A semi-automated algorithm to quantify scarp morphology (SPARTA): application to normal faults in southern Malawi

Abstract

Abstract. 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.

Structural inheritance and border fault reactivation during active early-stage rifting along the Thyolo fault, Malawi

“…In other studies, the Thyolo fault is considered inactive (Laõ-Dávila et al, 2015;Prater et al, 2016). Hodge et al (2019) combined remote sensing observations with field observations to show that the fault is active, by documenting a pseudo-continuous fault scarp and triangular facets at the southern end of the fault. However, the fault was divided into two separate faults, the Thyolo and Muona faults, based on the separation distance between their scarps (Hodge et al, 2019).…”

“…Hodge et al (2019) combined remote sensing observations with field observations to show that the fault is active, by documenting a pseudo-continuous fault scarp and triangular facets at the southern end of the fault. However, the fault was divided into two separate faults, the Thyolo and Muona faults, based on the separation distance between their scarps (Hodge et al, 2019). A Mw5.6 earthquake in March 2018 had a normal faulting focal mechanism with nodal planes aligned with the surface traces of faults in the Lower Shire Graben (Figure 1; Ekström et al, 2012).…”

“…We used scarp height as a proxy for displacement (e.g. Morewood and Roberts, 2000;Hodge et al, 2018bHodge et al, , 2019Wedmore et al, 2020) and identified segments based on local minima in the along-strike scarp height profile. We used adapted versions of the SPARTA scarp measuring tools (Hodge et al, 2019) to measure the height of the fault scarp along the Thyolo fault on the 12 m DEM.…”

“…Furthermore, thin syn-rift sediments mean that fault exposures formed during the current rift phase are not hidden by post-rift sediments (e.g. Hodge et al, 2019;Williams et al, 2019). We begin by describing the tectonic history of the region, before analysing the current activity, geometry and structure of the fault.…”

How Do Variably Striking Faults Reactivate During Rifting? Insights From Southern Malawi

“…In this study, we analyze oblique faulting by assuming a priori different stress states, and then interpreting their applicability in terms of fault reactivation potential. We use the southern end of the Malawi Rift ( Figure 1) as a case example, as geological maps (Bloomfield & Garson, 1965;Habgood et al, 1973;Walshaw, 1965), fault scarps (Hodge et al, 2018(Hodge et al, , 2019Jackson & Blenkinsop, 1997;Wedmore et al, 2019), and earthquake focal mechanisms (Delvaux & Barth, 2010) demonstrate that active faults switch from NW-SE striking in the Makanjira Graben to NNE-SSW in the Zomba Graben and then back to NW-SE in the Lower Shire Graben as the rift follows an arcuate bend in the high grade metamorphic foliation (Figure 2a). Furthermore, there is an inconsistency in the regional σ 3 trend when inferred from fault slickensides (Chorowicz & Sorlien, 1992;Delvaux et al, 2012), fault geometry (Mortimer et al, 2007), earthquake focal mechanisms (Delvaux & Barth, 2010), and geodesy (Stamps et al, 2018).…”