VARIATION IN THICKNESS OF THE LITHOSPHERE IN SOUTHERN KORAN PENINSULA INFERRED FROM TELESEISMIC P-WAVE TRAVELTIMES

To investigate variations in thickness and anisotropy of the lithosphere in southern Korean Peninsula, we analyzed P-wave traveltimes of 80 teleseismic events (Mw > 6.0) with epicentral distances of 30-90° occurred in 2016. Based on the IASP91 velocity model, P-wave traveltimes of the events were computed for 90 seismic stations on the Korea Meteorological Administration network. To reduce both high- and low-frequency seismic noise, we applied a frequency filter with a passband of 0.2 to 4.0 Hz to the 100-Hz continuous data before determination of P-wave traveltimes. Then static corrections for sensor elevations were applied using a velocity of 5.0 km/s which may represent P-wave velocities in the upper 1 km of depth. Total 2,302 P-wave time differences (P residuals) were obtained by subtracting observed P arrivals from the corresponding calculated time. Source effects and other event specific uncertainties were removed by subtracting the event mean from the P residuals at each station. Then the station mean of source-effect corrected P residuals for all events was computed to analyze variation in thickness of the lithosphere. Nearly all P residuals are negative with an average of -0.85 s, indicating relatively thicker lithosphere than the normally assumed based on the IASP91 model. Tectonically, the minimum P residual of -0.97 s is in Gyongsang basin and the maximum of -0.67 s is in Gyonggi massif. The relatively small absolute value in Gyonggi massif may relate to a root removal during the Mesozoic cratonic destruction. Azimuthal dependence of P residuals were also derived by fitting P residuals to sinusoidal curves as a function of back azimuth at each station. Significant azimuthal dependence was observed with early arrivals from the south and later arrivals from the north. The anisotropy direction may be not exact due to the limited azimuthal coverage.