We study the detailed mantle transition zone structure beneath the active Changbai intraplate volcano in Northeast China by using a receiver‐function method. A total of 3005 teleseismic receiver functions recorded by 70 broadband stations are obtained by using a common‐conversion‐point stacking method. For conducting the time‐to‐depth conversion, we use a three‐dimensional velocity model of the study region so as to take into account the influence of structural heterogeneities. Our results reveal significant depth variations of the 410, 520, and 660 km discontinuities. A broad depression of the 410 km discontinuity and a low‐velocity anomaly are revealed beneath the Changbai volcano, which may reflect a large‐scale hot mantle upwelling around the 410 km discontinuity with a positive Clapeyron slope. The 520 km discontinuity is identified clearly, and its uplift occurs above the stagnant Pacific slab. We also find a prominent depression of the 660 km discontinuity, which is elongated along the trend of deep earthquake clusters in a range of 39°N–44°N latitude, and the depression area has a lateral extent of about 400 km. Because the 520 and 660 km discontinuities correspond to positive and negative Clapeyron slopes, respectively, we think that the 520 uplift and the 660 depression are caused by the cold subducting Pacific slab. A part of the Pacific slab may have penetrated into the lower mantle and so caused the large‐scale 660 depression in front of the deep earthquake clusters. Our results also reveal a part of the upper boundary of the subducting Pacific slab in the mantle transition zone.

Seismic anisotropy records past and present tectonic deformations and provides important constraints for understanding the structure and dynamics of the Earth's interior. In this work, we use tremendous amounts of high‐quality P wave arrival times from local and regional earthquakes to determine a high‐resolution tomographic model of 3‐D P wave azimuthal anisotropy down to 1,000‐km depth beneath East Asia. Our results show that trench‐parallel fast‐velocity directions (FVDs) are visible in the shallow portion of the subducting Pacific slab (<80 km), whereas the deeper portion of the Pacific slab mainly exhibits trench‐normal FVDs, except for the stagnant slab in the mantle transition zone (MTZ) where obvious NE‐SW FVDs are revealed. The FVDs in the subslab mantle change from a subduction‐parallel trend at depths of 80–400 km to a subduction‐normal trend in the MTZ. Large‐scale low‐velocity anomalies are revealed beneath the Philippine Sea plate where the FVD is NE‐SW. The FVDs along the Izu‐Bonin arc and in a slab gap exhibit a striking anticlockwise toroidal trend. All these features may reflect complex 3‐D flows in the mantle wedge due to tearing and dehydration processes of the subducting Pacific slab. The subducting Pacific slab is split at ~300‐km depth under the Bonin arc and then penetrates into the lower mantle, whereas under East Asia the Pacific slab becomes stagnant in the MTZ and reaches the North‐South Gravity Lineament in China. The intraplate volcanoes in East Asia are caused by hot and wet upwelling flows in the big mantle wedge above the stagnant Pacific slab.

We study the detailed 3‐D crustal and upper mantle structure beneath the active Changbai intraplate volcano in Northeast China by conducting a trans‐dimensional Bayesian inversion of teleseismic receiver functions and Rayleigh‐wave group velocity dispersions from ambient noise. More than 12,000 teleseismic receiver functions recorded at 78 seismic stations and 1,573 group velocity dispersions are used in this study. Receiver‐function H‐κ stacking measurements reveal a thick crust (~40 km) with a high Vp/Vs ratio (~1.8) beneath the Changbai volcano. Our joint inversion results show a heterogeneous crustal structure in the study region. A low‐velocity body at depths of 8–15 km is revealed directly beneath the Changbai volcano, which has a lateral extent of ~100 km in the north–south direction and may reflect a large magma chamber in the midcrust. Our results also reveal a 5–10 km depressed Moho and a low‐velocity anomaly in the uppermost mantle beneath the Changbai volcano. These features may indicate an upwelling channel of the asthenospheric material with a high mafic composition, and the mafic intrusion attaches to the bottom of the crust and thus deepens the Moho beneath the volcano. Our results support the notion that the Changbai volcanism is caused by hot and wet mantle upwelling associated with subduction‐driven corner flow in the big mantle wedge above the stagnant Pacific slab in the mantle transition zone.

A high‐precision earthquake catalog is made from continuous waveform data recorded at 360 portable seismographs deployed for one month in the Changbaishan‐Tianchi volcanic area based on an automatic earthquake detection and location workflow. Then we investigate detailed 3‐D structures of P and S wave velocities (V_p and V_s) and V_p/V_s ratio of the crust beneath the study region. Our results reveal a high‐V_p/V_s zone accompanied with active volcanic earthquakes beneath the Tianchi caldera at ∼5 km depth, which may reflect a magma chamber. The intense seismic activities within 2 km depth beneath the Tianchi caldera from 2002 to 2005 occurred directly above the magma chamber revealed by this study, which could be related to upward migration of high‐pressure volatiles originating from the magma chamber. A potential middle‐crustal magma chamber might provide continuous magmatic materials feeding the upper‐crust magma chamber and lead to ruptures of surrounding rocks, resulting in two swarms of volcanic seismicity during the observation period. We deem that the diverse eruption styles of the Changbaishan volcano are closely associated with the multilevel structure of magma chambers in the upper and middle crust.

Seismic traveltime is critical information conveyed by seismic waves, widely utilized in various geophysical applications. Conventionally, the simulation of seismic traveltime involves solving the eikonal equation. However, the efficiency of traditional numerical solvers is hindered, as they are typically capable of simulating seismic traveltime for only a single source at a time. Recently, deep learning tools, particularly physics-informed neural networks (PINNs), have proven effective in simulating seismic traveltimes for multiple sources. Nonetheless, PINNs face challenges such as limited generalization capabilities across different models and difficulties in training convergence. To address these issues, we have developed a method for simulating multi-source seismic traveltimes in variable velocity models using a deep-learning technique, known as the physics-informed Fourier neural operator (PIFNO). The PIFNO-based method for seismic traveltime generator takes both velocity and background traveltime as inputs, generating the perturbation traveltime as the output. This method incorporates a factored eikonal equation as the loss function and relies solely on physical laws, eliminating the need for labeled training data. We demonstrate that our proposed method is not only effective in calculating seismic traveltimes for velocity models used during training but also shows promising prediction capabilities for test velocity models. We validate these features using velocity models from the Sibsbee2A velocity and OpenFWI dataset.