Thu, 08 Feb 2018 07:45:01 +0000
By Ronald Lwamba
Several Lusaka residents on 2nd November, 2017, panicked after the city experienced a tremor, i.e., a small earthquake in which the ground shakes slightly, measuring 4.7 on the Richter scale. The earthquake lasted for about 30 seconds leaving residents in panic mode as major buildings experienced major shakes but no damage was reported.
The incident happened around 11:04 hrs and people went on to express their feelings on social media. The Geological Survey Department Director, Chipilauka Mukofu, told ZNBC News that the earthquake occurred in Ngwerere area of Chibombo district in Central Province. He further disclosed that the preliminary results showed that the earthquake was recorded at a depth of 12 kilometres below the surface. He added that no infrastructure damage had been recorded so far. The last earthquake Zambia experienced with a magnitude of 6.1 was on 24th February, 2017, in Kaputa, at a depth of 10 kilometres.
Most of the tremors that have been felt in Lusaka have been caused by the Kariba Dam Reservoir, which I will discuss later. So, it is not surprising that when I first heard of this particular earthquake, I attributed it to Lake Kariba. The first earthquake or tremor I experienced due to the Kariba Dam Reservoir was in the 1970s, as an engineering student at UNZA, lying in bed in International Hall, an ideal situation for feeling earthquakes, which are usually easily felt when one is lying in bed.
The saying that a person “keeps his feet on the ground” means to have a sensible and realistic attitude to life. The fact that such a saying exists shows how much comfort we take in the idea that the ground beneath our feet is unmoving, unchanging and dependable. The idea that the ground we walk and sleep upon is liquid and mobile seems, therefore, to be really difficult for us to comprehend. Indeed, much of our civilization, from our houses and buildings to our energy, food and water sources, depends on an unmoving earth.
In truth, however, our planet’s seemingly stable ground is made up of enormous pieces of rock that are slowly but constantly moving. These pieces continually collide with and rub against each other, and sometimes their edges abruptly crack or slip and suddenly release huge amounts of pent-up energy. These unsettling events are called earthquakes, and small ones happen across the planet every day, without us even noticing them. But every so often, a big earthquake occurs, and when that happens, the amount of energy it releases, called seismic waves, can wreak havoc of unimaginable destruction and kill and injure many thousands of people.
The understanding of earthquakes can be traced to Alfred Wegener, a German climatologist and arctic explorer, who came up with the concept of continental drift in the early 1900s. The idea was that the continents move around on the Earth’s surface. His hypothesis was that the continents were once connected. Today, after a lot of scientific research and evidence that has been collected, we now know that Wegener was right.
It was in 1915 that he published his ideas in a book, Origins of the Continents and Oceans. Wegener thought that the continents we know today had once been part of an earlier supercontinent. He called this great landmass Pangaea (Greek for “all land”). According to the continental drift hypothesis, Pangaea broke apart and the pieces moved to their present places, becoming today’s continents. Continental drift helped explain issues in geology—like why South America and Africa seem to fit together. However, continental drift could not be accepted immediately by scientists because, at that time, there was no evidence to explain what force made the continents move.
This is because in the late 19th and early 20th centuries, geologists assumed that the Earth’s major features were fixed, and that most geologic features such as mountain ranges could be explained by vertical crustal movement. The assumption at the time was that the earth was solid, which made the various proposals difficult to explain.
The accepted explanation came in the form of the theory of plate tectonics. Alfred Wegener suggested that the present continents once formed a single land mass that drifted apart, thus releasing the continents from the Earth’s core and likening them to “icebergs” of low density granite floating on a sea of moredense basalt. But without detailed evidence and a force sufficient to drive the movement, the theory was not generally accepted: the Earth might have a solid crust and a liquid core, but there seemed to be no way that portions of the crust could move around. Later science supported theories proposed by the English geologist, Arthur Holmes, in 1920 that plate junctions might lie beneath the sea and Holmes’ 1928 suggestion of convection currents within the mantle as the driving force.
The outer layers of the Earth are divided into lithosphere and asthenosphere. The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which ride on the fluid-like (visco-elastic solid) asthenosphere. These plates move, typically, 10-40 mm per annum.
The plates are around 100 km thick and consist of lithospheric mantle overlain by either of two types of crustal material: oceanic crust or continental crust. The two types of crust differ in thickness, with continental crust considerably thicker than oceanic crust (50 km vs. 5 km).
The theory is that the Earth’s crust, or lithosphere, comprises many plates that slide over a lubricating asthenosphere layer. At the boundaries between these huge plates of rock and soil, the plates sometimes move apart, and magma, or molten rock, comes to the surface, where it’s called lava. It cools and forms new parts of the crust. The line where this happens is called a divergent plate boundary.
The plates can also push against each other. Sometimes, one of the plates will sink underneath the other into the hot layer of magma beneath it and partially melt. Other times, the edges of the two plates will push against each other and rise upward, forming mountains. This area is called a convergent plate boundary [source: Silverstein].
But in other instances, plates will slide by and brush against each other — a little like drivers on the highway sliding and rubbing past each other, but very, very slowly. This region between the two plates is called a transform boundary. It is here where earthquakes are usually caused when the rock underground suddenly breaks along a fault. This sudden release of energy causes the seismic waves that make the ground shake. When two blocks of rock or two plates are rubbing against each other, as they are being moved by convection current, they stick a little. They don’t just slide smoothly past each other. The rocks are still pushing against each other, but not moving. After a while, the rocks break because of all the pressure that has built up. When the rocks break, the earthquake occurs. During the earthquake and afterward, the plates or blocks of rock start moving again, and they continue to move until they get stuck again. The spot underground where the rock breaks is called the focus of the earthquake or hypocentre. The place right above the focus (on top of the ground) is called the epicentre of the earthquake. Most, though not all, earthquakes happen along transform boundary fault lines.
This little experiment performed by breaking foam rubber in half helps to understand how an earthquake occurs:
Break a block of foam rubber in half.
Put the pieces on a smooth table.
Put the rough edges of the foam rubber pieces together.
While pushing the two pieces together lightly, push one piece away from you along the table top while pulling the other piece toward you. See how they stick?
Keep pushing and pulling smoothly.
Soon a little bit of foam rubber along the crack (the fault) will break and the two pieces will suddenly slip past each other. That sudden breaking of the foam rubber is how an earthquake occurs and that is just what happens along a fault line.
The major continental plates shown in the map are:
North American Plate
South American Plate
African Plate
Arabian Plate
Eurasian Plate
Bismarc Plate
Indo-Australian Plate
Antarctic Plate
The major oceanic plates are:
Juan De Fuca Plate
Pacific Plate
Nazca Plate
Scotia Plate
Cocos Plate
Caribbean Plate
Philippine Plate
Fiji Plate
Carolina Plate
The author has worked as a Town Engineer for the then Municipal Council of Livingstone and ZESCO, initially as a Resident Engineer for Itezhitezhi rising to the post of Senior Manager, Civil Engineering where, among other things, he was in charge of the preparation of feasibility studies for hydropower projects. He was also a Part-time Lecturer in Renewable Energy at UNZA. He holds a Bachelor of Engineering Degree (Civil) from the University of Zambia (1974), Post Graduate Diplomas in Water Resources Development (University of Roorkee, India) and Hydropower Development (University of Trondheim, Norway) and a Master of Engineering in Water Resources Development (University of Roorkee, India). For comments: rbclwamba@gamail.com
