JGR–Solid Earth

No abstract is available for this article.

The continental mid-lithosphere discontinuity (MLD) is widely detected within cratons, with the dominant depth range of 70–100 km and a significant reduction of shear-wave velocity of 2%–12%. However, the formation mechanism and corresponding strength of the MLD are widely debated, which may strongly affect the roles of MLD in craton evolution. The comparisons among variable mechanisms indicate that the strength of the MLD varies from the relatively high viscosity of wet olivine to the rather low viscosity of antigorite. Thus, systematic numerical modeling has been conducted with the MLD of contrasting strengths, that is, the wet olivine-induced MLD or antigorite-induced MLD, to investigate the roles of MLD in the craton instability under variable tectonic regimes (stable, extension, compression, mantle flow traction, or mantle plume). The models show that the cratonic lithosphere with wet olivine-induced MLD maintains its stability under all the tectonic regimes. In contrast, the antigorite-induced MLD with lowest viscosity could significantly promote the decoupling of lithosphere, and facilitate the lithospheric deformation. However, lithospheric delamination only occurs with the rather weak MLD interacting with the sub-plate asthenosphere upwelling during craton extension or mantle plume activity. The sufficient amount of melts is essential for this process, which requires a large amount of extension or a mantle plume with rather high temperature anomaly and large size. Therefore, craton destruction is still difficult, and requires additional strict conditions. This may explain the general stability of most cratons with widespread MLDs.

To enhance our comprehension of the dynamic processes associated with complex plate subduction, volcanic magmatism, and lithosphere deformation beneath the Ryukyu–Taiwan–Philippines region, we utilized nonuniform inversion grids for Pn velocity and anisotropy tomography, and obtained the uppermost mantle structure of this area. The results demonstrate remarkable characteristics: cold oceanic subducting plates display high Pn velocities, whereas volcanic arcs, the extinct mid-ocean ridge in the South China Sea, and the Palawan region exhibit low Pn velocities. The extinct mid-ocean ridge also displays a low-velocity anomaly, indicating that residual heat persists even after approximately 15 million years since seafloor spreading. The occurrence of a discontinuous low-velocity beneath the Ryukyu arc supports the presence of a slab window at approximately 123°E. The volcanic arcs of all subduction zones within the study area displayed trench-parallel Pn anisotropy. The observed Pn fast directions beneath the Taiwan orogenic belt are consistent with crustal anisotropy, providing evidence for the crust–mantle coupled deformation. Moreover, our results shed light on the deep structural characteristics of the complex subduction zones beneath the Philippines region. Plate subduction causes partial melting due to dehydration; then, the melt ascends and accumulates at the uppermost mantle, showing low Pn velocities. However, the low-velocity anomaly is not widely distributed but corresponds to a narrow band. In particular, the east–west bidirectional subduction of the Philippines and Negros formed separate low-velocity anomalies. Low Pn velocities and trench-parallel anisotropy indicate the location of different subduction zones and finely characterize their impact on the uppermost mantle structure.

The Ries impact structure (Germany) contains well-preserved ejecta deposits consisting of melt-free lithic breccia (Bunte Breccia) overlain by suevite. To test their emplacement conditions, we investigated the magnetic properties and microstructures of 26 polymict breccia clasts and a stratigraphic profile from the clasts into the suevite at the Aumühle quarry. Remanent magnetization directions of the Bunte Breccia clasts fall into two groups: those whose directions mostly lie parallel to the reversed field during impact carried mostly by magnetite, and those whose directions vary widely among each clast carried by titanohematite. Basement clasts containing titanohematite acquired a chemical remanent magnetization (CRM) during the ejection process and then rotated during turbulent deposition. Clasts of sedimentary rocks grew magnetite after turbulent deposition, with CRM directions lying parallel to the paleofield. Suevite holds a thermal remanent magnetization carried by magnetite, except for ∼12 cm from the contact with the Bunte Breccia, where hematite concentrations increase due to hydrothermal alteration. These observations lead us to propose a three-stage model of (a) turbulent deposition of the melt-free breccia with clast rotation <580°C, (b) deposition of the overlying suevite, which acted as a semi-permeable barrier that confined hot (<300°C) oxidizing fluids to the permeable breccia zone, and (c) prolonged hydrothermal activity producing further alteration which ended before the next geomagnetic reversal. Basement outcrops have significantly different magnetic properties than the Bunte Breccia basement clasts with similar lithology. Two basement blocks situated near the inner ring may have been thermally overprinted up to 550°C.

