Nigerian Geological Survey Agency (NGSA) received the news of a disturbing ground shaking experienced by the inhabitants of Mpape and surrounding environments on the 5th of September. The Agency despatched a team of geologists and geophysicists on a fact finding mission.
From the account of people interviewed by the NGSA team as well as the reconnaissance survey carried out by the team, the following findings have been ascertained:
i. The vibration started around 2.30pm on 5th September, 2018 which lasted about a second but reoccurred 3 hours later around 5.30pm. The vibration was again felt around 8.00pm which lasted for about 10 seconds.
An earthquake could be generally defined as a sudden vibration, sometimes violent, of the earth’s surface that follows a release of elastic energy in the Earth’s crust. This energy is generated by a sudden dislocation of the crust when the built up stress exceeds the strength of the rock mass. The rock ruptures and assumes new position. The process of breaking generates vibrations called seismic waves that travel outward in all directions from the point of initial rupture.
Earthquakes tend to be concentrated in particular zones which coincide with the boundaries of the tectonic plates into which the earth is divided. Such boundaries may either be spreading zones, transform faults or subduction zones.
However, it is erroneous to think that the countries or zones that are not contiguous with plate boundaries are seismic.
It is being postulated that as plates continue to move and plate boundaries change over geologic time, weakened boundary regions become part of the interiors of the plates. These zones of weakness (sutures) within the continents can cause earthquakes in response to stresses that originate at the edges of the plate or in deeper crust. Chandra (1977) suggested that the orientation of the zones of weakness with respect to the ambient stress field may be an important factor in determining the fault along which future earthquakes are likely to occur.
Many of such earthquakes have been reported within the West African sub-region– Ghana, Guinea, Liberia, Ivory Coast, Cameroun Cases of earthquake occurrences in Nigeria, though largely unsubstantiated, had been reported in various parts of the country. It was however not until the 1984 earth tremor around Ijebu-Ode, S.W. Nigeria that the nation realized that the destructive earthquakes, read more from news items around the world, may after all be at our doorsteps. It was established that the tremors occurred at three distinct events, each lasting less than one minute at 1pm and 1.30pm on 28th July, 1984 and at 11.30am on 2nd August, 1984 within the area shown in (fig 5). From oral evidence the intensities differed from one locality to another with the maximum intensity of VI around Ijebu-Ode and minimum intensify of IV around Ibadan.
The possible causes postulate that the NE-SW, NW-SE conjugate sets of strike-slip faults must have played a critical role in the generation of the tremors. The faults are characterised by dextral sense of movement and are traceable to the coast line (McMurry, 1976). The faults are also suspected to be related to the ‘Chain’ transcurrent ocean fracture system. They are suspected to be part of the ‘Ife Wara’ fault traceable to Zungeru and even to Niger Republic
Earth tremors were also reported in Gembu in the Mambila Plateau in 1987 and at Ibadan in 1990.
The assumed epicentre (Mpape) lies at the boundary between Abuja and Gitata topographic Sheets (186 and 187). The geology of the area is largely dominated by migmatite gneiss complex making contact with schist belt units belonging to the Birnin-Gwari and Karu Schist Belt and plutons of Pan-African granitoids in the northwest portion of the map sheet.
Analysis of regional airborne geophysical data has identified the area to have undergone series of tectonic activities leading to structural emplacement. Some of these structures are deeply seated, broad and extensive and could serve as good channels for stress discipation. For instance, an extensive NE-SW trending linearment running through Kachia, kagarko in Kaduna state, linking karo, karshi in Nasarawa state is clearly distinguishable on the first vertical derivative data of area around Mpape. A verifyable evidence relating this regional structure and the assumed epicentre (Mpape) is yet to be established.
This preliminary investigation therefore associate the tremor to quarrying activities in the area which involve intense blasting that led to stress accumulation. The accumulated stress was then released as seismic energy triggering the ground shake. The stress may have travelled through secondary fractures to other areas where the tremor was experienced such as Gwarinpa and adjoining areas.
The intensity of the tremor is estimated to be between 3 and 3.5 on the Modified Mercalli scale since the investigating team did not observed any visible damage such as cracks, shuttering on windows or colouration in ground water.
The Agency shall keep a close monitoring team for the next 48 hours within which arrangement ought to have been concluded for a detail survey in the area involving reflection seismic and gravity measurement to fully unravel immediate and remote causes of the tremor.
The inhabitant of Mpape and the general public are once again advised not to panic as the situation is not yet out of control. The Federal government has recently procured six earth quake monitoring seismograms to enable proper monitoring of all ground disturbances. The process of installations of the seismograms has since begun in six geological zones of the country.