- Email: [email protected]
Seismicity of oceanic and continental rifts—a geodynamic approach
Physics of the Earth and Planetary Interiors 111 Ž1999. 253–266
Seismicity of oceanic and continental rifts—a geodynamic approach P.O. Sobolev a
a,)
, D.V. Rundquist
b
International Institute of Earthquake Prediction Theory and Mathematical Geophysics, Russian Academy of Sciences, WarshaÕskoye sh. 79, kor. 2, Moscow 113556, Russian Federation b Vernadsky State Geological Museum, Russian Academy of Sciences, ul. MokhoÕaya 11, Moscow 103009, Russian Federation
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 ŽMOR. 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, a , 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 wintracontinental incipient riftx – wintracontinental mature riftx – wintercontinental riftx – woceanic slow-spreading riftx – woceanic fast-spreading riftx, there is a gradual decrease of the role of rifts Žsensu 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. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Seismicity; Geodynamic approach; Rifts
)
Corresponding author. Fax: q7-095-310-70-32; E-mail: [email protected]
0031-9201r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 1 - 9 2 0 1 Ž 9 8 . 0 0 1 6 5 - 4
254
P.O. SoboleÕ, D.V. Rundquistr Physics of the Earth and Planetary Interiors 111 (1999) 253–266
1. Introduction Mid-oceanic ridges ŽMOR. are the most extensive global structures. They are edges where new oceanic crust is formed and plates diverge in opposite directions. In many respects, it is an ideal object to study the relations between seismicity and tectonics due to a relatively simple geodynamic environment. Another kind of divergent boundary is the intracontinental rift, the best known of which is the African– Arabian rift system. In the present article we have obtained seismic patterns for the whole world oceanic rift system and compared them with corresponding regularities of continental rifts. The main objectives are as follows: Ž1. to estimate the ratio between seismic and aseismic components of deformation for rift and transform parts of MOR; Ž2. to define the role of these two structural types in the seismicity of MOR; Ž3. to seek relationships between seismic moment and major structural characteristics of MOR; and Ž4. to compare seismicity of oceanic and intracontinental rifts.
2. The seismicity of MOR The discovery of the global system of rifts, including MOR is one of the most significant geological discoveries of our century; investigations of MOR continue to yield important data. On the one hand, some of the most fundamental problems of plate tectonics, petrology, and ore formation are connected with the study of these structures. On the other hand, rifts are active seismic belts. The largest as well as moderate oceanic earthquakes are located along MOR. The ridges are interconnected and form major global belts. Their cumulative length is more than 60,000 km. Slow- Žsuch as the Mid-Atlantic Ridge. and fast-spreading Že.g., the East Pacific Rise. ridges are differentiated on the basis of spreading rate values. MOR have two main structural types which alternate throughout the extent of the ridge. One type includes the rift zones. The other includes the transform faults. As was shown by Sykes Ž1967., earthquakes with normal fault mechanisms occur mainly
in the rift zone, while epicenters of earthquakes with strike–slip mechanisms are located inside active parts of transforms faults. Francis Ž1968. has established difference in some seismic regime parameters between rift and transform parts of the Mid-Atlantic Ridge, namely, the largest earthquakes are located along transforms and the rift events have larger values of b Žrate of slope of linear magnitude– frequency relation.. On the other hand, some relationships between seismic and tectonic characteristics were inferred from a study of the largest transform faults ŽBurr and Solomon, 1978; Solomon and Burr, 1979.. We analyzed data on the whole global system of MOR using digitized data of Stoddard Ž1992., who summarized many results. In spite of some inaccuracies, they are suitable for a global analysis. According to this model, there are 367 transform faults and the same number of rift zones, each of them belonging to one of the fifteen interplate boundaries ŽFig. 1.. First, the NEIC catalogue was used to evaluate the seismic moment release ŽGlobal Hypocenters Database, 1995.. All earthquakes with m b ) 4 from 1964 to 1995 that lie inside an 80-km wide zone along MOR have been considered. As a whole, there are 4732 events for transforms and 4385 events for rifts. For the same area around MOR, all earthquakes from the CMT catalogue were taken Ž1002 and 644 events, respectively.. From these data the parameters of a linear regression of log M0 on m b were calculated for m b ) 5, giving the following empirical equations: log M0 s 10.18 q 1.39m b Ž transforms. , log M0 s 11.16 q 1.17m b Ž rifts . . The theoretical value of the coefficient in a similar equation for Ms is equal to 1.5, corresponding approximately to 2.4 in the case of m b ŽKanamori and Anderson, 1975.. However, as was shown by Romanelli and Panza Ž1996., the coefficient depends on tectonic environments and depths of hypocenters. Also, the smaller value of the coefficient in the case of oceanic rifts confirms the smaller size of their earthquake sources relative to the transforms sources. After converting m b to M0 , the total seismic moment, Ý M0 , was calculated both for transforms
P.O. SoboleÕ, D.V. Rundquistr Physics of the Earth and Planetary Interiors 111 (1999) 253–266
255
Fig. 1. Major tectonic plates, MOR and the African–Arabian rift system.
and rifts for each pair of plates. The same value can be evaluated using the well-known formula of Brune Ž1968. for a simple plane fault:
AÕ s
1
mt
Ý M0
Ž 1.
where m is the shear modulus Ž5 = 10 10 Nrm2 ., t is the time interval, A is the fault area, and Õ is the velocity of displacement. One can compute the relative velocity for each part of a ridge using the global plate tectonic model NUVEL-1 ŽDeMets and Gordon, 1990.. The fault area was calculated by different ways for transforms and rifts, because the respective shapes differ. For the former case, it was supposed that the area of a transform susceptible to seismic failure is confined above the 4008C isotherm ŽStoddard, 1992.. The oceanic isotherms have the form of a parabola and depend on the spreading rate ŽSolomon and Burr, 1979.. So, the transform area above a given isotherm is proportional to a function
of transform length and velocity. We assume that the area of a rift fault is rectangular and its depth was evaluated as explained by Solomon et al. Ž1988.. The ratio of the catalogue-based seismic moment release to the corresponding value calculated on the bases of theoretical formula is called a seismic coupling coefficient, a ŽPacheco et al., 1993.. Commonly, it shows that part of the strain energy is released as earthquakes and part as aseismic deformations. The results of calculations are given in Table 1. The seismic coupling is a clear difference for rifts and transforms. The coefficient is higher for transforms and decrease with the growth of the spreading rate ŽFig. 2.. Only in the ‘fastest’ ridges of the East Pacific an a values almost the same for both types. Generally, a values vary between 1 and 10%. Thus, aseismic deformation is clearly dominant for the case of rift parts of MOR; the ratio of seismic and aseismic components of transform faults depends on the spreading rate. In both cases, these results suppose a controlling role of lithospheric thermal structure into MOR seismicity.
256
Number
Sector of MOR
N
L tr Žkm.
Lri Žkm.
V Žcmr year.
Ý M0tr Ž10 16 N m.
Ý M0ri Ž10 16 N m.
a tr Ž%.
a ri Ž%.
M0tr Ž10 16 N m.
