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9th International Congress of the Brazilian Geophysical Society
- Conference date: 11 Sep 2005 - 14 Sep 2005
- Location: Salvador, Bahia, Brazil
- Published: 11 September 2005
381 - 400 of 462 results
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O Uso do Geóide no Estudo de Mecanismos de Compensação Isostática
More LessIn this paper, we present some equations for the determination of the models of isostatic compensation, where geoid anomalies can be applied. This technique is based on the admittance function, which is related with geoid anomalies in the frequency domain by a simple mathematical formulation.
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Paleomagnetism of the post-Paleozoic alkaline magmatism in the Brazilian Platform: discussing ages and plate displacements
More LessA review of the available paleomagnetic data on the post-Paleozoic alkaline rocks in the Brazilian Platform in combination with other South American poles of same age allow the proposition of a new apparent polar wander path (APWP) revealing rotations of the plate associated with the emplacement of alkaline provinces. By means of an analysis of the magnetization polarity of the rocks some inferences of the relative ages of the igneous complexes were possible, as the investigated time interval comprises the long normal polarity interval of the Cretaceous (Cretaceous Normal Superchron). Absolute reconstructions of the drift movement of South America was achieved by means of paleomagnetic rotations and longitude control through the sea floor magnetic anomalies.
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Moho depth estimative at Balbina lake area, Amazonas, Brazil
Authors Cristiano Chimpliganond and Maria Fernanda Novo BarbosaThe Balbina Reservoir belongs to Eletronorte hydroelectric power company and is located at Amazonas State, Brazil. It’s volume is of 17,5 km3 and the dam has 42 m high, and the triggered seismicity started 2,5 years after the start of impoundment of the lake (1987). The broadband seismic station BALB was installed at the area in April 1998, equipped with a Guralp 40T sensor and a Reftek data logger sampling continuously at 100 samples per second. The Balbina Lake is located at the Amazonian Craton, between the geochronological provinces Amazonia Central (2.5 Ga) and Ventuari-Tapajós (1.95-1.8 Ga). The first comprise Paleoproterozoic felsic to intermediate volcanic rocks and granitoids, and the later granite-gnaisses (Tassinari & Macambira, 2004).
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Determinação dos parâmetros de fonte do sismo de 10/03/98 de Porto dos Gaúchos/MT, por análise espectral de sinais
More LessBrune’s source model is employed to determine the source parameters (stress-drop, seismic moment, peak frequency) and the seismic moment magnitude of the Porto dos Gauchos/MT main shock, occurred on March 10, 1998. The source parameters were determined from the spectral analysis of digital waveforms recorded by seven stations of a regional seismographic network. For all stations were determined displacement spectrum and from those the source parameters. The final values were taken as the average of the results. Three different types of magnitudes are presented: body wave magnitude (mb = 5.1), macrosseismic magnitude (mb = 5.1) and seismic moment magnitude (Mw = 5.2).
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Evolution of the Areado/MG seismic sequence - started in January, 2004
More LessA microearthquake seismic sequence, in course in the neighborhoods of the little Town of Areado, located in the southern part of Minas Gerais State is studied. This sequence started on January 21, with a 3.4 regional magnitude felt with a MMI V. After that, a local seismograph network with up to six stations detected more than nine hundred microearthquakes, 458 of these located using data of at least three components stations. A composite focal mechanism indicates a dextral transcurrent fault in NNE-SSW direction, in accordance with a hypocentral distribution of the microearthquakes. Results of preliminary studies of that sequence were presented in the first Symposium of the Brazilian Geophysical Society (SBGf) occurred in São Paulo in September 2004. In these studies, it was presented the analysis results of the data generated by the seismic network from January 28 to May 28. This paper intends to update the results presented on 2004 SBGf Symposium, in a more conclusive way, showing seismicity and evolution from January 28, 2004 to May 28, 2005.
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ESTUDO DE REFRAÇÃO SÍSMICA NA REGIÃO QUE ABRANGE A ESTAÇÃO SISMOGRÁFICA DE ANGRA DOS REIS (ESAR)
Authors Sabrina de Souza Lima Pierobou and Jesus BerrocalThe project studies the structural characteristics of the main layers of the crust in the region of the Seismographic Station of Angra dos Reis (ESAR), through the method of deep seismic refraction (RSP), defining the value of some seismological parameters, which are used for epicentral determination of the local seismic activity.
To complete the study of crust (it wasn’t possible to get all the RSP points wanted), the method Receiver Function was used to characterize the structure of the crust under ESAR. The results of both methods were used in the program TVEL that helped modeling the missing part of the RSP line. This model allowed to get relations between distance and the time S-P of the arrival of these waves.
