Physics of the Earth and Planetary Interiors最新文献

The dynamic processes associated with subducting tectonic plates and rising plumes of hot material are typically treated separately in dynamical models and seismological studies. However, various types of observations and related models indicate these processes overlap spatially. Here we use precursors to PP and SS reflecting off mantle transition zone discontinuities to map deflections of these discontinuities near three subduction zones surrounding the Caribbean Plate: 1) Lesser Antilles, 2) Middle America and 3) northern South American subduction zones. In all three regions slow seismic anomalies are present behind the sinking slab within the transition zone in tomographic images. Using array methods, we identify precursors and verify their in-plane propagation for M_W ≥ 5.8 events occurring between the years 2000 and 2020 by generating a large number of source receiver combinations with reflection points in the area, including crossing ray paths. The measured time lag between PP/SS arrivals and their corresponding precursors on robust stacks are used to measure the depth of the mantle transition zone discontinuities. In all three areas we find evidence for upward deflection of the 660 discontinuity behind the sinking slab, consistent with the presence of hot plume material (average temperature anomalies of 180 to 620 K), while there is not a corresponding downward deflection of the 410 km discontinuity. One interpretation of these disparate observations is suggested based on comparison to existing models of mantle convection and subduction: plume material rising across 660 km discontinuity could be entrained by lateral flow in the transition zone induced by the nearby sinking slab, and thus delaying the rise of hot material across the 410 km discontinuity.

We have investigated the spatial variation of body wave attenuation in Garhwal-Kumaun Himalaya region from the datasets of 465 well located earthquakes, recorded at 52 broadband stations between 2017 and 2020. The body wave attenuation parameters $Q_P$ and $Q_S$ were estimated at each station by applying the extended coda normalization method for five different central frequencies ranging from 1.5 to 18 Hz. Strong frequency-dependent body wave attenuation was observed at each station over the whole region. Also, we found a significant variation of $Q_P$ and $Q_S$ from north to south which is consistent with geotectonic diversity beneath the study region. We have also made separate estimations of frequency-dependent relations for Garhwal and Kumaun Himalaya region in order to investigate the lateral variation in attenuation characteristics, and obtained the frequency-dependent relations as follows: $Q_P=\left(30\pm 3\right)f^{\left(1.05\pm 0.05\right)}$, $Q_S=\left(143\pm 20\right)f^{\left(0.88\pm 0.07\right)}$ for the Garhwal Himalaya region and $Q_P=\left(31\pm 1\right)f^{\left(1.09\pm 0.02\right)}$, $Q_S=\left(121\pm 11\right)f^{\left(1.00\pm 0.04\right)}$ for the Kuamun Himalaya region. Obtained $Q$ values indicate strong body wave attenuation for both the region with no significant lateral variation. It may suggest the presence of crustal level folding, faulting, aqueous fluids, and metamorphic fluids below both the regions. Also, the ratios of $Q_S$/$Q_P$ are high ($>1$) for the entire analyzed frequency range, suggesting a significant level of heterogeneity and tectonic complexities in the crust of the study region.

A series of midcrustal moderate-size earthquakes occurred in the Korean Peninsula recently. A midcrustal $M_{L}4.9$ strike-slip earthquake with a fault-plane strike in N-S occurred on December 14, 2021 at the southwestern offshore region of Jeju Island, South Korea. The fault plane orientation and slip sense (faulting mechanism) hardly conform with the regional stress field. The deep focal depth and N-S directional strike-slip motion require transient changes in the medium properties and stress field. Strong ground motions of the midcrustal earthquake induce preferential dynamic stress changes in NE-SW direction, triggering subsequent aftershocks in NE-SW-directional adjacent faults. Both the static and dynamic stress changes caused by the mainshock contribute to the aftershock sequence. The number and focal depths of aftershocks decrease with distance from the mainshock. The different fault-plane orientations between the mainshock and aftershocks suggest earthquake nucleations in independent fault structures. The mainshock occurred in aseismic midcrustal paleovolcanic structure on the outskirt of a high seismicity region. The $M_{L}4.9$ earthquake suggests possible nucleation of earthquake in seismically-inactive paleotectonic structures, successively incurring aftershocks conforming to the ambient stress. The mainshock and aftershocks suggest that paleotectonic structures may behave as source structures to spawn earthquakes.

