Shear-wave velocity structure of Jeju Island, Korea

To reveal shear-wave velocity (v (s) ) structures of Jeju Island, the spatial autocorrelation (SPAC) method was applied to microtremors recorded at six sites on the island. At each site, four 3-component portable broadband seismometers were deployed at receiver intervals of 50-5944 m in a linear or an irregular array. SPAC functions of the vertical-component data were used to obtain dispersion curves defining phase velocities of Rayleigh waves. Shear-wave velocities derived by inverting the dispersion curves were compared with core data from nearby deep wells. The inversion models around the periphery of Mt. Halla indicate the existence of a low-velocity layer (LVL) beneath the extrusive volcanic outcrops of Quaternary age. This LVL correlates with Pleistocene marine sediments of the Seoguipo and U Formations that were sampled in nearby wells. Beneath the LVL at these sites, v (s) increases gradually with depth in welded tuffs that overlie a Cretaceous-Paleogene granite. The horizontal-to-vertical spectral ratio method was also applied to microtremor data to estimate depths to granitic basement rocks. The depths to the granitic basement rocks increase toward the center of the island, reaching a maximum of approximately 3.2 km below the summit of Mt. Halla.