Pub Date : 2025-11-01Epub Date: 2025-09-24DOI: 10.1016/j.pepi.2025.107456
Ming Gong , Michael I. Bergman
<div><div>Seismic attenuation can be intrinsic or due to scattering. The relative role of each for Earth's inner core is uncertain. Whereas intrinsic attenuation depends primarily on the material, temperature, and pressure, scattering is primarily a function of microstructure, that is, grain size, shape, texture, as well as single-crystal elastic anisotropy. Here we studied experimentally scattering of ultrasonic compressional waves in a hexagonal close-packed (hcp) Zn-rich Sn alloy, for two microstructures that are likely relevant to the inner core: textured, large columnar dendritic crystals typical of directional solidification, and untextured, equiaxed, ‘fine-grained’ crystals that can result from diffusion creep. We also studied the wavelength/grain size dependence of scattering for these two microstructures. We used a Zn-rich Sn alloy not because we expect it to have intrinsic attenuation similar to Fe under inner core conditions, but because its hcp crystal structure is the likely phase of the Fe alloy in the inner core, making it suitable for understanding the role of microstructure on scattering in the inner core. For the purpose of scaling the experiments to the inner core, pressure and temperature affect scattering primarily through their effects on the elastic constants of Fe and inner core growth dynamics, both of which we account for.</div><div>We developed an algorithm using the pulse-echo technique to experimentally determine a scattering quality factor <em>Q</em><sub><em>Z</em></sub>. We set criteria to determine, and measured, the energy per cycle in the first echo <em>T</em><sub><em>1</em></sub>, which is a measure of the transmitted energy, and the energy per cycle that is reflected before the first echo <em>R</em><sub><em>1</em></sub>, which represents the scattered energy. In order to facilitate comparison with seismic quality factors we defined a scattering quality factor <em>Q</em><sub><em>Z</em></sub> <em>= (R</em><sub><em>1</em></sub> <em>+ T</em><sub><em>1</em></sub><em>)/R</em><sub><em>1</em></sub>. Scaling <em>Q</em><sub><em>Z</em></sub> from the laboratory experiments to the inner core depends on the magnitude of the single-crystal wave speed anisotropy, which is known for Zn, but uncertain for Fe under inner core conditions, so we scaled the experimental results for single-crystal Fe elastic anisotropy between 5 and 20 %.</div><div>As expected, we found a directionally solidified microstructure has a highly anisotropic <em>Q</em><sub><em>Z</em></sub>, showing almost no scattering in the growth direction, whereas in the transverse directions scattering attenuation in the inner core may be comparable to intrinsic attenuation. Taking into account the anisotropy factor for scattering in polycrystalline, anisotropic material, our results predict randomly oriented, equiaxed 10 km-sized grains in the inner core would exhibit more scattering attenuation that the total inferred seismic attenuation, ruling out such a microstr
{"title":"An experimental ultrasonic method to determine a scattering quality factor, with application to earth's inner core","authors":"Ming Gong , Michael I. Bergman","doi":"10.1016/j.pepi.2025.107456","DOIUrl":"10.1016/j.pepi.2025.107456","url":null,"abstract":"<div><div>Seismic attenuation can be intrinsic or due to scattering. The relative role of each for Earth's inner core is uncertain. Whereas intrinsic attenuation depends primarily on the material, temperature, and pressure, scattering is primarily a function of microstructure, that is, grain size, shape, texture, as well as single-crystal elastic anisotropy. Here we studied experimentally scattering of ultrasonic compressional waves in a hexagonal close-packed (hcp) Zn-rich Sn alloy, for two microstructures that are likely relevant to the inner core: textured, large columnar dendritic crystals typical of directional solidification, and untextured, equiaxed, ‘fine-grained’ crystals that can result from diffusion creep. We also studied the wavelength/grain size dependence of scattering for these two microstructures. We used a Zn-rich Sn alloy not because we expect it to have intrinsic attenuation similar to Fe under inner core conditions, but because its hcp crystal structure is the likely phase of the Fe alloy in the inner core, making it suitable for understanding the role of microstructure on scattering in the inner core. For the purpose of scaling the experiments to the inner core, pressure and temperature affect scattering primarily through their effects on the elastic constants of Fe and inner core growth dynamics, both of which we account for.</div><div>We developed an algorithm using the pulse-echo technique to experimentally determine a scattering quality factor <em>Q</em><sub><em>Z</em></sub>. We set criteria to determine, and measured, the energy per cycle in the first echo <em>T</em><sub><em>1</em></sub>, which is a measure of the transmitted energy, and the energy per cycle that is reflected before the first echo <em>R</em><sub><em>1</em></sub>, which represents the scattered energy. In order to facilitate comparison with seismic quality factors we defined a scattering quality factor <em>Q</em><sub><em>Z</em></sub> <em>= (R</em><sub><em>1</em></sub> <em>+ T</em><sub><em>1</em></sub><em>)/R</em><sub><em>1</em></sub>. Scaling <em>Q</em><sub><em>Z</em></sub> from the laboratory experiments to the inner core depends on the magnitude of the single-crystal wave speed anisotropy, which is known for Zn, but uncertain for Fe under inner core conditions, so we scaled the experimental results for single-crystal Fe elastic anisotropy between 5 and 20 %.</div><div>As expected, we found a directionally solidified microstructure has a highly anisotropic <em>Q</em><sub><em>Z</em></sub>, showing almost no scattering in the growth direction, whereas in the transverse directions scattering attenuation in the inner core may be comparable to intrinsic attenuation. Taking into account the anisotropy factor for scattering in polycrystalline, anisotropic material, our results predict randomly oriented, equiaxed 10 km-sized grains in the inner core would exhibit more scattering attenuation that the total inferred seismic attenuation, ruling out such a microstr","PeriodicalId":54614,"journal":{"name":"Physics of the Earth and Planetary Interiors","volume":"368 ","pages":"Article 107456"},"PeriodicalIF":1.9,"publicationDate":"2025-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145221139","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-11-01Epub Date: 2025-10-22DOI: 10.1016/j.pepi.2025.107460