Estimating locking degree at faults is important for determining the spatial distribution of slip deficit at seismic gaps. Inverse methods of varying complexity are commonly used to estimate fault locking. Here we present an innovative approach to infer the degree of locking from surface GNSS velocities by means of supervised learning (SL) algorithms. We implemented six different SL regression methods and apply them in the Central Chile subduction. These methods were first trained on synthetic distributions of locking and then used to infer the locking from GNSS observations. We tested the performance of each algorithm and compared our results with a least squares inversion method. Our best results were obtained using the Ridge regression, which gives a root mean square error (RMSE) of 1.94 mm/yr compared to GNSS observations. The ML-based locking degree distribution is consistent with results from the EPIC Tikhonov regularized least squares inversion and previously published locking maps. Our study demonstrates the effectiveness of machine learning methods in estimating fault locking and slip, and provides flexible options for incorporating prior information to avoid slip instabilities based on the characteristics of the training set. Exploring uncertainties in the physical model during training could improve the robustness of locking estimates in future research efforts.

Starting from 2009, substantial improvements in the regional seismic network operating in Sulawesi, as part of the Meteorological, Climatological, and Geophysical Agency (BMKG) station deployment, allowed us to determine earthquake mechanism for 140 shallow (5–60 km) light to moderate size (3.8 ≤ M_w ≤ 6.1) earthquakes between 2009 and 2018 along the Palu-Koro and Matano faults, through moment tensor inversion. Ten (10) out of the 140 solutions obtained in this study were found in the International Seismological Centre Bulletin and used as validation. The double-couple mechanism of the best solutions (variance reduction ≥0.4 and condition number ≤ 7.5) is further used to analyze the Palu-Koro and Matano segments. We observed that the Donggala segment of the Palu-Koro fault shows variable earthquake mechanism types. Whereas the two subsequent segments, the Palu and Saluki segments, show predominate oblique strike-slip with normal and reverse mechanisms. The Matano fault is characterized by strike-slip mechanisms, with normal mechanisms also observed. The normal mechanisms depict the pull-apart system in the Matano segment, manifested as Matano Lake. The results of this work improve our understanding of regional tectonics.

The long-term evolution of the deep Earth depends on its initial temperature and composition. These were set by the large planetary collisions that formed the Earth. After each collision, the metallic core of the impactor fell into a molten silicate magma ocean. Previous investigations showed that, as it sank, the impactor core fragmented into drops. The overall fragmentation of the core controlled the efficiency of chemical transfers between the impactor metal and the magma ocean, and, as a consequence, the composition of the Earth's core and mantle. However, because previous studies lack an impact stage, it is unclear whether the projectile core fragmented during the impact at Earth's surface, or deeper in the magma ocean.

To answer this question, we conduct laboratory experiments modeling the collision of single-phase and two-phase impactors. In a first series of experiments, we investigate the impact of a single-phase centimetric liquid volume, representing the impactor core, onto a lighter immiscible liquid, representing the magma ocean. Our experiments approach the dynamical regime of planetary collisions for which inertia is large compared to surface tension. Varying the velocity and size of the impactor, we determine the conditions under which the impactor fragments into drops. We find that fragmentation occurs when the Froude number, which measures the relative importance of inertia to gravity, is larger than 40, regardless of surface tension. This fragmentation results from the growth of a turbulent Rayleigh-Taylor instability at the interface between the impacting liquid and the target pool. In contrast, when $\mathrm{Fr}<10$, the impactor remains coherent. In a second series of experiments, we use two-phase impactors to show that these results hold for impactors that are differentiated into a core and a mantle.

Applied to planet formation, our results suggest that the core of impactors less than 330 km in radius impacting at the escape velocity onto an Earth-sized planet fully fragments into droplets during the impact process, whereas the core of a giant Mars-sized impactor remains coherent. We derive a model for the depth at which the impactor core fragments in the magma ocean as a function of the impactor size and velocity. This model predicts that impactors with a radius less than 800 km fully fragment before reaching the bottom of the magma ocean. For velocities higher than twice the escape speed, some degree of fragmentation is unavoidable for any impactor size.

Northwestern India was affected by the Neoproterozoic Malani and Erinpura magmatism, Cretaceous rifting, and magmatism associated with the Deccan precursors. Receiver function analysis was done using 1704 RFs from 18 stations to image and decipher the imprints of the magmatic events and comprehend the crustal modifications in terms of variations in structure and composition. The H-κ grid search and neighbourhood inversion techniques are used to retrieve the crustal structure and Vp/Vs ratios ($\kappa$). The study reveals significant variations in the crustal thickness (H = 34 km to 43 km), composition ($\kappa$=1.74 to 1.92) and shear wave velocity structure across northwestern India. The terrains encompassing the Erinpura granite and the Malani igneous suite in the vicinity of the Barmer rift are characterized by a thick (H ≥ 41 km) crust with felsic to intermediate composition ($\kappa$ ≤ 1.81). The crust beneath the southern part of the Marwar basin is ≈ 38–40 km thick with mafic composition ($\kappa$> 1.81). The region around the Barmer rift has a thin crust (H = 34–36 km) with intermediate to mafic composition. The region hosting the Early Cretaceous to Paleogene alkaline complexes exhibits a high Vp/Vs ratio ($\kappa$= 1.91) that may be associated with mafic cumulates emplaced by magmatic events that overprint the signatures of the Malani event. The crust is heterogeneous with low/high velocity intracrustal layers that reflect the fractionation of magma at different depths. The mafic residue, together with magmatic intrusions, results in a mafic lowermost crust with high shear velocities of 3.9–4.0 km/s beneath most stations. Overall, northwestern India is characterized by a thick crust with intermediate crustal composition and intracrustal layering resulting from large scale magmatic events linked to the Neoproterozoic reorganization of plates and younger magmatic events.

