UNIVERSITY OF GOTHENBURG Department of Earth Sciences Geovetarcentrum/Earth Science Centre Earthquake frequency, distribution and the seismic hazard of Massawa - Eritrea Daniel Ghebrezghi ISSN 1400-3821 B653 Bachelor of Science thesis Göteborg 2011 Mailing address Address Telephone Telefax Geovetarcentrum Geovetarcentrum Geovetarcentrum 031-786 19 56 031-786 19 86 Göteborg University S 405 30 Göteborg Guldhedsgatan 5A S-405 30 Göteborg SWEDEN Contents 1. Introduction – page 2 1.1 Background – p.2 1.2 Area description – p. 3 1.3 Issues and purpose of study - p.6 1.4 Delimitation - p.6 2. Methodology – page 7 2.1 Seismic data – p.7 2.2 Geographic Information System – p.8 2.3 SEISAN – p.8 3. Results – page 9 3.1 Volcanic- and regional tectonic framework – p..9 3.2 Spatial distribution of seismic events – p.13 3.3 Magnitude and frequency - p.20 3.4 Massawa; vulnerable elements and the structural environment - p.23 4. Discussion – page 26 4.1 Source criticism; seismic data, magnitude and location of events - p.26 4.2 Vulnerability of Massawa - p.27 5. Conclusions – page 29 6. References – page 30 6.1 Written material – p.30 6.2 Internet – p.31 6.3 GIS data – p.32 7. Acknowledgements – page 33 8. Appendices – page 34 8.1 Appendix 1. Seismic- and volcanic history of Eritrea, interpretation of Gouin (1979). – p.34 8.2 Appendix 2. Data on seismic events retrieved from ISC. – p.49 8.3 Appendix 3. PDE Data on seismic events. – p.58 8.4 Appendix 4. SEISAN-Processed data from ASME and ABSE considering 1999.– p.65 8.5 Appendix 5. SEISAN-Processed data from ASME and ABSE considering 2001. – p.71 8.6 Appendix 6. Seismic data from GCMT.– p.80 Abstract The city of Massawa, hit by destructive estimated magnitude 6.2 and 5.8 earthquakes in 1884 and 1921, hosts Eritrea’s largest port and is administrative capital of the Northern Red Sea-region. The city´s population experience occasional tremors annually, and studying entries in earthquake catalogs there are events of close to moderate magnitude-range with epicenters in the greater area. Through a process in collecting and comparing data from different sources on seismic activity and plotting these events in GIS. Areas of where seismic activity have originated considering a period of up to fifty years back is displayed in cartographic material, this considering a study area covering Eritrea and the southern Red Sea. It appears none of these more recent events of activity in the region have posed threat to Massawa city, or met magnitude ranges equivalent to the quakes of 1884 and 1921. Through a review of the structural outlne; the conclusion is that the perception of the seismic hazard could still be regarded high. This if one takes into consideration the national importance of present day Massawa in terms of lifeline utilities present, and in some areas regarding the construction methods used. 1. Introduction 1.1 Background Earthquakes becomes a hazard when it through primary or secondary effects pose threats to humans or human activities. Earthquakes of course vary both in intensity and distribution, thus they occur in regions as well in size , where the effect and outcome do not pose such threat. Understanding the occurrence of natural earthquakes in a geographical context, one must have an insight to the geological background to understand a relationship between the spatial distribution of seismic events, and the region in which they frequent. It has to be stated that though earthquakes can be the result of many different factors, most are of natural processes which occur within the lithosphere. A common earthquake is the result in an abrupt release of elastic strain accumulated in bedrock. The internal strength within rock, or the frictional resistance between blocks of previously fractured rock in relation to the amount of strain implemented, is what decides time until- and size in an oncoming offset. The point of rupture is considered the earthquakes origin and is identified as the focus or hypocenter. When rupturing, accumulated strain within or between the blocks of rock is transformed into released energy. The energy leaves the point of rupture in form of seismic waves and these will cause an impact, depending on the amount of energy released, the type of seismic wave, and the medium in which they travel. This is what could cause the earth to quake. (Kearny & Vine 1996, p. 8-10, Ogubazghi & Woldehaimanot 1998, p. 68-71) 2 Earthquakes of natural origin are commonly divided into two groups, tectonic driven and those of volcanic origin. According to Kearny and Vine (1996, p.76), a view upon a global distribution in earthquake epicenters, the point at the earth´s surface right above the focus, one could roughly define boundaries between tectonic plates, and Ogubazghi and Woldehaimanot (1998, p. 5) states that up to 90% of all natural earthquakes are those of