25 research outputs found
Numerical simulation of site effects in the upper Aterno Valley array during the aftershock sequence of the 2009 L'Aquila earthquake
No full textOn April 6th, 2009 a Mw=6.3 earthquake jolted the Abruzzo region of Central Italy, very close to the urban center of L’Aquila.
Availability of high-quality recordings of the mainshock along with several aftershocks makes the seismic sequence the best recorded near-source events in Italy. In the present study, attention is devoted to the strong motion recordings of the upper Aterno River valley array, which is part of the Italian Accelerometric Network (Rete Accelerometrica Nazionale, RAN), deployed NW of L’Aquila. These data provide a better understanding of the role played by site effects in the seismic response of the valley in epicentral area. This was accomplished by comparing recordings with the results of 1D and 2D site response analyses. The subsoil model of the Aterno valley passing through the accelerometric stations was assumed from a previous study and was integrated with the results of dynamic tests carried out on reconstituted samples of coarse materials frequently encountered in the subsoil. First, the ground surface motion computed by assuming linear soil behavior was compared to the small-magnitude (ML=3-3.5) aftershocks recordings. It was found that 2D modeling provides a satisfactory understanding of the amplification phenomena in the array. Moreover, 2D analyses performed slightly better than 1D predictions. Based on this calibration study, further site response analyses were carried out and the computed ground motion was compared with the aftershock recordings of moderate magnitude (MW=4-5.6). In contrast, the results from these events do not show the analogous performance as obtained in the linear range. More specifically, shape of acceleration response spectra is generally satisfactorily simulated whereas discrepancies are observed in terms of
PGA as well as maximum spectral amplitude. It is speculated on the possible explanations of these discrepancies
Dynamic modeling of neotectonics and contemporary stress field in the India-Eurasia collision zone
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Seismic Hazard in the Himalayan Intermontane Basins: An Example from Kathmandu Valley, Nepal
No full textDetrital zircon ages and provenance of Neogene foreland basin sediments of the Karnali River section, Western Nepal Himalaya
No full textSite Characteristics of Kathmandu Valley from Array Microtremor Observations
No full text Array microtremor observations were conducted in Kathmandu Valley close to six seismic stations. Sedimentary layers from surface to deep basement rock were modeled according to the derived velocity structures for site response analysis. The records in horizontal component from the 2015 Gorkha earthquake main shock at deep sedimentary sites were compared with the predictions from analysis using the records from a shallow sedimentary site as input motions. Generally, the comparisons are in good agreement where spectral amplification at long periods and suppression at short periods could be justified by the velocity models. </jats:p
Fault development in the Thakkhola half graben : insights from numerical simulation
No full textGrabens of southern Tibet and the Himalaya represent the Cenozoic extensional tectonic phase, which has affected whole Tibet and northernmost part of the Himalaya. Thakkhola half graben, on the crest of the Himalaya, is one of the north trending grabens that define the Neogene structural pattern of the southern margin of the Tibetan Plateau and is seemingly enigmatic feature in a regionally contractional tectonic setting between the colliding plates. Two-dimensional, elastic, plane-strain, finite element models (FEMs) are generated to simulate the effects of geometry and rock properties on growth of graben faults and their configuration for the Thakkhola half graben evolution. The performed models have shown that the extensional graben faults form at the top of the overburden and propagate downward as we increased extensional displacement. Simulated models have clearly defined the graben bounding faults. Further they are able to suggest that natural grabens have multiple faults on each side rather than single fault. During progressive extensional displacement depth of faulting increases and deformation is mainly localized in downthrown block both in basement and syntectonic deposits. Syntectonic deposit is characterized by normal faulting in tensional tectonic stress field, which is a common feature of the small-scale graben at post rift deformation stage. The width and depth of graben are primarily controlled by the rheology of the upper elastic layer and syntectonic deposits. The applied rock layer properties are able to deduce the first order characteristics of the Thakkhola half graben. Therefore, this simulation constrains probable values for the rock layer properties controlling the Thakkhola half graben evolution. Assumption of the weak zone does not make significant difference on stress distribution and faulting. Thus it seems that the Thakkhola fault system only did not contribute to development of the half graben.紀要論
Numerical Modelling of Graben Faults with Special Reference to Thakkhola Half Graben, Central Nepal Himalaya
No full textThakkhola half graben is a product of Late Cenozoic extensional tectonics in the Himalaya-Tibetan region. A series of 2D finite element models are generated to simulate its first order characteristics using stress field and induced fault pattern as structural proxies. Extensional graben faults form in the upper weak layer and propagate downward with increasing extension. Clusters of failure elements at the two ends of the graben directly correspond to the graben bounding faults and the asymmetric feature is characterized by uneven development of faults therein. The syntectonic deposits are characterized by normal fauls in the tensional stress field, which is a common feature of the small-scale graben at post rift deformation stage. The proposed models suggest that depth of a graben growth fault is primarily controlled by the initial fault and density of the syntectonic deposits. Assumption of a weak zone (Thakkhola fault system) does not make significant difference in stress distribution and faulting. It is thus suggested that a weak zone only can not contribute to the development of a half graben. The spreading boundary condition could not simulate the existing fault configuration in the Thakkhola half graben. Our modelling results for the graben suggest gravitational collapse of the elevated plateau rather than mantle upwelling during spreading.論
FE modeling of contemporary tectonic stress in the India-Eurasia collision zone
No full textAn understanding of orientation of contemporary maximum horizontal compressive stress (σ_) is important to many aspects of earth sciences, e.g. seismicity, neotectonics, and plate driving mechanisms. Comparison of recent stress observations and the results of stress modeling provide a powerful approach to refine our understanding of geodynamics processes. This is especially important for complex area like Himalayan-Tibet orogen, a continental\ncollision zone between the Indian and Eurasian Plates. The frequently occurring earthquakes\nand other tectonic stress indicator have provided vast set of database on maximum\nhorizontal compressive stress (σ_) that can be useful to study contemporary stress\nsources, plate kinematics and ongoing geodynamics. In this contribution, taking advantage of elastic plane stress finite element modeling (FEM), and observed data on σ_, several models are presented to reproduce stress field. Simulated models show that the convergence normal to the orogen is essential to reproduce observed σ_, which in turn controls the magnitude and orientation of σ_. The kinematics equivalent to east-west tectonic escape did not reproduce the observed stress field. Therefore, the best-fit model of present day stress field is obtained only in three domains model with southeastward tectonic escape of\nthe Tibetan crust rather than eastward extrusion. There is, however, significant increase in σ_ magnitude with increasing crustal depth because of stress amplification.\nIncorporation of suture zones in the model did not change orientation of σ_, significantly.\nConsidering these facts, 'continuum tectonic model' is more preferable than the 'block\ntectonic model' for the active deformation of the Tibetan Plateau. Contemporary stress field\ndeduced from several tectonic stress markers reconciles with the predicted one giving\ninsights on their sources and ongoing plate kinematics of the continental collision zone\nbetween Indian and Eurasian Plates.紀要論
