The Earthquake Generating Stresses in the Western Rift Valley of Africa

J. Phys. Earth, 28, 45-57, 1980

THE EARTHQUAKE GENERATING STRESSES IN THE WESTERN RIFT VALLEY OF AFRICA

Kazuo TANAKA,* Shigeki HORIUCHI,** Toshiya SATO,** and Ndontoni ZANA***

*Department of Earth Sciences, University of Hirosaki, Hirosaki, Japan ** Akita Geophysical Observatory , Tohoku University, Akita, Japan *** Centre de Geophysique, Institut de Recherche Scientifique, Bukavu, Zaire (Received June 16, 1978; Revised October 26, 1979)

The earthquake generating stress fields of the Western Rift Valley of Africa are discussed in detail from the data obtained by the seismological network of IRS, Zaire, and by the seismological stations in Africa. The present analysis is based on the method of composite mechanism solution using the initial motions of P-waves from earthquakes which occurred during the period from 1958 to 1970. The stress fields are analyzed for ten sub-regions divided on the ground of the characteristics of seismicity. Quite stable solutions are obtained for the sub-regions of Lake Kivu and its vicinities, which are located in the central part of the Western Rift Valley, and shows that the stress fields are of the normal faulting type. The direction of tension axis of each solution is generally perpendicular to the rift system or to the local faults. The per- pendicularity is consistent with the results reported by several authors studying focal mechanisms of earthquakes occurring in the African Rifts and the ocean ridges. In contrast with this general conclusion, the strike-slip type of focal mechanism prevails in sub-regions of the southern part of the Western Rift Valley, and a heterogeneous stress field is suggested in the sub-region of the active volcanoes of Nyiragongo and Nyamuragira.

1. Introduction

The Rift Valleys of Africa form a part of the world-wide fracture zone which extends via the Red Sea and Gulf of Aden through the Indian Ocean to link up with the mid-ocean ridges (GIRDLER,1964). The seismicity either in the Rift Valleys or in the oceanic ridges is characterized by its activity being confined to shallow depths. However, the seismic belt within which earthquakes occur is narrow under the mid-oceanic ridges, while it is rather broad in the African Rifts. SYKES(1970) has suggested that these differences in areal distribution of earthquakes may be explained by one or more of the following hypotheses: (1) the pattern of failure is more complex in continental areas than in oceanic areas, perhaps because continents are older and contain many zones of weakness; (2) initial breakup of Africa is in an early stage south of the equator, and the initial pattern of breakup is normally complex and multibranched. The general

45 46 K. TANAKA, S. HORIUCHI, T. SATO, and N. ZANA seismicity of the African Rifts has been studied by many investigators (SYKESand LANDISMAN,1964; WOHLENBERG,1968, 1969; FAIRHEADand GIRDLER,1971, 1972; RYKOUNOV et al., 1972; BÅTH, 1975; ZANA, 1976). The local seismicity in the region of the Western Rift Valley has been investigated by SUTTONand BERG (1958), DE BREMAECKER(1959), LAHRand POMEROY(1970), BRAM(1972), MAASHA (1975 a, b), SATOet al. (1975), and ZANA(1977). The topography of the Western Rift Valley is characterized by a sequence of the Lakes of Mobutu, Amin, Kivu, Tanganyika, and Rukwa that are located from 2°N to 9°S in the form of an arched chain . The seismicity in the Western Rift appears to be more active and more closely related to the rift system in com- parison with the Eastern Rift (FAIRHEADand GIRDLER,1971). DE BREMAECKER (1959) studied the seismicity of the Western Rift by determining the epicenters of all the earthquakes whose magnitudes were greater than 2, and reported that most epicenters were located along the faults, which run along the east and west sides of the chain of lakes, or along their continuations. The results of focal mechanism of earthquakes occurring in and near the African continent, including the regions of the Red Sea and the Gulf of Aden , were summarized by FAIRHEADand GIRDLER(1971) . Additional results were supplemented by MAASHAand MOLNAR(1972). FAIRHEADand GIRDLERhave shown that the orientation of the tension axis is horizontal and perpendicular to the strike of the rift system. Most of mechanism solutions of earthquakes in and near the Red Sea and the Gulf of Aden are of the strike-slip type, while those i n the continent are of the normal faulting type. Among large earthquakes of which focal mechanisms were obtained , only two events were located in the Western Rift . They are the earthquake (M=6.7- 7.0 by IRS) of Ruwenzori area in 1966 and its aftershock (M=61/ 4-61/2). SYKES(196 7), and BANGHARand SYKES(1969) studied these earthquakes to find that the mechanism solutions are of the normal faulting type, the orientation of the tension axis being perpendicular to the rift system. Their solutions are similar to the mechanism solutions of earthquakes under the median of the oceanic ridges. Although both the rift valleys and the oceanic ridges are fracture zones ,th ere are some differences in seismicity, crustal structure , gravity field or magnetic field between them. A few seismological studies have been carried out on the stress system of the Western Rift Valley. These studies on stress systems give us important informations on the formation of rift valleys. It is very important to study further in detail the tectonic stress field of the western rift system by means of composite mechanism solutions of small earthquakes . KOYAMAet al. (1973), IZUTANIet al. (1975) and HORIUCHIet al. (1976) discussed the tectonic stress fields in the northeastern Japan arc , the Izu-Bonin-Mariana arc, and the Kuril-Kamchatka arc, respectively. They obtained the regional stress fields by the method of composite mechanism solution introduced originally by RITSEMA(1955) and developed by AKI (1966). This paper presents a detailed study of the earthquake generating stress fields in the seismic zone of the Western The Earthquake in the Western Rift Valley of Africa 47

