The Luangwa Rift Active Fault Database and fault reactivation along the southwestern branch of the East African Rift

. Seismic hazard assessment in slow straining regions is challenging because earthquake catalogues only record events from approximately the last 100 years, whereas earthquake recurrence times on individual faults can exceed 1,000 years. Systematic mapping of active faults allows fault sources to be used within probabilistic seismic hazard assessment, which overcomes the problems of short-term earthquake records. We use Shuttle Radar Topography Mission (SRTM) data to analyse surface deformation in the Luangwa Rift in Zambia and develop the Luangwa Rift Active Fault Database (LRAFD). The 5 LRAFD is an open-source geospatial database containing active fault traces and their attributes and is freely available at: https://doi.org/10.5281/zenodo.6513691. We identified 18 faults that display evidence for Quaternary activity and empirical relationships suggest that these faults could cause earthquakes up to Mw 8.1, which would exceed the magnitude of historically recorded events in southern Africa. On the four most prominent faults, the median height of Late Quaternary fault scarps varies between 12.9 ± 0.4 and 19.2 ± 0.9 m, which suggests they were formed by multiple earthquakes. Deformation is focused on 10 the edges of the Luangwa Rift: the most prominent Late Quaternary fault scarps occur along the 207 km long Chipola and 142 km long Molaza faults, which are the rift border faults and the longest faults in the region. We associate the scarp on the Molaza Fault with possible surface ruptures from two 20th Century earthquakes. Thus, the LRAFD reveals new insights into active faulting in southern Africa and presents a framework for evaluating future seismic hazard


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Earthquakes occur on active faults, and thus the systematic mapping of active faults is a major aim of seismic hazard research (Christophersen et al., 2015;Morell et al., 2020;Styron and Pagani, 2020;Williams et al., 2021Williams et al., , 2022. Within continental rifts, earthquakes on normal faults typically lead to high levels of shaking in their hanging wall basins, which are geomorphically suitable for human habitation and settlement (Bailey et al., 2000;Abrahamson et al., 2008). Consequently, normal faults inherently create conditions that lead to high seismic risk. Despite the seismic hazards associated with active continental 20 rifting, many active extensional regions around the world still lack systematic maps of active faults. This is particularly a 1 problem along many segments of the East African Rift, where there is a history of infrequent large magnitude earthquakes (Ambraseys and Adams, 1991;Meghraoui, 2016), but the location and activity rates of active faults is poorly known (Skobelev et al., 2004), and population growth over the past 20 years has been rapid (Gerland et al., 2014).
The creation of an active fault database involves defining a criteria to distinguish active faults, systematically map ::::::: mapping 25 all known faults that fit this criteria, and then collating their geomorphic attributes into a geospatial database (Styron and Pagani, 2020;Styron et al., 2020;Faure Walker et al., 2021;Williams et al., 2021Williams et al., , 2022. The use of active fault databases for fault-source seismic hazard is important in regions such as southern and eastern Africa, where the instrumental and historical records of earthquakes are short compared to the long recurrence times between earthquakes on individual faults (Hodge et al., 2015;. In recent years, the first active fault databases along the East African Rift System have been developed, 30 using the Malawi Rift as a case study (Williams et al., 2021(Williams et al., , 2022, but this has not yet been extended to other rift segments. In this paper, we map the active faults in one poorly studied rift segment, the Luangwa Rift in Zambia. Although it had been thought that the Luangwa Rift is inactive (Banks et al., 1995;Matende et al., 2021;Sun et al., 2021), recent plate modelling (Wedmore et al., 2021) and the evidence of Quaternary activity on the Chipola Fault (Daly et al., 2020) confirm this is an active rift system ( Figure 1). However, until now there has been no systematic map of active faults in the region.

