Uppermost Layer Research Articles

High‐Frequency Seismic Events on Mars Observed by InSight

AbstractThe seismometer deployed on the surface of Mars as part of the InSight mission (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) has recorded several hundreds of marsquakes in the first 478 sols after landing. The majority of these are classified as high‐frequency (HF) events in the frequency range from approximately 1 to 10 Hz on Mars' surface. All the HF events excite a resonance around 2.4 Hz and show two distinct but broad arrivals of seismic energy that are separated by up to 450 s. Based on the frequency content and vertical‐to‐horizontal energy ratio, the HF event family has been subdivided into three event types, two of which we show to be identical and only appear separated due to the signal‐to‐noise ratio. We show here that the envelope shape of the HF events is explained by guided Pg and Sg phases in the Martian crust using simple layered models with scattering. Furthermore, the relative travel times between these two arrivals can be related to the epicentral distance, which shows distinct clustering. The rate at which HF events are observed varies by an order of magnitude over the course of one year and cannot be explained by changes of the background noise only. The HF content and the absence of additional seismic phases constrain crustal attenuation and layering, and the coda shape constrains the diffusivity in the uppermost shallow layers of Mars.

Satellite Perspectives of Sea Surface Temperature Diurnal Warming on Atmospheric Moistening and Radiative Heating during MJO

AbstractDiurnal air–sea coupling affects climate modes such as the Madden–Julian oscillation (MJO) via the regional moist static energy budget. Prior to MJO initiation, large-scale subsidence increases (decreases) surface shortwave insolation (winds). These act in concert to significantly warm the uppermost layer of the ocean over the course of a single day and the ocean mixed layer over the course of 1–2 weeks. Here, we provide an integrated analysis of multiple surface, top-of-atmosphere, and atmospheric column observations to assess the covariability related to regions of strong diurnal sea surface temperature (dSST) warming over 44 MJO events between 2000 and 2018 to assess their role in MJO initiation. Combining satellite observations of evaporation and precipitation with reanalysis moisture budget terms, we find 30%–50% enhanced moistening over high-dSST regions during late afternoon using either ERA5 or MERRA-2 despite large model biases. Diurnally developing moisture convergence, only modestly weaker evaporation, and diurnal minimum precipitation act to locally enhance moistening over broad regions of enhanced diurnal warming, which rectifies onto the larger scale. Field campaign ship and sounding data corroborate that strong dSST periods are associated with reduced middle-tropospheric humidity and larger diurnal amplitudes of surface warming, evaporation, instability, and column moistening. Further, we find greater daytime increases in low cloud cover and evidence of enhanced radiative destabilization for the top 50th dSST percentile. Together, these results support that dSST warming acts in concert with large-scale dynamics to enhance moist static energy during the suppressed to active phase transition of the MJO.

Surface effects and turbulent pressure

The application of the full potential of stellar seismology is made difficult by the improper modelling of the upper-most layers of solar-like stars and their influence on the modelled frequencies. Our knowledge of these so-called ‘surface effects’ has improved thanks to the use of 3D hydrodynamical simulations, however, the calculation of eigenfrequencies relies on empirical models for the description of the Lagrangian perturbation of turbulent pressure, namely: the reduced-Γ1 model (RGM) and the gas-Γ1 model (GGM). Starting from the fully compressible turbulence equations, we derived both the GGM and RGM models by using a closure to model the flux of turbulent kinetic energy. We find that both models originate from two terms: the source of turbulent pressure due to compression produced by the oscillations and the divergence of the flux of turbulent pressure. We also demonstrate that they are both compatible with the adiabatic approximation and, additionally, that they imply a number of questionable assumptions, mainly with respect to mode physics. Among other hypotheses, it is necessary to neglect the Lagrangian perturbation of the dissipation of turbulent kinetic energy into heat and the Lagrangian perturbation of buoyancy work.

