Shallow seismic velocities in the summit crater of Mt. Halla on Jeju Island, Korea

To define shallow velocity structures in the summit crater, Baekrokdam, of Mt. Halla, seismic refraction and multi-channel surface-wave surveys were conducted along a northwest-southeast 195-m profile in June 2010. Small seismic signals were generated at 54 source locations by pounding a 5-kg sledgehammer on an aluminum plate. The signals were received by 24 4.5-Hz vertical geophones installed on the ground at 5-m intervals. P-wave velocities were derived by inversion of first-arrival times. Inversion of Rayleigh-wave dispersion curves using the common-midpoint cross-correlation method yielded one-dimensional S-wave velocity profiles that were combined to obtain a two-dimensional velocity section. A surficial layer of 11 +/- 3 m thickness has P-wave velocities less than 1200 m/s. S-wave velocities less than 100 m/s in an approximately 1-m interval at depths of 2-3 m and a steep increase in the velocity gradient near 5 m deep may indicate water-saturated soils that overlie an impermeable layer of sediments such as may account for the presence of the nearby pond in the crater. P-wave velocities between 1200 and 1500 m/s extending to depths of approximately 30 m in the northwest may indicate extensive weathering of the underlying Hallasan Trachyte. Near the southeastern end of the profile, higher dynamic Poisson's ratios may indicate the presence of volcanoclastic sedimentary rocks beneath the Baekrokdam Trachybasalt.