Passive Rifting of Thick Lithosphere in the Southern East African Rift: Evidence from Mantle Transition Zone Discontinuity Topography

To investigate the mechanisms for the initiation and early-stage evolution of the nonvolcanic southernmost segments of the East African Rift System (EARS), we installed and operated 35 broadband seismic stations across the Malawi and Luangwa rift zones over a 2 year period from mid-2012 to mid-2014. Stacking of over 1900 high-quality receiver functions provides the first regional-scale image of the 410 and 660 km seismic discontinuities bounding the mantle transition zone (MTZ) within the vicinity of the rift zones. When a 1-D standard Earth model is used for time-depth conversion, a normal MTZ thickness of 250 km is found beneath most of the study area. In addition, the apparent depths of both discontinuities are shallower than normal with a maximum apparent uplift of 20 km, suggesting widespread upper mantle high-velocity anomalies. These findings suggest that it is unlikely for a low-velocity province to reside within the upper mantle or MTZ beneath the nonvolcanic southern EARS. They also support the existence of relatively thick and strong lithosphere corresponding to the widest section of the Malawi rift zone, an observation that is consistent with strain localization models and fault polarity and geometry observations. We postulate that the Malawi rift is driven primarily by passive extension within the lithosphere attributed to the divergent rotation of the Rovuma microplate relative to the Nubian plate, and that contributions of thermal upwelling from the lower mantle are insignificant in the initiation and early-stage development of rift zones in southern Africa

Ranking and developing ground-motion models for Southeastern Africa

The southern East African Rift System (EARS) is an early-stage continental rift with a deep seismogenic zone. It is associated with a low-to-moderate seismic hazard, but due to its short and sparse instrumental record, there is a lack of ground-motion studies in the region. Instead, seismic hazard assessments have commonly relied on a combination of active crustal and stable continental ground-motion models (GMMs) from other regions without accounting for the unusual geological setting of this region and evaluating their suitability. Here, we use a newly compiled southern EARS ground-motion database to compare six active crustal GMMs and four stable continental GMMs. We find that the active crustal GMMs tend to underestimate the ground-motion intensities observed, while the stable continental GMMs overestimate them. This is particularly pronounced in the high-frequency intensity measures (.5 Hz). We also use the referenced empirical approach and develop a new region-specific GMM for southern EARS. Both the ranked GMMs and our new GMM result in large residual variabilities, highlighting the need for local geotechnical information to better constrain site conditions

Seismic Evidence for Plume‐ and Craton‐Influenced Upper Mantle Structure Beneath the Northern Malawi Rift and the Rungwe Volcanic Province, East Africa

