Body waves traversing the Earth’s interior from a seismic source to receivers on the surface carry rich information about its internal structures. Their travel time measurements have been widely used in seismology to constrain Earth’s interior at the global scale by mapping the time anomaly along their ray paths. However, picking the travel time of global seismic waves, suitable for studying Earth’s fine-scale structures, requires highly skilled personnel and is often fairly subjective. Here, we report the development of an automatic picker for PKIKP waves, traversing the Earth nearly along its diameters and through the inner core, based on the latest advances in supervised deep learning. A convolutional neural network (CNN) we developed automatically determines the PKIKP onset on vertical seismograms near its theoretical prediction of cataloged earthquakes. As high-quality manual onset picks of global seismic phases are limited, we employed a scheme to generate a synthetic supervised training dataset containing 300,000 waveforms. The PKIKP onsets picked by our trained CNN automatic picker exhibit a mean absolute error of ~0.5 s compared to 1,503 manual picks, comparable to the estimated human-picking error. In an integration test, the CNN automatic picks obtained from an extended waveform dataset yield a cylindrically anisotropic inner core model that agrees well with the models inferred from manual picks, which illustrates the success of this pilot model. This is a significant step closer to harvesting an unprecedented volume of travel time measurements for studying the inner core or other regions of the Earth’s deep interior.

Determining the seismic moment tensors (MT) from the observed waveforms, known as full-waveform seismic MT inversion, remains challenging for small to moderate-size earthquakes at regional scales. Firstly, there is an intrinsic difficulty due to a tradeoff between the isotropic (ISO) and compensated linear vector dipole (CLVD) components of MT that impedes resolving shallow explosive sources, e.g., underground nuclear explosions. It is caused by the similarity of long-period waveforms radiated by ISO and CLVD at regional distances. Secondly, regional scales usually bear complex geologic structures; thus, inaccurate knowledge of Earth’s structure should be considered a theoretical error in the MT inversion. However, this has been a challenging problem. So far, only the uncertainty of the 1D Earth model (1D structural error), apart from data errors, has been explored in the source studies. Here, we utilize a hierarchical Bayesian MT inversion to address the above problems. Our approach takes advantage of affine-invariant ensemble samplers to explore the ISO-CLVD tradeoff space thoroughly and effectively. Furthermore, we invert for station-specific time shifts to treat the structural errors along specific source-station paths (2D structural errors). We present synthetic experiments demonstrating the method’s advantage in resolving the ISO components. The application to nuclear explosions conducted by the Democratic People’s Republic of Korea (DPRK) shows highly similar source mechanisms, dominated by a high ISO, significant CLVD components, and a small DC component. The recovered station-specific time shifts from the nuclear explosions present a consistent pattern, which agrees well with the geological setting surrounding the event location.

We deployed 27 ocean-bottom seismometers (OBSs) and five Macquarie Island stations between 2020 and 2021 along the Australian-Pacific plate boundary in the Macquarie Ridge Complex (MRC) of the Southern Ocean. From the waveforms recorded on successfully recovered OBSs and the island stations, including permanent station MCQ, we generated a 3-D S-wave velocity model for the crust and uppermost mantle using full-waveform ambient noise tomography. Distinct surface waves are identified via inter-station cross-correlation of the vertical components. The new S-wave velocity model reveals a pronounced increase in velocity across expected crustal and uppermost mantle depths. Relatively high S-wave velocities (>3.8 km/s at 7 km below sea level) are consistent with the presence of upper mantle rocks at relatively shallow depths spread out along the ridge. Overall, our findings provide initial insights on the sub-surface structure of the MRC, a complex tectonic setting of potential subduction initiation.

The violent eruption of the Hunga (Tonga) submarine volcano on 15 January 2022 caused a 58 km-heigh ash plume, catastrophic tsunami, and significant global seismic and infrasound waves. However, the physical mechanism underpinning its multiple-explosive events remains unclear, and its resolvability relies on the seismic waveform source inversion. The studies of two different point-source models, the seismic moment tensor (MT) and the single force (SF), have been performed separately for this eruption, which, interestingly, can explain the seismic data adequately. Here, we use a joint inversion of MT and SF to unravel a composite source of an explosive MT and a significant upward force for the first major explosive event. Regarding the direction and magnitude, we propose that the upward force is likely a rebound force in response to the pressure drop on the seafloor because the water body above the volcano was abruptly uplifted by the shallow underwater explosion.

