# Tertiary Waves Measured during 2017 Pohang Earthquake Using an Underwater Glider
## 1 Introduction  
Seismic energy from submarine earthquakes is converted into acoustic energy at the seafloor-water boundary. A Tertiary wave (or T-wave) is the acoustic signal from these earthquakes. A T-wave typically has frequencies ranging from 4 to 50 Hz. T-waves propagate efficiently in the ocean compared to seismic waves through the earth and can be detected at great distances. Underwater sound observation techniques can directly observe oceanic earthquakes, landslides, volcanic eruptions, and corresponding underwater explosions in the ocean. Although not many, there have been some reports of direct observations of hydro-acoustic waves due to underwater seismic activity from an offshore observatory [1; 2; 3; 4]. These are the first direct measurements ever performed in a tsunami generation area.  
It is becoming increasingly important to acquire low-cost, high-efficiency, small-scale, unmanned, direct observation technology to replace existing high-cost, low-efficiency, large-scale, manned exploration platforms such as marine research vessels and stations in [1; 2; 3; 4; 5; 6]. Unmanned observation platforms such as Argo floats, Underwater Gliders, WaveGliders, Autonomous Underwater Vehicles and Autonomous Surface Vehicles have been widely used in marine surveys and military defense operations. Among these platforms, underwater gliders have been actively used for measurement of environmental underwater ambient noise. The addition of a single hydrophone sensor to an underwater glider provides a capability for measurement of underwater noise level [7; 8; 9; 10; 11; 12; 13; 14] and the addition of a hydrophone array enables surveillance of the underwater acoustic environment [15; 16].  
Research programs using these technologies have been conducted to investigate geophysical causes of seafloor earthquakes, landslides, volcanic eruptions and corresponding underwater explosions using hydrophone recorders [17; 18], an underwater glider [19]. Reference [19] observed underwater sounds at depths of 200 to 600 m using an underwater glider at a distance of 10 km from the Lau Basin submarine volcano. The sound frequency due to the volcano was 10-300 Hz, and the sound level was \(-25\) dB higher than the ambient noise level. The prototype and concept as ocean surface gateway (OSG) using WaverGliders for retransmitting data of ocean bottom package to shore were demonstrated in [20].  
In this paper we report the analysis of earthquake sounds recorded by an underwater glider equipped with a single hydrophone during the magnitude 5.5 earthquake of 15 November 2017, at Pohang in the Republic of Korea. The underwater glider measured the earthquake sounds at about 140 km from the epicenter, in a water depth of about 200 m during an underwater noise measurement survey from Uljin to Geoseong along the east coast of South KOREA. In order to distinguish the earthquake sound from the glider's operation noise, the noise sources and Sound Pressure Level (SPL) of the underwater glider were analyzed and measured. Next, the earthquake sounds measured by the gilder was analyzed in time and frequency domain. Finally, the time correlation of the seismic waves and earthquake sounds observed by the seismic land station and the glider were described.