Search

Active fault data are commonly used in seismic hazard assessments, but there are challenges in deriving the slip rate, geometry, and frequency of earthquakes along active faults. Herein, we present the open-access geospatial Malawi Seismogenic Source Model (MSSM; https://doi.org/10.5281/zenodo.5599616), which describes the seismogenic properties of faults that formed during ongoing east African rifting in Malawi. We first use empirically derived constraints to geometrically classify active faults into section, fault, and multifault seismogenic sources. For sources in the North Basin of Lake Malawi, slip rates can be derived from the vertical offset of a seismic reflector that dated lake cores indicate is 75 ka. Elsewhere, slip rates are constrained from advancing a systems-based approach that partitions geodetically derived rift extension rates in Malawi between seismogenic sources using a priori constraints on a regional strain distribution and a hanging wall flexural extension in magma-poor continental rifts. Slip rates are then combined with source geometry and empirical scaling relationships to estimate earthquake magnitudes and recurrence intervals, and their uncertainty is described from the variability in logic tree outcomes used in these calculations. Sources in the MSSM are 5–269 km long, which implies that large-magnitude (Mw 7–8) earthquakes may occur in Malawi. However, low slip rates (0.05–2 mm yr−1) mean that the frequency of such events will be low (recurrence intervals of ∼103–104 years). We also find that, for 9 out of 11 faults in Lake Malawi's North Basin, differences in the slip rates, when estimated independently from the geodetic data and the offset seismic reflector, are not statistically significant. The MSSM represents an important resource for investigating Malawi's increasing seismic risk and provides a framework for incorporating active fault data into seismic hazard assessment elsewhere in the East African Rift and other tectonically active regions.

Tectonic and paleo-environmental reconstructions of rift evolution typically rely on the interpretation of sedimentary sequences, but this is rarely possible in early-stage rifts where sediment volumes are low. To overcome this challenge, we use geomorphology to investigate landscape evolution and the role of different forcing mechanisms during basin development. Here, we focus on the humid Middle Shire River basin, located within the zone of progressive interaction and linkage between the southern Malawi Rift and Shire Rift Zone, East Africa. We used a digital elevation model to map knickpoints and knickpoint morphologies in the Middle Shire River basin and examined the relationships with pre-rift and syn-rift structures within the rift interaction zone. The main axial stream, Shire River, descends steeply, 372 m over a 50 km distance, across exposed metamorphic basement along the rift floor, exhibiting a strongly disequilibrated longitudinal elevation profile with both ‘mobile’ and ‘fixed’ knickpoints. In particular, we identify two clusters of mobile knickpoints, which we interpret as associated with baselevel fall events at the downstream end of the exposed basement that triggered knickpoint migration through the fluvial network since at least the Mid. Pleistocene. We infer that after the integration of the axial stream across the Middle Shire Basin, the knickpoints migrate upstream in response to fault-related subsidence in the Shire Rift Zone. Conversely, the fixed knickpoints are interpreted to reflect local differential bedrock erodibility at lithologic contacts or basement-hosted fault scarps along the basin floor. The results suggest that Middle Shire basin opening, associated with rift linkage, is likely a recent event (at least Mid. Pleistocene) relative to the Late Oligocene activation of Cenozoic rifting in the East African Rift's Western Branch. These findings support the hypothesis that the Western Branch developed from the gradual propagation, linkage and coalescence of initially nucleated distinct rift basins.

At the northern Hikurangi margin (North Island, New Zealand), shallow slow slip events (SSEs) frequently accommodate subduction-interface plate motion from landward of the trench to

During earthquakes on low (

Because quartz veins are common in fault zones exhumed from earthquake nucleation \ud temperatures (150°C–350°C), quartz cementation may be an important mechanism of strength recovery \ud between earthquakes. This interpretation requires that cementation occurs within a single interseismic period. \ud We review slip-related processes that have been argued to allow rapid quartz precipitation in faults, including: \ud advection of silica-saturated fluids, coseismic pore-fluid pressure drops, frictional heating, dissolutionprecipitation creep, precipitation of amorphous phases, and variations in fluid and mineral-surface chemistry. \ud We assess the rate and magnitude of quartz growth that may result from each of the examined mechanisms. \ud We find limitations to the kinetics and mass balance of silica precipitation that emphasize two end-member \ud regimes. First, the mechanisms we explore, given current kinetic constraints, cannot explain mesoscale \ud fault-fracture vein networks developing, even incrementally, on interseismic timescales. On the other hand, \ud some mechanisms appear capable, isolated or in combination, of cementing micrometer-to-millimeter thick \ud principal slip surfaces in days to years. This does not explain extensive vein networks in fault damage zones, \ud but allows the involvement of quartz cements in fault healing. These end-members lead us to hypothesize \ud that high flux scenarios, although more important for voluminous hydrothermal mineralization, may be of \ud subsidiary importance to local, diffusive mass transport in low fluid-flux faults when discussing the mechanical \ud implications of quartz cements. A renewed emphasis on the controls on quartz cementation rates in fault \ud zones will, however, be integral to developing a more complete understanding of strength recovery following \ud earthquake rupture

In low strain rate regions, extrapolation of incomplete instrumental earthquake records is a limitation for probabilistic seismic hazard analysis (PSHA). This limitation can be addressed by incorporating geologic and geodetic data into fault-based seismogenic sources, although it is not always clear how on-fault magnitude-frequency distributions should be described and, if the seismogenic layer is especially thick, how these 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, the seismogenic layer is 30-40 km thick, the instrumental catalog is ~60 years long, and fault-based sources were recently collated in the Malawi Seismogenic Source Database. Furthermore, Malawi is one of several countries along the East African Rift where exposure to seismic hazard is growing, but PSHA typically considers instrumental records alone. We use stochastic event catalogs to explore different fault-source down-dip extents and magnitude-frequency distributions. These indicate that hazard levels are highest for a Gutenberg-Richter on-fault magnitude-frequency distribution, even at low probabilities of exceedance (2% in 50 years), whilst seismic hazard levels are also sensitive to how relatively short (

