Here we report the intraplate seismicity in the Pernambuco Lineaments (PeL), a Neoproterozoic continuous and vertical ductile shear zone, 700 km long and 5 km wide similar to 600 Ma in intraplate Brazil. We observed one fault segment in the PeL, here defined as the Sao Caetano segment, to assess (1) the seismogenic reactivation of the ductile Precambrian basement fabric and (2) the correlation between seismological and structural data derived from satellite imagery to understand fault reactivation, and (3) the identification of a complex multi-fault zone architecture revealed by our seismological observations. In the Sao Caetano Fault segment, the overall spatial distribution of the seismic clusters follows approximately an E-W direction with a 64 degrees dip towards the N. However, the observed multi-fault architecture revealed by the hypocentral locations exhibits different clusters that do not form a single fault structure. Hence, composite solutions are not necessarily the only representative focal mechanism of an entire fault zone. The inversion of the stress tensor using the focal mechanism from the present study is consistent with S-1 nearly vertical and S-3 in the E-W direction. This indicates that the interplay between the stress field, inherited structures, and seismicity can be more complex and vary at the local scale of 10s of kilometers. In this intraplate region, we show that the stress field is variable, with long-wavelength variations overprinted by more local and subtle changes, which has profound implications for intraplate seismic hazard mapping since models to estimate hazard rely on simplified assumptions of the fault geometry.

The present study illustrates an example of methods and results of fault examination in a slow strain and elastic attenuation area in intraplate Brazil. We investigate the Iraucuba seismogenic fault, which occurs in the Precambrian crystalline basement of the Borborema Province, the Equatorial margin of Brazil. The focal mechanisms in the region show normal, reverse, and mainly strike-slip faulting indicating a regional strike-slip stress regime in the upper crust up to 12 km depth with maximum horizontal compression (SHmax) roughly following the coastline trend. Under such context, we present earthquake data from a swarm sequence, the Iraucuba fault, at which the seismicity does not correlate with surface features. However, events with a magnitude as small as 1.5 mb are felt by the local population, and, in some cases, events with magnitude 2.5 mb or greater have damaged poorly built constructions. Therefore, suitable aftershock monitoring demands detailed macroseismic surveillance and engagement of the affected population and the Civil Defense. This engagement is crucial for awareness of the affected cities and the sensor network design and sustainability of the many stages of the aftershock monitoring such as deployment and maintenance. The earthquake data reveals that the Iraucuba NW-SE-striking fault occurs at depths between 8 and 9 km and dips similar to 70 degrees to NE. This fault has not been previously reported and is solely defined by the spatial distribution of hypocentres that coincides with the focal mechanism solution, and no obvious surface expression is observed. The fault plane reveals an initial rupture at shallower depths that progressively migrated to deeper portions of the crust. Therefore, we suggest that the initial seismic period transferred stresses to other near-critical areas of the fault, which allowed pore pressure to migrate within permeable portions of the fault zone within the upper crust.

The understanding of earthquake occurrence in intraplate areas has been one of the most challenging tasks in Seismology. The attenuation of seismic waves in these areas is lower compared to border plate regions. Therefore, even moderate magnitude earthquakes can represent a great hazard. In this case, assessing the stress field in the intraplate areas is important to better understanding the earthquake generating mechanisms. We analyzed 241 earthquakes of the 2017 seismic sequence in the NW part of the Potiguar Basin and estimated the stress regime. Seismically defined faults were determined for four high-precision relocated NW-SE- and NE-SWtrending clusters. Faulting intersection acting in one of these clusters was representative for suggesting an intersection model for intraplate earthquakes to explain why moderate size earthquakes occur in the study region. Another cluster presented a seismically defined fault vertically segmented with a clear spatiotemporal evolution and occurrence of almost entire activity in a very short period, which suggest energy build-up and rapid release between its fault segments. We calculated the composite focal mechanism for three clusters, with predominant strike-slip faulting with a sinistral movement. The inversion of focal mechanisms reinforces evidence for a superposition of local and regional stresses, with major compressional axis subparallel to the continental equatorial margin.

