NATURAL DISASTERS AND HAZARDS IN THE LAKE KIVU BASIN, WESTERN RIFT VALLEY OF AFRICA.

D.M. Wafula1, M. Yalire1, M. Kasereka1, M. Ciraba1, M. Kwetuenda2 and H.

Hamaguchi3

1. Department of Geophysics, C.R.S.N./ Lwiro. D.S. Bukavu. D.R.C. 2. Centre de Recherche Géophysique de Kinshasa, B.P. 10.336, Kin.1, D.R.C.

3. Graduate School of Sciences, Tohoku University, Sendai 981, Japan. Abstract: The lake Kivu with altitude (1462m) and maximum depth (485m) is the most highest in the Western Rift. The lake Kivu basin is among the most active region in this Rift, dominated by the mountains. The earthquakes are making a lot of damages in villages and cities in the lake Kivu basin. The earthquakes are also generating landslides on slopes of mountains or hills surrounding the lake Kivu. In additional to the seismic activity prevailing, some factors such as: the regressive erosion, the presence of aquifer nape, the antropical action on the environment, the deforestation, and parcelling on the steep slopes are contributing to the landslides phenomenon and inundation in Bukavu. People are frequently injured or killed by the earthquakes, landslides or collapses of soil in the basin, mostly at Bukavu city located on the sout-western part of the lake. In the northern end of the lake is found the Virunga volcanic region with two volcaneos Nyiragongo and Nyamuragira which are among the most active in the world. The villages around the volcanoes Nyiragongo and Nyamuragira are frequently damaged by volcanic eruptions. During the volcanic eruptions of Nyamuragira, the atmosphere is polluted by volcanic volatiles products causing sicknesses, sometimes deaths and burning fields. The cows, lambs, goats are often killed when the eat polluted grasses. The fast lava flows from Nyiragongo destroy villages on its flanks and Goma city and Kill people. Since Nyiragongo fissures are very close to the cities of Goma and Gisenyi more big disaster might occurred during the futur eruption of this volcano if the prediction and the emergency plan fail. Recently the lava lake in the crater of Nyiragongo volcano is generating volcanic cloud with several gases, which mixed with the rain water generate acid rains that is falling on the zones where people are just living. Similar to the other lakes in the East African Rift Valley, the lake Kivu is stratified with three layers: epilimnion (0 -50m), metalimnion (50 - 75m) and hypolimnion (75m - ). The metalimnion is composed of mineral salts, however the hypolimnion is composed of gases such as carbon dioxide CO2 (75%) methane CH4 (15%) and 10% other gases. The quantity of CO2, is estimated to 256 km3, CH4 54 km3 and the volume of the lake water is 560 km3 on a 2,055Km² of surface. The two gases are intimately bound and physically dissolved in the hypolmnion waters. In the lake Kivu, the possibility of happening CO2 gas explosion as in the lake Nyos in 1986 is very small at present, because the dissolved gases carbon dioxide and methane do not reach to the saturation points in the hypolimnion and the temperature profile from bottom to the depth of 60 m, the fall of temperature is comparatively rapid, and than it works to increase the solubility with decreasing depth. The most cataclysmal explosion might occur if very fast lava flow entered rapidly in the lake to break the metalimnion natural barrier or generating water convection. Other

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possibilities for the gases explosion should be the occurrence of volcanic eruption of Nyiragongo in the lake close to the shore line or that of a big earthquake in lake generating Tsunamis. The gases explosion should be a big catastrophe because of the location of cities of Goma (D.R.C.) and Gisenyi (Rwanda) in the northern end of the lake, Bukavu (D.R.C.) and Cyagungu (Rwanda) in the southern end, and several villages on its the margins. More than one million of people might be affected. According to the analysis done in this report a big catastrophe may be expected in the future in this basin. 1. Introduction

The East African Rift Valley System is one of the most seismically active region in the African Continent and its geological and geophysical features are noted to be similar to those of the mid-oceanic ridges. Both the Rift Valley and the mid-oceanic ridges are characterized by the seismic activities confined to the shallow depths (Zana, 1977). In regard to the earthquake mechanism, it was shown that the earthquakes with normal faulting type prevail widely in the Western Rift, and that their directions of tension axis are approximately perpendicular to the strikes of the Rift (Zana and Tanaka, 1981) and (Wafula and Zana, 1990). The east African Rift is divided into two branches; the Eastern and the western (Fig.1a).