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Estudos preliminares da anisotropia sísmica em Senador Sá-CE
In the João Câmara and Açu areas, NE Brazil, the Precambrian fabric has been interpreted as predominantly controlling the crustal seismic anisotropy. This interpretation is due the polarization of the fastest S wave splitting which is parallel to Precambrian lineaments. The seismic activity in Senador Sá occurred between two shear zones. Initially, two stations presented the NE and NNE polarization; and delay time 8.16 ms/km and 2.56 ms/km. The investigation of regional lineaments shows that these features present a NE trend, which is compatible with our preliminary seismic anisotropy study. A detailed study using other stations located between the shear zones could shed further light on this issue and confirm or not the Precambrian-controlled seismic anisotropy in NE Brazil.
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Mecanismo Focal do Terremoto Principal de 10 de Março de 1998, Porto dos Gaúchos, Mato Grosso, Brasil
Authors Takato Nakayoshi and Vasile I MarzaThe active earthquake area of Porto dos Gaúchos, Mato Grosso state, Brazil, hereinafter called the Seismogenic Zone of Porto dos Gaúchos (whose Portuguese acronym is ZSPG) is placed at junction of the southern boundary of the Amazonian Craton with the northern part of the Parecis Basin. This area represents one of the most outstanding active seismic areas of Brazil intraplate seismicity, where it occurred in 1955 the largest known Brazilian earthquake and where recently, on March 23, 2005, happened another significant earthquake with magnitude 5.0 mb (USGS). The ZSPG quake of March 10, 1998, reaching a magnitude of 5.3 mb(ISC) is the second largest event of the ZSPG and the largest seismic event during the recent times of digital seismology, hence supplying a great amount of useful data due to the large number of seismographic stations recording it. Using the polarities of the incident P waves at 12 world-wide seismographic stations it was possible to workout the focal mechanism of the 1998 mainshock. This mechanism shows a predominant strike slip fault plane with a small reverse component. The preferred slipping plane has a strike of 303°, coincident with the kilometer extension lineaments (faults and shear zones) observed in the Amazonian Craton and with the structure of the Parecis Basin and also matching early aftershock lineament of the 1998 mainshock. The inferred fault mechanism is compatible with a model presenting an approximately EW compression coinciding with the direction of the predominant regional tectonic stresses acting in the South America Tectonic Plate.
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Representation of the Earth’s Gravity Field in the Southern part of the African Plate
Authors Ahmed M. A. Meguid and Eder C. MolinaA new digital gravity field model (SAGM2005) is presented, representing the free air anomaly and the height anomaly in the southern part of the African plate. A total of 12,203 gravity observations together with 5,486 free air GRACE-only data points were used in the process of Least-Squares Collocations (LSC) in order to carry the present representation out. A new computer algorithm was written, as a Matlab application, to calculate the observed covariance function as a primary step of the collocation procedure. The LSC method was applied as described by Moritz (1980) assuming a constant standard error of 3 mGal to the free air data points. The present work aimed to show the effect of introducing GRACE-only data with land observations on the precision of the calculated digital models, which represent the gravity field components in the studied area with spatial resolution of 0.1°, which is quite suitable to be used in regional studies.
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On gravity influence in fault patterns formation and westward displacement of the lithosphere
More LessMotivated by the lack of a unified theory for the driving force that causes plates movements on Earth and based on the success of some simple models to predict general features of Earth's seismic development we introduce outer gravitational fields as the possible energy providers for such enormous and permanent phenomena. Some known results on faults preferential orientations might give support to the hypothesis. Further experimental base for the consideration of external gravitational fields is supplied by the almost null tectonic activity nearby Earth's poles.
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Aspectos da evolução termo-mecânica da Bacia do Parnaíba
More LessIn this paper, we intend to contribute to the a better assessment of the thermo-mechanical evolution of the Parnaíba Basin and to investigate its capability for hydrocarbons generation and accumulation. The study of intracontinental sedimentary basins, such as Parnaíba, is also useful for the understanding of the thermo-mecanical evolution of the continental lithosphere. Based on the non-uniform lithospheric stretching model of Royden and Keen (1980), backstripped subsidence and paleotemperatures curves were constructed for 20 PETROBRÁS boreholes using the program BASTA (Friedinger, 1988) modified by De Sousa (1996).
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Comportamento da Anomalia Magnética do Atlântico Sul para os últimos 105 anos
Authors Gelvam A. Hartmann and Igor I. G. PaccaIn this paper, we discuss the characteristics of the South Atlantic Magnetic Anomaly (SAMA) for the last century. Some authors relate the origin of the SAMA to the offset of the eccentric dipole of the Earth’s magnetic field. There are several ways for defining the SAMA geographical location. We used in this work the minimum intensity values of the field. The change in the westward drift of the Anomaly may correspond to changes in the motion of the fluid in the outer core. The intensity curve shows three intervals with different behavior for the studied period. It may be speculated that these changes can be related to geomagnetic jerks. However, the SAMA and other similar anomalies should be studied in the thousands and the million years time scales in order to improve an interpretation in terms of the Geodynamo Theory.
