Seismic noise level variation in South Korea

Abstract

Variations of seismic background noise in South Korea have been investigated using power spectral analysis. We have estimated the power spectral density of seismic noise for 30 broadband stations from 2005 to 2007. In the frequency range 1–5 Hz, diurnal variations of noise level and daily differences in daytime noise level are observed at most stations, suggesting that cultural activities contribute to the noise level of a station. The variation in the number of stations detecting an event, however, suggests that cultural noise has little influence on the detection capability of events over magnitude 2.0. In the frequency range 0.1–0.5 Hz, a double-frequency (DF) peak is dominant at all stations. Clear seasonal variations of peaks are observed, while much less shown in the frequency range 1–5 Hz. DF noise levels are higher in winter than in summer. Strong DF peaks occur in summer when Pacific typhoons near the Korean Peninsula. The discrepancy in time between DF peaks at seismic stations and peaks of significant wave height at buoys, as well as the decrease of DF peaks with increased latitude, indicate that the likely source region of DF peaks is located in the Southern Sea of Korea.