Gidon Baer

The lowest place on Earth is subsiding—An InSAR (interferometric synthetic aperture radar) perspective

Since the early 1990s, sinkholes and wide, shallow subsidence features (WSSFs) have become major problems along the Dead Sea shores in Israel and Jordan. Sinkholes are readily observed in the field, but their locations and timing are unpredictable. WSSFs are often difficult to observe in the field. However, once identified, they delineate zones of instability and increasing hazard. In this study we identify, characterize, and measure rates of subsidence along the Dead Sea shores by the interferometric synthetic aperture radar (InSAR) technique. We analyze 16 SAR scenes acquired during the years 1992 to 1999 by the European Remote Sensing ERS-1 and ERS- 2 satellites. The interferograms span periods of between 2 and 71 months. WSSFs are observed in the Lisan Peninsula and along the Dead Sea shores, in a variety of appearances, including circular and elongate coastal depressions (a few hundred meters to a few kilometers in length), depressions in ancient alluvial fans, and depressions along salt-diapir margins. Phase differences measured in our interferograms correspond to subsidence rates generally in the range of 0–20 mm/yr within the studied period, with exceptional high rates that exceed 60 mm/yr in two specific regions. During the study period, the level of the Dead Sea and of the associated ground water has dropped by ∼6 m. This water-level drop within an aquifer overlying fine-grained, marly layers, would be expected to have caused aquifer-system consolidation, resulting in gradual subsidence. Comparison of our InSAR observations with calculations of the expected consolidation shows that in areas where marl layers are known to compose part of the upper 30 m of the profile, estimated consolidation settlements are of the order of the measured subsidence. Our observations also show that in certain locations, subsidence appears to be structurally controlled by faults, seaward landslides, and salt domes. Gradual subsidence is unlikely to be directly related to the sinkholes, excluding the use of the WSSFs features as predictable precursors to sinkhole formation.

Mechanisms of dike propagation in layered rocks and in massive, porous sedimentary rocks

The mechanisms of dike propagation are analyzed through detailed examination of small structures associated with the dike intrusion. Exposures in the Ramon area, Israel, offer an opportunity to evaluate theoretical models of tensile fracture propagation and to better understand the factors governing the dike geometry. Dike-related structures are studied in two Jurassic units: the Ardon Formation, a well-stratified sequence of shales and dolomites, and the Inmar Formation, which is composed of porous, massive sandstones with a few shale lenses. It is shown that for the different host rocks the propagation mechanisms and structures differ significantly. Dikes in the Ardon Formation are dominated by 1–20 m wide segments, confined to distinct layers. The mechanisms of segment containment are determined by calculating the stress intensity factor for an extension crack approaching a mechanical interface. It is apparent that dike segmentation and containment is controlled mainly by in situ stress and shear moduli differences between adjacent layers and partly by bedding plane slippage. Dikes in the Inmar Formation are dominated by smaller segments and by 1–10 cm wide and 10–100 cm long fingers with intermittent smooth patches and slickensides. Opposite dike walls display exact mirror images of these features. Dikes propagate in alternating stages of viscous flow and brittle deformation. The viscous stages include fluidization of the sandstone by the dike-related fluids and increase of the pore pressure in the rock. A viscous instability develops between the highly viscous fluidized sandstone and the less viscous dike-related fluids, forming a zone of viscous fingering in front of the dike. Stages of brittle fracturing occur when the pore pressure is released and the flux of fluids is insufficient to fluidize the rock.

