FOCAL MECHANISM SOLUTION OF THE 15TH MARCH 2008, NYAMANDLOVU EARTHQUAKE

B.T. SHUMBA, D.J. HLATYWAYO AND V. MIDZI 381 FOCAL MECHANISM SOLUTION OF THE 15TH MARCH 2008, NYAMANDLOVU EARTHQUAKE B.T. SHUMBA Goetz Observatory, P.O.Box AC65, Ascot, Bulawayo, Zimbabwe e-mail: shumbamajachani@yahoo.co.uk D.J. HLATYWAYO Physics Department, National University of Science and Technology, P.O.Box AC 939, Ascot, Bulawayo, Zimbabwe e-mail: djhlatywayo@gmail.com V. MIDZI Seismology Unit, Council for Geoscience, P. Bag X112 Pretoria 0001 South Africa e-mail: vmidzi@geoscience.org.za 2009 December Geological Society of South Africa ABSTRACT The focal mechanism solution of the 15th March 2008 earthquake (mb = 4.3) that occurred in the Nyamandlovu area, northwest of Bulawayo City, Zimbabwe, has been determined from P-wave first motion polarities. Results show normal oblique left lateral faulting. The earthquake mechanism bears a signature that is almost identical to those of events recorded in the Zambezi Branch of the East African Rift Zone. Synthetic seismograms compared with observed data from regional stations were employed for depth determination. This event had a shallow depth of 5 km. Earthquakes in the area tend to occur either after a significant drought or a wet season of considerable length. The area is underlain by thick Karoo sandstones that form an aquifer of high potential water storage. These events are therefore most probably induced by pore pressure differentials in the underlying rock. Introduction On 15th March 2008, at 0221 Hrs (UTC), a small magnitude earthquake (m b = 4.3), occurred in the Nyamandlovu area, approximately 135 km northwest of Bulawayo City. The epicenter of the event was 19.79 S and 27.39 E (Figure 1). The parameters were calculated at Goetz observatory, the national seismic data centre for Zimbabwe. The earthquake was felt in the entire city of Bulawayo, which has a population of over 1 million people. No injuries to the local population or destruction to structures and property were reported after the event. This is the largest earthquake on record from the area. The event was recorded as far as the Antarctica region. The Nyamandlovu area has been experiencing shaking from small to medium-size magnitude earthquakes (of undetermined mechanism) for some time now (Table 1). This study sought to establish the mechanism responsible for the occurrence of these events and infer the type of faulting in the area. Seismotectonics Compared to other seismically active parts of Zimbabwe, the seismicity of the Nyamandlovu area is low with few medium-size events that are generally believed to be caused by changes in pore pressure in the underlying rock formations (Hlatywayo and Midzi, 2005). A small fraction of Bulawayo city s water supply is obtained from underground water that is pumped from the Nyamandlovu aquifer. Surface lineaments trend north easterly (Hlatywayo, 1992). The geology of the Nyamandlovu area comprises mostly Kalahari Aeolian sands with white and red sandstones overlain by basaltic lavas approximately 700 m thick (Maufe, 1924; Vail, 1967; Reeves and Hutchins, 1975). To the south-east of the area, towards Bulawayo city, basaltic Karoos and Shamvanian metasediments dominate. Karoo sandstones cover the remainder of the area. The extreme east of the area under consideration comprises complex gneiss structures that are part of a well known gold-bearing belt. To the north of the area under study, the Karoo sediments of the Zambezi Basin extend south-westwards through the Hwange area into Botswana (Hlatywayo, 1992). Seismic event data for the Nyamandlovu area were taken from the Bulawayo Seismological Bulletins for the period 1970 to 2008. Events in the area (Figure 2) appear randomly distributed. Events that have previously occurred in the area were poorly recorded, making it very difficult to carry out meaningful focal mechanism studies based on polarity data. One such event, magnitude, m b = 4.2, occurred on the 26th of June 2004. It was felt widely over the whole of Bulawayo city and the surrounding areas (Hlatywayo and Midzi, 2005). However, there was no reported damage to property or buildings. In addition to the two events of 26 June 2004 and 15 March 2008, three more events of magnitude 4.0 occurred in the area over the period 1970 to 2008, all of which were poorly recorded., 2009, VOLUME 112 PAGES 381-386 doi:10.2113/gssajg.112.2-3.381

