Scholarly Works, Geosciences

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Estimating Wind Direction and Wind Speed Over Lakes With Surface Water Ocean Topography and Sentinel‐1 Satellite Observations
McQuillan, Katie A.; Allen, George H.; Fayne, Jessica; Gao, Huilin; Wang, Jida (American Geophysical Union, 2025-03-25)
Wind at the water‐air interface is an important driver of hydrologic and biogeochemical processes in lakes. Satellite synthetic aperture radar (SAR) is commonly used over the ocean to retrieve wind fields using backscatter coefficients which are sensitive to wind‐driven surface water roughness; however, its application to lakes has been largely unexplored. Here we assess the utility of SAR to retrieve wind fields specifically for lakes. We estimated wind direction from SAR backscatter using the Modified Local Gradient method for Surface Water Ocean Topography (SWOT) and Sentinel‐1 data. The estimated wind direction was then used as an input into a C‐band geophysical modeling function (GMF) to invert wind speed from Sentinel‐1 data. Comparisons between SWOT backscatter and in situ wind speeds were used to provide a foundation for understanding how SWOT could be used to study wind speeds. Using buoy data for validation, we found wind direction (1 km) mean absolute error (MAE) ranged from 31° to 40° for Sentinel‐1 and 28° to 38° for SWOT. Sentinel‐1 wind speed (100 m) MAE ranged from 1.05 to 2.09 m/s. These retrievals were more accurate and at higher resolution compared to global reanalysis dataset ERA5 (0.25°), with wind direction MAE from 23° to 50° and wind speed MAE from 1.49 to 2.35 m/s. SWOT backscatter sensitivity to wind speed depended on incidence angle, and demonstrated utility for developing a GMF for lakes. These methods could be used to better understand wind dynamics globally, especially over small lakes and in data poor regions.
Elucidating the role of InGaAs and InAlAs buffers on carrier dynamics of tensile strained Ge double heterostructures
Bhattacharya, Shuvodip; Johnston, Steven W.; Bodnar, Robert J.; Hudait, Mantu K. (ACS, 2024-06-06)
Extensive research efforts of strained germanium (Ge) are currently underway due to its unique properties, namely, (i) possibility of band gap and strain engineering to achieve a direct band gap, thus exhibiting superior radiative properties, and (ii) higher electron and hole mobilities than Si for upcoming technology nodes. Realizing lasing structures is vital to leveraging the benefits of tensile-strained Ge (ε-Ge). Here, we use a combination of different analytical tools to elucidate the effect of the underlying InGaAs/InAlAs and InGaAs overlaying heterostructures on the material quality and strain state of ε-Ge grown by molecular beam epitaxy. Using X-ray analysis, we show the constancy of tensile strain in sub-50 nm ε-Ge in a quantum-well (QW) heterostructure. Further, effective carrier lifetime using photoconductive decay as a function of buffer type exhibited a high (low) defect-limited carrier lifetime of ∼68 ns (∼13 ns) in 0.61% (0.66%) ε-Ge grown on an InGaAs (InAlAs) buffer. These results correspond well with the measured surface roughness of 1.289 nm (6.303 nm), consistent with the surface effect of the ε-Ge/III–V heterointerface. Furthermore, a reasonably high effective lifetime of ∼78 ns is demonstrated in a QW of ∼30 nm 1.6% ε-Ge, a moderate reduction from ∼99 ns in uncapped ε-Ge, alluding to the surface effect of the overlying heterointerface. Thus, the above results highlight the prime quality of ε-Ge that can be achieved via III–V heteroepitaxy and paves a path for integrated Ge photonics.