Rift Valley of Africa by using the computer program of HORIUCHI et al. (1972) for the mechanism solution.

2. Method of Analysisand Data Used

2.1 Method The procedure of focal mechanism determination adopted by us is as follows. The initial motions of P-waves from earthquakes occurring in a region are super- posed on a focal sphere. The tension (or pressure) axis is fixed at each of 181 lattice points taken at 10° interval on the focal hemisphere, and the minimum number of observed data inconsistent with the theory is found by rotating the pressure (or tension) axis around the tension (or pressure) axis under the orthogo- nality condition of the two axes. The minimum inconsistent number, which we call error, is plotted on each lattice point of the fixed axis. An area of the domain occupied by a small value of error expresses a reliability of the determined axis. The force system of type II by HONDA(1962) is presumed in this analysis.

2.2 Data The seismological observations of earthquakes occurring in East Africa have been carried out since 1953 by L'Institut de Recherche Scientifique du Zaire (IRS), formerly L'Institut pour 1a Recherche Scientifique en Afrique Centrale (IRSAC). The hypocenter determination was made for earthquakes whose magnitudes were greater than 3. The hypocenters determined and reported in the period from 1958 to 1970 are used in our analysis. For the period from July 1966 to December 1967, however, we use the hypocentral data given by the National Oceanic and Atmos- pheric Administration (NOAA), U.S.A., instead of those by IRS, since the latter has published no report on the hypocentral data for the above period. The seis-

Table 1. Seismological stations in Africa, of which data are used. Numerals correspond to ones in Fig. 1. 48 K. TANAKA, S. HORIUCHI, T. SATO, and N. ZANA mological stations in Africa whose initial P-wave motions are used are listed in Table 1 and illustrated in Fig. 1. The observational data of P-wave initial motions are obtained from the seismological bulletin of each of the above stations. Earth- quake Data Report of NOAA and Seismological Bulletin of the International Seismological Center (ISC), Great Britain. The data reported by IRS are re- examined by reading seismograms recorded at the station network and a number of additional data are made available.

Fig. 2. P-wave velocity models of the crust and the mantle of African Con- tinent. Model I is presented by BONJERet al. (1970), II is by GUMPER and POMEROY(1970), III is by MUEL- LERand BONJER(1973) and IV is a- dopted in this analysis.

Fig. 1. Seismological stations in Africa, of which data are used. Numerals attached to the circles correspond to ones in Table 1. Seismicity of square enclosed by thick solid lines is shown in Fig. 3.

