Temporal and spatial history of the 1999–2000 Endeavour Segment seismic series, Juan de Fuca Ridge

Abstract

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.

Fluid Phase Separation Processes in Submarine Hydrothermal Systems

“…Although this compositional variability is best indicated by the wide range of dissolved chloride concentrations of vent fl uids that greatly deviate from seawater values (Von Damm et al 1997;Von Damm 2000Seyfried et al 2003), confi rming evidence of phase separation processes can also be traced to the distribution and abundance of dissolved volatiles and other aqueous species known to fractionate between vapor and liquid when seawater phase separation and segregation occurs (Oosting and Von Damm 1996;Berndt and Seyfried 1997;Fornari et al 1998;Butterfi eld et al 1999;Lilley et al 2003;Von Damm et al 2003). Given the nature of magmatic and tectonic processes associated with the formation of the ocean crust at mid-ocean ridges, which can result in high temperatures at low to moderate pressure conditions (Wilcock and McNabb 1996;Bohnenstiehl et al 2004;Van Ark et al 2007), phase separation processes are not surprising, although the rate of change from vapor to liquid dominated systems and the compositional diversity expressed by ridge vent fl uids, remain enigmatic (Sohn et al 1999;Lilley et al 2003;Von Damm et al 2003;Larson et al 2007). …”

Oceanic lithosphere magnetization : marine magnetic investigations of crustal accretion and tectonic processes in mid-ocean ridge environments

“…Long term monitoring of seismicity in remote areas of the oceans using hydrophone arrays has proven to be very successful over the last decade. This is especially true at mid-ocean ridges where T-phase data are providing information on the overall seismicity of the ridge axis and are helping to constrain models of mid-ocean ridge crustal structure [e.g., Dziak andFox, 1999a, 1999b;Dziak et al, 1995Dziak et al, , 2004Schreiner et al, 1995;Fox et al, 1994Fox et al, , 1995Fox et al, , 2001Bohnenstiehl et al, 2002Bohnenstiehl et al, , 2004Smith et al, , 2003]. T-phase data are also an important source of information for studying hydroacoustic wave propagation in the oceans [Johnson et al, 1968;Pulli et al, 1999;Pulli and Upton, 2002;Harben et al, 2003].…”

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

“…the eNdeaVOur iNtegr ated StudieS Site In addition, the 1999-2000 seismic events and associated injection of melt beneath the Endeavour Segment (Davis et al, 2001;Lilley et al, 2003;Bohnenstiehl et al, 2004, Wilcock et al, 2009 (Clague et al, 2008). Indeed, one sulfide-hosted microbe recovered from the Mothra Hydrothermal Field grows at 121°C-only one organism has been cultured at higher temperatures (122°C) (Kashefi and Lovely, 2003;Takai et al, 2008).…”