Marcelo Assumpção

Upper mantle structure of South America from joint inversion of waveforms and fundamental mode group velocities of Rayleigh waves

[1] A new tomographic S wave velocity model for the upper mantle beneath South America is presented. We developed and applied a new method of simultaneously inverting regional S and Rayleigh waveforms and fundamental mode Rayleigh wave group velocities, to better constrain upper mantle S velocity structure and Moho depth. We used � 5700 Rayleigh wave group velocity dispersion curves and 1537 regional wave trains with paths principally passing through the South American continent. The joint inversion of this data set provided a new three-dimensional (3-D) upper mantle S velocity model and a Moho depth model for South America, which fits both the group velocity and regional waveform data sets well. New features of the final three-dimensional (3-D) S velocity and Moho depth model correlate well with known geotectonic units on a regional scale. The Moho depth ranges from 30 km in the central Chaco basin to 42 km beneath the Amazonian craton and 45–70 km beneath the orogenic Andean belt. The imaged S velocity indicates an average lithosphere thickness of around 160 km for the Amazonian craton. High velocities are imaged beneath the Amazon and part of the Parana´ and Parnao´ba basins down to about 150 km. Low to very low velocities are imaged ! ! !

Crustal thicknesses in SE Brazilian Shield by receiver function analysis: Implications for isostatic compensation

hypothesis are proposed to explain the data: (1) A lower density, by 30-40 kg/m 3 ,i n the lithospheric mantle under the Archean block of the Sao Francisco craton relative to the Proterozoic lithosphere is responsible for maintaining the high elevations in the plateau area. Relatively low density and high P wave velocity are compatible with a depleted (low FeO) composition for the Archean lithosphere. (2) Alternatively, if the density contrasts between Archean and Proterozoic lithospheres are smaller than the values above, then the crust beneath the Paranabasin must be more dense than that of the craton. Higher crustal density and high Poissons ratio would be consistent with magmatic underplating in the lower crust beneath the Paranabasin, as inferred from other studies. INDEX TERMS: 7205 Seismology: Continental crust (1242); 9360 Information Related to Geographic Region: South America; 7218 Seismology: Lithosphere and upper mantle; KEYWORDS: crust, upper mantle, Brazil, Parana basin, San Francisco craton

Shear wave splitting in SE Brazil: an effect of active or fossil upper mantle flow, or both?☆

We investigated the structure of the upper mantle beneath southeastern Brazil using teleseismic shear wave splitting measurements. Measurements were performed on seismic data recorded in the Ribeira and Brasilia Neoproterozoic belts, which wrap around the southern termination of the Sa ‹o Francisco craton and disappear westward under the Paranabasin. In the northern Ribeira belt, dominated by thrust tectonics, the fast shear wave polarization planes trend on average N080‡E, whereas in the central domain, dominated by strike-slip tectonics, fast shear waves are polarized parallel to the structural trend (N065‡E). Stations located above the main transcurrent fault display large delay times (s 2.5 s). Such values, among the largest in the world, require either an unusually large intrinsic anisotropy frozen within the lithosphere, or a contribution from both the lithospheric and asthenospheric mantle. Within the southern Brasilia belt, fast split shear waves are polarized parallel to the structural trend of the belt, at a high angle from the APM. Although part of our data set strongly favors an origin of anisotropy related to a fabric frozen in the lithospheric mantle since the Neoproterozoic, a contribution of the asthenospheric flow related to the present day plate motion is also required to explain the observed splitting parameters. > 2003 Elsevier Science B.V. All rights reserved.

Reservoir-induced Seismicity in Brazil

Abstract — A compilation of 16 cases of reservoir-induced seismicity in Brazil is presented with maximum magnitudes ranging from 1.6 ML to 4.2 mb. The compilation includes: location of the main epicentral area with respect to the reservoir (inside the lake, at the margin, or outside), predominant geology, and the temporal distribution of the main phase(s) of activity (initial or delayed in relation to impoundment). Data on the regional stress field for some reservoirs is also included. Four recent cases are discussed in more detail: Tucuruí, Nova Ponte, Miranda, and Serra da Mesa. A comparison with all other reservoirs deeper than 30 m and 50 m suggests that the hazard for induced-seismicity varies within Brazil: the NE part of the intracratonic Paraná basin has higher hazard as compared with the southern part of the same basin. No correlation of the induced hazard with variations in natural seismicity can be observed.

The tailings dam failure of 5 November 2015 in SE Brazil and its preceding seismic sequence

The collapse of a mine tailings dam and subsequent flood in SE Brazil on 5 November 2015 was preceded by a small-magnitude seismic sequence. In this report, we explore the spatiotemporal associations between the seismic events and the accident and discuss their possible connection. We also analyze the signals generated by the turbulent mudflow, as recorded by the Brazilian Seismographic Network (RSBR). In light of our observations, we propose as possible contributing factor for the dam collapse either ground shaking and/or soil liquefaction triggered by the earthquakes. The possibility of such a small-magnitude earthquake contributing to the collapse of a tailings dam raises important concerns regarding safety and related legislation of dams in Brazil and the world.

