skip to main content
US FlagAn official website of the United States government
dot gov icon
Official websites use .gov
A .gov website belongs to an official government organization in the United States.
https lock icon
Secure .gov websites use HTTPS
A lock ( lock ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites.

Explore Research Products in the PAR
It may take a few hours for recently added research products to appear in PAR search results.


Title: Permeability and Elastic Properties of Rocks From the Northern Hikurangi Margin: Implications for Slow‐Slip Events
Abstract Fluid flow and pore‐pressure cycling are believed to control slow slip events (SSEs), such as those that frequently occur at the northern Hikurangi margin of New Zealand. To better understand fluid flow in the forearc system we examined the relationship between several physical properties of Cretaceous‐to‐Pliocene sedimentary rocks from the Raukumara peninsula. We found that the permeability of the deep wedge is too low to drain fluids, but fracturing increases permeability by orders of magnitude, making fracturing key for fluid flow. In weeks to months, plastic deformation, swelling, and possibly not‐yet‐identified mechanisms heal the fractures, restoring the initial permeability. We conclude that overpressures at the northern HM might partly dissipate during SSEs due to enhanced permeability near faults. However, in the months following an SSE, healing in the prism will lower permeability, forcing pore pressure to rise and a new SSE to occur.  more » « less
Award ID(s):
1949171
PAR ID:
10499405
Author(s) / Creator(s):
; ; ; ; ; ; ;
Publisher / Repository:
AGU - Geophysical Research Letters
Date Published:
Journal Name:
Geophysical Research Letters
Volume:
51
Issue:
2
ISSN:
0094-8276
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. ; ; ; (, AGU)
    NA (Ed.)
    The Hikurangi Margin (HM) is a subduction zone along the east coast of the North Island of New Zealand where varying instances of slow slip events (SSEs) and earthquakes occur. These SSEs occur at different time scales and depths when comparing the northern and southern ends of the margin. Previous studies show that the rock comprising the accretionary wedge of the northern margin have low permeabilities, which could induce overpressures and modulate the occurrence of SSEs. Permeability rises when an SSE fractures the rocks within the deep wedge promoting fluid flow and thus dissipating the overpressures along and above the décollement. As fractures heal and permeability recovers overpressures build up once again. Although this cycle may explain the occurrence of SSEs along northern Hikurangi, it is not yet clear how intrinsic permeability varies in rocks above the décollement elsewhere along the margin. To better understand the disparity in SSE occurrence, rock samples from the northern and central part of the margin have been tested for permeability and elastic properties. We tested samples from the Weber, Whangai, Dannevirke and Wanstead formations, which are representative of the lithologies above the décollement in the central margin, and range in age from the Cretaceous to the mid to late Paleogene. We found that the Weber (PQ) and Whangai (PO) formation samples from central HM have higher permeability than northern HM rocks from the same formation in the north. This study provides insight into the mechanisms that lead to significantly fewer SSEs along the central HM. In the near future, we plan to conduct a suite of physical experiments that will include permeability recovery after fracturing, compaction, and ultrasonic velocity analysis to help further understand the stark differences in slip behavior observed along the margin. 
    more » « less
  2. ; (, Journal of Geophysical Research: Solid Earth)
    Abstract Fluids influence fault zone strength and the occurrence of earthquakes, slow slip events, and aseismic slip. We introduce an earthquake sequence model with fault zone fluid transport, accounting for elastic, viscous, and plastic porosity evolution, with permeability having a power‐law dependence on porosity. Fluids, sourced at a constant rate below the seismogenic zone, ascend along the fault. While the modeling is done for a vertical strike‐slip fault with 2D antiplane shear deformation, the general behavior and processes are anticipated to apply also to subduction zones. The model produces large earthquakes in the seismogenic zone, whose recurrence interval is controlled in part by compaction‐driven pressurization and weakening. The model also produces a complex sequence of slow slip events (SSEs) beneath the seismogenic zone. The SSEs are initiated by compaction‐driven pressurization and weakening and stalled by dilatant suctions. Modeled SSE sequences include long‐term events lasting from a few months to years and very rapid short‐term events lasting for only a few days; slip is ∼1–10 cm. Despite ∼1–10 MPa pore pressure changes, porosity and permeability changes are small and hence fluid flux is relatively constant except in the immediate vicinity of slip fronts. This contrasts with alternative fault valving models that feature much larger changes in permeability from the evolution of pore connectivity. Our model demonstrates the important role that compaction and dilatancy have on fluid pressure and fault slip, with possible relevance to slow slip events in subduction zones and elsewhere. 
