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Seismic Activity in Ghana: Past, Present and Future

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Seismic Activity in Ghana: Past, Present and Future View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Earth-prints Repository ANNALS OF GEOPHYSICS, VOL. 47, N. 2/3, April/June 2004 Seismic activity in Ghana: past, present and future Paulina Ekua Amponsah Geological Survey Department, Accra, Ghana Abstract Though Ghana is far away from the major earthquake zones of the world, it is prone to earthquake disaster. Ghana has records of damaging earthquakes dating as far back as 1615. The last three major events occurred in 1862, 1906 and 1939. This paper presents the main historical and current instrumental recorded earthquakes of Ghana and the steps being taken to mitigate the negative effects of such disastrous occurrences in the country. The discussion is based on historical and current data obtained from the seismological observatories in Accra and Kukurantumi. Historical earthquakes of magnitude greater than 6.0 and current local tremors with magni- tudes ranging from 1.0 to 4.8 on the Richter scale have been recorded since the establishment of the seismograph stations. Key words Ghana – seismicity mograph was in operation. In March 1973, a seismograph observatory equipped with a WorldWide Standard Seismograph Network 1. Introduction (WWSSN) system was established at Kukuran- tumi in Eastern Ghana. It operated continuous- Ghana is located on the southeastern margin ly until October 1974 and then intermittently of the West Africa craton and is far away from until continuous recording began again in 1977 the major earthquake zones that mark the pres- (Quaah, 1980). At present, there is a nine-sta- ent day lithospheric plate boundaries. However, tion radio telemetric network with a central a number of major and minor earthquakes have recording station at the Head Office of the Ge- struck the country in past and present. Earth ological Survey Department in Accra. It is an tremors of magnitude ranging from 1.0 to 4.8 analog recording system. Plans are underway to on the Richter scale have been recorded in re- procure a digital recording system. Heat pens cent times (fig. 1). (Hot-Stylus) are used for the recordings (Am- Instrumental recording of earthquakes in ponsah, 2002). Ghana began in 1914 when the colonial gov- ernment installed a Milne’s single-boom seis- mograph in the country. It operated until 1933 2. Geological setting and previous studies when it ceased recording (Banson, 1970) due to financial constraints. Several minor shocks About two thirds of the land surface of were recorded during the period when the seis- Ghana is covered by Birimian rocks that are of paleoproterozoic age. These rocks form the easternmost component of the Man Shield of West African craton that has remained stable Mailing Address: Dr. Paulina Ekua Amponsah, Geo- logical Survey Department, P.O. Box M. 80, Accra, Ghana; since 1.7 Ga. The eastern portion of the country e-mail: [email protected] is predominantly underlain by middle to late 539 Paulina Ekua Amponsah Fig. 1. Seismicity of southeastern Ghana. Proterozoic rock units that are differently boundary fault. It is evident from their results named as Dahomeyan, Togo, Buem and Voltain that there is a low level of seismic activity scat- belts (Leube, et al., 1990). tered along the Akwapim fault zone that could Work done by Bacon and Quaah (1981) in- be due to normal faulting suggested by Burke dicates that most of the earthquakes in Ghana (1969). In Quaah and Bacon’s view, the epicen- occur in the western part of Accra at the junc- tres of the located earthquakes are related to the tion of the two major fault systems namely, the level of activity of the faults. This they allege to Coastal boundary fault and Akwapim fault be due to the existence of an old thrust zone, zone. According to them most of the epicentres which has been reactivated. Burke (1971) asso- are located south of Weija suggesting that there ciated the seismic activity of Ghana with the is little activity north-eastward along the Ak- junction of the chain fracture zone and the wapim range and westward along the Coastal African continental margin. 