Seismicity of mid-oceanic ridges and its geodynamic implications: a review

The global system of mid-oceanic ridges (MOR) is one of the longest active seismic belts where most of the earthquake epicenters are located continuously within a narrow axial zone. We summarized the principal results of the studies in MOR seismicity and their implications for geodynamics. The studies of epicenter distribution along the ridges and of focal mechanisms make an important contribution to the development of plate tectonics. During recent decades, a great amount of information on MOR structure and sea-floor spreading has become available. Geological and geophysical observations revealed a partitioning of the ridge by numerous discontinuities of several orders, which is reflected in the seismicity. There is a clear difference in seismic regime between two kinds of first-order segments - transform faults and spreading centers. The first have seismic moment release one-two orders higher; their contribution into the total seismic budget of MOR increases with higher spreading rate. The relationships between the seismic moment release, fault length and spreading rate are quite different for transform and rift parts of MOR; this confirms the difference in the geometry of their respective earthquake source volumes. In both these cases, the principal factor controlling the ridge seismicity is the thermal structure of the lithosphere. Microseismic experiments using ocean bottom seismometers, hydrophones and sonobuoys have perceptibly reduced the cutoff magnitude of complete reporting and the location uncertainty. Large numbers of microearthquakes are usually recorded almost everywhere near the ridge axis, even during relatively short recording times. More detailed location of epicenters and focal depths acquired great significance in the studies of magmatism, hydrothermal circulation and fracturing on MOR. Some spatial relationships seem to emerge between epicenter distribution and the location of hydrothermal sulfide deposits along the MOR. Several hierarchical levels could be distinguished; the nature of the relationship depends on the level of interest. (C) 2002 Published by Elsevier Science B.V.