# Tertiary Waves Measured during 2017 Pohang Earthquake Using an Underwater Glider
## 3 Results
### Pohang 2017 Earthquake: Analysis of in Situ Observations
#### 3.2.3 Time and Frequency Analysis of Earthquake Underwater Sound  
It is most common for noise signals to be represented in the frequency domain as noise spectral levels plotted as a function of frequency [25; 26]. The data for ambient noise is usually expressed as spectral density (SD) levels or power spectral density (PSD), where the data in each frequency band has been normalized by dividing by the bandwidth of the frequency band. The units of the levels in each band are then dB re 1 uPa2/Hz. We used the following analysis method to distinguish earthquake underwater sounds. Extraction of necessary timing data from the recording files; Calculation of Power Spectrum Density (PSD) using a time window of chosen length in each recording. The length and overlap of FFT window for 1s and 50 %, respectively; Correction of PSD for recorder's sensitivity, gain and, when possible, frequency response to get the output data in sound pressure units (uPa).  
Figure 8 shows PSD of a 1 h recording started at 05:29:35 (UTC) collected by the glider when the earthquake occurred at 05:29:32 (UTC). It contains underwater earthquake sound as well as the  
Figure 7: Underwater glider survey track (red line) map (seismic station: (**a**) Busan (BUS), (**b**) Chungju (CHI)), (**c**) Namwon (NAWB), (**d**) Seawha (SEHB), (**e**) Seoul (SEC)).  
operating noise of the glider's machinery mentioned in Section 3, e.g., piston and pitch motors, fin, airbladder, altimeter, pumped CTD. The glider descended with pumped CTD and altimeter turned on until it reached the turning depth (200 m), and it then turned on the piston and pitch motor for ascending and inflated the external air bladder to begin surfacing for communication. The pumped CTD and altimeter were turned off when the glider was ascending. The earthquake sounds were dominated below about 50 Hz.  
The time sequence of the acoustic signal has been reconstructed, from the wav-file records starting at 05:29:35 (UTC) in Figure 9a. This time is the start reference time \(t_{0}\) = 0 s (i.e., when the recording starts at 05:29:35); 16 s later (i.e., \(t_{1}\) = 05:29:51), is the time of first arrival of the earthquake sounds at the glider's position, at about 200m depth. The arrival time (\(t_{1}\)) at about 19 s from earthquake correspond to about 7 km/s which may be the speed of seismic waves in the crust just up to the bottom of detection point, then in the water column and finally detected by the underwater glider. These sounds were observed for over 30 s.  
Spectrum and spectrogram analyses of the earthquake sound have been carried out. By carefully examining the frequency spectrum of the record in Figure 9b, it can be seen that the dominant frequency of the earthquake sounds was about 10 Hz. Figure 9c shows that after the earthquake occurred, the average ambient noise level at frequencies below 50 Hz was approximately 60 dB higher than the ambient noise level before the earthquake occurred. The earthquake acoustic signal can be distinguished from glider's self-noises of fin, pumped CTD and altimeter. The fin noise exists at frequencies of 800-950 Hz every 3 to 4 s. The pumped CTD sensor noise exists at discrete frequencies of 357, 410, 710, 1450 Hz. The dominant CTD noise frequency is 710 Hz. The altimeter noise exists at frequencies above 100 Hz.  
Figure 8: Spectrogram of earthquake underwater sound recording (started at 05:29:35 UTC).