Carrier Recombination Dynamics of Surface Passivated Epitaxial (100)Ge, (110)Ge, and (111)Ge Layers by Atomic Layer Deposited Al₂O₃
Hudait, Mantu K.; Johnston, Steven W.; Das, Manash R.; Karthikeyan, Sengunthar; Sahu, Partha P.; Das, Jagat; Zhao, Jing; Bodnar, Robert J.; Joshi, Rutwik (ACS, 2023-05-24)
Germanium (Ge) and its heterostructures with compound semiconductors offer a unique optoelectronic functionality due to its pseudo-bandgap nature, that can be transformed to a direct bandgap material by providing strain and/or mixing with tin. Moreover, two crystal surfaces, (100)Ge and (110)Ge, that are technologically important for ultralow power fin or nanosheet transistors, could offer unprecedented properties with reduced surface defects after passivating these surfaces by atomic layer deposited (ALD) dielectrics. In this work, the crystallographically oriented epitaxial Ge/AlAs heterostructures were grown and passivated with ALD Al2O3 dielectrics, and the microwave photoconductive decay (μ-PCD) technique was employed to evaluate carrier lifetimes at room temperature. The X-ray photoelectron spectroscopy analysis reveals no role of orientation effect in the quality of the ALD Al2O3 dielectric on oriented Ge layers. The carrier lifetimes measured using the μ-PCD technique were benchmarked against unpassivated Ge/AlAs heterostructures. Excitation wavelengths of 1500 and 1800 nm with an estimated injection level of ∼1013 cm-3 were selected to measure the orientation-specific carrier lifetimes. The carrier lifetime was increased from 390 ns to 565 ns for (100)Ge and from 260 ns to 440 ns for (110)Ge orientations with passivation, whereas the carrier lifetime is almost unchanged for (111)Ge after passivation. This behavior indicates a strong dependence of the measured lifetime on surface orientation and surface passivation. The observed increase (>1.5×) in lifetime with Al2O3-passivated (100)Ge and (110)Ge surfaces is due to the lower surface recombination velocity compared to unpassivated Ge/AlAs heterostructures. The enhancement of carrier lifetime from passivated Ge/AlAs heterostructures with (100)Ge and (110)Ge surface orientations offers a path for the development of nanoscale transistors due to the reduced interface state density.
Lattice matched GeSn/InAlAs heterostructure: Role of Sn in energy band alignment, atomic layer diffusion and photoluminescence
Karthikeyan, Sengunthar; Joshi, Rutwik; Zhao, Jing; Bodnar, Robert J.; Magill, Brenden A.; Pleimling, Yannick; Khodaparast, Giti A.; Hudait, Mantu K. (Royal Society Chemistry, 2023-07-20)
Germanium alloyed with α-tin (GeSn) transitions to a direct bandgap semiconductor of significance for optoelectronics. It is essential to localize the carriers within the active region for improving the quantum efficiency in a GeSn based laser. In this work, epitaxial GeSn heterostructure material systems were analyzed to determine the band offsets for carrier confinement: (i) a 0.53% compressively strained Ge0.97Sn0.03/AlAs; (ii) a 0.81% compressively strained Ge0.94Sn0.06/Ge; and (iii) a lattice matched Ge0.94Sn0.06/In0.12Al0.88As. The phonon modes in GeSn alloys were studied using Raman spectroscopy as a function of Sn composition, that showed Sn induced red shifts in wavenumbers of the Ge-Ge longitudinal optical phonon mode peaks. The material parameter b representing strain contribution to Raman shifts of a Ge0.94Sn0.06 alloy was determined as b = 314.81 ± 14 cm−1. Low temperature photoluminescence measurements were performed at 79 K to determine direct and indirect energy bandgaps of Eg,Γ = 0.72 eV and Eg,L = 0.66 eV for 0.81% compressively strained Ge0.94Sn0.06, and Eg,Γ = 0.73 eV and Eg,L = 0.68 eV for lattice matched Ge0.94Sn0.06 epilayers. Chemical effects of Sn atomic species were analyzed using X-ray photoelectron spectroscopy (XPS), revealing a shift in Ge 3d core level (CL) spectra towards the lower binding energy affecting the bonding environment. Large valence band offset of ΔEV = 0.91 ± 0.1 eV and conduction band offset of ΔEC,Γ-X = 0.64 ± 0.1 eV were determined from the Ge0.94Sn0.06/In0.12Al0.88As heterostructure using CL spectra by XPS measurements. The evaluated band offset was found to be of type-I configuration, needed for carrier confinement in a laser. In addition, these band offset values were compared with the first-principles-based calculated Ge/InAlAs band alignment, and it was found to have arsenic up-diffusion limited to 1 monolayer of epitaxial GeSn overlayer, ruling out the possibility of defects induced modification of band alignment. Furthermore, this lattice matched GeSn/InAlAs heterostructure band offset values were significantly higher than GeSn grown on group IV buffer/substrates. Therefore, a lattice matched GeSn/InAlAs material system has large band offsets offering superior carrier confinement to realize a highly efficient GeSn based photonic device.
