes on the endeavour segment of the juan de fuca ridge 1 we report the results of a 55 day microearthquake experiment on the endeavour segment of juan de fuca ridge the network covered a 5 km section of the ridge axis centered on the main vent field and extended 15 km off axis on the west flank the ridge axis and flanks were seismically active and 1750 earthquakes were located with a minimum of five travel time picks including at least one s wave over half the earthquakes occurred in swarms and a waveform cross correlation technique was used to obtain relative locations on the western flank the hypocenters for four swarms at midcrustal depths are compatible with steeply dipping fault planes that strike at 035 050 n and oblique to the abyssal hills focal mechanisms determined from p wave first motions and p s amplitude ratios are predominantly strike slip with north south compression and appear to be affected by the reorganization of the explorer plate earthquakes beneath the ridge axis are concentrated in a band of intense seismicity at 1 5 3 5 km depth to the north of high rise vent field the seismicity defines a plane striking parallel to the ridge axis and dipping east at 70 and the earthquakes appear to extend beneath an axial reflector previously imaged at 2 3 km depth farther south the hypocenters are not compatible with a single fault plane focal mechanisms are characterized by subhorizontal tension axes oriented in all directions except parallel to the ridge and suggest a stress field that is about equally influenced by ridge spreading and hydrothermal cooling

feeding volcanoes of the kluchevskoy group from the results of local earthquake tomography 1 we present a seismic model of the area beneath the kluchevskoy volcano group kamchatka russia based on the tomographic inversion of more than 66000 p and s arrival times from more than 5000 local earthquakes that occurred in 2004 and that were recorded by 17 permanent stations below a depth of 25 km beneath the kluchevskoy volcano we observed a very strong anomaly in the vp vs ratio that reached as high as 2 2 this is a probable indicator of the presence of partially molten material with a composition corresponding to deeper mantle layers the upper part of this anomaly at a depth of 25 30 km coincides with a cluster of strong seismicity that can be explained by strong mechanical stresses in the lowermost crust due to magma ascension water release and or phase transitions in the crust we observed regular seismicity clusters that link the mantle anomaly with the kluchevskoy volcano and most likely indicate the paths of magma migration between depths of 8 and 13 km we see several patterns of high vp vs ratios interpreted as intermediate depth magma storages directly below the kluchevskoy volcano we observed a shallow body of high vp vs which probably represents the activated magma chamber just beneath the volcano cone which erupted in the beginning of 2005 the existence of three levels of magma storage based on results of local earthquake tomography may explain the variety of the lava composition and eruption regimes in different volcanoes of the kluchevskoy group

axial seamount an active ridge axis volcano on the central juan de fuca ridge axial seamount some of the manuscripts in this special section refer to the edifice with the more precise name of axial volcano a large ridge axis volcano is located on the central segment of the juan de fuca ridge approximately 250 nautical miles west of the washington oregon british columbia coast currently both volcanically and hydrothermally active axial lies directly at the intersection of the cobb eickelberg seamount chain and the juan de fuca ridge figure 1 the volcanic activity associated with the seamount formation strongly interacts with and is affected by the normal seafloor spreading processes at the intersection because of this unique geologic setting its proximity to west coast ports and oceanographie institutions and its shallow depth axial has become the focus of a large number of scientific investigations over the past decade

seismic structure of the southern east pacific rise seismic data from the ultrafast spreading 150 to 162 millimeters per year southern east pacific rise show that the rise axis is underlain by a thin less than 200 meters thick extrusive volcanic layer seismic layer 2a that thickens rapidly off axis also beneath the rise axis is a narrow less than 1 kilometer wide melt sill that is in some places less than 1000 meters below the sea floor the small dimensions of this molten body indicate that magma chamber size does not depend strongly on spreading rate as predicted by many ridge crest thermal models however the shallow depth of this body is consistent with an inverse correlation between magma chamber depth and spreading rate these observations indicate that the paradigm of ridge crest magma chambers as small sill like midcrustal bodies is applicable to a wide range of intermediate and fast spreading ridges

