Seismicity of oceanic and continental rifts—a geodynamic approach

Abstract

Two major kinds of divergent structures—oceanic and intracontinental rifts—were compared in principal seismic and tectonic characteristics. First, the role of main components of the mid-oceanic ridges (MOR) was estimated for the whole Earth. We considered two levels of the MOR segmentation. The first-order structures are the segments of MOR between triple junctions and the second-order structures are a transform faults and rift parts of MOR. The seismic catalogues NEIC and CMT were used to assess the seismic moment release. The seismic moment release was calculated another way using the global plate tectonic model NUVEL-1 and Brune's formulae. Comparison of these two values shows that the seismic coupling coefficient, α, varies from 1 to 10% for most of MOR and is always higher for transform faults. Most of the deformation, therefore, is aseismic slip. Most seismicity of MOR is confined to transform faults. The energy contribution of transform faults is one to two orders magnitude higher than that of the rift, and increases with the spreading rate. There is a strong correlation between the seismic moment release of strike–slip faults and their total lengths. The correlation shows that the seismic moment release depends on the total transform area and confirms the simple thermal model of transform seismicity that was given by Burr and Solomon. The seismic moment release and the spreading rate have opposite patterns. For the rifts, there is an inverse correlation between the seismic moment of normal faults and spreading velocity, while it seems for transforms that these parameters are independent. Finally, these results show that the seismicity of transforms and rifts depends first of all on the thermal structure of oceanic lithosphere. In the case of continental rifts, one can distinguish in the degree of seismic activity depending on the stage of rifting. Hence, analysis of the continental seismicity requires the consideration of factors of a geological evolution that play practically no role in the case of oceanic lithosphere. The comparison of geological and seismic data for the East African region has allowed us to outline the regular changes of the seismic regime during development of the rift zone from the stage of incipient rift to mature oceanic rift. In the evolutionary series [intracontinental incipient rift]–[intracontinental mature rift]–[intercontinental rift]–[oceanic slow-spreading rift]–[oceanic fast-spreading rift], there is a gradual decrease of the role of rifts (sensu stricto, as tension structures) and increase of the role of strike–slip faults. Epicenters concentrate along major faults as well. The level of seismic energy becomes lower, although the rate of deformation increases.