High-κ Gate Dielectric on Tunable Tensile Strained Germanium Heterogeneously Integrated on Silicon: Role of Strain, Process, and Interface States
Hudait, Mantu K.; Clavel, Michael B.; Karthikeyan, Sengunthar; Bodnar, Robert J. (ACS, 2023-08-17)
Tensile strained germanium (ϵ-Ge) layers heterogeneously integrated on Si substrates are of technological importance for nanoscale transistors and photonics. In this work, the tunable tensile strained (0% to 1.2%) ϵ-Ge layers were grown by solid source molecular beam epitaxy using GaAs and linearly graded InxGa1-xAs as intermediate buffers, and their structural and metal-oxide semiconductor capacitor (MOS-Cs) properties were analyzed as a function of strain and process conditions. X-ray topography measurements displayed no visible thermal crack and a low thermal stress of ∼50 MPa. Temperature dependent strain relaxation properties, studied by X-ray and Raman analyses, confirmed that the tensile strain amount of 1.2% was well preserved within the ϵ-Ge layer when annealed up to 550 °C. Further, transmission electron microscopic study revealed a good quality 1.2% strained ϵ-Ge/In0.17Ga0.83As heterointerface. In addition, unstrained Ge (0% ϵ-Ge) MOS-Cs with atomic layer deposited Al2O3 and thermally grown GeO2 composite gate dielectrics of varying oxidation times (0.25-7.5 min) at 550 °C exhibited a low interface state density (Dit) of ∼2.5 × 1011 eV-1 cm-2 at 5 min oxidation duration. The minimum oxidation time needed for good capacitance-voltage (C-V) characteristics on 0.2% ϵ-Ge is inadequate to accomplish similar C-V characteristics on 1.2% ϵ-Ge MOS-C, due to the higher strain field impeding the formation of the GeO2 interface passivation layer at lower oxidation duration. In addition, with the trade-off between the minimum Dit and minimum equivalent oxide thickness values, ∼1.5 min is found to be an optimum oxidation time for good quality 1.2% ϵ-Ge MOS-C. The minimum Dit values of 1.36 × 1011 and 2.06 × 1011 eV-1 cm-2 for 0.2% and 1.2% ϵ-Ge, respectively, were determined for 4 nm Al2O3 with 5 min thermal oxidation at 550 °C. Therefore, the successful monolithic integration of tunable tensile strain Ge on Si with structural defects and MOS-Cs analyses offer a path for the development of tensile strained Ge-based nanoscale transistors.
GeSn-on-GaAs with photoconductive carrier lifetime >400 ns: role of substrate orientation and atomistic simulation
Karthikeyan, Sengunthar; Johnston, Steven W.; Gayakwad, Dhammapriy; Mahapatra, Suddhasatta; Bodnar, Robert J.; Zhao, Jing; Joshi, Rutwik; Hudait, Mantu K. (Royal Society Chemistry, 2024-04-04)
Group IV GeSn quantum material finds application in electronics and silicon-compatible photonics. Synthesizing these materials with low defect density and high carrier lifetime is a potential challenge due to lattice mismatch induced defects and tin segregation at higher growth temperature. Recent advancements in the growth of these GeSn materials on Si, Ge, GaAs, and with substrate orientations, demonstrated different properties using epitaxial and chemical deposition methods. This article addresses the effect of GaAs substrate orientation and misorientation on the materials’ properties and carrier lifetimes in epitaxial Ge0.94Sn0.06 layers. With starting GaAs substrates of (100)/2°, (100)/6°, (110) and (111)A orientations, Ge0.94Sn0.06 epitaxial layers were grown with an intermediate Ge buffer layer by molecular beam epitaxy and analyzed by several analytical tools. X-ray analysis displayed good crystalline quality, and Raman spectroscopy measurements showed blue shifts in phonon wavenumber due to biaxial compressive strain in Ge0.94Sn0.06 epilayers. Cross-sectional transmission electron microscopy analysis confirmed the defect-free heterointerface of Ge0.94Sn0.06/Ge/GaAs heterostructure. Minority carrier lifetimes of the unintentionally doped n-type Ge0.94Sn0.06 epilayers displayed photoconductive carrier lifetimes of >400 ns on (100)/6°, 319 ns on (100)/2°, and 434 ns on (110) GaAs substrate at 1500 nm excitation wavelength. On the other hand, Ge0.94Sn0.06 layer showed poor carrier lifetime on (111)A GaAs substrate. The observed differences in carrier lifetimes were correlated with the formation energy of the Ge on (100)/6° and (100)/2° GaAs heterointerface using Stillinger-Weber interatomic potential model-based atomistic simulation with different heterointerfacial bonding by Synopsys QuantumATK tool. Total energy computation of 6280-atom Ge/GaAs supercell on (100)/6° leads to lower formation energy than (100)/2°, making it more thermodynamically stable. Hence, the growth of the GeSn/III-V material system using misoriented (100) substrates that are more thermodynamically stable will enhance the performances of optoelectronic devices.