Ján Šimkanin
Changes in the geomagnetic field corresponding to the Earth’s inner core growth are numerically investigated. The Geodynamo is driven by thermochemical convection in the Earth’s outer core, with the codensity gradient serving as the primary driving force. Simulations begin with a small inner core (‘Past’), which progressively enlarges until reaching a stage where the inner core becomes dominant (‘Future’). During the ’Past’ stage, the Geodynamo model generates a multipolar geomagnetic field, which gradually transitions into a predominantly dipolar field as the inner core grows. These transitions are also accompanied by shifts between weak-field and strong-field dynamos and vice versa. The ratio of magnetic to kinetic energy emerges as a more reliable parameter for controlling the transition from multipolar to dipolar dynamos. The dipole component for a small inner core proves unstable, with frequent polarity reversals. As the inner core grows, the frequency of these reversals decreases until the ‘Present’ case, where polarity reversals cease entirely. It is important to note that during the ‘Past’, fluctuations in dipole polarity are observed even in a dipole-dominated magnetic field. In the ‘Future’ stage, representing a potential scenario for the Earth’s geomagnetic field, the hydromagnetic dynamo produces a dipole-dominated magnetic field without polarity reversals. However, if the Earth’s liquid outer core becomes exceedingly small, convection diminishes, causing the Geodynamo to fail. This leads to a slow decay of the magnetic field due to magnetic diffusion. During the ’Future’ stage, the emergence of subcritical dynamos is observed. It is important to note that the results of the present analysis are more strongly influenced by the supercriticality of the flow than by the inner core size, as the latter is determined by the selected solution parameters.
{"title":"Geomagnetic field and the growth of the Earth’s inner core: Past, present and future","authors":"Ján Šimkanin","doi":"10.1016/j.pepi.2025.107460","DOIUrl":"10.1016/j.pepi.2025.107460","url":null,"abstract":"<div><div>Changes in the geomagnetic field corresponding to the Earth’s inner core growth are numerically investigated. The Geodynamo is driven by thermochemical convection in the Earth’s outer core, with the codensity gradient serving as the primary driving force. Simulations begin with a small inner core (‘Past’), which progressively enlarges until reaching a stage where the inner core becomes dominant (‘Future’). During the ’Past’ stage, the Geodynamo model generates a multipolar geomagnetic field, which gradually transitions into a predominantly dipolar field as the inner core grows. These transitions are also accompanied by shifts between weak-field and strong-field dynamos and vice versa. The ratio of magnetic to kinetic energy emerges as a more reliable parameter for controlling the transition from multipolar to dipolar dynamos. The dipole component for a small inner core proves unstable, with frequent polarity reversals. As the inner core grows, the frequency of these reversals decreases until the ‘Present’ case, where polarity reversals cease entirely. It is important to note that during the ‘Past’, fluctuations in dipole polarity are observed even in a dipole-dominated magnetic field. In the ‘Future’ stage, representing a potential scenario for the Earth’s geomagnetic field, the hydromagnetic dynamo produces a dipole-dominated magnetic field without polarity reversals. However, if the Earth’s liquid outer core becomes exceedingly small, convection diminishes, causing the Geodynamo to fail. This leads to a slow decay of the magnetic field due to magnetic diffusion. During the ’Future’ stage, the emergence of subcritical dynamos is observed. It is important to note that the results of the present analysis are more strongly influenced by the supercriticality of the flow than by the inner core size, as the latter is determined by the selected solution parameters.</div></div>","PeriodicalId":54614,"journal":{"name":"Physics of the Earth and Planetary Interiors","volume":"368 ","pages":"Article 107460"},"PeriodicalIF":1.9,"publicationDate":"2025-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145416692","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-11-01Epub Date: 2025-09-20DOI: 10.1016/j.pepi.2025.107439
Sunil K. Roy, M. Ravi Kumar
This study comprehensively examines the shear wave splitting measurements of XKS (SKS and PKS) - SKKS pairs on the same seismograms recorded at 357 broadband stations spanning India, to characterize anisotropy in the lowermost mantle. This resulted in the identification of 104 XKS-SKKS pairs at 62 stations, of which 27 pairs were found to be discrepant, based on the difference in splitting intensity of XKS and the corresponding SKKS phases. These discrepant pairs dominantly sample a portion of the lowermost mantle beneath Southeast Asia and the Indian Ocean. The majority of these pairs represent null-split and split-split cases, with the delay time of SKKS being larger than that of XKS for the latter. This suggests that the XKS phases primarily sample the isotropic (weakly anisotropic) or anisotropic regions with a cancelling effect in the lowermost mantle, while the corresponding SKKS phases sample the anisotropic region of the D layer. In addition, there are three discrepant pairs in the split-null category, suggesting anisotropy in the vicinity of southern Tibet, where discrepant pairs from other cases are not observed. This implies an apparent change in the anisotropy of the D layer for the regions sampled by XKS and SKKS, although they are associated with high-velocity anomalies. In these regions, the fast polarization azimuths of the discrepant pairs are in the NE-SW and ENE-WSW, and NNE-SSW directions, respectively. These do not coincide with the trend of mantle flow in the lowermost mantle, suggesting an association with paleo-subducted slabs. The observed deformation is probably due to phase transformation of bridgmanite to a more stable post-perovskite, causing Crystallographic Preferred Orientation of the lowermost mantle, which is the candidate mechanism for lowermost mantle anisotropy beneath Southeast Asia and the Indian Ocean.
