Influence of topography and basement depth on surface rupture morphology revealed from LiDAR and field mapping, Hope Fault, New Zealand

Highlights

• A detailed structural geomorphic map was produced for the Hope Fault using LiDAR.

• The surface rupture pattern is complex despite optimal orientation for strike-slip.

• The fault deformation zone varies in width from a few metres to up to 500 m.

• Topography and thickness of cover sediments influence the fault deformation zone.

• Fault proximity to major river canyons increases the fault deformation zone width.

Abstract

High-resolution airborne LiDAR and field mapping were used to investigate a 29 km-long section of the Hurunui segment of the Hope Fault concealed beneath beech forest. Approximately 20 km of the dextral strike-slip principal slip zone (PSZ) was identified as a series of 69 individual fault strands on the LiDAR DEM. Mapping revealed 70 normal, 55 dextral-reverse, and 100 secondary faults, many of which were previously unrecognized. Secondary faults are kinematically linked with the PSZ and comprise a complex surface fault deformation zone (FDZ). A Rose diagram weighted by the lengths of the PSZ strands shows that the Hurunui segment strikes between 070° and 075° and is optimally oriented for dextral strike-slip within the regional stress field. The observed fault zone complexity is thus unlikely to result from large-scale fault mis-orientation with respect to regional stresses. FDZ width measurements from 415 locations reveal a spatially-variable, active FDZ up to ~ 500 m wide with an average width of 200 m. FDZ width increases with increased hanging wall topography and increased topographic relief (e.g., adjacent to high topography with deeply incised streams), suggesting that along-strike topographic perturbations to fault geometry and stress states increase fault zone complexity and width. Where adjacent PSZ strands strike between 070° and 075°, the FDZ is ≤ 150 m wide, however, FDZ width increases where the tips of adjacent PSZ segments locally vary in strike by ≥ 10°. FDZ width and surface fracture density also appear to increase with increasing thickness of alluvial deposits overlying bedrock. Our results indicate that spatial variations in near-fault topography and geology can generate along-strike variability in the morphology of surface ruptures, even in the case of fast-slipping, structurally mature faults where more confined, simplistic ruptures are expected at seismogenic depths.

Introduction

Fault deformation zones (FDZs) typically consist of narrow (< 2–5 m) principal slip zone(s) (PSZ) along which maximum fault slip occurs and a wide (> 10² m) zone of smaller faults, fractures and/or distributed folding (e.g., Schulz and Evans, 2000, Shipton and Cowie, 2001). Field data (e.g., Joussineau and Aydin, 2009, Kim et al., 2004, Martel et al., 1988, Rockwell and Ben-Zion, 2007, Sagy et al., 2007, Stirling et al., 1996, Tchalenko and Ambraseys, 1970), numerical models (e.g., Aydin and Schultz, 1990, Richard et al., 1991), and analogue experiments (e.g., Richard, 1991, Richard et al., 1995, Riedel, 1929, Tchalenko, 1970) predict that the FDZ should narrow in width and evolve from structurally complex to simple through-going fracture patterns as strain localizes with progressive slip, although alternative models for fault zone widening with cumulative slip have been proposed (e.g., Ben-Zion and Andrews, 1998). In general, the majority of studies suggest that structurally mature active faults with large accommodated strain, fast slip rates and more frequent surface ruptures should have relatively narrow and simple rupture morphologies compared to less evolved, more segmented faults with slower slip rates. Although discrete, structurally simple rupture zones of < 30 m width are common along some segments of major fast-slipping active faults (e.g., Lin et al., 2012, Sieh and Jahns, 1984, Zhou et al., 2010), rupture zones along many active faults, including evolved and fast-slipping faults in plate boundary zones, commonly exceed 10²–10³ m and contain complex surface rupture morphologies (Table 1). Progressive rotation and/or structural overprinting of faults misaligned with regional stresses (Scholz et al., 2010), fault zone segmentation and termination (e.g., Elliott et al., 2009, Kim et al., 2004, Oglesby, 2005, Wesnousky, 1988), and variations in the thickness and material properties of the faulted media (e.g., Barth et al., 2012, Norris and Cooper, 1997, Oskin et al., 2012, Richard et al., 1991, Richard et al., 1995, Shipton and Cowie, 2003) all offer explanations for the width and complexity observed in surface rupture morphology. Shallow (< 1–4 km depth) stress perturbations resulting from topographic loading from mountain ranges and unloading associated with valley systems (Barth et al., 2012, Eusden et al., 2000, Eusden et al., 2005, Norris and Cooper, 1995) may also influence surface rupture morphology. Understanding the morphology and causative mechanisms influencing surface ruptures on active faults is important for assessing future coseismic displacements and related fault rupture hazard to infrastructure adjacent to known active faults (e.g., Honegger et al., 2004, Van Dissen et al., 2013) including design of fault set-back distances (e.g., Villamor et al., 2012).

Active faults have traditionally been mapped using field techniques and aerial photography, however faults in areas of dense vegetation and/or subtle features such as small, secondary faults comprising fault zones were typically challenging to detect (Chan et al., 2007). Recently, airborne LiDAR (light detection and ranging) data has improved the detection of faults in densely vegetated areas and the resolution with which faults can be mapped (Arrowsmith and Zielke, 2009, Barth et al., 2012, Begg and Mouslopoulou, 2010, Chan et al., 2007, Duffy et al., 2013, Gold et al., 2013, Haddad et al., 2012, Langridge et al., 2013, Langridge et al., 2014, Nissen et al., 2012, Oskin et al., 2012, Quigley et al., 2012, Zachariasen and Prentice, 2008). In this paper we use LiDAR data to map the surface rupture patterns associated with major earthquakes on the Hurunui segment of the Hope Fault in New Zealand's South Island. We map and classify the structures within the FDZ, analyse their kinematics in the context of the regional stress field, and investigate how the FDZ width and geometry vary as a function of fault orientation, topography, and depth-to-bedrock. We discuss the applicability of our new fault zone maps for characterizing fault paleoseismicity. We provide explanations for why complex surface ruptures may form on structurally mature faults where kinematically simple slip confined to a discrete rupture may have been expected.