The continental margin of Brazil in intraplate South America exhibits a seismicity rate 70% higher than the average stable continental regions. This margin encompasses Northeastern Brazil, where most of the seismicity and active faults concentrate along the coast. The current study presents an integration of high-resolution aeromagnetic, seismological, and geological data to investigate the relationship between the seismicity and basement fabric in two areas in the northern part of the Precambrian Borborema Province. Our investigation indicates a relation between the metamorphic basement foliation and two seismogenic faults. In the first area adjacent to the Transbrasiliano Lineament, the largest continental suture zone in the South American platform, the magnetic anomaly patterns indicate that the regional basement grain and structural elements are E-W-oriented, which coincides with the strike of the E-W-striking, strike-slip Riacho Fundo Seismogenic Fault. In the second area, the magnetic anomaly patterns also coincide with the NE-SW-striking, strike-slip Samambaia Fault, and a swarm of chalcedony-quartz veins. Euler deconvolution used to estimate the source depth of the magnetic anomalies indicates that the faults in both cases could be longer and deeper than segments illuminated by the present-day seismicity. Although these findings are difficult to use as predictive tools, they indicate that the basement fabric could be reactivated by seismicity in continental interiors, as previously discussed in several cases in Brazil.

We model the contribution of the stresses generated by the topography and density heterogeneities in the stress field of NE Brazil and the interplay between these and regional stresses. Our model consists of a horizontally layered elastic slab (the lithosphere) that floats above an inviscid fluid (the asthenosphere), where the surface load is generated by the topography and bathymetry, and the internal loads are calculated from the Bouguer anomalies. The deviatoric flexural stresses are calculated in the Fourier domain. Our results showed local flexural stresses with magnitudes comparable to those of the plate-wide stresses, and mainly controlled by the long-wavelength Bouguer anomalies. Scenarios for the total stress field were calculated by superposing different values for an E-W-oriented regional compression. We predict the stress axes orientations and stress regimes for the equatorial margin and the Pernambuco Shear Zone (PESZ) region. However, we highlight the importance of spreading stresses, not considered here, in rotating the maximum horizontal compression (SHmax) predictions. The prediction of stress regime and stress axes orientation in PESZ using low to zero regional compression suggest that a decoupling of the shallow stresses from the plate-wide stresses might occur in this region. Another possible explanation found was that the spreading stresses might counterbalance the regional stresses. We conclude that the superposition of principally flexural and plate-wide stresses explains reasonably the observed stress regimes and SHmax orientations in a great part of NE Brazil. Furthermore, flexural stresses are playing an important role in the reactivation of the PESZ, possibly controlling the stress field.

As magnitudes dos principais sismos da série de João Câmara de 1986-1987 foram calculadas com estações regionais e telessísmicas. Correções das estações foram determinadas permitindo obter-se valores de magnitudes mais homogêneos e com menores desvios padrão. De agosto de 1986 a fevereiro de 1987, 30 sismos tiveram magnitudes maiores ou iguais a 3,5. A magnitude do maior sismo da série (30/11/86 às 05:19:48) foi m = 5,03 ± 0,05. Uma relação empírica entre magnitude e duração do sinal (m = c1 log D + c3) na estação JC01, em João Câmara, foi estabelecida permitindo um cálculo mais rápido de magnitude de microtremores. Para durações medidas até 1 mm pico-a-pico no sismograma, c1 = 2,05 e c3 = –1,61 para m ? 2. O exame das relações frequência-magnitude (log N = a – b m) indica que o coeficiente c1 deve ser menor para magnitudes abaixo de 2, aproximadamente. Para a atividade geral de João Câmara, foi encontrado um valor típico do parâmetro b de 1,12 ± 0,04. Não foi observada variação significativa no valor de b antes e depois do maior sismo de 30/11/1986.Palavras-chave: terremoto, onda de cauda, correções de estação, Rio Grande do Norte. DETERMINATION OF MAGNITUDES AND MAGNITUDE-FREQUENCY RELATION FOR THE EARTHQUAKES OF JOÃO CÂMARA, RNABSTRACT. Magnitudes of the major events of the 1986-1987 João Câmara earthquake swarm were calculated with regional and teleseismic stations. Station corrections were determined allowing more homogeneous magnitudes with smaller standard deviations. From August 1986 to February 1987, 30 events had magnitudes greater than 3.5. The largest (November 30, 1986 at 05:19:48) had m = 5.03 ± 0.05. An empirical relation between magnitude, m, and signal duration, D, (m = c1 log D + c3) at the local station JC01 was established allowing quick estimates of magnitudes for microearthquakes. For durations measured from the P arrival to coda amplitude of 1 mm peak-to-peak, c1 = 2.05 and c3 = –1.61 for magnitudes greater than about 2. The study of the frequency-magnitude relation (log N = a – b m) shows that the coefficient c1 must be smaller for magnitudes less than about 2. For the whole activity of João Câmara, a typical b-value of 1.12 ± 0.04 was found. No significant variation was observed in the b-value before and after the main event of November 30, 1986.Keywords: earthquake, coda wave, station corrections, Rio Grande do Norte State.

