Academic Programmes - Student Biographies
Georges Mavonga Tuluka
Ph.D. Degree Candidate
University of the Witwatersrand
I am from the Goma Volcano Observatory (GVO), which is part of the Department of Geophysics of the 'Centre de Recherche en Science Naturalles’ (CRSN) in the Democratic Republic of Congo (DRC). I started working in the Seismic Unit of the Department of Geophysics of CRSN in 1988. I completed postgraduate studies in seismology at the International Institute of Seismology and Earthquake Engineering, Tsukuba, Japan in 1991. In 1995 I completed the course ‘Assessing Natural Hazards and Monitoring Active Volcanoes’ at the University of Hawaii, Hilo, U.S. A. My current duty at GVO is to monitor tectonic and volcanic earthquakes in order to advise the Governor of the Province on issues related to earthquake and/or volcanic hazards and disasters in the region. In March 2007 I registered as a fulltime PhD student at the University of the Witwatersrand under the supervision of Professor Ray Durrheim, the incumbent of the recently established Wits/CSIR South African Research Chair in Exploration, Earthquake and Mining Seismology. I am funded by the Belgian Technical Cooperation agency (BTC).
My research topic ‘Monitoring Earthquakes and Natural Hazard Mitigation in DRC and Adjacent Areas’ forms part of the AfricaArray initiative.
The most prominent feature of the DRC’s geology is the Western Rift Zone (WRZ), which runs through its eastern border regions and neighbouring countries (e.g. Uganda, Rwanda, Burundi, Tanzania) between 28°E to 32°E and 4°N to 12°S. The WRZ extends over a 1600 km arc, including lakes Albert, Eduard, Kivu and Tanganyika, until it joins the eastern branch. The Western Rift Valley of Africa has experienced severe earthquakes and volcanic eruptions in recent historical times. Earthquakes with magnitude >=6 are not frequent, but may cause significant destruction. They occur mostly in DRC and neighbouring countries (e.g. Uganda and Tanzania). In 1991, IAVCEI selected the Nyiragongo volcano as the ‘African decade volcano’ for the International Decade of Natural Disaster Reduction (IDNDR) programme. Nyiragongo is located about 20 km north of Lake Kivu and 15 km north of Goma, a city of about 500,000 inhabitants. Goma is twinned to Gisenyi in Rwanda, which has a population of about 100,000. Several small villages are also located on the flank of the volcano. Population growth and poor or non-existent planning has led to relatively uncontrolled use of land for building, and the development of sites vulnerable to earthquake and/or volcanic risk.
The first part of my PhD study, a comprehensive earthquake hazard assessment of the DRC and adjacent areas, aims to minimize the risk to life as the population density continues to increase. Seismicity and source mechanisms of earthquakes in the DRC and neighbouring regions, especially in populated areas, must be evaluated to identify source zone(s) affecting the sites, formulate recurrence models that describe the seismicity of each zone, propose ground motion models that take the tectonics structure of the region into account, and ultimately produce a probabilistic seismic hazard map for the DRC and adjacent areas.
Our seismicity database will be built from the following sources: Gutenberg-Richter (1949), National Earthquake Information Centre (NEIC), Global Hypocenter Data Base CD-Rom, International Seismological Centre (ISC), previously International Summary (ISS) Bulletin, ISC monthly bulletin, Bulletins des Stations Seismographiques de l'Institut pour la Recherche Scientifique en Afrique Centrale (IRSAC), the CRSN (previously IRSAC) seismic network composed of 6 digital stations equipped since 2003 with 3-component Lennartz sensors (LE-3D/5sec) in 4 stations and 3- component broadband Trillium 40 sensors in 2 stations, and the USGS and ISC web sites.
Fault plane solutions of earthquakes in the DRC rift zone and adjacent areas will be analysed. The polarity of initial P-wave motions from ISC bulletins, stations of CRSN and neighbouring stations will be used to constrain the focal mechanisms of earthquakes for the period from 1990 to 2006. The events will be chosen only if they have sufficient polarity data (only earthquakes with a minimum 10 P arrivals will be considered). For events of Mb>=5.3, comparison will be made with centroid moment tensor (CMT HVRD) inversion method. Also, when waveform data is available (e.g. from the Incorporated Research Institutions for Seismology Data Management Centre (IRIS-DMC); GDSN, or the CRSN network), long period, broadband or short period seismograms of body and/or surface waves will be analysed by inversion of P and SH-waveform data. The least square inversion of the seismograms will be done to retrieve the fault mechanism, seismic moment, focal depth and rupture duration by minimizing the misfit between the observed and synthetic seismogram.
It is noteworthy that eruptions of the Nyiragongo and Nyamuragira volcanoes are believed to be enhanced or triggered by tectonic earthquakes occurring in the basin of Lake Kivu and vicinity. Thus, special attention will be given to the relationship between the focal mechanisms of earthquakes and volcanic activity in the Virunga volcanic area.
Recordings of small earthquakes will be used to developed strong ground motion attenuation models. The recordings of small earthquakes adjacent to the expected large earthquakes will be treated as empirical Green’s function between the source and that site. These recordings contain the site response, scattered waves, surface waves and other path effects that can be summed to produce realistic time histories at that site for a large earthquake.
The second part of my Ph.D. thesis will focus on the structure of the deep interior of the earth. Receiver function analysis will be used to determine a P- and S-wave velocity structure specific for the Virunga volcanic area. This will improve the locations of earthquake hypocenters in the Virunga area, and aid the location and assessment of magmatic activity. Data will be provided by 2 digital CRSN stations located in the Virunga volcanic area (KNN and KBB) equipped with 3-component broadband Trillium 40 sensors, the AfricaArray broadband stations MBAR, KMBO, Kigali, Lubumbashi, and selected stations which will be installed in Uganda and Tanzania during the period from 08/2007 to 12/2009 for the investigation of the shear velocity structure beneath these stations.