M0ri Ž10 16 N m.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
afr–ant afr–ara afr–aus afr–ind afr–nam afr–sam ant–aus ant–pac ant–sam coc–naz coc–pac nam–eur nam–pac naz–ant pac–naz
742 18 366 471 1199 1702 765 299 164 15 335 1682 404 481 474
4320 670 1390 1010 1490 5850 3460 3590 1310 830 390 2020 2870 2820 1110
4360 1560 1780 2290 3200 7520 7140 5080 720 2180 1850 5340 730 2210 4430
1.42 2.19 4.20 3.32 2.42 3.38 6.77 7.27 1.75 5.94 11.00 2.23 4.84 5.93 14.85
15,160
7300 120 2750 4040 5910 9610 6030 2000 1740 110 1240 4740 620 1350 2200
17.7
8.5 0.3 3.4 4.7 5.8 3.2 1.5 0.7 10.2 0.1 1.5 3.4 1.7 1.1 3.0
7540 870 3210 1710 3750 26,560 10,350 4860 1250 6150 1425 375 9680 6860 1960
1046 226 103 443 947 1055 50 21 127 77
3810 4850 9300 26,260 12,380 7790 10,490 1140 6630 2940 7500 2130
4.7 5.7 9.2 8.7 3.0 2.8 61.5 1.4 4.7 7.9 6.2 2.9
408 114 312
Plates: afr s African; ant s Antarctic; ara s Arabian; aus s Australian; coc sCocos; eur s Eurasian; nam s North American; naz s Nazca; pac s Pacific; sam sSouth American. N s number of earthquakes in the NEIC catalogue. Ltr s total length of transform faults. Lri s total length of MOR rift zones. V saverage spreading rate. Ý M0tr , Ý M0ri s the seismic moment sum for transform and rift zones of MOR, respectively. a tr , a ri sseismic coupling coefficient for transform and rift zones of MOR, respectively. M0tr , M0ri saccumulated seismic moment for earthquakes with strike–slip, normal and oblique faults mechanisms, respectively.
P.O. SoboleÕ, D.V. Rundquistr Physics of the Earth and Planetary Interiors 111 (1999) 253–266
Table 1 Some seismological and structural characteristics of MOR
P.O. SoboleÕ, D.V. Rundquistr Physics of the Earth and Planetary Interiors 111 (1999) 253–266
Fig. 2. Seismic coupling coefficient for the plate boundaries vs. average spreading rate.
The next stage of analysis was based on the global Harvard Centroid Moment Tensor Catalogue Ž1995. containing most events with M W ) 5.0 since 1977 to 1994 Ž M W is seismic moment magnitude.. The important advantage of this catalogue is that earthquakes can be discriminated on the basis of their mechanisms and, therefore, there is an opportunity to evaluate the contribution of each kind of fault into MOR seismicity. Previous estimates were based on the geographical location of the epicenters only, but there were great uncertainties near the intersections of rifts and transforms. It is possible to determine the type of earthquake using the value of rake Žthe angle between slip vector and horizontal line measured on the fault plane., because almost all focal plane solutions have dip angles more than 308. There are four kinds of mechanisms: normal faults Žy120 F rake F y60., thrusts Ž60 F rake F 120., strike–slip faults Žangles that lie within 308 of 180 or y180. and oblique ones. Most earthquakes belong either to normal faults or strike–slip faults, while earthquakes with oblique mechanisms and thrusts are a minority. The distribution of epicenters shows that, usually normal-fault earthquakes are confined to rifts, and strike–slip earthquakes occur near transform
257
faults. Earthquakes with oblique mechanisms occur much more rarely, and their position with relation to the basic elements of the MOR is rather irregular. Therefore, the earthquakes with strike–slip mechanisms can be used for the estimation of the seismic moment of transforms and, correspondingly, the seismic moment of rifts can be calculated using data for normal faults. The results are given in Table 1. It is clearly seen that the values of seismic moment release on transforms are higher than on rifts. It should be noticed that the energy contribution of transforms increases with the growth of spreading rate, even though their length is smaller. We tried to outline some relationships which connect Ý M0 with some structural factors, namely fault length and spreading rate. The dependence of the seismic moment release on the total length of faults is shown in Fig. 3. A power relationship should be expected between these parameters from theoretical reasoning about proportionality of M0 to fault area and the evaluation of this area on the basis of Eq. Ž1.. However, the real curve goes lower and is approximated rather by an exponential function. There is no clear-cut dependence of Ý M0 on length for rift zones. Evidently, the leading factor is the spreading rate in this case. A quite reliable logarithmic relation between Ý M0 and V is seen there, except for one
Fig. 3. Total seismic moment release for strike–slip faulting earthquakes vs. the total length of transform faults.
258
P.O. SoboleÕ, D.V. Rundquistr Physics of the Earth and Planetary Interiors 111 (1999) 253–266
Fig. 4. Total seismic moment release for the normal faulting earthquakes vs. average spreading rate.
point of the Pacific–Nazca boundary with an extremely high spreading rate ŽFig. 4.. The following conclusions can be drawn based on our analysis of global patterns for MOR. Ž1. The decisive role in MOR seismicity belongs to transform faults. The energy contribution of transform faults is one to two orders higher than that of rifts and increases with the increasing spreading rate; Ž2. Seismic coupling coefficient a varies from 1 to 10% for most of MOR, and it is invariably higher for the transform faults; Ž3. Relations between the seismic moment release, fault length, and spreading rate are different for transform and rift parts of MOR; this confirms the difference in shape and origin between their earthquake sources. Anyway, in both cases, the major factor of the MOR seismicity is the thermal structure of the lithosphere.
3. The seismicity of the African–Arabian rift system The African–Arabian rift system is an example of a continental rift. Notions of the structure and evolution of these rifts have changed considerably as a
result of complex geological and geophysical researches carried out in recent years Že.g., Chorowitz et al., 1987; Rosendahl, 1987.. It was found that the degree of evolution of riftogenic structures steadily decreases from north to south, which is explained by the different times of their formation. The African– Arabian rift system consists of a series of structural zones crossing the entire African continent. The triple junction of the Red Sea, Aden, and the Ethiopian rifts is located in the north. The so-called Eastern branch, integrating the Ethiopian and Kenyan rifts, continues further to the south. The Western branch, located along the chain of African lakes, is crossed by the Azva transversal zone in the north and the Tanganyika–Rukva–Malawi fault zone in the south. The Malawi Rift is located further south. The Kerimba fault zone is situated on the southern most part of the continent. All structures have different relief, type and character of faults, and deep structure. This is explained by the different times of origin of Cenozoic rifts and, hence, by the different degrees of their evolution ŽChorowitz et al., 1987.. The asymmetry of rift structures is revealed in the development of systems of subsidiary dislocations on the western side. The substantial influence of preCenozoic ŽPrecambrian and Permian–Triassic. rift structures and their confinement to zones between the most ancient granite–gneiss cores of Precambrian crust is common to all modern rifts ŽChorowitz et al., 1987.. Seven sectors were distinguished within the East African rift system Žboundaries of rifts and faults are from the Map of Fault Tectonics of Africa and Arabia by Yarmoluk, 1984. and a comparative evaluation of the seismicity was carried out ŽFig. 5.. The characteristics of these sectors are given in Table 2. Despite the uncertainty of the boundaries of these sectors, one can see that they differ rather sharply in all major features of geological structure and reflect the different stages in the evolution of a rift zone. Gradual thinning of the continental crust, accompanied by an increase of heat flow, expansion of the area of grabens, uplift of near-slope parts, and increasing subsidence of the graben floor, occur during the process of rifting. In the later stages, the oceanic lithosphere is eventually formed and a mid-oceanic ridge comes into existence. This process can be observed in the Gulf of Aden. In this general series,
P.O. SoboleÕ, D.V. Rundquistr Physics of the Earth and Planetary Interiors 111 (1999) 253–266
259
Fig. 5. Rift valleys, major faults, and epicenters of earthquakes with m b G 4.0 Ž1964–1995.. Numbers correspond to numbers of sectors in Table 2.