It must be noticed that most of the aftershocks of the strong earthquake of 1960 in Uvira area and that of 1966 in Ruwenzori area are excluded from the analysis. We are interested in obtaining the average field of tectonic stress rather than the focal mechanism of the aftershocks. The crustal structure in the African continent and the Rift Valley region has been investigated by several workers. DOPP (1964) obtained P-wave velocity in the Western Rift Valley from the observed travel time curves along the rift system near LWI. The models of crustal structure were presented by many authors: e.g., by BONJERet al. (1970), shown as model I in Fig. 2, from the crustal response due to long-period body waves at the stations AAE, NAI, and LWI; by GUMPER and POMEROY(1970), model II in Fig. 2, from phase and group velocities of seismic surface waves as well as the travel time data of short period P-waves for the African continent; and by LONGet al. (1973) from the travel time data of S-waves for the The Earthquake in the Western Rift Valley of Africa 49

East African Rift zone. MUELLERand BONJER(1973) have also derived an average crustal-mantle model (model III in Fig. 2) of the African continent from P-wave travel-time anomalies and surface wave dispersion. We derive a model, in which the crust and the mantle, respectively, have continuous functions of P-wave velocity with respect to depth, so as to satisfy approximately an average structure of models I, II, and III. The model, denoted by model IV in Fig. 2, is used to determine the angle of incidence of the ray at the focus for the first arrival waves. In the determination of hypocenters by IRS the JEFFREYS-BULLENtables (1948) were used at earlier periods (WOHLENBERG,1969). Later, the tables were modified by IRS, and the revised tables have been used up to now. The IRS determines the epicenters of earthquakes by assuming the focal depths as 15km except for nearby earthquakes. The angle of incidence calculated from the hypocentral parameters given by the IRS depends upon this assumption. However, it does not seem to affect the observations at greater distances to any serious extent on the focal mechanism determination. Composite focal mechanism solutions are not sensitive to changes in the velocity gradient or thickness of the upper layer in this case.

3. Result

It is interesting to know whether the stress fields reflect the directions of local fault traces or not. The seismic zone is divided into ten sub-regions A to J in alphabetical order (Fig. 3). This division is done in part by considering the seismic areas defined by De Bremaecker and the seismic zones by Bram. Each sub-region is selected or defined to exclude fault traces of different directions, and to include more than 50 observation data, except in sub-region J where the number of data is less than 50. The earthquake generating stress field in each sub-region is examined from the domains of permissible variation of pressure and tension axes. The domains are shown by two contours of errors that are larger by 10 and 20 percents than the error of the best fit solution, The reliabilities of the axes are graphically expressed by the areas of the domains in Fig. 5. The lower hemisphere of the focal sphere is projected by the equal area projection.

3.1 Sub-region A (which is almost identical with the Ruwenzori area by DE BREMAECKER(1959)) It includes the Ruwenzori Mountains consisting of pre-cambrian rocks (CAHEN, 1954). This sub-region is one of the most seismically active areas in the Western Rift Valley. A big earthquake (M=6.7-7.0 by IRS) which was accompanied with many foreshocks and aftershocks took place near the mountains in 1966. Three hundreds of P-wave initial motion data are used in the analysis on this sub-region. The obtained results of the earthquake generating stress field are illustrated in Fig. 5(A). The pressure axis is in the direction of northwest to southeast, but its plunge angle cannot be determined definitely. On the other 50 K. TANAKA, S. HORIUCHI, T. SATO, and N. ZANA

Fig. 3. Distribution of epicenters in the Rift Valley of Africa determined by IRS for the period 1960 to 1970 except for July 1966 to December 1967. Earthquake generating stress fields are obtained for the ten sub-regions from A to J. hand, the tension axis is nearly horizontal in the direction of N45°E. 3.2 Sub-regionB (the Lake Amin area) Lake Amin (Edward) and the escarpment on both sides of the rift valley are includedin this sub-region. As DEBREMAECKER noted, earthquakes are mostly on the west side of the lake. The results of calculation illustrated in Fig. 5(B) indicate that the pressure axis is nearly vertical and the tension axis is horizontal in the direction of northwest to southeast. The focal mechanismis characterized by normal faulting in this sub-region.

3.3 Sub-region C (the area southwest of Lake Amin) The obtained results, as illustrated in Fig. 5(C), show that the domain of existence of the pressure axis disperses into three. The tension axis is determined to be horizontal in the direction of east to west.

