Shallow Part Research Articles

Adaptive Feedback Convolutional‐Neural‐Network‐Based High‐Resolution Reflection‐Waveform Inversion

AbstractFull‐waveform inversion (FWI) applies non‐linear optimization to estimate the velocity model by fitting the observed seismic data. With a smooth starting velocity model, FWI mainly inverts for the shallower background velocity model by fitting the observed direct, diving and refracted data, and updates the interfaces by fitting the observed reflected data. As the deeper parts of background velocity model cannot be effectively updated by fitting the reflected data in FWI, the deeper interfaces are less accurate than the shallower interfaces. To update the deeper background velocity model, many reflection‐waveform inversion (RWI) algorithms were proposed to separate the tomographic and migration components from the reflection‐related gradient. We propose a convolutional‐neural‐network‐based reflection‐waveform inversion (CNN‐RWI) to repeatedly apply the iteratively updated convolutional neural network (CNN) to predict the true velocity model from the smooth starting velocity model (the tomographic components), and the high‐resolution migration image (the migration components). The CNN is iteratively updated by the more representative training data set, which is obtained from the latest CNN‐predicted velocity model by the proposed spatially constrained divisive hierarchical k‐means parcellation method. The more representative the training velocity models are, the more accurate the CNN‐predicted velocity model becomes. Synthetic examples using different portions of the Marmousi2 P‐wave velocity model show that CNN‐RWI inverts for both the shallower and deeper parts of velocity models more accurately than the conjugate‐gradient FWI (CG‐FWI). Both the CNN‐RWI and the CG‐FWI are sensitive to the accuracy of the starting velocity model and the complexity of the unknown true velocity model.

Focal Mechanism Inversion on the Aftershock Sequence of the 25 October, 2010 Mw 7.8 Mentawai Earthquake: A Preliminary Result

An Mw 7.8 tsunami earthquake ruptured the shallow part of the Mentawai megathrust on 25 October 2010 and generated a large tsunami that hit the Mentawai Islands. The earthquake continued a partial rupture sequence of the 2007 Mw 8.5 and 7.9 Bengkulu earthquake and the 2008 Mw 7.2 North Pagai earthquake. The aftershock activity gives the opportunity to investigate the geometry of the subduction interface, which is critical to understanding the propagation and termination of a large earthquake. However, it is necessary to have a precise location and mechanism of the aftershock distribution to clearly image the fault geometry. In this preliminary study, we have applied the Cut-and-Paste (CAP) focal mechanism inversion method to 20 aftershocks (M ≥ 4.9) of the 2010 Mentawai earthquake. The seismic waveforms recorded by the BMKG broadband seismic network on the regional distance were used in the inversion. They successfully obtained ten focal mechanisms more than that provided in the GCMT catalog. The result exhibits a distinctly different thrust and normal faulting aftershocks activity. The thrust faulting aftershocks are mainly distributed in the downdip of the Mentawai 2010 mainshock rupture, indicating the stresses that have not been released during the mainshock. In the shallow region near the trench, the aftershocks are dominated by a normal faulting mechanism which shows the tensile stress caused by the bending of the subducting plate.

Was the 2018 Hokkaido Eastern Iburi Earthquake in Japan Related to Carbon Capture and Storage (CCS)-CO2 Injection? Insights From Geomechanical Analysis

In a recent paper [“Groundwater anomaly related to CCS-CO2 injection and the 2018 Hokkaido Eastern Iburi earthquake in Japan” by Sano et al. (Front. Earth Sci., 2020, 8)], the authors claimed that CO2-enriched fluid may have initially migrated through permeable channels, blocking the fluid flow from the source region, increasing pore pressure in the focal region and triggering a natural earthquake where the brittle crust was already critically stressed. The proposed model is very interesting, but the authors have not shown any quantitative evaluation supporting their conclusion. Here, through geomechanics model analysis, even under extreme conditions, which overestimate the impact of the injection, the impact of the CO2 injection on the Iburi earthquake fault, whether the deep section or shallow part of the fault, is much lower than that caused by Earth tides. In addition, no convincing mechanism exists that would allow fluid channels to heal within a short period of time and block the natural fluid flow along the fault. Therefore, the occurrence of earthquakes was not related to CO2 injection. Geological storage of CO2 is expected to become an effective option for global warming countermeasures, and the assessment of its environmental impact must be carefully conducted.