260
Table 2 General characteristics of the areas of the African–Arabian Rift system Area
Time of the beginning of rifting
Neogene–quaternary magmatism
Deep structure
Structural characteristics
q
L
S
N
a
b
1
Aden
Eocene– Oligocene
Entire oceanic crust, mantle on the depth of 5–8 km
6–7
1060
502
9.97
1.48
Red Sea
Eocene– Oligocene
more than 250
5–7
570
271
6.96
0.98
3
East Rift
Late Oligocene
Escarpments, marginal depressions, clearly expressed mid-ocean ridge Escarpments, marginal depressions, the mid-ocean ridge is in the southern part only Broad dome uplifts, wide continuous graben
more than 150
2
more than 100 –150
3–5
230
81
8.73
1.42
4
West Rift
Early Miocene
135
319
8.18
1.19
5
Malawi
Middle–Late Miocene
85
171
8.01
1.24
2.1
Initial rift
6
Kerimba
Late Miocene
Active volcanism in the axial zone —tholeiitic basalts, also occurrences in the marginal parts Active modern volcanism in the axial graben –Tholeiitic basalts, occurrences of alkaline and subalkaline basalts in the marginal parts Broad fields of the volcanic rocks of basalt–trahite association, carbonatites Occurrences of the volcanic rocks of basalt– trahite–phonolite association, carbonatites Single ring basic– ultrabasic alkaline intrusions Absent Ž?.
9.69
1.63
3.2
Initial rift
7
Limpopo
The present timeŽ?.
Absent Ž?.
N s number of earthquakes. M0 s total seismic moment release Ž10 18 N m.. Ssarea of rifts Ž1000 km2 .. Ls total displacement in normal faults Žkm.. q saverage heat flow in the axial part ŽmWrm2 .. a,bs coefficients of the equation log N s ay bm b .
The parts of oceanic crust, forming a ‘chain’
Strongly thinned continental crust Thinned continental crust
Narrow half-dome uplifts, intermittent system of grabens
60–75 3–4
Normal continental crust Normal continental crust Normal continental crust
System of grabens
50–75 4
Separate grabens
60
?
5
77
Faults
50–60 ?
0
224
11.19 1.81
M0
8.9
19
3.0
37
Stage of rifting Oceanic
Initial oceanic
Mature rift
Typical rift
0.63 Pre-rift Ž?.
P.O. SoboleÕ, D.V. Rundquistr Physics of the Earth and Planetary Interiors 111 (1999) 253–266
Number
P.O. SoboleÕ, D.V. Rundquistr Physics of the Earth and Planetary Interiors 111 (1999) 253–266
Fig. 6. Histogram of rake value distribution for the earthquakes in the African–Arabian rift system Ža. and for all MOR Žb..
261
262
P.O. SoboleÕ, D.V. Rundquistr Physics of the Earth and Planetary Interiors 111 (1999) 253–266
magmatism changes regularly from basic complexes and carbonatites to alkaline and to tholeiitic basalts. Shallow earthquakes, with m b - 6, dominate in the area. Their epicenters are confined more often to faults with either a northwestern or northeastern strike and are located within the rift valleys or subsidiary faults, which become more numerous to the south. An analysis of focal mechanisms Žthe Harvard catalogue for the 1977–1994 period was used. shows that normal faults prevail in Africa. Most strike–slip movements are related to faults of the Aden and Red Sea rifts, and they are almost lacking inside the continent. This is explained by the prevalence of tension in the northwest direction approximately perpendicular to the strike of most riftbounding faults ŽKebebe and Kulhanek, 1991.. Types of mechanisms can be identified by the rake value. The diagram of distribution of this parameter shows a clear prevalence of normal faults ŽFig. 6., which make up more than half the total number of solutions and includes all large earthquakes. Diffuse seismicity prevails at the earlier stages of rifting, while almost all epicenters of earthquakes are located within rift troughs at the later stages. A
decrease in the b-value Žin Gutenberg–Richter’s relation. is evident: sectors where rifting begins are characterized by the greatest values of b. The smallest values of b and, hence, the greatest concentrations of stress are in the Red Sea and the Western Rift. In contrast, the Eastern Rift is characterized by a great number of smaller earthquakes; as corroborated by data from local seismic studies ŽTongue et al., 1994.. Earthquakes from the NEIC catalogue Ž m b G 4.0, 1964–1995. were used to calculate the seismic moment release. The Western Rift has the maximum density of released seismic energy. Most of the energy is released on subsidiary faults of the main rift valleys. Since the area of grabens reflects the degree of rifting, one can try to estimate very roughly the character of seismicity evolution during the geological development of intracontinental rifts ŽFig. 7.. It is likely that initially, seismicity increases quickly, reaches a maximum at the stage of a typical rift Žthe Western Rift., and then gradually decreases. An increase in the role of earthquakes located within the rift valleys is typical, suggesting the localization of epicenters in precisely these structures.
Fig. 7. The total seismic moment release vs. total area of grabens for the African–Arabian rift system.
P.O. SoboleÕ, D.V. Rundquistr Physics of the Earth and Planetary Interiors 111 (1999) 253–266
The most pronounced differences in seismic regimes are typical of the two important zones— Western and Eastern branches of the East African rift system. The geological structure zones strongly differ. The crust under the Kenyan and Ethiopian rifts ŽEastern branch. is much thinner. Based on geophysical data ŽGirdler, 1983., the lithosphere of this zone is characterized by increased plasticity. This is corroborated by the existence of large, active modern volcanism and increased heat flow. In addition, this sector has the maximum total length of rifts ŽTable 2.. All of these facts testify to a relatively high rate of deformation within the Eastern branch and the prevalence of the aseismic component. The situation in the Western branch is different. Here the crust is insignificantly thinned under rift valleys, volcanism is almost absent, and numerous faults deviate laterally from the rifts. It is likely that the lithosphere here is more fragile, and the deformations are accompanied by the release of seismic energy in the form of larger earthquakes. Evidently, the distinction between these two branches is mainly caused by their different positions. The Eastern branch represents the continuation of an oceanic rift inside the continent, whereas the Western branch can serve as an example of an ‘actual’ intracontinental rift.
4. Discussion: comparison of the seismicity of oceanic and continental rifts Continental and oceanic rifts are different stages of one prolonged tectonic process of rifting. This process begins with incipient rifting inside a continent, and then the crust transforms from continental to oceanic crust. A new oceanic lithosphere is born of the transformation complete but it can stop at any stage. Taking the African–Arabian rift system as an example, one can see almost complete evolutionary series of this process and it is probable that modern oceanic rifts Žmid-oceanic ridges, MOR. have gone through all these stages in the geological past. Hence, one of the major that links both kinds of rifts is their common origin. They reflect the evolution of divergent boundaries in time. However, there are some principal distinctions that cause their different geodynamic Žand so, seismic. patterns.
263
First of all, oceanic rifts are much more widespread than are continental ones. There are only few regions with active continental rifting, the larger of these being the African–Arabian, Baikal, RioGrande, and Rhein rift systems. The extent of MOR is many times larger. Although both kinds of rifts are divergent boundaries, in the case of oceans, the spreading rate is higher by at least an order of magnitude. Another principal difference consists in an absolutely different deep structure of continents and oceans. The lithosphere differs in composition, internal structure, and thickness. Also the thermal structure Žwhich strongly affects the elasticity properties. of the lithosphere is quite different for typical oceanic and continental rifts. The thickness of the brittle layer in oceans near the ridge is no more than 5–10 km, whereas for continents, it reaches 20–40 km. All these patterns are reflected in the seismicity of rifts. The general tendency in the distribution of epicenters is the following: their concentration increases from earlier stages of rifting to later ones. It is clearly seen in the case of Africa. During incipient rifting in South Africa the seismicity is very scattered, there is no preferred orientation of active faults. At later stages ŽWestern and Eastern Rifts. almost all epicenters are located within rift troughs and on the border faults. When a continental rift turns into an oceanic rift, all seismicity is confined to MOR. Lastly, one can notice that most epicenters are located along transform faults during the transition from slow-spreading ridges to fast-spreading ridge. Another tendency is the increasing role of transverse faults during rifting. It was shown above for African rifts that the most active zones are normal faults and the largest seismic events occur there. There are some large zones of transverse faults as well, but their seismicity is many times less, and as a whole, strike–slip earthquakes are less frequent than normal faults. A different picture is in the oceans. Here, the greater part of the seismic moment release occurs on transform faults and their relative contribution increases with the spreading rate. It is clearly seen on the summary histogram of the rake value for the two kinds of rifts ŽFig. 6.. Probably, it is worth estimating the most general characteristics of seismicity. The first of these is the total seismic moment release. The NEIC catalogue
264
P.O. SoboleÕ, D.V. Rundquistr Physics of the Earth and Planetary Interiors 111 (1999) 253–266
Fig. 8. The magnitude–frequency relations for the interplate earthquakes. Ž1. All oceanic rift zones; Ž2. all oceanic transform zones; Ž3. Red Sea and Aden rifts; Ž4. African rifts.
