The ManicaPost
Abraham Mudefi
The 6.5 earthquake that recently struck nearly 250 km north-northwest of Gaborone in Botswana was a wakeup call for most countries in Southern Africa. In 2016 alone, Zimbabwe experienced about 50 earthquakes that measured between 1.5 and 4.6 on the Rector Scale.
The Zimbabwe Government moved swiftly to form an Earthquake Committee that will study the country’s vulnerability and state of preparedness in the face of future earth quakes (seismic activities).
This week we feature the science behind earthquakes. The science of seismology was first propounded by Alfred Wegner in 1915 through his “theory of continental drift”. Alfred a German Geologist and Meteorologist believed the earth’s crust floated on molten magma. This led to the development of the theory of plate tectonics. The earth is believed to be made up of continental plates that are constantly in motion.
Plates go beyond continents. They constantly move towards each other (convergent plates), away from each other (divergent plates) or against each other (transforming plates). Sometimes this movement is not smooth, rocks and earth may halt this free movement causing the earth to snap due to the built up of stress and energy. When this occurs, the earth shakes, this is called an earthquake. Earthquakes occur alone fault lines or the edges of continental or oceanic plates. They are many other various reasons why earthquakes or tremors may occur sometimes within the major continental plates.
Isostatic adjustments is one other reason why earthquakes can occur. Some earthquakes can be caused by anthropological factors. Activities like building dams, building very high buildings, coal mining, drilling and injecting liquids into wells can also cause earthquakes. When the Kariba dam was constructed and commissioned in 1959, the weight of the water caused tremors. This shaking was a compensatory action or isostatic adjustment caused by the heavy load that suddenly was upon the earth in that locality.
The recent Botswana on the 3rd of April 2017 (at 1740hrs, magnitude 6.5 Mw and depth 29 km) earthquakes is suspected to have been caused or exacerbated by men during hydraulic fracking activities in the Central Kalahari Game Reserve. The Botswana Geoscience Institute has refuted this claim and stated that the depth 29 km can only be caused by a natural earthquake.
Earthquakes can come with other hazards such as landslides, volcanic eruptions or flooding. Sometimes if an earthquake occurs in an ocean it can result in a Tsunami which is a series of large water waves that can result in damage on coastal lands. Earthquakes can also be caused by volcanic activities, landslides, mine blasts and nuclear tests.
Apart from the movement of continental plates any human or natural activities that causes shocks or breaks the lithosphere or causes an imbalance due to removing of or deposition of the soil/water/debris, results in an imbalance that is compensated through isostatic adjustment. Volcanic activities usually occur along plate boundaries due to the breaking of the lithosphere releasing hot molten magma in the form of volcanoes.
The San Andreas Fault in California, America (which is formed by the sliding boundary between the North America Plate and the Pacific Plate) is a good example of such a region with a string of volcanoes. Earthquakes can serve as a warning of future eruptions of volcanoes. And likewise volcanic eruptions can be warning signs of the occurrences of earthquakes.
It is possible to predict seismic activities (earthquakes and volcanoes). Earthquake prediction can be very concise stating precise times, location and magnitude of future earthquakes. Alongside earthquake prediction, earthquake engineering provides vital information on the construction of building that will have very minimum damage during earthquakes. Earthquake insurance helps in replacing damaged property when disasters of such nature occur.
Earthquake early warning systems also help people become aware of the imminent dangers before it strikes.
Earthquakes are closely related to tremors. Tremors are the movements that happen before or after an earthquake. The extent of the shaking is dependent on the magnitude, distance, depth of focus, topography and the local conditions at the time of the earthquake.
The occurrences of earthquakes and volcanoes is not without its benefit to men. The movement of the earth’s surface results in the occurrences of volcanoes. When molten lava comes to the earth’s surface it brings with it rich soils. This supports agricultural activities. Minerals can also be brought closer to the earth surface resulting in a lot of gains in the mining sector.
It is becoming more important for homes and commercial buildings to be built with adherence to earthquake guidelines. This will minimise damage in the event of an earthquake. People should be prepared for an earthquake as the causes of such catastrophes are getting more complex with time. Some of the general precautions that can be take include keeping a fire extinguisher in the home.
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