Shear-wave velocity structure of the shallow sediments in the Bohai Sea from an ocean-bottom-seismometer survey

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 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 m/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 m/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 m/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.