Ž m b G 4.0. for the period 1964–1995 was used to calculate seismic moments and their sum for transform and rift zones of MOR, and for the African– Arabian rift system. The major difficulty for direct
comparison is that the seismicity of MOR has a practically linear distribution, whereas a seismicity spreads over some area in the case of continental rifts. Therefore, the density of seismic moment release was used Žthe ratio of Ý M0 to the total length of faults.; beside the African rifts, the perimeter of rift valleys was used as the denominator. The calculation of this quantity Župper and lower values, which were calculated with and without consideration of crest–transform intersections. gave the following results: 3.33 and 1.73 = 10 13 N for all transforms, 0.99 and 0.33 = 10 13 N for the rifts of MOR, and 0.20 = 10 13 N for the whole Africa–Arabian rift system Žthe values are very close for its ‘oceanic’ and ‘continental’ part.. Hence, the order of released seismic energy is approximately the same for continental rifts and MOR crest zone, but it is higher for the transform faults. It should be noted that these values generally decrease with the spreading rate for both kinds of MOR. It is notable that the values of the specific seismic moment are very close for continental and oceanic rifts, although the velocities of spreading are quite different, being several centimeters per year at oceanic rifts and no more than 0.4 cmryear at continental rifts ŽJestin et al., 1994.. The magnitude–frequency relation reflects regular variations of seismic regime during the process of rifting as well. The b-value increases in the transformation series—oceanic rift zones–transitional rifts–
Fig. 9. The evolutionary series of rifts.
P.O. SoboleÕ, D.V. Rundquistr Physics of the Earth and Planetary Interiors 111 (1999) 253–266
265
Table 3 Main differences between oceanic and continental rifts Mid-oceanic ridges
Continental rifts
Relatively high velocities of tectonic processes Thin homogeneous lithosphere Earthquakes are localized strictly in axial parts The leading role of transform faults in seismicity, a great number of strike-slip faults Seismicity shows no relation to the time of ridge formation, the only factors being the rate of spreading and the ratio between transform and rift parts
Relatively low velocities of tectonic processes Relatively thick inhomogeneous lithosphere The seismicity is rather scattered Seismicity linked mainly with normal faults, a lack of strike-slip faults Seismicity is determined by the evolutionary stage of rift development
intracontinental rifts ŽFig. 8.. It seems that the shape of the curve changes as well, becoming concave instead of convex.
5. Conclusions A comparative analysis of seismic and tectonic characteristics of two major kinds of divergent structures—oceanic and intracontinental rifts—is essentially qualitative in character. Nevertheless, it seems not to be absolutely useless. First, the estimation of the role of main components of MOR was given for the whole Earth. There exist regular interrelations between some principal seismic and tectonic characteristics of MOR within the whole range of the spreading rate, which mirrors the common nature of the seismic process for these geodynamic environments and the important role of thermal structure. On the other hand, for continental rifts, one notes a difference in the degree of seismic activity depending on the stage of rifting. Hence, analysis of continental seismicity requires consideration of factors of 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 regular changes of the seismic regime during development of the rift zone from the stage of incipient rift to the mature oceanic rift. In the evolutionary series wintracontinental incipient riftx – wintracontinental mature riftx – wintercontinental riftx – woceanic slow-spreading riftx – woceanic fast-spreading riftx, one can observe a gradual decrease of the role of rifts Žsensu stricto, as extension structures. and increase of the role of strike–slip
faults ŽFig. 9.. A concentration of epicenters along major faults occurs as well. In the general case, the level of seismic energy becomes lower, although the rate of deformation increases. Only for continental rifts the seismicity depends on evolutionary stage, while in oceans the age of a ridge does not matter and the main factors are spreading rate and correlation between rift and transform parts. The main facts that distinguish of continental and oceanic rifting are summarized in Table 3.
Acknowledgements We thank V.I. Keilis-Borok and A.V. Lander for their participation in discussions and A. Johnson and A.L. Petrosyan for their remarks. The work was supported by the International Center of Scientific and Technical Cooperation Žproject no. 415-96. and by the Russian Foundation for Basic Research Žproject no. 96-05-66021..
References Brune, J.N., 1968. Seismic moment, seismicity, and rate of slip along major fault zones. J. Geophys. Res. 73 Ž2., 777–784. Burr, N.C., Solomon, S.C., 1978. The relationship of source parameters of oceanic transform earthquakes to plate velocity and transform length. J. Geophys. Res. 83 ŽB3., 1193–1205. Centroid Moment Tensor Catalogue, 1977–1994, 1995. Harvard University, Dep. Earth Planet. Sci. Chorowitz, J., Le Fornier, J., Vidal, G., 1987. A model for rift development in Eastern Africa. Geological Journal 22, 495– 513, Them. Issue. DeMets, C.R., Gordon, G. et al., 1990. Current plate motions. Geophys. J. Int. 101, 425–478.
266
P.O. SoboleÕ, D.V. Rundquistr Physics of the Earth and Planetary Interiors 111 (1999) 253–266
Francis, T.J.G., 1968. The detailed seismicity of mid-oceanic ridges. Earth Planet. Sci. Lett. 4 Ž1., 39–46. Girdler, R.W., 1983. Processes of planetary rifting as seen in the rifting and break up of Africa. Tectonophysics 94, 241–252. Global Hypocenters Database, 1995. CD-ROM NEICrUSGS.II, Denver, CO. Jestin, F., Huchon, P., Gaulier, J.M., 1994. The Somalia plate and the East African rift system: present day kinematics. Geophys. J. Int. 116, 637–654. Kanamori, H., Anderson, D.L., 1975. Theoretical basis of some empirical relations in seismology. Bull. Seismol. Soc. Am. 65 Ž5., 1073–1095. Kebebe, F., Kulhanek, O., 1991. Recent seismicity of the East African Rift system and its implications. Phys. Earth Planet. Int. 68, 259–273. Pacheco, J.F., Sykes, L.R., Scholz, C.H., 1993. Nature of seismic coupling along simple plate boundaries of the subduction type. J. Geophys. Res. 98 ŽB8., 14133–14159. Romanelli, F., Panza, G.F., 1996. Effect of source depth correction on the estimation of earthquake size. Geophys. Res. Lett. 22 Ž9., 1017–1019.
Rosendahl, B.R., 1987. Architecture of continental rifts with special reference to East Africa. Annu. Rev. Earth Planet. Sci. 15, 445–503. Solomon, S.C., Burr, N.C., 1979. The relationship of source parameters of ridge-crest and transform earthquakes to the thermal structure of oceanic lithosphere. Tectonophysics 55 Ž1., 107–126. Solomon, S.C., Huang, P.Y., Meinke, L., 1988. The seismic moment budget of slowly spreading ridges. Nature 334, 58–60. Stoddard, P.R., 1992. On the relation between transform fault resistance and plate motion. J. Geophys. Res. 97 ŽB12., 17637–17650. Sykes, L., 1967. Mechanism of earthquakes and nature of faulting on the mid-oceanic ridges. J. Geophys. Res. 72, 2131–2153. Tongue, J., Maguire, P., Burton, P., 1994. An earthquake study in the Lake Baringo basin of the central Kenya Rift. Tectonophysics 236, 151–164. Yarmoluk, V.A. ŽEd.., 1984. Map of Fault Tectonics of Africa and Arabia, 1:5,000,000 scale. Moscow.
