Decomposed SH-, SV-, and P-wavefields and their visualizations for interpretation of reproduced synthetic waves in the Osaka sedimentary basin, Japan, due to a Mw 5.6 earthquake beneath its edge

Abstract We decomposed complex synthetic wavefields in an inhomogeneous sedimentary basin into P-, SV-, and SH-wavefields, and quantitatively evaluated the amplitude, propagation velocity, and propagation direction of coherent waves in each decomposed wavefield within the 0.125–1 Hz frequency band. In sedimentary basins with irregular subsurface structures, P-, SV-, and SH-waves can coexist at the same location and time, propagating as either body waves or surface waves, where SH-waves manifest as Love waves and P- and SV-waves combine to form Rayleigh waves. The relative amplitudes of these wave types depend on both the source radiation pattern and the subsurface geometry. To accurately evaluate the propagation characteristics, such as amplitude and directional variation, of each wave type, it is necessary to first decompose the wavefield by wave type. To date, no studies have addressed this issue from such a perspective. We fully decomposed the reproduced strong-motion waveforms from the 2018 Mw 5.6 earthquake beneath the margin of the Osaka sedimentary basin in Japan—the target event of this study—into P-, SV-, and SH‑wave components using Helmholtz decomposition. By applying semblance analysis to the decomposed wavefields, we quantitatively evaluated the propagation processes of each wave type in the three-dimensional sedimentary basin. Using the derived propagation characteristics, we conducted pseudo-trajectory analysis (PTA) to visualize wave propagation paths, analogous to streamlines in fluid dynamics. We noted spatial differences in the SH‑ and SV‑wavefields. For example, during an early time window, ground motions were oriented northwest–southeast on both sides of the north–south fault zone in the Osaka Plain. These motions result from southwestward-propagating SH-waves in the western region and southeastward-propagating SV-waves in the eastern region. Later, in the western region, Love waves dominated in the 0.125–0.25 Hz band, while Rayleigh waves dominated in the 0.25–0.5 Hz band. The spatiotemporal amplitude variations of these wave types depend on the combined effects of the source radiation pattern and the subsurface structure as noted above. The proposed method can also be applied to identify, where and what types of waves are likely to be generated. Graphical Abstract