influence of magma supply and spreading rate on crustal magma bodies and emplacement of the extrusive layer insights from the east pacific rise at lat 16 n seismic reflection data from the east pacific rise at lat 16 n which is spreading at the high end of intermediate rates suggest that the depths at which axial magma chambers reside do not vary smoothly as a function of spreading rate rather magma chamber depths form two distinct populations each associated with a distinct axial morphology and with an abrupt transition occurring within the intermediate spreading rate range our data 1 show that the melt lens at high intermediate spreading ridges lies at a shallow level similar to lens depths at faster spreading ridges and 2 provide further support for the spreading rate invariance of ridges with axial highs noted in other ridge properties the axial morphology of the two ridge segments within the study area differs markedly and a large contrast in magma supply is inferred the ridge segment with greater magma supply is associated with a broader and more continuous melt lens a wider region over which the extrusive crust accumulates and a thicker extrusive layer off axis where supply to the ridge segment appears to be centered however on axis the extrusive layer is thinnest where magma supply is robust and a shallower melt lens is observed consistent with a model in which magma pressure controls the thickness of the extrusive layer accumulated above the magma lens

hydrothermal seismicity beneath the summit of lucky strike volcano mid atlantic ridge abstract we present two years july 2007 july 2009 of earthquake locations from a local seismological network on the lucky strike volcano mid atlantic ridge whose summit hosts one of the most active and largest deep sea hydrothermal fields known two clusters of small m l

seismic constraints on shallow crustal emplacement processes at the fast spreading east pacific rise we present the results of nine on bottom seismic refraction experiments carried out over young east pacific rise crust the experiments are unusual in that both the source and receiver are located within a few meters of the seafloor allowing high resolution determinations of shallow crustal structure three experiments were located within the axial summit caldera asc over zero age crust the seismic structure at these three locations is fundamentally the same with a thin 60 m surficial low velocity 2 5 km s layer a 100 to 150 m thick transition zone with velocities increasing by 2 5 km s and a layer with velocities of 5 km s at a depth beneath the seafloor of 130 190 m the surficial low velocity layer and transition zone are defined as seismic layer 2a and the 5 km s layer is defined as the top of layer 2b both the surficial low velocity layer and the transition zone double in thickness within 1 km of the rise axis we model layer 2a as the extrusive sequence and transition zone and the 2a 2b boundary as the top of the sheeted dikes the primary implication of this interpretation is that the depth to the top of the sheeted dikes deepens from 150 m to 300 m within 1 km of the asc the thickening of the extrusive layer is interpreted to be due to lava that either overflows the asc walls is emplaced through eruptions outside of the asc or travels laterally from the asc through conduits the most probable cause for the thickening of the transition zone is sill emplacement outside of the asc either from magma that does not reach the surface in an off axis eruption or magma that is transported laterally during the drainage process creating the asc we suggest that the mechanism controlling the magnitude and rate of the dike subsidence is the mechanism that determines the thickness of the extrusive section and the total thickness of layer 2a

repeat bathymetric surveys at 1 metre resolution of lava flows erupted at axial seamount in april 2011 at frequently active submarine volcanoes it is difficult to distinguish between new and pre existing lava flows a combination of high resolution bathymetric surveys taken before and after an eruption at axial seamount in 2011 allows detailed mapping of the 2011 lava flows and highlights the tendency of new flows to mimic older ones

three dimensional p and s wave velocity structure of redoubt volcano alaska the three dimensional p and s wave structure of redoubt volcano alaska and the underlying crust to depths of 7 8 km is determined from 6219 p wave and 4008 s wave first arrival times recorded by a 30 station seismograph network deployed on and around the volcano first arrival times are calculated using a finite difference technique which allows for flexible parameterization of the slowness model and easy inclusion of topography and source receiver geometry the three dimensional p wave velocity structure and hypocenters are determined simultaneously while the three dimensional s wave velocity model is determined using the relocated seismicity and an initial s wave velocity model derived from the p wave velocity model assuming an average vp vs ratio of 1 78 convergence is steady with approximately 73 and 52 reduction in p and s wave arrival time rms respectively after 10 iterations the most prominent feature observed in the three dimensional velocity models derived for both p and s waves is a relative low velocity near vertical pipelike structure approximately 1 km in diameter that extends from 1 to 6 km beneath sea level this feature aligns axially with the bulk of seismicity and is interpreted as a highly fractured and altered zone encompassing a magma conduit the velocity structure beneath the north flank of the volcano between depths of 1 and 6 km is characterized by large lateral velocity variations high velocities within this region are interpreted as remnant dikes and sills and low velocities as regions along which magma migrates no large low velocity body suggestive of a magma chamber is resolved in the upper 7 8 km of the crust

