Subsurface imaging is key to understanding the origin of intraplate volcanos. The Changbaishan volcano, located about 2000 km away from the western Pacific subduction zone, has several debated origins. To investigate this, we compared regional seismic tomography with the electrical resistivity results and performed high-resolution 1D and quasi-2D velocity-depth profiles. We show that the upper mantle is characterized by two anomalies exhibiting distinct features which cannot be explained by the same mechanism. We document a localized low-velocity anomaly atop the 410-km discontinuity, where the P-wave velocity is reduced more than that of the S-wave (i.e., low Vp/Vs). We propose that this anomaly is caused by the reduction of the effective moduli during the phase transformation of olivine. The other anomaly, located between 300 km and 370 km depth, reveals a significant reduction of the S-wave velocity (i.e., high Vp/Vs), associated with a reduction of the electrical resistivity, altogether consistent with partial melting.

The nature of deep earthquakes with depths greater than 70 km is enigmatic because brittle failure at this high-temperature and the high-pressure regime should be inhibited. Three main hypotheses have been proposed to explain what causes deep earthquakes within the subducting slabs, dehydration embrittlement, phase transformational faulting, and thermal runaway instability. However, the existing seismological constraints can’t yet definitively distinguish between these hypotheses because the fine 3D slab structures are not well constrained in terms of slab upper interface, thickness, and internal fine layering. To better image the slabs in the Western Pacific subduction zones, this study employs a full waveform inversion (FWI) that minimizes waveform shape misfit between the synthetics and the observed waveforms from a large dataset, with 142 earthquakes recorded by about 2,400 broadband stations in East Asia. A 3-D initial model that combines two previous FWI models in East Asia (i.e., FWEA18 and EARA2014) are iteratively updated by minimizing the misfit measured from both body waves (8–40 s) and surface waves (30–120 s). Compared to the previous models, the new FWI model (EARA2020) shows much stronger wave speed perturbations within the imaged slabs with respect to the ambient mantle, with maximum perturbation of 8% for Vp and 13% for Vs. Furthermore, the slab thickness derived from EARA2020 exhibits significant downdip and along-strike variations at depths greater than 100 km. The large intra-slab deep earthquakes (Mw>6.0) appear to occur where significant slab thinning happens. This observation suggests that the significant deformation (or strain accumulation) of the slab is likely the first-order factor that controls the distribution of large deep earthquakes within the slab regardless of their triggering mechanism.

The lithospheric structure of the contiguous US and surrounding regions is significant in revealing the historical tectonic deformations and interactions between subducting slabs and cratons. In this paper, we present a new radially anisotropic shear wave speed model of this region, constrained by seismic full-waveform inversion. The new model (named CUSRA2021) utilizes frequency dependent travel time measured from waveforms of 160 earthquake events recorded by 5,280 stations. More earthquakes located in contiguous US are incorporated to improve the data coverage in eastern US. The final model exhibits clear and detailed shear wave speed anomalies that correlate very well with tectonic units such as North America Craton (high-Vs), Cascadia subduction zones (high-Vs), Columbia Plateau (low-Vs), Basin and Range (low-Vs), etc. In particular, the detail of the North America Craton beneath Illinois is revealed, and the depth of high-Vs anomaly beneath the North America Craton correlates well with S-to-P receiver function and SH reflection studies. The radial anisotropy also shows a layering of Craton lithosphere.