540 Seismic activity in Ghana: past, present and future Blundell and Banson (1975), Blundell scale of India, unless otherwise stated. It ranges (1976), suggested that continuous strike – from I to X (Junner, 1941). There are no records slip movement on the Romanche fracture of earthquakes in Ghana between 1636 and zone adjacent to the West African continental 1858 but there is the possibility that there were margin may be the cause of the earthquakes. some minor shocks during the period. In 1858, Ambraseys and Adams (1986) studied the however, an earthquake was reported to have history of earthquakes in West Africa and been felt in Accra (Ambraseys and Adams, were of the view that Accra, the capital of 1986). Ghana is the most seismically active area in In 1862 a very strong earthquake struck the the region. capital city of Accra and caused considerable A recent review of geological and instru- damage to many important structures. Its maxi- mental recordings by Amponsah (2002) shows mum intensity and magnitude (ML) were esti- that earthquakes have occurred in the past and mated to be IX and 6.5 respectively (Quaah, are still liable to occur within the vicinity of the 1980). Ambraseys and Adams (1986) obtained intersection of the Akwapim fault zone and the a surface wave magnitude greater than or equal Coastal boundary fault. to 6.5 and a maximum intensity of IX (uncer- Microseismic studies in southern Ghana in- tain) on the MSK scale for this event. The cas- dicated that the seismicity is associated with ac- tle (the seat of government) and some forts were tive faulting (Essel, 1997) between the east- rendered uninhabitable, together with all stone west trending Coastal boundary fault and a buildings (Junner, 1941). Three people were northeast-southwest trending Akwapim fault killed in Accra. The earthquake shock was felt zone, defined by a number of active faults. A along the coast, east of Togo and in Benin, which geophysical study by Essel indicated that the are neighbouring countries to Ghana. An earth- seismic activity is related to deep-seated faults. quake, which was felt in Accra in 1863, appears to have been an aftershock of the 1862 earthquake but there is no detailed information on the event to 3. Earthquakes in Ghana confirm this. Two severe shocks rocked Accra in 1871 and 1872. The magnitudes of these events Junner (1941) gave a report on historical were 4.6 and 4.9 respectively and the maximum earthquakes in Ghana. Ambraseys and Adams intensities were VI and VII (Ambraseys and (1986) studied the seismicity of West Africa, Adams, 1986). The 1872 event caused damages to which included the history of earthquakes in buildings in Accra. In 1883 there was another mi- Ghana. Their findings on the history of earth- nor event, which was reported to have been felt in quakes in Ghana are presented in the paper. Accra (Junner, 1941). The first earthquake in Ghana according to Two severe shocks were felt in Eastern Ambraseys and Adams occurred in 1615. The Ghana and Togo at about 9:00 p.m. and 9:20 fortress of Sao Jorge at Elmina was destroyed. p.m. on 20 November 1906. The maximum in- This was followed by a few aftershocks a cou- tensity of the shocks was estimated to be VIII ple of months later. The earliest recorded earth- at Ho in the Eastern region of Ghana (Junner, quake in Ghana occurred at about 2:00 p.m. on 1941). The magnitude (ML) estimation was 6.2 18 December 1636 in the Axim district in (Quaah, 1980), however, Ambraseys and southwestern Ghana near the border between Adams’s estimation of the surface magnitude Ghana and the Ivory Coast. Its surface magni- (Ms) was 5.0 and the maximum intensity VIII tude (Ms) was 5.7 and the maximum intensity on the MSK scale, which they considered un- was IX on the MSK scale (Ambraseys and certain. Many buildings were greatly affected Adams, 1986). The buildings and underground by this event, some cracked and others partly workings of a gold mine at Aboasi, northeast of destroyed. No casualties were reported for this Axim collapsed burying many of the miners event, also felt by people in Togo and Benin. (Claridge, 1915). The intensities mentioned in During the same month several minor shocks the paper are based on the Modified Mercalli were felt lasting for three weeks. 541 Paulina Ekua Amponsah On 11 February 1907 a fairly strong shock events caused panic among the people in the af- was felt in Accra and Lome. An earthquake lo- fected areas (Amponsah, 2002). The National cated out to sea and accompanied by tidal Earthquake Information Centre (NEIC) in the waves destroyed the wharf at Lome in Togo on USA located the 6 March event at 5.518N, 11 May 1911. During the same day at 3:21 0.313W, body magnitude (MB) at 4.4 and at a p.m., an earthquake was felt in Accra but did depth of 10km. The local magnitude was 4.8 on not cause any damage (Junner, 1941). the Richter scale. The most destructive earthquake in Ghana Many minor tremors were recorded from that caused a lot of damage and loss of life and 1998 to 2002 with magnitudes ranging from property occurred on 22 June 1939. The earth- 1.0 to 3.0 on the Richter scale. The intensities quake occurred at about 7:20 p.m. and was felt range from I to IV. The magnitudes of the for about 20 to 30 seconds.
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