The generalized nonvolcanic tremor has been proposed for characterizing the seismic process of megathrust earthquakes. A tremor signal with a vibrational velocity of microns per second exhibit the dominant frequency of 1 Hz to 10 Hz in the Fourier amplitude spectrum. Paying attention to the negative curvature of the spectrum, we generalize the tremor and define alpha-tremor as the degree of the negative curvature of the spectrum in the frequency range of 2.97 Hz to 9.80 Hz. Significant tremors and background vibrations are respectively represented by large positive alpha-tremors and non-positive alpha-tremors. Alpha-tremor is evaluated for ground vibration data acquired every 0.05 seconds for approximately 10 years at three seismic stations. At the station 188 km away from the epicenter of the Great East Japan Earthquake (GEJE) of magnitude 9, symmetries regarding the seismic process of the GEJE have been found. Among the observed 9 prominent peaks of the positive alpha-tremor, the first and last peak appears 3 years before and after GEJE, respectively. The frequency distributions of the alpha-tremor during 3 years before and after GEJE match by 99.95%. The statistical distribution that properly approximates the frequency distribution of the positive alpha-tremor is found to be the Gumbel distribution, rather than the Gaussian distribution. The time evolution of the frequency distribution of the alpha-tremor at the seismic station 1170 km away from the GEJE epicenter suggests that GEJE may have affected the ground motion there, and initiated the seismic process of the M7.3 earthquake that occurred near the station.

Probable maximum precipitation (PMP) is one of the most important key indices in hydrological design, and it should be estimated without the risk of overestimation or underestimation. However, observed extreme rainfall is insufficient to evaluate the estimated PMP in most regions, and there are no conclusive criteria to evaluate the possibility of PMP over- or underestimation. Our study aims to evaluate the reasonability of PMP estimation using a moisture-maximization method by comparing estimated PMPs based on the conventional method using surface dew points (SDP) with values based on actual precipitable water obtained from upper-air data (UAD). Furthermore, the estimated PMPs were evaluated with extreme-scale reference precipitation estimated from large ensemble climate simulation data (d4PDF) to check the possibility of PMP over- and underestimation. This reference value corresponds to a 3,000-year return period. The UAD-based estimation showed a reasonable PMP with a low deviation to the reference precipitation in most target areas, and the SDP-based estimation showed overestimated PMPs to the reference precipitation in areas with low SDPs. To control PMP overestimation of the SDP-based approach, the upper bound of the moisture maximizing ratio (MMR) in the SDP-based approach was limited to 2.0. Consequently, the SDP-based approach, which limits the upper bound of MMR, decreased deviations between estimated PMPs and reference values in areas with low SDPs. The SDP approach could reduce the possibility of PMP overestimation by limiting the upper bound of MMR. This evaluation could be conducted from extreme-scale reference precipitation under sufficient extreme events by utilizing the d4PDF.

The 2020 COVID-19 pandemic provided a unique opportunity to sample atmospheric gases during a period of very low industrial/human activity. Over 1000 Whole Air Samples were collected in over 30 cities and towns across the United States from April through July 2020 as part of the NASA Student Airborne Research Program (SARP). Sample locations leveraged the geographic distribution across the United States of the undergraduate and graduate students, faculty, and NASA personnel associated with the internship program (44 people total). Each person collected approximately 24 air samples in their city/town with the goal of characterizing local emissions with time during the pandemic. Samples were collected in 2-Liter stainless steel evacuated canisters at approximately 2 meters above ground level. The canisters were shipped to the Rowland/Blake Laboratory at the University of California Irvine and analyzed for methane, carbon dioxide, carbon monoxide, non-methane hydrocarbons, and halocarbons using the gas chromatographic system described in Colman et al. (2001) and Barletta et al. (2002). Initial samples collected in April coincided with the peak of stay-at-home/social distancing orders across most of the United States while samples collected later in the spring and early summer reflect the easing of these measures in most locations. Overall trends in emissions with time across the United States during the pandemic (in several large metro areas as well as rural locations) will be discussed.

