Hydraulic fracturing is often used in oil industry to incre ase the productivity of hydrocarbon reservoirs, which suffer hydrocarbon pressure and permeability of gas/oilbearing sediments. The generated hydraulic fracture is accompanied by shear cracks, which microseismic events usually with magnitude below -0.5 (Mw). These microseismic events can be monitored with OBS (Ocean Bottom Seismograph) in deep-ocean. We present in this work a numerical simulation of hypocentral errors and seismic image resolution of small fractures.

We present a crustal thickness map of Brazil and adjacent areas based on a compilation of data published in the literature as well as new measurements. We used crustal thicknesses mainly derived from seismic datasets such as deep seismic refraction experiments, receiver function analyses, and surface-wave dispersion velocities. Crustal thicknesses derived from modelling gravity anomalies commonly depend on assumptions, such as constant density contrast across the Moho interface, which are not always easily verifiable and were considered only along the continental shelf to fill large gaps in the seismic data. Our compilation shows that the crust in the stable continental area onshore has an average thickness of 39 +/- 5 km (1-sigma deviation) and that no clear difference can be observed between low altitude, intracratonic sedimentary basins, NeoProterozoic foldbelts (except for the Borborema Province), and cratonic areas. The thinnest crust is found in the Borborema Province of NE Brazil (30-35 km) and along a narrow belt within Tocantins Province (similar to 35 km), roughly parallel to the Eastern border of the Amazon craton, while the thickest crust is found in the Amazon and Sao Francisco cratons (41 +/- 4 km), and the Parana Basin (42 +/- 4 km). Both the Ponta Grossa and the Rio Grande Arches are areas of thinned crust, and the western border of the Brazilian platform, near the sub-Andean region, seems to be characterized by a crustal thickness of less than 40 km. Although sparse in data coverage, we expect the resulting crustal thickness map to be useful for future studies of isostasy, dynamic topography, and crustal evolution of the country. (C) 2013 Elsevier Ltd. All rights reserved.

O nível de atividade sísmica no Brasil é classificado como baixo, com ocorrência média de menos de dois sismos com magnitudes maiores que 4 mb por ano, um sismo com magnitude maior que 5 mb a cada seis anos, e um sismo de magnitude 6 mb a cada 45 anos. Mesmo com frequência muito baixa, não é impossível a ocorrência de sismos com magnitudes de até 7,5 mb, sendo que, nesse caso, o período de recorrência desse tipo de evento no Brasil é em torno de 885 anos. Neste trabalho iremos explorar a elaboração de cenários sísmicos considerando eventos com magnitudes comuns (m ≤ 5), moderadas (5 < m ≤ 6) e grandes (6 < m ≤ 7,5). Para satisfazer a curiosidade das pessoas e ajudar a entender a grandeza dos eventos sísmicos, também serão mostrados cenários sísmicos para eventos com magnitudes improváveis e extremas (maiores que 8 MW) sobre regiões densamente povoadas e com um grande número de obras importantes, como usinas hidrelétricas, nucleares e grandes fábricas.

Knowing the best 1D model of the crustal and upper mantle structure is useful not only for routine hypocenter determination, but also for linearized joint inversions of hypocenters and 3D crustal structure, where a good choice of the initial model can be very important. Here, we tested the combination of a simple GA inversion with the widely used HYPO71 program to find the best three-layer model (upper crust, lower crust, and upper mantle) by minimizing the overall P- and S-arrival residuals, using local and regional earthquakes in two areas of the Brazilian shield. Results from the Tocantins Province (Central Brazil) and the southern border of the São Francisco craton (SE Brazil) indicated an average crustal thickness of 38 and 43 km, respectively, consistent with previous estimates from receiver functions and seismic refraction lines. The GA+HYPO71 inversion produced correct Vp/Vs ratios (1.73 and 1.71, respectively), as expected from Wadati diagrams. Tests with synthetic data showed that the method is robust for the crustal thickness, Pn velocity, and Vp/Vs ratio when using events with distance up to about 400 km, despite the small number of events available (7 and 22, respectively). The velocities of the upper and lower crusts, however, are less well constrained. Interestingly, in the Tocantins Province, the GA+HYPO71 inversion showed a secondary solution (local minimum) for the average crustal thickness, besides the global minimum solution, which was caused by the existence of two distinct domains in the Central Brazil with very different crustal thicknesses.

