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Tectonic Stress (tectonic + stress)

Distribution by Scientific Domains

Earth and Environmental Science	83%

Selected Abstracts

A Bayesian approach to estimating tectonic stress from seismological data

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2007
Richard Arnold
SUMMARY Earthquakes are conspicuous manifestations of tectonic stress, but the non-linear relationships between the stresses acting on a fault plane, its frictional slip, and the ensuing seismic radiation are such that a single earthquake by itself provides little information about the ambient state of stress. Moreover, observational uncertainties and inherent ambiguities in the nodal planes of earthquake focal mechanisms preclude straightforward inferences about stress being drawn on the basis of individual focal mechanism observations. However, by assuming that each earthquake in a small volume of the crust represents a single, uniform state of stress, the combined constraints imposed on that stress by a suite of focal mechanism observations can be estimated. Here, we outline a probabilistic (Bayesian) technique for estimating tectonic stress directions from primary seismological observations. The Bayesian formulation combines a geologically motivated prior model of the state of stress with an observation model that implements the physical relationship between the stresses acting on a fault and the resultant seismological observation. We show our Bayesian formulation to be equivalent to a well-known analytical solution for a single, errorless focal mechanism observation. The new approach has the distinct advantage, however, of including (1) multiple earthquakes, (2) fault plane ambiguities, (3) observational errors and (4) any prior knowledge of the stress field. Our approach, while computationally demanding in some cases, is intended to yield reliable tectonic stress estimates that can be confidently compared with other tectonic parameters, such as seismic anisotropy and geodetic strain rate observations, and used to investigate spatial and temporal variations in stress associated with major faults and coseismic stress perturbations. [source]

Non-double-couple mechanisms in the seismicity preceding the 1991,1993 Etna volcano eruption

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2001
A. Saraò
Summary The temporal evolution of the complete source moment tensor is investigated for 28 earthquakes that occurred at Mt Etna in the period August 1990,December 1991 preceding the biggest eruption of the last three centuries. We perform several tests to check the robustness of the results of inversion considering different frequency ranges and different groups of stations. As well as the selection of good-quality data, the error analysis, statistically significant at the 95 per cent confidence level, is employed to validate the findings of the inversion and to distinguish between physical solutions and artefacts of modelling. For events between 0.3 and 10 km depth, strike-slip mechanisms prevail on normal, inverse and dip-slip mechanisms; this is possibly due to the dyke-induced stress dominating the overall stress field at the surface, producing a continuous switch of the tensile and compressive axes. The regional E,W tension prevails at depth, as indicated by the prevalence of normal mechanisms. An increment of the non-double-couple components is observed immediately before the eruption and can be related to movements of fluids, even though, for some events, the complex interaction between tectonic stress and volcanic activity cannot be excluded. The source time functions retrieved are in general simple and short but some show complexities, as one would expect in volcanic seismicity. From the seismic scalar moment found, we extrapolate an empirical moment,magnitude relation that we compare with other relations proposed for the same area and computed for the duration magnitude and the equivalent Wood,Anderson magnitude. [source]

Transpressional tectonics of the Mineoka Ophiolite Belt in a trench,trench,trench-type triple junction, Boso Peninsula, Japan