Dikes emplaced into fractured basement, Timna Igneous Complex, Israel

Dikes are usually envisioned as arrays of parallel segments dilated perpendicular to the direction of the least compressive stress. We describe here four dikes of highly irregular shape intruded in the fractured basement in the Timna Igneous Complex, southern Israel. The dikes include a doleritic dike, 2.3 km long and 1.6 m to 32 m thick, and three andesitic dikes, up to 1.5 km long and 8 m thick. The dikes each display significant variations of dip (up to 60°), strike (up to 160°) and thickness. The thickness variations correlate better with the segment attitude than with the position along the dikes. We show that the irregular shapes of the Timna dikes are the result of emplacement into fractured host rock under different paleostress states and driving pressures. Three dilation styles that differ by the geometry of the initial cracks are analyzed: an array of randomly oriented cracks (style A), a single linear crack (style B), and an array of interconnected, nonparallel cracks (style C). The analysis of style A provides the stress state during dike emplacement, including the orientations of the three principal stresses (σ1 ≥ σ2 ≥ σ3), the stress ratio ϕ = (σ2 - σ3)/(σ1 - σ3), and the normalized driving pressure R = (Pm - σ3)/(σ1 - σ3). The stress ratio ϕ indicates the shape of the stress ellipsoid and it ranges from ϕ = 0 for σ2 = σ3 (prolate ellipsoid) to ϕ = 1 for σ1 = σ2 (oblate ellipsoid). The normalized driving pressure R indicates the relative magnitude of the internal magma pressure Pm with respect to the tectonic stresses, and it ranges from R = 0 for Pm = σ3 to R = 1 for Pm = σ1. We found that for three dikes in Timna, ϕ ∼ 0.25, indicating small differences between the two horizontal principal stresses, and for one dike ∼ 0.9, indicating a large difference between the two horizontal principal stresses. The normalized driving pressure R is about 0.08 in two horizontally propagating dikes and about 0.25 in two vertically propagating dikes. Style B predicts an elliptical thickness profile along the dike due to dilation of a linear crack; this prediction agrees with the profile of one of the dikes. The predicted thicknesses due to dilation of the interconnected array of cracks (style C) are in good agreement with the thickness variations of the doleritic dike, and in fair agreement with two of the andesitic dikes. Deviations from the ideal geometry suggest separate stages of propagation and dilation in some of the dike segments.

Coseismic deformation associated with the November 1995, MW=7.1 Nuweiba earthquake, Gulf of Elat (Aqaba), detected by synthetic aperture radar interferometry

The November 22, 1995, MW = 7.1 Nuweiba earthquake occurred along one of the left-stepping segments of the Dead Sea Transform in the Gulf of Elat (Aqaba). Although it was the largest earthquake along this fault in the last few centuries, little is yet known about the geometry of the rupture, the slip distribution along it, and the nature of postseismic deformation following the main shock. In this study we examine the surface deformation pattern during the coseismic phase of the earthquake in an attempt to better elucidate the earthquake rupture process. As the entire rupture zone was beneath the waters of the Gulf, and there is very little Global Positioning System (GPS) data available in the region for the period spanning the earthquake, interferometric synthetic aperture radar (INSAR) provides the only source of information of surface deformation associated with this earthquake. We chose four synthetic aperture radar (SAR) scenes of about 90 × 90 km each spanning the rupture area, imaged by the ERS-1 and ERS-2 satellites. The coseismic interferograms show contours of equal satellite-to-ground range changes that correspond to surface displacements due to the earthquake rupture. Interferograms that span the earthquake by 1 week show similar fringe patterns as those that span the earthquake by 6 months, suggesting that postseismic deformation is minor or confined to the first week after the earthquake. A high displacement gradient is seen on the western side of the Gulf, 20–40 km south of Elat and Aqaba, where the total satellite-to-ground range changes are at least 15 cm. The displacement gradient is relatively uniform on the eastern side of the Gulf, and the range changes are less than 10 cm. To interpret these results, we compare them to synthetic interferograms generated by elastic dislocation models with a variety of fault parameters. Although selecting the best fit fault parameters is nonunique, we are able to generate a group of simplified model interferograms that provide a reasonable fit to the coseismic interferogram and serve to constrain the location of the fault. The present analysis shows that if the rupture reached the Gulf-bottom surface, the mean sinistral slip along the fault is constrained to about 1.4 m. If surface rupture did not occur, the average sinistral slip is constrained to the range of 1.4–3 m for a fault patch buried 0–4 km below the Gulf-bottom surface, respectively, with a minor normal component.