Abstract 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

Malawi Seismogenic Source Model: v1.1

The Malawi Seismogenic Source Model (MSSM) is a geospatial database that documents the geometry, slip rate and seismogenic properties (ie earthquake magnitude and frequency) of active faults in Malawi. Each geospatial feature represents a potential earthquake rupture of 'source' and is classified based on its geometry into one of three types: sectionfaultmulti-fault Source types are mutually exclusice, and so if incorporated into a PSHA, they should be assigned relative weightings. The MSSM is the first seismogenic source database in central and northern Malawi, and represents an update of the South Malawi Seismogenic Source Database (SMSSD; Williams et al., 2021a) because it incorporates new active fault traces (Kolawole et al., 2021; Williams et al., 2021b; 2022 - MAFD), new geodetic data (Wedmore et al., 2021) and a statistical treatment of uncertainty, within a logic tree approach. The seismogenic sources in this model are adapted from the faults in the Malawi Active Fault Database (Williams et al., 2021b; 2022). Prior to publication please cite this database using the following two references: Williams, J. N., Wedmore, L. N .J., Fagereng, Å., Werner, M. J., Biggs, J., Mdala, H., Kolawole, F., Shillington, D. J., Dulanya, Z., Mphepo, F., Chindandali, P., Wright, L. J. M.., Scholz, C. A. Geological and geodetic constraints on the seismic hazard of Malawi's active faults: the Malawi Seismogenic Source Model (MSSM). Manuscript submitted to Natural Hazards and Earth System Sciences Williams, Jack N., Wedmore, Luke N. J., Fagereng, Åke, Werner, Maximilian J., Biggs, Juliet, Mdala, Hassan, Kolawole, Folarin, Shillington, Donna J., Dulanya, Zuze, Mphepo, Felix, Chindandali, Patrick R. N., Wright, Lachlan J. M., & Scholz, Christopher A. (2021). Malawi Seismogenic Source Model [Data set]. Zenodo. https://doi.org/10.5281/zenodo.5599616 Database Design and File Formats The MSSM is a geospatial database that consists of two separate components: A 3D geometrical model of fault seismogenic sources in MalawiThe mapped trace of each source in a GIS vector format, with associated source attributes (Data Table). Each fault is associated with a source in the 3D geometrical model that is listed in a comma-separated-values (csv) file. The sections, faults and multi-faults that make up the individual seismogenic sources are described in separate geospatial files that describe the map-view geometry and metadata that control each sources earthquake magnitude and frequency for seismic hazard purposes. The sections, faults and multi-faults in this database are provided in a variety of GIS vector file formats. GeoJSON is the version of record, and any changes should be made in this version before they are converted to other file formats using the script in the repository that uses the GDAL tool ogr2ogr (the script is adapted from https://github.com/cossatot/central_am_carib_faults/blob/master/convert.sh - we thank Richard Styron for making this publicly available). The other versions available are ESRI ShapeFile, KML, GMT, and GeoPackage.   List and brief description of the fault geometry, slip rate estimates and earthquake source attributes in the GIS vector format files that make up the MSSM.AttribugeTypeDescriptionNotesMSSM_IDintegerUnique numerical reference ID for each seismic sourceID 00-300 is section rupture ID 300-500 is fault rupture ID 600-700 is a multi-fault rupturenamestring Assigned based on previous mapping or local geographic feature. For sections and faults, the name of the fault (flt_name) and larger multi-fault (mflt_name) system they are hosted on are given respectively.basinstringBasin that source is located within.Used in slip rate calculationsclassstringintrarift or border fault length (Ls)real numberstraight-line distance in km between fault tips; sum of Lsec for segmented faults; sum of Lfault for multi-faultsmeasured in km to 1 decimal place. Must be greater than 5 km (except for linking sections).areaintegerCalculated from Ls multiplied by Eq. 1 or based on fault truncation.measured in km2strikeintegerAzimuth of straigth line between the fault tips. azimuth is <180° Used as input for slip rate estimates in Eq. 2 dip_lowerintegerlower range of dip valueWhen no previous measurements of dip are available, a nominal value of 45° is used.dip_intintegerIntermediate dip valueIn the MSSM