2:30 PM - 2:45 PM
[SCG47-04] Distribution of submarine hot spring discharge and their fluctuation in temprature along the coast of Sakurajima volcano
★Invited Papers
Keywords:ground water, temperature logging, Submarine ground water discharge, crustal fluid
In active volcanoes, there are various monitoring techniques to observe their volcanic activity that further provides us clues for hazard prediction. Eruption activity and ejecta provide relatively low-frequency intermittent data, thus acquiring higher resolution data becomes crucial. Observations of crustal activity using instruments such as seismometers, inclinometers, and extensometers have proven successful in capturing subtle changes in activity. Fluids are also being studied as a more detailed monitoring target. While volcanic gas sampling from craters poses challenges, the accessibility of hot springs and groundwater at the foothills gives advantages. We are focusing on hot spring and groundwater in the vicinity of Sakurajima, one of the most active volcanoes. While the distribution of hot springs and groundwater on land are limited, it appears that hot spring discharge under the sea along the coast is hardly known. Therefore, in this study, we investigated the emergence and distribution of volcanic fluids (submarine hot springs) along the coast of Sakurajima and further observed fluctuations in these fluid parameters.
To elucidate the distribution of submarine hot springs, we conducted interviews and field surveys. We obtained specific information from scientific diving experts and fisheries personnel along the coast of Sakurajima. Additionally, we conducted on-site surveys mainly at the evacuation ports, conducting visual inspections. Based on this information, we created a map of submarine hot spring distribution consisting of 12 discharge points covering almost the entire coastal area of Sakurajima. From this map, we selected four observation points, St. 1 to 4. At these observation points, we filmed, photographed, collected water samples, conducted measurements (pH, electrical conductivity), and installed temperature loggers. Temperature measurements using loggers were conducted from December 7, 2023, to February 10, 2024.
Submarine discharge fluids we recovered show clearly lower pH and higher temperature than surrounding seawater which suggests that their origin is from volcanic thermal sources. Results from continuous temperature observations using loggers revealed unique trends at each observation point. At St. 1, a temperature increase of approximately 15 degrees Celsius was recorded over a period of about 4 days in mid-December. The periods before and after were characterized by relatively constant temperatures. St. 2 to 4 exhibited short-term oscillations, with St. 2 and 3 showing large swings towards higher temperatures and St. 4 towards lower temperatures. This should be the influence of tides, with the former indicating temperature increases due to shallowing and associated depletion of seawater components and the latter indicating temperature decreases due to exposure during low tide leading the influence of air temperature. Focusing on baseline fluctuations excluding tidal effects, a pulsating trend with a period of approximately 14 to 20 days was observed. These variations are thought to reflect some volcanic or crustal activity on the way in the flow path of the fluid at each observation point, rather than regional environmental factors such as tides or weather conditions.
To elucidate the distribution of submarine hot springs, we conducted interviews and field surveys. We obtained specific information from scientific diving experts and fisheries personnel along the coast of Sakurajima. Additionally, we conducted on-site surveys mainly at the evacuation ports, conducting visual inspections. Based on this information, we created a map of submarine hot spring distribution consisting of 12 discharge points covering almost the entire coastal area of Sakurajima. From this map, we selected four observation points, St. 1 to 4. At these observation points, we filmed, photographed, collected water samples, conducted measurements (pH, electrical conductivity), and installed temperature loggers. Temperature measurements using loggers were conducted from December 7, 2023, to February 10, 2024.
Submarine discharge fluids we recovered show clearly lower pH and higher temperature than surrounding seawater which suggests that their origin is from volcanic thermal sources. Results from continuous temperature observations using loggers revealed unique trends at each observation point. At St. 1, a temperature increase of approximately 15 degrees Celsius was recorded over a period of about 4 days in mid-December. The periods before and after were characterized by relatively constant temperatures. St. 2 to 4 exhibited short-term oscillations, with St. 2 and 3 showing large swings towards higher temperatures and St. 4 towards lower temperatures. This should be the influence of tides, with the former indicating temperature increases due to shallowing and associated depletion of seawater components and the latter indicating temperature decreases due to exposure during low tide leading the influence of air temperature. Focusing on baseline fluctuations excluding tidal effects, a pulsating trend with a period of approximately 14 to 20 days was observed. These variations are thought to reflect some volcanic or crustal activity on the way in the flow path of the fluid at each observation point, rather than regional environmental factors such as tides or weather conditions.