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Horizontal Stress (horizontal + stress)
Selected AbstractsPresent-day stress in the surroundings of 2009 L'Aquila seismic sequence (Italy)GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2010Maria Teresa Mariucci SUMMARY The axial zone of the Apenninic belt in central Italy is a tectonically active region affected by post-orogenic Quaternary extension. The present-day stress field is characterized by a minimum horizontal stress (Shmin) , NE,SW oriented, derived mainly from earthquake focal mechanisms and secondarily from borehole breakouts and fault data. The paper describes the computation of the Shmin orientation along two deep boreholes located in the vicinity of the area hit by the 2009 April 6, Mw 6.3 L'Aquila earthquake. The analysed wells show breakout zones at a depth range between 1.4 and 4.6 km, giving precious information on a depth interval usually not investigated by any other data. The results show an Shmin N81 ± 22° and N74 ± 10° oriented for Varoni 1 and Campotosto 1 wells, respectively. The comparison among the breakouts, the 2009 seismic sequence, the past seismicity and the Quaternary faults indicates a small rotation of Shmin orientation from , NE, in the southern, to , ENE in the northern sector of the study area, where the wells are located. These differences are linked both to the natural variations of data and to the orientation of the main tectonic structures varying from NW,SE in the Abruzzi region to , N,S moving toward the Umbro-Marchean Apennines. The identification of constant Shmin orientations with depth derived from all the examined active stress data, confirms the breakouts as reliable stress indicators also for aseismic areas. [source] Strike-slip earthquakes in the oceanic lithosphere: observations of exceptionally high apparent stressGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2002George L. Choy Summary The radiated energies, ES, and seismic moments, M0, for 942 globally distributed earthquakes that occurred between 1987 to 1998 are examined to find the earthquakes with the highest apparent stresses (,a=,ES/M0, where , is the modulus of rigidity). The globally averaged ,a for shallow earthquakes in all tectonic environments and seismic regions is 0.3 MPa. However, the subset of 49 earthquakes with the highest apparent stresses (,a greater than about 5.0 MPa) is dominated almost exclusively by strike-slip earthquakes that occur in oceanic environments. These earthquakes are all located in the depth range 7,29 km in the upper mantle of the young oceanic lithosphere. Many of these events occur near plate-boundary triple junctions where there appear to be high rates of intraplate deformation. Indeed, the small rapidly deforming Gorda Plate accounts for 10 of the 49 high- ,a events. The depth distribution of ,a, which shows peak values somewhat greater than 25 MPa in the depth range 20,25 km, suggests that upper bounds on this parameter are a result of the strength of the oceanic lithosphere. A recently proposed envelope for apparent stress, derived by taking 6 per cent of the strength inferred from laboratory experiments for young (less than 30 Ma) deforming oceanic lithosphere, agrees well with the upper-bound envelope of apparent stresses over the depth range 5,30 km. The corresponding depth-dependent shear strength for young oceanic lithosphere attains a peak value of about 575 MPa at a depth of 21 km and then diminishes rapidly as the depth increases. In addition to their high apparent stresses, which suggest that the strength of the young oceanic lithosphere is highest in the depth range 10,30 km, our set of high- ,a earthquakes show other features that constrain the nature of the forces that cause interplate motion. First, our set of events is divided roughly equally between intraplate and transform faulting with similar depth distributions of ,a for the two types. Secondly, many of the intraplate events have focal mechanisms with the T -axes that are normal to the nearest ridge crest or subduction zone and P -axes that are normal to the proximate transform fault. These observations suggest that forces associated with the reorganization of plate boundaries play an important role in causing high- ,a earthquakes inside oceanic plates. Extant transform boundaries may be misaligned with current plate motion. To accommodate current plate motion, the pre-existing plate boundaries would have to be subjected to large horizontal transform push forces. A notable example of this is the triple junction near which the second large aftershock of the 1992 April Cape Mendocino, California, sequence occurred. Alternatively, subduction zone resistance may be enhanced by the collision of a buoyant lithosphere, a process that also markedly increases the horizontal stress. A notable example of this is the Aleutian Trench near which large events occurred in the Gulf of Alaska in late 1987 and the 1998 March Balleny Sea M= 8.2 earthquake within the Antarctic Plate. [source] Hydraulic pathways in the crystalline rock of the KTBGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2000Günter Zimmermann Fracture systems and fluid pathways must be analysed in order to understand the dynamical processes in the upper crust. Various deterministic as well as stochastic fracture networks in the depth section of the Franconian Lineament (6900 to 7140 m), which appears as a brittle ductile shear zone and prominent seismic reflector, were modelled to simulate the hydraulic situation at the two boreholes of the Continental Deep Drilling Program (KTB). They led to estimations of the hydraulic permeability in crystalline rock. The geometrical parameters of the fractures, such as fracture locations and orientations, were determined from structural borehole measurements, which create an image of the borehole wall. The selection of potentially open fractures was decided according to the stress field. Only fractures