ABSTRACTA revised catalogue of earthquakes, which occurred in the Northeastern region of Brazilup to 1980, is presented. Most of the data come from historical macroseismic informationcollected in old publications and newspapers. Recent instrumental data is used to deriveempirical relations between magnitude and felt area in order to estimate mb magnitudesfor the events not recorded by seismographic stations. Macroseismic data for two largeevents (Pereiro 23.02.68, and Pacajus 20.11.80) are presented in detail. Epicentres of fourrecent events recorded at NAT were determined by combining the macroseismic andinstrumental data and used to define a preliminary value of the ratio K=Vp/Vs = 1.77 forthe region under study. A swarm of activity at Parazinho, Rio Grande do Norte, wasstudied in more detail to estimate the parameter b of the Gutenberg and Richter frequency-relation.

The Brazilian Seismographic Network (RSBR) began operating in 2011 with the joint effort of four different institutions: Universities of Sao Paulo (USP), Brasilia (UnB), Rio Grande do Norte (UFRN), and the National Observatory (ON). Initially funded by Petrobras (Brazilian State Oil Company) in a large-scale infrastructure project that started in 2009, RSBR is now sponsored by Brazilian Geological Survey (CPRM). Station installation began in 2011 in southeast (SE) Brazil and finished in 2014 in the Amazon. The network is composed of 84 stations (as of December 2017) operated by those four institutions. CPRM now coordinates the general maintenance and compilation of the Brazilian Seismic Bulletin. RSBR has lowered the detection threshold down to m(b )3.5 for earthquakes in Brazil as a whole, and in some areas, such as northeast (NE) and SE Brazil, detection thresholds are about m(b )3.0. Besides monitoring Brazilian and South American seismicity, RSBR serves as a backbone for temporary deployments to study crustal and upper-mantle structure.

Seismic activity beneath the Acu reservoir, northeast Brazil, is considered a classic, clear example of reservoir-induced seismicity (RIS) and has been investigated during the last two decades. After a quiescent period from April 1997 to June 2012, recent seismic activity was recorded by a seismic network deployed in the area during the seismic sequence, which lasted from July 2012 to April 2013. We located 50 seismic events from this recent sequence with M-w ranging from 0.1 to 1.4 and obtained well constrained relative locations and a composite focal mechanism for 35 of these events. The earthquakes occurred mainly beneath the lake at a depth of 3.5 km in a subvertical east-northeast-striking fault composed of two clusters similar to 250 m apart, defining a fault zone 150 m wide and similar to 1.5 km long, and were associated with the reactivation of the basement on a new fault cross cutting the main northeast-southwest-striking mylonitic structure. We estimate hydraulic transport properties within seismogenic faults. Depth-averaged and within-fault-plane permeabilities are 10(-15) and 10(-16) m(2), respectively, near the lower bounds of values found in the literature. We observe the seismicity at 3-4 km depth in the 2012-2013 sequence period; this was likely related to the long-term decrease in the water level. As the water level fell steadily because of drought in the region since 2011, the pore pressure due to diffusion (and the undrained response at a fraction of the water load) decreased, and the average load pressure decreased even more, leading to failure. Thus, the east-northeast-striking fault remained active until the pore pressure became less than the average load. This interplay between water level variation and pore pressure could explain the episodic behavior of the seismicity in a different active fault region in the 2012-2013 period.

