Current Research Projects:

Contemporary interpretation of rock faulting is still strongly influenced by the work of Coulomb's (1773) as introduced to the geological literature by Anderson (1905). Yet, these widely accepted works are far from being complete and I explored two central issues that remained unsolved. The first issue is faulting processes under three-dimensional strain. While 3-D strain is the general situation in the field and experiments, faulting theories use plane- strain assumptions. I presented the first experimental observations of fault patterns in granites, sandstone and limestones, deformed under three-dimensional strain (polyaxial loading). Further, following Taylor (1938), I derived a model which is based on the assumption that slip along faults accommodates the applied strain field. The model prediction fit well the experimental observations and explains some anomalies of fault patterns in the field. The second issue is fault evolution: nucleation and propagation of a single fault and the development of fault systems; these topics became the center of my faulting research during the last ten years. My early studies included clay cake experiments in which I examined the growth mechanisms of individual faults and the interaction of adjacent faults. The experiments indicated new relations between geometry of fault patterns and host material rheology. I used these results to explain the structure of pull-apart basins along the Dead Sea transform and the properties of fragmented rocks. Later, as part of the analysis of triaxial fault experiments of Westerly granite, I developed a model for nucleation and propagation of faults in brittle rocks. The model is based on the interaction among tensile microcracks that develop in a self-organized pattern. This model predicts that faults should propagate in their own plane, making an angle of 20-30 degrees with the axis of maximum compression (in agreement with thousands of field and laboratory observations). The model provides the first physical explanation for the 'internal friction', the empirical parameter of Coulomb criterion.

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I have studied the seismic and tectonic activity of the last few millennia along the Dead Sea transform. This work included paleoseismic analysis of the Jericho segment during the last 2000 years, Holocene faulting in the northwestern part of the Dead Sea basin, and Holocene activity of the Elat segment. The later study included underwater mapping of active faults by using a submersible. In a second stage, we measured in-situ stresses around the Dead Sea transform including determination of the stresses from caliper logs of oil boreholes, logging with a Televiewer system, and hydrofracturing in geotechnical wells. The in-situ stress measurements generated some stimuton nature of current faulting processes in Israel and adjacent areas. Currently, I am involved in thanalysis of earthquakes in non-linear viscoelastic crust. We investhe cycle of great earthquakes along the San Andreas fault for a crust with nonlinear, depth-dependeviscoela.The mpredictions fit geodetic observations in California and indthe in-situ viscosity of crustal rocks is 3-6 orders of magnitude less then the viscosity determined in the laboratory.

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Knowing the crustal stress field is essential for regional and mechanical structural analyses. My studies of this topic dealt with field measurements, development of numerical methods and mechanical synthesis. We determined the first detailed map of the tectonic paleo-stresses in the Sinai-Israel plate, I have recently measured in-situ stresses around the Dead Sea transform. These studies defined paleo- and current structural domains in Israel and refined the regional tectonic history. I developed a new stress inversion method for accurate determination of the complete stress tensor from fault slip data and microearthquakes. The method that incorporates friction along small faults, provides a tool for regional analysis and strength evaluation of the crust.

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In recent years I became engaged in studies of dike and laccolith emplacement. Our field work and analyses departed from the crustal idealized models of uniform elastic host rocks; they include layered sequences of sedimentary rocks, depth-dependent distribution of tectonic stresses, depth-dependence variations of rock rheology, and fractured basement rocks. These analyses led to new mechanisms for dike segmentation, dike segment rotations, segment termination and flow directions within dikes. We also explored the effect of preexisting fractures on dike orientations and we developed the first model for dike emplacement into 3-D fractured basement.

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Asymmetric folds range in size from small kink-bands through monoclines to lithosphere bending; structures of all these scales were studiesd by me. I started with the mechanisms of kinking in well laminated rocks, and showed in experiments and theory the vital effect of shear stress parallel to layering on the growth and stability of kink-bands. I then studied monoclines in the field and experiments and developed a comprehensive model for their development. The model incorporates three mechanisms: buckling, draping and kinking. The relative dominance of each mechanism depends on the nature of the tectonic loading and the rheology of the deformed rocks. This model was also used in the interpretation of dome above laccoliths, the lithosphere flexures of the Arabian plate, and the surface deformation associated with some large recent earthquakes occurred along blind faults in California.

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We will explore two general approaches for fault nucleation. One approach suggests that faults nucleate as a tiny initial nucleus surrouneded by a small process zone and grows when a specified stress state is exceeded at fault tips; this approach is similar to Barnblatt-Dugdale concept for tensile fractures. The second approach, based on distributed damage model, assumes that faults nucleate when critical mechanical conditions formulated in terms of rock damage cause damage localization in a narrow zone. Investigating these processes is the objective of the present work that will explore several aspects of faulting.

