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Subduction interfaces exhibit various slip styles, including slow slip events (SSEs). We use a micromechanics‐based approach to calculate the effective rheology of a shear zone containing ellipsoidal amphibolite clasts deforming by dislocation creep within an interconnected linear‐viscous phyllosilicate‐dominated matrix. Frictional failure occurs if local stress exceeds Mohr‐Coulomb yield strength. At moderate fluid overpressure, mixed‐frictional‐viscous behavior emerges at ∼${\\sim} $ 350–560°^{\\circ}$ C, consistent with a broad zone of mixed fault slip behavior without requiring extreme fluid overpressures. Increasing stress in this transition zone promotes local frictional failure and raises bulk strain rate. If, however, the bulk strain rate increases by more than one order of magnitude, system‐wide frictional sliding becomes preferable. This strain rate increase is insufficient to explain the slip rates observed in geophysically detectable SSEs. Therefore, viscous matrix flow as modeled here cannot explain SSEs without either invoking dynamic weakening within a frictional‐viscous flow or a mechanism switch to dominantly frictional sliding. Plain Language Summary Subduction plate boundaries are locked near the Earth's surface and will release the stored energy as earthquakes. Subduction zones creep steadily and viscously at deeper depths where temperatures and pressures are high. At the depth of the transition from earthquakes to steady creep, episodic aseismic slip is often observed. Rocks from this region are mixtures of strong, fractured clasts surrounded by a weak matrix. The observations of exhumed rocks suggest that the episodic, aseismic slip may nucleate when local frictional failure occurs in strong clasts, but the surrounding weak matrix stops this failure from generating major earthquakes. However, it is unclear how much the small‐scale rock behavior could be linked to the large‐scale slip. We use a numerical model to simulate the interplay between frictional and viscous creep and calculate the overall behavior of the subduction zone plate boundary. We explore how the slip style changes with depth and determine the transition zone's depth/temperature range. In the transition zone, a small increase in stress or decrease in strength can lead to a change from pure viscous flow to frictional sliding. This study overcomes the scale challenge between the small‐scale features preserved on outcrops and the large‐scale geophysical observations. Key Points Frictional‐viscous flow in a two‐phase shear zone modeled by a multiscale approach occurs at ∼350–560°C with moderate fluid overpressure Stress loading and/or fluid pressure weakening can cause a switch from steady viscous creep to transient frictional slip Viscous creep modeled here can accommodate tectonic strain rates but not slow slip events without invoking a switch to frictional sliding

Interactions between upper plate deformation and plate interface seismicity in subduction zones remain poorly understood, but growing evidence indicates that fluid flow along splay faults modulates upper‐plate faulting. Field observations from two exhumed splay faults define end‐member scenarios where impermeable faults trap fluids in their footwall, whereas permeable faults channel fluids along them. Using finite element poroelastic models, with slip mode inferred solely from stress–pore‐fluid pressure patterns, we define two end‐member behaviors: (a) Impermeable, clay‐rich, mature splay faults favor footwall fluid flow, promoting low differential stress, dilation, and vein formation in the upper plate, while reducing pore‐fluid pressure and coupling the megathrust downdip of the intersection. (b) Permeable, less mature faults allow distributed upper‐plate fluid flow, increasing fluid flux, and differential stress, while maintaining plate interface overpressure and promoting creep. These models provide a framework for prism‐scale effects of splay fault permeability on shallow subduction zone deformation and seismicity.