tectonic origin. Earthquakes of tectonic origin is the result of stress built up by the same forces that cause the relative movement of tectonic plates, thus earthquakes are common features in margin areas and for example around zones of plate divergence, convergence or strike-slip motion. Though most earthquakes actually occur in the vicinity and/or along margins of large-scale tectonic features, the presence of intra-plate earthquakes concludes that natural seismic events by no means are bound to such areas. (Kearny & Vine 1996, p.78) Natural intra-plate earthquakes can have a tectonic origin as stress in plate interiors could derive from its margins spread horizontally, or vertically driven by a regional rise in the underlying asthenosphere. (Weiran, Zuoxun, Dewei, Jishan, Jie & Wenxing 2009). Earthquakes on plate interiors could as well be a result of volcanism, though areas of volcanic activity are often found along margin-areas and plate boundaries. Volcanic driven earthquakes is considered of different nature but have similar characteristics as those driven by the mechanisms of plate tectonics. Eggert and Walter (2008) states there are indications on a relationship between tectonic earthquakes and volcanic activity. Though the origin of stress built up in rock in the case of volcanic seismicity is often a result of migrating magma, inflating chambers and dikes, which creates and accumulates strain and further could cause sudden rupture of surrounding bedrock. Sequences of seismicity around volcanic centers are thus often a precursor to an oncoming eruption at the site (Roman & Cashman 2006). 1.2 Area description The area studied; constituted by Eritrea and its adjacent coastal waters, is located on the north-eastern peninsula of the African sub-continent known as Africa´s horn (figure 1). Liberated in 1991 and internationally recognized as a sovereign state in 1993, Eritrea on the political map shares its national boundaries with the countries of Sudan, Djibouti and Ethiopia. Across the Red Sea-divide it borders Yemen and Saudi-Arabia. According to the National Statistics Office of Eritrea, the country host an estimated population of 4,106,168 inhabitants (population figure as of 2010), and according to FAO (web; 2011) it is by land area constituted of approximately 124,000 km2. Located in the Sahel-region of Africa largely presenting a semiarid climate, climatic changes with topography relates to a population divide where Eritrea largely present higher concentrations in the less dry highland-areas. The topography of Eritrea presents a steep gradient with elevation ranging from 3 areas below sea level to up to 3000 m s l. After the physical outline, the country have been commonly divided into three sub-regions; the central highlands and the eastern- respectively western lowlands. Eritrea acts the northernmost outpost of the East African Rift-system, also known as the Great rift valley, and with its approximate 1.000 km long coastline it borders the Red Sea. When located, in terms of plate tectonics; a triple-junction area, where the East African-, the Red Sea- and the Gulf of Aden-rifts meet, the country face presence of associated phenomena such as events of volcanism and seismic activity. Overlooking figure 1, the region in the African context stands out as one of the most probable to be struck by destructive earthquakes on the continent. Thus, the country does have a history of earthquakes, some with severe outcomes considering damage to the built environment as well as loss of human lives. Tough Eritrea’s location in a seismic prone region and earth-tremors historically have frequented the country, damage and destruction caused by earthquakes seem to have been rather uncommon and events recorded instrumentally mainly seem to fall in minor- up to moderate-magnitude ranges. More so, areas actually affected by tremors and the locations of Figure 1. Zoning of Africa after the 1956 version of the Modified Mercalli Intensity-scale, estimated epicenters seem data based on the 1988 Munich Re publication; world map of natural hazards. in general to be constituted by either adjacent coastal waters, or sparsely populated arid lands perhaps of less interest in the viewpoint of human settlement or future development. Despite above, the port town of Massawa is in the Eritrean context somewhat of an exception. While there is instrumental recordings of earthquakes adjacent to-, and historical reports on earth-tremors from other Eritrean population centers (see appendix 1), none seem to have faced a seismic hazard in terms of damage and destruction comparable to that of Massawa. 4 Located along one of the world’s busiest waterways regarding trade, the natural deep-water port of Massawa is situated approximately on latitude 15.6° N, longitude 39.5° E at the south wester-section of the African Red Sea-coast.