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Axial Seamount is the most volcanically active site of the northeast Pacific, and it has been monitored with a growing set of observations and sensors during the last two decades. Accurate imaging of the internal structure of volcanic systems is critical to better understand magma storage processes and to quantify mass and energy transport mechanisms in the crust. To improve the three‐dimensional velocity structure of Axial Seamount, we combined 469,891 new traveltime arrivals, from 12 downward extrapolated seismic profiles, with 3,962 existing ocean‐bottom‐seismometers traveltime arrivals, into a joint tomographic inversion. Our approach reveals two elongated magma reservoirs, with melt fraction up to 65%, representing an unusually large volume of melt (26–60 km3), which is likely the result of enhanced magma supply from the juxtaposition of the Cobb hot spot plume (0.26–0.53 m3/s) and the Axial spreading segment (0.79–1.06 m3/s). The tomographic model also resolves a subsided caldera floor that provides an effective trap for ponding lava flows, via a “trapdoor” mechanism. Our model also shows that Axial's extrusive section is thinnest beneath the elevated volcano, where anomalously thick (11 km) oceanic crust is present. We therefore suggest that focused and enhanced melt supply predominantly thickens the crust beneath Axial Seamount through diking accretion and gabbro crystallization. Lastly, we demonstrate that our three‐dimensional velocity model provides a more realistic starting point for relocating the local seismicity, better resolving a network of conjugate outward and inward dipping faults beneath the caldera walls.©2018. The Authors.

[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.

In order to study the long-term stability of fin whale (Balaenoptera physalus) singing behavior, the frequency and inter-pulse interval of fin whale 20 Hz vocalizations were observed over 10 years from 2003–2013 from bottom mounted hydrophones and seismometers in the northeast Pacific Ocean. The instrument locations extended from 40°N to 48°N and 130°W to 125°W with water depths ranging from 1500–4000 m. The inter-pulse interval (IPI) of fin whale song sequences was observed to increase at a rate of 0.54 seconds/year over the decade of observation. During the same time period, peak frequency decreased at a rate of 0.17 Hz/year. Two primary call patterns were observed. During the earlier years, the more commonly observed pattern had a single frequency and single IPI. In later years, a doublet pattern emerged, with two dominant frequencies and IPIs. Many call sequences in the intervening years appeared to represent a transitional state between the two patterns. The overall trend was consistent across the entire geographical span, although some regional differences exist. Understanding changes in acoustic behavior over long time periods is needed to help establish whether acoustic characteristics can be used to help determine population identity in a widely distributed, difficult to study species such as the fin whale.