Surface Microgravity Monitoring of Underground Water Migration: A Case Study in Wieliczka, Poland

Excessive water inflow in a mine poses a great threat to its operation, especially in the case of a salt mine. In 1992, a rapid outflow of water occurred in the Mina traverse in the Wieliczka Salt Mine, and a number of investigations were undertaken to assess the causes of the outflow and the condition of the rock mass, including the gravity and microgravity surveys discussed in this paper. The first of these was to investigate the rock mass with respect to its geological, hydrogeological properties and mining. The aim of study was to monitor the changes in the rock mass density and the impact of these changes on the subsidence of the ground surface. The surveys provided information on the geological structure of the study area and helped to identify possible routes for water migration. The first data confirmed density changes in the shallow parts of the rock mass, manifested by subsidence of the land surface. However, the subsequent measurements failed to show any significant density changes in the shallow parts of the rock mass, despite the subsidence of the land surface. Therefore, it can be argued that the processes in the rock mass did not cause voids in the shallow parts of the rock mass that could lead to discontinuous deformations. These processes run deep and caused only continuous deformations in the form of subsidence basins.

Combined geophysical surveys using a novel approach to characterize ancient burnt soils: A field experiment at Liangzhu city site in Hangzhou, China

Ancient burnt soils are valuable records in environmental and archaeological studies, and thus it is important to prospect such buried historical remains. By far, the study on geophysical exploration of the burnt soil is still rare. To investigate the effectiveness of geophysical methods for investigating such ancient relics, we designed a controlled field experiment at Liangzhu city site in Hangzhou, China, where there are plenty of Neolithic burnt remains in the shallow subsurface. We acquired common-offset ground penetrating radar (GPR) and electrical resistivity tomography (ERT) data, performed archaeological drilling and measured the physical properties of soil cores. The GPR and ERT data were first analyzed individually and compared with the borehole logs. We observe that the major reflected energy in the GPR data come from the vicinity of the burnt soil layer, while the energy below that is generally weak. The conductivity estimated from the ERT data is overall small in the shallow part but has a sudden increase from the lower interface of the burnt soil layer. The slopes estimated from two types of geophysical data show local similarities and complementarity, based on which we propose a novel workflow for geophysical data fusion based on the technique of predictive painting. The scheme can generate easy-to-understand images of soil layers with low computational costs, and its extension to 3D case is conceptually straightforward. It is also feasible to extend our work to large-scale surveys of ancient burnt soils.

The Presence of a Substrate Strengthens The Jet Generated by Upside-Down Jellyfish

Upside-down jellyfish, Cassiopea, are prevalent in warm and shallow parts of the oceans throughout the world. They are unique among jellyfish in that they rest upside down against the substrate and extend their oral arms upwards. This configuration allows them to continually pull water along the substrate, through their oral arms, and up into the water column for feeding, nutrient and gas exchange, and waste removal. Although the hydrodynamics of the pulsation of jellyfish bells has been studied in many contexts, it is not clear how the presence or absence of the substrate alters the bulk flow patterns generated by Cassiopea medusae. In this paper, we use three-dimensional (3D) particle tracking velocimetry and 3D immersed boundary simulations to characterize the flow generated by upside-down jellyfish. In both cases, the oral arms are removed, which allows us to isolate the effect of the substrate. The experimental results are used to validate numerical simulations, and the numerical simulations show that the presence of the substrate enhances the generation of vortices, which in turn augments the upward velocities of the resulting jets. Furthermore, the presence of the substrate creates a flow pattern where the water volume within the bell is ejected with each pulse cycle. These results suggest that the positioning of the upside-down jellyfish such that its bell is pressed against the ocean floor is beneficial for augmenting vertical flow and increasing the volume of water sampled during each pulse.

Characterization of the Kinematic Evolution of the Hongshiyan Landslide Using the Material Point Method

In 2014, a landslide disaster occurred at Hongshiyan in Ludian County, Yunnan Province, China, causing the landslide-dames in Niulanjiang River. Scholars have studied the characteristics and causal mechanisms of this landslide and obtained substantial results. However, the kinematic evolution characteristics of the landslide remain relatively unclear. To resolve this problem, we introduced the Material Point Method (MPM) to study large-scale deformations for simulation analysis. The results showed that the movement of the slide body could be divided into three stages: accelerated sliding, decelerated sliding, and stabilizing. The velocity field and displacment field of the slide body is closely related to its spatial distribution. The velocity and displacement of the surface part higher than that of the shallow part. The internal friction angle and friction coefficient significantly affected the kinematic characteristics of landslide. The decrease in the internal friction angle and friction coefficient reduced the energy consumed by the friction between particles and between the particles and the sliding bed, raising the landslide velocity and displacement.