high sensitivity of ocean ridge thermal structure to changes in magma supply the galapagos spreading center abstract we explore the physical mechanisms for the observed apparent sensitivity of ridge axis topography and crustal magma systems to small changes in magma supply at intermediate spreading rates numerical experiments were carried out to simulate crustal temperature structure of the galapagos spreading center which spreads at intermediate spreading rates with its various sections appear to have been influenced to different degrees by the nearby galapagos hotspot model results show a strong threshold effect as the crustal thickness decreases from 7 4 km at the 92 w area with an axial high westward to 6 0 km at the 94 w area with a transitional topography the depth to the top of a magma lens is calculated to increase from 1 7 to 2 5 km in contrast at the 97 w area where crustal thickness is only 5 6 km and a rift valley is present the model results predict no steady state magma lens in the crust these model calculations provide a simple physical explanation for the recent observations along the galapagos spreading center where abrupt changes in both magma lens and axial morphology occur within a short distance but crustal thickness changes only modestly results of this investigation illustrate the critical importance of hotspots in influencing mid ocean ridge crustal thickness and the associated changes in thermal structure especially for ridges that spread at the sensitive range of intermediate spreading rates

crustal structure of deception island volcano from p wave seismic tomography tectonic and volcanic implications 1 deception island 62 59 s 60 41 w is an active volcano located in the bransfield strait between the antarctic peninsula and the south shetland islands the island is composed of rocks that date from 0 75 ma to historical eruptions 1842 1967 1969 and 1970 and nowadays most of its activity is represented by vigorous hydrothermal circulation slight resurgence of the inner bay floor and intense seismicity with frequent volcano tectonic and long period events in january 2005 an extensive seismic survey took place in and around the island to collect high quality data for a high resolution p wave velocity tomography study a total of 95 land and 14 ocean bottom seismometers were deployed and more than 6600 air gun shots were fired as a result of this experiment more than 70 000 travel time data were used to obtain the velocity model which resolves strong p wave velocity contrasts down to 5 km depth the joint interpretation of the vp distribution together with the results of geological geochemical and other geophysical magnetic and gravimetric measurements allows us to map and interpret several volcanic features of the island and surroundings the most striking feature is the low p wave velocity beneath the caldera floor which represents the seismic image of an extensive region of magma beneath a sediment filled basin another low velocity zone to the east of deception island corresponds to seafloor sedimentary deposits while high velocities to the northwest are interpreted as the crystalline basement of the south shetland islands platform in general in the tomographic image we observe ne sw and nw se distributions of velocity contrasts that are compatible with the regional tectonic directions and suggest that the volcanic evolution of deception island is strongly conditioned by the bransfield basin geodynamics

melt to mush variations in crustal magma properties along the ridge crest at the southern east pacific rise the determination of along axis variations in melt properties within the crustal axial magma chamber beneath fast spreading axes is important for understanding melt delivery from the mantle eruption history along the ridge crest and the process of crustal accretion seismic reflection images1 2 3 4 have shown the molten sill to be continuous along the ridge crest for many tens of kilometres with varying widths 250 4 500 m but variations in its seismic properties and thickness have remained elusive despite several attempts to constrain these properties5 6 7 here we report that the melt sill along the southern east pacific rise which is about 50 m thick undergoes abrupt changes in its internal properties ranging from pure melt to mush the 60 km long ridge crest segment near 14 00 s is characterized by three 2 4 km sections containing pure melt embedded within a magma chamber rich in mush these small pure melt pockets may represent a fresh supply of magma from the mantle capable of erupting and forming the upper crust conversely the 80 90 of the magma chamber which is mushy is unlikely to erupt and may influence the lower crustal accretion

three dimensional geometry of axial magma chamber roof and faults at lucky strike volcano on the mid atlantic ridge we present results from three dimensional 3 d processing of seismic reflection data acquired in june 2005 over the lucky strike volcano on the mid atlantic ridge as a part of the seismic study for monitoring of the mid atlantic ridge survey we use a 3 d tomographic velocity model derived from a coincident ocean bottom seismometer experiment to depth convert the poststack time migrated seismic volume and provide 3 d geometry of the axial magma chamber roof fault reflectors and layer 2a gradient marker we also generate a high resolution bathymetric map using the seismic reflection data the magma chamber roof is imaged at 3 4 0 4 km depth beneath the volcano and major faults are imaged with dips ranging between 33 and 50 the magma chamber roof geometry is consistent with a focused melt supply at the segment center and steep across axis thermal gradients as indicated by the proximity between the magma chamber and nearby faults fault scarps on the seafloor and fault dip at depth indicate that tectonic extension accounts for at least 10 of the total plate separation shallow dipping reflectors imaged in the upper crust beneath the volcano flanks are interpreted as buried lava flow surfaces

