The Nyasa/ Malawi rift is characterized by poor magma with relatively large earthquakes. There has been a controversy as to the stress kinematics of the rift, some considering it as part of the transform fault and some considering it as a rift structure characterized by normal faulting. To review this controversy, we collect fault slip data from the central to the southern end of the rift and integrate our results with published focal mechanisms fault slip data on the rift. Results show that the central part of the rift is under radial extension whereas the southern half is under oblique NNE-SSW transtensive tectonic regime with the horizontal axis of minimum extension = 020˚. Further south, the obliquity extension rotates by about 15˚ reaching N-S with Shmin = 175˚. The level of structural penetration and intensity of faulting show that the N-S opening is more important and prominent in the south than towards the north. We also find that the faults that dip to the east and trending NW-SE are characterized by sinistral sense of movement whereas those that dip to the southwestern side are characterized by dextral sense of movements. This implies that regionally, the rift is essentially under normal faulting regime but with a significant strike –slip component – hence the obliquity kinematics. Tectonic regimes obtained from fault-slip data are related to lithospheric scale and involve both the crust and the upper mantle. Thus, the pure NNW-SSE extension related to focal mechanism data are crust deformation related events.

Fracturing and gouge formation absorb ≥50% of earthquake energy on low displacement (<1-2 km) faults in isotropic crust. To assess how these processes absorb earthquake energy in anisotropic crust, we performed field and microstructural investigations on the 110 km long, 0.4-1.2 km displacement, Bilila-Mtakataka Fault (BMF), Malawi. Where the fault is parallel to surface metamorphic fabrics, macroscale fractures define a 5-20 m wide damage zone. This is narrow relative to where the BMF is foliation-oblique (20-80 m), and to faults with comparable displacement in isotropic crust (~40-120 m). There is minimal evidence for cataclasis and microfracturing along the BMF; therefore, despite its 110 km length and geomorphic evidence for MW 7-8 earthquakes, widespread fault zone fracturing has not occurred. We attribute lack of damage to fault growth along shallow and deep-seated pre-existing weaknesses. This conclusion implies that earthquake energy dissipates differently along incipient faults in isotropic and anisotropic crust.

We present the Malawi Active Fault Database (MAFD), a geospatial database of 114 active fault traces in Malawi, and in neighboring Tanzania and Mozambique. The MAFD has been developed from a multidisciplinary dataset: high resolution digital elevation models, field observations, aeromagnetic and gravity data, and seismic reflection surveys from offshore Lake Malawi. Active faults longer than 50 km are found throughout Malawi, where seismic risk is increasing due to its rapidly growing population and its seismically vulnerable building stock. The MAFD also provides an opportunity to investigate the population of normal faults in an incipient continental rift. We find that the null hypothesis that the distribution of fault lengths in the MAFD is described by a power law cannot be rejected. Furthermore, a power-law distribution of faults in Malawi is consistent with its thick seismogenic crust (~35 km), and low (<8%) regional extensional strain that is predominantly (50-75%) accommodated across relatively long hard-linked border faults. Cumulatively, the data and inferences drawn from the MAFD highlight the importance of integrating onshore and offshore geological and geophysical data to develop active fault databases along the East African Rift and similar continental settings, both to understand the regional seismic hazard and tectonic evolution.