Intermediate layers (ILs) are regions of enhanced electron density located in the ionospheric valley that extends from the peak altitude of the daytime E-region to the bottom side of the F-region. This work presents the daytime behavior of the ILs parameters (the virtual height - h’IL, and the top frequency - ftIL) for the deepest solar minimum of the last 500 years. In such a unique condition, this research reveals for the first time the ILs’ quiet state seasonal behavior as well as its responses to moderate changes in the geomagnetic activity. Among the finds, it is highlight the annual periodicity of the ftIL while the h’IL presents semiannual component. The results also show that even small variations of geomagnetic activity (quantified by the planetary Kp index) are able to modify the dynamics of the ILs parameters. For the first time, it was observed that during December solstice and September equinox, the h’IL/ftIL decrease/increase rapidly with the increase of geomagnetic activity at the beginning of the day. As the day progresses, smoothed rise in the h’IL is observed at the same time in which a considerable decrease in the ftIL occurs, except during June solstice when a different behavior is observed both in relation to the annual as the seasonal average values of the ftIL.

A novel Bayesian Hierarchical Network Model (BHNM) for ensemble forecasts of daily streamflow that uses the spatial dependence induced by the river network topology and hydrometeorological variables from the upstream contributing area between station gauges is presented. Model parameters are allowed to vary with time as functions of selected covariates for each day. Using the network structure to incorporate flow information from upstream gauges and precipitation from the immediate contributing area as covariates allows one to model the spatial correlation of flows simultaneously and parsimoniously. An application to daily monsoon period (July-August) streamflow at four gauges in the Narmada basin in central India for the period 1978 – 2014 is presented. The covariates include daily streamflow from upstream gauges or from the gauge above of the upstream gauges depending on travel times and daily, 2-day, or 3-day precipitation from the area between two stations. The model validation indicates that the model is highly skillful relative to climatology and relative to a null-model of linear regression. We applied the BHNM out of sample to two high flooding years. High skill in both the timing and magnitude of the events is demonstrated.

Dissolved inorganic carbon (DIC) and total alkalinity (TA) were sampled in December, 2018 and May, 2019 in the Exclusive Economic Zone (EEZ) of Qatar in the Arabian Gulf. pCO2 calculated in surface seawater averaged 459 ± 61 matm and was supersaturated with respect to the atmosphere. The region was degassing CO2 to the atmosphere and the flux was about 1.25 mmol C m-2 d-1. The origin of this excess CO2 must be due to CaCO3 precipitation. The horizontal relationship between salinity-normalized total alkalinity (NTA) and dissolved inorganic carbon (NDIC) showed that CaCO3 formation was more important, relative to net biological productivity, than in the open ocean. The tracer Alk* has values primarily determined by CaCO3 formation and values of Alk* ranged from -50 to -310 mmol kg-1, which is consistent with substantial CaCO3 formation. DAlk* increased with increasing distance northward from Hormuz. The rate of calcification calculated from the air-sea flux of CO2 (5.6 mmol C kg-1 y-1) and from DAlk* (5.9 mmol C kg-1 y-1) agreed well. However, CaCO3 formation by net calcification in coral reefs is unlikely as they have limited distribution and have been severely damaged by past coral bleaching. There are high concentrations of excess particulate Ca in the water column that cannot be accounted for by input of CaCO3-rich Qatari dust. Carbonate forming plankton are absent in the water column. We propose that abiological, heterogeneous calcite precipitation (HCP) may be occurring. The mechanism is unknown but nucleation by CaCO3-rich Qatari dust may assist this process.

The long-term variations in the South Asian monsoon have been inferred based on the variations in the ocean productivity along the western coast of the Arabian Sea. The variations in ocean productivity were previously thought to be primarily influenced by the intensity of upwelling. Here, using idealized precession experiments in fully coupled climate models, we have shown that the area as well as the region of maximum upwelling change with precession. When summer occurs at perihelion (stronger summer insolation and monsoon precipitation) the area of upwelling is narrow. In contrast, during summer at aphelion (weaker summer insolation and monsoon precipitation), upwelling occurs over a broader region. This is due to the effect of convective heating over northeastern Africa and the western equatorial Indian ocean on the width and meridional location of the low-level jet. Therefore, the upwelling inferred from proxies does not necessarily indicate the Indian summer monsoon strength.