{"title":"Evidence for lowermost mantle anisotropy from discrepant splitting intensity of XKS and SKKS phases recorded in India","authors":"Sunil K. Roy, M. Ravi Kumar","doi":"10.1016/j.pepi.2025.107439","DOIUrl":"10.1016/j.pepi.2025.107439","url":null,"abstract":"<div><div>This study comprehensively examines the shear wave splitting measurements of XKS (SKS and PKS) - SKKS pairs on the same seismograms recorded at 357 broadband stations spanning India, to characterize anisotropy in the lowermost mantle. This resulted in the identification of 104 XKS-SKKS pairs at 62 stations, of which 27 pairs were found to be discrepant, based on the difference in splitting intensity of XKS and the corresponding SKKS phases. These discrepant pairs dominantly sample a portion of the lowermost mantle beneath Southeast Asia and the Indian Ocean. The majority of these pairs represent null-split and split-split cases, with the delay time of SKKS being larger than that of XKS for the latter. This suggests that the XKS phases primarily sample the isotropic (weakly anisotropic) or anisotropic regions with a cancelling effect in the lowermost mantle, while the corresponding SKKS phases sample the anisotropic region of the D<span><math><msup><mrow></mrow><mrow><mo>″</mo></mrow></msup></math></span> layer. In addition, there are three discrepant pairs in the split-null category, suggesting anisotropy in the vicinity of southern Tibet, where discrepant pairs from other cases are not observed. This implies an apparent change in the anisotropy of the D<span><math><msup><mrow></mrow><mrow><mo>″</mo></mrow></msup></math></span> layer for the regions sampled by XKS and SKKS, although they are associated with high-velocity anomalies. In these regions, the fast polarization azimuths of the discrepant pairs are in the NE-SW and ENE-WSW, and NNE-SSW directions, respectively. These do not coincide with the trend of mantle flow in the lowermost mantle, suggesting an association with paleo-subducted slabs. The observed deformation is probably due to phase transformation of bridgmanite to a more stable post-perovskite, causing Crystallographic Preferred Orientation of the lowermost mantle, which is the candidate mechanism for lowermost mantle anisotropy beneath Southeast Asia and the Indian Ocean.</div></div>","PeriodicalId":54614,"journal":{"name":"Physics of the Earth and Planetary Interiors","volume":"368 ","pages":"Article 107439"},"PeriodicalIF":1.9,"publicationDate":"2025-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145120915","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-11-01Epub Date: 2025-10-08DOI: 10.1016/j.pepi.2025.107459
Clemens Kloss
Studies of Earth’s magnetic field and its sources rely on accurate geomagnetic field models derived from ground and satellite-based magnetic data. During the field model estimation, data errors are usually assumed to be uncorrelated in time and independent of position. However, limitations in the field model parameterization, especially regarding ionospheric and magnetospheric fields, lead to data errors that are not only larger than the expected measurement noise but are also correlated in time and vary with position. As a result, the obtained model uncertainties are often underestimated, making it more challenging to evaluate the reliability of recovered signals in the field models.
This study investigates the effect of including correlated data errors in field modeling. The approach involves building a stochastic data error model to treat correlated errors due to unmodeled magnetospheric fields within the CHAOS geomagnetic field modeling framework. The error model parameters are estimated using empirical covariances computed from vector residuals between the satellite magnetic observations made by the Swarm satellites and the CHAOS geomagnetic field model. Field modeling experiments are performed with and without including the data error covariances described in the stochastic error model.
The inclusion of data error covariances due to unmodeled magnetospheric fields leads to only small changes in the estimated internal field, but also a noticeable increase in model uncertainty for the sectoral coefficients. This highlights the significant impact of unmodeled magnetospheric fields and the importance of accurately defining data errors, including the covariances between observations, for interpreting the retrieved magnetic signals in geomagnetic field modeling.