the self organizing map a new approach to apprehend the madden julian oscillation influence on the intraseasonal variability of rainfall in the southern african region the madden julian oscillation mjo is the major mode of intraseasonal variability 30 60 days in the tropics having large rainfall impacts globally and possibly on southern africa however the latter impact is not well understood and needs to be further explored the life cycle of the mjo known to be asymmetric has been nevertheless analyzed usually through methods constrained by both linearity and orthogonality such as empirical orthogonal function analysis here we explore a non linear classification method the self organizing map som a type of artificial neural network used to produce a low dimensional representation of high dimensional datasets to capture more accurately the life cycle of the mjo and its global impacts the classification is applied on intraseasonal anomalies of outgoing longwave radiation within the tropical region over the 1980 2009 period using the som to describe the mjo is a new approach complimentary to the usual real time multivariate mjo index it efficiently captures this propagative phenomenon and its seasonality and is shown to provide additional temporal and spatial information on mjo activity for each node the subtropical convection is analyzed with a particular focus on the southern africa region results show that the convection activity over the central tropical indian ocean is a key factor influencing the intraseasonal convective activity over the southern african region enhanced suppressed convection over the central indian ocean tends to suppress enhance convection over the southern african region with a 10 day lag by modulating the moisture transport

geometrical and mechanical constraints on the formation of ring fault calderas ash flow plate subsidence piston like calderas are bounded by a set of arcuated sub vertical collapse faults named ring faults experimental studies on caldera formation performed mostly using spherical or cylindrical magma chamber geometries find that the resulting ring faults correspond to steeply outward dipping reverse faults and show that pre existing fractures developed during pre eruptive phases of pressure increase may play a major role in controlling the final collapse mechanism a situation that should be expected in small to medium sized ring fault calderas developed on top of composite volcanoes or volcanic clusters on the other hand some numerical experiments indicate that large sill like elongated magma chambers may induce collapse due to roof bending without fault reactivation as seems to occur in large plate subsidence calderas formed independently of pre existing volcanoes also numerical experiments allow the formation of nearly vertical or steeply inward dipping normal ring faults in contrast with most of the analogue models using a thermoelastic model we investigate the geometrical and mechanical conditions to form ring fault calderas in particular the largest ones without needing a previous crust fracturing results are given in terms of two dimensionless geometrical parameters namely and e the former is the chamber extension to chamber depth ratio whereas the latter stands for the chamber eccentricity we propose that the e pair determinates two different types of ring fault calderas with different associated collapse regimes ring fault region a is related to large plate subsidence calderas i e andean calderas or western us calderas for which few depressurisation is needed to set up a collapse initially governed by flexural bending of the chamber roof in contrast ring fault region b is related to small to moderate sized calderas i e composite volcano calderas for which much depressurisation is needed our opinion is that collapse requires in the latter case reactivation of pre existing fractures and it is therefore more complex and history dependent

anatomy of an active submarine volcano most of the magma erupted at mid ocean ridges is stored in a mid crustal melt lens that lies at the boundary between sheeted dikes and gabbros nevertheless images of the magma pathways linking this melt lens to the overlying eruption site have remained elusive here we have used seismic methods to image the thickest magma reservoir observed beneath any spreading center to date which is principally attributed to the juxtaposition of the juan de fuca ridge with the cobb hotspot northwestern usa our results reveal a complex melt body which is 14 km long 3 km wide and up to 1 km thick beneath the summit caldera the estimated volume of the reservoir is 18 30 km3 more than two orders of magnitude greater than the erupted magma volumes of either the a d 1998 or 2011 eruption our images show a network of sub horizontal to shallow dipping 30 features that we interpret as pathways facilitating melt transport from the magma reservoir to the eruption sites

in situ ground deformation measurements from the summit of axial volcano during the 1998 volcanic episode since september 1987 seafloor instrumentation monitoring ground deformation has been maintained on the summit of axial volcano on the central juan de fuca ridge during a january 1998 volcanic episode pressure records from the central caldera showed a significant increase in pressure indicating caldera subsidence of over 3 m this observed subsidence is interpreted to represent volcanic deflation as magma is erupted and or injected into the volcanic flanks temperature anomalies of nearly 0 45 c were also observed a comparison of the subsidence record with hydroacoustic recordings indicates a continuous volcanic tremor signal coincident with the subsidence event the time coincidence of subsidence of the summit caldera with migration of epicenters into the volcanic rift zones confirms that during the 1998 episode the volcanic flanks were supplied by lateral dike injection from a central magma body rather than locally derived magma sources