ISLAND ARC, Issue 4 2005
Ryota Mori
Abstract Structures developed in metamorphic and plutonic blocks that occur as knockers in the Mineoka Ophiolite Belt in the Boso Peninsula, central Japan, were analyzed. The aim was to understand the incorporation processes of blocks of metamorphic and plutonic rocks with an arc signature into the serpentinite mélange of the Mineoka Ophiolite Belt in relation to changes in metamorphic conditions during emplacement. Several stages of deformation during retrogressive metamorphism were identified: the first faulting stage had two substage shearing events (mylonitization) under ductile conditions inside the crystalline blocks in relatively deeper levels; and the second stage had brittle faulting and brecciation along the boundaries between the host serpentinite bodies in relatively shallower levels (zeolite facies). The first deformation occurred during uplift before emplacement. The blocks were intensively sheared by the first deformation event, and developed numerous shear planes with spacing of a few centimeters. The displacement and width of each shear plane were a few centimeters and a few millimeters, respectively, at most. In contrast, the fault zone of the second shearing stage reached a few meters in width and developed during emplacement of the Mineoka Ophiolite. Both stages occurred under a right-lateral transpressional regime, in which thrust-faulting was associated with strike-slip faulting. Such displacement on an outcrop scale is consistent with the present tectonics of the Mineoka Belt. This implies that the same tectonic stress has been operating in the Boso trench,trench,trench-type triple junction area in the northwest corner of the Pacific since the emplacement of the Mineoka Ophiolite. The Mineoka Ophiolite Belt must have worked as a forearc sliver fault during the formation of a Neogene accretionary prism further south. [source]

Clarification of regional and local in situ stresses using the compact conical-ended borehole overcoring technique and numerical analysis

ISLAND ARC, Issue 3 2003
Seong-Seung Kang
Abstract Stress measurement is performed to estimate the states of in situ rock stress at the Torigata open-pit limestone mine in Japan using the compact conical-ended borehole overcoring (CCBO) technique. A set of back and forward analyses are then carried out to evaluate the states of regional and local in situ rock stresses and the mine-induced rock slope stability using a 3-D finite element model. The maximum horizontal local in situ rock stress measured by the CCBO technique acts in the northeast,southwest direction. The horizontal regional tectonic stresses obtained by the back analysis are in good agreement with those of the horizontal local in situ rock stress measured by the CCBO technique. However, the horizontal regional tectonic stress is more compressive than the horizontal local in situ rock stress. This is because the horizontal regional stress due to gravity is not considered in the back-analyzed horizontal regional tectonic stress, but it is included in the local in situ rock stress measured by the CCBO technique. The local stress obtained by the forward analysis, especially its horizontal components, is in good agreement with the horizontal local in situ rock stress measured by the CCBO technique, and the magnitude of the vertical normal stress increases more rapidly than those of the horizontal normal stresses with depth. As a result, the ratio of the horizontal normal stress to the vertical normal stress is largest at the nearest excavation level and decreases with depth. This means that the stress field within the mine-induced rock slope is affected by the horizontal components of the local in situ rock stress. [source]

Exploitation of High-Yields in Hard-Rock Aquifers: Downscaling Methodology Combining GIS and Multicriteria Analysis to Delineate Field Prospecting Zones

GROUND WATER, Issue 4 2001
Patrick Lachassagne
Based on research work in the Truyàre River catchment of the Massif Central (Lozàre Department, France), a methodology has been developed for delineating favorable prospecting zones of a few square kilometers within basement areas of several hundred, if not thousand, square kilometers for the purpose of siting high-yield water wells. The methodology adopts a functional approach to hard-rock aquifers using a conceptual model of the aquifer structure and of the functioning of the main aquifer compartments: the alterites (weathered and decayed rock), the underlying weathered-fissured zone, and the fractured bedrock. It involves an economically feasible method of mapping the thickness and spatial distribution of the alterites and the weathered-fissured zone, on which the long-term exploitation of the water resource chiefly depends. This method is used for the first time in hydrogeology. The potential ground water resources were mapped by GIS multicriteria analysis using parameters characterizing the structure and functioning of the aquifer, i.e., lithology and hydrogeological properties of the substratum, nature and thickness of the alterites and weathered-fissured zone, depth of the water table, slope, fracture networks and present-day tectonic stresses, and forecasted ground water quality. The methodology involves a coherent process of downscaling that, through applying methods that are increasingly precise but also increasingly costly, enables the selection of sites with diminishing surface areas as the work advances. The resulting documents are used for ground water exploration, although they can also be applied to the broader domain of land-use management. [source]