Seismicity of oceanic and continental rifts—a geodynamic approach P.O. Sobolev a
a,)
, D.V. Rundquist
b
International Institute of Earthquake Prediction Theory and Mathematical Geophysics, Russian Academy of Sciences, WarshaÕskoye sh. 79, kor. 2, Moscow 113556, Russian Federation b Vernadsky State Geological Museum, Russian Academy of Sciences, ul. MokhoÕaya 11, Moscow 103009, Russian Federation
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 ŽMOR. 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, a , 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 wintracontinental incipient riftx – wintracontinental mature riftx – wintercontinental riftx – woceanic slow-spreading riftx – woceanic fast-spreading riftx, there is a gradual decrease of the role of rifts Žsensu 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. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Seismicity; Geodynamic approach; Rifts
)
Corresponding author. Fax: q7-095-310-70-32; E-mail: [email protected]
0031-9201r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 1 - 9 2 0 1 Ž 9 8 . 0 0 1 6 5 - 4
254
P.O. SoboleÕ, D.V. Rundquistr Physics of the Earth and Planetary Interiors 111 (1999) 253–266
1. Introduction Mid-oceanic ridges ŽMOR. are the most extensive global structures. They are edges where new oceanic crust is formed and plates diverge in opposite directions. In many respects, it is an ideal object to study the relations between seismicity and tectonics due to a relatively simple geodynamic environment. Another kind of divergent boundary is the intracontinental rift, the best known of which is the African– Arabian rift system. In the present article we have obtained seismic patterns for the whole world oceanic rift system and compared them with corresponding regularities of continental rifts. The main objectives are as follows: Ž1. to estimate the ratio between seismic and aseismic components of deformation for rift and transform parts of MOR; Ž2. to define the role of these two structural types in the seismicity of MOR; Ž3. to seek relationships between seismic moment and major structural characteristics of MOR; and Ž4. to compare seismicity of oceanic and intracontinental rifts.
2. The seismicity of MOR The discovery of the global system of rifts, including MOR is one of the most significant geological discoveries of our century; investigations of MOR continue to yield important data. On the one hand, some of the most fundamental problems of plate tectonics, petrology, and ore formation are connected with the study of these structures. On the other hand, rifts are active seismic belts. The largest as well as moderate oceanic earthquakes are located along MOR. The ridges are interconnected and form major global belts. Their cumulative length is more than 60,000 km. Slow- Žsuch as the Mid-Atlantic Ridge. and fast-spreading Že.g., the East Pacific Rise. ridges are differentiated on the basis of spreading rate values. MOR have two main structural types which alternate throughout the extent of the ridge. One type includes the rift zones. The other includes the transform faults. As was shown by Sykes Ž1967., earthquakes with normal fault mechanisms occur mainly
in the rift zone, while epicenters of earthquakes with strike–slip mechanisms are located inside active parts of transforms faults. Francis Ž1968. has established difference in some seismic regime parameters between rift and transform parts of the Mid-Atlantic Ridge, namely, the largest earthquakes are located along transforms and the rift events have larger values of b Žrate of slope of linear magnitude– frequency relation.. On the other hand, some relationships between seismic and tectonic characteristics were inferred from a study of the largest transform faults ŽBurr and Solomon, 1978; Solomon and Burr, 1979.. We analyzed data on the whole global system of MOR using digitized data of Stoddard Ž1992., who summarized many results. In spite of some inaccuracies, they are suitable for a global analysis. According to this model, there are 367 transform faults and the same number of rift zones, each of them belonging to one of the fifteen interplate boundaries ŽFig. 1.. First, the NEIC catalogue was used to evaluate the seismic moment release ŽGlobal Hypocenters Database, 1995.. All earthquakes with m b ) 4 from 1964 to 1995 that lie inside an 80-km wide zone along MOR have been considered. As a whole, there are 4732 events for transforms and 4385 events for rifts. For the same area around MOR, all earthquakes from the CMT catalogue were taken Ž1002 and 644 events, respectively.. From these data the parameters of a linear regression of log M0 on m b were calculated for m b ) 5, giving the following empirical equations: log M0 s 10.18 q 1.39m b Ž transforms. , log M0 s 11.16 q 1.17m b Ž rifts . . The theoretical value of the coefficient in a similar equation for Ms is equal to 1.5, corresponding approximately to 2.4 in the case of m b ŽKanamori and Anderson, 1975.. However, as was shown by Romanelli and Panza Ž1996., the coefficient depends on tectonic environments and depths of hypocenters. Also, the smaller value of the coefficient in the case of oceanic rifts confirms the smaller size of their earthquake sources relative to the transforms sources. After converting m b to M0 , the total seismic moment, Ý M0 , was calculated both for transforms
P.O. SoboleÕ, D.V. Rundquistr Physics of the Earth and Planetary Interiors 111 (1999) 253–266
255
Fig. 1. Major tectonic plates, MOR and the African–Arabian rift system.
and rifts for each pair of plates. The same value can be evaluated using the well-known formula of Brune Ž1968. for a simple plane fault:
AÕ s
1
mt
Ý M0
Ž 1.
where m is the shear modulus Ž5 = 10 10 Nrm2 ., t is the time interval, A is the fault area, and Õ is the velocity of displacement. One can compute the relative velocity for each part of a ridge using the global plate tectonic model NUVEL-1 ŽDeMets and Gordon, 1990.. The fault area was calculated by different ways for transforms and rifts, because the respective shapes differ. For the former case, it was supposed that the area of a transform susceptible to seismic failure is confined above the 4008C isotherm ŽStoddard, 1992.. The oceanic isotherms have the form of a parabola and depend on the spreading rate ŽSolomon and Burr, 1979.. So, the transform area above a given isotherm is proportional to a function
of transform length and velocity. We assume that the area of a rift fault is rectangular and its depth was evaluated as explained by Solomon et al. Ž1988.. The ratio of the catalogue-based seismic moment release to the corresponding value calculated on the bases of theoretical formula is called a seismic coupling coefficient, a ŽPacheco et al., 1993.. Commonly, it shows that part of the strain energy is released as earthquakes and part as aseismic deformations. The results of calculations are given in Table 1. The seismic coupling is a clear difference for rifts and transforms. The coefficient is higher for transforms and decrease with the growth of the spreading rate ŽFig. 2.. Only in the ‘fastest’ ridges of the East Pacific an a values almost the same for both types. Generally, a values vary between 1 and 10%. Thus, aseismic deformation is clearly dominant for the case of rift parts of MOR; the ratio of seismic and aseismic components of transform faults depends on the spreading rate. In both cases, these results suppose a controlling role of lithospheric thermal structure into MOR seismicity.
256
Number
Sector of MOR
N
L tr Žkm.
Lri Žkm.
V Žcmr year.
Ý M0tr Ž10 16 N m.
Ý M0ri Ž10 16 N m.
a tr Ž%.
a ri Ž%.
M0tr Ž10 16 N m.
