Journal of Geophysical Research: Solid Earth最新文献

The response of antigorite deformation to preexisting fabrics in subduction zones is unclear. We deform antigorite schist with foliation and lineation (X is parallel to the lineation, Y is in the foliation and perpendicular to the lineation, and Z is normal to the foliation) at different angles to the maximum principal stress (σ₁) via a Paterson gas-medium apparatus under a confining pressure of 200 MPa, a temperature of 600°C, a strain rate of ∼10⁻⁵ s⁻¹, and a preheating time of 6 hr. The mechanical results indicate that σ(Z) ≫ σ(X) ≈σ(Y), indicating nearly transverse isotropy. Observations of the recovered samples revealed that less ductile and more brittle deformation occurred simultaneously, indicative of a brittle-to-semibrittle regime. Microcracks formed parallel to the foliation likely increase anisotropy. The slightly greater strength, more kinks and/or occurrence of slow stick-slip along the Y direction suggest that the a-axis is in a hard-friction direction, supporting the results of an atomic force microscopy (AFM) study by Campione and Capitani (2013, https://www.doi.org/10.1038/ngeo1905) on single-crystal antigorite under ambient conditions. In contrast, our previous deformation at 1.3 GPa shows that the b-axis is in a hard-slip direction, consistent with the results from transmission electron microscopy (TEM) observations by Amiguet et al. (2014, https://www.doi.org/10.1002/2013jb010791) on antigorite deformed under 1 and 4 GPa. This comparison suggests that mechanical anisotropy within the foliation changes with pressure, likely reconciling the opposite results obtained by the two studies and indicating brittle‒ductile transitions near the mantle wedge corner where deep slow earthquakes related to antigorite may have occurred.

An ongoing controversy revolves around the detailed structure of the subducting European and Adriatic plates under the Alps and the adjacent orogens. Mostly based on P-wave travel time tomographic images, slab break-off at different times, reversals of subduction polarity and segmentation of the slab into independent units have been proposed. These processes may have important geodynamic consequences such as rapid surface uplift, past magmatic events or changes in the style of continental collision. However, some of the tomographic results are contradictory, particularly evident in the uppermost mantle where teleseismic P waves traverse the medium almost vertically with few ray crossings and a stronger dependence on the crustal correction. In this work, we present the result of an innovative joint inversion approach using surface- and teleseismic body-wave travel times to mitigate some of the shortcomings in both data types. Applying a reversible-jump Markov chain Monte Carlo approach, we simultaneously constrain the $��v_P�$ and $��v_S�$ structure and their uncertainties in the crust and upper mantle. The results indicate a continuous slab structure from the crust-mantle boundary down to at least 400 km depth under the western, central and eastern Alps. The results, however, also suggest that fitting the data within their respective measurement uncertainties may not be sufficient to reliably determine the presence of a shallow slab break-off beneath the Alps.

With the increase in Global Navigation Satellite System (GNSS) observations, the requirement for objective and automated detection of slow slip event (SSE) signals hidden in displacement time series is increasing. However, machine learning for GNSS time series has rarely been attempted. Especially, the physical meanings of the spatio-temporal noise variations and their effects on the detection performance have been not so deeply discussed. In this study, we conducted a single-site SSE detection based on machine learning trained by real GNSS observations of southwest Japan to directly consider the complicated spatiotemporal characteristics of observational noise. Based on a catalog of 284 short-term SSEs, approximately 26,000 time series containing SSE signals or noises were extracted as training data. The signal data predominantly had an amplitude of 1.5–2.0 mm. The model architecture following the Generalized Phase Detection, which was originally proposed for seismic wave detection, was then adopted. We obtained an accuracy of 75% for the test data. As expected, the detectability were mainly controlled by the signal amplitude, and false positive appears to be caused primarily by the temporally correlated noise that resemble the onset or termination of the SSE signal. We examined the correlation between detection performance and noise properties at each site, such as standard deviation and slope of power spectrum. The analysis of this study is expected to facilitate a straightforward evaluation of the influence of noise characteristics on the detection performance, and clarify the crucial topics to improve detection precision.

Numerical modeling is a valuable approach for quantitative elucidation of the highly complex hydrothermal processes within active volcanoes. Various schemes have been developed for the numerical simulation of volcanic hydrothermal systems, generally employed in combination with other geophysical and geochemical monitoring techniques. However, a scheme for constructing realistic permeability structures (crucial for simulations) remains unexplored. As the first step toward establishing such a scheme, we conducted numerical simulations to explore the effect of lithology variations on hydrothermal circulation within active volcanoes. These simulation models were constructed based on the electrical resistivity structure model of the Kusatsu-Shirane Volcano (KSV) in Japan. Key factors include the permeability of the host rock and underlying basement, permeability reduction in the ductile region, and the geometry of a silica sealing layer. Of these, the silica sealing layer was a significant factor in reproducing the actual observations. The simulations indicated that the permeability and degree of closure of the silica sealing layer determined the pressure distribution within the region that the layer enclosed, and were, thus, responsible for the low resistivity of the subvertical conductors commonly found beneath volcanoes. The permeability structure used in this study was simple but systematically constructed, and simulations based on this permeability distribution successfully reproduced the key observations. The knowledge obtained from the numerical model of KSV can be used to explain the resistivity distribution of other active volcanoes, including higher- and lower-temperature systems. The results suggest the validity and potential broad applicability of the proposed modeling scheme.

The growth history of the Northeastern Tibetan Plateau (NETP) is enigmatic, with debates on when and how the NETP significantly uplifted. Here, we use a numerical landscape evolution model to quantitatively investigate the ∼20 Ma growth history of the NETP by studying the formation history of the upstream Yellow River (UYR). The long-term growth history of the NETP consists of an early block uplift (∼20–$��{12}_{�-�4�}^{�+�3�}�$ Ma) and a late outward propagation uplift ($��{12}_{�-�4�}^{�+�3�}�$–0 Ma), compared to the observed river profiles, erosion rates, the trend of acceleration time of deformation, and paleo-elevation data sets. Before $��{12}_{�-�4�}^{�+�3�}�$ Ma (middle Miocene), the NETP was uplifted via a block growth, with a major uplift in the south part. Subsequently, the high (∼5 km) NETP has been uplifted via a northward propagation until the present. We further suggest that pure headward erosion unlikely formed the observed river profile of the UYR over the past few million years. Our modeling thus reconciles the long-term outward growth of the NETP and the UYR profile, suggesting a downstream migration of high erosion rates, which is fundamentally different from the headward erosion of small mountain rivers. The downstream propagation of fluvial erosion may be common in the outward-growing plateaus elsewhere on Earth.

Long recurrence intervals of large earthquakes relative to the historical record mean that geological data are often utilized to inform forecasts of future events. Geological data from any particular fault may constrain the timing of past earthquakes (paleoearthquake data), or simply the time period over which a certain amount of fault displacement has occurred due to one or more earthquakes. These data are typically subject to large uncertainties, and available records often only constrain the timing of a few events. Variability in earthquake inter-event times (aperiodicity) has been observed for many faults, particularly in low seismicity regions, further hampering the utilisation of small data sets for developing forecasts. A challenge for earthquake forecasting therefore concerns how best to utilize all of the limited available data while fully considering uncertainties. Here we present a concise Bayesian model for developing time-dependent earthquake forecasts from geological data. Using the additive property of the Brownian passage time distribution, we make inference on the model parameters jointly from paleoearthquake and fault displacement data. Monte Carlo Markov Chain methods are used to sample the posterior distribution of model parameters, which is subsequently used to forecast future earthquake probabilities. The method incorporates data uncertainties and does not rely on a priori assumptions of quasiperiodic earthquake recurrence, allowing application in a wide range of tectonic settings. We demonstrate the method using data from two reverse faults in Otago, southern Aotearoa New Zealand, a region in which aperiodic earthquake recurrence has previously been observed.

