Geologic and geodetic constraints on the seismic hazard of Malawi’s active faults: The Malawi Seismogenic Source Database (MSSD)

Abstract

Abstract. 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 Database (MSSD), which describes the seismogenic properties of faults that have formed during East African rifting in Malawi. We first use empirical observations to geometrically classify active faults into section, fault, and multi-fault 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 is estimated to be 75 ka based on dated core. 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 regional strain distribution 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 of outcomes from a logic tree used in these calculations. We find that for sources in the Lake Malawi’s North Basin, where slip rates can be derived from both the geodetic data and the offset seismic reflector, the slip rate estimates are within error of each other, although those from the offset reflector are higher. Sources in the MSSD are 5–200 km long, which implies that large magnitude (MW 7–8) earthquakes may occur in Malawi. Low slip rates (0.05–2 mm/yr), however, mean that the frequency of such events will be low (recurrence intervals ~103–104 years). The MSSD represents an important resource for investigating Malawi’s increasing seismic risks and provides a framework for incorporating active fault data into seismic hazard assessment in other tectonically active regions.

Fault-based Probabilistic Seismic Hazard Analysis in Regions with Low Strain Rates and a Thick Seismogenic Layer: A Case Study from Malawi

“…In central and northern Malawi, the EAR has mostly been flooded by Lake Malawi, whilst in southern Malawi the rift is onshore and at its southern end has intersected and reactivated Karoo (i.e., Triassic-Jurassic) age faults in the Lower Shire Basin (Dulanya, 2017;Williams, Mdala, et al, 2021;Kolawole et al, 2021). Only negligible amounts of melt have been detected in Malawi's crust (Njinju et al, 2019;Accardo et al, 2020;Hopper et al, 2020), and so rift extension is primarily accommodated by normal fault earthquakes (Biggs et al, 2010;Hodge et al, 2015;Williams, Wedmore, Fagereng, et al, 2021a).…”

“…reflector in seismic reflection data in Lake Malawi (Shillington et al, 2020). These new geologic and geodetic data were combined into the Malawi Seismogenic Source Database (MSSD), a database that provides slip rate, earthquake magnitudes and recurrence interval estimates of faults included in the MAFD (Williams, Wedmore, Fagereng, et al, 2021a.…”

“…where c 1 is an empirically derived parameter and equals 17.5 m 1 3 for interplate dip-slip faults (Leonard, 2010), z is the seismogenic layer thickness in Malawi, set to an intermediate estimate of 35 km (Ebinger et al, 2019;Stevens et al, 2021), and L s and δ s are source length and dip. If two sources intersect downdip, the shorter source is assumed to be truncated by the longer one, and its W s accordingly revised (C. H. Scholz & Contreras, 1998;Williams, Wedmore, Fagereng, et al, 2021a).…”

“…Source slip rates (S s ) are estimated using a systems-based approach that incorporates geodetically-derived regional extension rates (Wedmore et al, 2021;Williams, Mdala, et al, 2021;Williams, Wedmore, Fagereng, et al, 2021a), except for some sources in Lake Malawi, where slip rates can be derived from offsets across a 75 Ka reflector in seismic reflection surveys (Shillington et al, 2020). Slip rate estimates are then combined with Ds to derive an earthquake recurrence interval (R s ) estimate for each source through the relationship R s = Ds /S s (Wallace, 1970;Stirling et al, 2012).…”

“…Malawi is located within a 900 km-long amagmatic section of the Western Branch of the East African Rift (EAR), and provides an ideal case study to investigate PSHA in a low strain rate region with a thick seismogenic layer as: (1) geodeticallyderived regional extension rates are low (0.5-2 mm/yr; Stamps et al, 2021;Wedmore et al, 2021), but nevertheless an order of magnitude higher than inferred from the last 50 years of instrumentally-recorded seismicity (Hodge et al, 2015;Ebinger et al, 2019), and (2) the seismogenic layer is ∼30-40 km thick (Jackson & Blenkinsop, 1993;Nyblade & Langston, 1995;Ebinger et al, 2019;Stevens et al, 2021;Craig & Jackson, 2021). Furthermore, constraints on the seismogenic potential of Malawi's active faults have improved with the collection of new geologic and geodetic data (Hodge et al, 2019;Shillington et al, 2020;Wedmore et al, 2021;Williams, Wedmore, Scholz, et al, 2021a;Kolawole et al, 2021), which have been synthesised for seismic hazard analysis in the Malawi Seismogenic Source Database (MSSD; Williams, Wedmore, Fagereng, et al, 2021a) .…”