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.
Earthquakeis the shaking of the Earth’s surface caused by rapid movement of the Earth’s rocky outer layer. Earthquakes occur when energy stored within the Earth, usually in the form of strain in rocks is suddenly released and transmitted to the surface of the Earth by earthquake waves. The effect is like the sudden breaking and snapping back of a stretched elastic band where waves radiate out from the rupturing fault starting at the focus of the earthquake. Most earthquakes are caused by the sudden slip along geologic faults or when pressure within the crust increases slowly over hundreds of years and finally exceeds the yield point of the rocks.
The size of an earthquake is normally indicated in units of
magnitude. There are many different ways that magnitude is measured from
seismograms because each method only works over a limited range of
magnitudes and with different types of seismometers. Some methods are
based on body waves (which travel deep within the structure of the
earth), some based on surface waves (which primarily travel along the
uppermost layers of the earth), and some based on completely different
methodologies. However, all of the methods are designed to agree well
over the range of magnitudes where they are reliable. Earthquake
magnitude is a logarithmic measure of earthquake size.
Earthquake waves make the ground move, shaking buildings and causing poorly designed or weak structures to partially or totally collapse. The ground shaking weakens soils and foundation materials under structures and causes dramatic changes in fine-grained soils. During an earthquake, water-saturated sandy soil becomes like liquid mud, an effect called liquefaction. Liquefaction causes damage as the foundation soil beneath structures and buildings weakens. Shaking may also dislodge large earth and rock masses, producing dangerous landslides, mudslides, and rock avalanches that may lead to loss of lives or property damage.
The destruction that can be caused by an earthquake depends on its magnitude and duration, or the amount of shaking that occurs. A structure’s design and the materials used in its construction also affect the amount of damage to the structure. Earthquakes vary from small, imperceptible shaking to large shocks felt over thousands of kilometers. Adequate precautions, such as education, emergency planning, and constructing stronger, more flexible, safely designed structures, can limit the loss of life and decrease the damage caused by earthquakes.
To date it has not been possible to predict the occurrence of an earthquake with much precision. However, one way of estimating the likelihood of future earthquakes is to study how fast strain accumulates. When plate movements build the strain in rocks to a critical level, like pulling a rubber band too tight, the rocks will suddenly break and slip to a new position. Scientists can measure how much strain accumulates along a fault segment each year, how much time has passed since the last earthquake along the segment, and how much strain was released in the last earthquake. This information is then used to calculate the time required for the accumulating strain to build to the levels that result in an earthquake. This simple model is complicated by the fact that such detailed information about faults is rarely available. Earthquakes, or seismic tremors, occur at a rate of several hundred per day around the world. A worldwide network of seismographs detects about 1 million small earthquakes per year. Very large earthquakes which cause millions of money in damage occur worldwide once every few years while moderate earthquakes occur about 20 times a year. Moderate earthquakes can also cause significant economic damage and loss of lives.Since earthquakes are not certainly predictable, it implies that earthquakes of any size may occur depending on the amount of strain energy that shall have been accumulated at that particular time and the nature of rock material through which seismic waves propagates.
Ground shaking leads to landslides and other soil movement. These are the main damage-causing events that occur during an earthquake. Primary effects that can accompany an earthquake include property damage, loss of lives, fire, and tsunami waves. Secondary effects, such as economic loss, disease, and lack of food and clean water, also can occur after a large earthquake.
Another post-earthquake threat is fire and the amount of damage
caused by post-earthquake fire depends on the types of building
materials used, whether water lines are intact, and whether natural gas
mains have been broken. Ruptured gas mains may lead to numerous fires,
and firefighting cannot be effective if the water mains are not intact
to transport water to the fires. Catastrophic earthquakes can create a
risk of widespread disease outbreaks, especially in underdeveloped
countries. Damage to water supply lines, sewage lines, and hospital
facilities as well as lack of housing may lead to conditions that
contribute to the spread of contagious diseases, such as influenza (the
flu) and other viral infections. In some instances, lack of food
supplies, clean water can create serious health problems as well.
Earthquakes cannot be prevented, but the damage they cause can be greatly reduced with communication strategies, proper structural design, emergency preparedness planning, education, and safer building standards. In response to the tragic loss of life and great cost of rebuilding after past earthquakes, many countries have established earthquake safety and regulatory agencies. These agencies require codes for engineers to use in order to regulate development and construction. Buildings built according to these codes survive earthquakes better and ensure that earthquake risk is reduced.
Tsunami early warning systems can prevent some damage because tsunami waves travel at a very slow speed. Seismologists immediately send out a warning when evidence of a large undersea earthquake appears on seismographs. Tsunami waves travel slower than seismic P and S wavesin the open ocean, they move about ten times slower than the speed of seismic waves in the rocks below. This gives seismologists time to issue tsunami alerts so that people at risk can evacuate the coastal area as a preventative measure to reduce related injuries or deaths.
Engineers minimize earthquake damage to buildings by using flexible, reinforced materials that can withstand shaking in buildings. Currently scientists and engineers have greatly improved earthquake-resistant designs for buildings that are compatible with modern architecture and building materials. They use computer models to predict the response of the building to ground shaking patterns and compare these patterns to actual seismic events. They also analyze computer models of the motions of buildings in the most hazardous earthquake zones to predict possible damage and to suggest what reinforcement is needed.
An earthquake can be a terrifying and dangerous event. Proper safety precautions such as education, emergency planning, and constructing stronger, more flexible, safely designed structures, can limit the loss of life and decrease the damage caused by earthquakes, you can help limit that terror through careful planning and organization. Hold occasional earthquake drills so your family or workers knows exactly what they should do in the event an earthquake. This will increase the odds that everyone survives unharmed. Earthquakes can trigger additional emergencies, and individuals should also be prepared to contend with these related natural hazards; Tsunamis near coastal areas, Landslides or mudslides in mountainous regions, fires if gas lines are ruptured or power lines spark blazes and flooding if dams break or rivers are diverted. These hazards will vary based on where the earthquake hits and how strong it is, but thorough safety precautions will address these additional disasters.
Earthquake mitigation measures can reduce earthquake damage which among others includes improved building codes and community awareness campaigns. It can also help to have valuable tips on ways to keep your family safe before, during and after an earthquake. The following are some of the action measures that can minimize damages, injuries, and other potential earthquake concerns:-
Before the Earthquake:
Some important things that families and individuals can do to prepare for an earthquake:
During an earthquake:
Earthquakes can last just a few seconds or several minutes, understanding of the safety measures on account of ground shaking can help prevent injuries:
After an earthquake: