Journal of Geophysical Research: Solid Earth

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ISSN / EISSN : 2169-9313 / 2169-9356
Published by: American Geophysical Union (AGU) (10.1029)
Total articles ≅ 5,106
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Weibin Song, Xuping Feng, Gaoxiong Wu, Gongheng Zhang, Ying Liu,
Journal of Geophysical Research: Solid Earth; https://doi.org/10.1029/2021jb022027

Abstract:
Ambient seismic noise cross-correlation has been widely applied in surface wave tomography at regional to global scales, including for seismic exploration of near-surface structures. Reliable seismic imaging requires the accurate selection of dispersion curves. However, manual picking has become cumbersome work with the increase in available correlation traces; it is even more difficult when the number of dispersion curves increases by using frequency-Bessel (F-J) transform. Here, we show that the neural network Res-Unet++ can automatically and accurately extract both fundamental dispersion curves and overtones from the F-J dispersion spectra after training the network. Results show that selected dispersion curves had high accuracies in the synthetic data (greater than 95%). The network could effectively extract both the fundamental and higher modes in real data, and transfer learning improved the adaptability of neural networks for different geological areas. The obtained dispersion curves from the real data agreed well with those acquired manually and were advantageous for generating more effective dispersion points.
, R. Leonhardt, R. Egli,
Journal of Geophysical Research: Solid Earth, Volume 126; https://doi.org/10.1029/2021jb022565

Abstract:
Although a large number of magnetic declination and inclination measurements are available for the historical period from 1400 to 1900 CE, even the evolution of the axial dipole moment, the most prominent quantity of the geomagnetic field, is highly disputed for this time interval. Here, a new iterative Bayesian spherical harmonic model is constructed that combines historical (direct) and archeomagnetic or volcanic (indirect) records to better constrain the geomagnetic field evolution over this historical epoch. The resulting geomagnetic field reconstruction (BIGMUDIh.1) benefits from recently published and revised data for critical periods and regions. The highly variable data uncertainties and qualities are tackled with a weighting and selection scheme. Model uncertainties are mainly controlled by selection criteria applied to indirect intensity records, and are estimated using a bootstrapping approach. A major improvement with respect to the established historical model gufm1 consists in the incorporation of archeointensities, which enables a direct estimation of the axial dipole evolution prior to 1840 CE. BIGMUDIh.1 yields a 2 µT (6%) decrease of the axial dipole component between 1500 and 1600 CE, followed by a relatively stable period until 1900 CE. Modeled declinations agree well with historical records and the gufm1 model derived from such records. Furthermore, the evolution of the South Atlantic Anomaly, driven by reverse flux patches (RFPs) at the core-mantle boundary, is outlined. Periods of accelerated movement of the northern magnetic dip pole appear to be connected to the formation and changes of RFPs in the Northern hemisphere.
Journal of Geophysical Research: Solid Earth, Volume 126; https://doi.org/10.1029/2020jb020799

Abstract:
New and published (U-Th)/He data on zircon, apatite and zircon fission track ages constrain the thermal overprint and cooling history of the eastern Aar Massif, Switzerland. The timing and pattern of cooling is in agreement with independent kinematic and age constraints from exposed shear zones. This suggests that the cooling ages mainly reflect exhumation and that long-term exhumation-dynamics were mainly controlled by crustal-scale tectonic processes. Results of a statistical inverse model reveal significant diachrony in the timing of exhumation in the along-strike direction. Maximum exhumation rates (1 mm/yr) were initially located in the central Aar Massif (from 22–10 Ma), then gradually migrated to the east between 10 Ma and present, while the central Aar Massif continued to exhume at slower rates (0.5 mm/yr). The diachrony in the timing of exhumation may be explained by lateral variations in the inherited thickness or the density of the accreted European crust. We attribute the increase in exhumation rates between 2 Ma and present to enhanced glacial erosion. Nevertheless, the post 2 Ma exhumation pattern reflects a continuation of non-cylindrical massif ”growth” in the eastward orogen-parallel direction. This indicates that – although at slow rates – thick-skinned and buoyancy-driven compressional deformation, likely enhanced by the presence of easily erodible flysch units at the surface, might still be ongoing especially in the eastern Aar Massif. Non-cylindrical massif-growth is likely to also affect other External Crystalline Massifs or orogens, but may be overlooked because studies often focus on single orogen-perpendicular transects.
, , , Naotaka Y. Chikasada, , Takeshi Nakamura, Hiroaki Tsushima
Journal of Geophysical Research: Solid Earth, Volume 126; https://doi.org/10.1029/2021jb022223