Damaging aftershock sequences often exhibit considerable spatio-temporal complexity. The stress changes associated with coseismic slip and aseismic afterslip are commonly proposed to drive aftershock sequences, but few systematic studies exist and do not always support strong, universal driving relationships. To investigate the roles that these two sources of stress changes may play in driving aftershocks, we assess the spatio-temporal relationships between coseismic slip, afterslip, and on-fault (within 5 km) aftershock density following seven M w 6.0–7.6 continental-settings earthquakes, using available high-quality slip models and regional seismic data. From previous empirical work and frictional considerations, near the mainshock we expect coseismic slip and afterslip to be anti-correlated, and aftershocks to occur where coseismic slip is low/zero, near high slip gradients, and/or to migrate with afterslip. However, we find that spatial relationships between afterslip and coseismic slip, and between afterslip and aftershock density differ between earthquakes. Aftershock density correlates with coseismic slip following five of the earthquakes, and with total cumulative slip (coseismic slip + afterslip) following six: indicating that on-fault aftershock distributions may be approximated by total slip (at current resolutions). Additionally, we find that the gradients of coseismic slip and afterslip (proxies for new stress concentrations) do not clearly correlate with aftershock distributions and that the choice of spatial domain over which relationships are tested can affect results significantly. A possible explanation of these results is that fault zones contain considerable fine-scale structural and frictional heterogeneity. Nonetheless, the empirical evidence for frequently assumed relationships between coseismic slip, afterslip and on-fault aftershocks is mixed.

Distributed acoustic sensing (DAS) using existing optical fiber cables facilitates high-density seismic observation. However, few studies have examined the reliability of the seismic waveform amplitude recorded by DAS. In this study, a DAS network was connected to optical fiber cables installed over a distance of 75 km along a high-speed train (Shinkansen) railway in the Kumamoto prefecture, Japan. We successfully observed strong motions of the Mj6.6 earthquake (approximately 150 km from the fiber) on 22 January 2022, in Hyuga-nada, in addition to several small local earthquakes. The observed strong motions from the Mj6.6 earthquake, using DAS, exhibited cycle skipping (clipping) issues due to dynamic range limitations at numerous channels. To address this, we estimated the shaking map, representing maximum strain distributions for Mj6.6, by replacing the clipped data with information from nearby unclipped channels and scaling their RMS amplitudes based on S-coda (unclipped). Furthermore, we verified the reliability of the amplitude information obtained from DAS by estimating the distance attenuation of seismic waves while correcting for the differences in the structure type and coupling as much as possible. The distance attenuation property of local earthquakes was consistent with that of the peak ground velocities obtained from seismometers, indicating that DAS data acquired using fibers installed on infrastructure (various structures) can also be utilized to assess the spatial distribution of the relative amplitude values along the fiber. Obtaining high-density seismic motion distributions is important for earthquake early warning and accurate damage estimation of strong motions.

Processes of magma generation and transportation in global mid-ocean ridges are key to understanding lithospheric architecture at divergent plate boundaries. These magma dynamics are dependent on spreading rate and melt flux, where the SW Indian Ridge represents an end-member. The vertical extent of ridge magmatic systems and the depth of axial magma chambers (AMCs) are greatly debated, in particular at ultraslow-spreading ridges. Here we present detailed mineralogical studies of high-Mg and low-Mg basalts from a single dredge on Southwest Indian Ridge (SWIR) at 45°E. High-Mg basalts (MgO = ∼7.1 wt.%) contain high Mg# olivine (Ol, Fo = 85–89) and high-An plagioclase (Pl, An = 66–83) as phenocrysts, whereas low-Mg basalts contain low-Mg# Ol and low-An Pl (Fo = 75–78, An = 50–62) as phenocrysts or glomerocrysts. One low-Mg basalt also contains normally zoned Ol and Pl, the core and rim of which are compositionally similar to those in high-Mg and low-Mg basalts, respectively. Mineral barometers and MELTS simulation indicate that the high-Mg melts started to crystallize at ∼32 ± 7.8 km, close to the base of the lithosphere. The low-Mg melts may have evolved from the high-Mg melts in an AMC at a depth of ∼13 ± 7.8 km. Such great depths of magma crystallization and the AMC are likely the result of enhanced conductive cooling at ultraslow-spreading ridges. Combined with diffusion chronometers, the basaltic melts could have ascended from the AMC to seafloor within 2 weeks to 3 months at average rates of ∼0.002–0.01 m/s, which are the slowest reported to date among global ridge systems and may characterize mantle melt transport at the slow end of the ridge spreading spectrum.