3.4 Sub-region D (the Virunga Volcanoes area) Active volcanoes, Nyiragongo and Nyamuragira, are located on the southern border of this sub-region. The seismic activity in the eastern area of the volcanoes was significantly high in 1957, showing a swarm type activity. The results of calculation show that either pressure or tension axis has two separated domains. The stress field being not definite, the pressure axis is nearly horizontal in the The Earthquake in the Western Rift Valley of Africa 51

Fig. 4. Fault map of the Western Rift Valley. Open circles denote the seismological stations of IRS.

direction of northeast to southwest, and the tension axis is nearly vertical. This suggests a focal mechanism of the reverse fault type. This sub-region differs in fault type from the others. 52 K. TANAKA, S. HORIUCHI, T. SATO, and N. ZANA

Fig. 5 The Earthquake in the Western Rift Valley of Africa 53

3.5 Sub-region E (the Masisi area) Masisi is located northwest of Lake Kivu. The directions of the fault traces found in this sub-region are southeast to northwest (Fig. 4). The obtained results clearly indicate that the average focal mechanism is normal faulting with the tension axis perpendicular to the strikes of fault traces.

3.6 Sub-region F (the Gweshe area) Gweshe is located southwest of Lake Kivu. Several fault traces with a strike in the direction of southwest to northeast are observed in this sub-region. The orientation of the pressure axis is nearly vertical and that of the tension axis is horizontal. Although the trend of the tension axis is not definitely determined, the stress field evidently indicates normal faulting type. The direction of southeast to northwest, which is perpendicular to the fault traces, is a reasonable esti- mation for the trend of tension axis in this area.

3.7 Sub-region G (the Lake Kivu area) This sub-region includes the northern part of the Ruzizi plain in DE BREMAECKER'spaper. The hypocenters in this sub-region are determined with an accuracy higher than those in others, since the focal regions is within the station net of observation. There are a few strong earthquakes in the sub-region. Most of the surface traces of faults strike about north to south. The results in Fig. 5(G) indicate that the pressure axis is nearly vertical and the tension axis is horizontal in the east to west direction. The stress fields of normal faulting is clearly found in this sub-region as in sub-regions E and F, though there are distinct differences in the trend of tension axis among the three sub-regions.

3.8 Sub-region H (Uvira-Usumbra area) This sub-region includes the southern part of the Ruzizi plain. Three earthquakes (M=5.5 in 1954, 5.0 in 1956, 61/4-61/2in 1960) were followed by many aftershocks. In the case of the latter two events foreshock activities were also observed. The surface traces of faults run from north to south along both sides of Lake Tanganyika. As seen in Fig. 5(H), the earthquake generating stress field appears to be normal faulting, although the domains of both axes are wide in area. The tension axis directs from northeast to southwest.

3.9 Sub-region I (Lake Tanganyika between Uvira and Kalemie, and the southern border of Burundi and the Luama plain) Many earthquakes of larger magnitude took place in this sub-region. The

Fig. 5. Composite mechanism solutions for the sub-regions defined in Fig. 1. The right and left figures in each sub-region show the domains of pressure and tension axes, respectively. Numerals 1 and 2 indicate the values 10 and 20 percents larger than the error from the best fit solution. Numerals below each figure indicate the number of the observed data and the number of the inconsistent stations in the best fit solutions. 54 K. TANAKA, S. HORIUCHI, T. SATO, and N. ZANA

directions of both axes are poorly determined mainly because of the poor accuracy in hypocenter determination (Fig. 5(I)). The stress field may be of the strike-slip faulting type.

3.10 Sub-region J There are three seismically active zones in the southwest of Lake Tanganyika according to BRAM(1972), the zone of the Upemba Graben, the zone of Lake Moero and Luapula, and the zone of Katanga Meridional and Western Zambia. The first two of the three zones are included in this sub-region. The results show that the pressure axis is nearly horizontal, while the tension axis is also nearly horizontal in the direction of northwest to southeast. The stress field seems to difine a focal mechanism with considerable strike-slip component. Since the trends of the tectonic lines in the Upemba Graben and Lake Moero are northeast to southwest, the tension axis is perpendicular to the tectonic lines.