Abstract:
Tsunamis with amplitudes of up to 40 cm, related to the Mw 7.1 normal-faulting earthquake off Fukushima, Japan, on November 21, 2016, were clearly recorded by a new offshore wide and dense ocean-bottom pressure gauge network, S-net, with high azimuthal coverage located closer to the focal area. We processed the S-net data and found some stations included the tsunami-irrelevant drift and step signals. We analyzed the S-net data to infer the tsunami source distribution. A subsidence region with a narrow spatial extent (∼40 km) and a large peak (∼200 cm) was obtained. The other near-coastal waveforms not used for the inversion analysis were also reproduced very well. Our fault model suggests the maximum stress drop across the fault plane of > ∼10 MPa and the average of 4.2 MPa, whereas the shear stress increase along the fault caused by the 2011 Tohoku earthquake was only ∼2 MPa. Past studies have suggested that horizontal compressional stress around this region switched to horizontal extensional stress after the Tohoku earthquake due to its stress perturbation. The present result, however, suggests that the horizontal extensional stress was locally predominant at the shallowest surface around the focal area even before 2011. The present study demonstrates that the S-net high-azimuthal-coverage pressure data provides a significant constraint on the fault modeling, which enables us to discuss the stress regime within the overriding plate at the offshore. Our analysis provides an implication for crustal stress states, which is important for understanding generation mechanisms of intraplate earthquakes.
Journal of Geophysical Research: Solid Earth, Volume 126; https://doi.org/10.1029/2021jb022322

Abstract:
Seismic anisotropy in the Earth's mantle inferred from seismic observations is usually interpreted in terms of intrinsic anisotropy due to Crystallographic Preferred Orientation (CPO) of minerals, or extrinsic anisotropy due to Shape Preferred Orientation (SPO). The coexistence of both contributions confuses the origins of seismic anisotropy observed in tomographic models. It is thus essential to discriminate CPO from SPO. Homogenization/upscaling theory provides means to achieve this goal. It enables computing the effective elastic properties of a heterogeneous medium, as seen by long-period waves. In this work, we investigate the effects of upscaling an intrinsically-anisotropic and heterogeneous mantle. We show analytically in 1-D that the observed radial anisotropy parameter is approximately the product of the intrinsic and the extrinsic components: This law is verified numerically in the case of a homogenized 2-D marble cake model of the mantle in the presence of CPO obtained from a micro-mechanical model of olivine deformation. Our numerical findings predict that for wavelengths smaller than the scale of deformation patterns, tomography may overestimate intrinsic anisotropy due to significant extrinsic anisotropy. At longer wavelengths, intrinsic anisotropy is always underestimated due to spatial averaging. Therefore, we show that it is imperative to homogenize a CPO model first before drawing comparisons with tomographic models. As a demonstration, we use our composite law with a homogenized CPO model of a plate-driven flow underneath a mid-ocean ridge, to estimate the SPO contribution to an existing tomographic model of radial anisotropy.
, C. B. Connor, P. Wetmore, , L. J. Connor, M. Rodgers,
Journal of Geophysical Research: Solid Earth; https://doi.org/10.1029/2021jb022507

Abstract:
The Blackfoot Reservoir volcanic field (BRVF), Idaho, USA, is a bimodal volcanic field that has hosted silicic eruptions during at least two episodes, as recently as 58 ka. Using newly collected ground and boat-based gravity data, two large negative anomalies ( mGal) are modeled as shallow ( km) intrusions beneath a NE-trending alignment of BRVF rhyolite domes and tuff rings. Given the trade-off between density contrast and model volume, best-fit gravity inversion models yield a total intrusion volume of ; a density contrast of kg results in two intrusions, each km km and about 0.5 km thick, with cumulative volume of 100 . A network of trending faults lies directly above and on the margins of the mapped gravity anomalies. Most of these faults have m throw; one has throw up to m. We suggest that the emplacement of shallow sill-like intrusions produced this fault zone and also created a ENE-trending fault set, indicating widespread ground deformation during intrusion emplacement. The intrusions and silicic domes are located km E of a regional, 20 mGal step in gravity. We interpret this step in gravity as thickening of the Upper Precambrian to lowermost Cambrian quartzites in the Meade thrust sheet, part of the Idaho-Wyoming Thrust Belt. Silicic volcanism in the BRVF is a classic example of volcanotectonic interaction, influenced by regional structure and creating widespread deformation. We suggest volcanic hazard assessments should consider the possibility of large-volume silicic eruptions in the future.
, , , Yi Cai, Mason Andrew Kass, Marcelo Leão‐Santos
Journal of Geophysical Research: Solid Earth; https://doi.org/10.1029/2021jb022668