Distributed Acoustic Sensing (DAS) is becoming a powerful tool for earthquake monitoring, providing continuous strain-rate records of seismic events along fiber optic cables. However, the use of standard seismological techniques for earthquake source characterization requires the conversion of data in ground motion quantities. In this study we provide a new formulation for far-field strain radiation emitted by a seismic rupture, which allows to directly analyze DAS data in their native physical quantity. This formulation naturally accounts for the complex directional sensitivity of the fiber to body waves and to the shallow layering beneath the cable. In this domain, we show that the spectral amplitude of the strain integral is related to the Fourier transform of the source time function, and its modeling allows to determine the source parameters. We demonstrate the validity of the technique on two case-studies, where source parameters are consistent with estimates from standard seismic instruments in magnitude range 2.0–4.3. When analyzing events from a 1-month DAS survey in Chile, moment-corner frequency distribution shows scale invariant stress drop estimates, with an average of Δσ = (0.8 ± 0.6) MPa. Analysis of DAS data acquired in the Southern Apennines shows a dominance of the local attenuation that masks the effective corner frequency of the events. After estimating the local attenuation coefficient, we were able to retrieve the corner frequencies for the largest magnitude events in the catalog. Overall, this approach shows the capability of DAS technology to depict the characteristic scales of seismic sources and the released moment.

Geoscientists use observed data to estimate properties of the Earth's interior. This often requires non-linear inverse problems to be solved and uncertainties to be estimated. Bayesian inference solves inverse problems under a probabilistic framework, in which uncertainty is represented by a so-called posterior probability distribution. Recently, variational inference has emerged as an efficient method to estimate Bayesian solutions. By seeking the closest approximation to the posterior distribution within any chosen family of distributions, variational inference yields a fully probabilistic solution. It is important to define expressive variational families so that the posterior distribution can be represented accurately. We introduce boosting variational inference (BVI) as a computationally efficient means to construct a flexible approximating family comprising all possible finite mixtures of simpler component distributions. We use Gaussian mixture components due to their fully parametric nature and the ease with which they can be optimized. We apply BVI to seismic travel time tomography and full waveform inversion, comparing its performance with other methods of solution. The results demonstrate that BVI achieves reasonable efficiency and accuracy while enabling the construction of a fully analytic expression for the posterior distribution. Samples that represent major components of uncertainty in the solution can be obtained analytically from each mixture component. We demonstrate that these samples can be used to solve an interrogation problem: to assess the size of a subsurface target structure. To the best of our knowledge, this is the first method in geophysics that provides both analytic and reasonably accurate probabilistic solutions to fully non-linear, high-dimensional Bayesian full waveform inversion problems.

Repeating earthquakes repeatedly rupture the same fault asperities, which are likely loaded to failure by surrounding aseismic slip. However, repeaters occur less often than would be expected if these earthquakes accommodate all of the long-term slip on the asperities. Here, we assess a possible explanation for this slip discrepancy: partial ruptures. On asperities that are much larger than the nucleation radius, a fraction of the slip could be accommodated by smaller ruptures on the same asperities. We search for partial ruptures of repeating earthquakes in Parkfield using the Northern California earthquakes catalog. We find 3991 individual repeaters which have 4468 partial ruptures. The presence of partial ruptures suggests that the asperities of repeating earthquakes are much larger than the nucleation radius. However, we find that partial ruptures could accommodate only around 25% of the slip on repeating earthquake patches. A 25% increase in the slip budget can explain only a small portion of the long recurrence intervals of repeating earthquakes.

We use sparse dictionary learning to develop transformations between seismic velocity models of different resolution and spatial extent. Starting with data in the common region of both models, the method can enhance a regional lower-resolution model to match the style and resolution of local higher-resolution results while preserving its regional coverage. The method is demonstrated by applying it to two-dimensional V S and three-dimensional V P and V S regional and local velocity models in southern California. The enhanced reconstructed regional results exhibit clear visual improvements, especially in the reconstructed V P /V S ratios, and better correlations with geological features. Moreover, the reconstructed regional V P , V S models outperform the original ones in comparison of simulated earthquake waveforms to observations. The improved fitting to observed waveforms extends beyond the domain of the overlapping region. The developed dictionary learning approach provides physically interpretable results and offers a powerful tool for additional applications of data enhancement in earth sciences.