The quality control of seismic data is extremely important for the accuracy of seismological analysis. The software PQLX can be used to determine the efficiency of seismographic stations to perform continuous record, assess instrumental failures, signal quality, noise levels and errors in metadata. Furthermore it is a good tool to estimate spectral and identification of patterns of environmental seismic vibrations and its temporal variations. In the present work, we show the results of PQLX for three seismographic stations (APOB2, PAZB and PORB), with different problems. The good definition of the graph PDF with the data used, show how the software is effective in pointing out where seismographic stations need improvement.

Triggered seismicity is commonly associated with deep water reservoirs or injection wells where water is injected at high pressure into the reservoir rock. However, earth tremors related solely to the opening of groundwater wells are extremely rare. Here we present a clear case of seismicity induced by pore‐pressure changes following the drilling of water wells that exploit a confined aquifer in the intracratonic Paraná Basin of southeastern Brazil. Since 2004, shallow seismic activity, with magnitudes up to 2.9 and intensities V MM, has been observed near deep wells (120–200 m) that were drilled in early 2003 near the town of Bebedouro. The wells were drilled for irrigation purposes, cross a sandstone layer about 60–80 m thick and extract water from a confined aquifer in fractured zones between basalt flow layers. Seismic activity, mainly event swarms, has occurred yearly since 2004, mostly during the rainy season when the wells are not pumped. During the dry season when the wells are pumped almost continuously, the activity is very low. A seismographic network, installed in March 2005, has located more than 2000 microearthquakes. The events are less than 1 km deep (mostly within the 0.5 km thick basalt layer) and cover an area roughly 1.5 km × 5 km across. The seismicity generally starts in a small area and expands to larger distances with an equivalent hydraulic diffusivity ranging from 0.06 to 0.6 m 2 /s. Geophysical and geothermal logging of several wells in the area showed that water from the shallow sandstone aquifer enters the well at the top and usually forms waterfalls. The waterfalls flow down the sides of the wells and feed the confined, fractured aquifer in the basalt layer at the bottom. Two seismic areas are observed: the main area surrounds several wells that are pumped continuously during the dry season, and a second area near another well (about 10 km from the first area) that is not used for irrigation and not pumped regularly. The main area displays cyclic annual activity, but the second area does not. We explain the earthquake swarms as being triggered by pore pressure diffusion in the fractured basalt layer due to additional pressure from the newly connected surface aquifer. This reaches critically prestressed areas up to a few kilometers away from the wells. During periods of continuous pumping, the reduction of pore pressure in the confined aquifer stops the seismic activity. Our study suggests that this kind of activity may be more common than previously thought and implies that many other cases of small tremors associated with the drilling of water wells may have gone unnoticed.

The Earth's ambient noise has been used over the past four decades for sedimentary basin studies, but only in the past ten years have geophysicists begun to apply ambient seismic noise, a technique known as passive seismic, to hydrocarbon exploration.

Intraplate earthquakes in stable continental areas have been explained basically by reactivation of pre-existing zones of weakness, stress concentration, or both. Zones of weakness are usually identified as sites of the last major orogeny, provinces of recent alkaline intrusions, or stretched crust in ancient rifts. However, it is difficult to identify specific zones of weakness and intraplate fault zones are not always easily correlated with known geological features. Although Northeastern Brazil is one of the most seismically active areas in the country (magnitudes 5 roughly every 5 yr), with hypocentral depths shallower than ∼10 km and seismic zones as long as 30–40 km, no clear relationship with the known surface geology can be usually established with confidence, and a clear identification of zones of weakness has not yet been possible. Here we present the first clear case of seismic activity occurring as reactivation of an old structure in Brazil: the Pernambuco Lineament, a major Neoproterozoic shear zone. The 2004 earthquake swarm of Belo Jardim (magnitudes up to 3.1) and the recurrent activities in the nearby towns of São Caetano and Caruaru (magnitudes up to 4.0 and 3.8), show that the Pernambuco Lineament is a weak zone. A local seismic network showed that the Belo Jardim swarm of 2004 November occurred by normal faulting on a North dipping, E–W oriented fault plane in close agreement with the E–W trending structures within the Pernambuco Lineament. The Belo Jardim activity was concentrated in a 1.5 km (E–W) by 2 km (downdip) fault area, and average depth of 4.5 km. The nearby Caruaru activity occurs as both strike-slip and normal faulting, also consistent with local structures of the Pernambuco Lineament. The focal mechanisms of Belo Jardim, Caruaru and S. Caetano, indicate E–W compressional and N–S extensional principal stresses. The NS extension of this stress field is larger than that predicted by numerical models such as those of Coblentz & Richardson and we propose that additional factors such as flexural stresses from the nearby Sergipe-Alagoas marginal basin could also affect the current stress field in the Pernambuco Lineament.

