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.

The Arxan volcanic group is one of the largest active volcanic fields in China. Its last eruption occurred ∼2,000 years ago, but it still has a potential of eruption in the future. The spatial distribution and migration channel of its magmatic reservoirs are still unclear. Here we precisely locate microearthquakes and study the detailed 3‐D velocity structure of the volcanic area using a large number of high‐quality P and S wave arrival‐time data recorded at 227 portable seismometers. A swarm of microearthquakes (M < 1.5) related to magmatic activity were recorded, which lasted about 4 days during 13–16 September 2020. Our tomographic results show that 12 Arxan volcanoes share the same magmatic system. The melt fraction of a magma chamber below the Tianchi volcano at ∼0–8 km depths is estimated to be ∼6.1%–8.6% from the seismic velocity reductions. Our results indicate that the Arxan magmatic system is still active at present.

The Changbaishan volcano (CBV) is the largest Cenozoic intraplate stratovolcano in Northeast China. Although many seismic imaging studies have been conducted in the CBV area during the past two decades, the detailed crustal structure beneath the Tianchi caldera is still poorly understood due to the sparse seismic observations limited by the harsh natural conditions. In this study, we deployed dense linear seismic arrays along three slopes of the Changbaishan‐Tianchi volcanic area for the first time to obtain unprecedented high‐resolution images of the crustal structure by using the teleseismic receiver‐function imaging technique. Our results reveal detailed variations of crustal interface characteristics beneath the Tianchi caldera and adjacent areas. Several intracrustal interfaces are clearly revealed that correspond well to geological bodies, which might indicate boundaries of consolidated igneous bodies related to the primitive magmatic activities. A continuous Moho discontinuity at ∼37 km depth is revealed, whereas a weak Moho zone with a vertical offset of ∼4 km appears directly beneath the Tianchi caldera, which shows a clear positive correlation with the average crustal Vp/Vs ratio. We deem that the prominent crustal thickening and the higher Vp/Vs ratio beneath the Tianchi caldera might be closely associated with the complicated mantle‐derived mafic materials underplating at the crust‐mantle boundary. The joint effect of the multilevel magmatic plumbing system and a narrow magma conduit beneath the Tianchi caldera could account for the distinctive bimodal volcanic eruption history of the CBV.

The 1975 Ms 7.3 Haicheng earthquake occurred in northwestern Liaodong Peninsula, which was successfully forecasted and marked a landmark achievement in seismology. However, the seismogenic mechanism of this earthquake remains poorly known. In this study, we obtain high‐precision earthquake catalog and 3‐D models of crustal P‐ and S‐wave velocities (Vp, Vs) and their ratio (Vp/Vs) in the Haicheng seismic zone (HSZ) from an unprecedentedly dense seismic array that we recently deployed. Our results show the seismicity pattern in the HSZ is predominantly controlled by an interconnected conjugate fault system, comprising the WNW‐striking Haichenghe fault and its NE‐striking concealed secondary fault. Furthermore, a prominent fluid‐rich zone characterized by low Vp, low Vs and high Vp/Vs ratio is clearly revealed in the middle crust at ∼15 km depth, directly beneath the hypocentral region of the 1975 Haicheng earthquake. A distinct seismic cluster is also observed adjacent to the main shock hypocenter, exhibiting spatial correlation with the underlying crustal fluid reservoir. This geometric correspondence indicates a fluid‐driven triggering mechanism that contributes to the localized seismicity. We deem that the reactivation of preexisting seismogenic faults induced by crustal fluid migration could be a possible mechanism for the nucleation of such a large intraplate earthquake.

We applied teleseismic tomography to investigate the 3D P-wave velocity (Vp) structure of the crust and upper mantle at depths of 50–400 km beneath the Yarlung–Tsangpo suture (YTS), by using 6164 P-wave relative travel-time residuals collected from 495 teleseismic events recorded at 20 three-component broadband seismograms. A modified multi-channel cross-correlation method was adopted to automatically calculate the relative arrival-time residuals of all teleseismic events, which significantly improved the efficiency and precision of the arrival-time data collection. Our results show that alternating low- and high-Vp anomalies are visible beneath the Himalayan and Lhasa blocks across the YTS, indicating that strong lateral heterogeneities exist beneath the study region. A significant high-Vp zone is visible beneath the southern edge of the Lhasa block at 50–100 km depths close to the YTS, which might indicate the rigid Tibetan lithosphere basement. There exists a prominent low-Vp zone beneath the Himalayan block to the south of the YTS extending to ~150 km depth, which might be associated with the fragmentation of the underthrusting Indian continental lithosphere (ICL) and induce localized upwelling of asthenospheric materials from the upper mantle. In addition, significant low-Vp anomalies were observed beneath the Yadong–Gulu rift and the Cona–Sangri rift extending to ~300 km depth, indicating that the tearing of the subducted ICL might provide pathways for the localized asthenospheric materials upwelling, which contributes to the widespread distribution of north–south trending rifts and geothermal activities in southern Tibet.