Elucidating the role of InGaAs and InAlAs buffers on carrier dynamics of tensile strained Ge double heterostructures
Bhattacharya, Shuvodip; Johnston, Steven W.; Bodnar, Robert J.; Hudait, Mantu K. (American Chemical Society, 2024-06-06)
Extensive research efforts of strained germanium (Ge) are currently underway due to its unique properties, namely, (i) possibility of band gap and strain engineering to achieve a direct band gap, thus exhibiting superior radiative properties, and (ii) higher electron and hole mobilities than Si for upcoming technology nodes. Realizing lasing structures is vital to leveraging the benefits of tensile-strained Ge (ϵ-Ge). Here, we use a combination of different analytical tools to elucidate the effect of the underlying InGaAs/InAlAs and InGaAs overlaying heterostructures on the material quality and strain state of ϵ-Ge grown by molecular beam epitaxy. Using X-ray analysis, we show the constancy of tensile strain in sub-50 nm ϵ-Ge in a quantum-well (QW) heterostructure. Further, effective carrier lifetime using photoconductive decay as a function of buffer type exhibited a high (low) defect-limited carrier lifetime of ∼68 ns (∼13 ns) in 0.61% (0.66%) ϵ-Ge grown on an InGaAs (InAlAs) buffer. These results correspond well with the measured surface roughness of 1.289 nm (6.303 nm), consistent with the surface effect of the ϵ-Ge/III-V heterointerface. Furthermore, a reasonably high effective lifetime of ∼78 ns is demonstrated in a QW of ∼30 nm 1.6% ϵ-Ge, a moderate reduction from ∼99 ns in uncapped ϵ-Ge, alluding to the surface effect of the overlying heterointerface. Thus, the above results highlight the prime quality of ϵ-Ge that can be achieved via III-V heteroepitaxy and paves a path for integrated Ge photonics.
Mapping the Ge/InAl(Ga)As interfacial electronic structure and strain relief mechanism in germanium quantum dots
Hudait, Mantu K.; Bhattacharya, S.; Karthikeyan, S.; Zhao, J.; Bodnar, Robert J.; Magill, Brenden A.; Khodaparast, Giti A. (Royal Society Chemistry, 2024-09-12)
Tensile-strained germanium (ϵ-Ge) has attracted significant interest due to its unique properties in emerging optoelectronic devices. High tensile-strained Ge materials with superior quality are still being investigated due to the intrinsic instability of ϵ-Ge against the formation of stacking faults (SFs). This work seeks to improve the understanding of these limits by closely examining, experimentally, the mechanisms by which tensile strain is relaxed in Ge. Here, ϵ-Ge layers were grown on highly mismatched In0.53Ga0.47As and In0.51Al0.49As virtual substrates (f = 3.4%), formed as quantum dots (QDs) by molecular beam epitaxy, and their strain relaxation mechanism was analyzed. Both In0.51Al0.49As and In0.53Ga0.47As growth templates were created using an Al0.49In0.51x(Ga0.51)1−xAs linearly graded metamorphic buffer on GaAs(001)/2° and InP(001)/0.5° substrates, respectively. Fully 3D growth (Volmer-Weber growth mode) due to high tensile strain resulted in Ge QDs with an average diameter and height of ∼50 nm and ∼20 nm, respectively, and a uniform density of ∼320 μm−2. Analysis of the interfacial electronic structure using high-resolution transmission electron microscopy collected from the Ge QDs indicated that minimal tensile strain was retained in Ge due to SF formation, corroborated via the Raman results. All Ge QDs contain multiple SFs of the close-packed {111} planes nucleated by Shockley partial dislocations with Burger vectors b = ⅙〈112〉. The presence of additional misfit dislocations at the Ge/In0.51Al0.49As or Ge/In0.53Ga0.47As heterointerface, not associated with SFs, indicates further relaxation by perfect dislocations with Burger vectors b = ½〈110〉. The tensile misfit of 3.4% in Ge revealed instability against SF formation, and the availability of a defect type must have the effect of lowering the critical layer thickness for ϵ-Ge layers. Thus, the above results suggest that a maximum tensile strain amount >3.4% is not achievable in Ge without the formation of Shockley partial dislocations.
A framework for developing a real-time lake phytoplankton forecasting system to support water quality management in the face of global change
Carey, Cayelan C.; Calder, Ryan S. D.; Figueiredo, Renato J.; Gramacy, Robert B.; Lofton, Mary E.; Schreiber, Madeline E.; Thomas, R. Quinn (Springer, 2024-09-20)
Phytoplankton blooms create harmful toxins, scums, and taste and odor compounds and thus pose a major risk to drinking water safety. Climate and land use change are increasing the frequency and severity of blooms, motivating the development of new approaches for preemptive, rather than reactive, water management. While several real-time phytoplankton forecasts have been developed to date, none are both automated and quantify uncertainty in their predictions, which is critical for manager use. In response to this need, we outline a framework for developing the first automated, real-time lake phytoplankton forecasting system that quantifies uncertainty, thereby enabling managers to adapt operations and mitigate blooms. Implementation of this system calls for new, integrated ecosystem and statistical models; automated cyberinfrastructure; effective decision support tools; and training for forecasters and decision makers. We provide a research agenda for the creation of this system, as well as recommendations for developing real-time phytoplankton forecasts to support management.
