White Paper Title: 
Seismicity and Upper-Crustal Structure of the Endeavour ISS

Research and Overview:

A suite of seismic investigations conducted at the Endeavour ISS provides insights to tectonic and magmatic processes associated with ridge-spreading and crustal accretion at intermediate-rate spreading segments.  I have compiled a 38-month catalog of automatically located hypocenters recorded between 2003-6 using a unique ocean bottom seismometer (OBS) network [Weekly et al., 2008] that documents a substantial segment-wide decline in the number of earthquakes occurring per day following a sequence of seismic swarms located near the Endeavour – West Valley OSC during January and February 2005.  Later this year, I will submit a paper that details the spatial-temporal characteristics of these swarms as the first chapter of my PhD thesis.  In the paper, I posit that these swarms represent the terminus of a multi-year non-eruptive spreading event that initiated with the swarm sequence of 1999-2000 that was located by the SOSUS network on the southern portion of the Endeavour [Bohnenstiehl et al., 2004; Dziak et al., 2006].  I establish background seismicity levels of sub-regions of the Endeavour Segment prior to the 2005 swarms, detail the complex spatial-temporal distribution of earthquakes observed during the swarms, and show that the subsequent ~80% decrease in seismic activity is observed at both the vent-field and segment scale (see figure).  Observed spatial-temporal hypocenter distributions and patterns of seismic moment release are consistent with non-eruptive magmatic extension near the southern tip of the West Valley Segment during the January swarm and along the northern portion of the Endeavour Segment during the February swarm.  Additionally, I find increased seismicity levels around the Main Endeavour and High Rise fields during the January swarm, and near the High Rise and Salty Dawg fields during the February swarm.  During each swarm, increases in vent field seismicity occur following a ~2.5 day delay of the swarm onset.  Hooft et al [2010] propose that these delays are consistent with along-axis diffusion of a pore-fluid pressure perturbation in the upper crustal aquifer.  The automated catalog I have created will facilitate further investigations including tidal triggering mechanisms, spatial and temporal patterns of seismic b-values, and rate-state modeling to better understand the relationship between stress change and swarm sequences.

Integration and Synthesis:

The focus of the second thesis chapter is to determine the three-dimensional seismic structure of the upper oceanic crust using compressional-wave travel-times collected from an August 2009 tomography experiment that comprised ~5,500 airgun shots recorded at 64 OBS sites.  Analysis is still in its nascent stages, but my objective is to obtain a high-resolution image of the upper crust P-wave velocity structure using only waves that have turned in the upper crust.  Ultimately, analysis will be expanded to include refracted arrivals from waves that interact with the top of the axial magma chamber.  The upper-crust velocity model will detect variations in the structure of layer 2, which can be interpreted in terms of along- and cross-axis differences in volcanic accretion processes.  Additionally, this model will provide tighter constraints on the structure of the thermal boundary layer separating the magma chamber from the upper crust.  Correlating along-axis variations in thermal structure with variations in seismicity levels and hydrothermal fluid flux at particular vent fields will provide insights into how hydrothermal systems mine heat from a midcrustal magma source.  This also has implications for inter- and intra-field variations in porosity structure.  Obtaining a segment-scale model for the seismic velocity structure is critical to identifying the scale and intensity of ridge segmentation and melt distribution, which is one of the fundamental questions being addressed in several thematic working groups.

The final chapter of my thesis will combine the earthquake and tomography data to provide geophysical constraints on heat-transfer mechanisms within the hydrothermal uptake zone.  An obvious application for the improved three-dimensional compressional-wave velocity model is to relocate the earthquakes.  Additionally, I will use the method of joint hypocenter-velocity inversion to simultaneously improve uncertainties associated with shear-wave crustal velocity model and earthquake relocations.  Improved constraints on the compressional- and shear-wave velocity structure in the upper crust will further improve the accuracy of earthquake depths and their position relative to the axial magma chamber in the upper crust.

For the upcoming R2K meeting, I am particularly interested in using the Events group to compare seismic characteristics of extensional plate-spreading events along different mid-ocean ridge segments.  Continuing to make progress on synthesis of the recently collected tomographic data will also be one of my top priorities throughout the upcoming academic year, so meeting with attendees to discuss geophysical inversion strategies and improvement of starting velocity models will be another of my goals.  The results of this inversion will be particularly appropriate for the Crustal Controls/OSC Magmatism group, so I will be keen on interacting with the community about expected levels of resolution and how other recently collected datasets might be used to complement the tomographic models.

Bohnenstiehl, D. R., R. P. Dziak, M. Tolstoy, C. G. Fox, and M. Fowler (2004), Temporal and spatial history of the 1999-2000 Endeavour Segment seismic series, Juan de Fuca Ridge, Geochem. Geophys. Geosyst., 5, 14.

Dziak, R., W. W. Chadwick, Jr, J. P. Cowen, E. Baker, R. Embley, D. R. Bohnenstiehl, and J. A. Resing (2006), Detecting volcanic events in the Northeast Pacific, EOS Trans. AGU, 87(4), 37, 42.

Hooft, E.E.E., W.S.D. Wilcock, H. Patel, D. R. Toomey, K.Becker, D.A. Butterfield, E.E. Davis, and M.D. Lilley (2010), A complex seismic swarm associated with local and regional hydrothermal and hydrological perturbations: the northern Endeavour segment February/March 2005, Geochem. Geophys. Geosyst., submitted.

Weekly, R. T., W. S. Wilcock, D. R. Toomey, E. E. Hooft, and P. R. McGill (2008), An automatically generated earthquake catalog for the Endeavour Segment of the Juan de Fuca ridge: Linkages between segment and vent-field scale seismicity, Eos Trans. AGU, 89(53), Fall Meet. Suppl. Abstract V54B-02.