MONITORING OF GEOLOGICAL HAZARDS IN TANZANIA

Geological hazards are naturally occurring geological phenomena that include landslides, rock-fall, flooding, land subsidence, volcanism, earthquakes, radiations and gas emanation from the ground and tsunami. However, in some cases, geological hazards can also be induced by human activities and can cause great impact to society including damage to infrastructure (sources of water, electrical networks, damage to roads, railways and other communications networks), outbreak of lethal diseases, damage and loss of property, damage to the environment, human and animal deaths and bring fear to the public. Throughout history mankind has been in conflict with naturally occurring events of geological, hydro-geological and atmospheric. This conflict has been demonstrated repeatedly when people construct structures at water body edges, in or near active fault systems capable of generating earthquakes, on steep slopes and near active volcanoes.

The East African Rift System (EARS) is one of the major tectonic structures of the earth which extends for about 6,500 km from the Middle East (Dead Sea-Jordan Valley) in the north to Malawi and Mozambique in the south. It is a major geological structure at the scale of African continent and a classic intercontinental rift. The thickness of the continental crust under the main rifts is reduced to 30-35km as compared to normal crustal thickness of 40-45km outside the rift system. The Rift is one of the most important zones of the world where the heat energy of the interior of the earth escapes to the surface in the form of volcanic eruptions, earthquakes and the upward transport of heat by hot spring and natural vapor emanations. It is a rare example of an active continental diffuse zone of crustal extension between Africa and Somalia plate spreading apart at a rate of several millimeters per year as a result of the actions of numerous normal (dip-slip) faults which are typical for all tectonic rift zones.
Tanzania is cross-cut by the EARS with two Arms; the Western Rift System (WRS) and the Eastern Rift System (ERS). The WRS stretches through Kagera, Kigoma, Katavi, Rukwa, Mbeya, Njombe and Ruvuma regions. The ERS which is about 60km wide extends from north-east through Mara, Arusha, Manyara, Singida, Dodoma and Iringa regions. The two Rift Arms intersects on the Rungwe volcanic Mountain complex in Mbeya region where they form a triple junction southward into Lake Nyasa though still there is a debate about this triple junction. Because of the tectonic activity from this active rift zone Tanzania have experienced a number of earthquake events reaching to a magnitude of 7.4 in the Richter scale of which some caused serious damage to property and dislodge of earth and rock masses and loss of lives.

The rift system is also associated with a number of hotspot including the active volcanic mountains of Oldonyo Lengai, Meru and Rungwe. The late Holocene record indicates that the Rungwe volcano erupts after around every 1000 years where the most recent was about 1200 years ago but currently is still dormant. The Oldonyo Lengai volcano is a stratovolcano ejecting natrocarbonatitic and nephelinite tephra where the last eruption was in 2007 through 2009.

MONITORING OF EARTHQUAKES:

The Government of Tanzania through the Geological Survey of Tanzania (GST) an Executive Agency under the Ministry of Energy and Minerals have installed 9 Permanent seismic stations for monitoring seismic activities. The stations are installed in Dodoma, Mbeya, Mtwara, Kondoa, Kibaya, Singida, Arusha, Geita and Babati. GST is responsible for the collection of seismic data from those seismic stations, processing, archiving and disseminating seismic activities reports to the public including occurrences of earthquakes and advice on the necessary mitigation measures.

Public awareness campaigns are normally conducted in workshops, community meetings, interview with TV and Radio stations and during the assessment in the event of an earthquake. Further to that, brochures and posters are published and distributed to the public. From the fact that the effects of damage and destruction caused by those hazard events can be limited through mitigation, GST has set a plan that shall comprise a comprehensive program to address geological hazards which burden threat to human health and safety. The program shall also encompass of other geological hazards like landslide, rock-fall, volcano and radiations and toxic gases emanating from the ground.

Further to the above the Geological Survey of Tanzania in collaboration with the Pennsylvania State University under AfricaArray Program installed a total of five (5) GPS stations in Dodoma, Mbeya, Mtwara, Arusha and Geita. The GPS stations are used to monitor crustal deformation by measuring crustal movement with precision of 0.3m for both horizontal and vertical position of relative points on the Earth's surface. The data is collected over a period of months after which it is determined on how the station has moved and ground deformation is calculated the information which is useful in determining the active tectonic zones.

It is fundamental to have a sound technical framework for addressing geological hazards so that citizens residing in those fragile areas are not at risk. It is important for the public to be aware of geological hazards and their impact to lives and property and importantly the possible mitigation measures affordable to a normal citizen in order to reduce risks. Efforts are underway for the launch of TV and Radio programs to streamline effectiveness of the awareness campaign for the mitigation measures of geological hazards.

Earthquakes are among the most devastating natural disastersand this means that mitigation planning before disaster strikes is essential. Generally earthquake events have occurred in many parts of the country particularly within the Rift System have been recorded since 1900 to date with magnitude up to 7.4 (Ms) some of which caused considerable damage. So far the September 2016 earthquake event which occurred in Kagera region with magnitude of 5.9 in Richter scale is one of the most devastating earthquakes that have occurred in Tanzania after the 2002 event in Bariadi (5.5).Earthquakes to be reckoned are the events of 1910 in Lake Tanganyika with magnitude of 7.4 (Ms), Mbulu/Babati in 1964 with magnitude of 6.4 (Mb), Lake Rukwa in 1994 with magnitude of 5.9 (Mb), Lake Tanganyika in 2000 with magnitude of 6.6 (Mb), Bariadi in 2000 with magnitude of 5.5 (Mb), Rungwe earthquake swarm in 2000 and the Oldoinyo Lengai earthquake swarm in 2007 with magnitude of up to 5.9 (Mb). These are some few examples which signify seismic activities within the Rift System in Tanzania. Despite that the 13th December 1910 earthquake event in Lake Rukwa region had the largest magnitude ever in the east African Rift System, it did not cause any serious damage and there was no loss of life.


Map showing earthquake epicentres (red dots; magnitude ≥ 2)