Tectonophysics 680 (2016) 1–7

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Tectonophysics

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Causes of intraplate seismicity in central Brazil from travel timeseismic tomography

Marcelo Peres Rocha a,⁎, Paulo Araújo de Azevedo a, Giuliano Sant'Anna Marotta a,Martin Schimmel b, Reinhardt Fuck a

a Institute of Geosciences, University of Brasília, Brasília, DF, Brazilb Institute of Earth Sciences “Jaume Almera”, CSIC, Barcelona, Spain

⁎ Corresponding author at: University of Brasília, Inst70910-900 Brasília, DF, Brazil.

E-mail addresses: marcelorocha.unb@gmail.com, marcpaulopaico@hotmail.com (P.A. Azevedo), marotta@unb.brschimmel@ictja.csic.es (M. Schimmel), reinhardt@unb.br

http://dx.doi.org/10.1016/j.tecto.2016.05.0050040-1951/© 2016 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 16 September 2015Received in revised form 1 May 2016Accepted 3 May 2016Available online 11 May 2016

New results of travel time seismic tomography in central Brazil provide evidence that the relatively high seismic-ity in this region is related to the thinner lithosphere at the limit between the Amazonian and São Franciscopaleocontinents. The transition between these paleocontinents is marked by low velocity anomalies, spatiallywell correlated with the high seismicity region, which are interpreted as related to the lithospheric thinningand consequent rise of the asthenosphere, which have increased the temperature in this region. The low-velocity anomalies suggest a weakness region, favorable to the build-up of stress. The effective elastic thicknessand the strain/stress regime for the study area are in agreement with tomographic results. A high-velocity trendis observed beneath the Parnaíba Basin, where low seismicity is observed, indicating the presence of a cratoniccore. Our results support the idea that the intraplate seismicity in central Brazil is related to the thin lithosphereunderlying parts of the Tocantins Province between the neighboring large cratonic blocks.

© 2016 Elsevier B.V. All rights reserved.

Keywords:Seismic tomographyIntraplate seismicityTocantins ProvinceSão FranciscoPaleocontinentAmazonian PaleocontinentParnaíba Basin

1. Introduction

1.1. Motivation

Most of the seismically active regions on Earth, including where thelargest earthquakes occur, coincide with the limits of tectonic plates.Over 90% of theworld earthquakes occur at the edge of oceanic and con-tinental plates. Unlike plate boundary regions, where the seismicity isrelatively concentrated and the causes are well understood, intraplateseismicity represents diffuse deformation in relatively stable tectonicregions (Zoback, 1992), and their origins cannot be explained simply,given that they depend on the local tectonic context (Assumpçãoet al., 2014).

Most common models proposed to explain intraplate seismicity arerelated to pre-existing weakness zones, such as extended crust inaborted rifts or continental margins (e.g. Johnston, 1989; Schulte andMooney, 2005), or stress concentration in the upper crust due to struc-tural inhomogeneities (e.g. Sykes, 1978; Talwani, 1989; Talwani andRajendran, 1991; Kenner and Segall, 2000). Liu and Zoback (1997)

itute of Geosciences — IG, CEP

elorocha@unb.br (M.P. Rocha),(G.S. Marotta),(R. Fuck).

showed that stress concentration in the upper crust of the NewMadrid seismic zone is the result of a weaker subcrustal lithosphere.Based on seismic tomography results, Assumpção et al. (2004) pro-posed that lithospheric thinning could provide favorable conditionsfor stress concentration in the brittle upper crust, which may explainthe epicentral distribution within the South American Platform.