{"title":"Accounting for correlated data errors in geomagnetic field modeling using Swarm magnetic observations","authors":"Clemens Kloss","doi":"10.1016/j.pepi.2025.107459","DOIUrl":"10.1016/j.pepi.2025.107459","url":null,"abstract":"<div><div>Studies of Earth’s magnetic field and its sources rely on accurate geomagnetic field models derived from ground and satellite-based magnetic data. During the field model estimation, data errors are usually assumed to be uncorrelated in time and independent of position. However, limitations in the field model parameterization, especially regarding ionospheric and magnetospheric fields, lead to data errors that are not only larger than the expected measurement noise but are also correlated in time and vary with position. As a result, the obtained model uncertainties are often underestimated, making it more challenging to evaluate the reliability of recovered signals in the field models.</div><div>This study investigates the effect of including correlated data errors in field modeling. The approach involves building a stochastic data error model to treat correlated errors due to unmodeled magnetospheric fields within the CHAOS geomagnetic field modeling framework. The error model parameters are estimated using empirical covariances computed from vector residuals between the satellite magnetic observations made by the <em>Swarm</em> satellites and the CHAOS geomagnetic field model. Field modeling experiments are performed with and without including the data error covariances described in the stochastic error model.</div><div>The inclusion of data error covariances due to unmodeled magnetospheric fields leads to only small changes in the estimated internal field, but also a noticeable increase in model uncertainty for the sectoral coefficients. This highlights the significant impact of unmodeled magnetospheric fields and the importance of accurately defining data errors, including the covariances between observations, for interpreting the retrieved magnetic signals in geomagnetic field modeling.</div></div>","PeriodicalId":54614,"journal":{"name":"Physics of the Earth and Planetary Interiors","volume":"368 ","pages":"Article 107459"},"PeriodicalIF":1.9,"publicationDate":"2025-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145416691","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-11-01Epub Date: 2025-08-22DOI: 10.1016/j.pepi.2025.107437
Satoshi Kaneshima
Seismic signals in P coda originating from deep mantle heterogeneity have not yet been investigated extensively, except for the observations of the waves reflected presumably at the top of the D″ layer. We show in this study that array processing of seismograms of deep earthquakes at Tonga-Fiji and Solomon Islands recorded by seismograph networks at southern California reveals strong off-great circle arrivals in P coda 5 to 10 s after direct P waves. We also show that the large arrivals observed for the Tonga-Fiji events are P-to-P reflected waves at a dipping interface in the mid-lower mantle beneath central Pacific. The reflector is located south-southeast of Hawaii around 2000 km depth and dips down to southeast by nearly 35°. The observed amplitude and polarity of the reflected waves could be explained if the Vp of the underlying side of the reflector is 1 to 2 % faster than the overlying side. The small Vp anomaly may not necessarily contradict the absence of a noticeable Vp anomaly in previous seismic tomography models at the site of the reflector. We also find that the reflected wave is approximately concomitant with a weaker arrival from a mid-mantle scattering object located nearly 1000 km closer to the hypocenters. The heterogeneous object causing the anomalous arrivals for the Solomon Islands events, although the properties of the object are less well constrained than the Tonga-Fiji reflector, also likely represents another dipping reflector at 2400 km located approximately below the Hawaiian hotspot. As in the Tonga-Fiji case the signals are occasionally followed by a weaker signal from a mid-mantle scattering object located nearer to the hypocenters. The mid-mantle reflection/scattering objects do not indicate the presence of a global discontinuity but must represent localized strong heterogeneities. It is notable that the localized heterogeneities are all located near the edges of a large low Vs body (the Pacific LLSVP) resolved by global tomography, up to 500 km above the LLSVP itself. The relation between the locations of the reflector/scatterers and the large scale Vp structure is unclear, probably reflecting poorer tomography images of Vp structure associated with the LLSVP. We discuss possible tectonic implications of these mid-mantle heterogeneities on the structure and evolution of the LLSVP.
{"title":"Seismic reflectors in the mid-lower mantle beneath central Pacific: The relationship with the Pacific LLSVP","authors":"Satoshi Kaneshima","doi":"10.1016/j.pepi.2025.107437","DOIUrl":"10.1016/j.pepi.2025.107437","url":null,"abstract":"<div><div>Seismic signals in P coda originating from deep mantle heterogeneity have not yet been investigated extensively, except for the observations of the waves reflected presumably at the top of the D″ layer. We show in this study that array processing of seismograms of deep earthquakes at Tonga-Fiji and Solomon Islands recorded by seismograph networks at southern California reveals strong off-great circle arrivals in P coda 5 to 10 s after direct P waves. We also show that the large arrivals observed for the Tonga-Fiji events are P-to-P reflected waves at a dipping interface in the mid-lower mantle beneath central Pacific. The reflector is located south-southeast of Hawaii around 2000 km depth and dips down to southeast by nearly 35°. The observed amplitude and polarity of the reflected waves could be explained if the Vp of the underlying side of the reflector is 1 to 2 % faster than the overlying side. The small Vp anomaly may not necessarily contradict the absence of a noticeable Vp anomaly in previous seismic tomography models at the site of the reflector. We also find that the reflected wave is approximately concomitant with a weaker arrival from a mid-mantle scattering object located nearly 1000 km closer to the hypocenters. The heterogeneous object causing the anomalous arrivals for the Solomon Islands events, although the properties of the object are less well constrained than the Tonga-Fiji reflector, also likely represents another dipping reflector at 2400 km located approximately below the Hawaiian hotspot. As in the Tonga-Fiji case the signals are occasionally followed by a weaker signal from a mid-mantle scattering object located nearer to the hypocenters. The mid-mantle reflection/scattering objects do not indicate the presence of a global discontinuity but must represent localized strong heterogeneities. It is notable that the localized heterogeneities are all located near the edges of a large low Vs body (the Pacific LLSVP) resolved by global tomography, up to 500 km above the LLSVP itself. The relation between the locations of the reflector/scatterers and the large scale Vp structure is unclear, probably reflecting poorer tomography images of Vp structure associated with the LLSVP. We discuss possible tectonic implications of these mid-mantle heterogeneities on the structure and evolution of the LLSVP.