Seismic observation and subsurface imaging using an urban telecommunication optic-fiber cable

<p indent=0mm>Seismic observation and subsurface imaging are of key importance in mitigating the seismic hazards and exploiting subsurface spaces in urban areas. However, the urban seismic observation is, to some extent, hindered by more and more populations and facilities in urban areas. To develop new methods for urban seismic observation, we carried out several field experiments in Binchuan city, Yunnan Province. In the experiments, we applied the distributed acoustic sensing (DAS) technique to an urban telecommunication optic-fiber cable to monitor ground motions. The DAS technique was rapidly developed and applied to seismic investigations of different scales during the past several years. Binchuan city is located in the Binchuan basin, which is one of the largest sedimentary basins in western Yunnan. The Binchuan basin is well studied and equipped with seismic instruments, including a Fixed Airgun Signal Transmission Station (FASTS). The FASTS can radiate highly repeatable seismic signals with seismic energy equivalent to a magnitude 0.7 earthquake; the seismic signal can be used to test the sensitivity of seismic instruments during the seismically quiet period. During the experiment, a <sc>5.2 km</sc> long optic-fiber cable buried along two local roads was parted into ~1000 sensing channels with a channel spacing of several meters, and the cable vibration was continuously digitized with a sampling rate of 5000 sps (samples per second). The DAS registered the ground motions originating from various sources. We clearly identified some vibrations moving with a speed of ~<sc>40 km/h,</sc> which likely originate from moving vehicles along the road. During the experiment, we fired 111 airgun shots. The seismic signal from each single airgun shot was invisible in the DAS records, but clearly emerged after stacking. The airgun signals recorded by the DAS show similar features as recordings from co-located short period seismometers. To test the feasibility of imaging the subsurface with DAS, we also hammered the ground at 48 points along the road with an <sc>8 kg</sc> hammer. The DAS clearly recorded the hammer signal traveling with apparent velocities of <sc>300−400 m/s</sc> to a distance of <sc>~100 m.</sc> Furthermore, we reconstructed the empirical Green’s function from continuous DAS record. We are able to retrieve high-quality surface wave signals and corresponding dispersion curves (the relationship between surface wave velocity and frequency) from 20-min long noise recordings. Then based on the dispersion curves, we obtained the shear wave velocity structure of the uppermost <sc>200 m.</sc> The shear wave velocity of the uppermost layer is important for the soil classification in engineering seismology. The shear wave velocities of the uppermost <sc>200 m</sc> range from ~280 to <sc>430 m/s,</sc> indicating a class II soil in the Binchuan basin. Our preliminary results revealed the feasibility of seismic observation using an urban telecommunication optic-fiber cable, which provides a novel method for many urban seismic applications, such as fine subsurface structure imaging, traffic monitoring, and earthquake early warning.

Modelling landslide hazards under global changes: the case of a Pyrenean valley

Abstract. Several studies have shown that global changes have important impacts in mountainous areas, since they affect natural hazards induced by hydrometeorological events such as landslides. The present study evaluates, through an innovative method, the influence of both vegetation cover and climate change on landslide hazards in a Pyrenean valley from the present to 2100. We first focused on assessing future land use and land cover changes through the construction of four prospective socioeconomic scenarios and their projection to 2040 and 2100. Secondly, climate change parameters were used to extract the water saturation of the uppermost layers, according to two greenhouse gas emission scenarios. The impacts of land cover and climate change based on these scenarios were then used to modulate the hydromechanical model to compute the factor of safety (FoS) and the hazard levels over the considered area. The results demonstrate the influence of land cover on slope stability through the presence and type of forest. The resulting changes are statistically significant but small and dependent on future land cover linked to the socioeconomic scenarios. In particular, a reduction in human activity results in an increase in slope stability; in contrast, an increase in anthropic activity leads to an opposite evolution in the region, with some reduction in slope stability. Climate change may also have a significant impact in some areas because of the increase in the soil water content; the results indicate a reduction in the FoS in a large part of the study area, depending on the landslide type considered. Therefore, even if future forest growth leads to slope stabilization, the evolution of the groundwater conditions will lead to destabilization. The increasing rate of areas prone to landslides is higher for the shallow landslide type than for the deep landslide type. Interestingly, the evolution of extreme events is related to the frequency of the highest water filling ratio. The results indicate that the occurrences of landslide hazards in the near future (2021–2050 period, scenario RCP8.5) and far future (2071–2100 period, scenario RCP8.5) are expected to increase by factors of 1.5 and 4, respectively.