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Identifying active faults in a region can reveal new insights into the seismotectonics of a region of active deformation. In southern Africa, this is important as there is debate over 1) the potential magnitude of future earthquake events given that faults may rupture completely or in segments (Jackson and Blenkinsop, 1997;Hodge et al., 2018) -and 2) why the continent is rifting given that tectonic forces are not thought to be sufficient to overcome the strength of the lithosphere (Kendall and Lithgow-Bertelloni, 2016;Rajaonarison et al., 2021). Mapping faults, and the way in which they are segmented, is vital to 40 addressing these debates. Firstly, the distribution of faults at the surface of the Earth can reveal the strength of the underlying lithosphere (Buck, 1991;Brun, 1999), and secondly fault segment boundaries may act as barriers to earthquake rupture (Aki, 1984;DuRoss et al., 2016). Thus, we develop the Luangwa Rift Active Fault Database (LRAFD), following the framework of the Global Active Fault Database (Styron and Pagani, 2020), and the Malawi Active Fault Database (Williams et al., 2022).
We use the Shuttle Radar Topography Mission (SRTM; Farr et al. (2007)) digital elevation model (DEM) alongside geological 45 maps and previously published analyses to study the tectonic geomorphology of the Luangwa Rift. Based on the discovery of steep fault scarps that offset Quaternary fluvial and alluvial sediments, and incised river valleys, we identify 18 active faults.
We then estimate the seismic source properties of these faults using empirical scaling laws (Leonard, 2010). We use the high resolution geomorphology of the fault scarps to identify evidence for fault segmentation and/or multiple earthquakes (Hodge et al., 2019(Hodge et al., , 2020. The LRAFD is fully open source and thus available for researchers and practitioners to implement within 50 future regional fault databases and probabilistic seismic hazard analyses (PSHA). By using remote sensing to identify active faults, the outcomes of this research provide targets for future ground-based studies of active tectonics in the Luangwa Rift.

Tectonic and Geologic Background
The southwestern branch of the East African Rift System (EARS) bifurcates from the western branch of the EARS in Tanzania and runs through Zambia, Botswana and into Namibia (Figure 1; Fairhead and Girdler, 1969;Reeves, 1972;Scholz et al., 55 1976; Fairhead and Henderson, 1977;Daly et al., 2020;Wedmore et al., 2021). The Luangwa Rift is situated in northeastern Zambia at the northern end of the southwestern branch of the EARS (Figure 1 & 2), and forms the eastern margin of the Central African Plateau (Daly et al., 2020). It remains unclear whether the onset of rifting along the southwestern branch of the EARS is contemporaneous with the Oligocene initiation of rifting along the western branch of the EARS (∼25Ma Roberts et al., 2012). Apatite fission track thermochronometry data from the southwestern branch suggest a period of regional cooling 60 between 38-22 Ma (Daly et al., 2020). However, Daly et al. (2020) suggest that rifting along the southwestern branch initiated in the Pliocene (5-3 Ma) at the same time as a period of regional uplift that formed the Central African Plateau.
The Luangwa Rift was active during the Permian-Jurassic breakup of Gondwana Banks et al., 1995;Matende et al., 2021) and during the Cretaceous (Daly et al., 2020). Up to 8,000 m of Permo-Traissic (ie Karoo period in southern Africa) mainly clastic sediments are unconformably capped by finer grained post-Karoo deposits (Banks et al., 1995).

