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Key Points:  AUV mapping along the north rift identifies hummocky mounds with pillows, channelized, and inflated flows formed during the 2015 eruption.  Impulsive sounds formed by lava/seawater interaction track mound growth over a 28 day period with an average extrusion rate of 22-45 m3s-1.  The sounds record the history of flow advancement and inflation from multiple eruptive centers, and is used to infer volcanic activity style. Quantifying eruption dynamics in submarine environments is challenging. During the 2015 eruption of Axial Seamount, the formation of hummocky mounds along the north rift was accompanied by tens‐of‐thousands of impulsive acoustic signals generated by the interaction of lava and seawater. A catalog of these sounds was integrated with detailed seafloor mapping to better understand eruptive processes in time and space. Mounds grew over a period of 28 days with average extrusion rates of 22 to 45 m3s‐1. The most distant mounds, ~ 9.5 to 15.5 km down rift from the caldera, grew primarily over the first few days of the eruption. The focus of eruptive activity then retreated ~5 km toward the caldera where it was sustained. Mounds are constructed as a series of superimposed lobes formed through alternating periods of flow inflation, generating up to 30‐m‐thick hummocks, and periods of flow advancement, with

In late 2007, two massive icebergs, C19a and B15a, drifted into open water and slowly disintegrated in the southernmost Pacific Ocean. Archived acoustic records show that the high-intensity underwater sounds accompanying this breakup increased ocean noise levels at mid-to-equatorial latitudes over a period of ∼1.5 years. More typically, seasonal variations in ocean noise, which are characterized by austral summer-highs and winter-lows, appear to be modulated by the annual cycle of Antarctic iceberg drift and subsequent disintegration. This seasonal pattern is observed in all three Oceans of the Southern Hemisphere. The life cycle of Antarctic icebergs affects not only marine ecosystem but also the sound environment in far-reaching areas and must be accounted for in any effort to isolate anthropogenic or climate-induced noise contributions to the ocean soundscape.

at NW Rota-1 are primarily H2O, SO2, and CO2. Instantaneous fluxes varied by a factor of � 100 over the deployment. Using melt inclusion information to estimate the concentration of CO2 in the explosive gases as 6.9 � 0.7 wt %, we calculate an annual CO2 eruption flux of 0.4 � 0.1 Tg a � 1 . This result is within the range of measured CO2 fluxes at continuously erupting subaerial volcanoes, and represents � 0.2–0.6% of the annual estimated output of CO2 from all subaerial arc volcanoes, and � 0.4–0.6% of the mid-ocean ridge flux. The multiyear eruptive history of NW Rota-1 demonstrates that submarine volcanoes can be significant and sustained sources of CO2 to the shallow ocean.

[1] During January–February 2009, an active-source seismic survey was performed over the Eastern Lau Spreading Center in the Lau Back-Arc Basin (21°S, 176°S). Acoustic signals generated by theR/V Langseth's 36-gun pneumatic source array were recorded within the deep sound channel at offsets of 29–416 km. The local ocean acoustic environment is everywhere bottom limited, with seafloor depths within the study domain ranging from ∼1700–2800 m. Low-frequency (4–125 Hz) sound levels are monitored using root-mean-square, energy-flux-density and zero-to-peak measurement techniques. From these field data, transmission loss is found to exceed the predictions of a geometric spherical spreading model. At similar ranges, arrival amplitudes vary by up to 20 dB and durations vary by a factor of three to six. The depth of the seafloor beneath the air gun source exhibits a positive correlation with arrival duration and a negative correlation with range-corrected amplitude, explaining up to 30% of the observed variation in both parameters. The strength of this correlation, however, varies for stations lying at different azimuths, highlighting the importance of seafloor aspect and slope in the coupling of bottom-interacting acoustic energy into the sound channel. Range-dependent ray tracing shows that shots deployed over shallower seafloor are more likely to produce sound channel trapped signals that propagate with limited bottom interaction. This results in arrivals that are more impulsive, with shorter durations and higher amplitudes. Shots deployed in deeper water typically undergo a larger number of bounces and are characterized by more emergent, longer duration and smaller amplitude arrivals.

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] The R/V Marcus G. Langseth is the first 3-D seismic vessel operated by the U.S. academic community. With up to a four-string, 36-element source and four 6-km-long solid state hydrophone arrays, this vessel promises significant new insights into Earth science processes. The potential impact of anthropogenic sound sources on marine life is an important topic to the marine seismic community. To ensure that operations fully comply with existing and future marine mammal permitting requirements, a calibration experiment was conducted in the Gulf of Mexico in 2007–2008. Results are presented from deep (∼1.6 km) and shallow (∼50 m) water sites, obtained using the full 36-element (6600 cubic inches) seismic source. This array configuration will require the largest safety radii, and the deep and shallow sites provide two contrasting operational environments. Results show that safety radii and the offset between root-mean-square and sound exposure level measurements were highly dependent on water depth.