Alternative afforestation options on sandy heathland result in minimal long-term changes in mineral soil layers

Extensive afforestation is currently being widely promoted as a key nature-based solution for climate change mitigation. Fundamental to this strategy is the sequestration of carbon into long-term stable storage, either in wood products or the soil. However, the long-term effects of tree planting on soil carbon, or other soil properties, has rarely been examined. Importantly, afforestation can take many different forms, with differing effects on soil properties. Here, we evaluate how the historical afforestation of sandy heathland adopting a range of management options – including different combinations of conifers and broadleaves in monocultures and mixtures – have affected soil pH, total carbon and nitrogen concentrations, the C:N ratio, and carbon and nitrogen stocks almost a century later. We analyse these properties at a range of soil depths through the organic (litter, F and grass layers) and upper mineral (0–5 cm, 5–10 cm and 10–20 cm depth) soil layers. In comparison to the historical heathland sites, afforestation decreased soil pH, most dramatically under conifers, and increased the C:N ratio. However, there was overall little difference in carbon and nitrogen concentrations between alternative management options. While the total carbon and nitrogen concentrations were much higher in the organic layers of the forest options compared to the open sites, this did not translate into differences in the mineral layers. Furthermore, although we found some evidence of the transferral of carbon and nitrogen into the uppermost soil mineral layers, this was minimal in comparison to the concentrations of the organic layers. The soils at our study site are low quality and sandy, and are therefore unfavourable for incorporating organic matter, but it is still notable how little was incorporated after nearly a century of afforestation. Given the current emphasis on tree planting as a means to tackle climate change, these results demonstrate the fundamental importance of the appropriate consideration of both the afforestation management option and underlying soil type.

Modeling Subsurface Drainage in Compacted Cultivated Histosols

Reclaiming histosols in Montéregie region, Québec, Canada, increases peat decomposition and compaction rate and decreases the effectiveness of subsurface drainage. The objective of this paper was to use HYDRUS-2D to model the behavior of subsurface drainage systems, in order to evaluate the compaction effect on drain depth and spacing, and to determine the compact layer thickness and saturated hydraulic conductivities (Ksat) resulting in an improvement of subsurface drainage]. The drainage model was calibrated [Nash-Sutcliffe efficiency coefficient (NSE) = 0.958, percent bias (PBIAS) = −0.57%] using Ksat, meteorological data, and matric potential (h) data measured on the project site from June 10 to July 19, 2017. The calibrated and validated model was used to analyze the variation of h values (Δh in cm d−1) as a function of drain spacing (2–7 m) and drain depth (1 and 1.2 m) and to identify the response surface of Δh to various compact layer thickness and Ksat combinations. The results showed that Δh was on average 58% greater below the compact layer than above it and that reducing drain spacing or increasing drain depth does not improve the drainage rate. The analysis of the compact layer thickness and Ksat effect on Δh showed that for a Δh of 40 cm d−1, Ksat actual values in the two uppermost layers should be multiplied by 50 for compact layer thickness varying from 12 to 35 cm. Water percolation in the soil is reduced by the compact layer. Soil management methods for improving Ksat should therefore be better than deepening the drains or and reducing the spacing.