382 FOCAL MECHANISM SOLUTION OF THE 15TH MARCH 2008, NYAMANDLOVU EARTHQUAKE Figure 1. Map of Nyamandlovu area with major geological boundaries Data Wave-form data of the 15 March, 2008 Nyamandlovu earthquake were recorded by seismic stations in South Africa, Zimbabwe, Malawi, Botswana, Ivory Coast and Namibia (Figure 3). In an attempt to improve the azimuthal coverage for the data, additional waveform data from seismic stations OPO (Madagascar), VNDA (Victoria lands) and CPUP (Paraguay) were used. To supplement the waveform data, P-wave first motion data recorded at other stations that could be sourced from the International Seismological Centre (ISC) bulletins were incorporated. In total, data from 63 seismic stations around the world were used (Figure 3). Table 1. Earthquakes felt in the Nyamandlovu area from 1959 to present (Data obtained from the Bulawayo Seismological centre Date Coordinates Magnitude Degrees Degrees South East 1973-08-02 19.9 28.1 2.9 1975-11-01 20.4 28.5 3.0 1975-11-01 20.0 29.0 2.9 1976-04-03 19.4 27.2 3.8 1976-11-05 19.8 28.0 3.9 1978-03-06 19.3 27.7 3.5 1982-06-02 19.4 28.4 3.8 1984-12-26 20.3 29.0 3.3 2004-06-25 19.9 28.4 4.2 2008-03-15 19.7 27.3 4.3 Hypocenter location To determine the hypocenter of the earthquake, we used the programme HYPOCENTER in SEISAN (Lienert et al., 1986; Lienert and Havskov, 1995). It can be used with all common crustal and global phases, and to locate teleseismic, regional and local events using the IASP91 model as well as a local model (Midzi et al., 2010). P and S wave arrival times were picked using the SEISAN MULPLT program (Havskov and Ottemöller, 2008). An epicenter located at 19.217S; 27.39E was determined with error values in the latitude and longitude of 0.2. This compared to locations obtained by USGS and Council for Geoscience (CGS), (-19.217 S, 27.480 E) and (-19.115 S, 27.494 E) respectively. The USGS calculated a moment magnitude value of 4.7 compared to our local magnitude of 4.3. It must be stressed that the focal depth is difficult to constrain given that we were only able to identify first arrival P and S phases. In order to determine the focal depth of the event, we computed three synthetic seismograms (Bouchon, 1981) at different depth values. The synthetic seismograms computed were then compared with the observed seismograms to select the best-fit seismograms. The method is based on a discrete wave number representation of the wave fields. The source is repeated periodically in space, so that integration over the k-domain is replaced by a series. Focal mechanism We used programme FOCMEC (Snoke et al., 1984) in

B.T. SHUMBA, D.J. HLATYWAYO AND V. MIDZI 383 SEISAN to determine the focal mechanism of the event. The programme performs an efficient systematic search of the focal sphere and reports acceptable solutions based on selection criteria for the number of polarity uncertainties. The selection criteria for both polarities and angles allow correction or weightings for near-nodal solutions. The velocity model by Midzi et al. (2010) was used in determination of take off angles. The complete description of how the programme works is found in the manual by Snoke et al. (1984). The data were weighted according to the nature of the polarity onset. That is if the P-wave polarity was clear, the weight of that polarity was given a unity value but if the onset was not very clear, the weight was assigned to half. Data from ISC bulletins were given half weight since we cannot vouch for the quality of the actual picking of the polarities. Results The focal mechanism solution of the earthquake obtained from P-wave arrivals shows normal oblique left lateral faulting (Figure 4). The first nodal plane s parameters are: Strike = 285.7, Dip = 27.99 and Rake = -43.22 while the second nodal plane has the parameters: strike = 55, Dip = 48 and Rake = -95 (Figure 4). To determine the actual fault plane of the event, we compared the solution to known existing faults and lineaments in the area. The main lineaments and faults are generally oriented in a north easterly direction (Hlatywayo, 1992; 1995). The second nodal plane (strike = 55, Dip = 48 and Rake = -95 ) was then selected as the likely solution from the comparison results. The depth of the event was determined by comparing synthetic waveforms to observed P waveforms for three stations (Matopos (MATP, Lobatse (LBTB) and Boshof (BOSA). Waveforms from these three stations were found to have best signal to noise ratios that were easier to fit. Figure 5 shows typical examples of the comparison of the synthetic and observed seismograms at different depths. All other parameters were fixed with only depth varying. By comparing the synthetics at various depths with the observed waveforms, we deduced that the best-fit is obtained at around 5 km, which compares very well with the value of 5.8 km determined by the USGS. Discussion The mechanism of the 15th March 2008 event is normal oblique left lateral with one of the nodal planes striking northeast to southwest and the other striking northwest to southeast. Without using additional information such Figure 2. Seismicity map of the Nyamandlovu area (Data obtained from the Zimbabwe Meteorological Services, Goetz Observatory).

