Some of my research interests include:
* Soft condensed matter physics, granular physics, nonlinear dynamics and pattern formation;
   image left: patterns in perturbed granular fluid;
   image right top: models of granular and amorphous friction;
   image right bottom: surface flow on a granular pile inside a rotating drum; source: link
• Landscape evolution; image: hillslope and river channel morphology near Te Puka Valley, Willington, New Zealand; source: link
•• Fault mechanics and earthquake (geo)physics; image: San Andreas Fault zone, California, USA; source: Tom Bean/Corbis
••• Sediment transport and surficial patterns in gravel bed rivers; image: gravel bar formation and armoring (grain sorting, coarsening) phenomenon in Elbow river, Canada; source: link
•••• Fluvial geomorphology; image: Rhône river flowing from Valais into Lake Geneva, bringing fresh muddy sediments to its delta; source: link
••••• (Aeolian?) Transport and surficial patterns on planetary bodies and its relation to their forcings and climatic history; image: dunes of Mars; source: link
•••••• Aeolian transport on Earth; image: sand dunes in Death Valley, California, USA; source: link

Intro↓                     Publications                    CV                    Blog

Welcome to my webpage! I hope you find the materials and brief discussions presented here interesting. This page is being regularly updated, so please feel free to come back soon! Also, if you have any questions or have some related or unrelated ideas or suggestions for potential collaborations, please do not hesitate to get in touch with me. My contact details are in blue color below.
I am Behrooz Ferdowsi, currently a Hess Fellow and Postdoctoral Research Associate at the Department of Geosciences, Princeton University. I am primarily working with Prof. Allan M. Rubin on constitutive laws for earthquake fault friction (also known as rate- and state-dependent friction laws) over the earthquake cycle, and the physics of the seismic cycle. I further collaborate with Prof. Troy Shinbrot (Rutgers University) on several problems in granular physics, including pattern formation in granular flows. I have a long-standing interest in granular friction and granular flows, and Earth- and planetary-surface and subsurface processes. Before coming to Princeton, I was a Postdoctoral Researcher at the University of Pennsylvania, Sediment Dynamics Laboratory (PennSeD) and a Synthesis Postdoctoral Fellow of the National Center for Earth-Surface Dynamics (NCED). At PennSeD, I worked with Prof. Douglas J. Jerolmack (UPenn, Earth and Environmental Science) on bimodal sediment transport, subsurface to surface evolution of riverbeds and also granular controls of hillslope creep and landscape evolution. I received my PhD (Dr. sc.) from ETH Zurich (Switzerland), Department of Civil, Environmental and Geomatic Engineering. For my PhD research, I studied the influence of vibrations on the frictional behavior of sheared granular layers and implcations of the phenomenon for triggering of earthquakes by other (seismic) sources.


Research interests

cloud.png I have a broad research interest in several (and expanding) areas of geosciences and geophysics, including (i) constitutive laws for earthquake fault friction, fault zone processes, and the physics and the mathematical modelings of the seismic cycle, (ii) hillslope and landscape evolution, geohazards and their environmental implications, (iii) fluvial geomorphology and aeolian transport, and (iv) sedimentology. I am further excited about ways that geomorphic and seismic observations can complement each other, and help with a better understanding of Earth-surface and subsurface processes in isolation and in coupling. I am also interested in broad engineering applications and implications of these research areas including implementation of the latest scientific discoveries in hazard assessment and mitigation methods. The unifying concept of my research and scientific interests however lies in soft condensed matter physics and the statistical physics of particles and fields. I search for laws (preferrably universal but they could be also non-universal) that can describe spatiotemporal evolution of different sections (and cross-sections) of the Earth and other planets across the scales of time (sub-second to million years) and space (asperities and grains to geo/log/phy/ical scales). For my works, I often use a combination of laboratory-scale experiments, numerical (Discrete Element Method and Molecular Dynamics) simulations, and analytical (sometimes continuum) modeling, with continuous inspiration and insight from direct (by myself) and indirect (previously published, collected, documented) field observations.