Wie tief ist der Boden? Von belebten Strukturen und Kreaturen des Untergrunds

The biologically active part of the earth‘s upper crust is not limited to a layer of the earth‘s surface only a few meters thick but extends much more heterogeneously in its vertical dimension. It extends locally down to a depth of 2 kilometres or more in caves and, in any case, several decametres through gaps, fissures, and crevices. With numerous examples from several continents, this article provides an insight into the diversity of subterranean habitats and the species that colonize them. The term „soil“ is interpreted broadly and discussed in the context of the subterranean part of the Earth‘s „critical zone“. Reference is also made to the potential refugial and corridor function of subterranean habitats in the context of climate change. Soil protection and the protection of subterranean habitats must be considered in a more differentiated and broader way than the „classic“ view, which often focuses on productive, biomass-rich, habitats that are preferred by higher plants. Extreme site conditions often play an important role for biodiversity on the soil, in its uppermost layers and in the „deep soil“; without or with only low suitability for higher plants and without accumulation of biomass, partly also with a dynamic through recurring processes that interrupt soil succession in small (mosaic-like) or large areas. In any case, in terms of biodiversity, this must be given equal consideration or case-by-case, and area-by-area, even priority over other possible soil functions

Combined Use of Zirconium-Modified Bentonite Capping and Calcium Nitrate Addition to Control the Release of Phosphorus from Sediments

In this study, sediment incubation experiments were carried out to investigate the efficiency and mechanism of the control of phosphorus (P) release from sediments. The results showed that under anoxic conditions, P could be released from the sediment into the pore water first and then the dissolved P in the pore water could be transported into the overlying water, leading to high concentrations of soluble reactive P (SRP) and diffusive gradient in thin-films (DGT)-labile P in the overlying water. However, the combined use of calcium nitrate (CN) addition and zirconium-modified bentonite (ZB) capping could effectively control the release of P from sediment, resulting in the low concentrations of SRP and DGT-labile P in the overlying water. Furthermore, the combined use of CN addition and ZB capping could significantly decrease the concentrations of SRP and DGT-labile P in the sediment. In addition, the combined utilization of CN addition and ZB capping also could result in a reduction of redox sensitive P (BD-P) in the uppermost sediment layer. The reduction of pore water SRP, DGT-labile P, and BD-P in sediment solids is of great importance to the control of sediment-P liberation by the combined use of CN addition and ZB capping. The reduction efficiency of overlying water SRP by combined CN addition/ZB capping technology was higher than that of single CN addition technology. Compared to that of single CN addition technology, the reduction efficiencies of pore water SRP, SRP diffusion flux across the sediment/overlying water interface (SWI), and BD-P in the sediment by the combined use of CN addition and ZB capping were also higher. The combined technology based on CN addition and ZB capping had a higher reduction efficiency of overlying water SRP during the late stage of sediment remediation than the single technology based on ZB capping, and the former had higher reduction efficiencies of pore water SRP, DGT-labile P, and SRP diffusion flux across the SWI and apparent P diffusion flux through the SWI than the latter. The results of this work indicate that the combined use of CN addition and ZB capping is a very promising method for the control of P release from sediments.

Spatial and Seasonal Pattern of Fine Root Biomass and Turnover Rate in Different Land Use Systems in Central Himalaya, India

Variation in fine root biomass (FRB) and turnover rate (TOR) were examined in four land use systems at two altitudes by using ingrowth core method. The aim of the study was to analyse the effect of altitude, depth and distance from tree stem on FRB and TOR. Significant variations (p < 0.05) were observed among land use systems, over seasons, depth and distances from the tree stem. The FRB varied between 47.09 and 528.59 kg ha–1. It was maximum in agri-horticulture (AH) system in bhabhar region and in homegarden (HG) system in hill region. In each land use system at both the altitudes FRB was maximum at uppermost layer (0‒20 cm) during rainy season at 50 cm distance from tree stem while, TOR did not showed any particular trend with soil depth and distance from the tree bole. FRB and turnover rate showed significance correlation with distance from tree bole at both the altitudes i.e. (r2 = 0.898 for low and r2 = 0.899 for high altitude).