Coseismic Rupture Model and Tectonic Implications of the January 7 2022, Menyuan Mw 6.6 Earthquake Constraints from InSAR Observations and Field Investigation

A Mw 6.6 earthquake struck Menyuan, Qinghai, China, on 7 January 2022. To determine the rupture parameters of this event, the coseismic InSAR deformation fields were mapped and further employed to estimate the focal mechanism. The best-fitting solution emphasized that the 2022 Menyuan earthquake ruptured at the junction of the Tuolaishan fault and the Lenglongling fault. Both rupturing faults were dominated by sinistral strike-slip, and the main slip was concentrated on the shallow part of the rupture plane. The latter was the main rupture segment with a strike of 106° and a dip of 86°. The slip mainly occurred at depths of 0–8 km, and the rupture was exposed at the surface. The maximum slip reached ~3.5 m, which occurred mainly at a depth of 4 km. Joint analysis of the optimal slip model, relocated aftershocks, Coulomb stress change, and field observation suggested that the strain energy in the Tuolaishan fault may not have been fully released and needs further attention. Moreover, the 2022 Mw6.6 Menyuan earthquake caused a significant stress loading effect on the western Tuolaishan fault and eastern Lenglongling fault, which implies that the 2022 event increased the seismic hazard in these regions.

P-Wave Velocity Structure of the Lower Crust and Uppermost Mantle beneath the Sichuan–Yunnan (China) Region

Abstract We conduct a tomographic inversion for the 3D P-wave velocity structure in the lower crust and uppermost mantle of the Sichuan–Yunnan region in the southeastern margin of the Tibetan Plateau. A total of 43,450 reliable arrival times of P waves are picked from over 300,000 regional seismic records using an automatic algorithm based on deep learning. A two-stage iterative inversion process in which events are relocated, leading to a significant reduction in travel-time residuals, is adopted. A statistical resolution matrix analysis suggests that our model has an optimal spatial resolution length of ∼0.4° in the lower crust and ∼0.2° in the uppermost mantle. Our 3D model shows that both the lower crust and uppermost mantle in the region are characterized by strong lateral heterogeneities. The unusually low velocities in the lower crust may indicate the existence of lower crustal flow, whereas the high-velocity anomalies in the uppermost mantle in and around the Sichuan–Yunnan Rhombic block (SYRB) may be an important factor in preventing the ductile materials in the lower crust from moving eastward. Our model also indicates a coupling between the surface deformation and the material flow in the lower crust. Finally, the lower crustal flow may influence the materials in the shallow part of the uppermost mantle beneath the SYRB, and the crust–mantle transition zone beneath the Songpan–Ganzi block may be influenced by weak materials from both the uppermost mantle and the lower crust.

Modelling groundwater flow in the Plaine du Nord–Massacre shallow aquifer, Haiti

To assist in the sustainable management of water resources, a groundwater model of the shallow groundwater in the Plaine du Nord–Massacre transboundary aquifer system was developed. The Plaine du Nord–Massacre Aquifer is one of the largest aquifer systems of Haiti, supposedly bearing a high potential for water supply and irrigation. The shallower part is already tapped by hundreds of domestic wells and hand-pumped public and private boreholes. In the framework of the AVANSE project (Appui à la Valorisation du Potentiel Agricole du Nord, pour la Sécurité Économique et Environnementale), a USAID funded project aiming at developing sustainable agriculture in the region, a groundwater model was built to assist in decision making. The finite-difference-based MARTHE software was used for the model. Due to the lack of monitoring data, the calibration of the model could only be carried out under steady-state hydraulic conditions. It should thus be considered as a first step towards a better characterisation of the aquifer’s potential. However, the model computed a groundwater balance at the scale of the aquifer showing that significant water resources can be easily mobilized if one carefully manages to rule out local operational difficulties. The simulated groundwater drawdowns using transient simulation of irrigation scenarios are pluri-metric, reaching 6–7 m. New irrigation wells must be deeper than the current average depth of wells to support agricultural development projects.

