[ghsc-seminars] GHSC Seminar *NEXT WEEK* *HYBRID* - Chengrui Chang, U. Tokyo, December 20, 2022

[Llenos, Andrea L]

NEWLY ADDED SEMINAR!​

Speaker: Chengrui Chang
                 Department of Biomaterial Sciences
                 University of Tokyo
Location: HYBRID (online and in-person at 1711 Illinois St, Golden, CO, Rm 204)
Date/Time: Tuesday, December 20, 2022 10-11am MST

Professor Chengrui Chang (U. Tokyo) will present the following two ~20 minute talks on the frictional behavior of an experimental fault zone and landslide movements.

Title #1: Frictional Properties and Structural Evolution of A Growing Experimental Fault Zone of NaCl Medium

Abstract #1: A key motivation of this study is to better understand the frictional behavior of a growing fault zone that develops in granular medium. To this end, we employ a unique ring-shear configuration (Figure a) that has a large shear box (18 cm and 12 cm for outer and inner diameters, 10.9 cm for maximum sample height) to test a large volume of grains. Halite has been frequently used as an analogue for fault gouge (e.g., Shimamoto, 1986; Noda and Shimamoto, 2010; Buijze et al., 2017). We use halite (NaCl) grains to simulate grain comminution. We vary the total normal stress from 0.2 to 1.0 MPa and keep the constant slip rate at 0.05 cm/s. We used two types of granular halite (NaCl) with different initial sizes. The coarse-grained halite was characterized by a particle size of ~2-5 mm while the fine-grained halite was of ~0.25-2 mm. It should be emphasized that a large experimental fault zone is developed within the granular medium (Figures b-e).
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The sheared medium is characterized by drastic grain fragmentation, which exhibits a higher friction coefficient over a slip followed by a substantial frictional weakening. We use a piecewise mechanical model to scale the lengths of both the frictional stable and frictional weakening regimes and an exponential decay model to scale the length of compaction of the sheared medium. We find that all lengths are dependent on normal stress but only the scaling differs for compaction length upon grain size. We propose that comminuted fines may serve as a frictional fluid and the weakening starts until shear zone is saturated by fines.

Title #2: On the Slow-to-fast Transition of Landslide Movements and the Rate- and State-dependent Frictional Behavior in It

Abstract #2: Slow moving landslides can sometimes accelerate to catastrophic failure (e.g., Lacroix et al., 2020). A series of phenomenological creep-failure models have been established to describe the pre-failure acceleration that is commonly observed in landslide events and to achieve the goal of failure-time forecast of landslide, including Saito (1965, 1969), Fukuzono (1985), and Voight (1988) models. Among them, the Voight model (Ω ̈=AΩ ̇^α) is more generalized for various forms of failure and natural phenomena, where Ω denotes an appropriate measurable quantity (e.g., surface displacement, titling angle), the dot refers to time derivative, and α and A are two empirical parameters. It is, however, purely empirical and the relation between the parameters and underlying processes remains to be investigated. Particularly, the value of α is equal to 2 in the most landslides (e.g., Segalini te al., 2018), but it can be significantly smaller (Bozzano et al., 2014). In addition, α can be different for different acceleration events in the same place (e.g., Crosta and Agliardi, 2003).
Behavior of a tectonic fault is often modeled by a rate- and state-dependent friction law (RSF), which similarly shows diverse range of slip behavior. Here we investigate whether and how the RSF impact on the accelerations that universally arise from the pre-failure stage of landslides. Helmstetter et al. (2014) applied RSF in a form of the aging law to a creep behavior of an underlying shear zone of landslides and showed α=2.

We conduct creep tests on clayey soils in ring-shear configuration to reproduce the pre-failure accelerating movement. We suggest that the parameters in the Voight model change when shear localization sharpens acceleration (Chang and Wang, 2022). We then establish interpretation of the creep behavior from the viewpoint of the RSF. We have mathematically shown that α = 2 naturally arise in many standard versions of RSFs, and α ≠ 2 and the change in α may be caused by time-dependent loading conditions.

For questions, comments, or speaker suggestions, please contact the GHSC Seminar Committee: Andrea Llenos <allenos at usgs.gov>, Benjamin Murphy <bmurphy at usgs.gov>, Lauren Schaefer <lschaefer at usgs.gov>


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