Seismicity on Mid-oceanic Ridges: A Global Geodynamic Analysis

Abstract

The discovery of a worldwide system of mid-oceanic ridges (MOR) has been a major advance in geology dur­ ing the twentieth century and was largely responsible for the change of tectonic paradigms. The study of MOR continues, yielding ever new results that are im­ portant for tectonics, petrology, and metallogeny. For almost all of their length (more than 60,000 km) the MOR are also active seismic belts producing the over­ whelming majority of the earthquakes occurring be­ neath the oceans (excluding earthquakes in subduction zones that are not considered as oceanic) According to plate tectonics, MOR are divergent plate boundaries where new crust is generated and the adjacent plates are diverging in opposite directions. Depending on the rate of divergence, one distinguishes slowly spreading ridges (the Mid-Atlantic Ridge) from fast spreading ones (e.g., the East Pacific Rise). Thanks to the comparatively simple geodynamic setting, MOR are a perfect object of study to investigate relations between seismicity and geodynamics. A MOR roughly consists of two major tectonic features that alternate for all of its length, rifts in an extensional environment and transverse transform faults. A transform fault is narrowly defined as an ac­ tive segment lying between adjacent rifts, where shear­ ing movements are taking place. A broader definition includes inactive segments outside of the MOR. Sykes [1967]) first to found that earthquake mecha­ nisms on the two structural types were different. Fran­ cis [1968] pointed out that the seismicity regimes on rifts and transform faults in the Atlantic are different; he also remarked on the fact that large earthquakes tend to occur in transform fault zones and that the b value is greater for the rifts. These patterns were later con­ firmed by Drumya et al. [1990] and Ginsar [1987], the most recent summaries being given in the Geological and Geophysical Atlas of the Atlantic Ocean [Udintsev, 1989-1990] and Boldyrev's papers [Boldyrev, 1992, 1994]. Nevertheless, it still remains unclear what are the respective contributions of these two types of earth­ quakes into the worldwide MOR seismicity. However, some relations between seismicity and tectonic param­ eters seem to have been found from a study of individ­ ual transform faults [Burr and Solomon, 1978; Solomon and Burr, 1979]. The present study summarizes data for the worldwide MOR system and addresses the fol­ lowing problems: 13 Copyright American Geophysical Union P TATIO AL S I Y E Y A ICS L. 5 i i it i ni i : l l i l sis