The area is thus a “natural laboratory” of coseismic ground deformation-one where structures observed along the rupture can be attributed to known amounts and directions of displacement. Geodetic data and geological field surveys processed after the earthquake provide precise measures of the coseismic displacement vector at many points along this trace (e.g., Hamling et al., 2017 Kearse et al., 2018 Zinke et al., 2019 Howell et al., 2020). Dextral slip on this part of the fault varied along strike between 6 and 12 m and was accompanied by a small heave (typically <1 m) between slight transpression and slight transtension ( Kearse et al., 2018). Rupture on this fault propagated northeastward (e.g., Cesca et al., 2017 Holden et al., 2017) across a near-coastal landscape of rolling hills, alluvial terraces, and agricultural fields, much of it grass covered. Of these, the Kekerengu fault, a chiefly dextral strike-slip structure, experienced the largest coseismic surface displacement (as much as ~12 m Kearse et al., 2018). This earthquake ruptured a diverse assemblage of faults in the northeastern part of the South Island along an ~180 km length of the transpressional Pacific-Australia plate boundary ( Litchfield et al., 2018). In this paper, we focus on the rupture of the 14 November 2016 M w 7.8 Kaikōura earthquake in New Zealand. Understanding processes by which the ground progressively deforms to accommodate large coseismic displacements would facilitate accurate mapping and documentation of coseismic slip in the landscape and identification of ancient earthquakes in paleoseismic trenches. While it is well known that mole tracks initiate from localized compression of the ground in contractional stepovers between overlapping strike-slip fractures (e.g., Bergerat et al., 2003 Lin et al., 2004), little or no work has been done to evaluate: (1) how mole tracks and their bounding structures may evolve as a function of increasing fault displacement or (2) what morphology or structures characterize rupture zones that have accrued an especially large (e.g., 6–10 m) strike slip.
We use it here to refer to uplifted mounds of broken and fractured ground that form in a repeating pattern along strike-slip earthquake ruptures. Turf rafts on slightly transtensional segments of the fault were also bulged and shortened-relationships that can be explained by a kinematic model involving “deformable slats.” In a paleoseismic trench cut perpendicular the fault, one would observe fissures, low-angle thrusts, and steeply dipping strike-slip faults-some cross-cutting one another-yet all may have formed during a single earthquake featuring a large strike-slip displacement.Īlthough its meaning seems imprecise and varies between workers, the term “mole track” has been in common use since at least the 1970s. Driven by distortional rotation, this contraction of the rafts exceeds the magnitude of fault heave. On strike-slip parts of the fault, internal shortening of the rafts averaged 1–2 m parallel to the R faults and ~1 m perpendicular to the main fault trace. Along the fault, clockwise rotation of these turf rafts within the rupture zone averaged ~20°–30°, accommodating a finite shear strain of 1.0–1.5 and a distributed strike slip of ~3–4 m.
Eventually these blocks were dispersed into strongly sheared earth and variably rotated.
As slip accrued, turf rafts fragmented into blocks bounded by short secondary fractures striking at a high angle to the main fault trace that we interpret to have originated as antithetic Riedel (R′) faults. The bulges are flanked by low-angle contractional faults that emplace the shortened mass of detached sediment outward over less-deformed ground. As slip accumulated, near-surface “rafts” of cohesive clay-rich sediment, bounded by R faults and capped by grassy turf, rotated about a vertical axis and were internally shortened, thus amplifying the bulges. During the earthquake, fissured pressure bulges (“mole tracks”) initiated at stepovers between synthetic Riedel (R) faults. Combining post-earthquake field observations with analysis of high-resolution aerial photography and topographic models, we describe the structural geology and geomorphology of the rupture zone. To evaluate ground deformation resulting from large (~10 m) coseismic strike-slip displacements, we focus on deformation of the Kekerengu fault during the November 2016 M w 7.8 Kaikōura earthquake in New Zealand.