Omid Dorostkar


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Dorostkar

First Name

Omid

ORCID

0000-0002-7758-4919

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2017 - 201932020 - 20212
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Publications 1 - 5 of 5
  • Grain Friction Controls Characteristics of Seismic Cycle in Faults With Granular Gouge
    Item type: Journal Article
    Dorostkar, Omid; Carmeliet, Jan (2019)
    Journal of Geophysical Research: Solid Earth
    Mature faults at their core contain granular gouge, created due to communition of host rocks, which its frictional behavior controls earthquake nucleation and rupture patterns. In this work, we consider a fault system with granular gouge to study the effect of grain friction on the characteristics of seismic cycles. Our results show that particle friction controls the evolution of fault frictional strength as well as accumulation and release of elastic strain energy. Our discrete element simulations show that the stick-slip frictional strength and dilation of the fault, as well as their variations, nonlinearly increase with the particle friction, but at high particle friction saturate. By statistical analyses on a large number of slip events, we find that the average recurrence time and its variations decrease with particle friction. A fault with higher grain friction shows more small slip events and also contains a limited number of extreme events. High particle friction introduces a more complex nucleation phase with higher stored energy and many recurrent small failures. We analyze the pseudo acoustic emission, which is based on monitoring the velocity signal of particles, and find higher temporal and more spatially distributed pseudo acoustic emissions for fault with higher grain friction. Our findings in this study show that, in faults with granular gouge, where the fault zone walls are totally engaged to the gouge layer, the friction at grain-scale controls the characteristics of stick-slip cycles including timing and amount of energy release.
  • Betweenness centrality illuminates intermittent frictional dynamics
    Item type: Working Paper
    Dorostkar, Omid; Daniels, Karen E.; Strebel, Dominik André; et al. (2021)
    arXiv
    Dense granular systems subjected to an imposed shear stress undergo stick-slip dynamics with systematic patterns of dilation-compaction. During each stick phase, as the frictional strength builds up, the granular system dilates to accommodate shear strain, developing stronger force networks. During each slip event, when the stored energy is released, particles experience large rearrangements and the granular network can significantly change. Here, we use numerical simulations of 3D, sheared frictional packings to show that the mean betweenness centrality -- a property of network of interparticle connections -- follows consistent patterns during the stick-slip dynamics, showing sharp spikes at each slip event. We identify the source of this behavior as arising from the connectivity and contact arrangements of granular network during dilation-compaction cycles, and find that a lower potential for connection between particles leads to an increase of mean betweenness centrality in the system. Furthermore, we show that at high confinements, few particles lose contact during slip events, leading to a smaller change in granular connectivity and betweenness centrality.
  • On the Effect of Grain Fragmentation on Frictional Instabilities in Faults With Granular Gouge
    Item type: Journal Article
    Wang, Di; Carmeliet, Jan; Zhou, Wei; et al. (2021)
    Journal of Geophysical Research: Solid Earth
    The evolution of frictional strength during stick-slip dynamics of a fault system is key to understanding earthquake nucleation and rupture patterns. In mature faults, granular gouge is produced by wear, comminution or fragmentation during tectonic movements. In this work, we introduce a fragmentation model in the simulation of a sheared granular fault to explore the influence of grain breakage on the stick-slip dynamics. With fragmentation of highly stressed particles, the fault frictional strength increases accompanied by many small slip events triggered by particle breakage. The small fragments produced by particle breakage are not only stronger and more difficult to break, but they also change the distribution of contact forces, leading to the strengthening of the fault system. Based on statistical analyses on the size distribution of slip events with different particle strengths, we find that with lower particle strength, slip events become more correlated with particle fragmentation events and that the number of large slip events decreases. In addition, our analyses on the relationship between slip and particle fragmentation events reveal three types of correlations: In the first and second types, particle fragmentation events trigger micro or major slips, respectively. In the third category, large-scale particle fragmentations take place at the end of large slip events owing to stress localization during post-slip particle rearrangements. Our results in this work highlight the role of micromechanics of particle fragmentation in the failure of fault damage zones and help in understanding the relation between particle breakage and frictional failures.
  • Machine Learning Reveals the State of Intermittent Frictional Dynamics in a Sheared Granular Fault
    Item type: Journal Article
    Ren, Christopher X.; Dorostkar, Omid; Rouet-Leduc, Bertrand; et al. (2019)
    Geophysical Research Letters
  • On the micromechanics of slip events in sheared, fluid-saturated fault gouge
    Item type: Journal Article
    Dorostkar, Omid; Guyer, Robert A.; Johnson, Paul A.; et al. (2017)
    Geophysical Research Letters
    We used a three-dimensional discrete element method coupled with computational fluid dynamics to study the poromechanical properties of dry and fluid-saturated granular fault gouge. The granular layer was sheared under dry conditions to establish a steady state condition of stick-slip dynamic failure, and then fluid was introduced to study its effect on subsequent failure events. The fluid-saturated case showed increased stick-slip recurrence time and larger slip events compared to the dry case. Particle motion induces fluid flow with local pressure variation, which in turn leads to high particle kinetic energy during slip due to increased drag forces from fluid on particles. The presence of fluid during the stick phase of loading promotes a more stable configuration evidenced by higher particle coordination number. Our coupled fluid-particle simulations provide grain-scale information that improves understanding of slip instabilities and illuminates details of phenomenological, macroscale observations.
Publications 1 - 5 of 5