News, short notes and views  

Brief summary on current and past research

a) Evolution of riverbed surface in gravel-bed rivers (2015-2017)

In collaboration with Dr. Carlos P. Ortiz and Dr. Morgane Houssais and under direction of Prof. Douglas J. Jerolmack, I worked on sediment transport in bimodal (two major grain size) systems in an idealized laboratory river. Separation or segregation of grains with different size (and shape, mass, surface characteristics, etc.) is a frequently observed phenomenon in rivers, streams (where it is commonly known as sorting, coarsening, or armoring when larger grains cover the riverbed surface, and is known as gravel bars and fans when segregation and separation takes place intermittently) and all forms of granular materials and systems! Geologists and geomorphologists tend to think that formation of a coarse surface layer shields the finer underlying grains from erosion. They also traditionally assume that armor develops due to sorting of surficial grains by the fluid flow. In this research, we showed how motion of grains deep beneath the surface delivers larger grains to the surface. Using experiments in a laboratory river, and discrete and continuum models, we further demonstrated that river-bed armoring is driven by vertical granular segregation and that the fluid has little effect. Results also revealed different segregation mechanisms for deep (creeping) versus shallow (dense and rapidly flowing) grains, which has broader implications for all manner of granular flows. Please see our Nature Communications (2017) paper and University of Pennsylvania News article: `Brazil Nut Effect` Helps Explain How Rivers Resist Erosion on this work for more information.

↑ Phenomenology and setup. (A) Bed sediment of the River Wharfe, U.K., that shows a pronounced surface armor. Photo courtesy D. Powell [1]. (B) Sketch of the experiment, showing position of the camera and laser plane used for imaging inside the granular bed. (C-E) Snapshots during armor development for τs* = 3.8τ*cs. Also shown is the fluid boundary stress, which is computed as τ = ηUf/hf [2] where Uf and hf are the top-plate speed and flow depth, respectively. The red curve shows the long-term-averaged streamwise particle velocity ux(z), where I and II correspond to the bed load and creep zones, respectively. The directions x and z are indicated. (F) Temporal evolution of the thickness of the armored layer at different Shields number. Legend indicates Shields number associated with each curve. The brighter continuous lines are predictions from a modified version -to account for creep (slow flow) segregation- of advection-diffusion model [ Gray and Ancey, [2015], Gray and Thornton, [2005]], , commonly used to model size-segregation phenomenon in granular mixing. Note the first rapid stage of armoring, which is dependent on Shields number and is associated with bed-load transport, and the second slower stage that exhibits a nearly constant rate for all Shields numbers and is the result of creep.

b) Hillslope evolution and landscape dynamics over geological timescales (2016-present)

Soil creeps imperceptibly downhill, but also fails catastrophically to create landslides. Despite the importance of these processes as hazards and in sculpting landscapes, there is no agreed upon model that captures the full range of behavior. In this work, we have examined the granular origins of hillslope soil transport by Discrete Element Method simulations, studying deformation and transport in an experimental hillslope using dynamic light scattering technique, and re-analysis of sediment flux versus gradient measurements in natural landscapes around the world. We found creep for slopes below a critical gradient, where average particle velocity (sediment flux) increases exponentially with friction coefficient (gradient). At critical there is a continuous transition to a dense-granular flow rheology, consistent with previous laboratory experiments and theoretical developments in amorphous Earth materials and disordered media [ Fisher, 1998, Chauve et al., 2000, Dauchot et al., 2005, Reddy et al., 2011, Reichhardt and Reichhardt, 2016]. Slow earthflows and landslides thus exhibit glassy dynamics that is a characteristic of a wide range of disordered materials; they are described by a two-phase flux equation that emerges from grain-scale friction alone. This glassy model reproduces topographic profiles of natural hillslopes, showing its promise for predicting hillslope evolution over geologic timescales.

↑ Landslide and creep phenomenology. (A) Rapid landslide in San Salvador, El Salvador; and (B) Slow earthflow in Osh, Kyrgyzstan. (C) Ranges of surface velocities observed for various types of slow and rapid landslides. The datapoints in red, brown, magenta, and green correspond to the observations reported or documented by  Cruden and Varnes, [1996],  Hungr et al., [2001],  Hilley et al., [2004], and Saunders and Young, [1983], respectively. (D) Schematic cross section of a soil-mantled hillslope. Photo credits: (A) Associated Press/Wide World Photos, (B) Joachim Lent.

↑ General flow behavior and the glassy flux model. (A) DEM results showing normalized local downslope velocity ([(ux(z))/(ux(zc))]) as a function of normalized local friction coefficient ([(μ)/(μc)]) for four different inclinations, below and above the bulk angle of repose. (B) Field data of normalized flux qs/qsc - equivalent to normalized velocity us/usc - versus normalized gradient (S/Sc) for five different studies of natural hillslopes. Data cloud color represents the probability of all field observations, that takes into account not only the probability of an observation at a given value of flux-gradient in the collected datasets, but also the number of sites/hillslopes collected and involved in each study (please see this arXiv:1708.06032 submission for methods and details of the analysis, and Figs. S5 & S6 therein). Dashed line illustrates an exponential scaling for creep regime with critical gradient Sc = 0.4 and a power-law scaling for the range of large flux. The chosen values of exponential and power law scaling coefficients are not a fit to the data, because of inherent variability of the observations. They represent a qualitative comparison between expectations from theory, numerical simulations, and the field.