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Although now an amagmatic rift, the Karoo phase of rifting was concomitant with the emplacement of diamond bearing lamproites, suggesting that this was possibly a rare example of rifting of thick (180-200 km), cold (≤42 mW/m2) cratonic lithosphere (Ngwenya and Tappe, 2021). The basin also experienced folding during the Late Jurassic-Early Cretaceous (Banks et al., 1995). The post-Karoo deposits are up to 500 m thick at the northern end of the Rift, and in the southern part of the Rift, the upper-and post-Karoo sediments are indistinguishable in seismic reflection data (Banks et al., 1995). Negative Vs 70 anomalies observed in the top few km beneath the southern Luangwa Rift are suggestive of loose sediments (Wang et al., 2019), and are similar to the low Vs anomalies observed beneath the Malawi Rift, which has up to 5.5 km of syn-rift sediments from the current post-Miocene phase of rifting in East Africa (Wang et al., 2019;Scholz et al., 2020).
The Luangwa Rift is 130 km wide and 500 km long, with two main escarpments that are greater than 1 km high ( Figure   2). The orientation of the rift follows the surface trace of the Mwembeshi shear zone (also referred to as the Mwembeshi 75 Dislocation Zone; de Swardt et al., 1965), a lithospheric scale structure that may have reactivated along a suture between the Irumide and Southern Irumide orogenic belts and which accommodated ENE-WSW dextral displacement during the late Proterozoic Sarafian et al., 2018;Alessio et al., 2019). Little is known about the lithology of the Mwembeshi shear zone as it is largely obscured by the sediments in the Luangwa Rift, other than that it displays a NE trending magnetic fabric, and contains eclogite (i.e. mafic) intrusives (Vrána et al., 1975;Sarafian et al., 2018). The Nyamadzi shear zone is a 80 splay of the Mwembeshi shear zone and is comprised of planar, vertically dipping fabrics within a wide variety of lithologies including ultramylonitic granites, highly deformed quartzites and mafic igneous gabbros and amphibolites . Daly et al. (2020) found evidence that shows that the Luangwa Rift has been active during the Quaternary, whereas others suggest that rifting ceased in the Mesozoic (Banks et al., 1995;Matende et al., 2021;Sun et al., 2021). The rift is hosted in 85 150-160 km thick lithosphere (Priestley et al., 2018), with a crustal thickness of 41-45 km (Sun et al., 2021), and seismicity occurs down to 29 km (Craig et al., 2011;Craig and Jackson, 2021). Tectonic plate modelling of southern Africa suggests that the Luangwa Rift accommodates 0.7 ± 0.3 mm/yr of extension between the San and Nubian plates along an azimuth of 108 • (Wedmore et al., 2021), and historical earthquake data shows Mw 6.7 and Mw 6.5 earthquake events occurring in 1919and 1940(International Seismological Centre, 2021 We use a 30 m (1-arc second) Shuttle Radar Topography Mission (SRTM; Farr et al. (2007)) ::::::::::::::::::::: (SRTM; Farr et al., 2007) digital 95 elevation model (DEM) to map the active faults in the Luangwa Rift, which has an absolute height error (90%) of 5.6 m in Africa (Rodriguez et al., 2005). SRTM data has been successfully used for remote investigation and mapping of active faults in southern Africa (Kinabo et al., 2007;McCarthy, 2013;Laõ-Dávila et al., 2015), and is available for free with global coverage.
We georeferenced a 1:1,000,000 scale geological map of Zambia (Priday and Camps, 1960), and used this alongside previous academic publications and the topographic data to identify active faults. We combined these resources with Google Earth 100 imagery to correlate each fault in a range of different datasets.
Active faults in the LRAFD are defined as having a high likelihood of producing significant seismicity under the current tectonics regime (Styron and Pagani, 2020). We make this assessment based on whether the faults display evidence of offsetting Quaternary sediments in the Luangwa basin as it is not clear how long the current tectonic regime along the southwestern branch of the EAR ::::: EARS has been active (see discussion above). Although Quaternary sedimentation in the Luangwa Rift is minimal 105 compared to Karoo sediments (Dixey, 1937;Utting, 1988;Bishop et al., 2016), exploratory petroleum cores and cosmogenic dates from archaeological surveys identified 40 m of sediment that is Quaternary aged (Barham et al., 2011), which is of comparable thickness to the juvenile southern Malawi Rift (Wedmore et al., 2020b). We identified steep scarps that offset these Quaternary sediments, which demonstrates evidence of recent fault activity. Although Daly et al. (2020) suggest that these steep scarps are <10 ka in age, this is not based on any definitive geochronology, so we prefer the term 'Quaternary' for the 110 age of these scarps.
Fault traces more than 5 km apart are mapped as separate features, as these earthquakes are less likely to be able to breach a gap this big (Wesnousky, 2006(Wesnousky, , 2008. Although some faults may be one continuous structure at depth, we only joined these structures where evidence of linkage is visible at the surface. Consequently, the database includes both discrete faults and sets 140 of features that may be one fault at depth, but which we have recognized as separate traces based on their surface expression. Exposure and epistemic quality variables are represented by numeric rankings of 1-2. Lower values (1) indicate a high quality of exposure and confidence of faulting. A value of 2 represents a lack of strong fault exposure and reduced certainty a fault exists.
There might be strong evidence for an exposed feature on the landscape, but little confidence it is a fault (exposure quality = 1, epistemic quality = 2). Conversely, a fault may have a high confidence of activity but little exposure or representation on 145 the topography (epistemic quality = 1, exposure quality = 2). Activity confidence is assigned numerically from 1-4: 1 for high confidence and likelihood of recent activity, 4 suggesting the fault shows only weak evidence of activity. Although multiple variables are used to deduce activity confidence, including exposure quality, epistemic quality, and the number of indicators of active faulting, the assigned value remains subjective.
Some mapped faults show limited evidence of recent surface activity, but we include these faults in our database if they 150 strike between NNE-SSW and ENE-WSW, which means they would be favourably oriented for reactivation given the SE-NW extension direction inferred from focal mechanisms (Delvaux and Barth, 2010) and geodetic models of the motion between the San and Nubian Plates (Wedmore et al., 2021), assuming a moderate fault dip (following Williams et al., 2022). Some major topographical structures may represent inactive faults and therefore, some inactive faults may be included in the LRAFD. As with any active fault database, bias towards inclusiveness reduces the likelihood that potentially active faults are missed (Styron 155 et al., 2020), but complete mapping of all existing active faults is unlikely, and large earthquake events may occur on unmapped faults.