rift topography linked to magmatism at the intermediate spreading juan de fuca ridge new seismic observations of crustal structure along the juan de fuca ridge indicate that the axial rift topography reflects magma induced deformation rather than alternating phases of magmatism and tectonic extension as previously proposed contrary to predictions of the episodic models crustal magma bodies are imaged beneath portions of all ridge segments surveyed at average depths of 2 1 2 6 km the shallow rift valley or axial graben associated with each juan de fuca segment is 50 200 m deep and 1 8 km wide and is well correlated with a magma body in the subsurface analysis of graben dimensions height and width shows that the axial graben narrows and graben height diminishes where the magma body disappears rather than deepening and broadening as expected for rift topography due to tectonic extension we propose an evolutionary model of axial topography that emphasizes the contribution of dike intrusion to subsidence and fault slip at the seafloor in this model an evolving axial topography results from feedbacks between the rheology of the crust above a magma sill and dike intrusion rather than episodic magma delivery from the mantle

three dimensional p wave velocity structure of tungurahua volcano ecuador abstract tungurahua volcano in the ecuadorian andes is a large andesitic stratovolcano 5023 m that has been erupting mildly since 1999 we studied the three dimensional 3 d p wave velocity vp structure beneath the volcano down to 5 km below the summit we inverted 1708 p wave first arrival times from 263 volcano tectonic vt earthquakes recorded by 5 to 10 short period vertical seismic stations on the volcano from august 1999 to may 2003 a tomographic inversion method was used to image the velocity structure in which first arrival times were calculated with a finite difference method the root mean square of the arrival time residuals rms was reduced by 43 after running 10 iterations from the initial rms of 0 15 s the relocated hypocenters in our model are tightly clustered along a vertical structure at depths between sea level and the summit crater a high velocity zone exists above the central base of the volcano under the vertically aligned hypocenters and may be interpreted as the source zone for recharge of the shallow magmatic system high velocity zones are also identified under the lower northeastern and southern flanks of the edifice the southern high velocity anomaly lies close to the surface and is connected to the high velocity zone in the central base of the volcano a feature suggesting an old lateral dike system except for these high velocity zones in the central northern and southern flanks the volcanic edifice is composed of low velocity materials down to a depth of 2 km above sea level these low velocity zones correlate with young unconsolidated deposits and older highly fractured and or altered volcanic materials

establishing a new era of submarine volcanic observatories cabling axial seamount and the endeavour segment of the juan de fuca ridge abstract at least 70 of the volcanism on earth occurs along the 65 000 km network of mid ocean ridge mor spreading centers within these dynamic environments the highest fluxes of heat chemicals and biological material from the lithosphere to the hydrosphere occur during volcanic eruptions however because underwater volcanoes are difficult and expensive to access researchers are rarely if ever in the right place at the right time to characterize these events therefore our knowledge is limited about the linkages among hydrothermal chemical and biological processes during seafloor formation and crustal evolution to make significant advancements in understanding the evolution of mor environments the united states and canada have invested in the first plate scale submarine cabled observatory linked through the global internet spanning the juan de fuca tectonic plate these two networks include 1700 km of cable and 14 subsea terminals that provide 8 10 kw power and 10 gbs communications to hundreds of instruments on the seafloor and throughout the overlying water column resulting in a 24 7 365 presence in the oceans data and imagery are available in real to near real time the initial experimental sites for monitoring volcanic processes include the mor volcanoes called axial seamount and the endeavour segment that are located on the juan de fuca ridge axial a hot spot influenced seamount is the most robust volcano along the ridge rising nearly 1400 m above the surrounding seafloor and it has erupted twice in the last 15 years in contrast the endeavour segment is characterized by more subdued topography with a well defined axial rift and it hosts one of the most intensely venting hydrothermal systems known a non eruptive spreading event lasting 6 years was documented at endeavour between 1999 and 2005 hydrothermal venting intensity chemistry and temperature as well as associated biological communities at both sites were significantly perturbed by the magmatic and intrusive events this paper presents the similarities and differences between the axial and endeavour volcanic systems and identifies reasons why they are ideal candidates for comparative studies the u s has made a 25 year commitment for sustained observations using the cabled infrastructure the highly expandable nature of submarine optical networking will allow for the future addition of novel experiments that utilize ever evolving advancements in computer sciences robotics genomics and sensor miniaturization comprehensive modeling of the myriad processes involved will continue to assimilate and integrate growing databases yielding a new understanding of integrated processes that create the seafloor in the global ocean basins

