Evidence from high resolution topography for multiple earthquakes on high slip-to-length fault scarps: the Bilila-Mtakataka fault, Malawi

M. Hodge, J. Biggs, Å. Fagereng, H. Mdala, L. N. J. Wedmore, J. N. Williams

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Abstract

Geomorphological features such as fault scarps and stream knickpoints are indicators of recent fault activity. Determining whether these features formed during a single earthquake or over multiple earthquakes cycles has important implications for the interpretation of the size and frequency of past events. Here, we focus on the Bilila‐Mtakataka fault, Malawi, where the 20 m high fault scarps exceed the height expected from a single earthquake rupture. We use a high resolution digital elevation model (< 1 m) to identify complexity in the fault scarp and knickpoints in river profiles. Of 39 selected scarp profiles, 20 showed evidence of either multi‐scarps or composite scarps and of the seven selected river and stream profiles, five showed evidence for multiple knickpoints. A near uniform distribution of vertical offsets on the sub‐scarps suggests they were formed by separate earthquakes. These independent methods agree that at least two earthquakes have occurred with an average vertical offset per event of 10 and 12 m. This contrasts earlier studies which proposed that this scarp formed during a single event, and demonstrates the importance of high‐resolution topographic data for understanding tectonic geomorphology. We use a one‐dimensional diffusion model of scarp degradation to demonstrate how fault splays form multi‐scarps and estimate the diffusion age κt of the Bilila‐Mtakataka fault scarp to be 48 ± m25 m2, corresponding to 6400 ± 4000 years since formation. We calculate that a continuous rupture would equate to a MW 7.8 ± 0.3 earthquake, greater than the largest seismic event previously recorded in East Africa.
Original languageEnglish
Number of pages24
JournalTectonics
Volume39
Issue number2
Early online date18 Dec 2019
DOIs
Publication statusPublished - 1 Feb 2020

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