Stress tensor inversion using focal mechanisms of earthquakes recorded in the Rwenzori Region, Albertine Rift System

Abstract

The goal of this study is to determine the regional stress tensor in the Rwenzori region using the fault plane solutions. Rwenzori region is located in Uganda and an integral part of East African Rift System. The research was conducted using a total of 776 earthquakes sourced from the RIFTLINK project, covering 2006 and 2007. The stress tensor orientation was determined by the inversion of 776 fault plane solutions using the acquired events. The fault plane solutions were compiled from the P-wave first motion using the Seisan software and stress tensor was analyzed using the Zmap mapping tool. The stress tensor inversion for the fault plane solution map indicated the maximum principal stress (S1) in the NNE -SSW direction. The best fitting stress tensor inversion modelling results suggest transpressional and normal faulting styles. The stress tensor inversion for the fault plane solution map indicated the maximum principal stress (S1) in the NNE -SSW direction. The best fitting stress tensor inversion modelling results, suggest transpressional and normal faulting styles. The variance on best fitting stress tensor ranges from 0.2-0.3, which indicates a heterogeneous stress field in the region. The maximum principal stress (S1) is oriented 52º NNE (52º, N20ºE) and minimum principal stress (S3) indicate that the plunge lies 38º SSW (38º, S08ºW), the intermediate principal stress (S2) is oriented 06º WNW (06º, N78ºW) for the fault plane solution in figure 4.3. Figure 4.5 indicates the plunge and trend for (S1) is SSW but near the vertical and (S2) plunge trend lies about 11ºNNE (11º, N19ºE) direction, while (S3) plunge trend lies 7º ESE (7º, S70ºE). The stress tensor inversion in terms of depth indicates that, the seismic events in the Rwenzori region were located at shallow depths ranging from 5km to 22km, with the orientation of the maximum principal stress (S1) in the NE-SW direction. The stress inversion in figure 4.9 indicates that, the thrust faulting types are dominant in the north and southern zones with a variance ranging from 0.08-0.18; which indicates a pure homogeneous stress. Similarly, the central zone indicates that normal faulting was dominant with variance values ranging from 0.14 - 0.24, which indicate homogeneous to heterogeneous stresses. In figure 4.11, the grid search, variance values range from 0.25 - 0.3 indicating a pure heterogeneous stress with the normal faulting type dominant in the north and southern zones. The central zone variance ranges from 0.1 – 0.3 indicating a mixed homogeneous to heterogeneous stress. The models indicate variance ranges from 0.1 - 0.3 suggesting homogeneous to heterogeneous stress or relatively fit to poorly fit stresses in the Rwenzori region, which implies the stress is not uniform. The misfit values were between 0 - 0.3 for the fault plane solutions (strike, dip and rake) misfit which indicates a good fit on the stress model. In conclusion, the results for most of the models in the investigated region suggest a normal faulting type, thrust and strike-slip faults in order of dominance. The orientation of the maximum principal stress (S1) is in the NE-SW direction and the stress field is mostly heterogeneous.