seafloor deformation and forecasts of the april 2011 eruption at axial seamount the axial seamount submarine volcano exhibits an inflation deflation cycle comparable to similar volcanoes on land measurements of ocean bottom pressure document the entire inflation deflation cycle between eruptions at axial seamount in 1998 and 2011 and imply that the timing of submarine eruptions could be more predictable than that of their subaerial counterparts

geochemistry of hydrothermal fluids from axial seamount hydrothermal emissions study vent field juan de fuca ridge subseafloor boiling and subsequent fluid rock interaction hydrothermal fluids collected from the ashes vent field in 1986 1987 and 1988 exhibit a very wide range of chemical composition over a small area 60 m in diameter compositions range from a 300 c gas enriched 285 mmol kg co2 low chlorinity 33 of seawater fluid to a 328 c relatively gas depleted 50 mmol kg co2 high chlorinity 116 of seawater fluid the entire range of measured compositions at ashes is best explained by a single hydrothermal fluid undergoing phase separation while rising through the ocean crust followed by partial segregation of the vapor and brine phases other mechanisms proposed to produce chlorinity variations in hydrothermal fluids precipitation dissolution of a chloride bearing mineral or crustal hydration cannot produce the covariation of chlorinity and gas content observed at ashes there is good agreement of the measured fluid compositions with compositions generated by a simple model of phase separation in which gases are partitioned according to henry s law and all salt remains in the liquid phase significant enrichments in silica lithium and boron in the low chlorinity fluids over levels predicted by the model are attributed to fluid rock interaction in the upflow zone depletions in iron and calcium suggest that these elements have been removed by iron sulfide and anhydrite precipitation at some time in the history of the low chlorinity fluids the distribution of low and high chlorinity venting is consistent with mechanisms of phase segregation based on differential buoyancy or relative permeability the relatively shallow depth of the seafloor 1540 m and the observed chemistry of ashes fluids are consistent with phase separation in the sub critical or near critical region

mapping of seismic layer 2a 2b boundary above the sheeted dike unit at intermediate spreading crust exposed near the blanco transform 1 we present results from a study mapping the seismic layer 2a 2b boundary in young ocean crust located adjacent to the north wall of the blanco transform fault btf we review seafloor features geochemical and petrologic data seismic imaging and seismic structure and conclude that the btf seismic study region is representative of crust formed at the intermediate spreading juan de fuca ridge the mean layer 2a two way travel time beneath the seafloor is 0 37 0 10 s for the 42 12 km seismic survey area several regions are observed where layer 2a is consistently thin or thick over lateral distances of 5 10 km both in a ridge parallel and ridge perpendicular direction layer 2a thicknesses appear more variable in the ridge parallel isochron direction than the ridge perpendicular flow line direction there is no systematic pattern of layer 2a thickness variability with distance from the btf nor is there a correlation between seafloor topography and layer 2a thickness calculated mean layer 2a thickness for the seismic study region is 485 135 m assuming an interval velocity of 2 65 km s the imaged layer 2a 2b boundary projects on average 600 650 m above the top of the sheeted dike complex mapped on the adjacent btf north wall we argue that the geologic context of the layer 2a 2b boundary will vary as competing processes of magmatic construction and fracturing increasing porosity and crustal thickening compaction dike intrusion and hydrothermal sealing decreasing porosity vary as the crust spreads laterally from the ridge axis to ridge flanks

poisson s ratio structure of young oceanic crust we have applied full waveform inversion to wide aperture seismic reflection data from the southern east pacific rise near 14 s the data contain clear compressional wave and doubly converted shear wave arrivals that provide good constraints on the p and s wave velocities vp vs and hence poisson s ratio and seismic attenuation qp qs structure of seismic layer 2 layer 2a is highly attenuating qp 18 30 and qs 8 15 and layer 2b is moderately attenuating qp 30 50 and qs 20 25 our results show high at the seafloor and in layer 2a 0 48 across the top of the 2a b transition the rapid increase in vp is accompanied by a sharp drop in to 0 25 within just 200 m of the seafloor we perform simple calculations to gain an insight into the porosity and crack distribution with depth these calculations suggest that porosity is in excess of 30 in layer 2a but reduces to 6 7 at the top of the 2a b transition and to about 5 at a depth of 600 m below seafloor within layer 2b our results suggest that there is an increase in the average aspect ratio with depth across the 2a b transition the most likely explanation is that numerous thin cracks either mechanically close or are infilled at depth our results show there to be an abrupt change in the pore structure across the 2a b transition which is consistent with a lithologic transition from extrusives to dykes but is equally consistent with a transition mechanical or hydrothermal within the extrusive pile

