3D P and S-wave tomography around the southern part of the Korean peninsula

A 3D tomographic inversion method is applied using earthquake data to investigate the characteristics of P- and S-wave velocities in a southern part of the Korean peninsula. The study area ranges from 34°N to 39°N in latitude and from 125°E to 131°E in longitude. To set up the initial velocity model for the 3D tomographic inversion, "minimum 1D model" was estimated from five previously published models. The minimum 1D model shows two major discontinuities at depths of 15 km and 35 km. It is assume that the discontinuity at the 15-km depth is the boundary between the upper and the lower crusts, and that at the 35-km depth is the Moho discontinuity between the crust and the upper mantle. Utilizing the optimal 1D reference model as an initial condition, a total of 10,332 data items (6,454 P-arrivals and 3,878 S-arrivals from 601 events) is used for the 3D travel time tomographic inversion. According to the results of P- and S-wave velocity images obtained at the depth of 17 km, high-velocity anomalies are developed mainly along tectonic boundaries in the middle part of Okcheon belt and northeastern part of Gyeongsang basin and Yeongnam massif. This may correspond to the result from a wide emplacement of intrusive granite by orogenic movements. It is believed that the Moho depth of 37 km at Yeongnam massif is deeper than that of Gyeonggi massif, since the velocity pattern of S-wave around Yeongnam massif clearly shows a low-velocity distribution. In general, the vertical cross section tends to show that P-wave velocity increases with increasing depth and relatively low velocity zones exist at the Gyeongsang basin and along the boundary between Yeongnam massif and Okcheon belt in NE direction. The Gyeongsang basin can be clearly bounded by thickness of low velocity anomaly. From the west boundary of Gyeongsang basin the low velocity anomaly tends to abruptly become thicker to the east. This thicker low-velocity anomaly seems to develop along the NE direction of the boundary between Gyeongsang basin and Yeongnam massif. The low-velocity anomaly at the west side is thicker than that at the east side of Gyeongsang basin.