Feed aggregator

Abstract

We use seismic ambient noise recorded by dense ocean bottom nodes (OBNs) in the Gorgon gas field, Western Australia, to compute time-lapse seafloor models of shear-wave velocity. The extracted hourly cross-correlation (CC) functions in the frequency band 0.1–1 Hz contain mainly Scholte waves with very high signal-to-noise ratio. We observe temporal velocity variations (dv/v) at the order of 0.1% with a peak velocity change of 0.8% averaged from all station pairs, from the conventional time-lapse analysis with the assumption of a spatially homogeneous dv/v. With a high-resolution reference (baseline) model from full waveform inversion of Scholte waves, we present an elastic wave equation based double-difference inversion (EW-DD) method, using arrival time differences between the reference and time-lapsed Scholte waves, for mapping temporally varying dv/v in the heterogeneous subsurface. The time-lapse velocity models reveal increasing/decreasing patterns of shear-wave velocity in agreement with those from the conventional analysis. The velocity variation exhibits a ∼24-hr cycling pattern, which appears to be inversely correlated with the diurnal variations in sea level height, possibly associated with dilatant effects for porous, low-velocity shallow seafloor and rising pore pressure with higher sea level. This study demonstrates the feasibility of using dense passive seismic surveys and wave-equation time-lapse inversion for quantitative monitoring of subsurface property changes in the horizontal and depth domain.

Abstract

Antigorite is the high-temperature member of the serpentine group minerals and is broadly considered a primary carrier of water in the subducting oceanic lithosphere. It has a wavy crystal structure along its a-axis and several polysomes with different m-values (m = 13–24) have been identified in nature. The m-value is defined as the number of tetrahedra in one wavelength and is controlled by the misfit between the octahedral and tetrahedral layers. The degree of misfit primarily depends on the volumes of the MgO6 octehedra and SiO4 tetrahedra within the layers, which vary as a function of pressure and temperature. However, it is not well understood which m-values of antigorite are stable at different pressure and temperature conditions. To investigate the pressure dependence of the stability of different m-values in antigorite, we performed first-principles calculations for several polysomes (m = 14–19) at high pressure from 0 to 14 GPa and compared their enthalpies at static 0 K. We found that although the energy differences between polysomes are small, polysomes with larger m-values are more stable at ambient pressure, while polysomes with smaller m-values are more stable at elevated pressures. This suggests that the structure of antigorite in the oceanic lithosphere subducting into the deep Earth may gradually evolve into a different polysome structure than the antigorite samples observed at ambient or near-surface pressure conditions. These changes in the m-values are accompanied by a minor dehydration reaction. By modulating the available amount of free water in the system, antigorite polysomatism may influence the distribution of intermediate-depth seismicity, such as the observance of double seismic zones.

Abstract

Tectonic plate motions feed the earthquake cycle—a process whereby stress along crustal faults slowly increases over decade– or century–long periods, to then suddenly drop during earthquakes. Steadiness of plate motions during such cycles has long been a central tenet in models of earthquake genesis and of faults seismic potential, and can be tested against measurements of contemporary plate motions available from Global Navigation Satellite Systems (GNSS). Here we present analyses of GNSS data from Central and Northern Italy that illuminate the motion of the Adria microplate over a period of 6 years preceding the M W 6.3, 6 April 2009 L’Aquila (Italy) earthquake. We show that the motion of the whole Adria microplate changed before the 2009 earthquake, and slowed down by around 20%. We demonstrate with quantitative models that the torque required upon Adria in order to drive such a kinematic change is consistent with what is imparted to Adria by temporal stress variations occurring during the late interseismic phase of the 2009 L’Aquila earthquake cycle. The inference that plate motions can be influenced by, and thus sensitive to, earthquake cycles offers an additional perspective to assessing the seismic potential of tectonic margins.