</div></div>","PeriodicalId":54614,"journal":{"name":"Physics of the Earth and Planetary Interiors","volume":"368 ","pages":"Article 107437"},"PeriodicalIF":1.9,"publicationDate":"2025-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144997579","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Subsurface oceans and brines beneath the thick icy crust of large icy satellites such as Europa, Ganymede, Callisto, and Titan are among the most promising targets for exploring potential habitability. The physical properties of these liquids, particularly viscosity, play a fundamental role in governing fluid dynamics, as well as material and heat transport occurring within high-pressure environments. Although magnesium sulfate (MgSO4) is likely one of the primary dissolved salts in these extra-terrestrial oceans, its viscosity under high-pressure conditions remains poorly understood. In this study, a falling-sphere viscometer was developed with a diamond anvil cell (DAC) to measure the viscosity of 10 wt% MgSO4 solutions at pressures up to 1100 MPa and temperatures ranging from 263 to 313 K. Our results showed that MgSO4 solutions exhibited viscosities more than 1.5 times as high as that of pure water at the same pressure and temperature conditions. At low temperature, the viscosity of MgSO4 solutions increased monotonically with pressure, whereas pure water exhibited a minimum viscosity at ∼200 MPa. This difference reflects the strong ionic effects on the disruption of water structure and construction of hydration shell by Mg2+ and SO42− ions. By extrapolating our findings to subsurface ocean conditions, we estimated that 10 wt% aqueous MgSO4 oceans/brines in icy satellites would have viscosities between 1 and 13 mPa·s at pressures below 700 MPa. This finding suggests that aqueous MgSO4 fluids potentially present in icy satellites can exhibit higher viscosities compared with pure water, whose viscosities are typically limited to the narrow range of 1–2 mPa·s.
{"title":"Viscosity measurements of aqueous magnesium sulfate solutions under high pressure: Implications for subsurface fluids in large icy satellites","authors":"Shunsuke Nozaki , Seiji Kamada , Shin Ozawa , Akio Suzuki","doi":"10.1016/j.pepi.2025.107450","DOIUrl":"10.1016/j.pepi.2025.107450","url":null,"abstract":"<div><div>Subsurface oceans and brines beneath the thick icy crust of large icy satellites such as Europa, Ganymede, Callisto, and Titan are among the most promising targets for exploring potential habitability. The physical properties of these liquids, particularly viscosity, play a fundamental role in governing fluid dynamics, as well as material and heat transport occurring within high-pressure environments. Although magnesium sulfate (MgSO<sub>4</sub>) is likely one of the primary dissolved salts in these extra-terrestrial oceans, its viscosity under high-pressure conditions remains poorly understood. In this study, a falling-sphere viscometer was developed with a diamond anvil cell (DAC) to measure the viscosity of 10 wt% MgSO<sub>4</sub> solutions at pressures up to 1100 MPa and temperatures ranging from 263 to 313 K. Our results showed that MgSO<sub>4</sub> solutions exhibited viscosities more than 1.5 times as high as that of pure water at the same pressure and temperature conditions. At low temperature, the viscosity of MgSO<sub>4</sub> solutions increased monotonically with pressure, whereas pure water exhibited a minimum viscosity at ∼200 MPa. This difference reflects the strong ionic effects on the disruption of water structure and construction of hydration shell by Mg<sup>2+</sup> and SO<sub>4</sub><sup>2−</sup> ions. By extrapolating our findings to subsurface ocean conditions, we estimated that 10 wt% aqueous MgSO<sub>4</sub> oceans/brines in icy satellites would have viscosities between 1 and 13 mPa·s at pressures below 700 MPa. This finding suggests that aqueous MgSO<sub>4</sub> fluids potentially present in icy satellites can exhibit higher viscosities compared with pure water, whose viscosities are typically limited to the narrow range of 1–2 mPa·s.</div></div>","PeriodicalId":54614,"journal":{"name":"Physics of the Earth and Planetary Interiors","volume":"368 ","pages":"Article 107450"},"PeriodicalIF":1.9,"publicationDate":"2025-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145120913","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-10-01Epub Date: 2025-08-25DOI: 10.1016/j.pepi.2025.107421
Erwan Thébault , Gauthier Hulot
Detailed mapping of Earth’s lithospheric magnetic field provides important insights into the composition, dynamics, and geological history of the crust. This field can be modeled using satellite and near-surface magnetic measurements. However, structures larger than approximately 2500 km in scale are obscured by the dominant magnetic signal generated by the Earth’s core. The superposition of core and crustal magnetic fields introduces ambiguities in both geodynamo and crustal magnetic source studies. Previous efforts to address this issue have included statistical estimates of upper and lower bounds on the long-wavelength components of the crustal field, as well as more deterministic predictions based on geophysical priors such as crustal magnetization and seismic Moho depth models. These approaches, however, have often produced contradictory results. In this study, we adopt a two-step strategy. The first step involves a series of regional spherical spectral analysis of the World Digital Magnetic Anomaly Grid (WDMAM2.2), without relying on any prior information from seismic or magnetization models. This approach, applied to the 5 km × 5 km WDMAM2.2 grid across 6000 regions uniformly distributed over the Earth’s surface, allows us to estimate the probability distributions of three key parameters statistically characterizing crustal magnetization in each of the 6000 regions: magnetization amplitude, magnetic layer thickness, and a power-law exponent. The resulting world map of magnetic layer thickness differs from existing Moho depth models but indicates that, statistically, there is no significant evidence of magnetic sources located below the Moho at the studied length scales. In the second step, the ensemble of regional magnetization models is used to generate a set of large-scale spherical harmonic models of the lithospheric magnetic field (degrees 1 to 50). This set allows us to quantify the extent to which the lithospheric field contaminates both the static and time-varying components of the core magnetic field. We find that this contamination is substantial between spherical harmonic degrees 12 and 15 for the static core field, and from degree 21 onward for the secular variation.
