Compliance

Compliance measurements made in 1994, 1999 and 2007 near 9°48'N on the EPR depend on the distribution of magma with depth through the crust. The 1994 measurements require a large, shallow body of melt beneath

the rise axis consistent with many other observations. The data also require a substantial melt region deep in the crust or uppermost mantle. A 1999 measurement (although 2 km south) shows a reduced deep melt body

suggesting melt has moved since 1994 upward through the crust into the shallow axial melt lens. During 2005-2006, the EPR near 9°48'N erupted a thick layer of lavas onto the seafloor along 18 km of the rise axis.

Measurements from 2007 show a large decrease in compliance compared to the previous measurements consistent with removal of much of the melt from the shallow axial magma chamber. Observations from other volcanic regions suggest the melt lens beneath this eruption site is likely to re-inflate on a time scale of a few years, which might make it possible to use repeated compliance measurements to constrain the extent of the

source region of the 2005-2006 eruption, discriminating between the two eruption models.

Vertical Deformation

Eruptions and dike intrusions produce rapid crustal deformation (on a time of scale of hours to days to weeks) and are associated with intense seismicity.  These episodic events are often followed by a decrease in seismicity and a gradual deformation signal due to redistribution of subsurface magma on the scale of months to years. Understanding the spatial and temporal characteristics of magma transport and intrusion at mid ocean ridges therefore requires geodetic observations that cover a wide range of time scales.In June 2008, a focused network of 10 concrete geodetic benchmarks was installed on the seafloor to enable precisely measuring the vertical deformation in the area of the 2005/6 eruption at 9°50'N on the East Pacific Rise.  The benchmark locations form a cross with a long axis extending 9 km east and perpendicular to the ridge axis. The relative heights of the benchmarks were determined from campaign style pressure measurements made using a mobile pressure recorder (MPR) carried by the submersible Alvin and placed on top of the benchmarks during measurements.  The survey consisted of 26 measurements made on top of the 10 benchmarks; the repeated measurements were used to correct for gauge drift during the survey and to determine the measurement uncertainty.  A nearby bottom pressure recorder BPR (Cormier, Webb, and Buck) was recovered, and the data was used to make tidal corrections to the campaign style measurements.  The BPR was then redeployed on axis.  The survey uncertainty was ~2cm based on the repeatability of the drift and tide corrected MPR pressure data.  The MPR measurements will compliment episodic deformation data obtained from the BPRs.

          

Quantifying the temporal and spatial patterns of vertical deformation should allow us to begin to address the following questions:  Seismic evidence indicates that continuous axial magma chambers underlay most fast-spreading and many intermediate-spreading ridges, but what is the along axis continuity of magma supply?  How does the linearity of the ridge system affect the subsurface movements of magma. What drives the observed asymmetry between the two sides of the ridge axis? Is there active extensional faulting ongoing near the ridge axis at 9º50' N?  What is the rheology of the crust near axis?  Recent studies of microseismicity indicate a relationship between micro-earthquakes and the locations of hydrothermal vent systems [Tolstoy et al., 2008].  How does the subsurface movement of magma tie into and drive this relationship?  Are changes in the character of vent properties and biological communities associated with these movements and if so, what is the time scale of response?