Linking Stress Changes And Hydrothermal Activity During A Non-Eruptive Spreading Event

E. Hooft¹*, W. Wilcock², R. Weekly², D. Toomey¹, P. McGill³

Corresponding author: emilie@uoregon.edu
¹Department of Geological Sciences, University of Oregon, Eugene, OR 97403
²School of Oceanography, University of Washington, Seattle, WA
³Monterey Bay Aquarium Research Institute, Monterey, CA

Abstract:
A recent development at the Endeavour Integrated Studies Site (ISS) is the first observation of a multi-year non-eruptive spreading event. This consisted of a 6-year sequence of seismic swarms that was bookended by the two largest swarms in 1999 and 2005 and was followed by a pronounced drop in background seismicity. A unique seismic network that we deployed around the vent fields from 2003-6 recorded the latter part of this spreading event. For the first year of the deployment, we interpret the characteristics of seismicity beneath the vent fields in terms of steady inflation of the axial magma chamber [Wilcock et al., 2009]. In late January and February 2005, two large swarms concentrated to the north of the network have complex seismic signatures, triggered small micro-earthquakes beneath the vent fields and perturbed hydrothermal flow [Hooft et al., 2009]. Following the swarms, both regional and local rates of seismicity declined to <20% of pre-swarm values (see abstract by R. Weekly). Although none of the swarms were accompanied by a seafloor eruption, they clearly have a magmatic component and we interpret this as a 6-year non-eruptive spreading event that cumulatively ruptured most of the segment and relieved extensional stresses. We have just been funded to: (1) Use rate-state modeling and studies of b-values to infer spatial and temporal patterns of stress changes associated with swarms in late January and February 2005; (2) Understand the impact of this regional spreading event on seismicity beneath the vent fields and how the changes are linked to the evolution of the hydrothermal systems and magmatic segmentation; and (3) Determine how the stress changes due to this regional spreading event affect upper crustal seismic anisotropy and tidal triggering.

Keywords:
Endeavour ISS, seismicity, stress, responses, spreading event

Contributions to Integration and Synthesis:
We encourage other researchers to use our seismic analyses; the seismic catalogue for the three-year period will be submitted to the R2K database by the end of 2009. We plan to integrate our work with (1) time series records of borehole pressure data from the east flank and Middle Valley which constrain the strain of extensional events thus complementing our stress analysis; (2) the SOSUS catalog which provides a longer but less detailed record of seismicity that captures the whole 1999-2005 spreading event; (3) time series of hydrothermal temperature and chemical perturbations that were recorded in 2005-6 and (4) studies to reconcile spatial variations in rock petrology, bathymetry and seismicity in terms of the differing characteristics of the hydrothermal field and the segmentation of the magma chamber apparent in the multi-channel seismic data. Other data that will contribute to the integration and synthesis phase of the Ridge2000 program at the Endeavour include: an upcoming tomography experiment to image the uppermost mantle, crustal magmatic structure and hydrothermal circulation zone; hydrothermal temperature and resistivity time series; repeat hydrothermal fluid chemistry and microbial sampling; macrobiological studies; bottom imagery; extensive basalt and sulfide petrology samples; precise heat flux measurements and repeat CTDs, detailed seafloor mapping. In addition it is now possible to start expanding our understanding of magmatic and tectonic processes on the Endeavour beyond the 10-km scale of the vent fields to the segment scale. Finally, by collaborating with colleagues working on the East Pacific Rise (EPR) ISS our goal is to compare the seismic characteristics of two contrasting spreading events, a non-eruptive spreading event on the Endeavour and the 2005-6 sequence of eruptions at the EPR.

Figures:
Figure 1. Along-axis vertical cross section showing ~2800 Hypoinverse hypocenters (red dots) determined using Hypoinverse, the position of the AMC reflector (green line) with vent fields (yellow stars) labeled with the heat fluxes measured in summer 2004 [Thompson et al., 2005]. Hooft_fig1.jpg
Figure 2. Heat flow measured in 2004 [Thompson et al., 2005] plotted against the moment sum of micro-earthquakes beneath each vent field in 2003-2004. Hooft_fig2.jpg
Figure 3. (a) Vertical cross-section aross the ridge axis showing hypocenters and focal mechanisms within 0.3 km of the profile. Also shown axial magma chamber and the predicted stress perturbations above a pressurized crack. (b) Stress perturbations above one side of a pressurized horizontal crack [Pollard and Segall, 1987] of width 0.5 km showing maximum compressional and extensional normal stress perturbations, σ´ (red and blue lines, respectively,) and maximum shear stresses, τ´ (shaded contours) all normalized to the overpressure in the crack. A dashed line shows the transition from normal to reverse faulting on a ridge-parallel fault dipping at 45°. Hooft_fig3.jpg
Figure 4. Temperature in the Mothra diffuse vent (top, D. Butterfield) and pressure in sealed ODP boreholes (bottom, E. Davis and K. Becker). Black dots show the magnitudes of earthquakes detected by PGC. Vertical yellow lines indicate the start of two seismic swarms. Hooft_fig4.jpg
Figure 5. Histogram of tidal phase of ~2800 vent field earthquakes in 2003-4. The rate of earthquakes increases by more than a factor of 2 at low tide (180° phase) compared with high tide. Hooft_fig5.jpg