LRAFD Availability and Data Format
The

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The criteria and indicators listed in Table 1 and described for the Chipola fault above were applied throughout the Luangwa Rift to create the LRAFD. Faults with a similar strength of evidence to that of the Chipola Fault are also mapped with the highest confidence (e.g. the Molaza and Kabungo faults; Figures 6 & 7). Scarps and escarpments are prominent on the DEM and Google Earth (e.g. Figure 5a & 7c), and slope maps highlight steep (>20 • ) fault scarps that have formed at the base of many of the escarpments (Figure 5b, 5f, 6, 7b & 7d). Figures 5-7 show slope maps of the Chipola, Mkumpa and Molaza faults,  Figure 7d-e), indicating recent fault activity and rupture events. We also observed river incision and channel steepening in the footwall of the Mukopa, Chitumbi, Kapampa, Chipola, and Molaza faults (e.g. Figure 6c).
Overall, ten faults had exposure quality scores of 1 indicating they are well exposed, whereas eight faults scored 2 meaning

Seismic Source Properties
Using the fault scaling laws set out in Leonard (2010) depth :::::: extent :::::: (RDE) of 6 +3/-2 to 49 +28/-14 km (Table 3). With these calculations, which assume a reasonable fault dip of 53 • , only the Chipola Fault produces a rupture depth ::::: extent that would exceed the crustal thickness of the region (∼45 km; Sun 280 et al., 2021), and only the Chipola and Molaza faults produce a rupture depth that exceeds the maximum depth of seismicity recorded in the region (∼30 km; Craig et al., 2011;Craig and Jackson, 2021). Potential earthquake magnitudes for whole-fault ruptures average M w 7.0 but vary between M w 5.8 and 8.1 (Table 3; Figure 8).

Recurrence Intervals
Using eq. 5 and 6 with a logic tree approach ( The large uncertainties associated with these estimates represent the large epistemic uncertainties inherent when propagating uncertainties from empirical scaling relationships to fault recurrence estimates in the systems-based approach (Williams et al., 2021).