Foreshocks are known to occur before certain large earthquakes, but the physical mechanism is still in debate. Recent field and laboratory studies have supported either cascade-triggering, pre-slip or a combination of both models. Here we use a dense seismic and geodetic network deployed in 2018 in Southwest China to quantify the long-term background seismicity, short-term spatio-temporal evolutions of foreshocks and transient deformation before the 2021 MW 6.1 Yangbi earthquake. We find multiple episodes of migrating foreshocks and repeating earthquakes in the last 4 days of the Yangbi mainshock. A rapid migration of microseismicity started in the last 30 minutes toward the eventual initiation point of the Yangbi mainshock, well beyond the rupture zone of the largest MW 5.2 foreshock at that time. The Coulomb stress changes due to relatively large-size foreshocks at the mainshock hypocenter are positive but relatively small (0.005–0.05 MPa). We also observe continuous geodetic signals with ∼20 mm of cumulative displacement at a nearby GPS station coinciding with the last hour of intense foreshocks. The inverted aseismic moment is equivalent to a MW 5.56 event with the maximum slip of ∼200 mm at 6 km depth, larger than the cumulative seismic slip of ∼60–120 mm inverted from waveforms of several large M4+ foreshocks. In addition, the predicted surface displacements based on the coseismic slips of the M4+ foreshocks are unobservable in the GPS data. We propose a migratory slow-slip model where a transient slow-slip event drives the last hour of foreshock sequence and eventually trigger the Yangbi mainshock.

(Mg, Fe, Al)(Si, Al)O3 bridgmanite is the most abundant mineral of Earth′s lower mantle. Al is incorporated in the crystal structure of bridgmanite through the Fe3+AlO3 and AlAlO3 charge coupled (CC) mechanisms, and the MgAlO2.5 oxygen vacancy (OV) mechanism. Oxygen vacancies are believed to cause a substantial decrease of the bulk modulus of aluminous bridgmanite based on first-principles calculations on the MgAlO2.5 end-member. However, there is no conclusive experimental evidence supporting this hypothesis due to the uncertainties on the chemical composition, crystal chemistry, and/or high-pressure behavior of samples analyzed in previous studies. Here, we synthesized high-quality single crystals of bridgmanite in the MgO–AlO1.5–SiO2 system with different bulk Al contents and degrees of CC and OV substitutions. Suitable crystals with different compositions were loaded in resistively heated diamond anvil cells and analyzed by synchrotron X-ray diffraction at pressures up to approximately 80 GPa at room temperature and 35 GPa at temperatures up to 1,000 K. Single-crystal structural refinements at high pressure show that the compressibility of bridgmanite is mainly controlled by Al–Si substitution in the octahedral site and that oxygen vacancies in bridgmanite have no detectable effect on the bulk modulus in the compositional range investigated here, which is that relevant to a pyrolytic lower mantle. The proportion of oxygen vacancies in Al-bearing bridgmanite has been calculated using a thermodynamic model constrained using experimental data at 27 GPa and 2,000 K for an Fe-free system and extrapolated to pressures equivalent to 1,250 km depth using the thermoelastic parameters of Al-bearing bridgmanite determined in this study.

Earthquake swarms represent a particular mode of seismicity, not directly related to the occurrence of large earthquakes (e.g., aftershocks) but rather driven by external forcing such as aseismic deformation or fluid migration in fault systems. Sometimes their occurrence overlaps with observable geodetic signals in space and time, indicating a direct link. However, the low resolution of geodetic observations tends to obscure the small scale spatial and temporal dynamics of swarms. In this work, we automatically extract clusters of seismicity related to the 2014 Alto Tiberina swarm sequence (Italy) using an unsupervised clustering approach that exploits space and time information of the seismicity. The quantitative characterization of each cluster indicates that the overall swarm is composed of spatially and temporally confined (sub) swarms each of which could potentially be driven by small-scale aseismic deformation process. This observation aligns with similar findings during slow slip events in subduction zones.

In this study we recast surface wave traveltime tomography as an inverse problem constrained by an eikonal equation and solve it using the efficient adjoint-state method. Specifically, recognizing that large topographic variations and high surface wave frequencies can make the topographic effect too significant to ignore, we employ an elliptically anisotropic eikonal equation to describe the traveltime fields of surface waves on undulated topography. The sensitivity kernel of the traveltime objective function with respect to shear wave velocity is derived using the adjoint-state method. As a result, the newly developed method is inherently applicable to any study regions, whether with or without significant topographic variations. Hawaii is one of the most seismically and magmatically active regions. However, its significant topographic variations have made it less accurate to investigate using conventional surface wave traveltime tomography methods. To tackle this problem, we applied our new method to invert ambient noise Rayleigh wave phase traveltimes and construct a 3D shear wave velocity model. Our results reveal features that are consistent with geological structures and previous tomography results, including high velocities below Mauna Loa Volcano and Kilauea Volcano, and low velocities beneath the Hilina Fault Zone. Additionally, our model reveals a high-velocity anomaly to the South of Hualalai's summit, which may be related to a buried rift zone. Our findings further demonstrate that including topography can lead to a correction of up to 0.8% in the shear wave velocity model of Hawaii, an island spanning approximately 100 km with volcanoes reaching elevations exceeding 4 km.

