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News: comments, Endeavour earthquake swarm [TCS]

March 2005

  • March 28, 2005: Bill Chadwick (28 KB pdf)
  • March 24, 2005: Bob Dziak (80 KB pdf)
  • March 24, 2005: Del Bohnenstiehl (40 KB pdf)
  • March 23, 2005: Ken Macdonald (45 KB pdf)
  • March 18, 2005: Del Bohnenstiehl (712 KB pdf)
  • March 17, 2005: Bob Dziak, Jim Cowen, Ed Baker, Bob Embley, Bill Chadwick, Joe Resing, John Lupton (see below)
  • March 11, 2005: Ken C Macdonald (see below)

Other news related to this Endeavour earthquake swarm:

Reply to Ken Macdonald’s Response, Bob Dziak et al.

Many in the R2K community are likely curious about the decision making process that lead to the recent Endeavour segment response effort. The fact that the earthquake swarm was located, for the most part, within an overlap zone was a cause of concern when evaluating whether or not this was a magmatic event. However, SeaMarcII sidescan data (Davis and Currie, 1993, Can. J. Earth Sci., v. 30, pp. 278–300), clearly shows evidence that the northern extension of the Endeavour segment within the overlap zone contains recent lava flows. The earthquake locations from this swarm are not precise enough to distinguish whether the events were located along the volcanic segment’s limbs or within the tectonic overlap zone. It appeared possible, therefore, that a dike could have been injected from the axial high at the Endeavour Vent field into the overlap zone, resulting in the earthquake swarm. Indeed, a dike injection/seafloor eruption event at the Cleft segment (southern JdFR) during the 1980s (see papers in JGR, v. 99, March 10, 1994) produced lava flows within the overlap zone between the Cleft and Vance segment to the north. This event demonstrates that a seafloor eruption and event plume can be produced in an overlap zone. In addition, as mentioned previously, the decay-rate modeling (as well as other factors) indicated the Endeavour swarm was not a typical mainshock-aftershock (tectonic) sequence and may have had a magmatic component. All this evidence, combined with the proximity of the seismic events to the Endeavour Integrated Studies Site, contributed to our decision to respond to this event.

We were also aware that the earthquake swarm might have produced changes at the known Endeavour vent fields, and several CTD profiles and water-column samples were collected over 4 of the 5 known vent fields to investigate this possibility. In 1999 a large earthquake swarm was centered west of the Endeavour vent fields and Axial Magma Chamber, yet still produced significant fluid-flux and temperature changes at the on-axis vent sites (Johnson et al 2000; Dziak and Johnson, 2002; Bohnenstiehl et al., 2002; Lilley et al., 2003). Thus it still may be possible to show that the Endeavour AMC was involved with, or affected by, this earthquake swarm, although shipboard analyses indicated no compositional changes to the Endeavour vent fluids.

We thank Ken Macdonald for his post-cruise comments on the recent earthquake swarm on the Endeavour segment. The earthquake analysis process and our findings will be discussed in more detail in an upcoming message. At this point, though, we’d briefly like to comment that care should be taken when comparing OBS recorded micro-earthquakes (< 2.5 M) with the small-to-moderate size events (2.5–5.1 M) recorded by SOSUS. The Galapagos-Rift OBS study lasted for only 21 days, had a low peak event count of 17 per day with no indication of swarm activity, and was therefore most likely a purely tectonic episode along a propagating rift.

Regards,
Bob Dziak, Jim Cowen, Ed Baker, Bob Embley, Bill Chadwick, Joe Resing, John Lupton

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Ken MacDonald, response to intense earthquake swarm

The fact that this swarm occurs mostly in the overlap region between the Endeavor segment and the next segment north means that it is almost certainly caused by lithospheric deformation between the two segment tips (probably bookshelf faulting) and not by magmatic or volcanic activity. The distribution of the events in time and in space is very similar to the microearthquake swarm measured at the Galapagos propagating rift near 95 W years ago (by Duenebeir’s OBS array), those too were entirely tectonic in nature. Visual effects (new lava flows) are most unlikely; local hydrothermal activity is not likely either. A hydrothermal response may occur due to distant shaking and opening of new fluid pathways at the main Endeavor site away from the earthquake swarm, along the lines argued by Johnson et al 2003 and Haymon et al 2005. No mention of this likely scenario was made in the blog you sent us, so I thought it might be helpful to point out this possibility.

Ken C Macdonald
Professor, Dept. Geological Sciences at the University of California, Santa Barbara