Carbonate rocks are widely used for paleomagnetic reconstructions of continental block rifting and geodynamic evolution. However, the detection of remagnetization in carbonate rocks is challenging. Uncertainty of remagnetization timing hinders our ability to understand the geomagnetic behavior during critical Phanerozoic transitions, such as the period across the Middle-Permian Guadalupian to Late-Permian Lopingian boundary (GLB). To test whether primary magnetization is preserved, we conducted paleomagnetic, calcite U-Pb geochronological and mineralogical studies at the Penglaitan section in southern China. The rock-magnetic results indicate that magnetite, maghemite and rare hematite are dominant remanence carriers. Calcite U-Pb ages of approximately 126 Ma, 223 Ma and 251 Ma, respectively, are consistent with the timing of three remanent magnetizations (RM1, RM2, RM3). This indicates that calcite U-Pb dating can provide robust support to constrain the age magnetizations. The consistency indicates that RM3 was acquired during the limestone deposition, whereas component RM1 and RM2 were due to early Cretaceous and late-Triassic remagnetization, respectively. In addition, the primary component RM3 was isolated from tuffaceous limestone and yielded a mean direction of D_s/I_s = 195.3°/+5.6° (α_95s = 5.3°, k_s = 22.8, n = 34) after tilt-correction. It defined a reversed magnetozone from the top of conodont Jinogondolella granti Zone to the lower part of the Clarkina dukouensis Zone, straddling the GLB. The calcite U-Pb dating and paleomagnetic results provide new insights into Mesozoic multi-remagnetization in the South China Block and refine the GLB positioned in a reversed magnetozone.

Spectral analysis (SA) for image processing, utilizing the Fast Fourier Transform (FFT), computes the 2D power spectrum to capture the amplitude of each frequency component of an image. Recent studies have applied SA on digital elevation models (DEMs) to characterize repetitive and spatially homogeneous landforms in terms of their orientation, frequency, and amplitude. Here, we advance the application of SA by introducing a new preprocessing step and an appropriate windowing function, tailored to analyze heterogenous and complex topographies and derive lineament spatial distributions. The validation of our approach involved two phases: (a) testing on synthetic images, and (b) application to a case study. The synthetic image validation illustrated the length-weighted characteristics of SA-derived rose diagrams and the robustness of the method, evidenced by a 99% similarity across 1,000 synthetically generated lineament networks. The case study consisted of three areas characterized by different topographic patterns within the Oslo region of Norway. The SA-derived results were compared to lineaments automatically extracted using a conventional peak-and-valley seeking algorithm that mimics manual tracing of lineaments inside a 3D map domain. The comparison showed similarity better than 90%. Lastly, we addressed a key pitfall of SA by locating signatures observed in the power spectrum on the map through cross-correlation (CC) of profiles. Although CC results are not consistently perfect, they provide a promising avenue for further development.

The subglacial geology beneath Devon Ice Cap (DIC) is not well understood. An airborne radar study published in 2018 suggested the presence of a hypersaline, subglacial lake beneath DIC where geologic modeling suggested that the source of the brine was an underlying evaporite-rich sedimentary unit. However recent surface based seismic and electromagnetic data have revealed the absence of subglacial water beneath the center of DIC. Continued studies of this subglacial environment require knowledge of the sediments and bedrock beneath the ice. In this study we combine previously published geology and geothermal studies with new surface-based magnetotelluric, airborne gravity and aeromagnetic data, to investigate the subglacial geology under DIC. The integrated results show that beneath the center of DIC there is likely a frozen sedimentary unit (3,000–6,000 Ωm) overlying unfrozen crystalline basement rocks of the Canadian shield (400–2,000 Ωm), at depths of 1,500 m–2,000 m. This agrees with recent studies of ice dynamics on DIC, where glacier velocities are low (<20 m a⁻¹), within the interior regions of DIC implying the ice is dominantly frozen to the bed. Furthermore, relatively low-density sedimentary rocks (∼2.2 g/cm³) are the likely cause of the gravity low (−50 to −70 mgal) observed in the northeast of the ice cap and could have implications for future ice dynamics.

The Namche Barwa massif (NBM) in the Eastern Himalayan Syntaxis (EHS) hosts some of the most active Cenozoic crustal deformation, magmatism, metamorphism, and extreme topographic relief on earth. The crust-mantle dynamics responsible for the tectono-magmatism in the NBM remains debated. We image the detailed crustal structures under the NBM using the recordings of a dense seismic array of 400 3-component node geophones and 4,290 P-wave receiver functions (PRFs). Our results show a wide range of negative seismic phases in the middle and lower crust and weak Moho signal in the northern part of the NBM, under the Yarlung-Zangbo River (YZR). The upper crustal discontinuities show “hut” shape structures, and these features were validated with the forward simulation and PRFs from three broadband seismic stations in the region. These crustal structures indicate the presence of magma underplating in the upper mantle under the NBM.