Phosphorus accumulation in a southern Brazilian Ultisol amended with pig manure for nine years

ABSTRACT: This study evaluated P pools after nine years of successive application of either pig slurry (PS) or deep pig litter (DL) in a no-till Ultisol from southern Brazil. The experiment was established in Dec 2002 with the treatments control, application of 90 and 180 kg N ha−1 N as PS and as DL. In Mar 2010, soil samples were taken at six layers up to 30 cm deep. Total organic and inorganic P were assessed by the ignition method, and P compounds classes were evaluated by 31P-NMR spectroscopy. Total soil P increased proportionally with the P amount applied via DL and PS. Only DL application increased soil organic P, mainly at the highest dose and in the uppermost soil layers. The application of high doses of manure to these soils under no-till to meet crop N demands significantly increased P accumulation at the soil surface, especially with DL. This, in turn, increases the risk of contamination of water bodies due to P transfer from soil to rivers via runoff. The ignition method overestimates organic P compared to P-NMR. The highest proportion of organic P estimated by the ignition and P-NMR methods, at surface layers in the control suggests that inorganic P is added to the plots treated, increasing inorganic P and decreasing organic P. Moreover, with no P additions to the control, inorganic soil P is removed by plants, causing an apparent increase in the organic P proportion.

The New Swedish Cyprus Expedition 2020 (The Söderberg Expedition): Excavations in the cemetery of Hala Sultan Tekke

In autumn 2020, the 11th season of excavations at the Late Bronze Age city of Hala Sultan Tekke took place in extramural Area A, where, based on indications provided by magnetometer surveys, tombs, possible ritual pits, and numerous wells were found, but no architectural remains. Tomb RR, which had been partly exposed in 2018 and 2019, was further excavated. It revealed a preliminary minimum total of 54 individuals (MNI) and numerous artefacts including, inter alia, complete ceramic vessels, figurines, jewellery, ivory objects, a scarab and a cylinder seal. The life span of the tomb can, on the evidence to date, be assigned from the LC IIA1 to LC IIC1 period, i.e., roughly from the last quarter of the 15th to the end of the 14th or the beginning of the 13th century BCE. Just to the south of Tomb RR another magnetic anomaly, Tomb SS, was investigated. Only the uppermost burial layer was reached, which was covered by two large deposits of several hundred intact or fully restorable ceramic vessels. These include various types of Cypriot tableware, as well as imports from the Aegean, Anatolia and the Levant. The preliminary date of these contexts is LC IIA˗B, i.e., roughly the last quarter of the 15th to the last quarter of the 14th century BCE. Since neither tomb has been completely exposed, excavations in the protected area will continue in spring 2021.

Assessment of potential impact of invasive vegetation on cohesive sediment erodibility in intertidal flats

In-situ erodibility experiments were conducted to reveal the effects of vegetation on sediment stability in an intertidal flat. Spartina alterniflora (Spartina), one of the most widespread types of vegetation, led to complexity in sediment erodibility. The long stems and leaves of Spartina, which grew to approximately 156 cm from May to November 2019, were effective in trapping suspended sediments in the water columns, eventually promoting the deposition of approximately 2.3 cm within its communities. Sediments eroded by increasing bed shear stress (τb) mainly originated from sediments that were adhered to the stems and leaves of Spartina (May: 76%; November: 54%). They protected subsequent bed erosion against τb. However, this was only an apparent effect because the Spartina caused the erosion rate (E) to stagnate by suppressing the outflow of eroded mass from the bed. As the protective effect of the stems was removed, the uppermost sediment layers in the Spartina communities became more vulnerable to erosion by τb, with the initial erosion thresholds lowered to 0.1 Pa (May) and 0.05 Pa (November). Despite continuous sedimentation by sediment trapping, the sediment bed in Spartina communities had not been consolidated under repeated tidal inundation, showing no distinct development of the critical shear stress for erosion. Thus, the differences in E between cases with or without Spartina's stems reached approximately 1.22 × 10−6 (May) and 1.83 × 10−6 kg m−2 s−1 (November) at τb = 0.6 Pa. Results suggest that the increase in thickness of erodible layers mainly contributed more than sediment volume fraction to the enhancement of erosion potential with τb. This study highlights the necessity to assess both positive and negative effects of Spartina on the stability of sediment beds in intertidal flats.