Mechanics of emplacement and tectonic implications of the Ramon Dike Systems, Israel

A radial system comprising more than 200 basaltic and trachytic dikes and two minor systems of parallel dikes intruded the Ramon area, southern Israel, during the Early Cretaceous. Field relations between dikes and fractures in the radial system indicate that the dikes intruded self-generated fractures, and thus indicate the directions of the tectonic stresses. Other field observations indicate that the dikes propagated in subhorizontal directions up to distances of 15 km from their source. Our analysis of the emplacement mechanics of these dikes shows that the horizontal propagation is best explained by the density differences between the intruding magma and the host rocks. The measured mean density for basement rocks at depths greater than 2.4 km is 2.55±0.07 g/cm3, and it is 2.36±0.21 g/cm3 for the sedimentary cover above. For the magma to be propagating horizontally at its neutral buoyancy level requires a mean magma density of about 2.5 g/cm3. The large distance of horizontal propagation requires a low viscous pressure drop, below 0.1 MPa/km within the dike, and an overpressure of about 1 MPa in the magma chamber. We computed stress trajectories using a two-dimensional elastic model for a pressurized hole in a regional stress field and compared it to the Ramon radial system. The model reveals that the dikes originated at a central intrusion of about 3 km diameter and intruded under a predominantly radial state of stress with negligible regional stress field. This period of weak tectonic stresses and intensive magmatism falls between the early Mesozoic extensional regime and the late Mesozoic-Cenozoic compressional regime in Israel. The calculated center of the radial system is offset from a large magnetic anomaly south of the Ramon area, suggesting a 3 km right-lateral displacement along the Ramon fault after the intrusion of the radial system.

Sinkhole precursors along the Dead Sea, Israel, revealed by SAR interferometry

The water level in the Dead Sea (Israel and Jordan) has been dropping at an increasing rate since the 1960s, exceeding one meter per year during the last decade. This drop has triggered the formation of sinkholes and widespread land subsidence along the Dead Sea shoreline, resulting in severe economic loss and infrastructural damage. In this study, the spatiotemporal evolution of sinkhole-related subsidence and the effect of human activities and land perturbation on sinkhole development are examined through interferometric synthetic aperture radar measurements and field surveys conducted in Israel during 2012. Interferograms are generated using COSMO-SkyMed satellite images and a high-resolution (0.5 m/pixel) elevation model obtained from LiDAR measurements. As a result of this unique combination of high-resolution data sets, millimeter-scale subsidence has been resolved in both natural and human-disturbed environments. Precursory subsidence over a period of a few months occurred before the collapse of all three sinkhole sites reported in this study. The centers of the subsiding areas migrated, possibly due to progressive dissolution and widening of the underlying cavities. Filling of newly formed sinkholes with gravel, and mud injections into drill holes, seem to enhance land subsidence, enlarge existing sinkholes, and form new sinkholes. Apart from shedding light on the mechanical process, the results of this study may pave the way for the implementation of an operational sinkhole early-warning system.

Mechanical modeling and InSAR measurements of Mount Sedom uplift, Dead Sea basin: Implications for effective viscosity of rock salt

We present a mechanical model for the growth of an emerging salt diapir in a tectonically active basin. The analytical model is applied to and serves to constrain the effective viscosity of rock salt and strain rates during diapirism of the wall-shaped Mount Sedom rock salt diapir, Dead Sea basin. The model is based on one-dimensional flow of Newtonian viscous fluid (salt) in a vertical channel that has been driven by the load of the overburden and affected by shear along the channel walls. Because the Poiseuille (channel) flow profile is parabolic and the Couette (shear) flow profile is linear, a one-dimensional model provides three sets of predicted profiles: topography, uplift rate, and shear strain. The present topography of Mount Sedom represents the shape of the Sedom diapir, and hence the effective viscosity of rock salt can be constrained by a model that best fits the present topography of the mountain. The resulting Sedom rock salt viscosity is determined to be between 2 and 3 × 1018 Pa s, and the associated strain rate is between 5 and 6 × 10−13 s−1. Geological structures indicate strain rates of 9 × 10−13 s−1 and 3 × 10−14 s−1 during the Holocene emerging stage and at the Plio-Pleistocene pre-emergent stage of the Sedom diapir, respectively. The uplift history of Mount Sedom predicted by the model and the current topography are compared to Interferometric Synthetic Aperture Radar (InSAR) measurements of salt uplift. The maximum uplift rates of Mount Sedom are 8.3 and 5.5 mm/yr for its northern and southern parts, respectively. The InSAR uplift profiles resemble topographic profiles obtained along the same traverses, implying that the uplift history during the last 14,000 years is stable. Steep uplift gradients observed by InSAR along the western margin of the diapir are higher than predicted by modeling of Newtonian viscous flow. This could imply that flow of power law viscous fluid may be more suitable than that of Newtonian viscous fluid for the Sedom rock salt at high strain rates above 8 × 10−13 s−1.

