A systems-based approach to parameterise seismic hazard in regions with little historical or instrumental seismicity: The South Malawi Active Fault Database

Abstract

Abstract. Seismic hazard is frequently characterised using instrumental seismic records. However, in regions where the instrumental record is short relative to earthquake repeat times, extrapolating it 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 settings and carries large inherent uncertainties. Here, we outline how fault slip rates and recurrence intervals can be estimated through an approach that combines fault geometry, earthquake-scaling relationships, geodetically derived regional strain rates, and geological constraints of regional strain distribution. We then apply this approach to the southern Malawi Rift where, although no on-fault slip rate measurements exist, there are theoretical and observational constraints on how strain is distributed between border and intrabasinal faults. This has led to the development of the South Malawi Active Fault Database (SMAFD), the first database of its kind in the East African Rift System (EARS) and designed so that the outputs can be easily incorporated into Probabilistic Seismic Hazard Analysis. We estimate earthquake magnitudes of MW 5.4–7.2 for individual fault sections in the SMAFD, and MW 6.0–7.8 for whole fault ruptures. These potentially high magnitudes for continental normal faults reflect southern Malawi's 11–140 km long faults and thick (30–35 km) seismogenic crust. However, low 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 intrabasinal faults. Sensitivity analysis indicates that the large range of these estimates can be reduced most significantly from an improved understanding of the rate and partitioning of rift-extension in southern Malawi, earthquake scaling relationships, and earthquake rupture scenarios. Hence these are critical areas for future research. The SMAFD provides a framework for using geological and geodetic information to characterize seismic hazard in low strain rate settings with few on-fault slip rate measurements, and could be adapted for use elsewhere in the EARS or globally.

Determining Histories of Slip on Normal Faults With Bedrock Scarps Using Cosmogenic Nuclide Exposure Data

“…Fault activity constrained over multiple earthquake cycles and across fault systems helps to address fundamental questions of how faults interact (Cowie et al., 2017; Mueller, 2017; Nixon et al., 2016; Wedmore et al., 2017), how tectonic strain accumulates and is released on brittle faults (Ferry et al., 2011; Hergert & Heibach, 2010), and how fault slip varies in time and space (Dolan et al., 2016; Nicol et al., 2010). Fault slip rates can be measured or inferred using a variety of tools, including geodesy (Bendick et al., 2000; Hussain et al., 2016; Walters et al., 2013; Williams et al., 2020), palaeoseismology and historical records (Cinti et al., 2019; Galli et al., 2008; Pantosti et al., 1996), and dating of offset geological, geomorphological, and man‐made features (R. D. Gold et al., 2017; Gregory et al., 2014; Mechernich et al., 2018; Phillips et al., 2004; Wang et al., 2011; Zechar & Frankel, 2009). Each method has different spatial and temporal coverage and resolution, and as a whole provide insight into tectonic processes occurring over a range of scales.…”