Thawing of mountain permafrost in response to rising temperatures degrades the stability of rock walls and thereby affects infrastructure integrity in Alpine terrain. In this study, we use 15 years of passive seismic data from a single station deployed near a known permafrost body on Mt. Zugspitze (Germany), to monitor freeze-thaw processes. The recordings reveal a persistent cultural seismic noise source, which we utilize to compute single-station cross-correlations and extract relative seismic velocity changes. We find that parts of the cross-correlations show seasonal velocity variations (∼3% peak-to-peak amplitude) and a long-term velocity decrease (∼0.1%/yr). Comparison with meteorological data and a previous electrical resistivity tomography study suggests that these velocity changes are caused by active-layer freeze-thaw cycles and by permafrost degradation, respectively. The results demonstrate the potential of passive seismology for permafrost monitoring and suggest that denser instrumentation will provide detailed spatio-temporal insights on permafrost dynamics in future studies.

Flow through partially frozen pores in granular media containing ice or gas hydrate plays an essential role in diverse phenomena including methane migration and frost heave. As freezing progresses, the frozen phase grows in the pore space and constricts flow paths so that the permeability decreases. Previous works have measured the relationship between permeability and volumetric fraction of the frozen phase, and various correlations have been proposed to predict permeability change in hydrology and the oil industry. However, predictions from different formulae can differ by orders of magnitude, causing great uncertainty in modeling results. We present a floating random walk method to approximate the porous flow field and estimate the effective permeability in isotropic granular media, without solving for the entire flow field in the pore space. In packed spherical particles, the method compares favorably with the Kozeny-Carman formula. We further extend this method with a probabilistic interpretation of the volumetric fraction of the frozen phase, simulate the effect of freezing in irregular pores, and predict the evolution of permeability. Our results can provide insight into the coupling between phase transitions and permeability change, which plays important roles in hydrate formation and dissociation, as well as in the thawing and freezing of permafrost and ice--bed coupling beneath glaciers.

Seagrasses and mangroves are crucial sources of atmospheric methane (CH4) from coastal areas. To study the dynamics of CH4 cycling at subtropical seagrass and mangrove, we studied diurnal CH4 emissions at the sea-air and sediment-water interfaces and related environmental parameters in August 2019 at lagoonal estuaries of southern Texas, USA, northwest coast of the Gulf of Mexico. Although seagrass meadows and mangroves locate at closely connected subtropical estuaries, they displayed distinct mechanisms in CH4 cycling. Dissolved CH4 concentration at the seagrass meadow decreased in the daytime and increased overnight, expressing a tight relationship with photosynthesis and respiration of seagrass. Plant mediation of seagrass played a crucial role in CH4 production, oxidation, and transport from sediment to water column. In comparison, the diel variation of dissolved CH4 concentration at the mangrove creek was controlled by tidal progression. The maximum CH4 level occurred during ebb due to the export of CH4 from inside the mangrove to the outside bay. Tidal pumping and tidal inundation were essential conduits for dissolved CH4 exchange between water and porewater. In both areas, sea-air CH4 fluxes were significantly affected by wind speeds, which hid related diurnal variations caused by physiological or tidal cycles. Our study also revealed a more significant contribution from seagrass to the local CH4 budget than from mangroves, indicating CH4 released from subtropical seagrass needs further investigation.

We report here a slow-moving landslide revealed by Sentinel-1 interferometric time-series analysis. Located along the western coast of the Aral Sea, with a >80-km length and 4-km width, this is the world’s largest active landslide complex reported so far with a constant velocity of 40 mm/yr. Systematic subsidence up to 5 mm/yr, is also observed along narrow strips of terraces that appear to result from rotations of fault-bounded blocks. The horizontal deformation does not correlate with the annual variations of the water level in the Aral Sea over the same period, indicating a long-term forcing of this landslide that might be caused by the long-term sea-level drop. The lateral spreadings involve the competent limestone beds lying horizontally on plastic clay- and evaporite-rich layers. We propose a conceptual model for the mechanism of landslides that appear to be controlled by the attitude of bedding, lithological sequence, hydrogeology, and low angle faults.

