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Oceanic plates experience extensive normal faulting as they bend and subduct, enabling fracturing of the incoming lithosphere. Debate remains about the relative importance of pre‐existing faults, plate curvature and other factors controlling the extent and style of bending‐related faulting. The subduction zone off the Alaska Peninsula is an ideal place to investigate controls on bending faulting as the orientation of the abyssal‐hill fabric with respect to the trench and plate curvature vary along the margin. Here, we characterize faulting between longitudes 161°W and 155°W using newly collected multibeam bathymetry data. We also use a compilation of seismic reflection data to constrain patterns of sediment thickness on the incoming plate. Although sediment thickness increases over 1 km from 156°W to 160°W, most sediments were deposited prior to the onset of bending faulting and thus should have limited impact on the expression of bend‐related fault strikes and throws in bathymetry data. Where magnetic anomalies trend subparallel to the trench (<30°) west of ∼156°W, bending faults parallel magnetic anomalies, implying that bending faults reactivate pre‐existing structures. Where magnetic anomalies are highly oblique (>30°) to the trench east of 156°W, no bending faults are observed. Summed fault throws increase to the west, including where pre‐existing structure orientations are constant (between 157 and 161°W), suggesting that another factor such as the increase in slab curvature must influence bending faulting. However, the westward increase in summed fault throws is more abrupt than expected for gradual changes in slab bending alone, suggesting potential feedbacks between pre‐existing structures, slab dip, and faulting. Plain Language Summary: Subduction zones are plate boundaries where two tectonic plates converge, and the oceanic plate is bent and forced below the other plate. Oceanic plates are faulted as they bend, and these "bending faults" are thought to be important for controlling the deep water cycle on Earth and influencing the generation of large earthquakes in subduction zones. The amount and style of bending faulting varies between and within subduction zones around the world, and debate remains about what causes this variability. Possible controls include the overall curvature of the oceanic plate as it bends and subducts and pre‐existing weaknesses in the oceanic plate from when it formed. We use bathymetry data across the Alaska subduction zone to characterize bending faults here and understand the controls on their formation. This is an ideal study area because the curvature of the plate and the pre‐existing weaknesses vary in this region. The amount of bending faulting increases abruptly to the west and appears to result from a feedback between favorably oriented pre‐existing weaknesses and increased curvature of the oceanic plate. These results can be used to understand bending faulting in other subduction zones. Key Points: Bathymetry data reveal variations in the orientation and summed throws of bending faulting outboard of the Alaska subduction zoneThe westward increase in the number and summed throws of bending faults is due to favorably oriented pre‐existing structures and increased slab dipVariable orientations of bend faulting and volcanic constructs updip of 2020 M7.6 intraplate earthquake implies complex stresses in the slab [ABSTRACT FROM AUTHOR]

We present the Malawi Active Fault Database (MAFD), an open‐access (https://doi.org/10.5281/zenodo.5507190) geospatial database of 113 fault traces in Malawi and neighboring Tanzania and Mozambique. Malawi is located within the East African Rift's (EAR) Western Branch where active fault identification is challenging because chronostratigraphic data are rare, and/or faults are buried and so do not have a surface expression. The MAFD therefore includes any fault that has evidence for displacement during Cenozoic East African rifting or is buried beneath the rift valley and is favorably oriented to the regional stresses. To identify such faults, we consider a multidisciplinary data set: high‐resolution digital elevation models, previous geological mapping, field observations, seismic reflection surveys from offshore Lake Malawi, and aeromagnetic and gravity data. The MAFD includes faults throughout Malawi, where seismic risk is increasing because of population growth and its seismically vulnerable building stock. We also investigate the database as a sample of the normal fault population in an incipient continental rift. We cannot reject the null hypothesis that the distribution of fault lengths in the MAFD is described by a power law, which is consistent with Malawi's relatively thick seismogenic layer (30–40 km), low (<8%) regional extensional strain, and regional deformation localization (50%–75%) across relatively long hard‐linked border faults. Cumulatively, we highlight the importance of integrating onshore and offshore geological and geophysical data to develop active fault databases along the EAR and similar continental settings both to understand the regional seismic hazard and tectonic evolution. Plain Language Summary: Earthquakes represent the occurrence of slip along cracks in the Earth's crust. Therefore, mapping these cracks, or "faults," is important when assessing earthquake hazards. However, faults are challenging to identify as they may not be visible at the surface. Fault mapping also requires recognizing which faults have slipped in earthquakes in the recent geologic past, as these "active" faults are the most likely faults to have future earthquakes. Here, we describe how we identified active faults in Malawi, which is located along the tectonically active East African Rift. Faults under Lake Malawi were mapped using images of lake sediments that were generated from sound waves. Onshore, some faults were mapped from their expression in the landscape. Other faults, not visible at the surface, were identified from aeromagnetic data, which image the spatial distributions of magnetic minerals in the Earth's crust. Faults are considered active if that show evidence for slip during East African rifting in Malawi. We combined the active faults identified from these analyses into the Malawi Active Fault Database, a freely available geospatial database. We suggest that this database will be useful for seismic hazard planning in Malawi, where population growth and vulnerable buildings are increasing earthquake risk. Key Points: Digital elevation models, offshore seismic reflection surveys, and aeromagnetic data are combined to identify active faults in MalawiMapped faults are incorporated into the Malawi Active Fault Database, an open‐access geospatial databaseThe mapped faults follow a power law length distribution, which is consistent with strain localization onto a few long (>50 km) faults [ABSTRACT FROM AUTHOR]