Study on Rubber-Pad Formation for Sheet Metal

This article introduces four rubber-pad formation techniques for the pressing and drawing of components from thin sheet metals. The Guerin process is the simplest technique of rubber formation, which enables the production of shallow sheet metal parts with complex contours and bends. Free formation is another technique used to draw near-hemispherical shells using a rubber medium. Different bending processes can also be conducted using a rubber pad as a flexible die. Multipoint formation is applied to form complex three-dimensional sheet metal parts with discrete dies and rubber cushions. The fundamentals of these techniques as well as the influence of process parameters on the results are investigated in this paper

Two Improved Acquisition Systems for Deep Subsurface Exploration

Present land seismic surveys mainly focus on acquiring reflection data. The maximum offset is usually 1–1.5 times the depth of targets. Limited offset results in that the acquired diving waves only penetrate the shallow parts of the Earth model, far from targets. Thus, the reflection data are used to build the deep part of the velocity model with migration velocity analysis. However, two issues challenge the success of velocity model building. First, incomplete information. Limited offsets lead to a narrow aperture of observation, which results in an under-determined inversion system. One manifestation is the trade-off between the depth of interfaces/reflectors and the average velocity above them. Second, low signal-to-noise (S/N) ratios. Complex near-surface conditions and geologic structures lead to low S/N ratios for reflection data, which fails to build velocity with reflection data. The fundamental solution to these two issues is to acquire better data with an improved acquisition system. In this work, we propose two types of modified geometries to enhance the penetration depth of the diving waves, especially the first arrivals, which can be used to build a deeper velocity model effectively. Type-I geometry adds extra sparse sources on the extension line of the normal acquisition geometry, whereas Type-II geometry deploys extra sparse receivers on the extension line. Consequently, the new acquisition system includes ultra-large offsets, which acquire diving waves from the deep subsurface. These diving waves, including waveform and first-break time, are particularly useful for recovering deeper velocity, which has paramount significance for the exploration of deep and ultra-deep hydrocarbon reservoirs. Synthetic and field data examples preliminarily demonstrate the feasibility of this improved acquisition system.

Behaviors of lithium and its isotopes in groundwater with different concentrations of dissolved CO2

Lithium (Li) isotopes have shown large potential in tracing weathering in various water bodies, but there is limited study on Li isotopes in subsurface conditions where CO2 has been largely consumed. In this study, we use a thick sandstone aquifer in the Ordos Basin, NW China, as a natural setting to investigate the behaviors of Li isotopes in hydrogeochemical conditions with different concentrations of dissolved CO2. For young groundwater in the recharge area (group R) where CO2 is abundant (mean PCO2 = 10-2.5 atm), clay formation accompanying with weathering leads to the enrichment of 7Li in groundwater. The four deep samples in the recharge area have uniform Li/Na ratios (with a mean of 2.52 μmol/mmol) and δ7Li (with a mean of 25.0‰), corresponding to a mean Li removal rate of 81.2% compared with the sandstone leachate. For groundwater in the shallow part of the discharge area (group D1), Li was firstly removed by clay formation during weathering in the recharge area and was later removed by physisorption when CO2 becomes much lower (mean PCO2 = 10-3.1 atm). Different degrees of weathering lead to a wide range of δ7Li varying from 19.7‰ in the deepest well to 33.0‰ in the shallowest well. The proportion of Li removal caused by physisorption is found to increase with groundwater age. After the stage of Li removal by adsorption, Li was released in the deeper part of the discharge area (group D2), and the positive correlation of δ7Li versus Li/Na is explained by a ternary mixing model. The endmember of water brought by cation exchange is inferred to have a heavier δ7Li than sandstone leachate, demonstrating that cation exchange could cause an enrichment of 7Li in water. This study enhances our understanding of the controlling factors of Li isotopes in deep groundwater with low dissolved CO2, which have implications for the application of Li isotopes in subsurface water.

