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S U M M A R YThis is a global survey of seismic moment release rates (scalar moment/length/time) along five categories of plate boundaries; for shallow earthquakes we evaluate divergent, transcurrent and convergent boundaries; for deep earthquakes we consider separately boundaries where the deepest seismicity is intermediate (<300 km) or deep-focus (500-600 km). The objective is to evaluate the typical range of rates observed and present the results in a form where it is straightforward to compare factors that affect rates in the different environments. For earthquakes occurring 1977-2005, the data are scalar moments from the global centroid moment tensor catalogue; for earthquakes 1900-1976, we use a standard formula to obtain moment from magnitudes in the Engdahl-Villaseñor Centennial catalogue. Moment release rates correlate with relative velocity between plates along divergent and transcurrent boundaries; focal depth influences moment release rates along intermediate and deep-focus boundaries. After accounting for these effects and adjusting rates for the influence of the short (29-106 yr) data record, within each category the typical observed variation is about an order of magnitude. We estimate seismic coupling coefficients, that is, the proportions of plate tectonic slip accommodated as earthquake rupture and reflected in the moment rates. Coupling may be 0.7 or greater along shallow convergent boundaries where at least one plate is continental. For all other boundary categories coupling is generally less than 0.4; it is 0.2 or less along most divergent boundaries and for intermediate and deep boundaries at depths exceeding 100 km. These results confirm that there are fundamental mechanical differences in the failure processes acting along different categories of plate boundaries. One implication is that only along a few highly coupled boundaries is it plausible that analysis of seismicity alone might lead to successful algorithms to predict large earthquakes.

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Preliminary analysis of near-bottom, high-resolution DSL-120 sidescan and bathymetric data from the median valley in the TAG (Trans-Atlantic Geotraverse) segment near 26°N on the Mid-Atl antic Ridge, and Argo-Il photography from the associated actively venting hydrothermal mound, allows us to evaluate the structural, volcanic, and hydrothermal processes occurring in this area, and to provide baseline constraints on the pre-drilling status of the morphology of the mound and the distribution of venting. These data were collected just 2 months before ODP drilling of the active TAG mound, one of the largest volcanichosted, mid-ocean ridge hydrothermal deposits yet discovered. Sharp discontinuities and strong asymmetry in the morphology and age distribution about the spreading axis suggest a recent episode of preferential accretion of crust to the east. Such episodes have likely contributed to the long-term spreading asymmetry (13 mm/yr to the east, 11 mm/yr to the west) of the TAG segment. The active mound lies within the intersection of a zone of actively developing ridge-parallel (north-northeast) fissures and a series of pre-existing obliquely oriented (east-northeast) faults. The east-northeast faults are not continuous with faults previously described from the upper rift valley walls. This intersecting fault pattern may be important in localizing hydrothermal activity. Contemporaneous tectonic deformation and hydrothermal deposition continue to modify the three-dimensional structure and hydrogeology of the active TAG mound.

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