PreviousNext No Access15th International Congress of the Brazilian Geophysical Society & EXPOGEF, Rio de Janeiro, Brazil, 31 July-3 August 2017Characterization of the seismogenic Samambaia Fault based on aeromagnetic data: preliminary resultsAuthors: Gilsijane V. RamosFrancisco H. R. BezerraDavid L. de CastroJoaquim Mendes FerreiraGilsijane V. RamosPrograma de Pós-Graduação em Geodinâmica e Geofísica - UFRNSearch for more papers by this author, Francisco H. R. BezerraPrograma de Pós-Graduação em Geodinâmica e Geofísica - UFRNSearch for more papers by this author, David L. de CastroPrograma de Pós-Graduação em Geodinâmica e Geofísica - UFRNSearch for more papers by this author, and Joaquim Mendes FerreiraPrograma de Pós-Graduação em Geodinâmica e Geofísica - UFRNSearch for more papers by this authorhttps://doi.org/10.1190/sbgf2017-202 SectionsAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail Abstract This research integrates geophysical, geological and seismological data to characterize the Samambaia seismogenic fault and its host crystalline basement. New airborne aeromagnetic data were used to identify magnetic lineaments and their relationship with the crystalline basement fabric and the seismogenic fault. The main results indicate that the Samambaia fault presents a length greater than that illuminated by seismicity and that the seismicity reactivates the basement fabric. Keywords: basement, aeromagnetic, mappingPermalink: https://doi.org/10.1190/sbgf2017-202FiguresReferencesRelatedDetails 15th International Congress of the Brazilian Geophysical Society & EXPOGEF, Rio de Janeiro, Brazil, 31 July-3 August 2017ISSN (online):2159-6832Copyright: 2017 Pages: 1876 publication data© 2017 Published in electronic format with permission by the Brazilian Geophysical SocietyPublisher:Society of Exploration Geophysicists HistoryPublished Online: 03 Aug 2017 CITATION INFORMATION Gilsijane V. Ramos, Francisco H. R. Bezerra, David L. de Castro, and Joaquim Mendes Ferreira, (2017), "Characterization of the seismogenic Samambaia Fault based on aeromagnetic data: preliminary results," SEG Global Meeting Abstracts : 1038-1042. https://doi.org/10.1190/sbgf2017-202 Plain-Language Summary KeywordsbasementaeromagneticmappingPDF DownloadLoading ...

A atividade sísmica em Pedra Preta (RN), localizada no Nordeste do Brasil, região de maior sismicidade intraplaca do país, teve início em dezembro de 2010. Antes disso, não havia registro de sismos na área. Durante o ano de 2013 e início de 2014, a rede sismográfica de Pedra Preta registou 273 sismos locais em 3 ou mais estações. Desses eventos foram selecionados os 50 com melhor leitura dos tempos de chegada das ondas P e S, sendo determinado o modelo de velocidades (Vp/Vs = 1,72 e Vp = 5,90 km/s) e calculados os hipocentros, com o programa HYPO71. Para a determinação do mecanismo focal composto foi realizada uma nova seleção de sismos obedecendo critérios mais restritos para os hipocentros (epicentros localizados dentro da rede com resíduo médio de tempo de chegada ≤ 0,01 s, número de observações ≥ 10, erro médio na horizontal < 0,1 km, erro médio na vertical < 0,1 km), tendo sido selecionados por esses critérios 24 eventos. Os hipocentros desses sismos mostram que a falha sismogênica tem aproximadamente 3 km de extensão, com sismos de 2,3 a 5,8 km de profundidade. Os parâmetros da falha sismogênica foram obtidos pela combinação do método dos mínimos quadrados e do programa FPFIT (strike = 254º, dip = 67º e rake = -66º), caracterizando uma falha normal. Os epicentros e o mecanismo focal foram utilizados para verificar se havia ou não possível correlação com feições geológicas mapeadas na área. A conclusão é que não existem feições geológicas mapeadas que possam estar relacionadas diretamente com a atividade sísmica estudada.

The Borborema province in NE Brazil is characterized by seismic sequences with small earthquakes that can last 10 yr or more. The seismicity in this region is concentrated in three main seismic zones. In this work, we investigate the stress field in one of these zones, the AcaraA(0) Seismic Zone, which is located in the NW part of the Borborema province. This seismic zone exhibits earthquake sequences that contain repeated earthquakes with similar waveforms and a shallow depth. Using a local network, we investigated a seismic sequence close to the town of Santana do AcaraA(0) from December 2009 to December 2010, and we present detailed results (velocity model, hypocentres and focal mechanism) from this network. In addition, we inverted seven focal mechanisms, including six that were used in previous studies, and determined the directions of the three main axes of the regional stress field. Selecting a very precise set of 12 earthquakes, we found an active seismic zone with a depth between 3.5 and 4.8 km and with a horizontal dimension of approximately 2.5 km in the NW-SE direction (azimuth of 118A degrees) and a strike-slip focal mechanism. The new seismic fault and some of the previous seismic faults determined in previous studies occur near the continental-scale Transbrasiliano lineament, but they exhibit no direct relationship with that ancient structure. The stress field is characterized by NW-SE trending compression and NE-SW trending extension. This result suggests that the rheological contrast between the continental-oceanic crusts created flexural stresses with maximum horizontal compression parallel to the continental margin. This stress pattern occurs along the Potiguar basin and continues west as far as the Amazon fan along the Equatorial margin of Brazil. This stress field and related seismicity may be a characteristic of this type of passive margin that is generated during the transform shearing between the South America and Africa plates and that exhibits an abrupt oceanic-continent transition, steep continental slopes and high bathymetric gradients.