M0ri Ž10 16 N m.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
afr–ant afr–ara afr–aus afr–ind afr–nam afr–sam ant–aus ant–pac ant–sam coc–naz coc–pac nam–eur nam–pac naz–ant pac–naz
742 18 366 471 1199 1702 765 299 164 15 335 1682 404 481 474
4320 670 1390 1010 1490 5850 3460 3590 1310 830 390 2020 2870 2820 1110
4360 1560 1780 2290 3200 7520 7140 5080 720 2180 1850 5340 730 2210 4430
1.42 2.19 4.20 3.32 2.42 3.38 6.77 7.27 1.75 5.94 11.00 2.23 4.84 5.93 14.85
15,160
7300 120 2750 4040 5910 9610 6030 2000 1740 110 1240 4740 620 1350 2200
17.7
8.5 0.3 3.4 4.7 5.8 3.2 1.5 0.7 10.2 0.1 1.5 3.4 1.7 1.1 3.0
7540 870 3210 1710 3750 26,560 10,350 4860 1250 6150 1425 375 9680 6860 1960
1046 226 103 443 947 1055 50 21 127 77
3810 4850 9300 26,260 12,380 7790 10,490 1140 6630 2940 7500 2130
4.7 5.7 9.2 8.7 3.0 2.8 61.5 1.4 4.7 7.9 6.2 2.9
408 114 312
Plates: afr s African; ant s Antarctic; ara s Arabian; aus s Australian; coc sCocos; eur s Eurasian; nam s North American; naz s Nazca; pac s Pacific; sam sSouth American. N s number of earthquakes in the NEIC catalogue. Ltr s total length of transform faults. Lri s total length of MOR rift zones. V saverage spreading rate. Ý M0tr , Ý M0ri s the seismic moment sum for transform and rift zones of MOR, respectively. a tr , a ri sseismic coupling coefficient for transform and rift zones of MOR, respectively. M0tr , M0ri saccumulated seismic moment for earthquakes with strike–slip, normal and oblique faults mechanisms, respectively.
P.O. SoboleÕ, D.V. Rundquistr Physics of the Earth and Planetary Interiors 111 (1999) 253–266
Table 1 Some seismological and structural characteristics of MOR
P.O. SoboleÕ, D.V. Rundquistr Physics of the Earth and Planetary Interiors 111 (1999) 253–266
Fig. 2. Seismic coupling coefficient for the plate boundaries vs. average spreading rate.
The next stage of analysis was based on the global Harvard Centroid Moment Tensor Catalogue Ž1995. containing most events with M W ) 5.0 since 1977 to 1994 Ž M W is seismic moment magnitude.. The important advantage of this catalogue is that earthquakes can be discriminated on the basis of their mechanisms and, therefore, there is an opportunity to evaluate the contribution of each kind of fault into MOR seismicity. Previous estimates were based on the geographical location of the epicenters only, but there were great uncertainties near the intersections of rifts and transforms. It is possible to determine the type of earthquake using the value of rake Žthe angle between slip vector and horizontal line measured on the fault plane., because almost all focal plane solutions have dip angles more than 308. There are four kinds of mechanisms: normal faults Žy120 F rake F y60., thrusts Ž60 F rake F 120., strike–slip faults Žangles that lie within 308 of 180 or y180. and oblique ones. Most earthquakes belong either to normal faults or strike–slip faults, while earthquakes with oblique mechanisms and thrusts are a minority. The distribution of epicenters shows that, usually normal-fault earthquakes are confined to rifts, and strike–slip earthquakes occur near transform
257
faults. Earthquakes with oblique mechanisms occur much more rarely, and their position with relation to the basic elements of the MOR is rather irregular. Therefore, the earthquakes with strike–slip mechanisms can be used for the estimation of the seismic moment of transforms and, correspondingly, the seismic moment of rifts can be calculated using data for normal faults. The results are given in Table 1. It is clearly seen that the values of seismic moment release on transforms are higher than on rifts. It should be noticed that the energy contribution of transforms increases with the growth of spreading rate, even though their length is smaller. We tried to outline some relationships which connect Ý M0 with some structural factors, namely fault length and spreading rate. The dependence of the seismic moment release on the total length of faults is shown in Fig. 3. A power relationship should be expected between these parameters from theoretical reasoning about proportionality of M0 to fault area and the evaluation of this area on the basis of Eq. Ž1.. However, the real curve goes lower and is approximated rather by an exponential function. There is no clear-cut dependence of Ý M0 on length for rift zones. Evidently, the leading factor is the spreading rate in this case. A quite reliable logarithmic relation between Ý M0 and V is seen there, except for one
Fig. 3. Total seismic moment release for strike–slip faulting earthquakes vs. the total length of transform faults.
258
P.O. SoboleÕ, D.V. Rundquistr Physics of the Earth and Planetary Interiors 111 (1999) 253–266
Fig. 4. Total seismic moment release for the normal faulting earthquakes vs. average spreading rate.
point of the Pacific–Nazca boundary with an extremely high spreading rate ŽFig. 4.. The following conclusions can be drawn based on our analysis of global patterns for MOR. Ž1. The decisive role in MOR seismicity belongs to transform faults. The energy contribution of transform faults is one to two orders higher than that of rifts and increases with the increasing spreading rate; Ž2. Seismic coupling coefficient a varies from 1 to 10% for most of MOR, and it is invariably higher for the transform faults; Ž3. Relations between the seismic moment release, fault length, and spreading rate are different for transform and rift parts of MOR; this confirms the difference in shape and origin between their earthquake sources. Anyway, in both cases, the major factor of the MOR seismicity is the thermal structure of the lithosphere.
3. The seismicity of the African–Arabian rift system The African–Arabian rift system is an example of a continental rift. Notions of the structure and evolution of these rifts have changed considerably as a
result of complex geological and geophysical researches carried out in recent years Že.g., Chorowitz et al., 1987; Rosendahl, 1987.. It was found that the degree of evolution of riftogenic structures steadily decreases from north to south, which is explained by the different times of their formation. The African– Arabian rift system consists of a series of structural zones crossing the entire African continent. The triple junction of the Red Sea, Aden, and the Ethiopian rifts is located in the north. The so-called Eastern branch, integrating the Ethiopian and Kenyan rifts, continues further to the south. The Western branch, located along the chain of African lakes, is crossed by the Azva transversal zone in the north and the Tanganyika–Rukva–Malawi fault zone in the south. The Malawi Rift is located further south. The Kerimba fault zone is situated on the southern most part of the continent. All structures have different relief, type and character of faults, and deep structure. This is explained by the different times of origin of Cenozoic rifts and, hence, by the different degrees of their evolution ŽChorowitz et al., 1987.. The asymmetry of rift structures is revealed in the development of systems of subsidiary dislocations on the western side. The substantial influence of preCenozoic ŽPrecambrian and Permian–Triassic. rift structures and their confinement to zones between the most ancient granite–gneiss cores of Precambrian crust is common to all modern rifts ŽChorowitz et al., 1987.. Seven sectors were distinguished within the East African rift system Žboundaries of rifts and faults are from the Map of Fault Tectonics of Africa and Arabia by Yarmoluk, 1984. and a comparative evaluation of the seismicity was carried out ŽFig. 5.. The characteristics of these sectors are given in Table 2. Despite the uncertainty of the boundaries of these sectors, one can see that they differ rather sharply in all major features of geological structure and reflect the different stages in the evolution of a rift zone. Gradual thinning of the continental crust, accompanied by an increase of heat flow, expansion of the area of grabens, uplift of near-slope parts, and increasing subsidence of the graben floor, occur during the process of rifting. In the later stages, the oceanic lithosphere is eventually formed and a mid-oceanic ridge comes into existence. This process can be observed in the Gulf of Aden. In this general series,
P.O. SoboleÕ, D.V. Rundquistr Physics of the Earth and Planetary Interiors 111 (1999) 253–266
259
Fig. 5. Rift valleys, major faults, and epicenters of earthquakes with m b G 4.0 Ž1964–1995.. Numbers correspond to numbers of sectors in Table 2.
