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The Lower TagusValley is located in the Lower TagusSedimentary Basin. Occurrence ofearthquakes in the area indicates thepresence of seismogenic structures at depththat are still deficiently known due to athick Cenozoic sedimentary cover. It isusually assumed that regional M = 6–7earthquakes are generated by the LowerTagus Valley fault. However, this structurehas never been characterised, and there isevidence for the presence of other faultsthat have the potential of generating largeseismic events. The hidden character ofmany of the structures in the Lower TagusBasin necessitates the use of indirectmethodologies for their study. Availableseismic reflection lines acquired for oilexploration in the Lower Tagus Valleyregion were thus examined, with emphasis onthe recognition of faults displacing theCenozoic and particularly the youngergeological units. The foreseen complexityof the basin structure was confirmed.Evidence for a continuous `Lower TagusValley fault' is absent. Instead, N-S toNNE-SSW trending, en echelon,sub-vertical and reverse faults showingsignificant post-Miocene offsets, linked bytransverse (NW-SE) faults that accommodatelarge displacements, were recognised in theseismic sections that cross the Tagusvalley. These faults range in length from10 km to a maximum of ca. 40 km, leading toan estimation of 6.2–6.9 maximum credibleearthquake magnitudes, which fall in themagnitude range of the regional historicalseismicity. 相似文献
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E. Matias Schneiter Eduardo de la Fuente Pablo F. Velázquez 《Monthly notices of the Royal Astronomical Society》2006,371(1):369-374
We present a single-explosion model study of the 3C 400.2 supernova remnant (SNR), made with two-dimensional axisymmetric simulations. The numerical simulations were made with the adaptive grid code yguazú-a . Several initial scenarios have been tested in order to reproduce the centrally peaked X-ray emission observed for this remnant. This study reveals that the explosion of the SN inside a pre-existing stellar wind bubble successfully generates the observed morphology, when thermal conduction is included in the model. The best morphological fit is obtained at an evolution time of 21 000 yr, when the total luminosity is 1.2 × 1034 erg s−1 . 相似文献
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N.A. Dias L. Matias N. Loureno J. Madeira F. Carrilho J.L. Gaspar 《Tectonophysics》2007,445(3-4):301-317
The Azores archipelago (Portugal) is located on an oceanic plateau, in a geodynamic environment prone to intense seismic and volcanic activity. In order to investigate the crustal structure in this region, we have conducted a local earthquake tomography study in the area of the islands of Faial, Pico and S. Jorge using data recorded in July 1998. The July 9th 1998 earthquake, near Faial Island, triggered an aftershock sequence of thousands of events that lasted for several months and were recorded by a total of 14 stations located on the three islands surrounding the epicentral area. In the upper crustal layers, consistency is seen between the tomographic results and the islands' surface volcanic units. Beneath the Faial central volcano a low Vp (< 6.0 km/s) anomaly roughly located at 3–7 km depth, suggests a connection to the plumbing system, possibly the presence of a magma chamber. In NE Faial, a high Vp (> 6.3 km/s) body was found located at mid-lower crust, most likely an intrusion of gabbroic composition, that is bordered by the registered seismic activity; its shape suggesting a tectonic controlled mechanism. The relocated hypocenters, together with the overall analysis of the Tomographic model, suggest a tectonic segmentation of Faial Island. The crustal thickness under the islands volcanic buildings of the Faial–Pico area was estimated at around 14 km. 相似文献
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Omar Benazzouz Luis M. Pinheiro Luis M. A. Matias Alexandra Afilhado Daniel Herold Seth S. Haines 《Mathematical Geosciences》2018,50(5):569-584
The positioning of ocean bottom seismometers (OBS) is a key step in the processing flow of OBS data, especially in the case of self popup types of OBS instruments. The use of first arrivals from airgun shots, rather than relying on the acoustic transponders mounted in the OBS, is becoming a trend and generally leads to more accurate positioning due to the statistics from a large number of shots. In this paper, a linearization of the OBS positioning problem via the multilateration technique is discussed. The discussed linear solution solves jointly for the average water layer velocity and the OBS position using only shot locations and first arrival times as input data. 相似文献
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This study investigates sediment transport at a very low‐energy backbarrier beach in southern Portugal, from a spring‐to‐neap tide period, during fair‐weather conditions. Rates and directions of transport were determined based on the application of fluorescent tracer techniques. Wind and currents were collected locally, whereas the dominant small and short‐period wind waves were characterized using a morphodynamic modelling system coupling a circulation model, a spectral wave model, and a bottom evolution model, well validated over the study area. For the recorded conditions sediment transport was small and ebb oriented, with daily transport rates below 0.02 m3 day‐1. Tidal currents (mainly ebb velocities) were found to be the main causative forcing controlling sediment displacements. Transport rates were higher during spring tides, tending towards very small values at neap tides. Results herein reported points towards the distinction between tracer advection and tracer dispersion in this type of environment. Transport by advection was low as a consequence of the prevailing hydrodynamic conditions (Hs < 0.1 m, and max. current velocity of 0.5 m s‐1) and the tracer adjustment to the transport layer, whereas dispersion was relatively high (few metres per day). Tracer techniques allowed distinguishing the broad picture of transport, but revealed the need for refinement in this type of environments (bi‐directional forcing by ebb and flood cycles). Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
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D. Dusunur J. Escartín V. Combier T. Seher W. Crawford M. Cannat S. C. Singh L. M. Matias J. M. Miranda 《Marine Geophysical Researches》2009,30(2):105-120
The crust at mid-ocean ridges is formed through a combination of magmatic and tectonic processes. Along slow-spreading ridges,
magmatism is inferred to be discontinuous and episodic, and lithospheric faulting may strongly interact with the melt supply
system. These interactions can be studied for the first time at the Lucky Strike segment along the Mid-Atlantic Ridge (MAR),
where a 3.4 km deep magma chamber (AMC) extending ~6 km along-axis is found at its centre (Singh et al. in Earth Planet Sci
Lett 246:353–366, 2006). With an array of ocean bottom seismometers we have detected along this ridge segment approximately 400 microseismic events
during a total of 6 days, and located 71 of them, whose local magnitudes ranged from 0.2 to 1.8. While most of the events
were concentrated at non-transform offset and inside corners, three events with well-constrained locations were detected beneath
the central volcano and at the edges of the AMC. Two of the microearthquakes, which occur in a brittle lithosphere and therefore
at temperatures lower than 750°C, are deeper than the AMC and therefore very steep thermal gradients both along- and across-axis.
Regionally seismicity deepens from ~6 km at the segment center to >10 km towards the ends. 相似文献
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