    more » « less
  3. ; ; ; ; (, AGU)
    The northern part of the Hikurangi margin (HM) regularly experiences shallow slow-slip events (SSEs), possibly extending into the thrust faults of the sedimentary prism. For example, offshore Gisborne SSEs occur every 1-2 years and can last several weeks, during which 5-30 cm of slip may be accommodated. Understanding what controls the timing of such events will help the comprehension of HM deformation and earthquake mechanics in general. One hypothesis for a slow slip mechanism is that the low permeability of the HM prism rocks and the large fluid volumes dragged deep into the subduction zone cause over-pressures along the megathrust and prism splay faults. Overpressure induces SSEs that locally increase permeability. After an SSE, swelling clays and ductile deformation reduce permeability within months, resetting the conditions for developing overpressure. We tested such a hypothesis by measuring the hydraulic permeability of fractured sedimentary rocks making up the core of the accretionary prism. Tests were performed using a newly developed X-ray transparent pressure vessel mounted inside a micro-computed tomography scanner (mCT) that allowed in-situ observation of fracture evolution as a function of confining pressure, time, and exposure to water. The tested rocks were probably subducted to ~7.5 km and are calcareous-glauconitic fine-grained sandstones with a silty matrix from the Late Cretaceous-to-Paleocene Tinui Group containing ~15% vol% of clay minerals. After exposure to high confining pressure and water, the samples regained pre-fracture permeability in tens of days. mCT imagery suggests that fracture clogging, possibly due to clay expansion, controls healing. We propose that slow slip events in the northern HM open fault fractures and allow drainage at the beginning of the slip cycle, followed by fracture clogging due to swelling clays and ductile deformation, with the duration of the cycle regulated by the interplay of these processes. 
    more » « less
  4. ; ; ; ; (, Geophysical Research Letters)
    Abstract We investigate the occurrence patterns of SSEs along the shallow (15 km) portion of the Hikurangi subduction zone. First, we build a manual catalog constraining timing and length of 92 SSEs between 2006 and 2024. Then, we investigate SSE occurrence patterns by fitting a renewal process, using Bayesian inference to obtain the posterior distribution of model parameters. Our results show that SSE recurrence intervals vary along the Hikurangi margin; less frequent SSEs occur in the southern part of the margin. The periodicity of SSEs also changes along strike. SSEs in the northern part of the margin occur more regularly than those at the central part. Finally, we do not find conclusive evidence that 2016 7.8 Kaikōura earthquake had a lasting effect on SSE occurrence patterns. 
    more » « less
  5. ; ; ; ; ; (, Proceedings of the International Ocean Discovery Program)
    null (Ed.)
    Slow slip events (SSEs) at the northern Hikurangi subduction margin, New Zealand, are among the best-documented shallow SSEs on Earth. International Ocean Discovery Program Expeditions 372 and 375 were undertaken to investigate the processes and in situ conditions that underlie subduction zone SSEs at the northern Hikurangi Trough. We accomplished this goal by (1) coring and geophysical logging at four sites, including penetration of an active thrust fault (the Pāpaku fault) near the deformation front, the upper plate above the SSE source region, and the incoming sedimentary succession in the Hikurangi Trough and atop the Tūranganui Knoll seamount; and (2) installing borehole observatories in the Pāpaku fault and in the upper plate overlying the slow slip source region. Logging-while-drilling (LWD) data for this project were acquired as part of Expedition 372, and coring, wireline logging, and observatory installations were conducted during Expedition 375. Northern Hikurangi subduction margin SSEs recur every 1–2 y and thus provide an ideal opportunity to monitor deformation and associated changes in chemical and physical properties throughout the slow slip cycle. In situ measurements and sampling of material from the sedimentary section and oceanic basement of the subducting plate reveal the rock properties, composition, lithology, and structural character of material that is transported downdip into the SSE source region. A recent seafloor geodetic experiment raises the possibility that SSEs at northern Hikurangi may propagate to the trench, indicating that the shallow thrust fault (the Pāpaku fault) targeted during Expeditions 372 and 375 may also lie in the SSE rupture area and host a portion of the slip in these events. Hence, sampling and logging at this location provides insights into the composition, physical properties, and architecture of a shallow fault that may host slow slip. Expeditions 372 and 375 were designed to address three fundamental scientific objectives: 1. Characterize the state and composition of the incoming plate and shallow fault near the trench, which comprise the protolith and initial conditions for fault zone rock at greater depth and which may itself host shallow slow slip; 2. Characterize material properties, thermal regime, and stress conditions in the upper plate directly above the SSE source region; and 3. Install observatories in the Pāpaku fault near the deformation front and in the upper plate above the SSE source to measure temporal variations in deformation, temperature, and fluid flow. The observatories will monitor volumetric strain (via pore pressure as a proxy) and the evolution of physical, hydrological, and chemical properties throughout the SSE cycle. Together, the coring, logging, and observatory data will test a suite of hypotheses about the fundamental mechanics and behavior of SSEs and their relationship to great earthquakes along the subduction interface. 
    more » « less
  • Citation Formats
  • MLA
  • APA
  • Chicago
  • BibTeX
  • Save / Share this Record
  • Send to Email