1D velocity structure beneath broadband seismic stations in the Cretaceous Gyeongsang Basin of Korea by receiver function analyses

Abstract

The crustal velocity structures beneath four broadband seismic stations (GKP, GSU, HDB, and BUS) in the Cretaceous Gyeongsang Basin, southeastern Korea, are estimated by using receiver function analyses employing teleseismic waveforms. The genetic algorithm is adopted to avoid the inherent non-uniqueness problem of the inversion. The inversion results are constrained by surface-wave dispersions to complement the shortcoming of the receiver function. The selected records of earthquakes distributed in three quadrants from each seismic station in the years from 2002 to 2005 are analyzed. Although limited quantity of data is used due to short operation periods, the resolution and confidence level are expected to be better than those using less data. The Moho depth under GKP is estimated to be 30 km showing no distinctive indication of dipping. This depth is well coincident with those in nearby velocity cross-sections obtained from crustal-scale seismic profiles compared to the 36-km deep Moho previously reported by other independent work. At GSU the Moho appears at the 32-km depth and a distinct low-velocity anomaly is detected at the depth of about 10 km. They agree with those in the velocity tomogram of the nearby survey line. An adakitic intrusion which results from the partial melting of a young and hot subducted oceanic crust in the basin during the Cretaceous is suggested as a possible geologic interpretation of the low-velocity zone. The Moho beneath HDB is 28-km deep, and agrees with those in nearby velocity cross-sections obtained from crustal-scale seismic profiles. This Moho depth at HDB is rather shallower than those of other stations. The significant P S phase amplitude and arrival time differences in the radial receiver functions conclude the Moho to dip southwestwardly. This is supported by polarities of direct P and P S in transverse receiver functions. Beneath BUS, P wave velocity increases gradually from the 32-km depth and it reaches 7.6 km/s at the 36-km depth. The Moho discontinuity beneath BUS is thought to be at an around 35-km depth.