| dc.description.abstract | Okinawa Trough is the back-arc basin formed behind the Ryukyu arc-trench system. In such extensional environments, magmatism and hydrothermal activities are very common. Many gas plumes and submarine volcanoes have been detected in the South Okinawa Trough (SOT), and several potential hydrothermal sites are mapped based on the various geophysical and geochemical survey data, which include Fire Dragon Volcano 1 (FDV-1), Fire Dragon Volcano 2 (FDV-2), Geolin Mounds (GLM), Penglai Fault Zone (PFZ), and Yonaguni Knoll IV (YK4-1). In order to monitor hydrothermal activity in the southwestern part of the SOT, a network of 6 OBSs was deployed during the OR2-2231 cruise in 2017, and the recording period was from April 7, 2017 to April 18, 2017. Harmonic tremor, a seismic signal that is generally associated with volcanic activity, was recorded at three of the OBSs. In this study, the methods used to study harmonic tremor include time-frequency analysis, model simulation, source direction analysis and quality factor estimation.
The result of time-frequency analysis of different OBSs demonstrates similar pattern, which shows a fundamental frequency of about 4 Hz. Each harmonic tremor event was not recorded in more than one station despite the fact that the distance between the stations is less than 7 km, suggesting that these signals should have originated very close to each receiver. In order to understand the source properties, the model simulation method proposed by Girona et al. (2019) was applied in this study. The result suggests that gas supply must contain certain degree of periodicity for the generation of harmonic tremor, and the variation of the dominant frequency may infer the condition of the hydrothermal sites. The source direction determined from the largest energy distribution shows a clear correlation with the gas plumes and submarine volcanoes position, inferring their causal relationship. However, the time-dependent variation for the source direction and the fundamental frequency may be associated to the different activity level of different sources or the fluid migration in the hydrothermal systems. Finally, the Q values are mainly lower than 20, which suggests that the composition of the hydrothermal fluids is stable during the studied period. | en_US |