Experimental and numerical simulation of lightning damage development on composites with/without a carbon-based protection layer

We analyzed a three-dimensional thermal-electrical coupled model based on COMSOL to characterize the thermal damage response of a woven carbon fiber reinforced polymer (CFRP) composite with and without a lightning strike protection (LSP) system. Fuzzy fiber (FF) fabric serving as a carbon-based protection layer was attached to the outermost ply of the CFRP composite to fabricate a fuzzy fiber reinforced polymer (FFRP) composite. CFRP and FFRP composites with temperature-dependent properties were inspected to predict lightning-induced damage resulting from a 20- and 40-kA peak current for 100 μs. The predicted area of thermal damage and the appearance of the composite surface agreed fairly well with post-lightning damage observed from experiments, thus demonstrating the credibility of the numerical model. LSP characteristics were evaluated for a range of CFRP composite properties in the outermost layer by enhancing the functional conductivity of the top layer in the in-plane and out-of-plane directions. The irreversible thermal damage region in the in-plane and thickness directions was dramatically mitigated by enhanced electrical conductivity, whereas a slight matrix decomposition damage change was observed by varying the thermal conductivity. Due to the increased electrical conductivity and integration of the lightweight FF carbon-based protection layer into the uppermost composite layer, the depth and area of damage can be limited by decreasing the thermal damage penetration through the underlying composites. These results reveal that a highly conductive FF layer may serve as a lightweight and effective anti-lightning strike layer for protecting the underlying composite.

S-wave velocity structures at Yedang Reservoir Dam inferred from amplification characteristics determined using H/V spectral ratios with background noise

Local shear wave (S-wave) velocity structure and sedimentary cover thickness are essential parameters that control local amplification of ground motion and associated seismic hazard during earthquakes. Recently, analysis of environmental background noise from individual stations has been used to estimate horizontal-to-vertical spectral ratio (H/V) curves. In the present study, we inverted H/V curves using environmental noise immediately prior to earthquake P-wave arrival to retrieve S-wave velocity (or shear velocity, V S) profiles at four temporary seismic monitoring stations (YDF, YDS, YDU, and YDD) near the Yedang Reservoir Dam. In the first step, we used a random search algorithm to constrain the subvolume of the parameter space (S-wave velocity structure) where the minimum of the misfit was located. In the second step, we independently applied two non-linear processes (a Monte Carlo sampling algorithm and a simulated annealing algorithm) to force the inversion towards an optimal solution, using the minimum misfit model determined in the first step as an initial estimate; we then compared the results. The feasibility and effectiveness of this two-step approach were verified by inversion of H/V curves for seismic noise recorded at four seismic monitoring stations near the Yedang Reservoir Dam. Borehole and topographical data from the four stations provided a well-constrained estimation of the local shear wave profile. Comparisons of synthetic and observed H/V curves showed that combining the two inversion algorithms efficiently overcame the extreme non-linearity of the inversion problem and provided a good resolution of S-wave structures at the Yedang Reservoir Dam. The S-wave velocity profile at the YDF station, which is situated on fresh, uniform bedrock, ranged from ∼2300 to 2700 m/s, which was consistent with the borehole data. Both the YDU station (which exhibited fundamental and first-order resonance frequency harmonics) and the YDD station (which exhibited fundamental, first-, and second-order resonance harmonics) showed significant stepwise velocity profiles for the entire depth. Thus, the entire depth can be regarded as three layers, including bedrock. At the YDS station, located near the dam spillway, the uppermost layer showed the lowest V S values among all four stations (∼726–728 m/s), suggesting that the concrete dam body is well weathered or partially infiltrated by water. Because of these low V S values of the concrete dam body, the Yedang Dam may require a full safety diagnosis in the near future. Analysis of the velocity profiles of all four stations near the Yedang Dam indicated that the bedrock, composed of fresh rock (∼V S > 2700 m/s), varies greatly in depth among stations.

