Yingfeng Ji

Key Points: • Slab geometry with upper slab termination at shallow depths beneath the Pamirs is incorporated into kinematic modeling. • The occurrence of intermediate-depth earthquakes aligns with cold slab regions near the slab tear-off below the southwestern Pamirs. • The lateral asymmetry in slab temperature possibly explains the mechanism of lateral tearing and differential slab-mantle coupling.

The resolution of lithospheric magnetic field models over the Tibetan Plateau is highly improved by incorporating data from over 200 ground-based stations. Moderate to strong earthquakes (M ≥ 5) predominantly occur in regions where the vertical component (Z) of the lithospheric magnetic field is negative, particularly within the range of-50 to 0 nT. The lithospheric magnetic field is a key indicator for revealing deep thermodynamic conditions and identifying seismic hazard zones. Abstract： The National Geophysical Data Center (NGDC) of the United States collected aeromagnetic data for input into a series of geomagnetic models to improve the resolution, but in the Tibetan Plateau region the ground-based observation data are missing and do not clearly reflect the characteristics of lithospheric magnetism. In this study, we evaluate the lithospheric magnetism of the Tibetan Plateau by using a 3D surface spline model and the observation of >200 newly constructed stations to determine the spatial distribution of plateau geomagnetism, as well as its correlation with the tectonic features of the region. We analyze the relationships between M ≥ 5 earthquakes and the lithospheric magnetic field variation at the Tibetan Plateau, and identify regions susceptible to the occurrence of strong earthquakes. We compare the geomagnetic results with those from an enhanced magnetic model (EMM) and shed light on improving the lithospheric magnetic field calculation in the Tibetan Plateau region. Further research reveals that this magnetic anomalies exhibit distinct differences from magnetic-seismic correlation mechanisms observed in other tectonic settings. It is primarily governed by the combined effects of compressional magnetism, thermal magnetism, and deep thermal-stress condition. This study provides new evidence and physical interpretation for identifying seismic hazard zones on the Tibetan Plateau using geomagnetic anomalies.

Plate subduction exerts a strong influence over the tectonic evolution of the eastern Himalayan syntaxis (EHS) due to the substantial heterogeneity of plate contacts over space and time. The variability in tectonic escapes around EHS presents challenges for understanding the primary drivers. Here we use 3D models to mimic the ≥ 40 Myr subduction of India based on different leading edges and quantitatively analyze the associated viscous flows at various depths across the eastern Tibet. We find that the continuous, northeastward indentation of India enhances clockwise rotation and that the difference between shallow subduction in the north and steep subduction in the south of the EHS is critical to tectonic extrusion. The dynamic mechanism of this bottom-up process leads to significant lateral mantle flow and contributes to > 1000 km of material escaping southward.

Due to the steep subduction of a highly concave slab, researchers have characterized megathrusts under the Marianas as among the coldest and curviest plate coupling interfaces in various circum-Pacific subduction zones. Seismic tomography indicates that the heterogeneous underlying plate varies markedly in its subduction angle, velocity, and flexure along the strike and dip, while their effects on the thermal structure and intraslab earthquake occurrence remain enigmatic. By incorporating the 3-D MORVEL velocity and state-of-the-art slab geometry into thermomechanical modeling, we estimated the 3-D subduction thermal state and hydrothermal regime below the Marianas. We find that (1) the concave slab geometry and the complexity of the intraslab velocity variation in the Marianas are associated with a heterogeneous along-strike thermal regime and a cold mantle wedge beneath the central Marianas; (2) amphibolitization and eclogitization of subducted oceanic crust cause variations in fluid pressure and fluid release from the subduction interface, which may influence the distribution of interface seismicity in the Mariana system; (3) the concentration of active hydrothermal vents in the trench > 8 km deep is accompanied by a large temperature gradient and subsequent remarkable slab dehydration in the southern Marianas; and (4) slab dehydration (> 0.02 wt%/km) from 30 to 80 km indicates notable fluid release and potential fluid migration in subduction channels, which may correspond to the large water flux at depth beneath the Marianas.

• The plate geometry of the Nazca Plate strongly affected the along-strike thermal structure of the Colombia-Ecuador zone. • The positions of 100-200°C and 350-450°C isotherms on the plate interface determine the updip and downdip limits of the Colombia-Ecuador seismogenic zone. • The released fluids from young Nazca plate influences the spatial distribution of Colombia-Ecuador interplate earthquakes.

• An integrated three-dimensional thermal model indicates that the geometry of subducting slabs has a substantial impact on the thermal structure of megathrusts globally. • The model highlights that slab obliquity and depth variations play a crucial role in shaping the thermal regime and earthquake potential, as observed in the Makran Subduction Zone. • Results suggest a strong correlation between zones of slab dehydration and the distribution of large crustal earthquakes, implying that slab metamorphism and fluid accumulation influence earthquake occurrence.

Largescale volcanic eruptions and earthquakes are occurring frequently in the Philippines, and research has shown that slab metamorphism and diversity alter the impacts of subducted oceanic plates by changing water‒carbon productivity and interplate stability. Within the framework of the thermal evolution history of subducting slabs, the relationships between subduction zone seismicity characterized by both regular megathrust earthquakes and slow slip events of various magnitudes and long-term slab dehydration–decarbonation evolution in the Philippines remain poorly understood. Here, we constructed a comprehensive thermal model incorporating 3-D slab geometric data for the incoming plate and a 3-D subduction velocity field based on the MORVEL plate motion dataset for the Philippine subduction zone with high spatial and temporal resolutions. Our findings reveal that subduction seismicity and arc volcanism are prominent in belt-shaped regions with high thermal gradients (> 5 °C/km) and large-scale slab dehydration (> 0.05 wt%/km). Dehydration of serpentinite in ultramafic rocks in the subducting slab and decarbonation of carbonate minerals preferentially contribute to the generation and transport of fluids and carbonate melts, thus facilitating seismicity and carbon-rich magmatism. Our results suggest that slab geometry diversity-induced juxtaposed slab dehydration-decarbonation processes play a vital role in the generation of megathrust earthquakes below the forearc.

Compelling evidence has shown that geomagnetic disturbances in vertical intensity polarization before great earthquakes are promising precursors across diverse rupture conditions. However, the geomagnetic vertical intensity polarization method uses the spectrum of smooth signals, and the anomalous waveforms of seismic electromagnetic radiation, which are basically nonstationary, have not been adequately considered. By combining pulse amplitude analysis and an experimental study of the cumulative frequency of anomalies, we found that the pulse amplitudes before the 2022 Luding M6.8 earthquake show characteristics of multiple synchronous anomalies, with the highest (or higher) values occurring during the analyzed period. Similar synchronous anomalies were observed before the 2021 Yangbi M6.4 earthquake, the 2022 Lushan M6.1 earthquake and the 2022 Malcolm M6.0 earthquake, and these anomalies indicate migration from the periphery toward the epicenters over time. The synchronous changes are in line with the recognition of previous geomagnetic anomalies with characteristics of high values before an earthquake and gradual recovery after the earthquake. Our study suggests that the pulse amplitude is effective for extracting anomalies in geomagnetic vertical intensity polarization, especially in the presence of nonstationary signals when utilizing observations from multiple station arrays. Our findings highlight the importance of incorporating pulse amplitude analysis into earthquake prediction research on geomagnetic disturbances.