Fig.1a: Map of the East African Rift Valleys System.

The Virunga Volcanic Region, located in the central part of the Western Rift, just at the Northern end of lake Kivu, consists of three subgroups of eight major volcanoes: the eastern (Muhavura, Gahinga and Sabinyo), the central (Visoke, Karisimbi and Mikeno) and the western (Nyiragongo and Nyamuragira) (Fig.1b).

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Fig.1b: Virunga volcanic region in the Western Rift Valley

The volcanoes are aligned in nearly an east-west direction and are nearly perpendicular to strike of the Rift System. The volcanoes in the western group are in the rift floor and are considered to be among the most active in the world; however, the other volcanoes in the eastern and central groups are believed to be in the dormant stage (Hamaguchi and Zana,1983). The lake Kivu with geographical coordinates 2° S an 29°E is one of the six lakes ranged from north to south in the Western Rift; namely: Albert, Edward, Kivu, Tanganyika Rukwa, and Nyasa (Malawi). This lake with altitude (1462m) is the most highest in the Western Rift. The maximum depht of the lake is (485m) and the volume of the lake water is 560 km3 on a 2,055Km² of surface. The lake Kivu basin is among the most active region in this Rift (Zana et. al., 1989) and (Zana et al., 1992). Similar to the other lakes in the East African Rift Valley, the lake Kivu is stratified with three layers: epilimnion (0 -50m), metalimnion (50 - 75m) and hypolimnion (75m - ). The tectonic process of lake Kivu might be in a nascent stage of development (Wong and Von Herzen , 1974).

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Let us notice that Goma (D.R.C.) and Gisenyi (Rwanda) in the north and Bukavu (D.R.C.) and Cyagungu (Rwanda) in the south western are the main cities on the lake Kivu margin (Figs.2 ,3), respectively. Several villages are also located on this margin.

Fig.2: Goma city northern part of lake Kivu (Mar.22, 2002), with Gisenyi city in the background. No natural border between the two cities. .

Fig. 3: Bukavu city southern part of lake Kivu, with Cyagungu in the background (Jan. 4, 2007). The two cities are separated by the Ruzizi river.

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The main aim of this paper is to evaluate the natural disasters already occurred in the lake Kivu basin and assess the natural hazards threatening the local inhabitants. The paper should be focussed on many factors generating the natural disasters and hazards in the basin. 2. Methods and investigations

The lake Kivu basin is among the most seismically active region observed in the Western Rift Valley. This region is characterised just at its northern end by a very active volcanism concentrated recently on the volcanoes Nyiragongo and Nyamuragira. The lake Kivu contains some gases such as: carbon dioxide CO2, (75%) and the methane CH4 (15%) and 10% other gases. The topography around the lake Kivu is dominated by the mountains. All those phenomenon already generated some disasters and presente some hazards in the region that should be evaluated in geology, geophysical and geochemical, etc..., point of view.

2.1. Geophysical and Geological Observations, Disasters and Hazards A. Seismic observations

For the seismic study in the Western Rift valley the “Centre des Recherches en Sciences Naturelles” (CRSN) of Lwiro constructed the station of Lwiro since 1953. Lwiro station was equipped with the Benioff short period seismographs (To= 1sec, Tg=0,25 sec, Mag= ca.100k). This station is located about 90km south of the Virunga region with Goma city and 25km north of Bukavu city, and contributes a lot to the investigations of volcanic activity in the Virunga and regional seismicity.

In 1979, a small seismic network with three stations (Lwiro, M’bayo and Muganzo) was installed around Lwiro in the western part of lake Kivu. Late, Muganzo station was moved to Bushumba. The seismic distribution obtained from this network shows clearly that the lake Kivu basin is an active region in the Western Rift Valley (Kavotha et al, 1990)(Fig.4). The seismic activity was mostly clustered during this period along the Idjwi island up to Bukavu city. We can notice also the concentration of earthquakes on the Westen side of the lake and in Masisi and Walikale areas.

