Seismic observations in volcanically active calderas are challenging. A new cabled observatory atop Axial Seamount on the Juan de Fuca ridge allows unprecedented real-time monitoring of a submarine caldera. Beginning on 24 April 2015, the seismic network captured an eruption that culminated in explosive acoustic signals where lava erupted on the seafloor. Extensive seismic activity preceding the eruption shows that inflation is accommodated by the reactivation of an outward-dipping caldera ring fault, with strong tidal triggering indicating a critically stressed system. The ring fault accommodated deflation during the eruption and provided a pathway for a dike that propagated south and north beneath the caldera's east wall. Once north of the caldera, the eruption stepped westward, and a dike propagated along the extensional north rift.

[1] Multichannel seismic reflection data collected in July 2002 at the Endeavour Segment, Juan de Fuca Ridge, show a midcrustal reflector underlying all of the known high-temperature hydrothermal vent fields in this area. On the basis of the character and geometry of this reflection, its similarity to events at other spreading centers, and its polarity, we identify this as a reflection from one or more crustal magma bodies rather than from a hydrothermal cracking front interface. The Endeavour magma chamber reflector is found under the central, topographically shallow section of the segment at two-way traveltime (TWTT) values of 0.9-1.4 s ($2.1-3.3 km) below the seafloor. It extends approximately 24 km along axis and is shallowest beneath the center of the segment and deepens toward the segment ends. On cross-axis lines the axial magma chamber (AMC) reflector is only 0.4-1.2 km wide and appears to dip 8-36°to the east. While a magma chamber underlies all known Endeavour high-temperature hydrothermal vent fields, AMC depth is not a dominant factor in determining vent fluid properties. The stacked and migrated seismic lines also show a strong layer 2a event at TWTT values of 0.30 ± 0.09 s (380 ± 120 m) below the seafloor on the along-axis line and 0.38 ± 0.09 s (500 ± 110 m) on the cross-axis lines. A weak Moho reflection is observed in a few locations at TWTT values of 1.9-2.4 s below the seafloor. By projecting hypocenters of well-located microseismicity in this region onto the seismic sections, we find that most axial earthquakes are concentrated just above the magma chamber and distributed diffusely within this zone, indicating thermal-related cracking. The presence of a partially molten crustal magma chamber argues against prior hypotheses that hydrothermal heat extraction at this intermediate spreading ridge is primarily driven by propagation of a cracking front down into a frozen magma chamber and indicates that magmatic heat plays a significant role in the hydrothermal system. Morphological and hydrothermal differences between the intermediate spreading Endeavour and fast spreading ridges are attributable to the greater depth of the Endeavour AMC and the corresponding possibility of axial faulting.Citation: Van Ark, E. M., R.

Mantle upwelling is essential to the generation of new oceanic crust at mid-ocean ridges, and it is generally assumed that such upwelling is symmetric beneath active ridges. Here, however, we use seismic imaging to show that the isotropic and anisotropic structure of the mantle is rotated beneath the East Pacific Rise. The isotropic structure defines the pattern of magma delivery from the mantle to the crust. We find that the segmentation of the rise crest between transform faults correlates well with the distribution of mantle melt. The azimuth of seismic anisotropy constrains the direction of mantle flow, which is rotated nearly 10 degrees anticlockwise from the plate-spreading direction. The mismatch between the locus of mantle melt delivery and the morphologic ridge axis results in systematic differences between areas of on-axis and off-axis melt supply. We conclude that the skew of asthenospheric upwelling and transport governs segmentation of the East Pacific Rise and variations in the intensity of ridge crest processes.

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Summary We have simultaneously inverted seismic refraction and wide‐angle Moho reflection traveltimes for the 2‐D crustal thickness and velocity structure of 150–300 kyr old crust along the East Pacific Rise (EPR) between the Siqueiros and Clipperton fracture zones (FZs). Our results show a strong correlation between ridge segmentation and upper‐ and mid‐crustal seismic velocities, with higher velocities near segment centres and lower velocities near segment ends. Low crustal velocities at the Clipperton and Siqueiros FZs are interpreted as fracturing resulting from brittle deformation of the crust in the transform domain. A relict overlap basin left on the Pacific Plate by the 9°03′N overlapping spreading centre (OSC) as it propagated southward is associated with a large (∼1 km s−1), negative upper‐ and mid‐crustal velocity anomaly. This anomaly is consistent with the presence of an unusually thick extrusive section within the basin and with tectonic alteration, fracturing and shearing arising from rotation of the basin as it was formed. The discordant zone left by this OSC on the Cocos Plate is characterized by moderately low crustal velocities, probably because of crustal fracturing as the OSC propagated into older crust. Higher crustal velocities near segment centres may reflect a higher ratio of dikes to extrusives in the upper crust, and lower‐intensity tectonic alteration of the crust, than near segment ends. The mean crustal thickness along the EPR between the Siqueiros and Clipperton FZs is 6.7–6.8 km. The thickest crust is found beneath the Lamont seamounts (∼9 km), and in a southward‐pointing, V‐shaped band located just north of the off‐axis trace of the 9°03′N OSC (7.3–7.8 km). The thinnest crust (<6 km) is found proximal to the Clipperton and Siqueiros FZs. The crust associated with the off‐axis trace of the 9°03′N OSC is not anomalously thin, suggesting that magma supply beneath the OSC is similar to that of the northern and southern segments. We see a similar pattern of crustal thickness variation to that determined using multichannel reflection data, including a gradual thickening of the crust from north to south along the northern ridge segment, and the location of the thickest crust just north of the 9°03′N OSC. However, the magnitude of the along‐axis crustal thickness variation we observe along the northern ridge segment between 9°50′N and 9°15′N(∼1.3–1.8 km, excluding the Lamont seamounts) is significantly less than the 2.3 km of variation previously reported, weakening the case for the existence of a low‐density mantle diapir at 9°50′N inferred from gravity data. The band of thick crust located just north of the off‐axis trace of the 9°03′N OSC suggests a close genetic link between this feature and the OSC. Thus we attribute the pattern of crustal thickness variations along the northern segment to the kinematics of the southward‐propagating 9°03′N OSC over the past 0.5 Myr, and not to along‐axis melt migration away from a mantle diapir as previously proposed.