Seismicity patterns and focal mechanisms in southeastern Brazil

The seismicity of SE Brazil is studied with a recent deployment of additional temporary digital and analog seismographic stations. Regional and local stations have allowed the determination of three new composite focal mechanisms for the earthquake series of Betim in 1992/93, Formiga in 1993, and the reservoir-induced events of Nova Ponte in 1995, all in Minas Gerais state. The focal mechanism data in the southern part of the Sao Francisco craton and adjacent Brasilia fold belt indicate both normal and reverse faulting with a common E-W to NE-SW orientation of the maximum horizontal stress. Far from the continental margin, the focal mechanisms are consistent with theoretical estimates of stress directions from finite-element modelings of the forces driving the South American plate. Near the Serra do Mar coastal range, the stress pattern seems more complex. A selection of the earthquake catalog, using threshold magnitudes varying in time to yield a data set with spatially uniform coverage, indicates two main seismic areas: 1) the offshore continental shelf , and 2) the southern part of the Brasilia fold belt and the Sao Francisco craton. The high topography areas of the Serra do Mar and Serra da Mantiqueira (in the Ribeira fold belt) and the Parana basin are much less seismically active.

Intraplate seismicity in mid-plate South America: correlations with geophysical lithospheric parameters

Abstract Mid-plate South America remains one of the least-studied regions of intraplate seismicity. Little is known about the origin and controlling factors that make this area the least seismically active intraplate region in the world. We analysed the distribution of intraplate seismicity and its correlation with several geophysical lithospheric parameters in an attempt to establish which factors might promote or inhibit the occurrence of intraplate earthquakes. We found that above-average seismicity occurs mostly in Neoproterozoic fold belts, associated with areas having a positive gravity anomaly, lower elastic thickness, higher heat flow, thinned crust and a negative S-wave anomaly at 100 km depth (associated with non-cratonic crust). Cratonic areas with a higher elastic thickness and lower heat flow are associated with low rates of seismicity. Our study suggests that the most important controlling factors are elastic thickness and heat flow. We propose that earthquake-prone areas with these favourable conditions correspond to regions of weakened lithosphere, where most of the regional lithospheric stresses are supported by the overlying brittle upper crust. These areas act as local concentrators of the regional compressional stress field, with the stress build-up then leading to the occurrence of intraplate seismicity. Supplementary material: contains additional statistics and figures considering different filters for the used catalogue as a mean of comparison with the figures presented in the main text. They are available at http://www.geolsoc.org.uk/SUP18872

Lithospheric Features of the São Francisco Craton

Studying the thick lithosphere of cratons is important to help understand their formation and the mechanisms for their preservation. We present a synthesis of the information available for the deep structure in Eastern Brazil, from seismological and gravity data, to characterize the Sao Francisco Craton (SFC) and help better define its lateral boundaries at depth. Crustal thicknesses of the SFC, known mainly from receiver function studies, range from 38 to 42 km, except for a localized thickening (up to 44 km) in the northern part, and crustal thinning towards the Atlantic continental margin in Bahia state. Overall, the crust is slightly thicker near the geologically-defined surface boundaries (40–42 km) and slightly thinner in the center (38–40 km), which is consistent with generally low Bouguer anomalies and high topography to the East and to the West of the craton probably defining the suture zones during the Gondwana amalgamation. Modeling of gravity anomalies with some seismic constraints indicates a relatively low-density lithospheric mantle for the SFC, despite higher Pn velocity, which is consistent with a Fe-depleted, buoyant lithosphere, which helps preserve the cratons’s root. Surface-wave continental-scale tomography suggested the thickest lithosphere, around 200 km, to be in the Archean southern part of the SFC, consistent with regional P- and S-wave tomography. Both the surface-wave and the body-wave tomographies show high upper mantle velocities beneath the Brasilia fold belt, next to the SFC’s surface limits, which is interpreted as a continuation at depth of the craton’s lithosphere, beneath the low-grade external metamorphic domain of the Brasilia fold belt. Analysis of the SFC seismicity shows that most earthquakes now occur on shallow (<2 km) normal faults formed during the formation of the Brasiliano continental margin, now reactivated under the present E–W compressional stresses.

Feições crustais determinadas pela análize azimutal da função do receptor, na região da estação sismológica de Rio Claro (RCLB)

This paper presents an analysis of receiver functions to estimate crustal structure beneath the Rio Claro Seismological Station, which has been in operation since October 2002. The depths obtained for the Moho discontinuity range from 36 to 44 km. A systematic variation of Moho depths with azimuth was observed. Larger values were obtained with events arriving from the North and the values of 36 km were observed with teleseisms arriving from the SW. This result shows that in the region there is an elevation of the Moho discontinuity near the Domo de Pitanga structural high. This result probably indicates a tectonic origin for this structure caused by deep deformation processes involving the whole lithosphere.

The 2009 earthquake, magnitude mb 4.8, in the Pantanal Wetlands, west-central Brazil

The main goal of this paper is to characterize the Coxim earthquake occurred in June 15th, 2009 in the Pantanal Basin and to discuss the relationship between its faulting mechanism with the Transbrasiliano Lineament. The earthquake had maximum intensity MM V causing damage in farm houses and was felt in several cities located around, including Campo Grande and Goiânia. The event had an mb 4.8 magnitude and depth was 6 km, i.e., it occurred in the upper crust, within the basement and 5 km below the Cenozoic sedimentary cover. The mechanism, a thrust fault mechanism with lateral motion, was obtained by P-wave first-motion polarities and confirmed by regional waveform modelling. The two nodal planes have orientations (strike/dip) of 300°/55° and 180°/55° and the orientation of the P-axis is approximately NE-SW. The results are similar to the Pantanal earthquake of 1964 with mb 5.4 and NE-SW compressional axis. Both events show that Pantanal Basin is a seismically active area, under compressional stress. The focal mechanism of the 1964 and 2009 events have no nodal plane that could be directly associated with the main SW-NE trending Transbrasiliano system indicating that a direct link of the Transbrasiliano with the seismicity in the Pantanal Basin is improbable.