Curved slickenlines preserve direction of rupture propagation

Abstract

Slip-parallel grooves (striations) on fault surfaces are considered a robust indicator of fault slip direction, yet their potential for recording aspects of earthquake rupture dynamics has received little attention. During the 2016 Kaikōura earthquake (South Island, New Zealand), >10 m of dextral strike-slip on the steeply dipping Kekerengu fault exhumed >200 m2 of fresh fault exposure (free faces) where it crossed bedrock canyons. Inscribed upon these surfaces, we observed individual striae up to 6 m long, all of which had formed during the earthquake. These were typically curved. Using simulations of spontaneous dynamic rupture on a vertical strike-slip fault, we reproduce the curved morphology of striae on the Kekerengu fault. Assuming strike-slip pre-stress, our models demonstrate that vertical tractions induced by slip in the so-called cohesive zone result in transient changes in slip direction. We show that slip-path convexity is sensitive to the direction of rupture propagation. To match the convexity of striae formed in 2016 requires the rupture to have propagated in a northeast direction, a prediction that matches the known rupture direction of the Kaikōura earthquake. Our study highlights the potential for fault striae to record aspects of rupture dynamics, including the rupture direction of paleo strike-slip earthquakes.

On‐Fault Geological Fingerprint of Earthquake Rupture Direction

“…Curved slickenlines were observed (Kearse et al, 2019) at multiple sites on the Kekerengu fault (Figures 9g and 10g). These slickenlines described a curved slip path, whereby the displacement was initially Figure 9.…”

“…In post-earthquake field investigations, slickenlines are commonly inscribed onto extremely fragile fault gouge, exhibit sharp micro-topographic relief across their tracks (e.g., Figure 13a), and therefore can be confidently attributed to the most recent earthquake (Spudich et al, 1998;Kearse et al, 2018Kearse et al, , 2019. Using these criteria, overlapping slickenlines, each with different slip directions, documented in these post-earthquake scenarios have been interpreted as forming in the same slip episode involving coseismic changes in slip direction (Kearse et al, 2018(Kearse et al, , 2019Otsubo, Shigematsu, et al, 2013;Pan et al, 2014) (e.g., Figure 13b). Relict slip planes adjacent to principle slip surfaces have been found to host slickenlines that appear much older than the most recent earthquake (e.g., Figure 13c), suggesting that there is potential for preserving slickenlines from more than one earthquake event, within a fault zone at a single location.…”

“…Extracting information about earthquake rupture dynamics from geological records remains one of the grand challenges in earthquake science. Striations on fault surfaces (referred to as slickenlines ) are commonly documented in the field after surface‐breaking earthquakes (Avagyan et al, 2003; Kearse et al, 2019; Otsubo, Shigematsu, et al, 2013; Shimamoto, 1996; Slemmons, 1957; Spudich et al, 1998; Xu et al, 2013) and form when one side of a fault scratches against the other during earthquake displacement. As such, slickenlines record the history and direction of coseismic slip and represent an important geological archive of dynamic faulting during past earthquakes.…”

“…Combinations of these refer to strongly oblique earthquake mechanisms. Modified from Kearse et al (2019).…”

“…A different view of curved slickenlines (Spudich et al, 1998) based on the theory of earthquake mechanics suggests that temporal changes in slip direction may instead be driven by dynamic stress changes on the sliding fault surface. This interpretation has led to the hypothesis that slickenline curvature could preserve the rupture propagation direction of past earthquakes (Figure 1a) (Kearse et al, 2019). Since rupture direction is an important parameter for our understanding of earthquake dynamics (Andrews & Ben‐Zion, 1997; Harris & Day, 2005) and seismic hazard (Gerstenberger et al, 2020; Somerville et al, 1997), the ability to uncover the rupture direction of large historical earthquakes would help address related questions, including whether elastic contrasts across bi‐material faults (Andrews & Ben‐Zion, 1997) or locked‐to‐creeping transitions (Barbot et al, 2012) exert first‐order controls on the direction of rupture propagation.…”