Amir Sagy

Evolution of fault-surface roughness with slip

Principal slip surfaces in fault zones accommodate most of the displacement during earthquakes. The topography of these surfaces is integral to earthquake and fault mechanics, but is practically unknown at the scale of earthquake slip. We use new laser-based methods to map exposed fault surfaces over scales of 10 µm to 120 m. These data provide the fi rst quantitative evidence that fault-surface roughness evolves with increasing slip. Thousands of profi les ranging from 10 µm to >100 m in length show that small-slip faults (slip <1 m) are rougher than large-slip faults (slip 10‐100 m or more) parallel to the slip direction. Surfaces of small-slip faults have asperities over the entire range of observed scales, while large-slip fault surfaces are polished, with RMS values of <3 mm on profi les as long as 1‐2 m. The large-slip surfaces show smooth, elongate, quasi-elliptical bumps that are meters long and as high as ~1 m. We infer that these bumps evolve during fault maturation. This difference in geometry implies that the nucleation, growth, and termination of earthquakes on evolved faults are fundamentally different than on new ones.

Nanograins form carbonate fault mirrors

Many faults are characterized by naturally polished, reflective, glossy surfaces, termed fault mirrors (FMs), that form during slip. Recent experiments also find that FMs form during rapid sliding between rock surfaces, and that FM formation coincides with pronounced friction reduction. The structure of FMs and the mechanism of their formation are thus important for understanding the mechanics of frictional sliding, particularly during earthquakes. Here we characterize the small-scale structure of natural carbonate FMs from three different faults along a tectonically active region of the Dead Sea transform. Atomic force microscopy measurements indicate that the FMs have extremely smooth surface topography, accounting for their mirror-like appearance. Electron microscope characterization revealed a thin (

From outcrop to flow simulation: Constructing discrete fracture models from a LIDAR survey

Terrestrial light detection and ranging (LIDAR) surveys offer potential enrichment of outcrop-based research efforts to characterize fracture networks and assess their impact on subsurface fluid flow. Here, we explore two methods to extract the three-dimensional (3-D) positions of natural fractures from a LIDAR survey collected at a roadcut through the Cretaceous Austin Chalk: (1) a manual method using the University of California, Davis, Keck Center for Active Visualization in the Earth Sciences and (2) a semiautomated method based on mean normal and Gaussian curvature surface classification. Each extraction method captures the characteristic frequencies and orientations of the primary fracture sets that we identified in the field, yet they extract secondary fracture sets with varying ability. After making assumptions regarding fracture lengths and apertures, the extracted fractures served as a basis to construct a discrete fracture network (DFN) that agrees with field observations and a priori knowledge of fracture network systems. Using this DFN, we performed flow simulations for two hypothetical scenarios: with and without secondary fracture sets. The results of these two scenarios indicate that for this particular fracture network, secondary fracture sets marginally impact (10% change) the breakthrough time of water injected into an oil-filled reservoir. Our work provides a prototype workflow that links outcrop fracture observations to 3-D DFN model flow simulations using LIDAR data, an approach that offers some improvement over traditional field-based DFN constructions. In addition, the techniques we used to extract fractures may prove applicable to other outcrop studies with different research goals.

Evolution of slip surface roughness through shear

A significant part of displacement in fault zones occurs along discrete shear surfaces. The evolution of fault surface topography is studied here in direct shear laboratory experiments. Matching tensile fracture surfaces were sheared under imposed constant normal stress and sliding velocity. The roughness evolution was analyzed using measurements of surface topography before and after slip. We show that shearing reduces the initial surface roughness at all measurement scales. At all wavelengths, the roughness ratio between initial and final roughness increases as a function of the slip distance. For a given test, the roughness ratio increases with wavelength up to a few millimeters, beyond which the ratio becomes wavelength independent. At this region the roughness measured after slip follows a power law similar to that of the initial tensile fracture surface. We interpret this geometrical evolution as a consequence of the deformation stage of interlocked asperities which is followed by shear-induced dilation.

Dynamic branched fractures in pulverized rocks from a deep borehole

Pulverized rocks observed in outcrops near large faults are considered to be products of intense fragmentation occurring during strong earthquakes. However, the exact mechanism and depth of pulverization are controversial. Here we present an analysis of pulverized rocks along an ∼930 m interval in a deep borehole in the vicinity of a buried fault. Cuttings of carbonate lithology from below 4980 m host well-preserved sets of dense fractures with a hierarchic branching geometry that was never affected by near-surface processes. Branches are typically symmetric with consistent orientations and hierarchic splitting. The main fractures are spaced at ∼0.1 mm, with angular fragments ranging from a few microns to submicrons, indicating further intense localized fragmentation. Splitting of fossils by fractures, in the absence of lateral shear, proves the tensile origin of the fractures. The fracture geometry and density are consistent with dynamic tensile fractures that propagate at critical velocities in laboratory experiments. Thus, these dense branching fractures are the first direct observation that pulverization in fault zones extends to a significant depth and originates from dynamic tensile fracturing.

Measuring the time and scale-dependency of subaerial rock weathering rates over geologic time scales with ground-based lidar

The evolution of roughness as a function of surface age was used to quantify weathering rates on rocky desert surfaces. Surface topography on eight late Quaternary alluvial terraces, which record the weathering of Holocene (5 ± 1 ka) boulder-strewn deposits into mature (87 ± 2 ka) desert pavements in the Negev desert of Israel, was measured with ground-based lidar. Roughness on each terrace was characterized with power spectral density (PSD) analysis, and changes in PSD as a function of length scale (λ ∼ 0.04–1.50 m) and surface age were used to estimate diminution/weathering rates of the surface rocks. We found PSD values that systematically increase as a power-law function of λ (roughness exponent of ∼2.0) and decrease as an inverse power-law function of surface age. This PSD evolution indicates a fragmentation rock weathering process driven by salt shattering throughout the 87 k.y. period examined. PSD analysis of the lidar data also revealed weathering rates that increase with rock size and decrease as an inverse power-law function of time, from initial values >20 mm/k.y. to <1 mm/k.y. within ∼60 k.y.