Theory of effective stress in soil and rock and implications for fracturing processes: A review

The effective stress principle (ESP) plays a basic role in geology and engineering prob-lems as it is involved in fundamental issues concerning strain and failure of rock and soil, as well as of other porous materials such as concrete, metal powders, biological tissues, etc. Although since its introduction in the 1920s the main ESP aspects have been unravelled and theoretically derived, these do not appear to have been always entirely perceived by many in the science community dealing with ESP‐related topics but having little familiarity with the complex theories of porous media and poroelasticity. The purpose of this review is to provide a guidance for the reader who needs an updated overview of the different theoretical and experimental approaches to the ESP and related topics over the past century, with particular reference to geological fracturing pro-cesses. We begin by illustrating, after some introductive historical remarks, the basic theory un-derlying the ESP, based on theory of elasticity methods. Then the different ESP‐related theories and experimental results, as well as main interpretations of rock jointing and fracturing phenom-ena, are discussed. Two main classical works are then revisited, and a rigorous ESP proof is de-rived. Such a proof is aimed at geologists, engineers and geophysicists to become more familiar with theories of porous media and poroelasticity, being based on the classical theory of elasticity. The final part of this review illustrates some still open issues about faulting and hydraulic fracturing in rocks

The ms 6.9, 1980 irpinia earthquake from the basement to the surface: a review of tectonic geomorphology and geophysical constraints, and new data on postseismic deformation

The MS 6.9, 1980 Irpinia earthquake occurred in the southern Apennines, a fold and thrust belt that has been undergoing post-orogenic extension since ca. 400 kyr. The strongly anisotropic structure of fold and thrust belts like the Apennines, including late-orogenic low-angle normal faults and inherited Mesozoic extensional features besides gently dipping thrusts, result in a complex, overall layered architecture of the orogenic edifice. Effective decoupling between deep and shallow structural levels of this mountain belt is related to the strong rheological contrast produced by a fluid-saturated, shale-dominated mélange zone interposed between buried autochthonous carbonates—continuous with those exposed in the foreland to the east—and the allochthonous units. The presence of fluid reservoirs below the mélange zone is shown by a high VP /VS ratio—which is a proxy for densely fractured fluid-saturated crustal volumes—recorded by seismic tomography within the buried autochthonous carbonates and the top part of the underlying basement. These crustal volumes, in which background seismicity is remarkably concentrated, are fed by fluids migrating along the major active faults. High pore fluid pressures, decreasing the yield stress, are recorded by low stress-drop values associated with the earthquakes. On the other hand, the mountain belt is characterized by substantial gas flow to the surface, recorded as both distributed soil gas emissions and vigorous gas vents. The accumulation of CO2-brine within a reservoir located at hypocentral depths beneath the Irpinia region is not only interpreted to control a multiyear cyclic behavior of microseismicity, but could also play a role in ground motions detected by space-based geodetic measurements in the postseismic period. The analysis carried out in this study of persistent scatterer interferometry synthetic aperture radar (PS-InSAR) data, covering a timespan ranging from 12 to 30 years after the 1980 mainshock, points out that ground deformation has affected the Irpinia earthquake epicentral area in the last decades. These ground motions could be a result of postseismic afterslip, which is well known to occur over years or even decades after a large mainshock such as the 23 November 1980, MS 6.9 earthquake due to cycles of CO2-brine accumulation at depth and its subsequent release by Mw ≥ 3.5 earthquakes, or most likely by a combination of both. Postseismic afterslip controls geomorphology, topography, and surface deformation in seismically active areas such as that of the present study, characterized by ~M 7 earthquakes. Yet, this process has been largely overlooked in the case of the 1980 Irpinia earthquake, and one of the main aims of this study is to fill such the substantial gap of knowledge for the epicentral area of some of the most destructive earthquakes that have ever occurred in Italy

Miocene fault-controlled sedimentation and thrust propagation in the previously faulted external zones of the Umbria-Marche Apennines, Italy

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Limitations posed by free DEMs in watershed studies: The case of river Tanaro in Italy