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d

Fig. 4: The seismicity of lake Kivu ba Aug. 1979 to Dec. 1980, mos Magnitude ≤ 3.5. Recently this station is equipped with a short

component SS-1 ranger seismometer (To=1s) connecteThe basic seismic network for volcanic observat

around the Nyiragongo and Nyamuragira volcanoes Japan cooperation by the CRSN. The number of statiwere progressively improved. These stations are recequipments than Lwiro station presently. In this kindneeded in the hypocenters determination.

During the simultaneous eruptions of NyiragNyamuragira on July 4, 1994 a telemeter network wregion. These stations used three components shorthorizontal). The seismic signals were sent from the seObservatory (GVO). The accuracy of time code ofprecise location of hypocenters. Unfortunately this net1996 war in D.R. Congo in 1996.

In May, 2004 a new telemeter network was insThe signals from these stations (three components; oare locally digitized from a data logger with sampling resolution of 24 bits, and telemetered to Goma base sin triggered and continuous files. Actually the seismic nshown in the figure (Fig.5).

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Idjwi Islan

sin for the period from t of earthquakes had

period Kinemetrics vertical d to PS-2 seismic recorder.

ions was permanently installed since 1983 under the Congo-ons and quality of equipments ently equipped with the same of network time correction is

ongo on June 23, 1994 and as introduced in the Virunga period (one vertical and two ismic station to Goma Volcano such a network allowed the work was destroyed during the

talled using the same stations. ne horizontal and two vertical) frequency of 50 Hz and a A/D tation where they are recorded etwork in the Virunga region is

Fig.5: Seismic network around the volcanoes Nyiragongo and Nyamuragira.

From seismic studies, field observations and testimonies of inhabitants in the

basin it is revealed that, since 1997 the lake Kivu basin had became more active than usually. Several moderate earthquakes with local magnitude more than 4.0 associated with foreshocks and aftershocks are recorded in basin every year. The earthquakes occurring in the lake Kivu basin are recorded at Lwiro station equipped with a digital seismograph short period “SEISDAS” (Fig.6). This figure shown two earthquakes of magnitude more than 4.0 were recorded on May 30, 1998 within 40 seconds only and followed 4 hours late by an after shock on May 31, 1998.

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Fig.6: Tectonic earthquakes recorded at Lwiro station on May 30,

within 40minutes 1998, followed by aftershock May 31, 1998. This seismic activity is supposed to be related to the reactivation of faults

observed in the basin. The earthquakes generate landslides and collapses, people are killed and houses destroyed by these phenomenon. B. Geological observations The geology of lake Kivu basin is mostly characterised by the rift escarpments dominated by regional active faults approximately in the N-S direction, some of them are roughly E-W direction perpendicular to the strike of the Rift in the N-S direction and others in NNW-SSE and NNE-SSW directions. At Nyamasasa and Kalungu fault the unsticking niche is visible indicating a normal fault (Fig.7).

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Fig.7 : Nyamasasa and Kalungu fault, 30km south of Goma city.

The Bukavu city in the south of lake Kivu is constructed on several steep slopes (Fig.8).

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Fig.8: Geological map of Bukavu

The geology of this city is constituted by very old lava flows, with alliterated

soils of thick clay-limon and limon-clay subdivided in three horizons: blackish, reddish to red ochre and greyish to whitish. This alliteration is probably due to the tropical climate hot and humid. In point of view tectonics, the city is cut by several faults orientated in the various directions; N-S like Ciderhwa, Karhale, Kwinja-Nyakavogo, Nguba 3 and Ruzizi faults, roughly E-W direction like Kasha fault, NNW-SSE direction like Buholo, Kahuha, Ndendere and Nyofu faults and NNE-SSW direction like Bwindi fault. These faults are considered to be still active in this part of the basin.

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By its nature and relief, the land of Bukavu is very favourable to the landslides. It is almost same for most of surrounding areas to Bukavu and Cyagungu the neighbouring city.

Goma and Gisenyi are constructed on more young lava flows of Nyiragongo volcano. Goma is delimited in the north by the Nyiragongo volcano and in the south by the lake Kivu. Around Goma can be seen several hills that are the old cones of Nyiragongo. The landslides are very frequent in Bukavu and surrounding areas. It is also important to notice that the landslides which were already observed in Bukavu were accelerated by the occurrence of moderate earthquakes since 1997. In additional to the seismic activity prevailing, some factors such as: the regressive erosion, the presence of aquifer nape, the antropical action on the environment, the deforestation, the destruction or the lack of water canalization and parcelling on the steep slopes are contributing to the landslides phenomenon and inundation in Bukavu (Figs. 9a,b).