Development of an integrated hydrochemical index for delineating livestock manure-derived groundwater plumes in agro-livestock farming areas

To delineate a livestock manure-derived groundwater plume (LDGP) in agro-livestock farming areas with the extensive use of chemical fertilizers, multilevel monitoring wells (MLWs) were installed for depth-specific sampling of groundwater, and then hydrochemical, nitrate N-O isotopic and microbiological data of shallow groundwater collected from the MLWs were evaluated with the help of multivariate statistical tools. The LDGP was distinguished from the pervasive agricultural contamination based on δ15Nnitrate (10‰). Fecal coliforms and Escherichia coli were not observed in the LDGP, whereas bovine enterovirus type 2 was detected at a sample collected at a depth of 18 m below ground level (bgl) downgradient manure piles, indicating a potential virus risk. Among hydrochemical parameters, Ca2+, Mg2+, Na+, Cl- and SiO2(aq) were shown to be effective indicators to trace the LDGP. On the other hand, nitrate was not effective to discriminate the LDGP because of denitrification in deep parts (>6 m bgl) within the LDGP, although nitrate contamination was serious at shallow parts (≤6 m bgl; 62.4 to 119.1 mg/L, median 97.9 mg/L). Thus, an integrated hydrochemical index consisting of Ca2+, Mg2+, Na+, Cl- and SiO2(aq) was suggested based on the result of principal component analysis with isometric log-ratio transformed data to delineate the LDGP in shallow unconsolidated aquifers overlying silicate bedrocks, and a threshold of the index was determined to be 1.17. The application of the index to three other agro-livestock farming areas with prevalent agricultural contamination successfully distinguished the groundwater samples within the LDGP (11 of 40 samples) that showed the index values exceeding the threshold and a median δ15Nnitrate of 12.5 ‰. The study results show that the hydrochemical index can be used for the quick evaluation of a LDGP in agro-livestock farming areas given its cost efficiency and easily accessible analysis devices for water chemistry compared to isotopes, although the broad use of the index to determine LDGPs needs to be further verified by application to various geology and land uses. Livestock manures should be carefully managed to protect groundwater resources, including impermeable covers above or beneath the manure piles, given the potential risk of virus at deep parts and nitrate at shallow parts within the LDGP. The methodology to select hydrochemical indicators for distinguishing a LDGP and the combination of the indicators to develop an index suggested in this study will be useful to build new indices adapted to local conditions.

Reconstructing Hydrothermal Fluid Pathways and Storage at the Solfatara Crater (Campi Flegrei, Italy) Using Seismic Scattering and Absorption

Imaging of fluid pathways is crucial to characterize processes taking place in hydrothermal systems, a primary cause of volcanic unrest and associated hazards. The joint imaging of seismic absorption and scattering is an efficient instrument to map fluid flow at crustal scale, and specifically in volcanoes; however, this technique has so far been applied to image volcanoes and hydrothermal systems at the kilometre scale. Here, we use data from a meter-scale, active seismic survey inside the shallow structure of the Solfatara crater to obtain the first frequency-dependent near-surface scattering and absorption model of a hydrothermal system. The Solfatara crater is the place used to monitor historic unrest at Campi Flegrei caldera (Italy), a high-risk volcano under continuous surveillance due to its closeness to a densely populated area. Improving the imaging of the shallow part of this system is crucial to broaden the understanding of unrest processes that are progressively characterizing other portions of the eastern caldera. The scattering contrasts highlight the primary structural feature, a fault separating the hydrothermal plume from zones of CO2saturation nearing fumaroles. While high-absorption anomalies mark zones of high soil temperatures and CO2fluxes, low-absorption anomalies indicate zones of very shallow upflow and are caused by contrasts between liquid-rich and vapour-rich fluids coming from mud pools and fumaroles, respectively. All maps show a SW-NE trend in anomalies consistent with fluid-migration pathways towards the eastern fumaroles. The results provide structural constraints that clarify mechanisms of fluid migration inside the crater. The techniques offer complementary geophysical images to the interpretation of hydrothermal processes and prove that seismic attenuation measurements are suitable to map fluid pathways in heterogeneous media at a detailed scale.