The hydropower fleet built in the Upper Mekong River, or Lancang, currently consists of eleven mainstream dams that can control about 55% of the annual flow to Northern Thailand and Laos. The operations of this fleet have become a source of controversy between China and downstream countries, with these dams often considered the culprit for droughts and other externalities. Assessing their actual impact is a challenging task because of the chronic lack of data on reservoir storage and operations. To overcome this challenge, we focus on the ten largest reservoirs and leverage satellite observations to infer 13-year time series of monthly storage variations. Specifically, we use area-storage curves (derived from a Digital Elevation Model) and time series of water surface area, which we estimate from Landsat images through a novel algorithm that removes the effects of clouds and other disturbances. We also use satellite radar altimetry data (Jason) to validate the results obtained from satellite imagery. Our results describe the evolution of the hydropower system and highlight the pivotal role played by Xiaowan and Nuozhadu reservoirs, which make up to ~85% of the total system’s storage in the Lancang River Basin. We show that these two reservoirs were filled in only two years, and that their operations did not change in response to the drought that occurred in the region in 2019-2020. Deciphering these operating strategies could help enrich existing monitoring tools and hydrological models, thereby supporting riparian countries in the design of more cooperative water-energy policies.

Ice shelf collapse could trigger widespread retreat of marine-based portions of the Antarctic ice sheet. However, little is known about the processes that control the stability of ice shelves. Recent observations have revealed that ice shelves have topographic features that span a spectrum of wavelengths, including basal channels and crevasses. Here we use ground-penetrating radar data to quantify patterns of roughness within and between ice shelves. We find that roughness follows a power-law with scaling exponent approximately constant between ice shelves. However, the magnitude of roughness varies by over an order of magnitude between different ice shelves. Critically, we find that roughness strongly correlates with basal melt rate, suggesting that increased basal melt not only leads to deeper melt channels, but also increased fracturing, rifting and de-creased ice shelf stability. This hints that the mechanical stability of ice shelves may be more tightly controlled by ocean forcing than previously thought.

A systematic dataset of petrography, mineralogy, geochronology, and geochemistry is reported for the enclave-bearing calc-alkaline I-type granitoids from the Chinese Altai, Central Asian Orogenic Belt (CAOB). Zircon U–Pb dating and geochemical data indicate that the MME and granitoids formed coevally at ~395 Ma in a subduction setting. Geochemical modelling and hybrid testing suggest that the granitoid parental magma was formed by mixing between a mafic and a felsic endmember that can be identified by isotopic compositions. The mafic rocks have (87Sr/86Sr)i of 0.7048 – 0.7062, εNd(t) of -0.5 – +2.6, and zircon εHf(t) of +2.3 – +5.4, while the host granitoids have similar Sr isotopic compositions ((87Sr/86Sr)i = 0.7054 – 0.7064), but generally lower whole-rock εNd(t) and zircon εHf(t) values (-2.2 – +0.4 and +0.6 – +4.6, respectively). The sharp decrease of An values from cores to rims (e.g., from ca. 80 to 40) of plagioclase phenocrysts points to polybaric crystallization accompanied by degassing, which is supported by the pressure and water content estimations based on amphibole compositions. Petrographic evidence and plagioclase in-situ Sr isotopic compositions ((87Sr/86Sr)i = 0.7053 - 0.7071) confirm the interaction of two isotopically different magmas during the mineral crystallization. A model for the formation of the enclave-bearing calc-alkaline plutons in an arc setting is presented: in-depth mantle and crustal melting and efficient magma mixing controlled the principal chemical compositions of the granitoid intrusions, while the later decompression-dominated crystallization, magma mingling and limited mixing in the higher crustal level finally determined the texture, mineral composition, and enclave morphology.