Fig. 9a: The “Institut Techinque Fundi Maendeleo” (ITFM) and “Lycée Wima” canyon in Bukavu city, Jan. 3, 2007.

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Fig.9b: Parcelling on the steep slope at Bukavu (Jan.5, 2007). Goma and the villages on the southern flank of Nyiragongo are frequently

threatened by the lava flows of Nyiragongo since longtimes (Fig.10). From this figure eighty percent of the lava flows are directed southward where people are just living.

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Fig.10: Map of Nyiragongo volcano complex with lava flows (Dement et al., 1994). In red lava flows from January 10, 1977, eruption. C. Disasters and Hazards due to the earthquakes, Landslides and Faults The seismic activity with magnitude at least 4.0 observed in the lake Kivu basin from 1960 up to now, indicates that most earthquakes are located in the northern part of the basin (Fig.11). All these seismic events were reported by the local inhabitants.

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Fig.11: The seismicity of lake Kivu basin for earthquakes with magnitude ≥ 4.0 for the period from 1960 to 2006. Blue solid circles are aftershocks associated to the October 24, 2002., encircled red solid circle.

Since May 1997, the seismic activity in the lake Kivu basin was remarkable (Munyololo et. al, 1999). Many earthquakes were felt by local inhabitants in the basin. Just on May 1997, a moderate earthquake generated landslides at two places supposed to be located along the same fissure in the direction NNE-SSO, in Bukavu, Bagira commune and surrounding area Hongo in the north. The landslide at Bagira destroyed several ten houses. This landslide is still in progression up to the present time(Figs.12a,b).

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Fig.12a: Bagira landslide, Bukavu city (July 23, 1997).

Horizon B

Horizon C

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Fig.12b: Bagira landslide in August 2006. The Hongo landslide with dimensions 80m x 40m, is still extending also

(Fig.13).

Horizon B

Fig.13: Hongo landslide in April 2003. In 1999 five children of the same family were killed by the collapse of a

supporting wall of a uphill house on house where the children were slipping. In the same year the wall of a church collapsed on two children; one was killed and another one injured. An earthquake of the series with magnitude Mw=6.2 occurred recently in the south-west of Goma and north of Bukavu on October 24, 2002 as shown in figure (11). The damages caused by this earthquake were recorded on houses in Bukavu, Lwiro, Kalehe, Nyabibwe, Kiroshe, Gisenyi, … and even in more far city like Kigali in Rwanda. Indeed, the most heavily damaged zone was in Kalehe territory where eight persons were killed, houses destroyed and a part of village went into the lake Kivu generating small Tsunamis. One church constructed during 15 years was destroyed (Fig.14).

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Fig.14: Whitsun church at Nyabibwe Kalehe territory destroyed by the earthquake on October 24, 2002. Very slight damages were observed in Goma, however in Bukavu the

environmental condition was affected. Building of the high school at Bagira commune was damaged and during the earthquake (Fig.15). The cracks are visible on the wall of the school. In that school, six students in the class room terrified by the opening of a fracture on the ground fainted from 8h AM and could recover at 6 PM in the hospital.

Fig.15: Bagira high school destroyed by the earthquake on October 24, 2002. In the Kasheke and Tchofi villages, in the Kalehe territory, 10 persons in total

were killed by the landslide that occurred during a heavy rain on the night from April 26 to 27, 2003 (Fig.16).

Fig.16: Kasheke landslide on April 26-27, 2003, 9 persons were killed and buried in the soil 1.5m deep.

On December 1st ,2006, three children were also killed by the collapse of a

supporting wall of the neighbouring house in uphill (Fig. 17) . The conditions are same than the case of five children in 1999.

Fig.17: House destroyed by a supporting wall of a uphill house, three children killed, Bukavu city. On February 8, 2007, an earthquake of magnitude more than 4.0 occurred in

the southern part of lake Kivu. On February 9, 2007, during a heavy rain, a supporting wall of neibouring house fallen on a house and killed 3 children. Probably in this case the wall should be affected by the earthquake that occurred the day before.

Several landslides are recorded in Bukavu and surrounding area with damages on houses. Since the seismic activity still prevailing in the lake Kivu basin more big damages are expected.