Silicified microfossils from the Ediacaran Doushantuo Formation along a shelf margin-slope-basin transect in Hunan Province, South China, with stratigraphical implications
Ouyang, Qing; Zhou, Chuanming; Xiao, Shuhai; Wu, Chengxi; Chen, Zhe; Lang, Xianguo; Shi, Hongyi; Sun, Yunpeng (2025)
Silicified microfossils are reported from nine stratigraphic sections of the Ediacaran Doushantuo Formation deposited in shelf margin, slope, and basin environments in Hunan Province of South China. These microfossils include sphaeromorphic and acanthomorphic acritarchs (15 genera and 29 species, including three new acanthomorph species, Bullatosphaera? colliformis n. sp., Eotylotopalla inflata n. sp., and Verrucosphaera? undulata n. sp.), multicellular algae, tubular microfossils, and other problematic forms, representing major fossil groups similar to those from the Doushantuo Formation in more proximal facies, e.g., inner shelf and shelf lagoon. A database of the abundance and occurrences of Doushantuo acanthomorphs is assembled and analyzed using quantitative and data visualization methods (i.e., rarefaction analysis, non-parametric multidimensional scaling, and network analysis). The results show that, at the genus- and species-level, taxonomic richness of Doushantuo acanthomorphs exhibits remarkable variation among facies, but this variation is largely due to sampling and taphonomic biases. The results also show that numerous acanthomorph taxa have broad facies distribution, affirming their biostratigraphic value. The analysis confirms that acanthomorphs in the Weng'an biota of shelf margin facies is transitional between Member II and Member III assemblages of shelf lagoon facies in the Yangtze Gorges area. The study shows the biostratigraphic potential of acanthomorphs in the establishment of regional biozones using the first appearance datum of widely distributed taxa, highlighting the importance of continuing exploration of undersampled Doushantuo sections in slope and basinal facies.
Identifying Barriers and Bridging Gaps Between Researchers and Decision Makers in Water Quality Modeling
Chowdhury, Mahabub; Carey, Cayelan C.; Figueiredo, Renato; Gramacy, Robert; Hoffman, Kathryn; Lofton, Mary; Patil, Parul; Schreiber, Madeline; Thomas, R. Quinn; Calder, Ryan S. D. (2024-12-12)
Spinel oxide enables high-temperature self-lubrication in superalloys
Zhang, Zhengyu; Hershkovitz, Eitan; An, Qi; Liu, Liping; Wang, Xiaoqing; Deng, Zhifei; Baucom, Garrett; Wang, Wenbo; Zhao, Jing; Xin, Ziming; Moore, Lowell; Yi, Yao; Islam, Md Rezwan Ul; Chen, Xin; Cui, Bai; Li, Ling; Xin, Hongliang; Li, Lin; Kim, Honggyu; Cai, Wenjun (Nature Research, 2024-11-20)
The ability to lubricate and resist wear at temperatures above 600 °C in an oxidative environment remains a significant challenge for metals due to their high-temperature softening, oxidation, and rapid degradation of traditional solid lubricants. Herein, we demonstrate that high-temperature lubricity can be achieved with coefficients of friction (COF) as low as 0.10-0.32 at 600- 900 °C by tailoring surface oxidation in additively-manufactured Inconel superalloy. By integrating high-temperature tribological testing, advanced materials characterization, and computations, we show that the formation of spinel-based oxide layers on superalloy promotes sustained self-lubrication due to their lower shear strength and more negative formation and cohesive energy compared to other surface oxides. A reversible phase transformation between the cubic and tetragonal/monoclinic spinel was driven by stress and temperature during high temperature wear. To span Ni- and Cr-based ternary oxide compositional spaces for which little high-temperature COF data exist, we develop a computational design method to predict the lubricity of oxides, incorporating thermodynamics and density functional theory computations. Our finding demonstrates that spinel oxide can exhibit low COF values at temperatures much higher than conventional solid lubricants with 2D layered or Magnéli structures, suggesting a promising design strategy for selflubricating high-temperature alloys.