geometrical model, only the intermediate measurements is considered. When no previous measurements of are available, a nominal value of 53° is assigned. No dip is assigned for multi-fault sources, as different participating faults may have different dips.dip_upperintegerUpper range of dip valueWhen no previous measurements of dip are availabe, a nominal value of 65° is used.dip_dirstringDip direction: compass quadrant that the fault dips in. slip_typestringSource kinematics (e.g. normal, thrust etc).All sources in the MSSM are assumed to be normal faults.slip_ratereal numberMean value from repeating Eq. 2 in Monte Carlo simulations (see manuscript for details).In mm yr-1. All sources in the MSSM are assumed to be normal so is equivalent to dip-slip rate. Reported to two significant figures.s_rate_errreal numberSlip rate error: 1σ error from Monte Carlo slip rate simlations. mag_lowerreal numberLower magnitude estimate. Calculated from Leonard (2010) scaling relationship (Eq. 4) for Ls or As, and using lower estimates of C1 and C2 constants in Leonard (2010).Reported to one decimal place.mag_medreal numberMean magnitude estimate. Calculated from Leonard (2010) scaling relationship (Eq. 4) for Ls or As, and using mean estimates of C1 and C2 constants in Leonard (2010).Reported to one decimal place.mag_upperreal numberUpper magnitude estimate. Calculated from Leonard (2010) scaling relationship (Eq. 4) for Ls or As, and using upper estimates of C1 and C2 constants in Leonard (2010).Reported to one decimal place.ri_lowerreal numberLower recurrence interval estimate. Calculated as 1σ below the mean of the Monte Carlo simulations (assuming a log normal distribution).Reported to two significant figures.ri_medreal numberMean recurrence interval. Mean value from log of recurrence interval Monte Carlo simulations.Reported to two significant figures.ri_upperreal numberUpper recurrence interval estimate. Calculated as 1σ above the mean of the Monte Carlo simulations (assuming a log normal distribution).Reported to two significant figures.MAFD_idlistList of integers of ID of equivalent structures in the Malawi Active Fault DatabaseMulti-fault sources have multiple ID's.   Version Control This version is intended to be "Live" and as such we encourage edits of the GeoJSON file and the submission of pull requests. Please contact Jack Williams [email protected] Luke Wedmore [email protected] or Hassan Mdala [email protected] for information, other requests or if you find any errors within the database. It is the intention that future versions of this database will include fault slip rates that have been determined from direct geological methods (e.g. offset stratigraphy that has been dated) rather than the systems based approach that is currently used.   References Kolawole, F., Firkins, M. C., Al Wahaibi, T. S., Atekwana, E. A., & Soreghan, M. J. (2021a). Rift Interaction Zones and the Stages of Rift Linkage in Active Segmented Continental Rift Systems. Basin Research. https://doi.org/10.1111/bre.12592 Leonard, M. (2010). Earthquake fault scaling: Self-consistent relating of rupture length, width, average displacement, and moment release. Bulletin of the Seismological Society of America, 100(5A), 1971-1988. https://doi.org/10.1785/0120090189 Wedmore, L. N. J., Biggs, J., Floyd, M., Fagereng, Å., Mdala, H., Chindandali, P. R. N., et al. (2021). Geodetic constraints on cratonic microplates and broad strain during rifting of thick Southern Africa lithosphere. Geophysical Research Letters. 48(17), e2021GL093785. https://doi.org/10.1029/2021GL093785 Williams, J. N., Mdala, H., Fagereng, Å., Wedmore, L. N. J., Biggs, J., Dulany, Z., et al. (2021). A systems-based approach to parameterise seismic hazard in regions with little historical or instrumental seismicity: Active fault and seismogenic source databases for southern Malawi. Solid Earth, 12(1), 187–217. https://doi.org/10.5194/se-12-187-2021 V1.1 Updates Updated seismic source files and model parameters. Changes are: Adding lower and upper dip estimates for sources (following a reviewer comment). This should be equivalent to Table 1 in the revised manuscript. Cleaning up the GIS files. In the old file there were some duplicate GIS features that are now removed Changing the name and acronyms from Malawi Seismogenic Source Database (MSSD) to Malawi Seismogenic Sources Model (MSSM).   Included a basic Matlab script to plot the MSSM geometrical polygon