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Example topographic profiles for the 4 faults with the highest confidence of activity (Chipola, Chitembo, Kabungo, and Molaza) are shown in Figure 9, with the corresponding along-strike scarp height profiles in Figure 10. The median scarp height of each fault ranged between 12.9 ± 0.4 (Molaza Fault) and 19.2 ± 0.9 m (Kabungo Fault - Figure :::::: Figures : 7 & 10c). The minimum resolvable scarp heights that we were :: are : able to measure using the SRTM data was :: is 2-3 m. However, the lower-resolution of SRTM (compared with TanDEM-X -see Wedmore et al., 2020b) meant that we were unable to identify clear fault segment The SE dipping 45 km long Kabungo Fault, which is 15 km east of the Chitembo Fault has the highest median scarp height of the faults that we measured 19.2 ± 0.9 m. The maximum scarp height is found to be 36.3 ± 0.08 ::: 0.  hazard assessments from other active fault and seismic source databases in southern/eastern Africa (Yang and Chen, 2010;Goda et al., 2016;Poggi et al., 2017;Williams et al., 2021). The LRAFD demonstrates that the framework for the future probabilistic seismic hazard analysis in southern Africa outlined by Williams et al. (2021) can be successfully applied to other regions using freely available, open access data such as SRTM. In addition, the mapped active faults provide an opportunity to analyse the seismotectonics of the Luangwa Rift and compare it to other amagmatic rifts in along the EARS, and this is the 335 focus of this discussion.
The two main border faults in the Luangwa Rift, the Chipola and Molaza faults, both follow Karoo-Basement contacts, but our analysis shows that these faults have also offset Quaternary sedimentary deposits (Figures 5 & 6). These relationships suggest that these are Karoo age structures that have been reactivated during the current phase of rifting. The Quaternary fault activity adds support to the notion that the southwestern branch of the East African Rift is active, and separates the Nubian 340 plate from smaller microplates (the San, Rovuma and possibly Angoni microplates) in southern and eastern Africa, as recently demonstrated by geodetic data (Wedmore et al., 2021).
Plate-scale modelling suggests that the extension direction across the San-Nubia plate boundary in the Luangwa Rift is 108 ± 8 • (relative to stable Nubia; Wedmore et al., 2021). Focal mechanism inversion shows that the σ3 ::: σ 3 , minimum compressive stress direction is 123 • (Delvaux and Barth, 2010). These extension directions are sub-perpendicular to the fault orientation 345 found here (mean strike: 045 ± 45 • ), which follows at the kilometer scale, the orientation of the Mwembeshi shear zone ( Figure 2). Both geodetic and seismological data suggest a NW-SE extension direction that is orientated sub-perpendicular to the orientation of the faults in the Luangwa Rift. However, the fault orientations are still consistent with a divergent boundary as we find no geomorphic evidence of horizontal offsets and it is not uncommon for normal faults in the EARS to reactivate at slightly oblique angles to the regional extension direction . Furthermore, the only available focal 350 mechanisms from the Luangwa Rift, from a M b 5.7 earthquake in 1976, shows a normal faulting mechanisms (Nyblade and Langston, 1995). Consequently, we consider that all faults in the LRAFD have pure normal kinematics but note that further work is needed to constrain the stress orientation in this region as the focal mechanism inversion of Delvaux and Barth (2010) is only based on six events, and the geodetic solution of Wedmore et al. (2021) is based on a continental scale GNSS network, with very few stations in the vicinity of the Luangwa Rift. Thus, the LRAFD demonstrates that the Luangwa Rift is an active 355 rift system that forms the extensional boundary between the Nubia and San plates in southern Africa, with faults that have reactivated Karoo-age structures aligned with the pre-existing lithospheric scale Mwembeshi shear zone.

Fault Activity in the Luangwa Rift and Comparison with other EARS basins
Active and inactive faults are typically distinguished by the age of the most recent earthquakes (Christophersen et al., 2015).
However, large magnitude earthquakes do not always result in surface rupture, especially in regions in southern Africa where 360 the crust can be seismogenic down to 40 km (Jackson and Blenkinsop, 1993;Nyblade and Langston, 1995;Kolawole et al., 2017;Craig and Jackson, 2021;Stevens et al., 2021). Furthermore, we are only aware of two palaeoseismic trenches along the whole of the East African Rift (Kervyn et al., 2006;Zielke and Strecker, 2009;Cohen et al., 2013) 2015)). Consequently, it is hard to definitively conclude that a fault is 'inactive' based on the absence of direct evidence of surface rupture alone. Thus, faults that do not fulfill all active criteria are still included in the database as we applied a broad definition of active faulting to reduce risk of excluding 'inactive' faults that :::: faults :::: that :::::: appear :: to :: be :::::::: inactive, ::: but :::::: which could rupture in a future earthquake, despite displaying limited evidence of 370 activity. In published maps of the region, no attempt was made to distinguish active and inactive faults in the Luangwa Rift (Banks et al., 1995;Daly et al., 2020). Here we classify 18 faults into varying degrees of activity confidence in a systematic active fault database.
The faults determined to have the highest confidence of activity (Chipola, Molaza, Chitumbi, Kabungo) all have prominent scarps, offset alluvial fans, and steeply incised rivers in the footwall ( Figure :::::: Figures : 5-7). We measured the height of the 375 prominently exposed fault scarps on each of these faults ( Figure :::::: Figures : 9 & 10), with the median scarp height between 13-19 m ( Figure 10). The Chipola Fault has a scarp height of 13.0 ± 0.4 m ( Figure :::::: Figures : 9a and 10). It has been suggested that a ∼12-14 m high scarp previously detected along the Chipola South Fault formed in the last 10 ka (although no evidence was provided for this time period; Daly et al., 2020), but the authors were unable to distinguish whether this resulted from a single, large magnitude earthquake, or a series of smaller events. Our measurements of scarp height exceed the average single event 380 displacement values from the Leonard (2010) scaling relationships. Thus, our results suggest that these scarps have formed from multiple earthquakes. Along the Bilila-Mtakataka fault in Malawi, Hodge et al. (2020) demonstrated that a 20 m high fault scarp was generated by at least two earthquakes with single event displacements possibly as high as 10-12 m, which also exceeds empirically derived single event displacement estimates. Evidence from the Hebron Fault in Namibia (Salomon et al., 2021) also suggests that normal faults that rupture thick crust may have higher single event displacement-length ratios than