Seismic hazard is frequently characterised using instrumental seismic records. However, in regions where the instrumental record is short relative to earthquake repeat times, extrapolating it to estimate seismic hazard can misrepresent the probable location, magnitude, and frequency of future large earthquakes. Although paleoseismology can address this challenge, this approach requires certain geomorphic settings and carries large inherent uncertainties. Here, we outline how fault slip rates and recurrence intervals can be estimated through an approach that combines fault geometry, earthquake-scaling relationships, geodetically derived regional strain rates, and geological constraints of regional strain distribution. We then apply this approach to the southern Malawi Rift where, although no on-fault slip rate measurements exist, there are theoretical and observational constraints on how strain is distributed between border and intrabasinal faults. This has led to the development of the South Malawi Active Fault Database (SMAFD), the first database of its kind in the East African Rift System (EARS) and designed so that the outputs can be easily incorporated into Probabilistic Seismic Hazard Analysis. We estimate earthquake magnitudes of MW 5.4–7.2 for individual fault sections in the SMAFD, and MW 6.0–7.8 for whole fault ruptures. These potentially high magnitudes for continental normal faults reflect southern Malawi's 11–140 km long faults and thick (30–35 km) seismogenic crust. However, low slip rates (intermediate estimates 0.05–0.8 mm/yr) imply long recurrence intervals between events: 102–105 years for border faults and 103–106 years for intrabasinal faults. Sensitivity analysis indicates that the large range of these estimates can be reduced most significantly from an improved understanding of the rate and partitioning of rift-extension in southern Malawi, earthquake scaling relationships, and earthquake rupture scenarios. Hence these are critical areas for future research. The SMAFD provides a framework for using geological and geodetic information to characterize seismic hazard in low strain rate settings with few on-fault slip rate measurements, and could be adapted for use elsewhere in the EARS or globally.

Along-strike variation in scarp morphology reflects differences in a fault's geomorphic and structural development and can thus indicate fault rupture history and mechanical segmentation. Parameters that define scarp morphology (height, width, slope) are typically measured or calculated manually. The time-consuming manual approach reduces the density and objectivity of measurements and can lead to oversight of small-scale morphological variations that occur at a resolution impractical to capture. Furthermore, inconsistencies in the manual approach may also lead to unknown discrepancies and uncertainties between, and also within, individual fault scarp studies. Here, we aim to improve the efficiency, transparency and uniformity of calculating scarp morphological parameters by developing a semi-automated Scarp PARameTer Algorithm (SPARTA). We compare our findings against a traditional, manual analysis and assess the performance of the algorithm using a range of digital elevation model (DEM) resolutions. We then apply our new algorithm to a 12m resolution TanDEM-X DEM for four southern Malawi fault scarps, located at the southern end of the East African Rift system: the Bilila-Mtakataka fault (BMF) and three previously unreported scarps-Thyolo, Muona and Malombe. All but Muona exhibit first-order structural segmentation at their surface. By using a 5m resolution DEM derived from high-resolution (50cmpixel-1) Pleiades stereo-satellite imagery for the Bilila-Mtakataka fault scarp, we quantify secondary structural segmentation. Our scarp height calculations from all four fault scarps suggest that if each scarp was formed by a single, complete rupture, the slip-length ratio for each earthquake exceeds the maximum typical value observed in historical normal faulting earthquakes around the world. The high slip-length ratios therefore imply that the Malawi fault scarps likely formed in multiple earthquakes. The scarp height distribution implies the structural segments of both the BMF and Thyolo fault have merged via rupture of discrete faults (hard links) through several earthquake cycles, and the segments of the Malombe fault have connected via distributed deformation zones (soft links). For all faults studied here, the length of earthquake ruptures may therefore exceed the length of each segment. Thus, our findings shed new light on the seismic hazard in southern Malawi, indicating evidence for a number of large (Mw 7-8) prehistoric earthquakes, as well as providing a new semi-automated methodology (SPARTA) for calculating scarp morphological parameters, which can be used on other fault scarps to infer structural development.

Themogene travertines are U-series-dateable, continental, CaCO3 precipitates often used for neotectonic, paleoclimate, paleohydrological-hydrothermal, and further geological reconstructions both in space and in time. Recognizing early (primary or depositional) and late (secondary, postdepositional, or diagenetic) structures of travertines is essential for their correct use in geology. This problem can be tackled in an easy and efficient way through open-air lessons opposite to travertine-made buildings in cities such as Rome, Italy, where huge quantities of the closely quarried Tivoli travertine have been used for millennia for construction and decoration. Five brief open-air lessons through Rome are proposed to provide indications on how to distinguish early and late structures in travertines. Chimney-like veins, matrix-supported breccias, and undulated veins are recognized as late structures on the basis of their cross-cutting/superimposition relationships with primary porous beds. Limitations of this method are explained. These lessons may be helpful to those willing to use thermogene travertines as recorders of geological processes. In the future, the presented instances, together with further building stones exposed in cities, may be included in open geographic databases for easy, efficient, and accessible field experiences and geological fieldwork in urban environments, as to increase sensibility in the geology, environment, and resources offered by our planet.