LukeWedmore/malawi_seismogenic_source_model: Malawi Seismogenic Source Model v1.2

The Malawi Seismogenic Source Model (MSSM) is a geospatial database that documents the geometry, slip rate and seismogenic properties (ie earthquake magnitude and frequency) of active faults in Malawi. Each geospatial feature represents a potential earthquake rupture of 'source' and is classified based on its geometry into one of three types: sectionfaultmulti-fault Source types are mutually exclusice, and so if incorporated into a PSHA, they should be assigned relative weightings. The MSSM is the first seismogenic source database in central and northern Malawi, and represents an update of the South Malawi Seismogenic Source Database (SMSSD; Williams et al., 2021a) because it incorporates new active fault traces (Kolawole et al., 2021; Williams et al., 2021b; 2022 - MAFD), new geodetic data (Wedmore et al., 2021) and a statistical treatment of uncertainty, within a logic tree approach. The seismogenic sources in this model are adapted from the faults in the Malawi Active Fault Database (Williams et al., 2021b; 2022). Prior to publication please cite this database using the following two references: Williams, J. N., Wedmore, L. N .J., Fagereng, Å., Werner, M. J., Biggs, J., Mdala, H., Kolawole, F., Shillington, D. J., Dulanya, Z., Mphepo, F., Chindandali, P., Wright, L. J. M.., Scholz, C. A. Geological and geodetic constraints on the seismic hazard of Malawi's active faults: the Malawi Seismogenic Source Model (MSSM). Manuscript submitted to Natural Hazards and Earth System Sciences Williams, Jack N., Wedmore, Luke N. J., Fagereng, Åke, Werner, Maximilian J., Biggs, Juliet, Mdala, Hassan, Kolawole, Folarin, Shillington, Donna J., Dulanya, Zuze, Mphepo, Felix, Chindandali, Patrick R. N., Wright, Lachlan J. M., & Scholz, Christopher A. (2021). Malawi Seismogenic Source Model [Data set]. Zenodo. https://doi.org/10.5281/zenodo.5599616 Database Design and File Formats The MSSM is a geospatial database that consists of two separate components: A 3D geometrical model of fault seismogenic sources in MalawiThe mapped trace of each source in a GIS vector format, with associated source attributes (Data Table). Each fault is associated with a source in the 3D geometrical model that is listed in a comma-separated-values (csv) file. The sections, faults and multi-faults that make up the individual seismogenic sources are described in separate geospatial files that describe the map-view geometry and metadata that control each sources earthquake magnitude and frequency for seismic hazard purposes. The sections, faults and multi-faults in this database are provided in a variety of GIS vector file formats. GeoJSON is the version of record, and any changes should be made in this version before they are converted to other file formats using the script in the repository that uses the GDAL tool ogr2ogr (the script is adapted from https://github.com/cossatot/central_am_carib_faults/blob/master/convert.sh - we thank Richard Styron for making this publicly available). The other versions available are ESRI ShapeFile, KML, GMT, and GeoPackage.   List and brief description of the fault geometry, slip rate estimates and earthquake source attributes in the GIS vector format files that make up the MSSM.AttribugeTypeDescriptionNotesMSSM_IDintegerUnique numerical reference ID for each seismic sourceID 00-300 is section rupture ID 300-500 is fault rupture ID 600-700 is a multi-fault rupturenamestring Assigned based on previous mapping or local geographic feature. For sections and faults, the name of the fault (flt_name) and larger multi-fault (mflt_name) system they are hosted on are given respectively.basinstringBasin that source is located within.Used in slip rate calculationsclassstringintrarift or border fault length (Ls)real numberstraight-line distance in km between fault tips; sum of Lsec for segmented faults; sum of Lfault for multi-faultsmeasured in km to 1 decimal place. Must be greater than 5 km (except for linking sections).areaintegerCalculated from Ls multiplied by Eq. 1 or based on fault truncation.measured in km2strikeintegerAzimuth of straigth line between the fault tips. azimuth is <180° Used as input for slip rate estimates in Eq. 2 dip_lowerintegerlower range of dip valueWhen no previous measurements of dip are available, a nominal value of 45° is used.dip_intintegerIntermediate dip valueIn the MSSM geometrical model, only the intermediate