The uppermost mantle is the key area for exchange of heat flux and material convection between the crust and lithospheric mantle. Spatial variations of lithospheric thinning and dynamic processes in the North China Craton could inevitably induce the velocity heterogeneity in the uppermost mantle. In this study, we used Pn arrivals from permanent seismic stations in North China and surrounding regions to construct a tomographic image of the North China Craton. The tomographic method with Pn travel time difference data were used to study the velocity variations in the uppermost mantle. Pn velocities in the uppermost mantle varied significantly in the Eastern, Central and Western blocks of the North China Craton. This suggests that the lithosphere beneath different blocks of the North China Craton have experienced distinct tectonic evolutions and dynamic processes since the Paleozoic. The current uppermost mantle has been imprinted by these tectonic and dynamic processes. Fast Pn velocities are prominent beneath the Bohai Bay Basin in the Eastern Block of the North China Craton, suggesting residuals of the Archean lithospheric mantle. Beneath the Tanlu Fault Zone and Bohai Sea, slow Pn velocities are present in the uppermost mantle, which can be attributed to significant lithospheric thinning and asthenospheric upwelling. The newly formed lithospheric mantle beneath Yanshan Mountain may be the dominant reason for the existence of slow Pn velocities in this region. Conversely, the ancient lower crust and lithospheric mantle already have been delaminated. In the Central Block, significant slow Pn velocities are present in Taihangshan Mountain, which also extends northward to the Yinchuan-Hetao Rift on the northern margin of the Ordos Block and Yinshan Orogen. This characteristic probably is a result of hot asthenospheric upwelling along the active tectonic boundary on the margin of the Western Block. The protracted thermal erosion and underplating of hot asthenospheric upwelling may induce lithospheric thinning and significant slow velocities in the uppermost mantle. Fast velocities beneath the Western Block suggest that the thick, cold and refractory Archean lithospheric keel of craton still is retained without apparent destruction.

To investigate the seismic velocity structure of the shallow sediments in the Bohai Sea of China, we conducted a shear-wave velocity inversion of the surface-wave dispersion data from a survey of 12 ocean-bottom seismometers (OBSs) and 377 shots of a $9000{\mathrm{in}}^3$ air gun. With OBS station spacing of approximately 5 km and air-gun shot spacing of approximately 190 m, high-quality Scholte-wave data were recorded by the OBSs within 0.4–5 km offset. We retrieved the Scholte-wave phase-velocity dispersion for the fundamental mode and first overtone in the frequency band of 0.9–3.0 Hz with the phase-shift method and inverted for the shear-wave velocity structure of the shallow sediments with a damped iterative least-squares algorithm. Pseudo-2D shear-wave velocity profiles with a depth of approximately 400 m revealed coherent features of relatively weak lateral velocity variation. We also estimated the uncertainty in shear-wave velocity structure based on the pseudo-2D profiles from six trial inversions with different initial models, which suggested a velocity uncertainty less than $30\mathrm{m}/\mathrm{s}$ for most parts of the 2D profiles. The layered structure with little lateral variation may be attributable to the continuous sedimentary environment in the Cenozoic sedimentary basin of the Bohai Bay basin. The shear-wave velocity of 200–300 $\mathrm{m}/\mathrm{s}$ in the top 100 m of the Bohai seafloor may provide important information for offshore site response studies in earthquake engineering. Furthermore, the very low shear-wave velocity structure (150–600 $\mathrm{m}/\mathrm{s}$) down to 400 m depth could produce a significant traveltime delay of approximately 1 s in the shear-wave arrivals, which needs to be considered to avoid serious bias in shear-wave traveltime tomographic models.

The shear-wave (S-wave) structures of shallow marine sediments are important for offshore geotechnical studies, deep crustal S-wave imaging, multicomponent seismic exploration, and underwater acoustics studies. We have applied the multicomponent Scholte-wave analysis technique to an active-source shallow marine seismic profile in the East China Sea. Scholte waves have been excited by shots from a 5450 inch3 air-gun array and their recordings have been conducted at the seafloor using ocean bottom nodes (OBNs). First, we extract the common-receiver gathers (CRGs) and correct for the time drift simultaneously using a forward and inverse fast Fourier transform resampling algorithm. Three CRGs of seismic sensors are used for Scholte-wave analysis. Raw sensor CRGs are rotated to the inline, crossline, and vertical coordinate system. The rotated tilt and roll angle are directed using the inner electric compass log value, and the shot inline azimuth is estimated using the particle motion method. Then, the velocity spectra are calculated from the inline and vertical components using the phase-shift method. Higher Scholte-wave modes dominate on the horizontal components, whereas the stronger fundamental mode dominates on the vertical components. The multicomponent velocity spectrum stacking method is adopted to produce the final dispersion energy image. Up to four modes of dispersion curves are retrieved within the 1.1–4.3 Hz frequency band. The multimode dispersion curve inversion is constructed for imaging the shallow sediments. The results suggest a low VS of 180–650 m/s and few lateral variations within the top 0.5 km of shallow marine sediments in the East China Sea. This model can provide an important reference for offshore geotechnical investigations, especially for OBN multicomponent seismic exploration data processing. The use of OBNs has high feasibility in VS imaging for shallow marine sediments when combined with the Scholte-wave dispersion-curve inversion.

We deployed the first dense onshore‐offshore seismic survey profile across the ultrahigh/high‐pressure metamorphic Sulu Orogenic Belt (SLOB). By applying multiple seismic and gravity modeling, we obtained the crustal P wave velocity and density model across the SLOB and extending into tectonic units on both sides, including the eastern Sino‐Korean Block (SKB), and the lower Yangtze Block (YZB). A ∼120‐km‐wide high‐velocity and high‐density upper crust in the southern SLOB reveals the distribution of ultrahigh/high‐pressure metamorphism, which should be linking with the deep subduction, detachment, and exhumation of YZB midcrust. In the northern SLOB, the U‐shaped velocity contours throughout the crust and the low‐density midcrust reveals the downward bending of the crustal deformation due to the collision between SKB and YZB. The crustal deformation and detachment in the SLOB revealed by our results indicate that the orogenic spatial distribution should be wider than what we know before.