Abstract

Scientists have observed the surface expression of creep events along the San Andreas Fault since the 1960s. However, the evolution of slip at depth has been examined relatively little. So here we probe that deep slip by analyzing strain observations just before and during hours- to day-long creep events at the northern end of the creeping section of the San Andreas Fault. We identify 71 strain offsets that are likely produced by few-hour bursts of slip at depth. Then, we grid search to determine the location, depth, and magnitude of these slip bursts. We find that the slip bursts occur at a range of along-strike locations, from 0 to 7 km away from the surface slip observations. Slip occurs at depths from 0 to 10 km; 42%–55% of the bursts are likely below 4 km depth. The bursts typically have moments equivalent to M w 3.2–4.1 earthquakes. These findings suggest that creep events are not just small shallow events; they are relatively large events that nucleate at significant depths and could play a prominent role in the slip dynamics of the creeping section.

Abstract

We validate the multiyear potential predictability of wintertime heavy precipitation potential in East Asia by combining initialized decadal hindcasts of the global climate model and large ensemble simulations from a high-resolution global atmospheric model. By analyzing a set of initialized hindcasts, the major predictive components of sea surface temperature (SST) variability beyond interannual timescales are identified as high-latitudes multidecadal variability and the so-called trans-basin variability (TBV). A set of 100 ensemble simulations using a high-resolution atmospheric model showed a significantly large signal-to-noise ratio for the wintertime heavy precipitation potential in East Asia, which is closely related to the TBV. When the SST around the maritime continent is higher, the anomalously low pressure in the northwestern Pacific enhances low-level cold air transport due to the winter monsoon. Consequently, the resultant weaker baroclinicity in the lower atmosphere reduces storm activity and wintertime heavy precipitation potential in East Asia.

Abstract

Reservoirs exert a profound influence on the cycling of dissolved organic matter (DOM) in inland waters by altering flow regimes. Biological incubations can help to disentangle the role that microbial processing plays in the DOM cycling within reservoirs. However, the complex DOM composition poses a great challenge to the analysis of such data. Here we tested if the interpretable machine learning (ML) methodologies can contribute to capturing the relationships between molecular reactivity and composition. We developed time-specific ML models based on 7-day and 30-day incubations to simulate the biogeochemical processes in the Three Gorges Reservoir over shorter and longer water retention periods, respectively. Results showed that the extended water retention time likely allows the successive microbial degradation of molecules, with stochasticity exerting a non-negligible effect on the molecular composition at the initial stage of the incubation. This study highlights the potential of ML in enhancing our interpretation of DOM dynamics over time.

Abstract

We study the sensitivity of South Asian Summer Monsoon (SASM) precipitation to Southern Hemisphere (SH) subtropical Absorbed Solar Radiation (ASR) changes using Community Earth System Model 2 simulations. Reducing positive ASR biases over the SH subtropics impacts SASM, and is sensitive to the ocean basin where changes are imposed. Radiation changes over the SH subtropical Indian Ocean (IO) shifts rainfall over the equatorial IO northward causing 1–2 mm/day drying south of equator, changes over the SH subtropical Pacific increases precipitation over northern continental regions by 1–2 mm/day, and changes over the SH subtropical Atlantic have little effect on SASM precipitation. Radiation changes over the subtropical Pacific impacts the SASM through zonal circulation changes, while changes over the IO modify meridional circulation to bring about changes in precipitation over northern IO. Our findings suggest that reducing SH subtropical radiation biases in climate models may also reduce SASM precipitation biases.

Statistical calibration of probabilistic medium-range fire weather index forecasts in Europe
Stephanie Bohlmann and Marko Laine
Nat. Hazards Earth Syst. Sci. Discuss., https//doi.org/10.5194/nhess-2024-57,2024
Preprint under review for NHESS (discussion: open, 2 comments)
Probabilistic ensemble forecasts of the Canadian Forest Fire Weather Index (FWI) can be used to estimate the possible risk for wildfires but requires post-processing to provide accurate and reliable predictions. We present a calibration method using non-homogeneous Gaussian regression to statistical post-process FWI forecasts up to 15 days. Calibration improves the forecast especially at short lead times and in regions with elevated FWI values.