Subduction megathrust slip speeds range from slow creep at plate convergence rates (centimetres per year) to seismic slip rates (metres per second) in the largest earthquakes on Earth. The deformation mechanisms controlling whether fast slip or slow creep occurs, however, remain unclear. Here, we present evidence that pressure solution creep (fluid-assisted stress driven mass transfer) is an important deformation mechanism in megathrust faults. We quantify megathrust strength using a laboratory-constrained microphysical model for fault friction, involving viscous pressure solution and frictional sliding. We find that at plate-boundary deformation rates, aseismic, frictional–viscous flow is the preferred deformation mechanism at temperatures above 100 °C. The model thus predicts aseismic creep at temperatures much cooler than the onset of crystal plasticity, unless a boundary condition changes. Within this model framework, earthquakes may nucleate when a local increase in strain rate triggers velocity-weakening slip, and we speculate that slip area and event magnitude increase with increasing spacing of strong, topographically derived irregularities in the subduction interface. The controls on fast versus slow fault slip in subduction zones are unclear. Rock deformation experiments suggest that faults within the seismogenic region of a subduction zone may slip aseismically due to pressure solution creep.

Aseismic megathrust slip downdip of the seismogenic zone is accommodated by either steady creep or episodic slow slip events (SSEs). However, the geological conditions defining the rheology of megathrust slip remain elusive. We examined exhumed subduction mélanges on Kyushu, Japan, which deformed at ∼370–500°C under greenschist to epidote‐amphibolite facies conditions, comparable to warm‐slab environments. The mélanges recorded fluid release and viscous shear localization associated with metasomatic reactions between juxtaposed metapelitic and metabasaltic rocks. Metasomatic reactions caused albitization of metapelite, resulting in depth‐dependent changes to megathrust rheology. In a mélange deformed at ∼370°C, very fine grained reaction products (metasomatic albite) facilitated grain boundary diffusion creep at stresses of ∼45 MPa, less than those in the surrounding metabasalt. Mineralogical and chemical changes during metasomatic reactions, and their field content, imply an onset of albite metasomatism at ∼350°C. Albite metasomatism therefore potentially contributed to decreased megathrust strength around the inferred thermally controlled base of the seismogenic zone. In a mélange deformed near the mantle wedge corner at ∼500°C, metasomatic reactions promoted local quartz vein formation and localized viscous shear at slow slip strain rates, during which the coarse‐grained metasomatic albite behaved as relatively rigid blocks in a viscous matrix. We suggest that albite metasomatism can facilitate changes in a megathrust slip mode with depth and may explain why slip mode changes from creep to SSEs with tremor with increasing depth. Plain Language Summary Along tectonic plate boundaries, where one plate slips beneath another, plate movement occurs by processes including large and devastating earthquakes slipping at meters/second, very small earthquakes called tectonic tremor, slow slip events (SSEs) slipping at millimeters/day, and steady creep slipping at centimeters/year. However, the factors controlling where these different slip styles occur remain poorly understood. On Kyushu, Japan, ancient plate boundary rocks have been exposed by uplift and erosion. Our measurements of structures and rock chemistry in these rocks revealed that chemical reactions between subducted basalts and sediments may influence the plate boundary slip behavior. In rocks that deformed near ∼370°C, chemical reactions produced very fine grained rocks that caused local weakening within the plate boundary. This could explain why the plate boundary slip behavior changes from frictional to viscous near the downdip of the seismogenic zone at ∼350°C. In rocks that deformed at ∼500°C, near where slow slip and tremor events occur, chemical reactions promoted quartz vein formation that may represent tremor and localized viscous shear at faster strain rates than in surrounding rocks. This could explain the occurrence of tectonic tremors and SSEs. Key Points Exhumed mélange shear zones deformed downdip of the seismogenic zone recorded albite metasomatism during subduction Very fine grained albite facilitated shear zone weakening by grain boundary diffusion creep near the base of the seismogenic zone Albite metasomatism promoted viscous shear localization at an increased strain rate near the mantle wedge corner