We conduct a high-resolution seismic tomography for the crustal P and S-wave velocities of Yunnan region in southwestern China. Waveforms recorded at 128 broadband stations from 131 regional earthquakes of moment magnitudes 3.9−5.5 occurring between 2009 and 2021 are used to obtain traveltime residuals by the cross-correlation between records and synthetics. Using the regional community velocity model SWChinaCVM‐1.0 as the initial model, we carry out a three-stage iterative adjoint tomography, progressing from the longer period band of 50–20 s to shorter-period bands of 30–10 s and 30–5 s. The final model shows general consistency in the spatial patterns of P- and S-wave velocity anomalies. Widespread low-velocity anomalies with high-Vp/Vs ratios in the mid and lower crust in the region suggest a mix of weak materials of the mid-lower crustal flow from under the Tibetan Plateau with hot materials of the upwelling from the deep mantle plume that led to the Emeishan Large Igneous Province. Localized velocity and Vp/Vs ratio anomalies also reveal that the Lijiang-Xiaojinhe Fault Zone appears to be confined in the upper crust, while the Anninghe-Zemuhe Fault Zone and the Xiaojiang Fault Zone are both whole-crust structures reaching the Moho interface. The Red River Fault Zone, a whole-crust fault, separates the Yangtze Craton to the northeast from the Indo-China Block to the southwest. The main fault zones, the decoupling between the crustal and uppermost mantle parts, and the wide-spreading weak mid-lower crustal materials mutually interact, all contributing to the tectonic evolution of the entire region.

We conduct a high-resolution seismic tomography for the crustal P and S-wave velocities of Yunnan region in southwestern China. Waveforms recorded at 128 broadband stations from 131 regional earthquakes of moment magnitudes 3.9−5.5 occurring between 2009 and 2021 are used to obtain traveltime residuals by the cross-correlation between records and synthetics. Using the regional community velocity model SWChinaCVM‐1.0 as the initial model, we carry out a three-stage iterative adjoint tomography, progressing from the longer period band of 50–20 s to shorter-period bands of 30–10 s and 30–5 s. The final model shows general consistency in the spatial patterns of P- and S-wave velocity anomalies. Widespread low-velocity anomalies with high-Vp/Vs ratios in the mid and lower crust in the region suggest a mix of weak materials of the mid-lower crustal flow from under the Tibetan Plateau with hot materials of the upwelling from the deep mantle plume that led to the Emeishan Large Igneous Province. Localized velocity and Vp/Vs ratio anomalies also reveal that the Lijiang-Xiaojinhe Fault Zone appears to be confined in the upper crust, while the Anninghe-Zemuhe Fault Zone and the Xiaojiang Fault Zone are both whole-crust structures reaching the Moho interface. The Red River Fault Zone, a whole-crust fault, separates the Yangtze Craton to the northeast from the Indo-China Block to the southwest. The main fault zones, the decoupling between the crustal and uppermost mantle parts, and the wide-spreading weak mid-lower crustal materials mutually interact, all contributing to the tectonic evolution of the entire region.

Earth’s microseisms are continuously recorded by seismographs worldwide. Yet, studies using broadband digital data to analyze microseisms in Australia have been rare. Building on initial research efforts that relied on the Warramunga array in the continent’s center, we expand the investigation of microseisms by utilizing seismic arrays in various locations and with distinct apertures and geometries, particularly spiral-arm arrays. Motivated by expanding knowledge of microseismic sources, we investigate the distribution and characteristics of microseisms. We process one-year continuous waveform data using beamforming at various periods. Using the back-projection, we then investigate the plausible source areas of surface waves and teleseismic P-waves generated by ocean activity. We also examine the seasonal variability of microseismic sources and their relationship with the ocean wave hindcast model by comparing our observations of Rayleigh (Rg) waves with modelled Rg wave sources and juxtaposing the back-projected P-waves with significant wave heights. Our results suggest that over the time interval of several months and longer, Rayleigh waves are the dominant component arriving from the nearby coastlines. They show a transition to higher mode Lg waves in the higher frequency bands. In contrast, on the time scale of days and weeks, teleseismic P-waves from the coastal and pelagic sources are observed particularly from the tropical and equatorial regions. We also identify new patterns of body waves from the perspective of the Southern Hemisphere. Our study highlights the importance of utilizing multiple arrays and elucidates the critical roles of the frequency range and bathymetry.

The structure of the lowermost mantle and the core-mantle boundary (CMB) has profound implications for Earth’s evolution and current-day dynamics. Whilst tomographic studies of Vs show good agreement in the lowermost mantle, consensus as to Vp and especially CMB radius has not yet been reached. We perform a hierarchical Bayesian inversion for Vp in the lowermost 300 km of the mantle and the radius of the core-mantle boundary using differential travel time data. Concurrent with finding Vp perturbations of 0.56% RMS amplitude that spatially agree with previous studies in areas of low posterior variance, we find 4.5 km RMS amplitude core-mantle boundary radius perturbations with a broadly north-south hemispherical character, with spherical harmonic power evenly distributed between degrees 1-3. These results suggest that CMB radial processes are set by a longer scale process than the Vp perturbations.