Evaluation of calibration performance of a low-cost particulate matter sensor using collocated and distant NO2
Kabseok Ko, Seokheon Cho, and Ramesh R. Rao
Atmos. Meas. Tech., 17, 3303–3322, https://doi.org/10.5194/amt-17-3303-2024, 2024
In our study, we examined how NO2, temperature, and relative humidity influence the calibration of PurpleAir PA-II sensors. We found that incorporating NO2 data from collocated reliable instruments enhances PM2.5 calibration performance. Due to the impracticality of collocating reliable NO2 instruments with sensors, we suggest using distant NO2 data for calibration. We demonstrated that performance improves when distant NO2 correlates highly with collocated NO2 measurements.

Identification of ice-over-water multilayer clouds using multispectral satellite data in an artificial neural network
Sunny Sun-Mack, Patrick Minnis, Yan Chen, Gang Hong, and William L. Smith Jr.
Atmos. Meas. Tech., 17, 3323–3346, https://doi.org/10.5194/amt-17-3323-2024, 2024
Multilayer clouds (MCs) affect the radiation budget differently than single-layer clouds (SCs) and need to be identified in satellite images. A neural network was trained to identify MCs by matching imagery with lidar/radar data. This method correctly identifies ~87 % SCs and MCs with a net accuracy gain of 7.5 % over snow-free surfaces. It is more accurate than most available methods and constitutes a first step in providing a reasonable 3-D characterization of the cloudy atmosphere.

Review article: Insuring the green economy against natural hazards – charting research frontiers in vulnerability assessment
Harikesan Baskaran, Ioanna Ioannou, Tiziana Rossetto, Jonas Cels, Mathis Joffrain, Nicolas Mortegoutte, Aurelie Fallon Saint-Lo, and Catalina Spataru
Nat. Hazards Earth Syst. Sci. Discuss., https//doi.org/10.5194/nhess-2024-82,2024
Preprint under review for NHESS (discussion: open, 0 comments)
There is a global need for insuring green economy assets against natural hazard events. But their complexity and low exposure history, means the data required for vulnerability evaluation by the insurance industry is scarce. A systematic literature review is conducted in this study, to determine the suitability of current, published literature for this purpose. Knowledge gaps are charted, and a representative asset-hazard taxonomy is proposed, to guide future, quantitative research.

Abstract

We study electromagnetic ion cyclotron (EMIC) waves based on observations from the ionosphere, magnetosphere, and ground during a geomagnetic storm recovery phase on 28 August 2018. In this case, multiple ducting EMIC waves in the ionosphere show higher frequencies in the post-midnight than those in the pre-midnight. Ionospheric EMIC wave frequency differences in magnetic local time (MLT) are consistent with MLT frequency differences in the equatorial magnetosphere, which are mainly caused by different background magnetic field at different L-shells. Moreover, we report the first observation of frequency range selections in ionospheric ducting EMIC waves and find that frequency selections depend on the magnetic field intensity in the main part of the ionospheric waveguide, with higher frequency corresponding to larger magnetic field. This study reveals the important role of background magnetic field in regulating ducting EMIC wave frequencies in the ionosphere.

In silico calculation of soil pH by SCEPTER v1.0
Yoshiki Kanzaki, Isabella Chiaravalloti, Shuang Zhang, Noah J. Planavsky, and Christopher T. Reinhard
Geosci. Model Dev., 17, 4515–4532, https://doi.org/10.5194/gmd-17-4515-2024, 2024
Soil pH is one of the most commonly measured agronomical and biogeochemical indices, mostly reflecting exchangeable acidity. Explicit simulation of both porewater and bulk soil pH is thus crucial to the accurate evaluation of alkalinity required to counteract soil acidification and the resulting capture of anthropogenic carbon dioxide through the enhanced weathering technique. This has been enabled by the updated reactive–transport SCEPTER code and newly developed framework to simulate soil pH.

