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Earthquake Focal Mechanisms (earthquake + focal_mechanism)
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] Simplified algorithms for calculating double-couple rotationGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2007Yan Y. Kagan SUMMARY We derive new, simplified formulae for evaluating the 3-D angle of earthquake double-couple (DC) rotation. The complexity of the derived equations depends on both accuracy requirements for angle evaluation and the completeness of desired solutions. The solutions are simpler than my previously proposed algorithm based on the quaternion representation designed in 1991. We discuss advantages and disadvantages of both approaches. These new expressions can be written in a few lines of computer code and used to compare both DC solutions obtained by different methods and variations of earthquake focal mechanisms in space and time. [source] A Bayesian approach to estimating tectonic stress from seismological dataGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2007Richard 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] Inversion of earthquake focal mechanisms to obtain the seismotectonic stress IV,a new method free of choice among nodal planesGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2002Jacques Angelier Summary A new method is presented, to obtain the stress state that best accounts for a set of double couple focal mechanisms of earthquakes. This method is based on the slip shear stress component (SSSC) criterion. The sum of the SSSC values is maximized as a function of four unknowns that describe the reduced stress tensor, including the orientations of the principal stress axes and the ratio between the principal stress differences. This new method combines two advantages. First, no choice between the nodal planes of each focal mechanism is needed, because of the intrinsic properties of the SSSC. Secondly, the runtime is negligible regardless of the size of the data set, because the inverse problem is solved by analytical means so that the numerical aspects are reduced to a minimum. For these reasons, the SSSC-based inversion is easily included in a variety of processes for separating or refining the data. A typical set of focal mechanisms of earthquakes in Taiwan is processed to illustrate the application and potential of the new method. [source] Analyses of the stress field in southeastern France from earthquake focal mechanismsGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2001Emmanuel Baroux Summary Owing to the apparent deformation field heterogeneity, the stress regimes around the Provence block, from the fronts of the Massif Central and Alpine range up to the Ligurian Sea, have not been well defined. To improve the understanding of the SE France stress field, we determine new earthquake focal mechanisms and compute the present-day stress states by inversion of the 89 available focal mechanisms around the Provence domain, including 17 new ones calculated in the current study. This study provides evidence of six distinct deformation domains around the Provence block, with different tectonic regimes. On a regional scale, we identify three zones characterized by significantly different stress regimes: a western one affected by an extensional stress (normal faulting) regime; a southeastern one characterized by a compressional stress (reverse to strike-slip faulting) regime with NNW- to WNW-trending ,1; and a northeastern one, namely the Digne nappe front, marked by a NE-trending compression. Note that the Digne nappe back domain is controlled by an extensional regime that is deforming the western Alpine core. This extensional regime could be a response to buoyancy forces related to the Alpine high topography. The stress regimes in the southeast of the Argentera Massif and around the Durance fault are consistent with a coherent NNW-trending ,1, implying a left-lateral component of the active reverse oblique slip of the Moyenne Durance Fault. In the Rhone Valley, an E-trending extension characterizes the tectonic regime, implying a normal component of the present-day N,^mes fault displacement. This study provides evidence for short-scale variation of the stress states, which arises from abrupt changes in the boundary force influences on upper crustal fragments (blocks). These spatial stress changes around the Provence block result from the coeval influence of forces applied at both its extremities, namely in the northeast the Alpine front push, and in the southeast the northward African plate drift. In addition to these boundary forces, the mantle plume under the Massif Central influences the western block boundary. [source] Origin of the in situ stress field in south-eastern AustraliaBASIN RESEARCH, Issue 3 2004Mike Sandiford The in situ stress field of south-eastern Australia inferred from earthquake focal mechanisms and bore-hole breakouts is unusual in that it is characterised by large obliquity between the maximum horizontal compressive stress orientation (SHmax) and the absolute plate motion azimuth. The evolution of the neotectonic strain field deduced from historical seismicity and both onshore and offshore faulting records is used to address the origin of this unusual stress field. Strain rates derived from estimates of the seismic moment release rate (up to ,10,16 s,1) are compatible with Quaternary fault,slip rates. The record of more or less continuous tectonic activity extends back to the terminal Miocene or early Pliocene (10,5 Ma). Terminal Miocene tectonic activity was characterised by regional-scale tilting and local uplift and erosion, now best preserved by unconformities in offshore basins. Plate-scale stress modelling suggests the in situ stress field reflects increased coupling of the Australian and Pacific Plate boundary in the late Miocene, associated with the formation of the Southern Alps in New Zealand. [source] | ||||||||||