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Numerical analysis of electromagnetoelastic waves propagation in heterogeneous media
Authors Lucas A. P. Gomes, Viatcheslav Priimenko and Mikhail VishnevskiiInteraction of electromagnetic fields with elastic bodies is the subject of many theoretical investigations in mechanics of continua for the last decades. Some variants of direct and inverse problems have been studied leading into the determination of some characteristics of a medium. It is very interesting to study this phenomenon due to the possibility of applying this theory to geophysical prospecting and study of earthquake sources. It is well known that when an electrical-conducting elastic body oscillates in an electromagnetic field, variations of the electrical and magnetic fields are observed as a result of this motion. Similar processes are also observed when seismic waves propagate in the Earth’s crust. Variations of the seismic and electromagnetic fields arising in this case are called electromagnetoelastic waves. Our work is dedicated to investigation of such interaction. The model considered here is based on a simple variant of combination of the Lamé and Maxwell equations. We form the basic equations for the description of some variant of this coupling and consider mathematical and numerical models of this process.
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CONSIDERAÇÕES SOBRE EVENTOS TERMOCINEMÁTICOS ASSOCIADO AO SOERGUIMENTO DE ÁREAS PRÉ CAMBRIANAS E A SUA RELAÇÃO COM A SUBSIDÊNCIA DA BACIA DO PARANÁ: TRAÇOS DE FISSÃO E PALEOTENSÕES
The Precambrian basement of Southeast Brazil along the northeast board of Paraná basin is characterized by many bittle reactivations of shear zones that developed during the end of the Brasiliano orogeny. This brittle activity occurred several times during the Phanerozoic, resulting in additional fault movements and uplift of the Pre-cambrian areas. To constrain the uplift (fission tracks) and paleostress phases of the Pre – cambrian areas, in the northeast Paraná basin margin, and establishment to relation with the subsidence events of the Paraná basin. The results of this study consists of two main domains, which record different episodes of uplift and reactivation of faults: The first domain is denominated Sorocaba, it is located near of the Paraná basin and consists of Neoproterozoic rocks and Paleozoic sedimentary units bounded by Campinas fault zone (NE), Jundiuvira fault zone (WNW) and Moreiras fault zone (NE); The second domain consists of Pre-cambrian rocks, Granites rocks and Tertiary sedimentary units, characterized by three principal faults Jundiuvira fault zone (EW), Camanducaia and Extrema fault zone (NE). This with many evidences of reativaction, where was recognized tectonic rocks as pseudotachylytes cataclasites and fault gouges. Based on fission track data, it is associated uplift of basement rocks around the NE margin of the Paraná Basin in southeast Brazil was related to the tectonic subsidence and consequent deposition of the Paraná basin. The evolution of this basin seems to have been strongly influenced by compressive and extensional events as suggested by Millani (1997) that developed during the subduction of Panthalassan plate under the southwestern Gondwana margin (Ramos,1988) These pulses are recorded as important unconformities in the stratigraphic framework of the Parana basin and the elucidation of the uplift and denudation histories of the basement shield is crucial to the understanding these tectonics effects.
Fission track studies (correct ages) indicate the correlation with significant events of cooling in 193Ma (Triassic); 126Ma (Early Cretacic); 89Ma (Late Cretacic) and 69Ma (Late Cretacic). The thermal histories are indicate tectonic uplift117 the Oligocene too.
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Recent progress in mesospheric gravity wave studies using nightglow imaging systems
During the past four decades a variety of optical remote sensing techniques have revealed a rich spectrum of wave activity in the upper atmosphere. Many of these perturbations, with periodicites ranging from ~5 min to several hours and horizontal scales of a few ten's of km to several thousands km, are due to freely propagating acoustic-gravity waves and forced tidal oscillations. Optical observations of the spatial and temporal characteristics of these waves in the mesosphere and lower thermosphere (MLT) region (~80-100 km) are facilitated by several naturally occurring, vertically distinct nightglow layers. This paper describes the use of state-of-the-art ground-based CCD imaging techniques to detect these waves in intensity and temperature. All-sky (180°) image measurements are used to illustrate the characteristics of small-scale, short period (< 1 hour) waves and to investigate their seasonal propagation and impact on the MLT region. These results will be contrasted with measurements of mesospheric temperature made using a separate imaging system capable of determining induced temperature amplitudes of much larger-scale wave motions and investigating night-to-night and seasonal variability in mesospheric temperature.