Postseismic deformation at convergent margins is controlled mainly by continuous slip on the fault (afterslip) and relaxation of the earthquake-induced stress in the viscoelastic upper mantle (viscoelastic relaxation). Study of these deformation processes provides insight into the rheological properties of upper mantle and slip behavior of the fault. We have constructed a three-dimensional finite element model to investigate the postseismic deformation of the 2018 Mw 7.9 Kodiak earthquake. We derived the first 2-year postseismic Global Positioning System observations to constrain afterslip and upper mantle rheology in the south-central Alaska. The upper mantle is separated into the mantle wedge and oceanic upper mantle topped by an 80-km thick asthenosphere layer by the subducting slab. Results show that afterslip generally occurred in areas adjacent to the rupture zone and has a small magnitude of a few tens of millimeters. The viscosities of the asthenosphere and mantle wedge are determined to be in a range of 1–4 × 1018 and 0.5–5 × 1019 Pa s with an optimal value of 2 × 1018 and 2 × 1019 Pa s, respectively. Model results reveal a localized weak mantle wedge of ∼1018 Pa s beneath Lower Cook Inlet that may be due to the fluids dehydrated from the slab. Coulomb stress changes show that the earthquake enhanced coseismic and postseismic stress loading of up to 0.9 and 0.1 bar, respectively, on the shallow subduction interface near Kodiak Island, but there is no obvious triggered seismicity, probably due to the low stress status already released by the 1964 Mw 9.2 Alaska earthquake.

Accurate assessment of the rate and state friction parameters of rocks is essential for producing realistic earthquake rupture scenarios and, in turn, for seismic hazard analysis. Those parameters can be directly measured on samples, or indirectly based on inversion of coseismic or postseismic slip evolution. However, both direct and indirect approaches require assumptions that might bias the results. Aiming to reduce the potential sources of bias, we take advantage of a downscaled analog model reproducing megathrust earthquakes. We couple the simulated annealing algorithm with quasi-dynamic numerical models to retrieve rate and state parameters reproducing the recurrence time, rupture duration and slip of the analog model, in the ensemble. Then, we focus on how the asperity size and the neighboring segments' properties control the seismic cycle characteristics and the corresponding variability of rate and state parameters. We identify a tradeoff between (a–b) of the asperity and (a–b) of neighboring creeping segments, with multiple parameter combinations that allow mimicking the analog model behavior. Tuning of rate and state parameters is required to fit laboratory experiments with different asperity lengths. Poorly constrained frictional properties of neighboring segments are responsible for uncertainties of (a–b) of the asperity in the order of per mille. Roughly one order of magnitude larger uncertainties derive from asperity size. Those results provide a glimpse of the variability that rate and state friction estimates might have when used as a constraint to model fault slip behavior in nature.

Seismic anisotropy provides essential information for characterizing the orientation of deformation and flow in the crust and mantle. The isotropic structure of the Antarctic crust and upper mantle has been determined by previous studies, but the azimuthal anisotropy structure has only been constrained by mantle core phase (SKS) splitting observations. This study determines the azimuthal anisotropic structure of the crust and mantle beneath the central and West Antarctica based on 8—55 s Rayleigh wave phase velocities from ambient noise cross-correlation. An anisotropic Rayleigh wave phase velocity map was created using a ray—based tomography method. These data are inverted using a Bayesian Monte Carlo method to obtain an azimuthal anisotropy model with uncertainties. The azimuthal anisotropy structure in most of the study region can be fit by a two-layer structure, with one layer at depths of 0–15 km in the shallow crust and the other layer in the uppermost mantle. The azimuthal anisotropic layer in the shallow crust of West Antarctica, where it coincides with strong positive radial anisotropy quantified by the previous study, shows a fast direction that is subparallel to the inferred extension direction of the West Antarctic Rift System. Fast directions of upper mantle azimuthal anisotropy generally align with teleseismic shear wave splitting fast directions, suggesting a thin lithosphere or similar lithosphere-asthenosphere deformation. However, inconsistencies in this exist in Marie Byrd Land, indicating differing ancient deformation patterns in the shallow mantle lithosphere sampled by the surface waves and deformation in the deeper mantle and asthenosphere sampled more strongly by splitting measurements.