{"title":"Unraveling the lithospheric magnetic field masked by the Earth’s main field by estimating the magnetization and magnetic crustal thickness using a statistical approach","authors":"Erwan Thébault , Gauthier Hulot","doi":"10.1016/j.pepi.2025.107421","DOIUrl":"10.1016/j.pepi.2025.107421","url":null,"abstract":"<div><div>Detailed mapping of Earth’s lithospheric magnetic field provides important insights into the composition, dynamics, and geological history of the crust. This field can be modeled using satellite and near-surface magnetic measurements. However, structures larger than approximately 2500 km in scale are obscured by the dominant magnetic signal generated by the Earth’s core. The superposition of core and crustal magnetic fields introduces ambiguities in both geodynamo and crustal magnetic source studies. Previous efforts to address this issue have included statistical estimates of upper and lower bounds on the long-wavelength components of the crustal field, as well as more deterministic predictions based on geophysical priors such as crustal magnetization and seismic Moho depth models. These approaches, however, have often produced contradictory results. In this study, we adopt a two-step strategy. The first step involves a series of regional spherical spectral analysis of the World Digital Magnetic Anomaly Grid (WDMAM2.2), without relying on any prior information from seismic or magnetization models. This approach, applied to the 5 km × 5 km WDMAM2.2 grid across 6000 regions uniformly distributed over the Earth’s surface, allows us to estimate the probability distributions of three key parameters statistically characterizing crustal magnetization in each of the 6000 regions: magnetization amplitude, magnetic layer thickness, and a power-law exponent. The resulting world map of magnetic layer thickness differs from existing Moho depth models but indicates that, statistically, there is no significant evidence of magnetic sources located below the Moho at the studied length scales. In the second step, the ensemble of regional magnetization models is used to generate a set of large-scale spherical harmonic models of the lithospheric magnetic field (degrees 1 to 50). This set allows us to quantify the extent to which the lithospheric field contaminates both the static and time-varying components of the core magnetic field. We find that this contamination is substantial between spherical harmonic degrees 12 and 15 for the static core field, and from degree 21 onward for the secular variation.</div></div>","PeriodicalId":54614,"journal":{"name":"Physics of the Earth and Planetary Interiors","volume":"367 ","pages":"Article 107421"},"PeriodicalIF":1.9,"publicationDate":"2025-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144896060","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-10-01Epub Date: 2025-08-26DOI: 10.1016/j.pepi.2025.107424
Naomi Shakespeare-Rees , Philip W. Livermore , Christopher J. Davies , Hannah F. Rogers , William J. Brown , Ciarán D. Beggan , Christopher C. Finlay
The Earth’s main geomagnetic field arises from the constant motion of the fluid outer core. By assuming that the field changes are advection-dominated, and that diffusion only plays a minor role, the fluid motion at the core surface can be related to the secular variation of the geomagnetic field, providing an observational approach to understanding the motions in the deep Earth. The majority of existing core flow models are global, showing features such as an eccentric planetary gyre, with some evidence of rapid regional changes. By construction, the flow defined at any location by such a model depends on all magnetic field variations across the entire core–mantle boundary: because of this nonlocal dependence of the flow on the magnetic field, it is very challenging to interpret local structures in the flow as due to specific local changes in magnetic field. Here we present an alternative strategy in which we construct regional flow models that rely only on local secular changes. We use a novel technique based on machine learning termed Physics-Informed Neural Networks (PINNs), in which we seek a regional flow model that simultaneously fits both the local magnetic field variation and dynamical conditions assumed satisfied by the flow. Although we present results using the Tangentially Geostrophic flow constraint, we set out a modelling framework for which the physics constraint can be easily changed by altering a single line of code. After validating the PINN-based method on synthetic flows, we apply our method to the CHAOS-8.1 geomagnetic field model, itself based on data from Swarm. Constructing a global mosaic of regional flows, we reproduce the planetary gyre, providing independent evidence that the strong secular changes at high latitude and in equatorial regions are part of the same global feature. Our models also corroborate regional changes in core flows over the last decade. In our models, we find that the azimuthal flow under South America has changed sign quasi-periodically, with a recent sign change in 2022. Furthermore, our models endorse the existence of a dynamic high latitude jet, which began accelerating around 2005 but has been weakening since 2017.
