Active fault scarps in southern Malawi and their implications for the distribution of strain in incipient continental rifts

The distribution of deformation during the early stages of continental rifting is an important constraint on our understanding of continental breakup. Incipient rifting in East Africa has been considered to be dominated by slip along rift border faults, with a subsequent transition to focussed extension on axial segments in thinned crust and/or with active magmatism. Here, we study high‐resolution satellite data of the Zomba Graben in southern Malawi, an amagmatic rift whose topography is dominated by the west‐dipping Zomba fault. We document evidence for five sub‐parallel fault scarps between 13 and 51 km long spaced ~10‐15 km apart. The scarps consist of up to five segments between 4‐18 km long, separated by minima in scarp height and river knickpoints. The maximum height of each fault scarp ranges from 9.5 ± 4.2 m to 35.3 ± 14.6 m, with the highest scarp measured on the intrabasin Chingale Step fault. We estimate that the scarps were formed by multiple earthquakes of up to Mw7.1, and represent a previously unrecognized seismic hazard. Our calculations show that 55 ± 24 % of extensional strain is accommodated across intrabasin faults within the ~50 km wide rift. This demonstrates that a significant proportion of displacement can occur on intrabasin faults during early stage rifting, even in thick continental lithosphere with no evidence for magmatic fluids

Fault-based probabilistic seismic hazard analysis in regions with low strain rates and a thick seismogenic layer: a case study from Malawi

Historical and instrumental earthquake catalogs in low strain rate regions are not necessarily indicative of the long-term spatio-temporal distribution of seismicity. This implies that probabilistic seismic hazard analysis (PSHA) should also consider geologic and geodetic data through fault-based seismogenic sources. However, it is not always clear how on-fault magnitude-frequency distributions (MFDs) should be described and, if the seismogenic layer is especially thick, how fault sources should be extrapolated down-dip. We explore these issues in the context of a new PSHA for Malawi, where regional extensional rates are 0.5–2 mm yr−1, the seismogenic layer is 30–40-km thick, the instrumental catalog is ∼60 yr long and fault-based sources were recently collated in the Malawi Seismogenic Source Model. Furthermore, Malawi is one of several countries along the East African Rift where exposure to seismic hazard is growing, but PSHA does not typically consider fault sources. We use stochastic event catalogs to explore different fault source down-dip extents and MFDs. Our PSHA indicates that hazard levels are highest for a Gutenberg–Richter on-fault MFD, even at low probabilities of exceedance (2 per cent in 50 yr), whilst seismic hazard levels are also sensitive to how relatively short (<50 km) fault sources are extrapolated down-dip. For sites close to fault sources (<40 km), seismic hazard levels are doubled compared to previous instrumental-seismicity based PSHA in Malawi. Cumulatively, these results highlight the need for careful fault source modelling in PSHA of low strain rate regions and the need for new fault-based PSHA elsewhere in the East Africa Rift

Spatial distribution, chemistry and subsurface temperatures of geothermal springs Nkhata bay, Malawi

No Abstract

A metric-based assessment of flood risk and vulnerability of rural communities in the Lower Shire Valley, Malawi

In response to the increasing frequency and economic damages of natural disasters globally, disaster risk management has evolved to incorporate risk assessments that are multi-dimensional, integrated and metric-based. This is to support knowledge-based decision making and hence sustainable risk reduction. In Malawi and most of Sub-Saharan Africa (SSA), however, flood risk studies remain focussed on understanding causation, impacts, perceptions and coping and adaptation measures. Using the IPCC Framework, this study has quantified and profiled risk to flooding of rural, subsistent communities in the Lower Shire Valley, Malawi. Flood risk was obtained by integrating hazard and vulnerability. Flood hazard was characterised in terms of flood depth and inundation area obtained through hydraulic modelling in the valley with Lisflood-FP, while the vulnerability was indexed through analysis of exposure, susceptibility and capacity that were linked to social, economic, environmental and physical perspectives. Data on these were collected through structured interviews of the communities. The implementation of the entire analysis within GIS enabled the visualisation of spatial variability in flood risk in the valley. The results show predominantly medium levels in hazardousness, vulnerability and risk. The vulnerability is dominated by a high to very high susceptibility. Economic and physical capacities tend to be predominantly low but social capacity is significantly high, resulting in overall medium levels of capacity-induced vulnerability. Exposure manifests as medium. The vulnerability and risk showed marginal spatial variability. The paper concludes with recommendations on how these outcomes could inform policy interventions in the Valley

A metric-based assessment of flood risk and vulnerability of rural communities in the Lower Shire Valley, Malawi

In response to the increasing frequency and economic damages of natural disasters globally, disaster risk management has evolved to incorporate risk assessments that are multi-dimensional, integrated and metric-based. This is to support knowledge-based decision making and hence sustainable risk reduction. In Malawi and most of Sub-Saharan Africa (SSA), however, flood risk studies remain focussed on understanding causation, impacts, perceptions and coping and adaptation measures. Using the IPCC Framework, this study has quantified and profiled risk to flooding of rural, subsistent communities in the Lower Shire Valley, Malawi. Flood risk was obtained by integrating hazard and vulnerability. Flood hazard was characterised in terms of flood depth and inundation area obtained through hydraulic modelling in the valley with Lisflood-FP, while the vulnerability was indexed through analysis of exposure, susceptibility and capacity that were linked to social, economic, environmental and physical perspectives. Data on these were collected through structured interviews of the communities. The implementation of the entire analysis within GIS enabled the visualisation of spatial variability in flood risk in the valley. The results show predominantly medium levels in hazardousness, vulnerability and risk. The vulnerability is dominated by a high to very high susceptibility. Economic and physical capacities tend to be predominantly low but social capacity is significantly high, resulting in overall medium levels of capacity-induced vulnerability. Exposure manifests as medium. The vulnerability and risk showed marginal spatial variability. The paper concludes with recommendations on how these outcomes could inform policy interventions in the Valley

Palaeolimnological reconstruction of recent environmental change in Lake Malombe (S. Malawi) using multiple proxies

Shallow inland water bodies in Malawi continue to be threatened by various environmental challenges despite their importance to the fisheries industry. Due to the complex interaction between natural and anthropogenic disturbances, disentangling the effect of the two may be a complicated process. The littoral zone of most water bodies is important in environmental reconstructions including pollution and lake level monitoring. This study used a littoral zone, transect-based approach employing multi-proxy palaeolimnological techniques to reconstruct recent environmental change (ca. 100 yrs.) in Lake Malombe in the Malawi Rift, East Africa. The results of the study could inform fisheries management in Lake Malombe, which experienced a catastrophic decline in fish stocks. Results support documentary evidence for the complete desiccation of the lake less than 100 years ago. Subsequently, there is evidence for accelerated eutrophication in the recent past. In light of these results, it is concluded that transect sampling approaches rather than relying on single core measurements, and the need for careful consideration of the types of proxy, are significant considerations in palaeo-environmental reconstructions