Evidence of Seismic Slip on a Large Splay Fault in the Hikurangi Subduction Zone

Abstract

The Hikurangi subduction zone is capable of producing moderate to large earthquakes as well as regularly repeating slow slip events. However, it is unclear what structures host these different slip styles along the margin. Here we address whether splay faults can host seismic slip at shallow (<1 km) depth by investigating the Pāpaku fault, sampled during International Ocean Discovery Program Expedition 375. We use biomarker thermal maturity to search for evidence of frictional heating within turbiditic sediments of the Pāpaku fault. Four zones of localized high temperature are found near the top of the fault zone, which are interpreted to be zones of localized seismic slip. Thermal modeling shows that the most likely maximum displacement on the shallow Pāpaku fault during each event was 14–17 m. Our results demonstrate that the Pāpaku fault, and potentially other splay faults along the margin, host coseismic slip and have the potential to produce large tsunami (e.g., runup heights of >1 m as observed in the 1947 Poverty and Tolaga Bay earthquakes.

Asymmetric Brittle Deformation at the Pāpaku Fault, Hikurangi Subduction Margin, NZ, IODP Expedition 375

“…Dynamic stresses during earthquake slip can impart stress asymmetry around a seismically active fault and give rise to persistent differences in damage zone thickness and decay. The Pāpaku fault shows evidence that earthquake slip has occurred at Site U1518 (Coffey et al., 2021). A propagating rupture can produce asymmetric damage, and depending on the fault geometry relative to the stress orientation, that damage can occur on either the compressive or extensive quadrant (Andrews & Ben‐Zion, 1997; DeDontney et al., 2011; Duan, 2008; Ma & Beroza, 2008; Templeton & Rice, 2008; Viesca et al., 2008).…”

“…Although SSEs within the area are usually modeled as occurring on the megathrust, some component of the slow slip could branch upwards along the Pāpaku fault, consistent with recent observations on more landward accretionary prism faults (Shaddox & Schwartz, 2019). Furthermore, biomarker thermal maturity anomalies within the Pāpaku fault indicate high frictional temperatures only achievable during earthquake rupture have occurred along this part of the fault in the past (Coffey et al., 2021; Rabinowitz et al., 2017; Savage et al., 2018). Initial results from drilling showed a mix of brittle and ductile structural features within the Pāpaku fault zone, which has been interpreted as possible evidence for a range of slip speeds or effective stresses (Fagereng et al., 2019).…”

Dueling dynamics of low-angle normal fault rupture with splay faulting and off-fault damage

“…Coseismic splay fault activity has been documented and modeled most extensively in subduction zones, where splay fault slip and inelastic deformation in the frontal wedge are intensely debated mechanisms that may amplify coseismic seafloor displacements and resulting tsunami heights of megathrust earthquakes like Tohoku-Oki 4 – 8 . Vitrinite reflectance and thermal biomarker paleoseismology confirm that such splays slip in large earthquakes 9 , 10 . The governing factors proposed to control the coseismic slip tendency of megathrust splays include the tectonic stress regime in the upper plate, the frictional properties of splay faults as well as sediments and gouges in the subduction interface, seismically transmitted dynamic stresses from the deeper ruptured megathrust, and enhanced shallow coseismic fault weakening due to thermal pressurization of fluids.…”

How Fault Rocks Form and Evolve in the Shallow San Andreas Fault

“…A second, not mutually exclusive, possibility is that initial complexity in fracture patterns associated with early development of the Mojave Segment gave rise to a network of fault strands forming a large‐scale anastomosing fault zone, and that these strands have been simultaneously active since their creation. Finally, the broad network of discrete gouge zones may be related in part to the repeated surface rupturing earthquakes, as mapping of the distribution of seismic slip surfaces within fault zones shows that ruptures do not always occur on the exact same fault surfaces (e.g., Coffey et al., 2021; Rabinowitz et al., 2020; Rowe et al., 2018; Savage & Polissar, 2019). All of these interpretations are supported by other research demonstrating that the number of strands within a fault zone increases with increasing displacement (e.g., Rowe et al., 2013; Savage & Brodsky, 2011), although there does appear to be an upper bound beyond which fault zone growth in width/complexity tapers off with increasing displacement (McKay et al., 2021; Savage & Brodsky, 2011).…”