{"title":"Local flow estimation at the top of the Earth’s core using Physics Informed Neural Networks","authors":"Naomi Shakespeare-Rees , Philip W. Livermore , Christopher J. Davies , Hannah F. Rogers , William J. Brown , Ciarán D. Beggan , Christopher C. Finlay","doi":"10.1016/j.pepi.2025.107424","DOIUrl":"10.1016/j.pepi.2025.107424","url":null,"abstract":"<div><div>The Earth’s main geomagnetic field arises from the constant motion of the fluid outer core. By assuming that the field changes are advection-dominated, and that diffusion only plays a minor role, the fluid motion at the core surface can be related to the secular variation of the geomagnetic field, providing an observational approach to understanding the motions in the deep Earth. The majority of existing core flow models are global, showing features such as an eccentric planetary gyre, with some evidence of rapid regional changes. By construction, the flow defined at any location by such a model depends on all magnetic field variations across the entire core–mantle boundary: because of this nonlocal dependence of the flow on the magnetic field, it is very challenging to interpret local structures in the flow as due to specific local changes in magnetic field. Here we present an alternative strategy in which we construct regional flow models that rely only on local secular changes. We use a novel technique based on machine learning termed Physics-Informed Neural Networks (PINNs), in which we seek a regional flow model that simultaneously fits both the local magnetic field variation and dynamical conditions assumed satisfied by the flow. Although we present results using the Tangentially Geostrophic flow constraint, we set out a modelling framework for which the physics constraint can be easily changed by altering a single line of code. After validating the PINN-based method on synthetic flows, we apply our method to the CHAOS-8.1 geomagnetic field model, itself based on data from Swarm. Constructing a global mosaic of regional flows, we reproduce the planetary gyre, providing independent evidence that the strong secular changes at high latitude and in equatorial regions are part of the same global feature. Our models also corroborate regional changes in core flows over the last decade. In our models, we find that the azimuthal flow under South America has changed sign quasi-periodically, with a recent sign change in 2022. Furthermore, our models endorse the existence of a dynamic high latitude jet, which began accelerating around 2005 but has been weakening since 2017.</div></div>","PeriodicalId":54614,"journal":{"name":"Physics of the Earth and Planetary Interiors","volume":"367 ","pages":"Article 107424"},"PeriodicalIF":1.9,"publicationDate":"2025-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144932544","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-10-01Epub Date: 2025-06-18DOI: 10.1016/j.pepi.2025.107403
David Bercovici, Jennifer Girard
The emergence of plate tectonics on the early Earth likely first requires subduction to initiate motion and to harness the mantle's convective gravitational energy as a power source. Whether such proto-subduction initiated as lithospheric drips (Rayleigh-Taylor like instabilities), or was triggered by mantle plumes (or even bolide impacts) remains a mystery. To infer whether drip instabilities and intermittent downwellings are possible causes of subduction initiation, we have developed a relatively simple two-and-a-half-dimensional (2.5-D) model of lithospheric instabilities. We specifically use this model to examine whether such instabilities, coupled to shear-localizing mechanisms like two-phase grain damage, can lead to subduction-like features, as well as semi-permanent weak zones that can be reactivated later to make new plate boundaries. Our model couples the physics of drip instabilities of amplitude in a horizontal 2-D layer with 2-D viscous lithospheric flexure (bending and folding) of amplitude . The flexure model is generalized from the Biot's classical 1-D thin-plate theory by accounting for all bending and twisting torques, as well as complex rheology. The drip and flexure models are coupled in that the drips act as a load on the bending lithosphere, while vertical flexure affects the heat transport and pressure gradients governing drip growth. The coupled model predicts least stable mode selection of drip and flexure instabilities, in some cases bimodal instabilities wherein one mode is oscillatory, thus predicting intermittent downwellings. With two-phase grain damage, drips localize into narrow features, often organizing into strings of drips, which induce lineated or arcuate weak zones suggestive of dormant and inheritable plate boundaries.