Aging dams, political instability, poor human decisions and climate change: recipe for human disaster
Shirzaei, Manoochehr; Vahedifard, Farshid; Sadhasivam, Nitheshnirmal; Ohenhen, Leonard; Dasho, Oluwaseyi; Tiwari, Ashutosh; Werth, Susanna; Azhar, Mohammed; Zhao, Yunxia; Nicholls, Robert J.; AghaKouchak, Amir (Springer Nature, 2025-01-16)
In Derna, Libya, a record-breaking storm and subsequent dam failures on September 10, 2023, caused over 11,000 deaths. Analyzing satellite data from 2016–2023, we found 1.8 mm/yr of differential settlement in dams contributed to their failure, and flooding damaged ~8570 buildings. We argue that the interplay of aging infrastructure, political instability, climate change, and human decisions drove this disaster, stressing the need for a holistic ‘healthcare’ management approach to prevent future catastrophes.
Effects of Using the Consistent Boundary Flux Method on Dynamic Topography Estimates
Williams, Karen; Stamps, D. Sarah; Austermann, Jacqueline; King, Scott D.; Njinju, Emmanuel (Oxford University Press, 2024-06-12)
Dynamic topography is defined as the deflection of Earth's surface due to the convecting mantle. ASPECT (Advanced Solver for Planetary Evolution, Convection, and Tectonics) is a continually evolving, finite element code that uses modern numerical methods to investigate problems in mantle convection. With ASPECT version 2.0.0 a consistent boundary flux (CBF) algorithm, used to calculate radial stresses at the model boundaries, was implemented into the released version of ASPECT. It has been shown that the CBF algorithm improves the accuracy of dynamic topography calculations by approximately one order of magnitude. We aim to evaluate the influence of the CBF algorithm and explore the geophysical implications of these improved estimates of dynamic topography changes along the East Coast of the United States. We constrain our initial temperature conditions using the tomography models SAVANI, S40RTS and TX2008, and combine them with a corresponding radial viscosity profile (2 for TX2008) and two different boundary conditions for a total of eight experiments. We perform simulations with and without the CBF method, which takes place during post-processing and does not affect the velocity solution. Our dynamic topography calculations are spatially consistent in both approaches, but generally indicate an increase in magnitude using the CBF method (on average ∼15 and ∼76 per cent absolute change in present-day instantaneous and rate of change of dynamic topography, respectively). This enhanced accuracy in dynamic topography calculations can be used to better evaluate the effects of mantle convection on surface processes including vertical land motions, sea level changes, and sedimentation and erosion. We explore results along the US East Coast, where a Pliocene shoreline has been deformed by dynamic topography change. An increased accuracy in estimates of dynamic topography can improve Pleistocene and Pliocene sea level reconstructions, which allow for a better understanding of past sea level changes and ice sheet stability.
Evidence for a prehistoric multifault rupture along the southern Calico fault system, Eastern California Shear Zone, USA
Vadman, Michael J.; Garvue, Max M.; Spotila, James A.; Bemis, Sean P.; Stamps, D. Sarah; Owen, Lewis A.; Figueiredo, Paula M. (Geological Society of America, 2023-09-08)
Geomorphic mapping and paleoseismologic data reveal evidence for a late Holocene multifault surface rupture along the Calico-Hidalgo fault system of the southern Eastern California Shear Zone (ECSZ). We have identified ~18 km of continuous surface rupture along the combined Calico and Hidalgo faults in the vicinity of Hidalgo Mountain in the southern Mojave Desert. Based on the freshness of geomorphic fault features and continuity of surface expression, we interpret this feature to reflect a simultaneous paleorupture of both faults. Displacement along the paleorupture is defined by 39 field measurements to be generally pure right-slip with a mean offset of 2.3 m. Scaling relationships for this offset amount imply that the original surface rupture length may have been ~82 km (corresponding to a M7.4 earthquake) and that much of the rupture trace was erased by subsequent erosion of sandy and unconsolidated valley alluvium. Eight luminescence ages from a paleoseismic trench across the paleorupture on the Hidalgo fault bracket the timing of the most recent rupture to 0.9–1.7 ka and a possible penultimate event at 5.5–6.6 ka. This timing is generally consistent with the known earthquake clusters in the southern ECSZ based on previous paleoseismic investigations. The ages of these earthquakes also overlap with the age brackets of the most recent events on the Calico fault 42 km to the north and the Mesquite Lake fault 40 km to the south from earlier work. Based on these age constraints and the expected surface rupture length, we propose that the Calico fault system experienced a major, multifault rupture that spanned the entire length of the fault system between the historical Landers and Hector Mine ruptures but preceded these events by ~1–2 k.y. Coulomb stress change modeling shows that the Calico paleorupture may have delayed the occurrence of the Landers-Hector Mine cluster by placing their respective faults in stress shadows and may have also prevented a triggered event from occurring on the Calico fault following the historic events. This work implies that closely spaced ruptures in complex shear zones may repel each other and thereby stretch out the duration of major earthquake clusters. These results also suggest that complex multifault ruptures in the ECSZ may not follow simple, repeatable patterns.