In the Kalehe territory is observed the Nyabibwe fault which is very active in

the N75°W direction (Fig.18a). This fault was very active during the Kalehe earthquake on October 24, 2002. According to the local inhabitants seismic shaking are felt more frequently at the present time. Niches of Landslides can be see at on the slope of the escarpments.

Fig.18a : Nyabibwe fault in the Kalehe Territory (Feb 21, 2007) This fault destroyed the bridge on the river Nyamishaga, on the road between

Bukavu and Goma (Fig.18b). This bridge was just located in the central part of the fault.

Fig.18b: The crossing of the Nyamishaga river bridge destroyed by the Nyabibwe fault (Feb.21, 2007).

D. Disasters and Hazards due to the volcanic activities

On January 10, 1977, Nyiragongo erupted on its flanks generating 4 lava flows. The longest flow was that in the south stopped at 1.5km of Goma international airport. Official 70 persons were killed by its fast lava flow. Several small villages were destroyed by the lava flow including field. On January 17, 2002, Nyiragongo erupted again on its flank. Is was reported that 45 were killed by the lava flows on January 17 and 60 by the explosion of fuel tank on January 18.

Based on field observations and satellite imageries, 13 % of Goma city were

burned with nearly all properties and 80% of the economic area were destroyed by the lava flow (Fig.19). The lava flow crossed the city of Goma from north to south into the lake Kivu (Wafula et al.).

Fig. 19: Lava flows from Nyiragongo flank fissure eruption, On January 17, 2002. Times of the opening of the fissures: A: 8h20’ AM, B: 9h00’ AM, C: 9h30’ AM, D: 3h30’ PM, E: 4h20’ PM.

The lava flows prologated about 100 m into the lake Kivu and went about 70m

deeper. A third of Goma international airport runway was destroyed by very slow (aa) lava flow, on January 18 to 19 (Fig.20). The thickness of the lava flows on the runway was estimated about 4m in average.

Fig.20: The runway of Goma airport attacked by low speed lava flows. The little boy is not afraid of lava flow because of its low speed.

Since, November 2002, the lava lake which completely disappeared during the flank eruption on January 17, reappeared at the summit crater of Nyiragongo generating almost permanently volcanic cloud mixed with rain water producing acid rain which is dangerous for human life.

Another fact is that the fissures related to January 17, 2002 eruption are very close to the city of Goma, this situation increases much the level of hazard. The volcano Nyamuragira being very active are disturbing very much the live and the environment in cities and villages around them. Since 1980, Nyamuragira volcano erupts every two years. The lava flows of Nyamuragira may cover a distance of 20 km and a large surface during an eruption (Fig.21).

Fig.21: Pyroclastic cone and lava flow from Nyamuragira (Mikombe-Ndakamaro) eruption on Sept. 20, 1991 (Kasahara). The atmosphere is polluted up to 10km high with volcanic cloud (Fig.22), and

the environment is attacked on a ray of more than 20 km from the eruption sites with volatiles volcanic products; scories, pele’s hairs and ashes (Fig. 23). People are not killed directly from lava flows but by scories and ashes the pollute water, crops and atmospher, and generate fiever, diarrhea, conjonctivite, breafing that can kill. Aborption is also observed for women. In 2000 Nyamuragira 18 persons were killed at Rusayu villages by driking polluated water. The high volcanic cloud is very dangerous for the plane with reactor. The cows, lambs, goats are often killed when the eat polluated grasses.

Fig.22: Lava fountain from Nyamuragira (Tumaini) parasitic cone during during the eruption on February 5, 2001. High volcanic cloud is seen.

Fig.23: Banana tree field destroyed by the scories, pele’s hairs and volcanic ashes from Nyamuragira eruption on November 27, 2006

2.2. Geochemestry The stratified lake Kivu contains in its second layer (metalimnion) mineral

salts, however the third layer (hypolimnion) gases such as carbon dioxide CO2 (75%) methane CH4 (15%) and 10% other gases. The quantity of CO2, is estimated to 256 km3, CH4 54 km3 . The two gases are intimately bound and are physically dissolved in the deep waters in the hypolimnion. A. Origin of gases in the lake Kivu a. methane gas

