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Rayleigh-Wave, Group-Velocity Tomography of the Borborema Province, NE Brazil, from Ambient Seismic Noise

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Rayleigh-Wave, Group-Velocity Tomography of the Borborema Province, NE Brazil, from Ambient Seismic Noise Pure Appl. Geophys. Ó 2014 Springer Basel DOI 10.1007/s00024-014-0982-9 Pure and Applied Geophysics Rayleigh-Wave, Group-Velocity Tomography of the Borborema Province, NE Brazil, from Ambient Seismic Noise 1 1,2 3 RAFAELA CARREIRO DIAS, JORDI JULIA` , and MARTIN SCHIMMEL Abstract—Ambient seismic noise has traditionally been Key words: Seismic interferometry, Ambient seismic noise, regarded as an unwanted perturbation that ‘‘contaminates’’ earth- Rayleigh-wave dispersion, surface wave tomography, Borborema quake data. Over the last decade, however, it has been shown that Province. consistent information about subsurface structure can be extracted from ambient seismic noise. By cross-correlation of noise simul- taneously recorded at two seismic stations, the empirical Green’s function for the propagating medium between them can be recon- structed. Moreover, for periods less than 30 s the seismic spectrum of ambient noise is dominated by microseismic energy and, 1. Introduction because microseismic energy travels mostly as surface-waves, the reconstruction of the empirical Green’s function is usually pro- Seismic noise has traditionally been regarded as portional to the surface-wave portion of the seismic wavefield. In this paper, we present 333 empirical Green’s functions obtained an unwanted signal in seismic recordings of the from stacked cross-correlations of one month of vertical compo- Earth’s ground motion and has frequently been nent ambient seismic noise for different pairs of seismic stations in omitted from detailed analysis. In recent years, the Borborema Province of NE Brazil. The empirical Green’s however, it has been shown that the empirical functions show that the signal obtained is dominated by Rayleigh waves and that dispersion velocities can be measured reliably for Green’s function of the propagating medium between periods between 5 and 20 s. The study includes permanent stations two points can be reconstructed by cross-correlating from a monitoring seismic network and temporary stations from seismic noise recorded simultaneously at those two past passive experiments in the region, resulting in a combined network of 34 stations separated by distances between approxi- points (LOBKIS and WEAVER 2001;CAMPILLO and PAUL mately 40 and 1,287 km. Fundamental-mode group velocities were 2003;SHAPIRO and CAMPILLO 2004;SNIEDER 2004; obtained for all station pairs and then tomographically inverted to SHAPIRO et al. 2005). Similar to recorded seismo- produce maps of group velocity variation. For short periods (5–10 s) the tomographic maps correlate well with surface geology, grams, cross-correlations of ambient seismic noise with slow velocities delineating the main rift basins (Potiguar, contain information about the distribution of seismic Tucano, and Recoˆncavo) and fast velocities delineating the location velocities within the propagating medium, and ana- of the Precambrian Sa˜o Francisco craton and the Rio Grande do lysis of ambient noise cross-correlations is now Norte domain. For longer periods (15–20 s) most of the velocity anomalies fade away, and only those associated with the deep routinely used to infer subsurface velocity structure Tucano basin and the Sa˜o Francisco craton remain. The fading of (SHAPIRO et al. 2005;SABRA et al. 2005;MOTTAGHI the Rio Grande do Norte domain fast-velocity anomaly suggests et al. 2013). Moreover, because ambient noise pro- this is a supracrustal structure rather than a lithospheric terrain, and places new constraints on the Precambrian evolution of the files are dominated by microseismic peaks at Borborema Province. approximately 0.05–0.10 and 0.1–0.2 Hz, and mi- croseisms propagate predominantly as Rayleigh waves (LACOSS et al. 1969;FRIEDRICH et al. 1998; BROMIRSKI 2001;BROMIRSKI and DUENNEBIER 2002; STEHLY et al. 2006), results from cross-correlation of 1 Programa de Po´s-Graduac¸a˜o em Geodinaˆmica e Geofı´sica, Universidade Federal do Rio Grande do Norte, Natal, Brazil. seismic ambient noise are dominated by the surface- E-mail: [email protected]; jordi@geofisica.ufrn.br wave portion of the Green’s function within those 2 Departamento de Geofı´sica, Universidade Federal do Rio frequency ranges. Dispersion velocities can thus be Grande do Norte, Natal, Brazil. 