Observation and inversion of very-low-frequency seismo-acoustic fields in the South China Sea.

Very-low-frequency (VLF) sound has significant potential for underwater detection and estimation of geoacoustic models of the ocean bottom structure. In marine settings, one type of VLF sound is the interface wave. These waves, trapped near the fluid-solid interface, are called Scholte waves, and this is the subject of this study. A field experiment was carried out in the South China Sea with the objective of exciting Scholte waves and investigating the propagation. The data were acquired by an ocean bottom seismometer, deployed on the seafloor. A large volume airgun array near the sea surface provided the sound source. The fundamental and three higher-order mode Scholte waves were excited. The Scholte waves are investigated by seismograms and a phase velocity inversion. The observed frequencies are in the range of 1.0-2.9 Hz. The energy attenuation is proportional to 1/r at the peak frequency 1.4 Hz. The shear wave speed structure, down to 600 m beneath the seafloor, is revealed from the dispersion curves by a least-squares inversion algorithm. The inversion result shows that the shear wave speed is below 300 m/s in the uppermost layer, which explains well the weak excitation of Scholte waves in this experiment.

Coherence-guided InSAR deformation analysis in the presence of ongoing land surface changes in the Imperial Valley, California

While the quality and availability of Interferometric Synthetic Aperture Radar (InSAR) data has dramatically improved in recent years, InSAR analysis and interpretation remain challenging in actively deforming regions with extensive agricultural activities, where vegetation changes and soil moisture variability can degrade data quality or introduce confounding signals. Here we use Sentinel-1 satellite imagery for the Imperial Valley, California, over the period of 2015–2019 to explore how factors specific to land surface changes may impact InSAR time series and to resolve time-varying deformation due to tectonic processes and geothermal energy production. We examine the temporal variability of data quality, via interferometric phase coherence, at high spatial resolution, taking into account the observation that some agricultural fields lie fallow for long time intervals punctuated by periods of cultivation. This strategy allows us to better distinguish signals and noise associated with agricultural activities, shoreline changes, or surface soil conditions. A series of masking, interpolation, and filtering steps facilitate phase unwrapping, and the unwrapped, unfiltered product is then recovered, reducing artifacts from spatial filtering. We adopt model-based tropospheric corrections to improve time series results, particularly in regions with high topographic relief, along with the use of distributed reference points to render a more uniform error structure. We validate InSAR observations with continuous GPS where available and find that the estimates of average line-of-sight (LOS) velocity over the valley from InSAR and GPS agree to ~3 mm/yr in areas with good data coverage. Discrepancies between the two estimates often exist in areas with lower InSAR data quality; in better-constrained areas, they appear to reveal signals attributable to surficial processes occurring in the uppermost soil layers. We observe a diverse suite of natural signals over multiple spatial scales, including steady interseismic deformation, seasonal lake-level-modulated signals at the southeastern Salton Sea shore, and transient slow slip on the Superstition Hills fault. In addition, we observe complex deformation at four geothermal fields within the valley. Extensive subsidence at the Salton Sea geothermal field suggests spatial overlap of anthropogenic and tectonic deformation, interspersed with potential surficial signals. Geothermal sites at East Mesa, North Brawley, and Heber exhibit smaller-amplitude, more localized deformation, often with nonlinear temporal trends. Our analysis demonstrates the need to assess whether InSAR signals result from surficial changes or deeper sources, and produces robust ground deformation data in support of efforts to study subsurface processes, manage geothermal operations, and improve hazard assessments.