In the South American continent, the regional stress field isdominated by E–W compression (Zoback, 1992). The origin of thisstress regime is mainly due to forces related with spreading in theMid-Atlantic Ridge and the resistive forces exerted by the Caribbeanplate to the north and the Nazca plate subduction to the west(Mendiguren and Richter, 1978; Coblentz and Richardson, 1996).Intraplate seismicity in Brazil is clearly not uniform and a few areas ofhigher activity have been identified (Assumpção et al., 2004, 2014).An example of the high seismic concentration can be observed in centralBrazil. Significant seismicity, with preferential epicenter distribution inthe SW–NE direction, is observed in the Tocantins Province. This isknown as the Goiás-Tocantins Seismic Zone — GTSZ (Berrocal et al.,1984; Fernandes et al., 1991). The seismicity of central Brazil may berelated to the limits between the São Francisco and Amazonianpaleocontinents, which could represent a region of lithospheric thin-ning. According to Assumpção et al. (2004), in regions of tectonic litho-spheric thinning, stress tends to focus on the crust, while in regions ofthicker lithosphere the stress is more distributed within the uppermantle.

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In seismic tomography results, regions with thinner lithosphereappear as low-velocity anomalies (Assumpção et al., 2004; Zhanget al., 2009; Rocha et al., 2011), which can be interpreted as regions ofrelatively higher temperature. Regions with thicker lithosphere, suchas cratons, are characterized by stability and low temperatures, andnormally appear as high-velocity anomalies in tomographic results. Inthis context, our objective is to relate new results of P-wave seismictomography in central Brazil (Tocantins Province) with the seismicitydistribution, in order to understand the causes of the high seismicityof the GTSZ.

1.2. Study area

The study area is locatedmostly in the Tocantins Province, in centralBrazil, covering also the southeastern part of the Amazonian Craton, thewestern part of the São Francisco Craton, the southern part of theParnaíba Basin and northeastern part of the Parana Basin. The area iscrossed by the Transbrasiliano Lineament (TBL), a major lithosphericdiscontinuity in the region, which defines the boundary of differentcrustal domains (Cordani and Sato, 1999; Cordani et al., 2013; BritoNeves and Fuck, 2014). Here, we focus on the area marked by thedashed rectangle (Fig. 1). The black and white triangles are the seismicstations which have been added to the study carried by Rocha et al.(2011) for this purpose.

Central Brazil, and especially the Tocantins Province, is one of themost complex tectonic regions in South America. The most important

Fig. 1. Study area with stations distribution. Black triangles are new stations, white triangles arRocha et al. (2011). Dashed rectangle is the interpreted area. The solid lines in gray and black areline is the Transbrasiliano Lineament (TBL). PRB— Paraná Basin; PB— Parnaíba Basin; TP— TocBelt; (02) Paraguay Fold Belt; (03) Brasilia Fold Belt.

event related to the formation of the Tocantins Province is the collisionbetween theAmazonian and the São Francisco cratons and a third stablearea, the Paranapanema block hidden beneath the Paraná Basin(Pimentel et al., 2000, 2004), giving rise to theNeoproterozoic Araguaia,Paraguay and Brasília fold belts (01, 02 and 03 in Fig. 1). To the north ofTocantins Province lies the Parnaíba Basin. This is one of the largestcratonic basins of South America (like the Paraná, Solimões andAmazonic basins). This basin occupies an area of more than600,000 km2 (Brito Neves, 1998), and covers a rigid lithosphere block,which probably represents its cratonic core (Brito Neves et al., 1984;Góes et al., 1993; Nunes, 1993; Castro et al., 2014; Daly et al., 2014).

2. Data and method

For this study, new data were acquired through a permanentnetwork recently installed in Brazil – Brazilian Seismographic Network(BSN) –with more than 80 stations distributed throughout the country(black triangles in Fig. 1) and providing, for this study, data between2011 and 2015. We used also data from a temporary network with 15stations (white triangles in Fig. 1), with events recorded between2007 and 2013 (Azevedo et al., 2015). These new data were includedin a database previously assembled by prior works (VanDecar et al.,1995; Schimmel et al., 2003; Rocha et al., 2011). We used records of Pand PKIKP phases for events with minimum magnitudes larger than4.6 and 5.4 (mb), respectively. Events were chosen in the epicentraldistance range of 30° to 95° for P-waves and 150° to 180° for PKIKP

e the stations included by Azevedo et al. (2015) and gray triangles are old stations used bythe boundaries of the countries and geological provinces, respectively. Dashed and dottedantins Province; SFC— São Francisco Craton; AC— Amazonian Craton; (01) Araguaia Fold

Fig. 2. Tomographic horizontal image at 150 kmdepth, and the seismicity of central Brazil.The thick lines indicate geological boundaries and the white circles represent epicenters(Brazilian Seismic Bulletin, Magnitudes N =3.0). The dashed-dotted line is theTransbrasiliano Lineament. White line Z-Z' indicates the vertical profile shown in Fig. 3cand the white square indicates the region that contains the events used in Figs. 3a and 5a.