4. Conclusion and Discussion

The earthquake generating stress field in the Western Rift Valley of Africa was studied in detail from the initial motions of P-waves by the method of composite mechanism solution. The general conclusion obtained is that the focal mechanism of normal faulting prevails widely in the Western Rift Valley, and that the directions of tension axes are perpendicular to the strike of the rift system.

Fig. 6. Map showing possible orientations of tention axes in the Western Rift Valley. The Earthquake in the Western Rift Valley of Africa 55

The normal faulting mechanism is evident particularly in Lake Kivu and its western vicinity, which occupy the central region of the Western Rift Valley. It is emphasized that the tension axes are perpendicular to the strikes of a group of fault traces which are small branches of the great rift in these regions. The directions of the tension axis in each region is summarized and illustrated in Fig. 6. The above results are in agreement with the results of the directions of tension axes of the African continent shown by FAIRHEADand GIRDLER(1971) and others , and with the results of the oceanic ridges shown by SYKES(1967, 1970). These results are also consistent with the suggestion of HOLMES(1965) that normal faulting is common in the East African Rift Valley, from geological investigations . Discussing the stress fields in detail, however, we find that they show particular features characteristic of each individual seismic region. The mechanism solutions of Ruwenzori earthquake of 1966 and one of its aftershocks by SYKES(1967) and BANGHARand SYKES(1969) show that the tension axis is in the direction of east to west and the pressure axis is about vertical. MAASHA(1975a, b) has studied the focal mechanisms of local microearthquakes in the northern Ruwenzori Mountains. He shows that the motion along the faults bounding the northern Ruwenzori tends to raise the mountain block relative to the surrounding country. According to his result from nine focal mechanism solutions, tension axes in most of them are in the direction of east to west. The present result for the tension axis in the Ruwenzori area is in the direction of northeast to southwest and differs from the above two results. It was obtained by the use of all the earthquakes that occurred in and around Mt. Ruwenzori. Considering this disagreement in the results, they may have some different focal mechanisms, which is also suggested by the fact that there are many faults of different orientations as shown in Fig. 3. Because of the poor accuracy of hypocenter location, we could not divide the earthquakes into several groups of different seismic zones which are associated closely with the faults. For the purpose of studying the local field of earthquake generating stresses in this area, it is necessary to determine the hypocenters of earthquakes with greater accuracy. The stress field of the sub-region that includes active volcanoes shows a feature different from those in other non-volcanic regions. The scattering of the pressure axis and tension axis found in the volcanic region implies a localized stress distribution due to the heterogeneity of crustal structure which is characteristics of seismicity in the region of earthquake swarm (MOGI, 1962). There is a possibility that earthquakes in sub-region H have two different mechanism solutions. ZANA(1977) has shown that aftershocks of the earthquake of 1960 (M=6.3-6.5) in Uvira are separated into two groups of different mechanism solutions. According to his results, the composite mechanism solution of northern Uvira group of aftershocks indicates that the faulting motion is a normal fault and is right lateral, if the northerly trending nodal plane is taken as the fault plane. On the other hand, the composite mechanism solution of southern Uvira group indicates a normal faulting and is left lateral for the northerly trending fault plane. 56 K. TANAKA, S. HORIUCHI, T. SATO, and N. ZANA

In the latter solution strike-slip component is predominant (ZANA,1977). Strike-slip component is predominant in the mechanism solutions of the earthquakes in sub-regions I and J, the southern part of the Western Rift Valley. Only one among the mechanism solutions obtained by the several authors for earthquakes which occurred in the Western and Eastern Rift Valleys was reported to be of the strike-slip type. The event is located in the Eastern Rift. Considering that the mechanism solutions of earthquakes in Western Zambia adjacent to these sub-regions are of the normal faulting type (FAIRHEADand HENDERSON,1977), the strike-slip type faulting in these sub-regions is an exceptional characteristics of the Rift Valley of Africa.

We wish to express our sincerethanks to ProfessorA. Takagi of Tohoku Universityfor his valuablesuggestions and kind encouragementsduring the courseof thisstudy. Sincerethanks are alsodue to ProfessorT. Hirasawaof TohokuUniversity for his criticalreading of the manu- script.

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