[1] An array of autonomous underwater hydrophones is used to investigate regional seismicity associated with the 22 January 2006 seafloor-spreading event on the northern East Pacific Rise near 9°50′N. Significant earthquake activity was observed beginning 3 weeks prior to the eruption, where a total of 255 earthquakes were detected within the vicinity of the 9°50′N area. This was followed by a series of 252 events on 22 January and a rapid decline to background seismicity levels during the subsequent 3 days. Because of their small magnitudes, accurate locations could be derived for only 20 of these events, 18 of which occurred during a 1-h period on 22 January. These earthquakes cluster near 9°45′N and 9°55′N, at the distal ends of the young lava flows identified posteruption, where the activity displays a distinct spatial-temporal pattern, alternating from the north to the south and then back to the north. This implies either rapid bilateral propagation along the rift or the near-simultaneous injection of melt vertically from the axial magma lens. Short-duration T wave risetimes are consistent with the eruption of lavas in the vicinity of 9°50′N on 22 January 2006. Eruptions on 12 and 15–16 January also may be inferred from the risetime data; however, the locations of these smaller-magnitude events cannot be determined accurately. Roughly 15 h after the last earthquakes were located adjacent to the eruption site, a sequence of 16 earthquakes began to the north-northeast at a distance of 25–40 km from the 9°50′N site. These events are located in vicinity of the Clipperton Transform and its western inside corner, an area from which the regional hydrophone network routinely detects seismicity. Coulomb stress modeling indicates that a dike intrusion spanning the known eruptive zone to the south (9°46′–9°56′N) would act to promote normal faulting or a combination of normal faulting and transform slip within this region, with stress changes on the order of 1–10 kPa.

[1] Deep-towed DSL-120 bathymetric data are used to investigate the pattern of lava dome formation along a superfast spreading portion of the southern East Pacific Rise (EPR), including the overlapping limbs of a giant (120 × 120 km) propagator near 29°S. Along the 670 km of the axis surveyed, 1172 small domes were identified using a closed contour algorithm. Their abundance, defined by spatial density, is well correlated with the along-axis relief of the ridge crest. Where the western and eastern limbs plunge toward the overlap zone, densities are high (3–6 km−2); however, where the axial depth profile is shallow and flat, densities are comparably low (0.4 km−2). Volcanic domes within the low abundance areas are characterized by lower ratios of height to basal radius (0.15 versus 0.22), smaller maximum heights (18 versus 40 m), and a larger relative percentage of small versus large mounds. The zone of high dome abundance encompasses the overlapping limbs of the rift and extends more than 100 km to the north and south beyond the overlap zone. Domes form dominantly during low effusion rate, point-source eruptions, which suggests that discontinuous melt lenses underlie the ridge axis proximal to the overlapper. Conversely, fissure-fed sheet flows dominate along the more distal segments, implying the presence of a more continuous axial magma lens. Throughout the survey area, dome abundance increases systematically near second-order segment boundaries. Within the high abundance zone, some third-order offsets also correlate with increased dome production, but local peaks in abundance are not tied exclusively to higher-order ridge offsets. Where dome abundance is low, domes are clustered tightly near second-order offsets and there is no increase in dome abundance near third-order segment boundaries.

[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.

[1] Mid to low southern latitude hydrophone stations within the Indian Ocean have recorded two distinct types of low-frequency (