Abstract

Bursty reconnection models predict that flux transfer events (FTEs) moving along the magnetopause launch fast mode compressional waves into the magnetosheath that push the bow shock outward. By contrast, increases in the solar wind density striking the bow shock should push that boundary inward and launch fast mode compressional waves that propagate across the magnetosheath, drive waves on the magnetopause, and generate transient events in the outer magnetosphere. Multipoint ACE, Wind, THEMIS, and GOES-11/12 solar wind, bow shock, and magnetospheric observations on 14 October 2008 provide direct evidence for solar wind pressure pulses producing a large amplitude indentation with crater FTE-like properties on the magnetopause.

Abstract

Recent study has demonstrated that electron cyclotron harmonic (ECH) waves can be excited by a low energy electron beam. Such waves propagate at moderately oblique wave normal angles (∼70°). The potential effects of beam-driven ECH waves on electron dynamics in Earth's plasma sheet is not known. Using two-dimensional Darwin particle-in-cell simulations with initial electron distributions that represent typical plasma conditions in the plasma sheet, we explore the excitation and saturation of such beam-driven ECH waves. Both ECH and electron acoustic waves are excited in the simulation and propagate at oblique wave normal angles. Compared with the electron acoustic waves, ECH waves grow much faster and have more intense saturation amplitudes. Cold, stationary electrons are first accelerated by ECH waves through cyclotron resonance and then accelerated in the parallel direction by both the ECH and electron acoustic waves through Landau resonance. Beam electrons, on the other hand, are decelerated in the parallel direction and scattered to larger pitch angles. The relaxation of the electron beam and the continuous heating of the cold electrons contribute to ECH wave saturation and suppress the excitation of electron acoustic waves. When the ratio of plasma to electron cyclotron frequency ω pe /ω ce increases, the ECH wave amplitude increases while the electron acoustic wave amplitude decreases. Our work reveals the importance of ECH and electron acoustic waves in reshaping sub-thermal electron distributions and improves our understanding on the potential effects of wave-particle interactions in trapping ionospheric electron outflows and forming anisotropic (field-aligned) electron distributions in the plasma sheet.

Abstract

We made observations of magnetic field variations in association with pulsating auroras with the magneto-impedance sensor magnetometer (MIM) carried by the Loss through Auroral Microburst Pulsations (LAMP) sounding rocket that was launched at 11:27:30 UT on 5 March 2022 from Poker Flat Research Range, Alaska. At an altitude of 200–250 km, MIM detected clear enhancements of the magnetic field by 15–25 nT in both the northward and westward components. From simultaneous observations with the ground all-sky camera, we found that the footprint of LAMP at the 100 km altitude was located near the center of a pulsating auroral patch. The auroral patch had a dimension of ∼90 km in latitude and ∼25 km in longitude, and its major axis was inclined toward northwest. These observations were compared with results of a simple model calculation, in which local electron precipitation into the thin-layer ionosphere causes an elliptical auroral patch. The conductivity within the patch is enhanced in the background electric field and as a result, the magnetic field variations are induced around the auroral patch. The model calculation results can explain the MIM observations if the electric field points toward southeast and one of the model parameters is adjusted. We conclude that the pulsating auroral patch in this event was associated with a one-pair field-aligned current that consists of downward (upward) currents at the poleward (equatorward) edge of the patch. This current structure is maintained even if the auroral patch is latitudinally elongated.