Prior information regarding subsurface patterns may be used in geophysical inversion to obtain realistic subsurface models. Field experiments require prior information with sufficiently diverse patterns to accurately estimate the spatial distribution of geophysical properties in the sensed subsurface domain. A variational autoencoder (VAE) provides a way to assemble all patterns deemed possible in a single prior distribution. Such patterns may include those defined by different base training images and also their perturbed versions, e.g. those resulting from geologically consistent operations such as erosion/dilation, local deformation and intrafacies variability. Once the VAE is trained, inversion may be done in the latent space which ensures that inverted models have the patterns defined by the assembled prior. Inversion with both a synthetic and a field case of cross-borehole GPR traveltime data shows that using the VAE assembled prior performs as good as using the VAE trained on the pattern with the best fit, but it has the advantage of lower computation cost and more realistic prior uncertainty. Moreover, the synthetic case shows an adequate estimation of most small scale structures. Estimation of absolute values of wave velocity is also possible by assuming a linear mixing model and including two additional parameters in the inversion.

The term “weather whiplash” was recently coined to describe abrupt swings in weather conditions from one extreme to another, such as from a frigid cold spell to anomalous warmth or from drought to prolonged precipitation. These events are often highly disruptive to agriculture, ecosystems, and daily activities. In this study we propose and demonstrate a novel metric to identify weather whiplash events (WWEs) and track their frequency over time. We define a WWE as a transition from one persistent large-scale circulation regime to another distinctly different one, as determined using an objective pattern cluster analysis called self-organizing maps (SOMs). We focus on the domain spanning North America and the eastern N. Pacific Ocean. A matrix of representative atmospheric patterns in 500-hPa geopotential height anomalies is created. We analyze the occurrence of WWEs originating with long-duration events (defined as lasting 4 or more days) in each pattern, as well as the associated extremes in temperature and precipitation. A WWE is detected when the pattern two days following a long-duration event is substantially different, measured using internal matrix distances and thresholds. Changes in WWE frequency are assessed objectively based on reanalysis and climate model output, and in the future with climate model projections. Temporal changes in the future under RCP 8.5 forcing are more robust than in recent decades, with consistent increases (decreases) in WWEs originating in patterns with an anomalously warm (cold) Arctic.

Although tremor is believed to consist of myriad Low-frequency Earthquakes (LFEs), it also contains longer-period signals of unknown origin. We investigate the source of some of the longer-period signals by locating tremor windows independently in relatively high-frequency (’HF’, 1.25–6.5 Hz, containing typical LFEs) and low-frequency (’LF’, 0.5–1.25 Hz) bands. We hypothesize that if tremor consists entirely of LFEs, such that the lower-frequency signals come from the non-uniform timing of higher-frequency ($\sim$2 Hz) LFEs, then contemporaneous LF and HF signals should be nearly co-located. Here we search for a systematic offset between the locations of contemporaneous LF and HF detections during rapid tremor migrations (RTMs). This first requires correcting for apparent offsets in location that arise simply from filtering in different passbands. To guard against possible errors in our empirical filtering effect corrections, we focus on a region of the subduction interface beneath southern Vancouver Island that hosts migrations propagating in nearly opposing directions. We find that the LF energy appears to occur roughly 500 m farther behind the propagating fronts of RTMs than the HF energy, whether those fronts propagate to the ENE or to the WSW. This separation is small compared to the location error of individual LF detections, but the result seems robust owing to the large number of detections. If this result stands, it suggests that tremor consists of more than just a collection of LFEs, with longer-period energy being generated farther behind the migrating fronts of RTMs, where slip speeds are presumably lower.

To help tackle drought-related farmer suicides, a new design of a rain simulator, recently patented is presented (Patent no. 351688, Govt. of India Patent Office). This is versatile and yields showers with near-identical drop-size distributions when compared to natural showers. It is useful for many applications requiring controlled hydration from showers instead of relying on root hydration alone. The apparatus is soon to be rolled out to farmers in the Indian State of Tamil Nadu. It is portable, and with a cost of under USD 1000 a piece, can alter patterns of cropland hydration even when a monsoon fails.