Intrarift fault interactions: Insights from coseismic stress redistribution from large seismogenic segment ruptures, Northern Malawi Rift
Njinju, Emmanuel A.; Kolawole, Folarin; Stamps, D. Sarah; Atekwana, Estella A.; Ghomsi, Franck Eitel Kemgang; Atekwana, Eliot A. (Elsevier, 2025-02-01)
The understanding of the factors influencing the active interaction and coalescence of intrarift fault segments in extending continental regions is limited. The 2009 Mw6.0 Karonga earthquake occurred in the westernmost portion of the Northern Malawi Rift, which hosts clustered intrarift faulting. The event ruptured the strongly coalesced southern segment of the St. Mary Fault (sSMF), and subsequently, moderate-magnitude events localized on poorly -coalesced segments that define the northern continuation of the fault. To investigate the role of coseismic stress redistribution on interacting faults, we explore the slip distribution of the 2009 event with realistic 3D strike-variable fault geometries, and compute coseismic Coulomb stress changes around the sSMF and neighboring faults. The results suggest that the down-dip intersections of the sSMF with the neighboring faults partition strain in a way that directs most of the deformation to >5 km depths. Additionally, the coseismic stress changes from the 2009 earthquake promoted interactions between the sSMF and adjoining northern segments of the fault at shallow (<5 km) depths, indicating that upper-crustal hard-linkage is underway in the poorly coalesced en-echelon sections of the northern segment. These results demonstrate how coseismic static stress transfer between evolving intrarift fault systems drive fault linkage over a single seismic cycle. Over successive slip events, such stress transfer processes may accelerate the linkage and coalescence of contiguous intrarift fault segments, amalgamate and deepen sub-basins along-strike, and promote across-rift basin compartmentalization. This process is relevant for fault coalescence over multi-seismic cycles, progressive maturation of rift basins, and transitions from juvenile continental rifting to the development of margins where break-up can initialize.
Crustal heterogeneity and fault parametrization effects on seismic hazards assessment: southeastern margin of the Tibetan Plateau case study
Rui, X.; Williams, C. A.; Stamps, D. Sarah; Fang, L. H. (Oxford University Press, 2024-11-21)
Assumptions about lithospheric composition and fault discretization schemes are crucial for estimating seismic hazards. This study aims to examine the impact of these two variables in the Southeastern Margin of the Tibetan Plateau, with a specific focus on the Daliangshan mountain area. This area, located along the central segment of the Xianshuihe–Xiaojiang fault system, has been considered in the last few decades to host the highest seismic hazards in mainland China given its historical recurrence intervals of large magnitude earthquakes. Previous studies have constrained the kinematics of the region, including estimates of fault slip and accumulated moment magnitudes, assuming (1) the crust is compositionally homogeneous and isotropic and (2) the discretized fault patches (nodes) are coupling independent. In this work, we use constraint from an updated GNSS velocity solution comprised of 287 sites to test the influence of a compositionally heterogeneous crust and two fault patch (node) parametrizations to assess predicted fault slip rates and several seismic hazard quantities. The two fault node parametrizations are: (1) node depth-dependent (NDD; locking coefficients of nodes along a depth profile are dependent) and (2) node depth-independent (NDI; locking coefficients of all nodes are independent). Statistical tests (F-tests) indicate that the NDD model fits GNSS data significantly better than the NDI model even with less than ∼50 per cent of to-be-estimated parameters. We further examine the impact of lithospheric composition on regional kinematics by incorporating data from high-resolution seismic tomography. Comparisons between homogeneous and heterogeneous models for our preferred NDD fault locking parametrization scheme suggest that the material heterogeneity has minimal influence on the predicted recurrence interval estimates of large (e.g. Mw = 7) earthquakes but some influence on discerning the details of the fault coupling distribution. These results show that the crustal medium and parametrization scheme have an impact on the description of fault kinematics, which should be considered as one of the sources of uncertainty in hazard assessment.