The methane gas in the lake Kivu is produced in two ways

1. The bacterial reduction of magmatic CO2 (70 %) CO2 + 2H2 CH4 + O2 2. The bacterial fermentation of acetate in the sediments (30%) : CH3COOH CH4 + CO2 b. The carbon dioxide 1. The oxidation of methane CH4 + O2 CO2 +2H2 2. The decomposition of magmatic carbonate CaCO3 CO2 + CaO 3. The decomposition organic materials 4. The contribution of carbon dioxide from the atmosphere 5. Etc,… (Tietze, 1974, Tietze, 1975). B. Gas in the Goma and Gisenyi cities, and surrounding areas The cities of Goma and Gisenyi are built on the old lava flows of Nyiragongo volcano. The notion of gases is well known by the people living in Goma and Gisenyi cities, and surrounding areas. The gases emission points are called “Mazukus” by the local inhabitants. Several emission points are observed in Goma, Gisenyi and surrounding areas. In some houses the gases escaped through toilets conduits. People are often fainted or killed by these gases especially by CO2. So it needs much care to pay about the gases in the region. The northern margin from the locality of Sake in the

West to Gisenyi East is observed many “Mazukus” points with concentration reaching 70%. The At Sake students are sometimes killed by gases at. The ”Mazukus” are located on the borders of slopes, the fractures and depressions. During and after the eruption people in Goma confronted a variety of gas emissions, from the lava flow in the city and from fractures that of occurred because of intense seismic activity associated to the eruption. Several gas bursts were reported by the population of Goma. Most gases were CO2 or CH4. The evidence of gas emission in Goma was confirmed by this observation; two women cleaning the church in bending position fainted because of CO2 emissions through fractures. From the observation, it was noticed throughout the city similar fractures are scattered. Minor explosions of phreatic origin also occasionally occurred in different places on old lava flows even in the houses. The gas measurements indicated the presence of carbon dioxide and methane with concentration between (1 to 5%) at the different points in Goma city.

On February 8, 2002, heavy rain occurred accompanied with storm in Goma city, very dense steam arose from the lava flows still hot from Nyiragongo volcano on January 17, 2002. Unfortunately the steam contented gases, so 8 persons were killed. To go back home people were obliged to cross hot lava all time after the Nyiragongo eruption on January 17, 2002 (Fig.24).

Fig. 24: People walk on the lava flow from Nyiragongo eruption

on January 17, 2002. The lava degassing hot steam

with gases. The boy on the right trying to protect him

self from gases.

In the lake Kivu, the possibility of happening CO2 gas explosion as in the lake Nyos in 1986 is very small at present, because the dissolved gases carbon dioxide and methane do not reach to the saturation points in the hypolimnion and the temperature profile from bottom to the depth of 60 m, the fall of temperature is comparatively rapid, and than it works to increase the solubility with decreasing depth (Nakao et al., 1988). The most cataclysmal explosion might occur if very fast lava flow entered rapidly in the lake to break the metalimnion natural barrier or the hot lava could disturb the lake stability by generating lake-water convection. Other possibilities for the gases explosion should be the occurrence of volcanic eruption of Nyiragongo in the lake close to the shore line or that of a big earthquake in lake generating Tsunamis. In the past Nyiragongo already erupted in the lake Kivu and on the shore line (Mountain Goma). No historical record is available in the present time about those past eruptions. The gases explosion should be a big catastrophe because of the location of cities of Goma (D.R.C.) and Gisenyi (Rwanda) in the northern end of the lake, Bukavu (D.R.C.) and Cyagungu (Rwanda) in the southern end, and several villages on its the margins. More than two million of people might be affected.

Let us notice that, the Nyiragongo eruption on January 17, 2002 did not break

the natural barrier, probably because the speed of the lava flows and its temperature which decreased during the 5 km of route from the effusion fissures at Munigi to the fall point in the lake. The lava crossing Goma city transported a lot of materials which decreased its speed and the temperature, so the impact with the lake water was not so strong. The location of Goma on a plateaus contributed much to the decrease of lava speed before to reach the lake.

For the prediction of future flank eruption, the CO2 measurements are

performed along several fissures on the south flanks of the volcano Nyiragongo. Data obtained are shown in table (1) and displayed in figure (25). From this figure, we may notice that the concentration of CO2 is stable. In the approach of lava outburst from fissure the concentration of CO2 should increase more significantly.