Shallow seismic activity has been observed in the Bebedouro rural area, Northeast of Sao Paulo State, Brazil, since 2004. A 6-station seismographic network, installed in March 2005, has recorded around 2,200 micro-earthquakes. The events with magnitudes up to 2.9 and intensities V MM (March-April 2005) are located near some groundwater wells (120-200m deep) drilled in early 2003 to extract water from a basalt fractured aquifer. The activity occurs as swarms of events mostly during the rainy season when the wells are not pumped. The third swarm occurred in April 2006 one month after four new wells were drilled. Migration of the epicenters away from the wells with the largest water flow, more than 150m/h, was clearly observed in 2005 with a “seismic diffusivity” of about 0.3 to 0.6m/s. The spatio-temporal evolution of the seismicity shows it was triggered by the drilling and operation of the waterwells.

Como se pode observar nas Figuras 4 e 6, é difícil evi-denciar qual é o mecanismo focal mais adequado para a região. Dados geológicos mostram que os movimentos antigos são compatíveis com o segundo caso, mas os epicentros próximos a estação BJ14 podem ter sido gerados pela mesma falha E-W evidenciada na Figura 4. Ambos os mecanismos focais estudados satisfazem bem os dados observados, embora o segundo conjunto de soluções seja mais compatível com a solução independente obtida apenas com a distribuição dos focos.

Considerando que a porosidade e a permeabilidade de um aqüífero fissural dependem da distribuição, abertura, e tamanho das fissuras do basalto (Feitosa e Manoel F°, 2000), é possível inferir que poços tubulares com vazões tão altas como as observadas (150-190 m3/h, poços P10 e P07) (ver Fig. 1), sejam indicativos de alta concentração de fraturas ou fendas no basalto na área epicentral.

Tremores de terra têm ocorrido no município de Bebedouro, Nordeste da Bacia do Paraná. O primeiro enxame de eventos foi sentido de janeiro a julho de 2004 e o segundo, de dezembro de 2004 a abril de 2005. Estações sismográficas instaladas na região registraram centenas de sismos, que atingiram magnitudes de até 2,9. Cobrem a área sedimentos da Formação Adamantina e o basalto da Formação Serra Geral, onde ocorrem os tremores. Os epicentros estão localizados nas proximidades de poços tubulares de 120-220 m de profundidade, perfurados em 2003 e 2004, que exploram água das fraturas do basalto. A atividade sísmica tende a ocorrer nas estações chuvosas, quando os poços não são bombeados. É possível que exista uma relação entre sismos e poços, considerando a proximidade dos epicentros e as ocorrências cíclicas dos tremores. Entretanto, para confirmar essa possibilidade e melhorar a determinação epicentral, serão necessários outros levantamentos geofísicos como os sísmicos e as perfilagens de poço, incluindo a térmica. Um mecanismo possível para a indução de tremores é quando a perfuração permite comunicação entre o aqüífero e fraturas secas. A pressão da água agindo sobre zonas fraturadas, previamente sob esforços, pode causar sismos. Dois casos comprovados de sismicidade induzida por poços ocorreram nas regiões nordeste e central da Bacia do Paraná, relacionados com poços de 200 m e 1.800 m, respectivamente.

Em julho de 2003 foram sentidos tremores no município de Correntina, BA.Alguns meses depois são instaladas quatro estações sismográficas na região epicentral, e são registrados mais de cem eventos com magnitudes (mD) entre 0,2 e 1,5, vários deles com ondas de superfície bem nítidas.Com dados de polaridade das ondas P e S, e modelagem das ondas de superfície obteve-se um modelo de velocidades para a porção mais rasa da crosta superior, e determinou-

Genetic Algorithms (GA) have been playing an increasingly important role on geophysical problem solving. GA was used in the inversion of hypocenter determination residues of seven regional seismic events in an structural model of the crust and upper mantle at Goiás region. The determination of the hypocenters was made with the arrival times of P and S waves of the seven events, according to crustal models evaluated by a computational implementation of GA. A brief discussion is made about the advantages and disadvantages of a blind search method (GA) compared with a full search in controlled space. These two methods present equivalent results, which show a 38 km thick crust, with P wave velocity of 8.2 km in the upper mantle and Vp/Vs ratio of 1.75. These values are consistent with a Pre-Cambrian shield region.