A new Probe into the Innermost Inner Core Anisotropy via the Global Coda-correlation Wavefield

Investigations of the Earth's inner core (IC) using seismic body waves are limited by their volumetric sampling due to uneven global distribution of large earthquakes and receivers. The sparse coverage of the IC leads to uncertainties in its anisotropy, the directional dependence of seismic velocity. Yet, detailed constraints on anisotropy, such as its magnitude, and spatial distribution, are required to understand the crystallographic structure of IC's iron and its solidification and deformation processes. Here, we present a new method to investigate the IC's anisotropic properties based on Earth's coda-correlation wavefield constructed from the late coda of large earthquakes. We perform a comprehensive travel time analysis of I2*, an IC-sensitive correlation feature identified as a counterpart of the direct seismic wavefield's PKIKPPKIKP waves, yet fundamentally different. Namely, I2* is a mathematical manifestation of similarity among specific seismic phases with the same slowness detected in global correlograms in the short inter-receiver distance range. Our new spatial sampling of the IC overcomes the shortage of direct seismic wavefield paths sensitive to the IC's central volume, also known as the innermost IC (IMIC). The observed I2*’s travel time variations relative to Earth's rotation axis (ERA) support a model of cylindrical anisotropy with 3.3% strength and a zonal pattern of slow axis oriented 55° from ERA. We thus find compelling evidence for a deep IC structure with distinct anisotropy, although we cannot resolve the depth at which the change occurs. This finding reinforces previous inference on the IMIC, with implications for Earth's evolution

Expressão geofísica-estrutural do lineamento transbrasiliano na porção central da Bacia do Parnaíba (Maranhão-Piauí)

