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1. Permeability healing of clay-rich sedimentary rocks from the Hikurangi margin: a possible mechanism to explain slow-slip event recurrence

   Alamoudi, O; Tisato, N; Van_Avendonk, H J; Garza, H; Bangs, N L (December 2023, 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. 

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2. Insights into the style of deformation in the Hikurangi subduction zone from rock physics measurements: elastic properties, permeability, and fracture healing

   Tisato, N; Bland, C; Van_Avendonk, H; Gase, A; Bangs, N (December 2022, AGU)

   The Hikurangi subduction zone exhibits a north-to-south variation in deformation style. The plate interface in the south is locked, and megathrust earthquakes could accommodate the long-term plate convergence. In contrast, the northern megathrust regularly experiences shallow slow-slip events possibly extending into the thrust faults of the sedimentary prism. Understanding such a difference could reveal slip behavior and seismic cycle controls and help earthquake forecasting globally. One hypothesis is that the prism rock properties and fluid pressures affect these different slip behaviors. To test such a hypothesis, we measured the physical properties of rocks from the northern Hikurangi margin, focusing on ultrasonic elastic properties, permeability, and fracture healing. Such lithologies are equivalent to rocks buried to a few km depths within the accretionary prism. We found that all rocks contain >18 vol% of clay minerals. The hydraulic permeability of rock samples that are proxies for the deep part of the prism (i.e., 5 to 10 km depth) is three to four orders of magnitude lower than the values estimated by different authors for the prism as a whole. The results suggest that active faults and fractures in the accretionary prism must play a key role in draining fluids from the base of the prism and potentially from the subducting plate. Tests on a fractured sample show that fractures heal in tens of days, and permeability decreases over a short period relative to slip cycles of just a few weeks. Microphotography and micro-CT images suggest that healing is achieved by clay expansion. The observed healing could be underestimated as achieved under high confining pressure (up to 200 MPa) but at room temperature and humidity. We conclude that slow slip events in the northern Hikurangi margin may have a critical role in briefly increasing permeability at the beginning of the slip cycle, thus regulating pore pressure in the prism and allowing drainage. 

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3. Radar imaging of fractures and voids behind the walls of an underground mine

   https://doi.org/10.1190/geo2020-0763.1  

   Abbasi Baghbadorani, Amin; Hole, John A.; Baggett, Jonathan; Ripepi, Nino (July 2021, GEOPHYSICS)

   Two- and three-dimensional rock-penetrating-radar data were acquired on the wall of a pillar in an underground limestone mine. The objective was to test the ability of radar to image fractures and karst voids and to characterize their geometry, aperture, and fluid content, with the goal of mitigating mining hazards. Strong radar reflections in the field data correlate with fractures and a cave exposed on the pillar walls. Large pillar wall topography was included in the steep-dip Kirchhoff migration algorithm because standard elevation corrections are inaccurate. The depth-migrated 250 MHz radar images illuminate fractures, karst voids, and the far wall of the pillar up to approximately 25 m depth into the rock, with a spatial resolution of <0.5 m. Higher frequency radar improved the image resolution and aided in the interpretation, but at the cost of shallower depth of penetration and extra acquisition effort. Due to the strong contrast in physical properties between the rock and the fracture fluid, fractures with apertures as thin as a 50th of a radar wavelength were imaged. Water-filled fractures with mm-scale aperture and air-filled fractures with cm-scale apertures produce strong reflections at 250 MHz. A strong variation in the reflection amplitude along each fracture is interpreted to represent the variable fracture aperture and the nonplanar fracture structure. Fracture apertures were quantitatively measured, but distinguishing water from air-filled fractures was not possible due to the complex radar wavelet and fracture geometry. Two conjugate fracture sets were imaged. One of these fracture sets dominates the rock mass stability and water inrush challenges throughout the mine. All of the detected voids and a large cave are at the intersection of two fractures, indicating preferential water flow and dissolution along conjugate fracture intersections. Detecting, locating, and characterizing fractures and voids prior to excavation can enable miners to mitigate potential collapse and flood hazards before they occur. 

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4. Evolution of Fault-Zone Hydromechanical Properties in Response to Different Cementation Processes

   https://doi.org/10.2113/2022/1069843  

   Romano, C. R.; Williams, R. T.; Cheng, ed., Feng (January 2022, Lithosphere)

   Abstract Progressive cementation and sealing of fault-localized fractures impact crustal mass transport and the recovery of fault strength following slip events. We use discrete fracture network (DFN) models to examine how fracture sealing during end-member cementation mechanisms (i.e., reaction- versus transported-limited cementation) influences the partitioning of fluid flow through time. DfnWorks was used to generate randomized fracture networks parameterized with fracture orientation data compiled from field studies. Single-phase flow simulations were performed for each network over a series of timesteps, and network parameters were modified to reflect progressive fracture sealing consistent with either reaction- or transport-limited crystal growth. Results show that when fracture cementation is reaction-limited, fluid flow becomes progressively channelized into a smaller number of fractures with larger apertures. When fracture cementation is transport-limited, fluid flow experiences progressive dechannelization, becoming more homogeneously distributed throughout the fracture network. These behaviors are observed regardless of the DFN parameterization, suggesting that the effect is an intrinsic component of all fracture networks subjected to the end-member cementation mechanisms. These results have first-order implications for the spatial distribution of fluid flow in fractured rocks and recovery of permeability and strength during fault/fracture healing in the immediate aftermath of fault slip. 

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5. Flow in porous media with low dimensional fractures by employing enriched Galerkin method

   https://doi.org/https://doi.org/10.1016/j.advwatres.2020.103620  

   Kadeethum, T; Nick, H.M; Lee, Sanghyun; Ballarin, F. (January 2020, Advances in water resources)

   This paper presents the enriched Galerkin discretization for modeling fluid flow in fractured porous media using the mixed-dimensional approach. The proposed method has been tested against published benchmarks. Since fracture and porous media discontinuities can significantly influence single- and multi-phase fluid flow, the heterogeneous and anisotropic matrix permeability setting is utilized to assess the enriched Galerkin performance in handling the discontinuity within the matrix domain and between the matrix and fracture domains. Our results illustrate that the enriched Galerkin method has the same advantages as the discontinuous Galerkin method; for example, it conserves local and global fluid mass, captures the pressure discontinuity, and provides the optimal error convergence rate. However, the enriched Galerkin method requires much fewer degrees of freedom than the discontinuous Galerkin method in its classical form. The pressure solutions produced by both methods are similar regardless of the conductive or non-conductive fractures or heterogeneity in matrix permeability. This analysis shows that the enriched Galerkin scheme reduces the computational costs while offering the same accuracy as the discontinuous Galerkin so that it can be applied for large-scale flow problems. Furthermore, the results of a time-dependent problem for a three-dimensional geometry reveal the value of correctly capturing the discontinuities as barriers or highly-conductive fractures. 

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