Grass species richness decreases along a woody plant encroachment gradient in a semi-arid savanna grassland, South Africa

Savanna grasslands are undergoing rapid land cover transformation as a consequence of woody plant encroachment. It still remains unclear how increasing woody plant density and cover in savannas influences tree-grass-soil interactions. This study determined the effect of increasing tree density on grass species richness, evenness, diversity and assessed how nutrients change along an encroachment gradient of a shallow plinthic savanna soil. Vegetation was quantified and soil samples analysed from eighteen randomly distributed 10 m × 10 m plots along a tree density gradient spanning from open, transitioning into intermediate and intensively encroached savanna grassland in northern South Africa. We found that an increase in tree density along the encroachment gradient decreased grass richness (GR) by 53% in the intensively woody-encroached grassland. Higher tree density and canopy cover led to a 90% dominance of Panicum maximum grass species. Conversely, higher tree density increased concentrations of carbon, nitrogen, phosphorus and exchangeable calcium and magnesium in the topsoil layer. This study highlights that woody plant encroachment decreases grass species richness and composition, but increases nutrients in the uppermost layer of a plinthic savanna soil. Such information is critical to improve understanding of the mechanisms underlying tree-grass interactions in savannas.

Room-Temperature Synthesis of 2D Janus Crystals and their Heterostructures.

Janus crystals represent an exciting class of 2D materials with different atomic species on their upper and lower facets. Theories have predicted that this symmetry breaking induces an electric field and leads to a wealth of novel properties, such as large Rashba spin-orbit coupling and formation of strongly correlated electronic states. Monolayer MoSSe Janus crystals have been synthesized by two methods, via controlled sulfurization of monolayer MoSe2 and via plasma stripping followed thermal annealing of MoS2 . However, the high processing temperatures prevent growth of other Janus materials and their heterostructures. Here, a room-temperature technique for the synthesis of a variety of Janus monolayers with high structural and optical quality is reported. This process involves low-energy reactive radical precursors, which enables selective removal and replacement of the uppermost chalcogen layer, thus transforming classical transition metal dichalcogenides into a Janus structure. The resulting materials show clear mixed character for their excitonic transitions, and more importantly, the presented room-temperature method enables the demonstration of first vertical and lateral heterojunctions of 2D Janus TMDs. The results present significant and pioneering advances in the synthesis of new classes of 2D materials, and pave the way for the creation of heterostructures from 2D Janus layers.

The impact of vertical resolution in reducing biases in sea surface temperature in a tropical Pacific Ocean model

Motivated by observations of fine scale vertical shear and its contribution to mixing in the tropical ocean, this study explores the impact of vertical resolution in an ocean model on sea surface temperature in the tropical Pacific Ocean. We conduct two model experiments that differ in the vertical discretization only, with the grid spacing in one being significantly smaller than the other in the upper ocean. We examine the temperature difference between the high and low vertical resolution experiments. We find that the difference in the upper-most layer is positive in the equatorial cold tongue and negative along the South American coast, thus reducing the commonly seen cool and warm biases in the two regions, respectively. The change in the structure of the vertical diffusivity, as determined by the K-profile parameterization, is identified as the primary cause in reducing the biases. In the central equatorial Pacific, the change in the vertical diffusivity from low to high vertical resolution in the upper pycnocline results in a positive temperature difference that propagates eastward as an equatorial Kelvin wave, rising to the sea surface in the central and eastern regions to increase the sea surface temperature there. In the far eastern equatorial Pacific, the change in the vertical diffusivity in the lower pycnocline produces a negative temperature difference that propagates poleward as coastal Kelvin waves along the west coast of the American continent, outcropping along the South American coast to reduce the sea surface temperature there. The high vertical resolution experiment captures much of the small-scale vertical velocity shear and resolves the fine details of the stratification in the upper ocean. Our analysis suggests that the shear-generated turbulence is the primary contributor to the change in the vertical diffusivity in the central region whereas stratification is the dominant factor in the far eastern region.