Three‐Dimensional Surface Displacements During the 2016 M_W 7.8 Kaikōura Earthquake (New Zealand) From Photogrammetry‐Derived Point Clouds

Abstract

High-resolution, three-dimensional (3-D) measurements of surface displacements during earthquakes can provide constraints on fault geometry and near-surface slip and also quantify on-fault and off-fault deformation. However, measurements of surface displacements are often hampered by a lack of high-resolution preearthquake elevation data, such as lidar. For example, preearthquake lidar for the 2016 M W 7.8 Kaikōura, New Zealand, earthquake only covers ≲10% of~180 km of mapped surface ruptures.To overcome a lack of preearthquake lidar, we measure 3-D coseismic displacements during the Kaikōura earthquake using point clouds generated from aerial photographs. From these point clouds, which cover the whole area of the 2016 surface ruptures, it is possible to measure 3-D displacements to within ±0.2 m. We measure coseismic slip and estimate the geometries of faults in the steep, inaccessible Seaward Kaikōura mountains, where postearthquake field observations are very sparse. The Jordan Fault (previously the Jordan Thrust) ruptured in 2016 as a moderate-to-steeply dipping (~50-80°), predominantly strike-slip fault. Slip on this fault in 2016-which included a normal-sense component in some areas-contrasts with field observations that indicate significant longer-term shortening across the Jordan Fault during the Holocene. It is therefore likely that different earthquakes on the Jordan Fault have very different slip vectors and that the fault does not exhibit "characteristic" slip behavior. For faults like the Jordan Fault, long-term time-averaged estimates of slip rate may not be reliable indicators of the sense and magnitude of slip in individual earthquakes.

Illuminating the Pre‐, Co‐, and Post‐Seismic Phases of the 2016 M7.8 Kaikōura Earthquake With 10 Years of Seismicity

“…Several key observations provide further constraints on the most likely rupture route for the Kaikōura earthquake, principally the occurrence of a small, localized tsunami (Gusman et al., 2018), and the inverted motion of the Jordan Thrust, which hosted normal motion rather than the reverse motion, as would be expected from the geological record (Howell et al., 2020; Van Dissen & Yeats, 1991). We propose that these two factors, alongside our observation that offshore thrust faulting spans the gap between the Hundalee Fault and the Papatea Fault, require that the earthquake propagated via the offshore route (Figure 6).…”

3D Coseismic Surface Displacements From Historical Aerial Photographs of the 1987 Edgecumbe Earthquake, New Zealand

“…The 3D point cloud differencing (e.g., as in Howell et al., 2020) achieved poor results due to relatively flat topography, sparse and small surface features (e.g., buildings and ditches), and high degree of non‐tectonic surface change (e.g., farm/crop modification). Similar problems were encountered with the 2D image correlation, with additional problems caused by different shadows from varying sun angles.…”

“…We use legacy aerial photos of the Rangitāiki Plains in the North Island of New Zealand (Figure 1) and photogrammetry software to generate pre‐ and post‐earthquake digital surface models (DSMs) and orthophoto mosaics from the 1987 Edgecumbe earthquake. Most existing studies that use similar remote sensing techniques (i.e., historical airplane‐based photos) focus on larger magnitude ( M w 7+) earthquakes where slip is more easily resolved (e.g., Barnhart et al., 2019; Howell et al., 2020; Zhou et al., 2016). The M w 6.5 Edgecumbe earthquake, conversely, represents a moderate magnitude earthquake with <2 m average surface slip (Beanland et al., 1989), and therefore tests the limits of this technique with a final topographic product resolution that approaches the scale of surface displacement.…”

“…Previous studies using legacy aerial photographs to characterize coseismic deformation are limited and primarily focus on 2D lateral displacements (e.g., Ayoub et al., 2009; Milliner et al., 2015). Vertical differencing and 3D displacement measurements are typically reserved for M w 7+ earthquakes because the large surface displacements yield higher elevation signal‐to‐noise ratios given typical photo resolutions (e.g., Barnhart et al., 2019; Howell et al., 2020; Zhou et al., 2016). Post‐event aerial photos have been supplemented with inferred pre‐earthquake topography to reduce noise and measure displacements (Lajoie et al., 2020).…”

Supplemental Material: The influence of off-fault deformation zones on the near-fault distribution of coseismic landslides

“…There are a number of explanations for these differences including the different spatial resolution of the datasets, different signal to noise ratios, and/or differences in the locations and interpretation of the fault perpendicular profiles. Further information on the ICP and OIC datasets themselves is available in Howell et al (2020).…”