↑ Hillslope topography of the Oregon Coast Range (OCR) derived from this publicly available airborne lidar data on opentopography platform (link to lidar data). (A) Regional perspective view, showing locations of two example hillslopes. (B) The elevation-distance and (C) gradient-distance relationships for representative profiles of hillslopes (1) (black dots) and (2) (red dots) in panel (A). Blue dashed line is the prediction of the "glassy" flux model with Sc = 0.5 and β = 5/2. Please see supplementary information in this arXiv:1708.06032 submission for more examples.

c) Physics of rate- and state-dependent friction laws for Earth materials with a focus on earthquake fault gouge and earthquake source (2015-present)

Numerical simulations of earthquake nucleation rely on constitutive rate and state evolution laws to model earthquake initiation and propagation processes. In these laws, the frictional response of an interface is described as:

τ = σ
f* + a  ln V

+ b  ln V*θ


where a and b in equation (1) are the coefficients of the rate- and-state "direct" and "evolution" effects, respectively, σ is the effective normal stress, f* and V* are reference (initial) values of the friction coefficient and sliding velocity, respectively, V is the current value of the sliding velocity, and θ is the "state" variable. Two empirical forms for the evolution of the state variable θ have been previously proposed by  Ruina, [1983], and are used more frequently:

= 1 −

                     (Aging law)



                     (Slip law)
In equations (2) and (3), Dc is a characteristic slip distance. The "state" variable θ is designed so as to reflect the product of the true contact area of asperities of the frictional interface and the intrinsic strength of the contacts in touch.
↑ (A) Most empirical frictional laws deal with the evolution of frictional resistance between two solid blocks pressed together and pulled (sometimes with a spring attached to one of the blocks) at a constant velocity or force parallel to solid-solid interface. The effective contact between the two interface is made of small localized patches whose total area is significantly less that the nominal area of contact. (B) The actual fault-fault interface is usually comprised of weak and fragmented rocks called "fault gouge". The rate- and state-dependent friction laws are supposedly designed to work for both solid-solid interface friction and the granular-interface friction. Despite decades of rigorous experimental, numerical, computational, observational and theoretical research by a broad group of researchers from geophysics to physics and tribology, the fundamental physics of frictional interfaces (including for fault physics) remain illusive for both types of interfaces until nowadays. The image is the courtesy of Kristie Bradford, (C) The figure shows the transient ("direct") and (toward "evolution") steady-state effect on friction of a change in loading velocity for a 3-mm–thick layer of quartz gouge sheared under nominally dry conditions at 25-MPa normal stress [Marone, 1998]. (D) The figure shows the behavior of velocity-step experiments (four order of magnitude velocity change compared to the initial imposed velocity) on a simulated granular layer, subject to confining pressure 5-MPa. The Hertzian normal contact law and Coulomb friction law govern the grain-grain contact interactions, and the mean grain diameter is 3 mm (polydisperse grain size distribution). Note that no time-dependent plasticity is implemented at the grain contact scale. One of our goals in this research program is to separate complex though important contributions of contact plasticity, then design a granular-physics based friction law for fault and Earth materials friction. We will next revisit the problem and contributions of contact plasticity on frictional behavior as related to wide range of geoscience problems including earthquake source and physics in collaboration with physicists, geophysicists, and tribologists.

The response of different state evolution laws to large velocity increases is an important feature of these constitutive relations that can significantly change the style of earthquake nucleation in numerical models. However, currently there is not a rigorous understanding of the physical origins of the response of bare rock or gouge-filled fault zones to large velocity increases. This in turn hinders our ability to design physics-based friction laws that can appropriately describe those responses. In this research program, we argue that most fault zones form a granular gouge after an initial shearing phase and that it is the behavior of the gouge layer that controls the fault friction. We perform numerical experiments of a confined sheared granular gouge under a range of confining stresses and driving velocities relevant to fault zones and apply 1-3 order of magnitude velocity steps to explore dynamical behavior of the system from grain- to macro-scales. We compare our numerical observations with experimental data from biaxial double-direct-shear fault gouge experiments under equivalent loading and driving conditions. Our intention is to first investigate the degree to which these numerical experiments, with Hertzian normal and Coulomb friction laws at the grain-grain contact scale and without any time-dependent plasticity, can reproduce experimental fault gouge behavior. We next compare the behavior observed in numerical experiments with predictions of the Dieterich (Aging) and Ruina (Slip) friction laws. Ultimately, the numerical observations at the grain and meso-scales will be used for designing a rate and state evolution law that takes into account recent advances in rheology of granular systems, including local and non-local effects [DeGiuli and Wyart, 2017,Zhang and Kamrin, 2017,Henann and Kamrin, 2013], for a wide range of shear rates and slow and fast deformation regimes of the fault gouge.