Peridotites with back-arc basin affinity exposed at the southwestern tip of the Mariana forearc

Peridotites at water depths of 3430 to 5999 m have been discovered using the submersible Shinkai6500 (dives 6K-1397 and 6K-1398) on the southwestern slope of the 139°E Ridge (11°12′N, 139°15′E), a small ridge at the southwesternmost tip of the Mariana forearc near the junction with the Yap Trench and Parece Vela Basin. The peridotites studied consist of 17 residual harzburgites and one dunite and show various textures with respect to their depths. Peridotites with coarse-grained (> 1 mm) textures were sampled from the shallowest part (3705–4042 m) of the dive area, and peridotites with fine-grained (< 0.5 mm) textures were sampled deeper (5996 m). Olivine crystal-fabrics vary with grain size, with (010)[100] A-type patterns for the coarse-grained peridotites, {0kl}[100] D-type patterns for the fine-grained peridotites, and various indistinct patterns in samples of variable grain sizes. Fine-grained peridotites with D-type olivine crystal-fabrics could result from deformation under relatively higher flow stresses, suggesting that a ductile shear zone in the lithospheric mantle could occur in the deepest part of 139°E Ridge. Spinel Cr# range from relatively low (0.36) to moderately high (up to 0.57), and correlate with Ti contents (0.07–0.45 wt.%). The trace element patterns of clinopyroxene similarly exhibit steepening slopes from the middle to the light REEs regardless of textural variations. These mineralogical and geochemical features would result from melt-rock interactions under conditions of relatively shallow lithospheric mantle, which are much more comparable with the Parece Vela Basin peridotites than the Mariana forearc peridotites. Consequently, the Parece Vela Basin mantle is more likely exposed on the inner slope of the westernmost Mariana Trench, presumably due to the collision of the Caroline Ridge.

Geochemical Characteristics and Process of Hydrocarbon Generation Evolution of the Lucaogou Formation Shale, Jimsar Depression, Junggar Basin

Lacustrine shale, represented by the Middle Permian Lucaogou Formation in the Jimsar Depression in the eastern Junggar Basin, has become one of the main areas of shale oil exploration in China. In this study, we used 137 samples of shale from the Lucaogou Formation, drawn from 14 wells in the Jimsar Depression, to investigate their characteristics of pyrolysis, organic carbon and soluble organic matter content, biomarkers, organic microscopic composition, and vitrinite reflectance. Basin simulation and hydrocarbon generation thermal simulation experiments were also conducted in a closed system. The results of this study indicate that the input of an algae source was dominant in the source rocks of the Lucaogou Formation, that the water in which the rocks were deposited had high salinity and strong reducibility, and that the source rocks were oil-prone. The Lucaogou source rocks generally had good hydrocarbon generation capability, but showed significant heterogeneity. At the end of the Cretaceous period, the shales in the Lucaogou Formation entered the oil-generation window as a whole. Currently, the shales of the Lucaogou Formation are generally in the high-maturity stage in the deep part of the depression, producing a large amount of high-maturity oil and condensate gas, while those in the shallow part have relatively low maturity and can only produce a large amount of conventional crude oil. The maximum crude oil generation rate of the Lucaogou Formation shale obtained from the thermal simulation results was 220.2 mg/g of the total organic carbon (TOC), and the maximum hydrocarbon expulsion efficiency was estimated to be 59.3–76.4%.

Experimental and analytical investigation on chloride ions transport in concrete considering the effect of dry-exposure ratio under diurnal tidal environment

For reinforced concrete structures in tidal zone, the dry-wet ratio (the ratio of drying time to wetting time in one cycle) of member varies with the elevation. Chloride ions transport in concrete is affected by coupling action of diffusion and convection under dry-wet cycle environment. The erosion of concrete corresponding elevation of the most unfavorable dry-wet ratio is the most intense, in which invade the most chloride ions. In existing studies, the most unfavorable dry-wet ratio determined by different scholars is not consistent, and there is a lack of quantitative indexes to describe the impact of convection action on chloride concentration distribution. In this research, a series of natural exposure experiments were carried out in submarine environment and dry-wet cycle environment using the indoor tidal automatic circulation device independently invented by the laboratory. A new index, dry-exposure ratio (the ratio of drying time to exposure time in a cycle), was proposed to quantify the dry-wet cycle environment. The mechanism of convection and the influence of different dry-exposure ratios on chloride ions transport in concrete were discussed, and the most unfavorable dry-exposure ratio was determined. Under the condition of diurnal tide (dry-wet cycle period is 24 h), while wetting time is 10 min, shallow part of concrete is the most seriously corroded by chloride ions due to the coupling action of convection and diffusion, while wetting time is 45 min, deep part of concrete is the most seriously corroded by chloride ions diffusion. The variations of peak chloride concentration, depth of convection zone and apparent chloride diffusion coefficient with the dry-exposure ratio and exposure duration were analyzed. The results show that the peak chloride concentration and depth of convection zone can be used as the indexes of quantitative convection. By introducing the depth of convection zone and modifying the peak chloride concentration and apparent chloride diffusion coefficient, a time-dependent model of chloride ions transport in concrete considering the effect of dry-exposure ratio was established. The proposed empirical model is expected to be useful in realistically predicting chloride ions transport in concrete under dry-wet cycle environment.