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waves, to avoid phase misidentification due to triplications, caused bythe mantle transition zone or inner core. The final database for all ofBrazil consists of 27,665 time measurements for phases P and PKIKPfrom 6141 events.

The tomographic method used is the linearized inversion approachof VanDecar et al. (1995), which is based on the ACH inversion method(Evans and Achauer, 1993). This method has been successfully used indifferent regions (VanDecar et al., 1995; Sol et al., 2002; Wolfe et al.,2002; Schimmel et al., 2003; Bastow et al., 2005, 2008; Benoit et al.,2006; Lees et al., 2007; Schmid et al., 2008; West et al., 2009; Rochaet al., 2011; Azevedo et al., 2015). In this method, travel-time relativeresiduals (see Evans and Achauer, 1993) of teleseismic waves areinverted simultaneously for three-dimensional velocity structure,earthquake relocations, and station terms. Observed times are obtainedby phase picking of the seismograms, and theoretical travel times havebeen computed for spherically symmetric Earthmodel IASP91 (Kennettand Engdahl, 1991). In order to obtain more accurate picks, we usedthe Multi-Channel Phase Cross Correlation (MCPCC) developed bySchimmel et al. (2003). This method is an extension of the Multi-Channel Cross Correlation approach (VanDecar and Crosson, 1990),and provides an amplitude unbiased phase cross-correlation ratherthan the classical cross-correlation (Schimmel, 1999).

The parametermodel has been discretized in a dense grid of knots in-terpolated with splines under tension (Cline, 1981; Neele et al., 1993).This interpolation scheme provides a smooth slowness distribution andtherefore permits an accurate ray tracing. The grid for the whole Brazilis composed of 449,625 knots: 33 knots in depth, 109 knots in latitudeand 125 knots in longitude. The spacing between the knots increasesfrom the center to the border of the grid, and in depth. The parameteriza-tionmodel extends outside the area of the stations to minimize themap-ping of noise and inconsistencies, as unrealistic structures into the centralarea. Further details about the inversion and data processing procedurescan be obtained in VanDecar et al. (1995), Schimmel et al. (2003),Rocha et al. (2011) and Azevedo et al. (2015).

We used in our results (Figs. 2 and 3) earthquake data of theBrazilian Seismic Bulletin (BSB, http://rsbr.gov.br/catalogo_sb.html).BSB include historical and instrumental events from 1724 toDecember/2013 (Version 2014.11), with magnitudes from 2.0 to 6.2.The completeness of the BSB is variable in time and space, mainly duethe inhomogeneous station density, which increased significantly onlyfrom the late 1970s with the installation of many stations in areasof hydroelectric reservoirs, and with another significant increasefrom 2011, with the installation of permanent stations of the BSN(Assumpção et al., 1997). According to Assumpção et al. (1997) theMagnitude of Completeness value for the BSB is 3.2 for events startingfrom 1980 and 2.8 for events starting from 1990 being higher for priorperiods (e.g. 4.5 for 1968, when beginning the operation of the stationsarrangement in the capital, Brasília). We used events with a minimummagnitude of 3.0 for our study area, considering that the majority ofstations in that region started operation from the 1990s.

3. Results and discussion

Our results (Figs. 2 and 3) are velocity perturbations with respect tothe IASP91 reference model (Kennett and Engdahl, 1991). The anoma-lies represent mainly lateral structure, where higher than averagevelocities are indicated by cold colors and lower than average velocitiesby hot colors. Areas without data are shown in black.