Some faults slip at high angles to the greatest principal compressive stress. The discovery of shear veins formed at angles of about 80 degrees relative to the greatest principal compressive stress in the Chrystalls Beach complex, New Zealand, suggests that slip can be facilitated by a pre-existing rock fabric under high fluid pressure. Some faults seem to slip at unusually high angles (>45°) relative to the orientation of the greatest principal compressive stress 1 , 2 , 3 , 4 , 5 . This implies that these faults are extremely weak compared with the surrounding rock 6 . Laboratory friction experiments and theoretical models suggest that the weakness may result from slip on a pre-existing frictionally weak surface 7 , 8 , 9 , weakening from chemical reactions 10 , elevated fluid pressure 11 , 12 , 13 or dissolution–precipitation creep 14 , 15 . Here we describe shear veins within the Chrystalls Beach accretionary mélange, New Zealand. The mélange is a highly sheared assemblage of relatively competent rock within a cleaved, anisotropic mudstone matrix. The orientation of the shear veins—compared with the direction of hydrothermal extension veins that formed contemporaneously—indicates that they were active at an angle of 80°±5° to the greatest principal compressive stress. We show that the shear veins developed incrementally along the cleavage planes of the matrix. Thus, we suggest that episodic slip was facilitated by the anisotropic internal fabric, in a fluid-overpressured, heterogeneous shear zone. A similar mechanism may accommodate shear at high angles to the greatest principal compressive stress in a range of tectonic settings. We therefore conclude that incremental slip along a pre-existing planar fabric, coupled to high fluid pressure and dissolution–precipitation creep, may explain active slip on severely misoriented faults.

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 Malawi Mapped faults are incorporated into the Malawi Active Fault Database, an open‐access geospatial database The mapped faults follow a power law length distribution, which is consistent with strain localization onto a few long (>50 km) faults

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.

Seismic hazard is commonly characterised using instrumental seismic records. However, these records are short relative to earthquake repeat times, and extrapolating 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 setting, is resource intensive, and can carry large inherent uncertainties. Here, we outline how fault slip rates and recurrence intervals can be estimated by combining fault geometry, earthquake-scaling relationships, geodetically derived regional strain rates, and geological constraints of regional strain distribution. We apply this approach to southern Malawi, near the southern end of the East African Rift, and where, although no on-fault slip rate measurements exist, there are constraints on strain partitioning between border and intra-basin faults. This has led to the development of the South Malawi Active Fault Database (SMAFD), a geographical database of 23 active fault traces, and the South Malawi Seismogenic Source Database (SMSSD), in which we apply our systems-based approach to estimate earthquake magnitudes and recurrence intervals for the faults compiled in the SMAFD. We estimate earthquake magnitudes of MW 5.4–7.2 for individual fault sections in the SMSSD and MW 5.6–7.8 for whole-fault ruptures. However, low fault 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 intra-basin faults. Sensitivity analysis indicates that the large range of these estimates can best be reduced with improved geodetic constraints in southern Malawi. The SMAFD and SMSSD provide a framework for using geological and geodetic information to characterise seismic hazard in regions with few on-fault slip rate measurements, and they could be adapted for use elsewhere in the East African Rift and globally.

Non‐volcanic tremor is a recently discovered fault slip style occurring with remarkable regularity in space near the down‐dip end of the locked zone on several subduction thrust interfaces. The physical mechanisms and the controls on the location of tremor have not yet been determined. We calculate the stable mineral assemblages and their water content in the subducting slab, and find that slab dehydration is not continuous, but rather restricted to a few reactions localised in pressure‐temperature space. Along geothermal gradients applicable to Shikoku and Cascadia ‐ where tremor has been relatively easy to detect ‐ tremor locations correlate with discontinuous and localised voluminous water release from the breakdown of lawsonite and chlorite + glaucophane respectively. The shape of the pressure‐temperature path for subducting slabs prevents fluid release at depths above and below where these dehydration reactions occur. We conclude that abundant tremor activity requires metamorphic conditions where localised dehydration occurs during subduction, and this may explain why tremor appears more abundant in some subduction zones than others. Key Points Discontinuous dehydration is localised to some points on the subduction P‐T path Abundant tremor activity correlates with localised voluminous water release Significant tremor activity requires specific metamorphic conditions

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 12 m 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 5 m resolution DEM derived from high-resolution (50 cm pixel−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.