The proposed work integrates field observations and microstructural analysis of faulted rocks with experimental studies. In the course of the study we will map fault related deformation in few scales ranging from 1:500 to 50:1. We will evaluate damage using ultrasonic velocities and insitu elastic modulus values. A series of rock mechanics experiments will establish the relationship between damage intensity strain history and mechanical properties. The field and laboratory results will be used to verify models of fault nucleation by utilizing numerical simulations of damage media.

We chose block of fine grained quartz-syenitic in Gevanim dome intrusions, Ramon as a suitable site for field work. The block contains thousands of small and intermediate size faults that are well exposed. These faults were mapped and the deformation around them was analyzed. We found that the faulted block is damaged microscopicaly as is evident by lower ultrasonic velocity, lower elastic rebound and intense microcracking. Additional mapping of the faults geometry and the microfabric pattern will be conduced in the course of the study. Rock mechanics experiments will establish the relationship between damage intensity, strain history and mechanical properties of Gevanim quarts-syenite. The results of the experimental work will enable the mapping of rock properties and damage distribution around the faults. This mapping is the essential tool for theoretical modeling of faulting mechanisms. We hope that the present new approach, of damage mapping in the field, will contribute to the understanding of faulting mechanisms.

Graduate student: Katz Oded (Ph.D.)
Research supervision: Ze'ev Reches, Vladimir Lyakhovsky & Gidon Baer

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Countless fractures appear in the earth's crust. Many of them grow unstably and at high velocities as evidenced by recent earthquakes and rockbursts. However, there is no systematic method to deduce the fracture growth velocity from its appearance in the field. We propose to analyze the mechanisms of unstable, dynamic fracturing in rocks by focusing on mode I, tensile fractures (joints and dikes). The analysis includes experimental work, field mapping of joints and dikes and numerical modeling. The planned work is based on a recent breakthrough in fracturing experiments by Sharon and Fineberg (1996). They established the relationships between fracture geometry, fracture velocity and energy-release rate. Their experiments were with 2-D plates of uniform, brittle materials, and we intend to expand this analysis to granular, heterogeneous materials, and to explore the effects of 3-D structures. We will develop geometric criteria for dynamic fracturing in rocks in laboratory tests, and will apply these to resolve a set of open questions related to joints and dikes in the field. Knowing growth velocity of fractures is important for practical purposes like seismic hazard evaluation, rockburst risk and geotechnical stability estimates.

Funded by BSF, 1999-2002.

Principal investigators: Reches Ze'ev, Germanovich Leonid and Fineberg Jay

Graduate student: Sagy Amir (Ph.D.)

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The Dead Sea basin (DSB) is a large pull-apart associated with the 105km of left-lateral slip along the Dead Sea transform. The DSB is an elongated depression, about 150km long and up to 25km wide. The present structural analysis of the western margins of the DSB indicates distinct strain partitioning: the horizontal component of slip is accommodated along a subvertical fault zone inside the basinal depression, whereas the vertical subsidence is accommodated by a belt of large normal faults that forms the basin margins.

This belt of faults trends in a N-S direction, it is about 100km long and 3-5km wide and its cumulative vertical displacement locally exceeds 10km. We analyzed the relationships between faults and joints of this belt. Joints were studied in 35 stations in which they were mapped at scale of 1:10 or measured along scanlines. Subsurface fractures were analyzed on CAST logs in one borehole. The regional, subvertical joints exhibit two prevailing sets, NNE and NNW, that display ribs, plumose and branching structures. The dominafaults ithe belt are oblique-normal with extensive zigzag segmentation. Four sets of segments in orthorhombsymmetry are recognized here; their orientations indicatethat they formed under 3D-strain state of vertical maximum shortening, horizontal N-S shortening and E-W extension.strikes of the fault sets are parallel to the strikes of the dominating joint sets, and these relations are explained by modeling the elastic stress fields associated with layer bending above zigzag fault systems. We also evaluated in-situ stress state within the belt from breakout analysis and stress-inversion of fault slip data.

The above structural features are compatible with E-W extension; however, none of them fits the N-S left-lateral shear of the Dead Sea transform. This strain partitioning is attributed to the following process: (1) subsidence of a "proto" DSB due to the pull-apart motion; (2) large scale bending of the margins of the Sinai-Israel subplate along the "proto" basin; (3) brittle yielding of the upper crust to develop a belt of normal faults; (4) continuous left-lateral slip and elongation of the pull-apart basin, and semicontemporaneous vertical subsidence along the normal fault. We use this model to evaluate the elastic thickness of the Sinai-Israel plate.

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