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The localized deformation across border faults justifies our approach of using the geodetically-derived regional extension rate to estimate the slip rate and earthquake recurrence interval of the border faults (Section 3.2.2). However, these should be treated as upper bounds on the slip rate and lower bounds on the recurrence interval. Numerical models indicate that rifts with localized deformation across border faults form in strong lithosphere, where the strength is dominated by the crust (Huismans and Beaumont, 2011). Low V p /V s ratios and high horizontal shear wave velocities suggest the absence of partial 410 melt, magmatic intrusions or significant levels of fluid and instead imply that the crust is strong beneath the Luangwa Rift (Wang et al., 2019;Sun et al., 2021). Furthermore, although faults in the rift follow the orientation of the foliated mylonitic gneiss and eclogites within Mwembeshi Shear Zone (or a splay of the shear zone Daly et al., 1989), experiments on similar mafic samples from the Malawi Rift suggest these rocks are unlikely to frictionally weak (Hellebrekers et al., 2019). Although the high-grade metamorphic shear zones such as the Mwembeshi Shear Zone are more likely to be viscously weak because of occurs on the same day as a M s 6.2 event recorded on 1st May 1919, which macroseismic damage reports initially suggest was located 250 km to the north (Ambraseys and Adams, 1991). It is unclear if these events are linked : , :::: thus :::: field ::::::::::: investigation :: of :: the ::::::: Molaza ::::: Fault ::::: should ::: be : a ::::::: priority :: to ::::::: establish ::::::: whether :::: this ::::::::: represents : a :::: rare ::::::: example :: of :: a :::: 20th :::::: century ::::::::: earthquake ::::::: surface :::::: rupture :: in ::::: Africa. Nevertheless, previous seismic hazard assessment in the region by definition considers the maximum possible earthquake magnitude to be 0.5 greater than the largest recorded historical earthquake (Poggi et al., 2017). However, this 435 seismic hazard assessment states that the maximum magnitude earthquake in this region is M 6.9 (Poggi et al., 2017). Our new finding of a M w 6.7 event on the Molaza Fault should therefore prompt a revision of the seismic hazard in the region.
Despite evidence for the activity on the Molaza Fault in the 20th century, there remains large portions of the 140 km long fault that have not ruptured recently. We estimate that the Molaza Fault is a seismic source capable of hosting a : an :::::::::: earthquake :::: with : a ::::::::: maximum ::::::::: magnitude :: of Mw 7.8 earthquake with ::: and a displacement of 3.1 m, which is an order of magnitude greater 440 than earthquakes recorded in the Luangwa region and more than the any event in the whole of southern/eastern Africa. The largest regional event was the 13th December 1910 Ms 7.4 Rukwa earthquake in Tanzania (Ambraseys, 1991a) Girdler and McConnell, 1994) and the 2006 M w 7.0 Mozambique earthquake (Copley et al., 2012). The LRAFD contains eight faults that exceed 50 km in length, 445 with two faults greater than 100 km. Seismic source attributes calculated from the LRAFD indicate that there are 12 faults that have the potential to rupture in M w ≥7.0 earthquakes (Figure 8), with the 207 km long Chipola fault capable of hosting :: up :: to a M w 8.1 earthquake. Global compilations of continental normal faulting earthquakes suggest that they rarely exceed rupture lengths of 50 km and low M w 7 (Neely and Stein, 2021), and there is only one event with a surface rupture length > 100km (Valentini et al., 2020). Thus, although M w 7+ events are likely rare, they should be considered possible in the Luangwa Rift 450 due to the long faults that are hosted in 45 km thick crust (Sun et al., 2021), which has recorded seismicity to ∼30 km depth (Craig et al., 2011;Craig and Jackson, 2021). Nonetheless, these large magnitude events likely occur infrequently as >100 km long normal faults in southern Africa are often segmented (Mortimer et al., 2016;Hodge et al., 2018;Wedmore et al., 2020a, b) and the regional b-value is ∼1 (Poggi et al., 2017) implying that smaller segmented ruptures are more likely than earthquakes that rupture an entire fault.

