Microearthquakes on the Endeavour segment of the Juan de Fuca Ridge

Abstract

[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.

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

“…Kellogg, 2011). 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).…”

“…This fault is also the western terminus of the current axial magma chamber and is the main structural control for fluid flow that feeds the southern hydrothermal fields (Delaney et al, 1992;Kelley et al, 2001b;Glickson et al, 2007). It has also been the locus of past seismic swarms (Wilcock et al, 2002). Surprisingly, both magmas and fluids appear to rise along fractures within this fault system (Gill and Michael, 2008).…”

Local Earthquake Magnitude Scale andb‐Value for the Danakil Region of Northern Afar

“…At the Mid-Atlantic ridge, b-values have been observed to vary, but they are consistently above 1 and can reach values of up to 1.5 (Kong et al, 1992;Barclay et al, 2001;Tilmann et al, 2004). Wilcock et al (2002) performed a microearthquake study on the Endeavour segment of the Juan de Fuca ridge and recorded a b-value of 1.06. These b-values suggest that seismic energy at ocean ridges is primarily released through swarms of relatively low-magnitude earthquakes.…”

Geochemistry of lavas from the 2005–2006 eruption at the East Pacific Rise, 9°46′N–9°56′N: Implications for ridge crest plumbing and decadal changes in magma chamber compositions

“…The local tensional stresses applied to the ridge generate sporadic pulses of extension focused along or near the ridge axis [e.g., Buck et al, 2005]. Also, extension and normal faulting may have been generated locally by thermal stresses from hydrothermal cooling [Sohn et al, 1999;Wilcock et al, 2002;Sohn et al, 2004] or by magma intrusion [e.g., Rubin, 1992]. The latter mechanism has been invoked by Wilcock et al [2009] to explain the observed microseismicity patterns and focal mechanism distribution along the Endeavor segment of the Juan de Fuca Ridge.…”