Earthquakes 101 with Emphasis on Accra’s Seismicity



Introduction This is the first in the series of articles intended to discuss earthquake issues and their related effects on civil society. This article will focus on the “hows” and the “whys” of earthquake generation particularly in the context of Accra. The article is presented in a question and answer form, and is intended to be a simple overview and not a detailed technical discussion.



Questions 1: What is the earth’s structure like? A cross-section through the earth’s interior shows five main sections: the outer and inner core, the lower and upper mantle, and the crust. The upper mantle and crust are termed the lithosphere, and are broken down into seven major regions called tectonic plates. The seven tectonic plates are the North American, African, Eurasian, Antarctic, Pacific, Indian and South American plates.



2: How and why do earthquakes occur?



Earthquakes develop when there is relative motion along a fracture within the lithosphere; such fractures are normally fault lines or tectonic plate boundaries. The location where this takes place is called the epicentre of the earthquake. This relative motion is associated with the release of energy, which travels in the form of waves away from the epicentre. This phenomenon is similar to waves that form when a stone is dropped in a pool of water. As the waves travel away from the source, their amplitude decreases with distance. The rate at which this happens is a function of a number of factors, but in general, areas within a 100 km radius of the epicentre could experience significant shaking.



There are generally two main types of earthquakes: inter-plate earthquakes and intra-plate earthquakes. Inter-plate earthquakes occur when two tectonic plates grind against each other. An example of such a case is the Cascadia subduction zone in the north-western part of North America, where one tectonic plate is sliding underneath another. These types of earthquakes generate enormous amounts of energy and have been the cause of major earthquakes around the world. Most earthquakes in the Pacific Ring of Fire region including the Great 2004 tsunami-generating Sumatra earthquake are inter-plate earthquakes. Intra-plate earthquakes, on the other hand, occur due to relative motion along active fault lines within mid-continental regions of the earth’s crust, and not along plate boundaries. Intra-plate faulting results in the generation of large earthquakes, but not to the scale that has been observed for inter-plate earthquakes.



3: What does the magnitude of an earthquake refer to?



The magnitude of an earthquake represents a measure of how large an earthquake is. In other words, it expresses how much energy is released during the earthquake. There are different earthquake magnitude measures, these are: the Richter magnitude; the Surface and Body Wave magnitudes, and the Moment magnitude. The most popular is the Richter magnitude; however, the Moment magnitude is generally accepted in the technical community as the most reliable magnitude scale.



Generally, earthquake magnitudes below 5 are considered as small earthquakes or earth tremors. Earthquakes with magnitudes above 6 are generally significant, whereas, those above 7 are considered as strong; great earthquakes are of magnitude 8 and above. To put the above magnitudes into perspective, the energy released by a magnitude 6 earthquake is a thousand times (three orders of magnitude) greater than that of a magnitude 5 earthquake.



Earth tremors generally happen more often and are normally not noticeable. Larger earthquakes on the other hand do not occur often, but have a significant impact on society. The epicenters of earthquakes are located with instruments called seismographs. In addition to seismographs, strong-motion equipment (accelerometers) are used to record earthquakes in the form of accelerations. The maximum value of the recorded acceleration is termed the peak ground acceleration (PGA). This is normally expressed as a ratio of the acceleration due to gravity, and is one of the important earthquake engineering design parameters (note: not the only).



4: What type of earthquake characterizes Accra’s Seismicity?



Accra’s seismic activity is due to intra-plate faulting. Most of the epicenters of earthquakes and earth tremors recorded in the Greater Accra Metropolitan Area are located along two major fault systems, namely: the north-easterly Akwapim fault zone and the north-easterly coastal boundary fault. These two faults are observed to meet in the Weija area.



5: What earthquakes has Accra experienced in the past?



Four significant earthquakes have struck Accra within the period of known history: 1636, 1862, 1906 and 1939. The best measured earthquake is the 1939 event that measured 6.5 on the Richter scale. Some researchers argue that the 1862 earthquake was more severe than the 1939 earthquake; the 1906 earthquake was, however, less severe than the 1939 one. The 1939 earthquake caused extensive damage to Accra. It was even felt in some regional capitals, and some coastal villages in the Central region were virtually destroyed. To put the 1939 earthquake into perspective, the recent earthquake in Haiti had a magnitude of 7.0 on the Richter scale.



In recent years, the earthquakes experienced in Accra have ranged between a magnitude of 3.8 and 4.8 on the Richter scale. Famous among these was the event that occurred during the independence-day anniversary celebration in 1997.



6: Can earthquakes be predicted?



The simple answer is no. To date, no rational approach has been developed to help predict when an earthquake would occur. The most extensive earthquake prediction project was conducted in the US in Parkfield, California in the 1980s. This project involved the extensive instrumentation of a section of the San Andreas fault, which is known to be the source of most of the earthquakes in the area. The results of this study were inconclusive and further studies have still not been able to answer this question.



However, some events have been known to occur before an earthquake event. For example, some animals such as horses that have a heightened sense of awareness have been noted to become hysterical. Also, and even more important, an increase in the emission of some gases such as radon gas from the ground, have been observed to occur before an earthquake. This was observed in the recent 2009 L’Aquila Italian earthquake. Radon gas measurements have been previously conducted in the Weija area, and the concentration of Radon gas was found to be above expected threshold levels. This in itself does not mean an earthquake is imminent, but further confirms the Weija area as an active seismic zone.



Conclusions In conclusion, Accra has experienced destructive earthquakes in the past, due predominantly to intra-plate faulting in the Weija area. The time of the next major earthquake in Accra cannot be predicted, however, it will certainly happen. For residents of Accra, and as Ghanaians as a whole, living our lives as though this event will never take place, is similar to a child playing with fire, hoping it will not burn him/her.



Nii Kwashie Allotey, Ph.D., P.Eng., Email: nii50@inbox.com