relationship between caldera collapse and magma chamber withdrawal an experimental approach collapse calderas have received considerable attention due to their link to earth s ore deposits and geothermal energy resources but also because of their tremendous destructive potential although calderas have been investigated through fieldwork numerical models and experimental studies some important aspects on their formation still remain poorly understood one key issue concerns the volume of magmas involved in caldera forming eruptions we perform analogue experiments to correlate the structural evolution of a collapse with the erupted magma chamber volume fraction the experimental device consists of a transparent box 60 60 40 cm filled with dry quartz sand and a water filled latex balloon as a magma chamber analogue evacuation of water through a pipe causes a progressive deflation of the balloon that leads to a collapse of the overlying structure the experimental design allows to record the temporal evolution of the collapse and to track the evolution of fractures and faults we study the appearance and development of specific brittle structures such as surface fractures or internal reverse faults and correlate each different structure with the corresponding removed magma chamber volume fraction we also determine the critical conditions for caldera onset experimental results show that at any stage of caldera developments the experimental relationship between volume fraction and chamber roof aspect ratio fits a logarithmic curve it implies that volume fractions required to trigger caldera collapse are lower for chambers with low aspect ratios shallow and wide than for chambers with high aspect ratios deep and small these results are in agreement with natural examples and previous theoretical studies

inconsistent correlation of seismic layer 2a and lava layer thickness in oceanic crust the oceanic crust makes up two thirds of the earth s surface and has been studied for many years yet the processes by which it is formed are still not fully understood one problem is inaccessibility only a tiny fraction of it has been subjected to drilling and sampling the tools of geology so much relies on remote sensing techniques such as seismic exploration the tools of geophysics these two approaches have led to some confusion over crustal structure christeson et al set out to reconcile the two by a direct comparison between seismic interpretations and geologic mapping the new data question the idea that seismic layer 2a correlates with lava layer thickness instead the layer 2a reflection seems to correspond to an alteration boundary within the lava layer at mid ocean ridges with fast to intermediate spreading rates the upper section of oceanic crust is composed of lavas overlying a sheeted dyke complex these units are formed by dykes intruding into rocks overlying a magma chamber with lavas erupting at the ocean floor seismic reflection data acquired over young oceanic crust commonly image a reflector known as layer 2a which is typically interpreted as defining the geologic boundary between lavas and dykes1 2 3 an alternative hypothesis is that the reflector is associated with an alteration boundary within the lava unit4 5 6 many studies have used mapped variability in layer 2a thickness to make inferences regarding the geology of the oceanic crust including volcanic construction dyke intrusion and faulting7 8 9 10 however there has been no link between the geologic and seismological structure of oceanic crust except at a few deep drill holes here we show that although the layer 2a reflector is imaged near the top of the sheeted dyke complex at fast spreading crust located adjacent to the hess deep rift it is imaged significantly above the sheeted dykes section at intermediate spreading crust located near the blanco transform fault although the lavas and underlying transition zone thicknesses differ by about a factor of two the shallow seismic structure is remarkably similar at the two locations this implies that seismic layer 2a cannot be used reliably to map the boundary between lavas and dykes in young oceanic crust instead we argue that the seismic layer 2a reflector corresponds to an alteration boundary that can be located either within the lava section or near the top of the sheeted dyke complex of oceanic crust

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

the 1998 eruption of axial seamount new insights on submarine lava flow emplacement from high resolution mapping 1 axial seamount an active submarine volcano on the juan de fuca ridge at 46 n 130 w erupted in january 1998 along 11 km of its upper south rift zone we use ship based multibeam sonar high resolution 1 m bathymetry sidescan sonar imagery and submersible dive observations to map four separate 1998 lava flows that were fed from 11 eruptive fissures these new mapping results give an eruption volume of 31 106 m3 70 of which was in the northern most flow 23 in the southern most flow and 7 in two smaller flows in between we introduce the concept of map scale submarine lava flow morphology observed at a scale of hundreds of meters as revealed by the high resolution bathymetry and an interpretive model in which two map scale morphologies are produced by high effusion rate eruptions inflated lobate flows are formed near eruptive vents and where they drain downslope more than 0 5 1 0 km they transition to inflated pillow flows these two morphologies are observed on the 1998 lava flows at axial a third map scale flow morphology that was not produced during this eruption pillow mounds is formed by low effusion rate eruptions in which pillow lava piles up directly over the eruptive vents axial seamount erupted again in april 2011 and there are remarkable similarities between the 1998 and 2011 eruptions particularly the locations of eruptive vents and lava flow morphologies because the 2011 eruption reused most of the same eruptive fissures 58 of the area of the 1998 lava flows is now covered by 2011 lava

