Discrimination of small earthquakes and artificial explosions in the Korean Peninsula using Pg/Lg ratios

Abstract

The purpose of this study is to develop a technique to discriminate artificial explosions from local small earthquakes (M ≤ 4.0) in the time-frequency domain. In order to obtain spectral features of artificial explosions and earthquakes, 3-D spectrograms (frequency, time and amplitude) have been used. They represent a useful tool for studying the frequency content of entire seismic waveforms observed at local and regional distances (Kim, Simpson and Richards 1994). P and S(Lg) waves from quarry blasts show that the frequency content associated with the dominant amplitude appears above 10 Hz and Rg phases are observed at close distances. P and S(Lg) waves from the Tongosan earthquake have strong amplitudes below 10 Hz. For the Munkyong earthquake, however, a broader frequency content up to 20 Hz is found. For discrimination between small earthquakes and explosions, Pg/Lg spectral ratios are used below 10 Hz, and through spectrogram analysis we can see different frequency contents of explosions and earthquakes. Unfortunately, because explosion data recorded at KSRS array are digitized at 20 sps, we cannot avoid analysing below 10 Hz because of the Nyquist frequency. In order to select time windows, the group velocity was computed using multiple-filter analysis (MFA), and free-surface effects have been removed from all three-component data in order to improve data quality. Using FFT, a log-average spectral amplitude is calculated over seven frequency bands: 0.5 to 3, 2 to 4, 3 to 5, 4 to 6, 5 to 7, 6 to 8 and 8 to 10 Hz. The best separation between explosions and earthquakes is observed from 6 to 8 Hz. In this frequency band we can separate explosions with log(Pg/Lg) above -0.5, except EXP1 recorded at SIHY1-1, and earthquakes below -0.5, except the Munkyong earthquake record at station KMH.