Ambient noise tomography of crustal shear-wave velocity structure in the Seoul metropolitan area

Seoul is the capital city of South Korea with a population of ~10 million. The crustal seismic velocity structure in the Seoul metropolitan area is poorly known. Understanding velocity structure is important for studying subsurface geological structure and seismic hazard estimation. The Korean Peninsula presents a low occurrence rate of major earthquakes, which causes difficulty to perform regional surface-wave tomography based on passive sources. It is useful to apply an ambient-noise cross-correlation analysis to image a seismic velocity structure for the area. We perform a high resolution surface-wave tomography based on ambient noises, and illuminate the shear-wave velocity structure. We collect continuous vertical seismic noises for 3 months at 49 stations (39 broadband velocity stations, 10 accelerometer stations) that were deployed over Seoul metropolitan area. We calculate the cross correlation for 1176 inter-station pairs, and obtain the Green's functions of Rayleigh waves. The group velocity dispersion curves of fundamental-mode Rayleigh waves between 0.2 Hz and 5 Hz were measured using the multiple-filter technique. We obtain two-dimensional group velocity maps for discrete frequency bands using a fast marching method. We construct a 3D shear velocity model based on the 1D velocity structures for grid cells. The 3D velocity model is correlated with known geological features and surface topography. The velocity model may be interpreted to be subsurface geological structures. The 3D velocity model can be confirmed by comparison with results from receiver function analysis. The 3D velocity model may be useful to indicate potential locations of subsurface faults in the Seoul metropolitan area with combination of additional information such as micro-seismicity and seismic exploration.