Although the deep, wide basins of the Western rift, Africa, have served as analogues for the evolution of half‐graben basins, the geometry and kinematics of the border, intrabasinal, and transfer fault systems have been weakly constrained. Despite the >100‐km‐long fault systems bounding basins, little was known of seismicity patterns or the potential for M > 7.5 earthquakes. Using our new local earthquake database from the 2013‐2015 Study of Extension and maGmatism in Malawi aNd Tanzania (SEGMeNT) seismic array (57 onshore, 32 lake‐bottom stations) and TANGA14 (13 stations), we examine the kinematics and extension direction of the Rungwe Volcanic Province and northern Malawi rift. We relocated earthquakes using a new 1‐D velocity model and both absolute and double‐difference relocation methods. Local magnitudes of 1,178 earthquakes within the array are 0.7 < ML < 5.2 with a b‐value 0.77 ± 0.03, and magnitude of completeness ML 1.9. Focal mechanism solutions for 63 earthquakes reveal predominantly normal and oblique‐slip motion, and full moment tensor solutions for ML 4.5, 5.2 earthquakes have centroid depths within 2 km of catalog depths. The preferred nodal planes dip more than 40° from surface to >25‐km depths. Extension direction from local earthquakes and source mechanisms of teleseismically detected earthquakes are approximately N58°E and N65°E, respectively, refuting earlier interpretations of a NW‐SE transform fault system. The low b‐value indicating strong coupling across crustal‐scale border faults, border fault lengths >100 km, and evidence for aseismic deformation together indicate that infrequent M > 7.5 earthquakes are possible within this cratonic rift system. Key Points: Steep nodal planes of earthquake focal mechanisms correspond to projections of border and intrabasinal faults to depths of 25 kmExtension direction across Rungwe volcanic province and Malawi rift is ENE, refuting interpretations of a NW‐SE transform fault systemLow b‐value, fault lengths >100 km, seismogenic layer 25‐30 km, and aseismic deformation suggest that infrequent M>7.5 earthquakes are possible [ABSTRACT FROM AUTHOR]

P and S wave tomographic models have been developed for the northern Malawi rift and adjacent Rungwe Volcanic Province (RVP) using data from the Study of Extension and maGmatism in Malawi aNd Tanzania project and data from previous networks in the study area. The main features of the models are a low‐velocity zone (LVZ) with δVp = ~−1.5–2.0% and δVs = ~−2–3% centered beneath the RVP, a lower‐amplitude LVZ (δVp = ~−1.0–1.3% and δVs = ~−0.7–1%) to the southeast of the RVP beneath the center and northeastern side of the northern Malawi rift, a shift of the lower‐amplitude anomaly at ~−10° to −11° to the west beneath the central basin and to the western side of the rift, and a fast anomaly at all depths beneath the Bangweulu Craton. The LVZ widens further at depths >~150–200 km and extends to the north beneath northwestern Malawi, wrapping around the fast anomaly beneath the craton. We attribute the LVZ beneath the RVP and the northern Malawi rift to the flow of warm, superplume mantle from the southwest, upwelling beneath and around the Bangweulu Craton lithosphere, consistent with high 3He/4He values from the RVP. The LVZ under the RVP and northern Malawi rift strongly indicates that the rifted lithosphere has been thermally perturbed. Given that volcanism in the RVP began about 10 million years earlier than the rift faulting, thermal and/or magmatic weakening of the lithosphere may have begun prior to the onset of rifting. Plain Language Summary: P and S wave tomographic models have been developed for the northern Malawi rift and adjacent Rungwe Volcanic Province (RVP) using data from the Study of Extension and maGmatism in Malawi aNd Tanzania project and data from previous networks in the study area. A low‐velocity anomaly is imaged under the RVP and northern Malawi rift. We attribute the low‐velocity anomaly to flow of warm mantle from the African superplume to the southwest of the study area, which has migrated around the side of thick Bangweulu Craton lithosphere and upwelled beneath the thinner mobile belt lithosphere to the east of the Bangweulu Craton. The observation that volcanism began in the RVP prior to the onset of rifting suggests that the lithosphere beneath the Malawi rift may have been thermally weakened prior to rifting. Key Points: Low‐velocity anomaly is imaged under Rungwe Volcanic Province and northern Malawi riftLow‐velocity anomaly is attributed to upwelling of warm mantle around side of Bangweulu Craton lithosphereLithosphere beneath the Malawi rift may have been weakened prior to rifting [ABSTRACT FROM AUTHOR]