From the installation of new stations, it was possible to observe anew low-velocity anomaly in central Brazil, elongated in the SW–NEdirection (Fig. 2) concurrent with the trend of the TranbrasilianoLineament. This anomaly is well resolved as shown by resolutionstests carried out by Azevedo et al. (2015). Further, this lineament isaccompanied by high seismicity, mainly in its central portion. The seis-micity decreases significantly at the boundary of the Parnaíba Basin,which is practically aseismic. The low-velocity anomaly at the southern

portion of the Parnaiba Basin seems to be divided in two branches. It ispossible that these low-velocity anomalies indicate the limits of acratonic block that would represent the basement of this basin (seeCastro et al., 2014), explaining the seismicity absence in its interior.

Based on the assumption that low-velocity anomalies, interpreted ashigher temperatures, could result in weakness zones (Assumpção et al.,2004), we could consider that the low-velocity anomaly in centralBrazil is a weaker region, and represents a lithospheric thinningand consequent asthenosphere rising, leading to the TranbrasilianoLineament reactivation, and consequent seismic activity in that region.However, we observe a high-velocity anomaly in the southwesternportion of the Tocantins Province, exactly in a region with absence ofseismicity. According to Alkmim et al. (1993) and Ussami (1993),based in gravimetric anomalies, the São Francisco Craton extendsbeyond its surface boundary to the west, below the Brasilia Belt Fold.This pattern is confirmed by the results of seismic tomography(Azevedo et al., 2015; Rocha et al., 2011). Therefore, the lithosphere inthat region has cratonic characteristics, not being a weakness region,not favoring the occurrence of high seismicity.

In Fig. 3 we show the vertical tomographic profile Z-Z' (Fig. 3c), asindicated in Fig. 2, and the model to summarize our interpretation ofthe inhomogeneous seismicity in the E-W direction in central Brazil(Fig. 3b). This model is based on our seismic tomography results and onthe model for the lithosphere structure in the region proposed bySoares et al. (2006) based in deep refraction results. The cumulativeevent number over a range of 200 km width is shown on Fig. 3a,centered in profile Z-Z'. Clearly, most seismic events occur over the low-velocity anomaly.

Fig. 3. Comparison of the P-wave tomographic results with seismicity distribution alongthe vertical cross-section Z-Z' shown in Fig. 2. (a) Cumulative epicenter distribution inZ-Z' profile over a range of 200 km for each sides. (b) Proposed interpretative model toexplain the inhomogeneous epicenter distribution in central Brazil (based on Soareset al., 2006) where red arrows are the stress distributed in depth, and blue arrows repre-sent the regional stress. (c) P-wave tomography vertical profile (Z-Z') crossing TocantinsProvince. Red dashed line represents the Lithosphere-Asthenosphere Boundary (LAB).(For interpretation of the references to color in this figure legend, the reader is referredto the web version of this article.)

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The depth of the Lithosphere/Asthenosphere Boundary (LAB) inFig. 3c was based on the maximum depths of cratons and mobile beltsindicated in the literature (e.g. Veeraswamy and Raval 2004), whereLAB in cratonic and mobile belt regions can reach depth intervals of180 and 250 km and 70 and 150 km, respectively. Heit et al. (2007)indicate a value of 160 km for the LAB depth beneath Tocantins Province(station BDFB, Brasília) using the receiver function method, corroborat-ing our results.

Some events occur within the São Francisco Craton, consistent withdiffuse nature of the intraplate seismicity (Zoback, 1992). If the SouthAmerican Platform was formed by a single continental block, existingseismicity would probably be diffusive as observed within the SãoFrancisco Craton. However, as this is not the case, it is expected thatthe stress distributed across all of the South American Platform, can berelieved in weaker regions, like the northwestern Tocantins Province(low-velocity tomographic anomalies, Fig. 2).