Topography is a critical element in the hydrological response of a drainage basin and its availability in the form of digital elevation models (DEMs) has advanced the modeling of hydrological and hydraulic processes. However, progress experienced in these fields may stall, as intrinsic characteristics of free DEMs may limit new findings, while at the same time new releases of free, high-accuracy, global digital terrain models are still uncertain. In this paper, the limiting nature of free DEMs is dissected in the context of hydrogeomorphology. Ten sets of terrain data are analyzed: the SRTM GL1 and GL3, HydroSHEDS, TINITALY, ASTER GDEM, EU DEM, VFP, ALOS AW3D30, MERIT and the TDX. In specific, the influence of three parameters are investigated, i.e., spatial resolution, hydrological reconditioning and vertical accuracy, on four relevant geomorphic terrain descriptors, namely the upslope contributing area, the local slope, the elevation difference and the flow path distance to the nearest stream, H and D, respectively. The Tanaro river basin in Italy is chosen as the study region and the newly released LiDAR for the Italian territory is used as benchmark to reassess vertical accuracies. In addition, the EU-Hydro photo-interpreted river network is used to compare DEM-based river networks. Most DEMs approximate well the frequency curve of elevations of the LiDAR, but this is not necessarily reflected in the representation of geomorphic features. For example, DEMs with finer spatial resolution present larger contributing areas; differences in the slope can reach 10%; between 5 m and 12 m H, none of the considered DEMs can faithfully represent the LiDAR; D presents significant variability between DEMs; and river network extraction can be problematic in flatter terrain. It is also found that the lowest mean absolute error (MAE) is given by the MERIT, 2.85 m, while the lowest root mean square error (RMSE) is given by the SRTM GL3, 4.83 m. Practical implications of choosing a DEM over another may be expected, as the limitations of any particular DEM in faithfully reproducing critical geomorphic terrain features may hinder our ability to find satisfactory answers to some pressing problems

PS-inSAR data analysis: Pre-seismic ground deformation in the 2009 l’aquila earthquake region

The accuracy of the millimetre-scale measurements made so far by the SAR systems, as well as the multi-temporal analysis methodologies, have provided impressive images of surface displacements in areas affected by strong earthquakes, and contributed to constrain the geometric and kinematic features of earthquake generating faults. The multi-temporal analysis of InSAR data is also being acknowledged as promising for the search of earthquake precursors. We have applied the multi-temporal PS-InSAR technique to the detection of pre- to post-seismic ground displacements in the region struck by the normal faulting 2009 L’Aquila earthquake. We have used ERS and ENVISAT PS-data sets from both ascending and descending orbits, covering a 20-year long time span. On the yearly-scale, we have identified a pre-seismic displacement pattern, which consists of opposite vertical motions that have affected the blocks in the hanging wall and footwall of the structure that is recognised as the surface trace of the earthquake-generating fault. In particular, we have highlighted a pre-seismic uplift for 4-5 years followed by subsidence (starting 6-8 months prior to the earthquake) of the hanging wall block, coeval to opposite vertical motions of the footwall block. We suggest that such a displacement pattern may represent an earthquake precursor signal

Disentangling multipole resonances through a full x-ray polarization analysis

Complete polarization analysis applied to resonant x-ray scattering at the Cr K-edge in K2CrO4 shows that incident linearly polarized x-rays can be converted into circularly polarized x-rays by diffraction at the Cr pre-edge (E = 5994 eV). The physical mechanism behind this phenomenon is a subtle interference effect between purely dipole (E1-E1) and purely quadrupole (E2-E2) transitions, leading to a phase shift between the respective scattering amplitudes. This effect may be exploited to disentangle two close-lying resonances that appear as a single peak in a conventional energy scan, in this way allowing to single out and identify the different multipole order parameters involved.Comment: 6 pages, 6 figure

Non-volcanic CO2 and CH4 degassing in an actively extending orogen, southern Apennines, Italy

The southern Apennines fold and thrust belt has been undergoing post-orogenic extension since ca. 700 kyr. Crustal extension controls active tectonics and seismogenesis in the mountain chain [1], with seismicity being characterized by low to moderate magnitude events punctuated by strong earthquakes [2]. Effective decoupling between deep and shallow structural levels is related to the strong rheological contrast produced by a fluid-saturated, clay-rich mélange zone interposed between buried autochthonous carbonates – continuous with those exposed in the Apulian foreland – and the allochthonous units. This mélange zone also acts as a seal preventing the migration of deep-seated aqueous fluids – as well as oil in the Basilicata region, which hosts the largest Europe’s onshore oil fields – towards the surface. On the other hand, the mountain belt is characterized by substantial gas flow, recorded as both distributed soil gas emissions and vigorous gas vents, associated with active faults at the surface. We measured a CO2 flux up to 34000 g/m-2 per day at a gas vent, as well as large amounts of He (up to 52 ppm), Rn (up to 228 kBq/m3) and CH4 (up to 5000 ppm). Overpressured CO2, which has been proposed as triggering normal fault earthquakes in the Apennines, has been interpreted as mostly of mantle origin. However, our new results from isotope analyses carried out on the carbon contained in both CO2 and CH4 indicate a dominant thermogenic origin for these gases, probably associated with the emplacement of magmatic sills within the lower section of the thick carbonate platform succession occurring at the base of the sedimentary cover in the southern Apennines. Our results bear major implication concerning the postulated occurrence of crustal faults allowing fluids to migrate directly from mantle depths to the surface