Ambient noise correlation analyses are largely used in seismology to map heterogeneities and to monitor the temporal evolution of seismic velocity changes associated mostly with stress field variations and/or fluid movements. Here we analyse a small earthquake swarm related to a main m(R) 3.7 intraplate earthquake in North-East of Brazil to study the corresponding post-seismic effects on the medium. So far, post-seismic effects have been observed mainly for large magnitude events. In our study, we show that we were able to detect localized structural changes even for a small earthquake swarm in an intraplate setting. Different correlation strategies are presented and their performances are also shown. We compare the classical auto-correlation with and without pre-processing, including 1-bit normalization and spectral whitening, and the phase auto-correlation. The worst results were obtained for the pre-processed data due to the loss of waveform details. The best results were achieved with the phase cross-correlation which is amplitude unbiased and sensitive to small amplitude changes as long as there exist waveform coherence superior to other unrelated signals and noise. The analysis of 6 months of data using phase auto-correlation and cross-correlation resulted in the observation of a progressive medium change after the major recorded event. The progressive medium change is likely related to the swarm activity through opening new path ways for pore fluid diffusion. We further observed for the auto-correlations a lag time frequency-dependent change which likely indicates that the medium change is localized in depth. As expected, the main change is observed along the fault.

The main objective of the study is the analysis of cross-correlation between water level and seismicity recorded over 10years at Açu reservoir (Brazil) to unravel the physical mechanisms driving the observed seismicity. The reservoir is a shallow (<35m deep) earth-filled dam, which is located in a rifted crystalline basement. Robust power spectral density of both monthly water level and earthquake counts show a significant yearly cycle, indicating the clear link between the two variables in terms of loading/unloading of the dam. The application of the singular spectrum analysis (SSA) has allowed to extract from both time series their significant component. The cross-correlation analysis confirms that the evolution of seismicity is mainly due to pore pressure diffusion and other processes due to fracture compaction (undrained response) are not strongly present in our data.

The Pernambuco Lineament (PeL) is an E–W-striking, 700km long shear belt located in the central part of the Borborema Province, Brazil. Seismic sequences have been monitored in the region since 1991, and they are clearly correlated with the main shear belt and its NE-trending branches. A new sequence of small earthquakes occurred in April 2010 away from the main belt in the southern block of the lineament. We monitored this seismicity using a ten station network for 154days and used the observations to determine the nature of the seismicity and the stress patterns. In addition, we used data from five previous local seismograph networks to determine the stress tensor. The results indicate that the seismicity is concentrated in three clusters, two of which are associated with normal faults: the first strikes 96°, dips 51°, and has a rake of −65°, and the second strikes 253°, dips 64°, and has a rake of −120°. These faults are 2.2km and 1.5km long and 2.0–3.0km and 3.0–3.5km deep, respectively, and are not correlated with the Pernambuco Lineament or its NE-trending branches. The inversion of focal mechanisms from our investigation and previous studies indicates E–W-trending compression and N–S-trending extension that affects both the northern and southern blocks of the PeL. The new data indicate that the fault behavior in the shear zone area is more complex than has previously been observed along reactivated mylonitic belts.

A sequence of earthquakes occurred in 2008 in the Meruoca granitic pluton, located in the northwestern part of the Borborema Province, NE Brazil. A seismological study defined the seismic activity occurring along the seismically-defined Riacho Fundo fault, a 081° striking, 8 km deep structure. The objective of this study was to analyze the correlation between this seismic activity and geological structures in the Meruoca granite. We carried out geological mapping in the epicentral area, analyzed the mineralogy of fault rocks, and compared the seismically-defined Riacho Fundo fault with geological data. We concluded that the seismically-defined fault coincides with ∼E-W-striking faults observed at outcrop scale and a swarm of Mesozoic basalt dikes. We propose that seismicity reactivated brittle structures in the Meruoca granite. Our study highlights the importance of geological mapping and mineralogical analysis in order to establish the relationships between geological structures and seismicity at a given area.