Ambient Seismic Noise Tomography for the Seoul Metropolitan Area

Abstract:

There were several studies on crustal seismic velocity structures in the Korean Peninsula using surface wave tomography. Large-scale structures were generally resolved from the studies. However, fine-scale crustal structures still remain unclear. There exist complex fault systems and geological structures in the northern Seoul metropolitan area where microearthquakes occur frequently. The comprehension of seismic velocity structures is important for understanding the subsurface geological structures and seismic hazard mitigation. The Korean Peninsula presents a low seismicity, causing difficulty to perform a tomographic inversion based on passive sources. We apply ambient-noise tomography to image the seismic velocity structures. We use seismic data from the dense seismic array combined with 60 portable broadband seismometers and 17 permanent stations in Seoul metropolitan area. We collect continuous vertical seismic noises for 3 months from the stations. We calculate the correlation functions of ambient seismic noises for 2926 inter-station pairs, and determine the Green’s functions of Rayleigh waves. The group velocity dispersion curves of fundamental-mode Rayleigh waves between 0.1 and 5 Hz were measured using a multiple-filter technique. We obtain two-dimensional group velocity maps for discrete frequency bands using fast marching method and subspace inversion method. The 3-D shear-wave velocity model at depths ~20 km is inverted using the group velocities. The model is compared with geological features and results of other studies. The velocity structures present lateral heterogeneities that may illuminate geological structures. The velocity structures are generally correlated with the distribution of surface faults and microseismicity. A combined interpretation of 3-D velocity model and other features may suggest the subsurface fault structure.