384 FOCAL MECHANISM SOLUTION OF THE 15TH MARCH 2008, NYAMANDLOVU EAR THQUAKE Figure 3. Red triangles represent global seismic stations whose data were used for determining the focal mechanism solution for the 2008, 15th March Nyamandlovu earthquake. The blue circle indicates the epicenter of the earthquake. Figure 4. The focal mechanism solution of the 15th March 2008 event obtained global data using SEISAN algorithm (Havskov and Ottemöller, 2008). The triangles shows seismic stations which has negative onset polarity where as the circles represents seismic stations with positive polarity as geology or aftershocks, it is difficult to determine which of the two possible solutions is the actual fault plane. An attempt to determine the actual fault plane is done here by comparing the obtained solution with notable major observed fault lineaments. Hlatywayo (1995; 1997) noted that the main lineaments in the Hwange, Zambezi Basin to the north of our study area, trend in the north-easterly direction further north into the Luangwa river valley in eastern Zambia. Furthermore, Vail (1967) showed the trend of lineaments from Botswana running northeasterly through Hwange and Kariba gorge into Luangwa valley. Scholtz et al. (1976), found concentrations of epicenters of earthquakes trending along a northeasterly direction from Botswana into the Deka-fault zone, Zambezi/ Luangwa valleys. The focal mechanism solution obtained is similar to other solutions which were obtained for the Lake Kariba area which is approximately 150 km north of Nyamandlovu (Sykes, 1967; Gupta et al., 1972; Shudofsky, 1985; Hlatywayo, 1995). Most of their results showed fault planes striking to the northeast. Fairhead and Girdler (1971) noted that the Zambian earthquake which occurred on December 2, 1968 has a normal fault mechanism with the faults striking north-easterly. Skyes (1967) also showed that solutions for two events that occurred on Lake Kariba were identical with both showing normal

B.T. SHUMBA, D.J. HLATYWAYO AND V. MIDZI 385 Figure 5. Comparison of the observed seismograms (red line) with the synthetic seismograms (blue line) using SEISAN algorithm (Havskov and Ottemöller, 2008) at depths labeled next to each set of traces

386 FOCAL MECHANISM SOLUTION OF THE 15TH MARCH 2008, NYAMANDLOVU EARTHQUAKE faulting mechanisms. The results obtained in this study are consistent with those obtained along the East African Rift, in particular, the mid-zambezi rift. Hlatywayo (1995), Skyes (1967) and Scholz et al. (1976) confirmed that fault plane solutions of earthquakes in the region are associated with rifting and show normal faulting with tensile axis directed to the southeast. We can then attribute the seismodynamics of the events of the Nyamandlovu earthquakes to be consistent with the mechanics of the rift systems. There is a possibility of pore-pressure changes being the major trigger of earthquakes in this area (Hlatywayo and Midzi, 2005). The government of Zimbabwe has drilled many boreholes in this area to provide water to residents in the nearby Bulawayo City. Hlatywayo and Midzi (2005) concluded that there is a possibility that excessive pore pressure built up by slow pore pressure changes is the driving mechanism and causes instability in small, low-permeability creeping joints, resulting in seismic activity. Conclusion The scope of this paper attempts to address the focal mechanism of the 15th March 2008 Nyamandlovu earthquake and to determine the depth of this event. The results from this research are consistence with the known seismotectonics of the area. This study is necessary in order to improve the understanding of the seismotectonics of the Nyamandlovu area and also to try to understand the causes of the Nyamandlovu tremors. The results obtained in this study are very important in future studies such as seismic hazard assessment. Acknowledgements The analysis described here, was performed using data from stations of the Council of Geoscience in South Africa, AfricaArray, International Seismological Centre and CTBTO. 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