Size of the red arrows in Fig. 3b is a schematic interpretation of thedepth distribution of the stress in the crust and upper mantle along

the Z-Z' profile. In this model, the depth distribution of the regionalstress (blue arrows) in thinner lithosphere is different if comparedwith in thicker lithosphere. In the thicker lithosphere most part of thestress is concentrated in the lithosphericmantle, while in regions of lith-ospheric thinning, stress is concentrated mainly in the crust, allowingconditions for the occurrence of the seismic events. Also, the crust inthe region of Tocantins Province is thinner (Soares et al., 2006) andweaker, probably due to the lithospheric thinning in the region and in-creasing of the temperature, which favors the occurrence of earth-quakes there instead of the neighboring thicker cratonic crust regions.

Based on gravity results andmodeling, Assumpção and Sacek (2013)proposed that the cause of the seismicity in the Tocantins Province isflexural deformation. The arguments presented by those authors arebased on numerical modeling, and indicate that the stress reaching~100 MPa, generated by flexure in central Brazil, due to a densitycontrasts in the lithosphere and topographic load, is sufficient togenerate earthquakes. This result does not eliminate the possibilitythat the seismicity in central Brazil can also be generated by regionalstress accumulation. Therefore, based on our results, we believe thatthe combination of weakness zones, caused by lithospheric thinning,and regional stress concentration, bears significant influence on thecauses of seismicity in central Brazil.

To corroborate our tomographic results, Fig. 4 shows the effectiveelastic thickness (Te), available from Bizzi et al. (2003) and the strain/stress regime in the study area, available from Assumpção et al. (1985,1997), Lima et al. (1997), Assumpção (1998), Chimpliganond et al.(2010), Marotta et al. (2013), and Agurto-Detzel et al. (2015).

Fig. 4a shows that most of Tocantins Province presents Te between20 and 55 km. This area is between two stronger regions (blue), locatedon the Amazonian (NW) and São Francisco (SE) cratons, which have Tevalues above 55 km. In the southern Tocantins Province (betweenParaná basin and São Francisco craton), we observed high Te values(coincident with high-velocity anomalies), which may be related tothe part of São Francisco Paleocontinent, as explained above. Thesmallest Te values (between 10 and 20 km) can be observed in part ofParaná and Parnaiba basins and northernmost São Francisco Craton.Most seismic events occur over regions with low-velocity anomalyand Te down to 55 km.

Regarding the strain and stress regime in the studied area, theprincipal strain rate (Fig. 4b) is concentrated on the SW–NE direction,and the stress (Fig. 4c) is concentrated in the E–Wdirection. Accordingto Marotta et al. (2013), these directions corroborate the finding thatthe regional stress field, on the passive margin of South AmericanPlatform, is related mainly with spreading in the Mid-Atlantic Ridgeand the resistive forces exerted by Nazca plate subduction.

If we consider only the Z-Z' profile region, the Te values shown inFig. 4a (low to intermediate values) and the direction of the stressesand principal strain rate on Fig. 4d (SW–NE direction), corroborate themodel presented in Fig. 3, which shows that the stress concentrationin lithospheric thinning regions is a favorable context for the occurrenceof high seismicity.

We calculated cross-correlations between Seismicity, Te,Tomographic Anomalies and Crustal Thickness pairs (Fig. 5d, e, f, g, h,i), and made a comparison between the values of these parameters,along profile Z-Z' (Fig. 5a, b, c). We observe reasonable correlation(over 67%) between different pairs of measurements, with the excep-tion of the pair Te × Crustal Thickness (about 41%). Probably the crustalthickness has low correlation with the Te, since Te measures containvariations beyond the crust. Also, the highest correlation between thetomographic anomalies and seismicity along profile Z-Z' shows thatthe origin of both is related.

