White Paper Title: 
Linking Stress Changes And Hydrothermal Activity During A Non-Eruptive Spreading Event

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. Although none of the swarms were accompanied by a seafloor eruption, they clearly have a magmatic component. We interpret this sequence as a 6-year non-eruptive spreading event that cumulatively ruptured most of the segment and relieved extensional stresses.

Integration and synthesis at the Endeavour ISS.

This is the first multi-year episode of intrusive plate spreading to be observed at any ridge. A unique seismic network that we deployed around the vent fields from 2003-6 recorded the seismicity associated with the latter part of this spreading event. To infer the associated spatial and temporal patterns of crustal stress changes we are applying rate-state modeling to seismicity rates and are using studies of b-values. We are also determining the spatial and temporal patterns of upper crustal seismic anisotropy and whether this relates to changes in crustal stress. To understand the influence of crustal stress changes on hydrothermal flow, we will integrate our observations of stress changes and crustal anisotropy with hydrothermal temperature and compositional perturbations. We will also combine our determinations of stress changes associated with the 1999-2005 non-eruptive spreading event with observations of regional borehole pressure perturbations to constrain the strain associated with extensional events.

Earlier this summer we submitted a paper to G3 that integrates a seismic analysis of the late February 2005 earthquake swarm with hydrothermal responses and hydrologic pressure perturbations to understand the tectonic and magmatic processes occurring during this seismic swarm. The spatial and temporal seismicity pattern is complex and located around the Endeavour – West Valley OSC. The start of the main episode of the swarm initiates a hydrologic pressure response at distances of 25-105 km that we attribute to a magmatic intrusion at the northern tip of the Endeavour axis. This is consistent with a RIDGE 2000 event response cruise that found no evidence of a seafloor eruption. While the amplitudes and signs of the hydrologic pressure perturbations imply that this intrusion is the main driving process for the seismic sequence it is largely aseismic. We infer that the intrusion changes stresses on the opposing limb of the Endeavour OSC causing tectonic deformation and giving rise to diffuse seismicity in the southwest Endeavour Valley as well as six larger strike-slip events. There may also be additional dike propagation from the West Valley segment to the south. These results are consistent with the eventual decapitation of the northern Endeavour ridge axis by the West Valley segment. Lastly we infer that a hydrologic pressure pulse diffuses away from the main magma intrusion and triggers seismicity beneath the Endeavour vent fields and perturbs flow at a diffuse hydrothermal vent in the Mothra field.

Integration with the EPR ISS. Ridge 2000 Integration and Synthesis also involves inter-comparisons between sites. At the Endeavour our seismic data in combination with SOSUS and the borehole pressure records will provide strong constraints on the stress and strain changes associated with a non-eruptive spreading event. At the EPR, Maya Tolstoy and her collaborators have recorded the seismicity associated with an eruptive spreading event in 2005-6. We intend to work with the EPR team to compare these two contrasting spreading events. One key issue is to determine why there were eruptions at the EPR but not at the Endeavour. If stressing rates are higher on the EPR as evidenced for example by comparisons of the patterns of focal mechanisms, temporal and spatial trends in b-values and stress changes and the levels tidal triggering, we might infer that higher volatile overpressures play a role in eruptions at the EPR. Alternatively, if the characteristics of the seismicity are similar, the different volcanic styles may be a direct result of differing average upper crustal densities.