d) Pattern formation and charge transfer in vibrated granular beds and implications for landforms on Earth and other planets (2017-present)

coming soon!

e) Role of dynamic stress and perturbation in triggering slip in frictional amorphous systems (2011-2014)

coming soon!


[Chauve et al. 2000]
Pascal Chauve, Thierry Giamarchi, and Pierre Le Doussal. Creep and depinning in disordered media. Physical Review B, 62 (10): 6241, 2000.
[Cruden and Varnes 1996]
David M Cruden and David J Varnes. Landslides: investigation and mitigation. chapter 3-landslide types and processes. Transportation research board special report, (247), 1996.
[Dauchot et al. 2005]
Olivier Dauchot, Guillaume Marty, and Giulio Biroli. Dynamical heterogeneity close to the jamming transition in a sheared granular material. Physical review letters, 95 (26): 265701, 2005.
[DeGiuli and Wyart 2017]
E. DeGiuli and M. Wyart. Friction law and hysteresis in granular materials. Proceedings of the National Academy of Sciences, 2017. doi: 10.1073/pnas.1706105114. URL
[Fisher 1998]
Daniel S Fisher. Collective transport in random media: from superconductors to earthquakes. Physics reports, 301 (1): 113-150, 1998.
[Gray and Ancey 2015]
JMNT Gray and C Ancey. Particle-size and-density segregation in granular free-surface flows. Journal of Fluid Mechanics, 779: 622-668, 2015.
[Gray and Thornton 2005]
JMNT Gray and AR Thornton. A theory for particle size segregation in shallow granular free-surface flows. In Proceedings of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, volume 461, pages 1447-1473. The Royal Society, 2005.
[Henann and Kamrin 2013]
David L Henann and Ken Kamrin. A predictive, size-dependent continuum model for dense granular flows. Proceedings of the National Academy of Sciences, 110 (17): 6730-6735, 2013.
[Hilley et al. 2004]
George E Hilley, Roland Bürgmann, Alessandro Ferretti, Fabrizio Novali, and Fabio Rocca. Dynamics of slow-moving landslides from permanent scatterer analysis. Science, 304 (5679): 1952-1955, 2004.
[Houssais et al. 2015]
Morgane Houssais, Carlos P Ortiz, Douglas J Durian, and Douglas J Jerolmack. Onset of sediment transport is a continuous transition driven by fluid shear and granular creep. Nature communications, 6, 2015.
[Hungr et al. 2001]
Oldrich Hungr, SG Evans, MJ Bovis, and JN Hutchinson. A review of the classification of landslides of the flow type. Environmental & Engineering Geoscience, 7 (3): 221-238, 2001.
[Marone 1998]
Chris Marone. Laboratory-derived friction laws and their application to seismic faulting. Annual Review of Earth and Planetary Sciences, 26 (1): 643-696, 1998.
[Powell 1998]
D Mark Powell. Patterns and processes of sediment sorting in gravel-bed rivers. Progress in Physical Geography, 22 (1): 1-32, 1998.
[Reddy et al. 2011]
KA Reddy, Y Forterre, and O Pouliquen. Evidence of mechanically activated processes in slow granular flows. Physical Review Letters, 106 (10): 108301, 2011.
[Reichhardt and Reichhardt 2016]
Charles Reichhardt and CJ Olson Reichhardt. Depinning and nonequilibrium dynamic phases of particle assemblies driven over random and ordered substrates: a review. Reports on Progress in Physics, 80 (2): 026501, 2016.
[Ruina 1983]
Andy Ruina. Slip instability and state variable friction laws. Journal of Geophysical Research: Solid Earth, 88 (B12): 10359-10370, 1983.
[Saunders and Young 1983]
Ian Saunders and Anthony Young. Rates of surface processes on slopes, slope retreat and denudation. Earth Surface Processes and Landforms, 8 (5): 473-501, 1983.
[Zhang and Kamrin 2017]
Qiong Zhang and Ken Kamrin. Microscopic description of the granular fluidity field in nonlocal flow modeling. Physical Review Letters, 118 (5): 058001, 2017.

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On 8 June 2018, 15:23 EST