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A statistical study of plasma bubble zonal drift velocity variations with geomagnetic activity and events of corotating and transient solar wind streams
Authors P.M. Terra, P. M., C.G.M. Brum, C. G. M., J.H.A. Sobral, J. H. A., M.A. Abdu, M. A., J.R. Souza and H. TakahashiA study of the plasma bubble zonal drift velocities is carried out using experimental airglow data registered at Cachoeira Paulista (22.5° S, 45° W, dip 30° S) during the period from October to March, between 1980-1994. This study is based upon 109 nights of zonal scanning photometer measurements of OI 630 nm airglow. The zonal velocity magnitudes of the ionospheric bubbles are investigated taking into account the geomagnetic effects. The geomagnetic activity condition in this study is based on the 3-hour geomagnetic index Kp. A detailed analysis of these plasma bubble velocities, as a function of geomagnetic activity, shows that the velocity variations expressed by Vfinal – Vinitial are smaller during quiet nights than during disturbed nights. In addition, it is noted that the mean velocities present the highest (lowest) values during quiet (disturbed) nights of solar maximum (minimum). Also it is observed that the tendency of the plasma bubbles is to appear earlier during corotating stream events than transient or no stream periods.
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Muon and Neutron Observations in Connection with the Corotating Interaction Region
Ground cosmic ray observations are used for studying several kinds of interplanetary structures. The cosmic ray data has different responses to each kind of interplanetary structure. This article has as objective to study cosmic ray muon and neutron signatures due to the passage of Corotating Interaction Region (CIR) in the interplanetary medium, and identify the signatures in the cosmic ray data due to these events. The cosmic ray muon data used in this work are recorded by the multidirectional muon detector installed at INPE’s Observatório espacial do Sul – OES/CRSPE/INPE-MCT, in São Martinho da Serra (Brazil) and the neutron monitor installed in Newark (USA). The CIR events were selected in the period from 2001 to 2004.
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Some important factors that affect the Doppler properties of vertical m-size echoes in the low latitude E region
Authors J.-P. St.-Maurice and R. K. ChoudharyMeter size irregularities are routinely studied with radars in the equatorial and low latitude regions. In both instances echoes from the E region (90 to 120 km altitude) are a common occurrence. The resulting echoes are labeled as so-called Type I or Type II according to their spectral signature. In this paper we show that the phase velocity of Type I echoes increases with decreasing altitude owing to thermal feedback effects taking place in the growth process. We also show that Type II echoes can be influenced by atmospheric neutral winds to the point of revealing the presence of Kelvin-Helmholtz billows, as shown by a recently studied example taken from the Gadanki radar in India.
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Observações de ondas de gravidade mesosféricas de baixas frequências via keograma da aeroluminescência OH IR
Authors Delano Gobbi, Fábio Augusto Vargas and Hisao TakahashiContinuos observation of airglow emissions obtained by an all-sky imager (INPE), installed at Cachoeira Paulista (230 S), have been used to investigate the characteristics on the major low frequencies oscillations. The images of the Near Infrared OH emissions produce keograms constructed from a north-south and east-west slice from the middle of every image joined over the whole night. The keograms are a convenient way to reveal wave activities over the whole night, on the other hand, the individual images are the best way to see short period gravity waves.
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F-region nocturnal zonal plasma drift velocities of the brazilian equatorial ionosphere during maximum solar
This study focused on the nocturnal zonal plasma drift velocities of ionosphere (F region) at equatorial latitude regions of the Brazil. The zonal plasma drifts result from equatorial latitude electrodynamics with influences from the E and F region conductivities and neutral wind shears in altitude and latitude. The analysis is interpreted using the simple plasma drift model (Eccles, 1998a,b) and monochromatic digital images of OI630nm emission, which allow direct visualization of ionospheric bubbles. The digital images were obtained by optical imager system operating at São João do Cariri (equatorial geographic latitudes: 7.4ºS; 36.5ºW; 19oS dip latitude) from January to December 2001 (maximum solar activity). In this period, 48 quiet nights ( KP<24o) presented the signature of ionospheric bubbles and the zonal drift velocities were calculated (observational results). The geophysical conditions of each quiet night were considered in equations of the simple plasma drift model in order to obtain the theoretical zonal plasma drift velocities between 18LT and 6LT. The theoretical and observational results were grouped seasonally. The theoretical results showed two humps around 20LT-21LT and 2LT–4LT that were present in observational results.
Observational results were smaller (greater) than theoretical results before (after) 22LT-23LT being earlier in the Summer. The discrepancies between the theoretical and observed results apparently were due to
the neutral wind magnitudes provided by HWM93 empirical model. The Summer 2001 season presented the theoretical and observed results in better agreement considering the bars of variance.
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