A seismic swarm and regional hydrothermal and hydrologic perturbations: The northern Endeavour segment, February 2005

Abstract

[1] The February 2005 swarm at the overlapping spreading center (OSC) on the northern end of the Endeavour segment is the first swarm on the Juan de Fuca Ridge recorded on a local seafloor seismic network. The swarm included several larger earthquakes and caused triggered seismicity and a hydrothermal response in the Endeavour vent fields as well as regional-scale hydrologic pressure perturbations. The spatial and temporal pattern of over 6000 earthquakes recorded during this seismic sequence is complex. Small-magnitude events dominate, and seismicity rates wax and wane, indicating a magmatic process. The main swarm initiates at the northern end of the Endeavour ridge. However, most of the moment release, including six strike-slip events, occurs in the southwest Endeavour Valley, where the swarm epicenters generally migrate south. The main swarm is contemporaneous with a hydrologic pressure response at four sealed seafloor boreholes, ∼25-105 km away. We infer that the seismic sequence is driven by a largely aseismic magma intrusion at the northern Endeavour axis. Resulting stress changes trigger slip on tectonic faults and possibly dike propagation at the opposing limb of the Endeavour OSC in the southwest Endeavour Valley, consistent with the eventual decapitation of the Endeavour by the West Valley segment. Furthermore, 2.5 days after the start of the main swarm, seismicity is triggered beneath the Endeavour vent fields, and temperature increases at a diffuse vent in the Mothra field. We infer that this delayed response is due to a hydrologic pressure pulse that diffuses away from the main magma intrusion.

Endeavour Segment of the Juan de Fuca Ridge: One of the Most Remarkable Places on Earth

“…This correlation suggests that the earthquakes are intimately linked to the processes of hydrothermal heat extraction, similar to findings at the EPR (Fornari et al, 2012, in (McClain et al, 1993) or that the axial earthquakes were a manifestation of thermal stresses induced by hydrothermal cooling (Wilcock et al, 2002). (Hooft et al, 2010). Interestingly, the migration of earthquakes suggests that the dike propagated southward toward the center of the segment from a magma source to the north, which is possibly associated with Endeavour Seamount at the east end of the Heck seamount chain (Weekly et al, 2008).…”

“…The 20-year SOSUS earthquake catalog (Dziak et al, 2011) It was the first event to be recorded by in situ short-period seismometers deployed in small boreholes and "seismonuments" (i.e., rectangular cement casings with holes in which short-period seismometers are placed to better couple the sensors to the seafloor) at Endeavour as part of the Keck observatory effort (Wilcock et al, 2009;Hooft et al, 2010).…”

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

“…Hooft et al . [] interpret this delay between swarm onset and observed thermal anomalies as the time required for a hydrologic pressure perturbation to diffuse along the ridge.…”

“…Two periods of particular interest are swarm sequences recorded between June 1999 and January 2000 along the central and southern portions of the Endeavour segment [ Bohnenstiehl et al ., ; Johnson et al ., ], and between January and March 2005 along the northern Endeavour and adjacent southern West Valley Segments [ Hooft et al ., ]. Although Johnson et al .…”

“…The majority of moment release, including the six largest earthquakes that have strike‐slip mechanisms, occurred at the southwest end of the Endeavour Valley near the southern extension of the West Valley Segment. However, the magnitude and polarity of borehole pressure anomalies and their timing relative to the onset of seismicity indicated a dike intrusion on the northern portion of the Endeavour Segment [ Hooft et al ., ]. The February 2005 swarm triggered earthquakes and a hydrologic response within the vent fields.…”