[1] We have collected 12 kHz SeaBeam bathymetry and 120 kHz DSL-120 side-scan sonar and bathymetry data to determine the tectonic and volcanic segmentation along the fastest spreading (∼150 km/Myr) part of the global mid-ocean ridge system, the southern East Pacific Rise between the Easter and Juan Fernandez microplates. This area is presently reorganizing by large-scale dueling rift propagation and possible protomicroplate tectonics. Fracture patterns observed in the side-scan data define structural segmentation scales along these ridge segments. These sometimes, but not always, correlate with linear volcanic systems defining segmentation in the SeaBeam data. Some of the subsegments behave cohesively, with in-phase tectonic activity, while fundamental discontinuities occur between other subsegments. We also collected hydrothermal plume data using sensors mounted on the DSL-120 instrument package, as well as CTDO tow-yos, to determine detailed structural and volcanic controls on the hydrothermal vent pattern observed along 600 km of the Pacific-Nazca axis. Here we report the first rigorous correlation between coregistered hydrothermal plume and high-resolution marine geophysical data on similar scales and over multisegment distances. Major plume concentrations were usually found where axial inflation was relatively high and fracture density was relatively low. These correlations suggest that hydrothermal venting is most active where the apparent magmatic budget is greatest, resulting in recent eruptions that have paved over the neovolcanic zone. Areas of voluminous acoustically dark young lava flows produced from recent fissure eruptions correlate with many of the major hydrothermal vent areas. Increased crustal permeability, as gauged by increased fracture density, does not enhance hydrothermal venting in this area. Axial summit troughs and graben are rare, probably because of frequent volcanic resurfacing in this superfast spreading environment, and are not good predictors of hydrothermal activity here. Many of the hydrothermal areas are found in inflated areas near the ends of segments, suggesting that abundant magma is being supplied to these areas.

[1] Two large clusters of earthquake activity in June of 1999 and January of 2000 have dominated recent seismicity along the Endeavour Segment of the Juan de Fuca Ridge. The impacts of the June 1999 sequence on the hydrologic system, which include changes in vent temperature and chemistry within the Main Endeavour Vent Field, have been well documented previously. Analysis of seismic and hydroacoustic data indicates that both sequences exhibit a swarm-like behavior, characterized by the absence of a dominant main shock event. The epicentral locations of events within the two swarms overlap spatially, with centroid positions near 47°49′ and 47°46′N latitude. During the June 1999 swarm, the initial activity spans the along-axis region where a shallow axial magma chamber reflector was later imaged. The epicenters then migrate ∼12 km to the south at a rate of 0.3 m/s, consistent with lateral dike propagation. A distinct subcluster of events also occurred in the vicinity of Surveyor Volcano on the overlapping portion of the Cobb Segment. Given its distance from the main swarm, this activity may represent a triggered response to dynamic shaking. The January 2000 swarm has a more limited along-axis extent, relative to the June 1999 swarm, with no indication of lateral migration. Much of this activity is concentrated in a region predicted to have undergone extension due to dike propagation in 1999. Although it contains fewer total events and is of shorter duration, relative to the June 1999 swarm, the January 2000 activity exhibits a higher peak rate of seismicity and greater mean event magnitude. As in situ temperature monitoring was not in place during January 2000 and vent fluids were not sampled until June 2000, the impacts of this swarm on the hydrothermal system are unknown. The southernmost tip of the Endeavour Segment also is found to be a region of repeating swarm activity. Although morphologic evidence indicates the Cobb Segment has been propagating northward recently, this seismic activity suggests that the western limb of the Endeavour-Cobb overlap zone remains active.

[1] Structural mapping from high-resolution sonar and bathymetric data collected near 19°30′S on the southern East Pacific Rise (EPR) reveals multiple packets of small, closely spaced outward-dipping faults that form antithetic to larger inward-dipping faults. The mean spacing and height of inward-dipping scarps are ∼1.6 and ∼1.9 times greater than those of outward-dipping faults, and the median spacing and height of inward-dipping scarps are ∼2.0 and ∼1.7 times greater, respectively. Accordingly, inward-dipping faults are less abundant (35–45%) but accommodate more (50–55%) of the roughly ∼4.0% strain. Although previous studies have shown that EPR fault populations exhibit exponential size-frequency distributions, one-dimensional throw and spacing data from the 19°30′S region are not described well by this model. The first major (>40 m scarp height) fault is observed within 1–4 km of the ridge crest and invariably dips inward. Further off-axis, a transition from greater inward-dipping to greater outward-dipping fault abundance constrains the minimum half width of the active plate boundary zone to be ∼5–15 km. Inspection of additional data sets from 16° and 18°30′S indicates a pattern of large inward-dipping faults with associated hanging wall deformation similar to that observed in the 19°30′S region. This pattern is distinct from the horst and graben morphology observed along the fast spreading northern EPR. The pervasive hanging wall deformation observed on the flanks of the southern EPR suggests master fault geometries that are more sharply curved or kinked at depth, relative to their abyssal hill equivalents on the northern EPR. Boundary element modeling indicates the pattern and geometry of faulting observed along the southern EPR is consistent with fault nucleation above a shallow, sill-like magma chamber with internal overpressure. Since the influence of the sill on fault growth is strongly dependent on the depth of the sill, the different patterns of faulting observed along the northern and southern EPR may reflect the small difference in axial magma chamber depth between these two regions, with a slightly shallower chamber beneath the southern EPR.