The magmatic process of continental intraplate volcanism (CIV) is difficult to understand due to heterogeneous interactions with the crust and the lithospheric upper mantle. Mount Baekdu (Changbai) volcano (MBV) is one of the prominent CIVs in northeast Asia that has shown a complex history of eruptions and associated magmatic structures. In addition, the relationship between the crustal magmatic structures and upper mantle phenomena are enigmatic due to the lack of consistent seismic constraints for the lithospheric structure. To enhance comprehensive understanding of the MBV magma evolution, we image the lithospheric structure beneath the MBV and surrounding regions using ambient noise data and the following two approaches: (1) multiple measures of ambient noise dispersion are acquired through different methods and (2) a transdimensional Bayesian inversion method is utilized to obtain unbiased results in joint analysis of the multiple data sets. The estimated Earth structure shows a thick crust (~40 km) and a crustal anomaly with relatively high S wave velocity in the depth range 20–40 km. This type of structure extends to ~100 km north from the MBV and is accompanied by the shallow and rapid S wave velocity decrease beneath the mantle lid (~80 km). Through a comparison with previous P wave models, we interpret this structure as a consequence of compositional partitioning by mafic underplating and overlying cooled felsic layers as a result of fractional crystalization.

SUMMARY Ambient noise tomography has been used to construct Rayleigh‐wave group velocity maps covering the northern (NMER), central (CMER) and southern (SMER) parts of the Main Ethiopian Rift (MER). In addition, dispersion curves, extracted from the group velocity maps, have been inverted to obtain a quasi‐3‐D model of crustal shear wave velocities. In comparison to crustal structure on the Ethiopian Plateau, we find (1) lower shear wave velocities at all crustal depths beneath the Yerer‐Tullu Wellel Volcanotectonic Lineament, (2) lower shear wave velocities throughout the MER at upper crustal depths (<10 km), (3) regions of lower shear wave velocities at mid‐ (10–20 km) crustal depths beneath the Wonji Fault Belt (WFB), in the transition between the NMER and CMER, and beneath the Silti‐Debre zeit Fault Zone (SDFZ) on the western side of the CMER, (4) an offset in the velocity pattern at mid‐crustal depths between the NMER and CMER coincident with the Boru‐Toru Structural High (BTSH) and (5) little evidence for lower shear wave velocities at mid‐ or lower‐crustal depths beneath the SMER. We attribute these findings primarily to along‐strike changes in crustal composition, melt content and thermal structure resulting from the Cenozoic to recent magmatism, and also, at upper crustal depths (<10 km), to basin structure and fill. Our findings corroborate a magmatic plumbing model for the MER that shows two major zones of magmatic activity, one beneath the WFB and the other beneath the SDFZ. The shear wave velocity patterns in our model show good correlation with the depth extent of seismicity, upper‐mantle seismic anomalies and seismic anisotropy, as would be expected if the along‐strike changes in shear wave velocity reflect the thermal and compositional structure of the crust.

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We reassess the mantle transition zone structure below the northeast Asia margin in the context of subduction of the Pacific plate below the Eurasian continent. We use two independent approaches of teleseismic imaging, namely, compressional‐to‐shear converted waves (receiver functions) and shear wave underside reflections (SS precursors), and compare them within their statistical uncertainties. We find localized complexity in the interfaces marking solid phase changes in mantle minerals, in terms of both apparent topography and reflectivity. The 660‐km discontinuity is doubled, with ∼80‐km maximum vertical distance between the interfaces, over an 890 × 350 km2 region between 36–44°N and 130–133°E at the tip of the subducted Pacific plate. A similar complexity exists on the 410‐km discontinuity, coinciding with the presence of a deep cluster of seismicity below the Japan Sea. Both methods suggest the presence of low‐velocity zones atop the 410, within the mantle transition zone, and below the 660. This complex seismic signature is related to the Pacific plate and interpreted in light of the subduction thermal regime and phase equilibria for a pyrolitic mantle composition. Phase changes manifest themselves as broad zones of velocity gradients with localized doubled or multiple first‐order discontinuities, associated with transitions in the olivine, pyroxene, and garnet systems. An average pyrolitic composition and local temperatures of 1000–1300 K can explain the observed velocity gradients and multiple discontinuities. We show that the dissolution of stishovite, a high‐pressure polymorph of SiO2, into the higher‐pressure perovskite mineral, is a possible explanation for the low‐velocity zones at the top of the lower mantle.

Intraplate volcanism adjacent to active continental margins is not simply explained by plate tectonics or plume interaction. Recent volcanoes in northeast (NE) Asia, including NE China and the Korean Peninsula, are characterized by heterogeneous tectonic structures and geochemical compositions. Here we apply a transdimensional Bayesian tomography to estimate high‐resolution images of group and phase velocity variations (with periods between 8 and 70 s). The method provides robust estimations of velocity maps, and the reliability of results is tested through carefully designed synthetic recovery experiments. Our maps reveal two sublithospheric low‐velocity anomalies that connect back‐arc regions (in Japan and Ryukyu Trench) with current margins of continental lithosphere where the volcanoes are distributed. Combined with evidences from previous geochemical and geophysical studies, we argue that the volcanoes are related to the low‐velocity structures associated with back‐arc processes and preexisting continental lithosphere.