O objetivo deste estudo foi caracterizar a expressão estrutural-geofísica do Lineamento Transbrasiliano na porção centro-leste da Bacia do Parnaíba. O Lineamento Transbrasiliano (LTB) corresponde a uma megazona de cisalhamento de idade neoproterozoica (Ciclo Brasiliano), com direção NE-SW e cinemática transcorrente dextral, ocorrendo subjacente (e exposta lateralmente nas bordas NE e SW) à seção sedimentar da Bacia do Parnaíba. No presente trabalho, a interpretação dos mapas de anomalias gravimétrica e magnética é analisada face a essa cinemática do LTB, sendo que a assinatura das anomalias geofísicas corresponde às etapas de evolução brasiliana a tardi-brasiliana, de temperatura alta e declinante. Verifica-se que o padrão das anomalias gravimétricas residuais é compatível com um par S-C dextral, moldando os corpos geológicos do embasamento heterogêneo. As bandas C, com direção NE, devem ser constituídas por fatias de gnaisses e granulitos (anomalias positivas), rochas graníticas ou metassedimentares de baixo grau e grabens pré-silurianos em estilo pull-apart (anomalias negativas). Já as anomalias de traços curvilíneos no mapa gravimétrico identificam trends contracionais (de superfícies S), incompatíveis com a sua interpretação como um graben pré-siluriano, restando as demais alternativas citadas. No tocante à interpretação dos trends no mapa de anomalias magnéticas (reduzidas ao polo), a maior parte destes é tentativamente associada a falhas ou zonas de cisalhamento de baixa temperatura (planos C), delimitando blocos distintos em termos de propriedades magnéticas, e/ou preenchidas por corpos básicos. É também possível que algumas anomalias magnéticas isoladas/pontuais correspondam a corpos ígneos de idade tardi-brasiliana ou mesozoicos. A configuração desses lineamentos no embasamento pode ser interpretada em analogia ao modelo de fraturas de Riedel, assumindo planos de mergulho acentuado e com seção de movimento sub-horizontal. Nesta dissertação, são também exploradas interpretações relativas a modelagens gravimétricas 2D combinadas com a interpretação de uma linha sísmica dip ao Lineamento Transbrasiliano. A seção de rochas equivalente ao Grupo Jaibaras mostrou anomalias gravimétricas discretas da bacia, conferindo assim uma maior influência às estruturas do embasamento nas respostas gravimétricas. A delimitação dos grabens sotopostos à seção paleozoica da bacia sofre restrições causadas pelas heterogeneidades e anisotropia do embasamento.The objective of this study was to characterize the structural-geophysical expression of the Transbrasiliano Lineament (TBL) in the east-central portion of the Parnaíba Basin. The TBL corresponds to a major Neoproterozoic NE-trending shear zone related to the Brasiliano orogenic cycle, with dextral strike-slip kinematics, underlying (but also laterally exposed in the NE and SW basin edges) the sedimentary section of the Parnaíba Basin. In this study, the interpretation of gravity and magnetic anomaly maps is consistent with the TBL kinematics, the signature of the geophysical anomalies corresponding to the high (plastic behaviour) and subsequent declining temperature (ductile to brittle behaviour) stages during Brasiliano and late Brasiliano times. The pattern of residual gravity anomalies is compatible with an S-C dextral pair shaping the geological bodies of an heterogeneous basement, such as slices of gneisses and granulites (positive anomalies), granitic and low-medium grade metasedimentary rocks (negative anomalies). Such anomalies curvilinear trends, ranging from NNE (interpreted as S surfaces) to NE (C surfaces), correspond to flattening surfaces (S), while the NE rectilinear trend must represent a C band. The narrower magnetic anomalies also display NNE to NE (S surfaces) trends and should correspond to similar (although narrower and more discontinuous) sources in the equivalent anomaly patterns. Pre-Silurian pull-apart style grabens may contribute to the NE negative gravimetric anomalies, although this interpretation demands control by seismic data analysis. On the other hand, the curvilinear anomalies associated to contractional trends are incompatible with their interpretation as pre-Silurian graben, in both maps. In the (reduced to the pole) magnetic anomalies map, most of these are again associated to low-temperature shear zones (C planes) and faults, juxtaposing distinct blocks in terms of magnetic properties, or eventually filled with basic bodies. It is also possible that some isolated magnetic anomalies correspond to igneous bodies of late-Brasiliano or Mesozoic age. The basement late discontinuities pattern can be interpreted in analogy to the Riedel fractures model, with steep dipping surfaces and a sub-horizontal movement section. This study also explored 2D gravity modeling controlled by the interpretation of a dip seismic line as regards to the Transbrasiliano Lineament. The rock section equivalent to the Jaibaras Group occupying a graben structure (as identified in the seismic line) corresponds to a discrete negative anomaly superimposed to a gravimetric high, once again indicating a stronger influence of older crystalline basement rocks as gravimetric sources, mainly reflecting the heterogeneities and anisotropies generated at high temperature conditions and their subsequent cooling along the TBL, during the Brasiliano cycle

An estimate of absolute shear-wave speed in the Earth’s inner core

Abstract Observations of seismic body waves that traverse the Earth’s inner core (IC) as shear (J) waves are critical for understanding the IC shear properties, advancing our knowledge of the Earth’s internal structure and evolution. Here, we present several seismological observations of J phases detected in the earthquake late-coda correlation wavefield at periods of 15–50 s, notably via the correlation feature I-J, found to be independent of the Earth reference velocity model. Because I-J is unaffected by compressional wave speeds of the Earth’s inner core, outer core, and mantle, it represents an autonomous class of seismological measurements to benchmark the inner core properties. We estimate the absolute shear-wave speed in the IC to be 3.39 ± 0.02 km/s near the top and 3.54 ± 0.02 km/s in the center, lower than recently reported values. This is a 3.4 ± 0.5% reduction from the Preliminary Reference Earth Model (PREM), suggesting a less rigid IC than previously estimated from the normal mode data. Such a low shear-wave speed requires re-evaluating IC composition, including the abundance of light elements, the atomic properties and stable crystallographic phase of iron, and the IC solidification process

Resumos concluídos - Ciências Biológicas

Resumos concluídos - Ciências Biológica

Resumos concluídos - Ciências Biológicas

Resumos concluídos - Ciências Biológica