Tabl.1: CO2 concentration at Munigi fissure for the period from 2004 to 2006. Dates

29/ 07/04

25/ 08/04

15/ 09/04

30/ 10/04

20/ 11/04

11/ 12/04

22/ 01/05

03/ 02/05

04/ 03/05

21/ 04/05

13/ 05/05

08/ 06/05

21/ 07/05

14/ 10/05

Concentration

19.6

22.5

22.9

25.2

25.8

27.1

24.4

24.3

23.5

23.7

24.5

21.2

23.4

27.4

Dates

11/ 11/05

28/ 12/05

02/ 01/06

23/ 02/06

25/ 03/06

07/ 04/06

05/ 05/05

15/ 06/06

19/ 07/06

23/ 08/06

21/ 09/06

19/ 10/06

19/ 12/06

Concentration

28.2

25.2

28.3

24.7

26.6

26.6

26.6

28.1

27.1

25.6

26.6

26.6

26.0

0

5

10

15

20

25

30

Dates

CO2

Conc

entra

tion

Fig.25: CO2 concentration at Munigi fissure for the period from 2004 to 2006. The Recently the lava lake in the crater of Nyiragongo volcano is generating

volcanic cloud containing several gases, such as: SO2, NO2, Cl2 and F which mixed with the rain waters generate by hydrolyse chemical process acid rains containing H2SO4, HNO3, HCl and HF, respectively (Fig.26). The rains are falling on the zones where people are just living. It was noticed by using the differential optical absorption spectrometer (DOAS), that the daily amount of SO2 ejected in the atmosphere by Nyiragongo volcano is varying between 3.000 and 52.000 tons.

Sure that the atmosphere in Goma, Gisenyi and surrounding area should be

affect by big amount gas emissions that should have impact also on the ozone which is needed for life.

Fig. 26: Volcanic cloud generated by the recent Nyiragongo

lava lake in the crater.

3. Conclusion and Discussion From seismic observations it is confirmed that the lake Kivu basin is among the most seismically active region in the Western Rift Valley. Since 1997, the seismic activity had increased accelerating the landslides already present in the basin.

Except some seismological and geological results available no serious studies are done in the lake Kivu basin on soil engineering; foundation soil, special treatment of slope instability and subsidence, and modification of strong-motion parameters. Every year several people are killed directly by earthquake or by landslides and inundations.

The villages around the volcanoes Nyiragongo and Nyamuragira are frequently damaged by volcanic eruptions During eruptions of Nyamuragira eruptions, the atmosphere is polluted by volcanic volatiles products causing sicknesses, sometimes deaths and burning fields. The cows, lambs and goats are always victimes of Nyamuargira eruption. The Nyiragongo lava lake is generating almost permanently volcanic cloud mixed with rain water producing acid rain which is dangerous for human life, the preventive mesearuments should be taken to assist the local inhabitants concerned by these kind of rains.

The lava flow of Nyamuragira can cover large areas, desvating the national park and kill animals. The lava flows from Nyiragongo just attacked the villages on its flanks and Goma city and kill people. More than half million of people are in trouble when Nyiragongo flank fissure eruption occurred.

The Goma experience from Nyiragongo eruption on January 17, 2002 was very severe one and all measures should be taken to avoid its repetition in the future, as the new lava lake already reappeared since November 2002 (Fig. 27).

Fig.27: Lava lake in the pit crater of Nyiragongo eruption volcano, on June 22, 2006 Another fact is that the fissures related to the January 17, 2002 are very close

to Goma city, this situation increase very much the level of hazard. The level of the new lava lake already accessed about 200m. So the monitoring system should be really improved, the Local Scientists motivated and trained and local authorities, medias, populations involved in the mitigation of volcanic hazards program. In addition detail knowledge of the topography around the volcano and preparation of a volcanic risk map are needed.