with the dip direction (azimuth) of the fracture plane perpendicular to the maximum horizontal stress field were assumed to be open. The motivation for this assumption is the fact that the maximum horizontal stress is higher than the vertical stress from the formation, indicating that the state of stress is a strike-slip faulting. Therefore, the probability of open fractures due to this particular stress field at the KTB sites is enhanced. Length scales for fracture apertures and extensions were stochastically varied and calibrated by hydraulic experiments. The mean fracture aperture was estimated to be 25 ,m, assuming an exponential distribution, with corresponding permeability in the range of 10,16 m2. Similar results were also obtained for log-normal and normal distributions, with a variation of permeability of the order of a factor of 2. The influence of the fracture length on permeability of the stochastic networks was also studied. Decreasing the fracture length beyond a specific threshold of 10 m led to networks with vanishing connectivity and hence vanishing permeability. Therefore, we assume a mean fracture length exceeding the threshold of 10 m as a necessary assumption for a macroscopic hydraulically active fracture system at the KTB site. The calculated porosity due to the fracture network is of the order of 10,3 per cent, which at first sight contradicts the estimated matrix porosity of 1 to 2 per cent from borehole measurements and core measurements. It can be concluded from these results, however, that if the fluid transport is due to a macroscopic fracture system, only very low porosity is needed for hydraulic flow with permeabilities up to several 10,16 m2, and hence the contribution of matrix porosity to the hydraulic transport is of a subordinate nature. [source] Active Faulting Pattern, Present-day Tectonic Stress Field and Block Kinematics in the East Tibetan PlateauACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 4 2009Yueqiao ZHANG Abstract: This paper examines major active faults and the present-day tectonic stress field in the East Tibetan Plateau by integrating available data from published literature and proposes a block kinematics model of the region. It shows that the East Tibetan Plateau is dominated by strike-slip and reverse faulting stress regimes and that the maximum horizontal stress is roughly consistent with the contemporary velocity field, except for the west Qinling range where it parallels the striking of the major strike-slip faults. Active tectonics in the East Tibetan Plateau is characterized by three faulting systems. The left-slip Kunlun-Qinling faulting system combines the east Kunlun fault zone, sinistral oblique reverse faults along the Minshan range and two major NEE-striking faults cutting the west Qinling range, which accommodates eastward motion, at 10,14 mm/a, of the Chuan-Qing block. The left-slip Xianshuihe faulting system accommodated clockwise rotation of the Chuan-Dian block. The Longmenshan thrust faulting system forms the eastern margin of the East Tibetan Plateau and has been propagated to the SW of the Sichuan basin. Crustal shortening across the Longmenshan range seems low (2,4 mm/a) and absorbed only a small part of the eastward motion of the Chuan-Qing block. Most of this eastward motion has been transmitted to South China, which is moving SEE-ward at 7,9 mm/a. It is suggested from geophysical data interpretation that the crust and lithosphere of the East Tibetan Plateau is considerably thickened and rheologically layered. The upper crust seems to be decoupled from the lower crust through a décollement zone at a depth of 15,20 km, which involved the Longmenshan fault belt and propagated eastward to the SW of the Sichuan basin. The Wenchuan earthquake was just formed at the bifurcated point of this décollement system. A rheological boundary should exist beneath the Longmenshan fault belt where the lower crust of the East Tibetan Plateau and the lithospheric mantle of the Yangze block are juxtaposed. [source] Stresses at sites close to the Nojima Fault measured from core samplesISLAND ARC, Issue 3-4 2001Kiyohiko Yamamoto Abstract The Nojima Fault in Awaji, Hyogo prefecture, Japan, was ruptured during the 1995 Hyogo-ken Nanbu earthquake (MJMA = 7.2). Toshima is located close to the fault segment, in which a large dislocation has been observed on the Earth's surface. Ikuha is near the southern end of the buried fault that extends from the surface rupture. Stresses are measured on core samples taken at depths of 310 m, 312 m and 415 m at Toshima and a depth of 351 m at Ikuha. The measured stresses show that both sites are in the field of a strike,slip regime, but compression dominates at Toshima. Defining the relative shear stress as the maximum shear stress divided by the normal stress on the maximum shear plane, the relative shear stress ranges from 0.42 to 0.54 at Toshima and is approximately 0.32 at Ikuha. While the value at Ikuha is moderate, those at Toshima are comparably large to those in areas close to the inferred fault of the 1984 Nagano-ken Seibu earthquake. Value amounts greater than 0.4 suggest that there are areas of large relative shear stress along faults, thus having the potential to generate earthquakes. Provided that the cores are correctly oriented, the largest horizontal stresses at shallow depths are in the direction from N113°E to N139°E at Toshima and N74°E at Ikuha, indicating that the fault does not orient optimally for the stress field at both sites. The slip is known to be predominant in the right-lateral strike,slip component. Although this slip may appear contradictory to the stress field at Toshima, the slip direction is found to be parallel to the measured stresses resolved on the fault plane for the first approximation. The ratio of shear stress to normal stress on the fault plane is roughly estimated to be greater than zero and smaller than 0.3 near Toshima. [source] | ||||||||||