{"title":"A two-and-a-half-dimensional flexing-drip model of lithospheric instabilities and proto-subduction (with two-phase grain-damage)","authors":"David Bercovici, Jennifer Girard","doi":"10.1016/j.pepi.2025.107403","DOIUrl":"10.1016/j.pepi.2025.107403","url":null,"abstract":"<div><div>The emergence of plate tectonics on the early Earth likely first requires subduction to initiate motion and to harness the mantle's convective gravitational energy as a power source. Whether such proto-subduction initiated as lithospheric drips (Rayleigh-Taylor like instabilities), or was triggered by mantle plumes (or even bolide impacts) remains a mystery. To infer whether drip instabilities and intermittent downwellings are possible causes of subduction initiation, we have developed a relatively simple two-and-a-half-dimensional (2.5-D) model of lithospheric instabilities. We specifically use this model to examine whether such instabilities, coupled to shear-localizing mechanisms like two-phase grain damage, can lead to subduction-like features, as well as semi-permanent weak zones that can be reactivated later to make new plate boundaries. Our model couples the physics of drip instabilities of amplitude <span><math><mi>h</mi></math></span> in a horizontal 2-D layer with 2-D viscous lithospheric flexure (bending and folding) of amplitude <span><math><mi>w</mi></math></span>. The flexure model is generalized from the Biot's classical 1-D thin-plate theory by accounting for all bending and twisting torques, as well as complex rheology. The drip and flexure models are coupled in that the drips act as a load on the bending lithosphere, while vertical flexure affects the heat transport and pressure gradients governing drip growth. The coupled model predicts least stable mode selection of drip and flexure instabilities, in some cases bimodal instabilities wherein one mode is oscillatory, thus predicting intermittent downwellings. With two-phase grain damage, drips localize into narrow features, often organizing into strings of drips, which induce lineated or arcuate weak zones suggestive of dormant and inheritable plate boundaries.</div></div>","PeriodicalId":54614,"journal":{"name":"Physics of the Earth and Planetary Interiors","volume":"367 ","pages":"Article 107403"},"PeriodicalIF":1.9,"publicationDate":"2025-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144772112","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2025-10-01Epub Date: 2025-08-21DOI: 10.1016/j.pepi.2025.107436
Tengfei Wu , Shuangxi Zhang , Meng Chen
The collision and convergence between the Yangtze Craton (YZC) and North China Craton (NCC) beneath the Qinling orogenic belt (QOB) have resulted in complex lithospheric deformation, the mechanisms of which remain unclear. In this study, we extracted 10–60 s Rayleigh-wave dispersion curves using the two-station method from vertical-component waveform data of 1087 teleseismic events, recorded at 110 seismic stations across the QOB and adjacent regions. Subsequently, anisotropic tomography was employed to reconstruct high-resolution isotropic and anisotropic phase velocity models of the crust and upper mantle beneath the QOB and surrounding regions. We focused on analyzing deformation patterns in four key subregions of the QOB. Our results demonstrated that crustal deformation is affected by multiple geological factors. Major tectonic activities, such as island arc collisions, oceanic basin closure, and orogenic events, have fundamentally shaped the regional structural framework. Building on this, crustal lithological features, thrust tectonic movements, and the strike of fault systems, which together control present-day deformation. Furthermore, our anisotropic model, in combination with previous geodetic and seismological observations, suggests that the clockwise rotation of the YZC during its convergence with the NCC plays a significant role in influencing crustal deformation. Upper mantle deformation is primarily driven by absolute plate motion, with additional influences from the northeastward escape of material in the Tibetan Plateau and mantle flow. Notably, our anisotropic model provides new seismological evidence supporting the clockwise rotation of the YZC crust, which is closely related to the tectonic development of the Sichuan basin and the formation of the Dabashan arcuate structure. Integrating with previous studies, we propose a conceptual model to explain the formation mechanism of the Dabashan arcuate structure, which we attribute to the combined effects of the clockwise rotation of the YZC crust during the Middle to Late Triassic and the ongoing convergence between the YZC and the NCC. These findings provide new insights into the lithospheric dynamic processes of the QOB.
{"title":"Complex deformation mechanisms of the Qinling orogenic belt lithosphere and clockwise rotation of the Yangtze craton crust: Insights from Rayleigh wave azimuthally anisotropic tomography","authors":"Tengfei Wu , Shuangxi Zhang , Meng Chen","doi":"10.1016/j.pepi.2025.107436","DOIUrl":"10.1016/j.pepi.2025.107436","url":null,"abstract":"<div><div>The collision and convergence between the Yangtze Craton (YZC) and North China Craton (NCC) beneath the Qinling orogenic belt (QOB) have resulted in complex lithospheric deformation, the mechanisms of which remain unclear. In this study, we extracted 10–60 s Rayleigh-wave dispersion curves using the two-station method from vertical-component waveform data of 1087 teleseismic events, recorded at 110 seismic stations across the QOB and adjacent regions. Subsequently, anisotropic tomography was employed to reconstruct high-resolution isotropic and anisotropic phase velocity models of the crust and upper mantle beneath the QOB and surrounding regions. We focused on analyzing deformation patterns in four key subregions of the QOB. Our results demonstrated that crustal deformation is affected by multiple geological factors. Major tectonic activities, such as island arc collisions, oceanic basin closure, and orogenic events, have fundamentally shaped the regional structural framework. Building on this, crustal lithological features, thrust tectonic movements, and the strike of fault systems, which together control present-day deformation. Furthermore, our anisotropic model, in combination with previous geodetic and seismological observations, suggests that the clockwise rotation of the YZC during its convergence with the NCC plays a significant role in influencing crustal deformation. Upper mantle deformation is primarily driven by absolute plate motion, with additional influences from the northeastward escape of material in the Tibetan Plateau and mantle flow. Notably, our anisotropic model provides new seismological evidence supporting the clockwise rotation of the YZC crust, which is closely related to the tectonic development of the Sichuan basin and the formation of the Dabashan arcuate structure. Integrating with previous studies, we propose a conceptual model to explain the formation mechanism of the Dabashan arcuate structure, which we attribute to the combined effects of the clockwise rotation of the YZC crust during the Middle to Late Triassic and the ongoing convergence between the YZC and the NCC. These findings provide new insights into the lithospheric dynamic processes of the QOB.</div></div>","PeriodicalId":54614,"journal":{"name":"Physics of the Earth and Planetary Interiors","volume":"367 ","pages":"Article 107436"},"PeriodicalIF":1.9,"publicationDate":"2025-10-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"144903878","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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