Data and early results from temporary seismic arrays for monitoring and investigating magmatic processes beneath Mt. Halla and Ulleung Island volcanoes, South Korea

Temporary seismic networks on Mt. Halla and Ulleung Island volcanoes were deployed, which employ broadband and geophone arrays to monitor potential volcanic activities and to estimate high-resolution magmatic structures beneath these volcanoes. The purpose of this paper is to introduce these networks and present early results through basic seismic analyses, suggesting the potential for future comprehensive seismological studies. The array in Mt. Halla volcano consists of five broadband sensors (JH array), and it has been operational around the Baengnokdam summit crater since October 2020. There was an additional linear geophone array (HL array) installed in September 2021 for detailed shallow subsurface imaging. Ulleung Island volcano had been under observation for two years since June 2021 with a network of nine broadband sensors (UL array) along its coast and in the Nari crater basin, complemented by a 52-geophone array (UG array) deployed in May 2022 for high-resolution subsurface studies. Despite the noisy environments typical of temporary setups, power spectral density analyses confirmed the quality of data as comparable to established reference noise models in permanent stations. Our study aimed to initiate studies uncovering seismic activities and structures beneath Mt. Halla and Ulleung Island volcanoes, specifically regarding volcanic activity. This approach detected no clear sign of volcanic seismicity on both islands, suggesting a period of magmatic dormancy. Seismic velocity variation (dv/v) analyses further indicated that environmental factors, rather than volcanic processes, influenced the changes in the physical properties of the underground structures. Conversely, the receiver function analysis and ambient noise data processing hinted at the presence of complex subsurface structures, potentially indicative of volcanic features, such as partial melting. Despite the lack of direct evidence for active magmatic processes, the collected seismic data provides a crucial baseline for future monitoring and a deeper understanding of the magmatic and tectonic dynamics beneath these volcanoes, offering valuable insights for ongoing volcanic research.