Fromvisual correlation (Figs. 4 and 5) it can be seen that the seismic-ity coincides with Te values smaller than 50 km in Tocantins Province,especially with values near this limit. From the Temap, one can observethat the greater seismicity region (intermediate Te) is in regions wherethe deformation is greater and consequently where the stress is

Fig. 4. Strain and stress found on the studied area. a) Effective elastic thickness (Te) for Brazil (with local Te values) and for study area, with distribution of the strain rate and SHmax(modified from Bizzi et al., 2003). b) Strain rate estimated by geodetic observations (Marotta et al., 2013). c) Stress estimated by focal mechanism (FM), breakout (BO), in situ measure-ments (IS) and geological fault analysis (GF), showed by Assumpção et al. (1985, 1997), Lima et al. (1997), Assumpção (1998), Chimpliganond et al. (2010), Marotta et al. (2013), andAgurto-Detzel et al. (2015). d) Predominant directions of strain rate and stress for the profile (vertical cross-section Z-Z') region shown in Figs. 2 and 3. (For interpretation of the referencesto color in this figure, the reader is referred to the web version of this article.)

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greatest. Thus, for the occurrence of seismicity in regions of lower Te,the existence of stress would be necessary. Apparently, the boundarybetween the Amazonian and São Francisco cratons is a favorable areafor concentration of stress. Perhaps the two cratons may be serving asa kind of director of regional stress, and the boundary between themwould be the most affected region.

4. Conclusions

In the South American Platform, tomographic low-velocity anoma-lies coincide with relatively high seismicity, mainly in central Brazil.These low-velocity anomalies likely indicate lithospheric thinning.

Tomographic anomalies can be related with changes in Te values,where, higher Te values (N55 km) are generally related with high-velocity anomalies (mainly in cratonic regions), and smaller and interme-diate Te values (between 10 and 55 km) are related with low-velocityanomalies, generally interpreted in our study area as weakness regions.

Regional stresses may be distributed evenly in the lithosphere, andin thinner regions, they tend to bemore intense in the crust when com-pared with thicker regions. The boundary between the Amazonian and

the São Francisco paleocontinents is a favorable area for concentrationof stress.

The relative absence of high seismicity in the adjacent cratonicregions could be related to the concentration of the most part of thediffusive regional stress in the upper mantle.

Cross-correlation analysis between Seismicity, Te, TomographicAnomalies and Crustal Thickness pairs suggests that the variationof each one has the same origin, with exception of the Te × CrustalThickness pair that shows low cross-correlation coefficient, probablydue to the fact that Te does not have the same information with thatof Crustal Thickness.

The almost total absence of seismicity in the Parnaíba Basin can berelated to the presence of a cratonic core that represents its basement.More stations need to be deployed in the Parnaíba Basin to confirmthis assertion.

Acknowledgments

The authors thank Coordenação de Aperfeiçoamento de Pessoal deNível Superior (CAPES) and the Instituto Nacional de Ciência e

Fig. 5.Comparison and cross-correlations between the values of the Seismicity, Te, Tomographic Anomalies andCrustal Thickness. (a) Cumulative epicenter distribution in Z-Z' profile overa range of 200 km for each side. (b) P-wave velocity anomaly along the profile Z-Z' for a depth of 150 km. (c) Dashed and solid lines are the Crustal Thickness and Te values along Z-Z'profile, respectively. (d), (e), (f), (g), (h) and (i) Calculated cross-correlations and distributions of the pairs of the analyzed parameters.

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Tecnologia — Estudos Geotectônicos (INCT-ET) for the MSc and PhDscholarships granted to P. A. Azevedo. ToCNPq for Research ProductivityGrant provided to R. Fuck and M. Rocha. To Petrobrás, for providingfunding through the projects Brazilian Seismographic Network (RSBR)and Lineamento Tranbrasiliano. M. Rocha thanks Prof. MarceloAssumpção for his suggestions and questionswith respect to the results.We used data from stations of the Seismological Center of University ofSão Paulo, Institute of Technological Research of São Paulo (IPT), Seis-mological Observatory and Laboratory of Lithospheric Studies of theUniversity of Brasilia, GTSN and GEOSCOPE Networks, and ETH-Zürich.John VanDecar kindly provided the Regional Seismic Tomographycode. We thank all staff and students that helped in the stations instal-lation. Finally, we thank the reviewers for all comments, which helpedimprove the paper substantially.

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