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 firs
t-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 rele
ase. 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, alpha, varies from 1 t
o 10% for most of MOR and is always higher for transform faults. Most of th
e deformation, therefore, is aseismic slip. Most seismicity of MOR is confi
ned to transform faults. The energy contribution of transform. faults is on
e to two orders magnitude higher than that of the rift, and increases with
the spreading rate. There is a strong correlation between the seismic momen
t release of strike-slip faults and their total lengths. The correlation sh
ows that the seismic moment release depends on the total transform area and
confirms the simple thermal model of transform seismicity that was given b
y Burr and Solomon. The seismic moment release and the spreading rate have
opposite patterns. For the rifts, there is an inverse correlation between t
he seismic moment of normal faults and spreading velocity, while it seems f
or transforms that these parameters are independent. Finally, these results
show that the seismicity of transforms and rifts depends first of all on t
he thermal structure of oceanic lithosphere. In the case of continental rif
ts, one can distinguish in the degree of seismic activity depending on the
stage of rifting. Hence, analysis of the continental seismicity requires th
e consideration of factors of a geological evolution that play practically
no role in the case of oceanic lithosphere. The comparison of geological an
d seismic data for the East African region has allowed us to outline the re
gular changes of the seismic regime during development of the rift zone fro
m the stage of incipient rift to mature oceanic rift. In the evolutionary s
eries [intracontinental incipient rift]-[intracontinental mature rift]-[int
ercontinental rift]-[oceanic slow-spreading rift]-[oceanic fast-spreading r
ift], there is a gradual decrease of the role of rifts (sensu stricto, as t
ension structures) and increase of the role of strike-slip faults. Epicente
rs concentrate along major faults as well. The level of seismic energy beco
mes lower, although the rate of deformation increases. (C) 1999 Elsevier Sc
ience B.V. All rights reserved.