Abstract

Magnetic field line curvature (FLC) scattering is an effective mechanism for collisionless particle scattering. In the terrestrial magnetosphere, the FLC scattering plays an essential role in shaping the outer boundary of protons radiation belt, the rapid decay of ring current, and the formation of proton isotropic boundary (IB). However, previous studies have yet to adequately investigate the influence of FLC scattering on charged particles in the Earth's dayside magnetosphere, particularly in the off-equatorial magnetic minima regions. This study employs T89 magnetic field model to investigate the impacts of FLC scattering on ring current protons in the dayside magnetosphere, with a specific focus on the off-equatorial minimum regions. We analyze the spatial distributions of single and dual magnetic minima regions, adiabatic parameter, and pitch angle diffusion coefficients due to FLC scattering as functions of Kp. The results show that the effects of FLC scattering are significant not only on the dusk and dawn sides but also in the off-equatorial minima regions on the noon. Additionally, we investigate the role of dipole tilt angle in the hemispheric asymmetry of FLC scattering effects. The dipole tilt angle controls the overall displacement of the dayside magnetosphere, resulting in different FLC scattering effects in the two hemispheres. Our study holds significance for understanding the FLC scattering effects in the off-equatorial region of Earth's dayside magnetosphere and for constructing a more accurate dynamic model of particles.

Abstract

The paper presents the results of a numerical assessment of the contribution of the ionospheric D region to the total electron content during six powerful X-ray flares that occurred in September 2017. The calculation of the electron concentration in the lower ionosphere was carried out using a plasma-chemical model of the ionospheric D region. This model was verified using the data of ground-based radiophysical measurements in the VLF (very low frequency) range and data of the incoherent scattering radar. To calculate the ionization rate at the D region heights, we used real data on the radiation flux measured by the GOES and SDO satellites during the considered flares. The total electron content was estimated using GNSS data. As a result of the analysis, it was found that the contribution of the lower ionosphere to the TEC change varied from 7% to 23% for flares with different spectra. A functional dependency has been obtained that can be used to estimate the contribution of the D region to the TEC increment depending on the spectrum of the flare.

Abstract

Fine-grained authigenic magnetite has been recognized increasingly in iron-rich marine environments affected by methane seepage and is a major sedimentary magnetization source. However, it is unknown whether this magnetite forms via microbial or abiotic processes. We report here abundant fine magnetite crystals, in close association with goethite, within coarse-grained sediments from two adjacent methane seepage sites in the South China Sea. The magnetite- and goethite-rich horizons have sharply increased Zr/Ti, Zr/Rb, Ti/Al, and Fe/Al ratios, probably reflecting deposition by turbidity currents. Deeper intervals have elevated pyrite content, positive δ34S excursions of chromium reducible sulfur, and low magnetic susceptibilities, which is consistent with past sulfate-driven anaerobic oxidation of methane in environments with dynamically variable seepage intensity. In magnetically extracted aggregates (>63 μm), magnetite particles are mainly clustered euhedral crystals with 0.2–0.8 μm sizes, which will likely impact sedimentary magnetic signals. The fine, euhedral crystalline nature of the magnetite suggests formation in sulfide-free, ferrous iron-rich sedimentary environments. Based on 16S rRNA gene sequences, anaerobic methanotrophic archaea coincide with pyrite rich horizons. In contrast, two co-occurring methanogenic archaea groups of the Methanomicrobia class (mainly Methanosarcina and Methanocella) are particularly abundant in turbidites but have low abundance in all other horizons. Increased Methanomicrobia abundances suggest that this class of archaea may be involved in microbial iron reduction in turbidites with abundant goethite as a reactive iron source, and that they apparently trigger magnetite formation. Our findings provide new clues to microbial magnetite formation in iron-rich marine sediments.

Abstract

Modeling the nonlinear interactions between flow, sediment, and vegetation is essential for improving our understanding and prediction of river system dynamics. Using simple numerical models, we simulate the key flow-sediment-vegetation interaction where the disturbance is intrinsically generated by the presence of vegetation. In this case, biomass growth modifies the flow field, induces bed scour, and thus potentially causes vegetation uprooting when erosion exceeds root depth. Our results show that this nonlinear feedback produces deterministic chaos under a wide range of conditions, with complex aperiodic dynamics generated by a period-doubling route to chaos. Moreover, our results suggest relatively small values of Lyapunov time, spanning 2–4 growth-flood cycles, which significantly restrict the predictability of riverbed evolution. Although further spatial and temporal processes may add complexity to the system, these results call for the use of ensemble methods and associated uncertainty estimates in ecomorphodynamic models.