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We :: To :::::::: estimate ::: the ::::::::: magnitude ::: of :::::: smaller ::::::::: segmented :::::::: ruptures, ::: we : attempted to identify fault segments for the four best exposed faults in the Luangwa Rift by systematically measuring along-strike fault scarp heights ( Figure 10). This approach has been successful in other East African rift basins using 12.5 m resolution TanDEM-X data (Hodge et al., 2018;Wedmore et al., 2020a, b). Using the 30 m resolution SRTM data, we were unable to identify clear segment boundaries in either the scarp height measurements or in notable changes in fault geometry (e.g. 90 • bends), making it challenging to assess the limits on rupture the Molaza Fault. Thus, Although ::::::: although : we do not directly observe evidence for fault segmentation in the Luangwa Rift, the data provided here can be used to incorporate small ruptures along these faults into seismic hazard assessment by combining the provided slip rate, fault area, and magnitude estimates with a regional b-value (Poggi et al., 2017) and the methodology 465 developed by Youngs and Coppersmith (1985) to develop continuous recurrence models for these sources (see Williams et al.).
However, it is also possible that multi-fault rupture may occur, which would increase the potential magnitude of future events.  indicates that building vulnerability in this region is higher than currently predicted by global models (e.g. the USGS WHE-PAGER model; Novelli et al., 2021;Giordano et al., 2021). We suggest that active fault mapping, such as has been carried out 480 here in the LRAFD and in other active fault databases in southern Africa (Williams et al., 2021(Williams et al., , 2022) provides a framework for accurate probabilistic seismic hazard assessment, and thus for increasing resilience to seismic hazard throughout southern and eastern Africa.

Conclusions
Using SRTM data, we have systematically mapped and compiled the attributes of 18 known active faults in the Luangwa Rift,

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Zambia to produce the Luangwa Rift Active Fault Database (LRAFD). The LRAFD is freely available open-source dataset that is aimed for use in future probabilistic seismic hazard assessment, as well as providing a resource of further scientific study of the Luangwa Rift. Empirical scaling relationships between fault length and earthquake magnitude suggest that the faults in the Luangwa Rift can host earthquakes greater than M w 7, up to M w 8.1, although we consider that these scenarios are unlikely or extremely rare. 490 We find evidence that all 18 faults mapped have been active during the Quaternary, with the four most prominent faults displaying well preserved linear fault scarps up to ∼20-30 m high. Systematic measurements of the height of the scarps on these four faults suggest that they were formed by multiple earthquakes, but using 30 m resolution SRTM data, we were unable to use along-strike scarp height profiles to identify fault segment boundaries. Within the Luangwa Rift, the two border faults (Chipola and Molaza), which have opposing polarity and have reactivated structures that were previously active during a Karoo 495 phase of rifting, appear to accommodate most of the surface deformation. This suggests that the 45 km thick crust is strong and does not contain any weaker mid-or lower-crustal layers, which is confirmed by other geophysical proxies. Although the orientation of the faults in the rift follows that of the underlying Mwembeshi shear zone, it remains unclear why this shear zone is weaker than the surrounding rocks. Nevertheless, we conclusively demonstrate that faults in the Luangwa Rift are active, and provide a pathway for the inclusion of active faults in the region into future probabilistic seismic hazard assessment.

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Data availability. All data generated in this manuscript is freely available and archived in online repositories.     (Utting, 1988;Banks et al., 1995;Bishop et al., 2016). Faults predominantly follow contacts between Karoo sediments and Basement     (Leonard, 2010). Scarp heights exceeding this, along with bimodal histogram peaks, suggest the potential for multiple earthquake events and composite scarps. This data has been archived and is available at the following location: https://doi.org/10.5281/zenodo.6513545 (Turner et al., 2022a).