measurements is considered. When no previous measurements of are available, a nominal value of 53° is assigned. No dip is assigned for multi-fault sources, as different participating faults may have different dips.dip_upperintegerUpper range of dip valueWhen no previous measurements of dip are availabe, a nominal value of 65° is used.dip_dirstringDip direction: compass quadrant that the fault dips in. slip_typestringSource kinematics (e.g. normal, thrust etc).All sources in the MSSM are assumed to be normal faults.slip_ratereal numberMean value from repeating Eq. 2 in Monte Carlo simulations (see manuscript for details).In mm yr-1. All sources in the MSSM are assumed to be normal so is equivalent to dip-slip rate. Reported to two significant figures.s_rate_errreal numberSlip rate error: 1σ error from Monte Carlo slip rate simlations. mag_lowerreal numberLower magnitude estimate. Calculated from Leonard (2010) scaling relationship (Eq. 4) for Ls or As, and using lower estimates of C1 and C2 constants in Leonard (2010).Reported to one decimal place.mag_medreal numberMean magnitude estimate. Calculated from Leonard (2010) scaling relationship (Eq. 4) for Ls or As, and using mean estimates of C1 and C2 constants in Leonard (2010).Reported to one decimal place.mag_upperreal numberUpper magnitude estimate. Calculated from Leonard (2010) scaling relationship (Eq. 4) for Ls or As, and using upper estimates of C1 and C2 constants in Leonard (2010).Reported to one decimal place.ri_lowerreal numberLower recurrence interval estimate. Calculated as 1σ below the mean of the Monte Carlo simulations (assuming a log normal distribution).Reported to two significant figures.ri_medreal numberMean recurrence interval. Mean value from log of recurrence interval Monte Carlo simulations.Reported to two significant figures.ri_upperreal numberUpper recurrence interval estimate. Calculated as 1σ above the mean of the Monte Carlo simulations (assuming a log normal distribution).Reported to two significant figures.MAFD_idlistList of integers of ID of equivalent structures in the Malawi Active Fault DatabaseMulti-fault sources have multiple ID's.   Version Control This version is intended to be "Live" and as such we encourage edits of the GeoJSON file and the submission of pull requests. Please contact Jack Williams [email protected] Luke Wedmore [email protected] or Hassan Mdala [email protected] for information, other requests or if you find any errors within the database. It is the intention that future versions of this database will include fault slip rates that have been determined from direct geological methods (e.g. offset stratigraphy that has been dated) rather than the systems based approach that is currently used.   References Kolawole, F., Firkins, M. C., Al Wahaibi, T. S., Atekwana, E. A., & Soreghan, M. J. (2021a). Rift Interaction Zones and the Stages of Rift Linkage in Active Segmented Continental Rift Systems. Basin Research. https://doi.org/10.1111/bre.12592 Leonard, M. (2010). Earthquake fault scaling: Self-consistent relating of rupture length, width, average displacement, and moment release. Bulletin of the Seismological Society of America, 100(5A), 1971-1988. https://doi.org/10.1785/0120090189 Wedmore, L. N. J., Biggs, J., Floyd, M., Fagereng, Å., Mdala, H., Chindandali, P. R. N., et al. (2021). Geodetic constraints on cratonic microplates and broad strain during rifting of thick Southern Africa lithosphere. Geophysical Research Letters. 48(17), e2021GL093785. https://doi.org/10.1029/2021GL093785 Williams, J. N., Mdala, H., Fagereng, Å., Wedmore, L. N. J., Biggs, J., Dulany, Z., et al. (2021). A systems-based approach to parameterise seismic hazard in regions with little historical or instrumental seismicity: Active fault and seismogenic source databases for southern Malawi. Solid Earth, 12(1), 187–217. https://doi.org/10.5194/se-12-187-2021 V1.1 Updates Updated seismic source files and model parameters. Changes are: Adding lower and upper dip estimates for sources (following a reviewer comment). This should be equivalent to Table 1 in the revised manuscript. Cleaning up the GIS files. In the old file there were some duplicate GIS features that are now removed Changing the name and acronyms from Malawi Seismogenic Source Database (MSSD) to Malawi Seismogenic Sources Model (MSSM).   Included a basic Matlab script to plot the MSSM geometrical polygons V1.2 Updates Updated fault source geometry .csv file due to compiling error