Episode 4 (2019\u20132020) Nishinoshima activity: abrupt transitions in the eruptive style observed by image datasets from multiple satellites

In December 2019, a new activity started at Nishinoshima volcano in the southern part of the Izu–Ogasawara arc, Japan. This is now referred to as Episode 4 of a series of activities that began in 2013. We analyzed the eruption sequence, including erupted volume and effusion rate, based on combined observations of thermal anomalies by Himawari-8 and topographic changes by ALOS-2. The total eruption volume during Episode 4 was ~ 132 × 106 m3, and the average effusion rate over the entire period was 0.51 × 106 m3 day−1 (5.9 m3 s−1), which was two to three times higher than that of Episode 1. Episode 4 had three stages. In Stage 1, effusive activity was dominant, and most of the lava erupted from a northeast vent at the foot of the pyroclastic cone to cover the northern half of the island. The average effusion rate was estimated to be 0.46 × 106 m3 day−1 (5.3 m3 s−1). In Stage 2, an intensive lava fountain with a high discharge rate developed, and it increased the size of the pyroclastic cone rapidly. The effusion rate temporarily reached 2.6 × 106 m3 day−1 (30 m3 s−1). Pyroclastic rocks accounted for 45–88% of the total erupted volume in this stage. Lava flows with rafted cone material were generated, and those possibly caused by intensive spatter falls on the slope were also formed. These lavas flowed down the southern half of the island. In Stage 3, continuous phreatomagmatic eruptions released ash and spread it over a wide area. The high effusion rate and the drastic change in the activity style in Episode 4 can be explained by deep volatile-rich magma being supplied to a shallower magma chamber prior to Episode 4. When the volatile-rich magma reached a shallow part of the conduit in Stage 2, fragmentation occurred due to rapid volume expansion to eject large amounts of magma and form the intensive lava fountain. Observations by satellite-borne ultraviolet–visible image sensors detected a rapid increase in SO2 emissions in response to the intensive lava-fountain activity. The less-differentiated nature of the ash fragments collected during Stage 2 may reflect the composition of the volatile-rich magma. Large-scale discolored-seawater areas appeared during the late period of Stage 1, which may have been caused by ascent of the volatile-rich magma.Graphical

Full-Likelihood Genomic Analysis Clarifies a Complex History of Species Divergence and Introgression: The Example of the erato-sara Group of Heliconius Butterflies.

Introgressive hybridization plays a key role in adaptive evolution and species diversification in many groups of species. However, frequent hybridization and gene flow between species make estimation of the species phylogeny and key population parameters challenging. Here, we show that by accounting for phasing and using full-likelihood methods, introgression histories and population parameters can be estimated reliably from whole-genome sequence data. We employ the multispecies coalescent (MSC) model with and without gene flow to infer the species phylogeny and cross-species introgression events using genomic data from six members of the erato-sara clade of Heliconius butterflies. The methods naturally accommodate random fluctuations in genealogical history across the genome due to deep coalescence. To avoid heterozygote phasing errors in haploid sequences commonly produced by genome assembly methods, we process and compile unphased diploid sequence alignments and use analytical methods to average over uncertainties in heterozygote phase resolution. There is robust evidence for introgression across the genome, both among distantly related species deep in the phylogeny and between sister species in shallow parts of the tree. We obtain chromosome-specific estimates of key population parameters such as introgression directions, times and probabilities, as well as species divergence times and population sizes for modern and ancestral species. We confirm ancestral gene flow between the sara clade and an ancestral population of Heliconius telesiphe, a likely hybrid speciation origin for Heliconius hecalesia, and gene flow between the sister species Heliconius erato and Heliconius himera. Inferred introgression among ancestral species also explains the history of two chromosomal inversions deep in the phylogeny of the group. This study illustrates how a full-likelihood approach based on the MSC makes it possible to extract rich historical information of species divergence and gene flow from genomic data. [3s; bpp; gene flow; Heliconius; hybrid speciation; introgression; inversion; multispecies coalescent]