Termination of a 6 year ridge‐spreading event observed using a seafloor seismic network on the Endeavour Segment, Juan de Fuca Ridge

Abstract

We present automatically determined epicenters and magnitudes for 36,523 earthquakes recorded along the Endeavour segment between August 2003 and October 2006 using a local ocean‐bottom seismometer (OBS) network. The catalog is dominated by two swarm sequences in January and February 2005 in the vicinity of the Endeavour overlapping spreading center, which included earthquakes in West Valley, the northern portion of the Endeavour segment, southwest Endeavour Valley, and the Endeavour vent fields. These swarms are attributed to volcanism including a dike intrusion on the northern Endeavour in February 2005 and smaller diking events on the propagating tip of the West Valley segment in both swarms. The dike on the northern Endeavour propagated to the south, which is inconsistent with magma sourced from the axial magma chamber beneath the elevated central portion of the segment. Following the swarms, seismic activity on the Endeavour segment decreased on average to ~15% of pre‐swarm values and almost ceased at the segment ends. We infer that a 6 year non‐eruptive event that started with a swarm in 1999 and finished with the 2005 swarms ruptured the entire segment and relieved plate‐spreading stresses. The inferred coupling between the 1999 and 2005 events, the observation of extensive precursory activity prior to the 2005 swarms, and the interaction between seismically active regions during the swarms is consistent with static triggering with delays influenced by viscoelastic relaxation, hydraulic diffusion, and magma withdrawal and replenishment.

Upper crustal seismic structure of the Endeavour segment, Juan de Fuca Ridge from traveltime tomography: Implications for oceanic crustal accretion

“…There are significant along‐axis variations in the magnitude of anisotropy on the spreading axis (Figures a–c). The high values near Y = 15 km (Figures a and b) and Y = 5 km (Figure c) lie at the southern end of the E‐WV OSC and the inferred southward propagating extension of the West Valley segment [ Weekly et al ., ] and are consistent with high levels of ongoing deformation within the OSC. Within the vent fields the strongest shallow anisotropy is observed between the Main Endeavour and High‐Rise fields (Figure a).…”

“…Studies of microearthquakes from the central axial region show a concentration of seismicity at ∼2 km depth just above the AMC [ Wilcock et al ., ; Weekly et al ., ], with the most intense seismicity occurring beneath the High Rise and Main Endeavour fields [ Wilcock et al ., ] which also have the largest area and highest heat flux [ Kellogg , ]. McClain et al .…”

Seismic structure and magmatic construction of crust at the ultraslow‐spreading Southwest Indian Ridge at 50°28'E

“…This layer is named layer 2 and composed of extrusive basalts and dykes [ Detrick et al , ]. It is cool and brittle as indicated by the prevalence of microearthquakes [ Barclay et al , ; Crawford et al , ; Weekly et al , ] and supported by the thermodynamic modeling [ Morgan and Chen , ]. The seismic velocity variation in this layer is closely related to porosity [ Berryman et al , ].…”

Near‐axis crustal structure and thickness of the Endeavour Segment, Juan de Fuca Ridge

“…The second smaller area of thicker crust lies within the overlap basin of the West‐Valley Endeavour OSC and is immediately north of a region of thin crust near the spreading axis that cuts into the plateau. The thin crust coincides with a region of intense ongoing seismicity that is interpreted as a region of extension at the southernmost limit of the West Valley propagator [ Weekly et al , ]. The thicker crust to the north is harder to explain—it is beneath the advancing part of the overlap basin rather than behind it as the OSC models predict [ Marjanović et al , ; VanderBeek et al , ] and is south of the region of enhanced mantle melt concentrations imaged beneath the center of the OSC [ VanderBeek et al , ].…”