3 Institut de Cie`ncies de la Terra ‘‘Jaume Almera’’, Centro measured in the cross-correlated time-series and, if Superior de Investigaciones Cientı´ficas, Barcelona, Spain. enough stations are available, tomographic inversion R. C. Dias et al. Pure Appl. Geophys. can be used to develop images of dispersion velocity was considered, which led to the emergence of variation (SHAPIRO et al. 2005;SABRA et al. 2005; Rayleigh waves in the reconstructed empirical VILLASEN˜ OR et al. 2007;BENSEN et al. 2008;MOT- Green’s functions. After obtaining the empirical TAGHI et al. 2013). Green’s functions, group velocities were measured on In the work reported here ambient noise tomog- the cross-correlated time series by using the multiple raphy was used to develop high-resolution filtering analysis (MFA) of DZIEWONSKI et al. (1969) tomographic images of fundamental-mode, Rayleigh- for periods between 5 and 20 s. Tomographic maps wave, group-velocities for the Borborema Province of lateral group-velocity variation were developed by of NE Brazil, with the objective of mapping shallow, using the fast marching surface tomography (FMST) sub-surface velocity variations in the region. Surface- inversion procedure of RAWLINSON (2005), which wave tomographic images of the Borborema Province combines the fast marching method (FMM) of published so far are only available from a few con- RAWLINSON and SAMBRIDGE (2005) for forward com- tinental-scale studies of South America (FENG 2004; putation of surface-wave group delays with the FENG et al. 2007;LLOYD et al. 2010;ASSUMPC¸A˜ O iterative subspace inversion scheme of KENNETT et al. et al. 2013), and are of low-resolution in the Province (1988) to map lateral variations in group velocity. because of the limited data available for the region. In Geologically, the Borborema Province is a struc- recent years, however, the Borborema Province has tural domain located in the northeastern-most corner been the focus of large multi-institutional, inter-dis- of South America (Fig. 1). It is characterized by ciplinary projects. Those include the Institutos do complex tectonic evolution that began during Pre- Mileˆnio—Tectonic and Geophysical Studies in the cambrian times and extended into the Cenozoic Borborema Province and the Instituto Nacional de (ALMEIDA et al. 1981, 2000;SANTOS et al. 2000;BRITO Cieˆncia e Tecnologia para Estudos Tectoˆ nicos NEVES and CORDANI 1991;TROMPETTE 1994). The (INCT-ET), both funded by the Brazilian Centro Province is criss-crossed by several east–west and Nacional de Desenvolvimento Cientı´fico e Tec- northeast–southwest trending shear-zones, suggested nolo´gico (CNPq), which deployed several temporary as marking the boundaries of smaller tectonic terrains broadband networks in the region. Moreover, since that amalgamated during the Brasiliano–Pan African 2011, the seismicity in NE Brazil has been monitored orogeny (BRITO NEVES and CORDANI 1991;JARDIM DE by use of the Rede Sismogra´fica do Nordeste SA´ et al. 1992;CORDANI et al. 2003). Some authors, (RSISNE), a permanent network of 16 broadband however, regard them as the surface expression of stations supported by the Brazilian oil company supracrustal deformations overlying a mostly coher- Petrobras. In total, the combined network of perma- ent Early Proterozoic basement (NEVES 2003;NEVES nent and temporary stations now available in the et al. 2000, 2006). Because of extension during con- Borborema Province and surrounding regions con- tinental breakup in Mesozoic times, a number of rift sists of 34 broadband stations with inter-station basins, now aborted, formed in the continental inte- distances between 40 and 1,287 km, approximately. riors. These include the Potiguar basin to the north, the This dramatic increase in the seismic coverage of the Araripe basin to the center-west, and the Tucano, Ja- Borborema Province provides a unique opportunity toba´, and Recoˆncavo basins to the south, with smaller for passive imaging of the Province’s subsurface rift basins scattered throughout the Province (Fig. 1). structure with unprecedented detail. Evolution of the province in the Cenozoic was marked Our study includes 333 cross-correlations by episodes of intraplate volcanism and uplift (MIZU- obtained from 1 month of continuous seismic noise SAKI et al. 2002;MORAIS NETO et al. 2009), which are recordings at several pairs of broadband stations in probably related to magmatic upwellings originating NE Brazil. For each station pair, multiple correlations from upper mantle sources (USSAMI et al. 1999; were obtained at one-day intervals and then stacked KNESEL et al. 2011;OLIVEIRA and MEDEIROS 2012; by using the time–frequency, phase-weighted meth- PINHEIRO and JULIA` 2014). odology of SCHIMMEL and GALLART (2007). Only the For short periods (5–10 s) our tomographic ima- vertical component of the seismic noise recordings ges clearly outline the major intra-continental rift Group-Velocity Tomography of the Borborema Province Figure 1 Topographic map of the Borborema Province and surrounding physiographic provinces with its Precambrian domains, Mesozoic rift-basins, and shear-zones superimposed. Cenozoic volcanic features along the Fernando de Noronha-Mecejana alignment (FNMA) and the Macau- Queimadas alignment (MQA) are also indicated. Adapted from DE CASTRO et al. (2008), OLIVEIRA (2008), and KNESEL et al. (2011) basins with slower-than-average group velocities. For overall lack of correlation between surface geology longer periods (15–20 s) the anomaly associated with and group velocity variation for longer periods, the Potiguar basin fades away, as expected from the throughout the Province, suggest the Precambrian shallow depth-extent of the basin, whereas the domains making up the Borborema Province may not anomaly associated with the Tucano–Recoˆncavo rift continue at depth. system remains. The Tucano–Recoˆncavo rift system is overlain by a thick layer of slow-velocity sedi- mentary rocks, and the persistence of the anomaly 2.
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