Vertical Displacements and Sea-Level Changes in Eastern North America Driven by Glacial Isostatic Adjustment: an Ensemble Modeling Approach
Williams, Karen; Stamps, D. Sarah; Melini, Daniele; Spada, Giorgio (Wiley, 2024-10)
Glacial isostatic adjustment (GIA) describes the response of the solid Earth, oceans, and gravitational field to the spatio-temporal evolution of ice sheets during a glacial cycle. Present-day vertical displacements and sea-level changes vary throughout eastern North America in response to the melting of the Laurentide Ice Sheet following the Last Glacial Maximum. We use the open-source software SELEN4.0 (a SealEveL EquatioN solver) to investigate the influence of GIA on vertical land motions and sea-level changes in eastern North America. Further, we evaluate the uncertainties associated with the lithospheric thickness and viscosity structure using an ensemble modeling approach (129,956 total simulations). We identify the best-fitting rheological profiles by comparing modeled vertical displacements to vertical velocity rates derived from Global Positioning System (GPS). We find a general pattern of subsidence (causing accelerated relative sea-level rise) in the eastern United States region and uplift (causing relative sea-level fall) in the eastern Canada region consistent with previous studies for two tested ice sheet models (ICE-6G(VM5a) and ICE-7G(VM7)). Overall, we find lower rates of modeled vertical displacement using ICE-6G(VM5a) compared with ICE-7G(VM7), which produces lower residuals when compared with the GPS-derived vertical velocity rates. Our ensemble analysis identifies adjustments to the nominal VM5a and VM7 viscosity models that improve fits to the GPS-imaged vertical velocity rates throughout eastern North America and on the North American Atlantic Coast. The differences in our best-fitting models for inland versus coastal regions highlight the importance of exploring lateral viscosity variations for GIA modeling throughout North America and elsewhere.
Impact of the 2008 M_W 7.9 Great Wenchuan earthquake on South China microplate motion
Iaffaldano, Giampiero; de Blas, Juan Martin; Rui, Xu; Stamps, D. Sarah; Bin, Zhao (Nature Portfolio, 2024-07-16)
Tectonic plate motions drive the earthquake cycle, as they result in the slow accrual and sudden release of energy along plate boundaries. Steadiness of plate motions over the earthquake cycle is a central tenet of the plate tectonics theory and has long been a main pillar in models of earthquake genesis, or of plate-margins seismic potential inferred from slip-deficit estimates. The advent of geodesy in the geosciences and the availability of multi-year-long series of position measurements permit tracking the motions of tectonic plates from before to after the time of significant seismic events that occur along their margins. Here, we present evidence that large earthquakes are capable of modifying the motions of entire microplates. We use high precision Global Navigation Satellite System (GNSS) position time-series covering the periods 2001–2004 and 2014–2017 to demonstrate that, contrary to the tenet above, the South China microplate motion changed after the 2008 MW 7.9 Great Wenchuan earthquake. The GNSS data and associated uncertainties indicate a plate motion slowdown of up to 20% that is beyond the possible impact of data noise and is thus tectonically meaningful. We use quantitative models of torque balance to show that generating this kinematic change requires a force upon the South China microplate compatible with that imparted by the Great Wenchuan earthquake of 2008. The existence of a kinematic signal linked to the earthquake cycle that impacts an entire microplate might offer an additional, novel perspective to assessing the hazards of earthquake-prone tectonic regions.
Vertically graded Fe-Ni alloys with low damping and a sizable spin-orbit torque
Maizel, Rachel E.; Wu, Shuang; Balakrishnan, Purnima P.; Grutter, Alexander J.; Kinane, Christy J.; Caruana, Andrew J.; Nakarmi, Prabandha; Nepal, Bhuwan; Smith, David A.; Lim, Youngmin; Jones, Julia L.; Thomas, Wyatt C.; Zhao, Jing; Michel, F. Marc; Mewes, Tim; Emori, Satoru (American Physical Society, 2024-10-21)
Energy-efficient spintronic devices require a large spin-orbit torque (SOT) and low damping to excite magnetic precession. In conventional devices with heavy-metal/ferromagnet bilayers, reducing the ferromagnet thickness to approximately 1 nm enhances the SOT but dramatically increases damping. Here, we investigate an alternative approach based on a 10-nm-thick single-layer ferromagnet to attain both low damping and a sizable SOT. Instead of relying on a single interface, we continuously break the bulk inversion symmetry with a vertical compositional gradient of two ferromagnetic elements: Fe with low intrinsic damping and Ni with sizable spin-orbit coupling. We find low effective damping parameters of αeff<5×10-3 in the Fe-Ni alloy films, despite the steep compositional gradients. Moreover, we reveal a sizable antidamping SOT efficiency of |θAD|≈0.05, even without an intentional compositional gradient. Through depth-resolved x-ray diffraction, we identify a lattice strain gradient as crucial symmetry breaking that underpins the SOT. Our findings provide fresh insights into damping and SOTs in single-layer ferromagnets for power-efficient spintronic devices.

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