Abstract:
Magnetic inversion methods based on structured grids have been used extensively in geomagnetic research. Nevertheless, structured grids limit the modeling capability and accuracy of models with arbitrary geometries. To address these challenges, 3-D magnetic numerical forward modeling and inversion methods using an unstructured tetrahedral grid based on a partial differential equation (PDE) framework are proposed. The methods are derived from Maxwell’s partial equations and constructed using the finite element method. Arbitrary undulating topographies, complex geometries, and demagnetization effects can be represented exactly, leading to high-accuracy forward modeling and inversion solutions. The proposed methods are suitable for both local- and global-scale magnetic data interpretation, for example, in mineral exploration and tectonic research. A synthetic example and real airborne magnetic survey data collected from Mount Iliamna, Alaska, USA, are tested. The results demonstrate that the novel methods significantly improve the capability and accuracy of magnetic data interpretation.
Siyuan He, , , Jingtao Xie,
Journal of Geophysical Research: Solid Earth, Volume 126; https://doi.org/10.1029/2021jb022611

Abstract:
Gravity surveys in regional geophysical research can be used to estimate the depth of the sediment-basement interface. In this paper, we investigate a novel method using the convolutional neural network (CNN) for depth-to-basement inversion directly from gravity data. Based on the Random-Midpoint-Displacement method (RMD) and the features of the observed gravity data, we can generate a large set of realistic sediment-basement interface models. This new method for model generation can significantly reduce the size of the training data sets which is usually considerably large to train a pervasive network. The application on synthetic models shows that the developed CNN inversion is able to capture the detailed features of the sediment-basement interface for the complex geological model. However, so far, the training set obtained from the proposed method is still continuous and the CNN inversion still cannot effectively recover the models such as abrupt faults. We also successfully applied the developed method and workflow to a field study. The proposed approach opens a new window for estimating the physical contrast interfaces using potential field.
Journal of Geophysical Research: Solid Earth; https://doi.org/10.1029/2021jb022632

Abstract:
A three-dimensional shear velocity model for the crust and upper mantle beneath the central United States is presented by inverting Rayleigh wave phase velocities from 20s to 100s periods. These phase velocities were determined using regional and teleseismic earthquakes recorded by the Northern Embayment Lithospheric Experiment stations, the CERI New Madrid Seismic Network, the Earthscope Transportable Array, and the Ozark Illinois INdiana Kentucky Flexible Array. A low Vs anomaly is imaged in the mantle below the Reelfoot Rift, which is the uppermost portion of connected low-velocity zones dipping toward the southwest below the rift and to the northwest below the Illinois Basin. According to the analysis in previous tomographic studies using both Vp and Vs anomalies, the elevation of temperature and the enrichment of iron, water, and orthopyroxene contents are required factors to explain the reduced seismic velocities. These low-velocity zones are produced by silica-rich fluids rising from the stalled Farallon slab. Two areas with low shear strength characterized by low Vs are imaged below the Ste. Genevieve and the Wabash Valley seismic zones. The low-velocity, weak areas may be responsible for stress concentration and thus the generation of intraplate seismicity.
Journal of Geophysical Research: Solid Earth; https://doi.org/10.1029/2021jb022650

Abstract:
Ambient noise interferometry is a powerful technique to continuously measuring crustal seismic velocity changes (dv/v) and studying crustal behaviors over time. However, the interpretation of such dv/v variations is not straightforward since multiple causes including internal (tectonic/magmatic) processes of the crust and external (environmental) factors could both affect dv/v simultaneously. To differentiate the interplay between the internal and external processes in dv/v variations is an essential step toward accurate crustal monitoring. In this study, we apply the single-station cross-component (SC) method to 15 selected stations from the Broadband Array in Taiwan for Seismology (BATS) to investigate the temporal evolution of crustal seismic velocities across Taiwan. We process the continuous BATS seismic recording from 1998 to 2019, construct the daily SC correlation functions, and compute dv/v values by the stretching technique in a frequency band of 0.1 to 0.9 Hz. We observe both strong annual dv/v variations and co-seismic velocity drops associated with regional moderate-to-large earthquakes. Systematic spectral and time-series analyses with the weather data suggest that the rainfall-induced pore-pressure change plays a predominant role in driving the dv/v seasonality, reflecting a diffusion process from meteoric water into shallow crust. The effects of other factors are relatively local and secondary. We also demonstrate how understanding and correcting rainfall effects could critically improve the resolution and accuracy of internal crustal damage related to earthquakes.
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