This paper presents new u-channel paleomagnetic secular variation (PSV) data for ODP Site 1233 (Chile margin) for the last 70 ky. This is the highest-resolution, long-term PSV record ever recovered. The u-channel study has carried out detailed af demagnetization of the natural remanence (NRM) and developed a characteristic remanence (ChRM – final PSV direction) for each sampling horizon. The u-channel rock magnetic studies have measured magnetic susceptibility and anhysteretic remanence (ARM) with af demagnetization and used them to normalize the NRM and develop a new, detailed relative paleointensity record. The paleomagnetic record contains three excursions: the Mono Lake Excursion, the Laschamp Excursion, and the Greenland/Norwegian-Sea Excursion. These excursions have nominal sampling intervals of 5–8 years. Both the Mono Lake and Greenland/Norwegian-Sea Excursions involve short duration intervals of excursional directions (<300 years). The excursions are distinctly different from similar excursion records from other parts of the World. The Laschamp Excursion exhibits a full local reversal of the field with fast (<200 yr) movement to/from reversed directions and ∼ 500 years of full reversed polarity directions. This pattern is distinctly different from Laschamp excursion records from other parts of the World. We have carried out a statistical analysis of the Site 1233 u-channel PSV record. Notable specific features of that analysis include documentation that the field spends >1/3 of its time with anomalously large amplitude directional variability (high angular dispersion). These intervals occur at∼25–45 ka and 55–65 ka. These intervals have lower-frequency (more sluggish) directional variability and are associated with lower than average paleointensity.

The intense and persistent seismic activity on the Island of Euboea (or Evia) was a unique opportunity to study anisotropy in a region without any notable prior seismicity. The study area is located in the central-western Aegean Sea and constitutes the endpoint where the North Aegean Trough and four parallel dextral strike-slip fault branches terminate against the Greek mainland, along with their sinistral counterparts. Even though seismicity has been recorded around the island, the affected area close to the town of Styra has no known reported earthquakes. In 2022, surprising activity was initiated by two moderate earthquakes (M_L 4.8 and M_L 5.0) that occurred on 29 November and were felt as far as Athens. The activity continued well into the second quarter of 2023, with over 1000 microearthquakes being recorded. A pre-planned nearby temporary installation of a broadband instrument (GR27), in the context of the AdriaArray project, offered recordings of the activity. Here, we investigated the occurrence and cause of shear-wave splitting beneath Styra. Based on focal mechanisms of local earthquakes we inferred the stress axes. A fully automated process was used to analyze data for shear-wave splitting and determine the polarization direction of the S_fast (φ), the time-delay (t_d) and the normalized time-delay (t_n). A total of 272 acceptable results showcased a unimodal distribution of φ with an average of N68°E and a mean t_d of 80 ms. We observed temporal variations of splitting parameters, associated with outbursts of seismicity, not with individual events. The determination of the shear-wave velocity anisotropy, jointly with observations of other splitting parameters, led us to hypothesize that anisotropy is the result of along-fault fluid processes. However, this would require a convincing selection of the NE-SW nodal plane as the preferred fault orientation. Further enrichment of the catalogue and an increase in splitting results is required to draw more robust conclusions.

The construction and assessment of model trajectories that link multiple mantle states is essential to constrain poorly known mantle convection parameters. Previously, volumetric approaches have been applied to assess the quality of constructed mantle flow trajectories. However, there is a need to assess these trajectories based on their dynamic topography predictions because mantle convection cannot be directly observed and may be inferred via its surface geological expressions. Typical metrics for assessing dynamic topography suffer from the double penalty problem — a prediction that is correct in intensity, size, and timing, but incorrect in location, results in large root-mean-square errors when compared to an observation. Here, we introduce metrics, gleaned from meteorology, that decompose any number of dynamic topography fields into their distinct objects after which the similarity between objects is compared. We find that this object-based approach overcomes double penalty and assesses models in a robust manner by providing the ability to assess separately the quality of match between subsidence and uplift areas. Additionally, the approach allows independent quality assessment of multiple aspects of a dynamic topography field, including amplitude and location of dynamic topography.