microearthquake patterns following the 1998 eruption of axial volcano juan de fuca ridge mechanical relaxation and thermal strain 1 ocean bottom seismic networks deployed following the 1998 eruption of axial seamount reveal an evolving pattern of microearthquake activity associated with subsurface magmatism and thermal strain seismicity rates decay steadily over 15 months of observation february 8 1998 to april 30 1999 consistent with a trend toward thermal and mechanical equilibrium in the shallow crust after the magmatic event immediately after the eruption seismicity rates were high for about 60 days in the southeast corner of the caldera where lava flows from the 1998 eruption were mapped a small burst of seismic activity was observed on the southeast shoulder of the volcano from 100 to 150 days after the eruption these events which are characterized by slip on nearly vertical faults in the shallow crust extend about 6 km from the southeast corner of the caldera and overlie a mid crustal low velocity zone after this episode seismicity rates remain low until the end of the observation period 455 days after the eruption shallow 0 7 km depth events consistent with thermal contraction and volume changes of 2 10 3 m3 in 5 m3 sources are observed in individual clusters beneath hydrothermal vents within the 1998 lava flow at the southeast edge of the caldera microearthquakes observed during the last 70 days of observation are distributed around the central caldera most likely representing small amounts of subsidence on caldera faults during the final stages of equilibration following melt withdrawal associated with the 1998 eruption

seismological evidence for magmatic and hydrothermal structure in long valley caldera from local earthquake attenuation and velocity tomography we compare the results of six different tomographic inversions for the three dimensional seismic attenuation and velocity structure in long valley caldera california in order to constrain interpretations on the location and character of magmatic and geothermal structure in the caldera crust p and s wave amplitudes and arrival times obtained in previous studies from seismograms of local earthquakes recorded on vertical and three component seismographis were inverted for q sv 1 q sh 1 q s 1 q p 1 and v p v s in the upper 9 km of teh caldera crust detailed comparisons of the results of the inversions suggest significant locations of high temperatures and perhaps magma and hydrothermal zones in the caldera a region of low q s with locally variable q p and v p v s is imaged in the 7 9 km depth range beneath the resurgent dome overlying and adjacent to this anomaly are significant low q p and low v p v s anomalies as well as smaller zones of low q s and high v p v s together these suggest zones of high temperature magma below and adjacent to zones of supercritical hydrothermal fluids at 4 5 km depth along the southern rim of the caldera just east of mammoth mountain a significant anomaly with relatively low q p and relatively low v p v s is imaged and is interpreted as a supercritical hydrothermal zone poorly resolved relatively low q p and relatively high v p v s are seen at 4 5 km depth beneath the inyo craters suggesting possible high temperatures and perhaps magma because of lack of data in the individual inversions the resultant images have spatially variable error and resolution however by comparing multiple images of different seismic parameters we have improved our ability to determine the significance of an constrain interpretation of seismic structure anomalies

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

magma storage beneath axial volcano on the juan de fuca mid ocean ridge axial volcano which is located near the intersection of the juan de fuca ridge and the cobb eickelberg seamount chain beneath the northeast pacific ocean is a locus of volcanic activity thought to be associated with the cobb hotspot1 the volcano rises 700 metres above the ridge has substantial rift zones extending about 50 kilometres to the north and south and has erupted as recently as 1998 ref 2 here we present seismological data that constrain the three dimensional velocity structure beneath the volcano we image a large low velocity zone in the crust consisting of a shallow magma chamber and a more diffuse reservoir in the lower crust and estimate the total magma volume in the system to be between 5 and 21 km3 this volume is two orders of magnitude larger than the amount of melt emplaced during the most recent eruption3 4 0 1 0 2 km3 we therefore infer that such volcanic events remove only a small portion of the reservoir that they tap which must accordingly be long lived compared to the eruption cycle on the basis of magma flux estimates we estimate the crustal residence time of melt in the volcanic system to be a few hundred to a few thousand years