Substantial Upper Plate Faulting Above a Shallow Subduction Megathrust Earthquake: Mechanics and Implications of the Surface Faulting During the 2016 Kaikoura, New Zealand, Earthquake

Abstract

The 2016 moment magnitude 7.8 Kaikoura, New Zealand, earthquake occurred at the southern end of the Hikurangi subduction zone where the upper plate above the shallow megathrust is exposed sub‐aerially. As a result, the substantial co‐seismic deformation in the upper plate above the megathrust rupture was observed geologically and geodetically. We explore the relationship between this surface faulting and the subduction megathrust rupture and find that the greatest upper plate fault slip occurred coincident (in time and location) with the megathrust rupture. Models of Coulomb stress change demonstrate that these surface faults become positively loaded as the upper plate rebounds during the megathrust event, favoring fault slip. In addition, during the megathrust rupture these faults terminate against an uncoupled subduction plate interface. We simulate the effects of decoupling at the base of these faults and find that very large fault slip is an expected consequence of this decoupling, allowing near‐complete strain release. In contrast, typical strike‐slip faults, pinned at their base, would have lower amounts of fault slip. These two conditions—increased Coulomb stress and basal decoupling—combine to produce the extreme co‐seismic upper plate faulting observed above the shallow Kaikoura megathrust earthquake. Similar conditions occur in other global subduction zones, but in most subduction zones the region above the coupled megathrust is underwater and poorly observed. Our analysis of the Kaikoura earthquake indicates a need to reevaluate patterns of strain accumulation and release in these regions, rather than assuming simple models of elastic rebound.

Next Generation Seismic Source Detection by Computer Vision: Untangling the Complexity of the 2016 Kaikōura Earthquake Sequence

“…One possibility is that reverse slip on the offshore thrust unclamped the inland crustal faults, allowing both pathways to rupture at the same time or in very quick succession. This simultaneous rupture scenario was first proposed by Herman et al (2023) on the basis of Coulomb stress change calculations and finite element models of fault slip, but with a linking role for the underlying Hikurangi subduction interface. However, this seems unlikely because the "slab2" geometry (Hayes, 2018) used in their study is ∼10 km too shallow and misaligned with regional intraslab seismicity (Aziz Zanjani et al, 2021).…”

Strain Partitioning and Fault Kinematics in the Northern Qilian Shan (NE Tibet) Determined From Bayesian Inference of Geodetic Data

“…Although the kinematic model and seismicity does not require any creep on shallow thrust faults during the interseismic period, there is geological and geomorphological evidence that the thrust faults separating the Qilian Shan and Alashan are active and have accommodated tens of kilometers of crustal shortening in the Cenozoic (Gaudemer et al, 1995;Meyer et al, 1998;Zuza et al, 2016). In essence, the short-term geodetic measurements do not reflect the kinematics of the locked parts of the thrusts in the northern Qilian Shan over geological timescales (see also Herman et al, 2023;Lamb, 2021;Lamb & Smith, 2013). Allen et al (2017) previously investigated the kinematics of strain partitioning in the northern Qilian Shan ∼150 km to north-west of our data profile.…”

“…Where convergence between a mountain range and its foreland is oblique the long-term deformation is often accommodated by a combination of thrust faulting along the range margins and strike-slip faulting within its interior-a kinematic configuration known as strain partitioning (Daout et al, 2016;McCaffrey, 1988;Murphy et al, 2014;Sanderson & Marchini, 1984;Schütt & Whipp, 2020). Faults accommodating strain partitioning can slip in individual earthquakes releasing strain piecemeal, or in multi-fault earthquakes that rupture thrust and strike-slip faults simultaneously releasing the strain in one large event (e.g., 2016 Mw 7.8 Kaikoura earthquake; Herman et al, 2023;Shi et al, 2017).…”

Integrating onshore and offshore paleoseismic evidence to reveal past large earthquakes on the weakly coupled central Hikurangi margin, New Zealand

“…OFFSHORE 5.5. DISCUSSION interface can help to propagate rupture across crustal features (Herman et al, 2023;Wallace et al, 2017a;Wang et al, 2018b). Once again, this correlation highlights that even when onshore deformation is consistent with upper plate faulting, isolated or widespread seismogenic slip on the plate interface may help to facilitate complex multi-fault ruptures.…”