260
Table 2 General characteristics of the areas of the African–Arabian Rift system Area
Time of the beginning of rifting
Neogene–quaternary magmatism
Deep structure
Structural characteristics
q
L
S
N
a
b
1
Aden
Eocene– Oligocene
Entire oceanic crust, mantle on the depth of 5–8 km
6–7
1060
502
9.97
1.48
Red Sea
Eocene– Oligocene
more than 250
5–7
570
271
6.96
0.98
3
East Rift
Late Oligocene
Escarpments, marginal depressions, clearly expressed mid-ocean ridge Escarpments, marginal depressions, the mid-ocean ridge is in the southern part only Broad dome uplifts, wide continuous graben
more than 150
2
more than 100 –150
3–5
230
81
8.73
1.42
4
West Rift
Early Miocene
135
319
8.18
1.19
5
Malawi
Middle–Late Miocene
85
171
8.01
1.24
2.1
Initial rift
6
Kerimba
Late Miocene
Active volcanism in the axial zone —tholeiitic basalts, also occurrences in the marginal parts Active modern volcanism in the axial graben –Tholeiitic basalts, occurrences of alkaline and subalkaline basalts in the marginal parts Broad fields of the volcanic rocks of basalt–trahite association, carbonatites Occurrences of the volcanic rocks of basalt– trahite–phonolite association, carbonatites Single ring basic– ultrabasic alkaline intrusions Absent Ž?.
9.69
1.63
3.2
Initial rift
7
Limpopo
The present timeŽ?.
Absent Ž?.
N s number of earthquakes. M0 s total seismic moment release Ž10 18 N m.. Ssarea of rifts Ž1000 km2 .. Ls total displacement in normal faults Žkm.. q saverage heat flow in the axial part ŽmWrm2 .. a,bs coefficients of the equation log N s ay bm b .
The parts of oceanic crust, forming a ‘chain’
Strongly thinned continental crust Thinned continental crust
Narrow half-dome uplifts, intermittent system of grabens
60–75 3–4
Normal continental crust Normal continental crust Normal continental crust
System of grabens
50–75 4
Separate grabens
60
?
5
77
Faults
50–60 ?
0
224
11.19 1.81
M0
8.9
19
3.0
37
Stage of rifting Oceanic
Initial oceanic
Mature rift
Typical rift
0.63 Pre-rift Ž?.
P.O. SoboleÕ, D.V. Rundquistr Physics of the Earth and Planetary Interiors 111 (1999) 253–266
Number
P.O. SoboleÕ, D.V. Rundquistr Physics of the Earth and Planetary Interiors 111 (1999) 253–266
Fig. 6. Histogram of rake value distribution for the earthquakes in the African–Arabian rift system Ža. and for all MOR Žb..
261
262
P.O. SoboleÕ, D.V. Rundquistr Physics of the Earth and Planetary Interiors 111 (1999) 253–266
magmatism changes regularly from basic complexes and carbonatites to alkaline and to tholeiitic basalts. Shallow earthquakes, with m b - 6, dominate in the area. Their epicenters are confined more often to faults with either a northwestern or northeastern strike and are located within the rift valleys or subsidiary faults, which become more numerous to the south. An analysis of focal mechanisms Žthe Harvard catalogue for the 1977–1994 period was used. shows that normal faults prevail in Africa. Most strike–slip movements are related to faults of the Aden and Red Sea rifts, and they are almost lacking inside the continent. This is explained by the prevalence of tension in the northwest direction approximately perpendicular to the strike of most riftbounding faults ŽKebebe and Kulhanek, 1991.. Types of mechanisms can be identified by the rake value. The diagram of distribution of this parameter shows a clear prevalence of normal faults ŽFig. 6., which make up more than half the total number of solutions and includes all large earthquakes. Diffuse seismicity prevails at the earlier stages of rifting, while almost all epicenters of earthquakes are located within rift troughs at the later stages. A
decrease in the b-value Žin Gutenberg–Richter’s relation. is evident: sectors where rifting begins are characterized by the greatest values of b. The smallest values of b and, hence, the greatest concentrations of stress are in the Red Sea and the Western Rift. In contrast, the Eastern Rift is characterized by a great number of smaller earthquakes; as corroborated by data from local seismic studies ŽTongue et al., 1994.. Earthquakes from the NEIC catalogue Ž m b G 4.0, 1964–1995. were used to calculate the seismic moment release. The Western Rift has the maximum density of released seismic energy. Most of the energy is released on subsidiary faults of the main rift valleys. Since the area of grabens reflects the degree of rifting, one can try to estimate very roughly the character of seismicity evolution during the geological development of intracontinental rifts ŽFig. 7.. It is likely that initially, seismicity increases quickly, reaches a maximum at the stage of a typical rift Žthe Western Rift., and then gradually decreases. An increase in the role of earthquakes located within the rift valleys is typical, suggesting the localization of epicenters in precisely these structures.
Fig. 7. The total seismic moment release vs. total area of grabens for the African–Arabian rift system.
P.O. SoboleÕ, D.V. Rundquistr Physics of the Earth and Planetary Interiors 111 (1999) 253–266
The most pronounced differences in seismic regimes are typical of the two important zones— Western and Eastern branches of the East African rift system. The geological structure zones strongly differ. The crust under the Kenyan and Ethiopian rifts ŽEastern branch. is much thinner. Based on geophysical data ŽGirdler, 1983., the lithosphere of this zone is characterized by increased plasticity. This is corroborated by the existence of large, active modern volcanism and increased heat flow. In addition, this sector has the maximum total length of rifts ŽTable 2.. All of these facts testify to a relatively high rate of deformation within the Eastern branch and the prevalence of the aseismic component. The situation in the Western branch is different. Here the crust is insignificantly thinned under rift valleys, volcanism is almost absent, and numerous faults deviate laterally from the rifts. It is likely that the lithosphere here is more fragile, and the deformations are accompanied by the release of seismic energy in the form of larger earthquakes. Evidently, the distinction between these two branches is mainly caused by their different positions. The Eastern branch represents the continuation of an oceanic rift inside the continent, whereas the Western branch can serve as an example of an ‘actual’ intracontinental rift.
4. Discussion: comparison of the seismicity of oceanic and continental rifts Continental and oceanic rifts are different stages of one prolonged tectonic process of rifting. This process begins with incipient rifting inside a continent, and then the crust transforms from continental to oceanic crust. A new oceanic lithosphere is born of the transformation complete but it can stop at any stage. Taking the African–Arabian rift system as an example, one can see almost complete evolutionary series of this process and it is probable that modern oceanic rifts Žmid-oceanic ridges, MOR. have gone through all these stages in the geological past. Hence, one of the major that links both kinds of rifts is their common origin. They reflect the evolution of divergent boundaries in time. However, there are some principal distinctions that cause their different geodynamic Žand so, seismic. patterns.
263
First of all, oceanic rifts are much more widespread than are continental ones. There are only few regions with active continental rifting, the larger of these being the African–Arabian, Baikal, RioGrande, and Rhein rift systems. The extent of MOR is many times larger. Although both kinds of rifts are divergent boundaries, in the case of oceans, the spreading rate is higher by at least an order of magnitude. Another principal difference consists in an absolutely different deep structure of continents and oceans. The lithosphere differs in composition, internal structure, and thickness. Also the thermal structure Žwhich strongly affects the elasticity properties. of the lithosphere is quite different for typical oceanic and continental rifts. The thickness of the brittle layer in oceans near the ridge is no more than 5–10 km, whereas for continents, it reaches 20–40 km. All these patterns are reflected in the seismicity of rifts. The general tendency in the distribution of epicenters is the following: their concentration increases from earlier stages of rifting to later ones. It is clearly seen in the case of Africa. During incipient rifting in South Africa the seismicity is very scattered, there is no preferred orientation of active faults. At later stages ŽWestern and Eastern Rifts. almost all epicenters are located within rift troughs and on the border faults. When a continental rift turns into an oceanic rift, all seismicity is confined to MOR. Lastly, one can notice that most epicenters are located along transform faults during the transition from slow-spreading ridges to fast-spreading ridge. Another tendency is the increasing role of transverse faults during rifting. It was shown above for African rifts that the most active zones are normal faults and the largest seismic events occur there. There are some large zones of transverse faults as well, but their seismicity is many times less, and as a whole, strike–slip earthquakes are less frequent than normal faults. A different picture is in the oceans. Here, the greater part of the seismic moment release occurs on transform faults and their relative contribution increases with the spreading rate. It is clearly seen on the summary histogram of the rake value for the two kinds of rifts ŽFig. 6.. Probably, it is worth estimating the most general characteristics of seismicity. The first of these is the total seismic moment release. The NEIC catalogue
264
P.O. SoboleÕ, D.V. Rundquistr Physics of the Earth and Planetary Interiors 111 (1999) 253–266
Fig. 8. The magnitude–frequency relations for the interplate earthquakes. Ž1. All oceanic rift zones; Ž2. all oceanic transform zones; Ž3. Red Sea and Aden rifts; Ž4. African rifts.