Damage zones around en echelon dike segments in porous sandstone

We investigate arrays of en echelon dike segments and their associated deformation in porous sandstone to infer the segmentation mechanism and the state of stress during dike emplacement. The en echelon arrays are interpreted as breakdown segments of planar parent dikes that propagated from greater depth under mixed-mode conditions. Typically, an array consists of either continuous nonoverlapping stepped segments (offset smaller than segment thickness) or overlapping connected segments (offset larger than segment thickness). The deformation associated with the nonoverlapping stepped segment arrays consists of newly documented fan-like patterns of deformation bands (lamellae of crushed detrital quartz grains), whereas the overlapping connected segment arrays consists of net-like patterns of deformation bands. Thus the patterns of deformation are related to offset geometry and are likely to be diagnostic of the states of stress. We simulated the stress and deformation fields around interacting breakdown segments by applying a continuum damage mechanics model. The simulation results mainly illustrate the stress dependence of the damage distribution and the sensitivity of the damage distribution to the geometry of the segment offset and the mutuality of segment propagation. By changing the applied stress and by controlling the segment tip growth, symmetric and asymmetric distributions of damage were produced. We describe which aspects of the generated damage zones satisfactorily correlate with field observations. Damage mechanics simulations are useful tools for studying the state of stress during dike emplacement.

Faults and their associated host rock deformation: Part I. Structure of small faults in a quartz–syenite body, southern Israel

We analyze pervasive and discontinuous deformation associated with small faults in a quartz – syenite body in southern Israel. The analysis includes detailed mapping, measurement of in-situ mechanical rock properties and microstructural study of the faults. The mapped faults have 1 – 100-m-long horizontal traces, consisting of linked, curved segments; the segmented nature of the faults is also apparent at the 1 – 10 mm scale. The observed deformation features are breccia, as well as intra- and inter-granular fractures; these features are accompanied by reduction of the Young modulus and uniaxial strength of the host rock. The deformation features are zoned from a central fault-core through a damage-zone to the protolith at distances of 0.05– 0.06 the fault length. Shear strains up to 300% were calculated from measured marker lines displacements and distortion in proximity to the faults. We argue here that the fault-related deformation during fault propagation is manifested by highly localized deformation in a process zone having a width of 0.001– 0.005 of the fault length (fault-related deformation due to subsequent slip along the existing faults is analyzed in Part II). The observed self-similarity of the discontinuities over five length orders of magnitude and the outstanding lack of tensile microcracks suggest fault initiation and growth as primary shear fractures. q 2003 Elsevier Science Ltd. All rights reserved.

The GIL network of continuous GPS monitoring in Israel for geodetic and geophysical applications

Wdowinski, S., Bock, Y., Forrai, Y., Melzer, Y., and Baer, G. 2001. The GIL network of continuous GPS monitoring in Israel for geodetic and geophysical applications. Isr. J. Earth Sci. 50: 39–47. GIL (GPS in Israel) is a network of 12 continuous GPS stations, of which 11 stations are fully operational and one station is to be installed in 2002. The network provides a reference frame for precise GPS measurements in Israel and serves basic and applied geophysical research, including (1) monitoring plate motion and crustal deformation across the Dead Sea Fault, (2) mapping atmospheric water vapor content, and (3) monitoring ionospheric total electron content. Results from 36 months of continuous GPS measurements reveal that the current displacement rate within the State of Israel is 1–4 mm/yr, reflecting interseismic deformation across the Dead Sea Fault due to 2–4 mm/yr of relative motion between Sinai and Arabia and possibly post-seismic deformation induced by the 1995 Nuweiba earthquake.