The place with high concentration of gases sould be detected and isolated from people. The toilet with concentration of CO2 should be closed. A present, in the lake Kivu, the possibility of happening CO2 gas explosion is very small. The situation may change if very fast lava flow entered rapidly in the lake to break the metalimnion natural barrier or generating water convection. Other possibilities for the gases explosion should be the occurrence of volcanic eruption of Nyiragongo in the lake close to the shore line or that of a big earthquake in lake generating Tsunamis. The gases explosion should be a big catastrophe because of the location of cities of Goma (D.R.C.) and Gisenyi (Rwanda) in the northern end of the lake, Bukavu (D.R.C.) and Cyagungu (Rwanda) in the southern end, and several villages on its the margins. More than one million of people might be affected. The stability of the lake should be surveyed, including vertical profiles of temperature, turbidity and other mesearements. The gases concentration must be in controle, It is also important to think about the mesearuments that should be taken in case of gases explosion. The gas methane is useful for domestic actvities. It is a great ressource since the lake is containing a huge amount.

4. References Dement, A., P., Lestrade, Lubala, R.T., Kampunzu, A.B., Durieux,J. 1994. Volcanological and Petrological evolution of Nyiragongo volcano, Virunga volcanic field, Zaire. Bull. Volcanol. 56, 47-61. Hamaguchi, H., Zana, N., 1983: Introduction to Volcanoes Nyiragongo and Nyamuragira; Volcanoes Nyiragongo and Nyamuragira, Geophysical Asp. In: Hamaguchi, H. (Ed.), Faculty of Science, Tohoku University, Sendai, Japan, 1-6. Kavotha, K.S., Zana, N., Yamamoto, A., 1990. Considérations sur la Séismicité Récente du Bassin du Lac Kivu (Rift Ouest-Africain). Rev. Sc. Nat. Vol. 1. Lwiro, D.R.C., 50-61. Munyololo, Y., Wafula, M., Kasereka, M., Ciraba, M., Mukambilwa, K., Mavonga,T., Cirimwami, M., Muhigirwa, B., Bagalwa, R., Mundala, M., 1999. Récrudescence des Glissements de Terrain suite à la Réactivation Séismique du Bassin du Lac Kivu, région de Bukavu (D.R.C.). Mus. Roy. Afr. Centr. Dépt. Géol. Min., Rapp. Ann. 1997 & 1998, 285-298. Nakao, K., Kwetuenda,M.K., Kakogozo, B. Mbwebwa, D., 1988. Stability in the Hypolimnion of lake Kivu. Ecology and Limnological on lake Tanganyika and its adjacent regions, Tokyo, Japan, Vol. V, 73-79. Tietze, K. 1974. The lake Kivu methane – Problems of Extraction and their Mathematical – Physical study. Conference paper, Regional Conference on Petroleum Industry and Manpower Requirements in the field of Hydrocarbons, United Nations Economic and Social Council, United Nations Economic Commission for Africa, E/CN.14/EP/INP.13, Tripoli, 7P. Tietze, K., Schröder, L.,1975. Rapport Intermédiaire sur le Projet Gaz Methane du lac Kivu (Zaïre, Rwanda). Report No. 68414, Bundesanstall für Geowissen - Schaften und Rohstoffe, Hnover, 25p. Wafula, M., D., Zana, N. 1990. Focal Mechanism Study of Earthquakes in the Western Rift Zone and Central Basin of Zaire. Rev. Sci. Nat., Vol. 1, No. 1, Lwiro, Zaire, 75-92. Wafula, M., Zana, N., Kasereka M., Hamaguchi, H. The Nyiragongo volcano. A case study for the Mitigation of Hazards on an African Rift Volcano Virunga Region, Western Rift Valley, Jaes Edit. Wong, H., K., Von R.,P.,Herzen, 1974. A Geophysical Study of Lake Kivu, East Africa, Geophys. J. R. Astr. Soc. 37, 371-389. Zana, N., 1977: The Seismicity of the Western Rift Valley of Africa and Related problems. Doctorat Thesis, Tohôku Univ. Sendai, Japan.

Zana, N. , Tanaka, K., 1981. Focal Mechanism of Major Earthquakes in the Western Rift Valley of Africa, Tohôku Geophys. Journ. (Sci. Rep. Tohôku Univ., Ser. 5). Vol. 28, Nos. 3-4, 119-129. Zana, N., Kamba, M., Katsongo, S., Janssen, T. 1989. Recent Seismic Activity Of the Kivu Province, Western Rift Valley of Africa. Physics, Earth, Planet. Interiors, 59, 52-60. Zana, N., Kavotha, K., Wafula, M., 1992. Estimation of Earthquake Risk in Zaire. Tectonophysics, Elsevier Sciences Publishers B.V. Amsterdam, 209, 321- 323.