Ž m b G 4.0. for the period 1964–1995 was used to calculate seismic moments and their sum for transform and rift zones of MOR, and for the African– Arabian rift system. The major difficulty for direct
comparison is that the seismicity of MOR has a practically linear distribution, whereas a seismicity spreads over some area in the case of continental rifts. Therefore, the density of seismic moment release was used Žthe ratio of Ý M0 to the total length of faults.; beside the African rifts, the perimeter of rift valleys was used as the denominator. The calculation of this quantity Župper and lower values, which were calculated with and without consideration of crest–transform intersections. gave the following results: 3.33 and 1.73 = 10 13 N for all transforms, 0.99 and 0.33 = 10 13 N for the rifts of MOR, and 0.20 = 10 13 N for the whole Africa–Arabian rift system Žthe values are very close for its ‘oceanic’ and ‘continental’ part.. Hence, the order of released seismic energy is approximately the same for continental rifts and MOR crest zone, but it is higher for the transform faults. It should be noted that these values generally decrease with the spreading rate for both kinds of MOR. It is notable that the values of the specific seismic moment are very close for continental and oceanic rifts, although the velocities of spreading are quite different, being several centimeters per year at oceanic rifts and no more than 0.4 cmryear at continental rifts ŽJestin et al., 1994.. The magnitude–frequency relation reflects regular variations of seismic regime during the process of rifting as well. The b-value increases in the transformation series—oceanic rift zones–transitional rifts–
Fig. 9. The evolutionary series of rifts.
P.O. SoboleÕ, D.V. Rundquistr Physics of the Earth and Planetary Interiors 111 (1999) 253–266
265
Table 3 Main differences between oceanic and continental rifts Mid-oceanic ridges
Continental rifts
Relatively high velocities of tectonic processes Thin homogeneous lithosphere Earthquakes are localized strictly in axial parts The leading role of transform faults in seismicity, a great number of strike-slip faults Seismicity shows no relation to the time of ridge formation, the only factors being the rate of spreading and the ratio between transform and rift parts
Relatively low velocities of tectonic processes Relatively thick inhomogeneous lithosphere The seismicity is rather scattered Seismicity linked mainly with normal faults, a lack of strike-slip faults Seismicity is determined by the evolutionary stage of rift development
intracontinental rifts ŽFig. 8.. It seems that the shape of the curve changes as well, becoming concave instead of convex.
5. Conclusions A comparative analysis of seismic and tectonic characteristics of two major kinds of divergent structures—oceanic and intracontinental rifts—is essentially qualitative in character. Nevertheless, it seems not to be absolutely useless. First, the estimation of the role of main components of MOR was given for the whole Earth. There exist regular interrelations between some principal seismic and tectonic characteristics of MOR within the whole range of the spreading rate, which mirrors the common nature of the seismic process for these geodynamic environments and the important role of thermal structure. On the other hand, for continental rifts, one notes a difference in the degree of seismic activity depending on the stage of rifting. Hence, analysis of continental seismicity requires consideration of factors of 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 regular changes of the seismic regime during development of the rift zone from the stage of incipient rift to the mature oceanic rift. In the evolutionary series wintracontinental incipient riftx – wintracontinental mature riftx – wintercontinental riftx – woceanic slow-spreading riftx – woceanic fast-spreading riftx, one can observe a gradual decrease of the role of rifts Žsensu stricto, as extension structures. and increase of the role of strike–slip
faults ŽFig. 9.. A concentration of epicenters along major faults occurs as well. In the general case, the level of seismic energy becomes lower, although the rate of deformation increases. Only for continental rifts the seismicity depends on evolutionary stage, while in oceans the age of a ridge does not matter and the main factors are spreading rate and correlation between rift and transform parts. The main facts that distinguish of continental and oceanic rifting are summarized in Table 3.
Acknowledgements We thank V.I. Keilis-Borok and A.V. Lander for their participation in discussions and A. Johnson and A.L. Petrosyan for their remarks. The work was supported by the International Center of Scientific and Technical Cooperation Žproject no. 415-96. and by the Russian Foundation for Basic Research Žproject no. 96-05-66021..
References Brune, J.N., 1968. Seismic moment, seismicity, and rate of slip along major fault zones. J. Geophys. Res. 73 Ž2., 777–784. Burr, N.C., Solomon, S.C., 1978. The relationship of source parameters of oceanic transform earthquakes to plate velocity and transform length. J. Geophys. Res. 83 ŽB3., 1193–1205. Centroid Moment Tensor Catalogue, 1977–1994, 1995. Harvard University, Dep. Earth Planet. Sci. Chorowitz, J., Le Fornier, J., Vidal, G., 1987. A model for rift development in Eastern Africa. Geological Journal 22, 495– 513, Them. Issue. DeMets, C.R., Gordon, G. et al., 1990. Current plate motions. Geophys. J. Int. 101, 425–478.
266
P.O. SoboleÕ, D.V. Rundquistr Physics of the Earth and Planetary Interiors 111 (1999) 253–266
Francis, T.J.G., 1968. The detailed seismicity of mid-oceanic ridges. Earth Planet. Sci. Lett. 4 Ž1., 39–46. Girdler, R.W., 1983. Processes of planetary rifting as seen in the rifting and break up of Africa. Tectonophysics 94, 241–252. Global Hypocenters Database, 1995. CD-ROM NEICrUSGS.II, Denver, CO. Jestin, F., Huchon, P., Gaulier, J.M., 1994. The Somalia plate and the East African rift system: present day kinematics. Geophys. J. Int. 116, 637–654. Kanamori, H., Anderson, D.L., 1975. Theoretical basis of some empirical relations in seismology. Bull. Seismol. Soc. Am. 65 Ž5., 1073–1095. Kebebe, F., Kulhanek, O., 1991. Recent seismicity of the East African Rift system and its implications. Phys. Earth Planet. Int. 68, 259–273. Pacheco, J.F., Sykes, L.R., Scholz, C.H., 1993. Nature of seismic coupling along simple plate boundaries of the subduction type. J. Geophys. Res. 98 ŽB8., 14133–14159. Romanelli, F., Panza, G.F., 1996. Effect of source depth correction on the estimation of earthquake size. Geophys. Res. Lett. 22 Ž9., 1017–1019.
Rosendahl, B.R., 1987. Architecture of continental rifts with special reference to East Africa. Annu. Rev. Earth Planet. Sci. 15, 445–503. Solomon, S.C., Burr, N.C., 1979. The relationship of source parameters of ridge-crest and transform earthquakes to the thermal structure of oceanic lithosphere. Tectonophysics 55 Ž1., 107–126. Solomon, S.C., Huang, P.Y., Meinke, L., 1988. The seismic moment budget of slowly spreading ridges. Nature 334, 58–60. Stoddard, P.R., 1992. On the relation between transform fault resistance and plate motion. J. Geophys. Res. 97 ŽB12., 17637–17650. Sykes, L., 1967. Mechanism of earthquakes and nature of faulting on the mid-oceanic ridges. J. Geophys. Res. 72, 2131–2153. Tongue, J., Maguire, P., Burton, P., 1994. An earthquake study in the Lake Baringo basin of the central Kenya Rift. Tectonophysics 236, 151–164. Yarmoluk, V.A. ŽEd.., 1984. Map of Fault Tectonics of Africa and Arabia, 1:5,000,000 scale. Moscow.