3:30 PM - 5:00 PM
[SGD01-P03] Continuous relative gravity change associated with atmospheric pressure change due to the Tonga's volcanic eruption
Keywords:LaCoste relative gravimeter, continuous gravity observation, Tonga volcanic eruption, atmospheric pressure change, Lamb wave, tidal gravity change
Continuous gravity observation is one of the most effective methods to understand spatiotemporal mass variations at the Earth's surface. Continuous gravity data have often been collected by superconducting and absolute gravimeters with high precision. However, only a small number of these gravimeters are installed in Japan, so it is difficult to maintain a continuous gravity observation network at all times. In contrast, there are many spring-type relative gravimeters in Japan, but their observation accuracy is said to be inferior to those of superconducting and absolute gravimeters. Although the gravity decrease of 5.86 microGal was observed by a continuous gravity observation with a Scintrex relative gravimeter during the inflation event at Sakurajima Volcano in 2015 (Kazama et al., 2016), no one has reported the detection of a smaller continuous gravity change using spring-type relative gravimeters. If such a small gravity signal of about 1 microGal is confirmed in relative gravity data, minute spatiotemporal gravity variations can be monitored by an observation network of spring-type relative gravimeters.
Therefore, we verified whether one of the spring-type relative gravimeters detected a small gravity change due to the atmospheric pressure change caused by the big eruption of Hunga Tonga-Hunga Ha'apai Volcano on January 15, 2022. The gravity change of approximately 0.5 microGal associated with the pressure change had been detected by a superconducting gravimeter installed in Matsushiro, Nagano Prefecture (Imanishi, 2022). Here we analyzed the continuous relative gravity data collected by the G31's LaCoste gravimeter at Kyoto University; the gravimeter outputs its spring position in voltage from its readout socket, and the voltage value is continuously recorded at the 1-Hz sampling rate and 0.1 mV voltage resolution. We first performed a tidal analysis using the Baytap08 software (Tamura and Agnew, 2008) on the continuous gravity data collected by the D58's LaCoste gravimeter at Kyoto University; the gravimeter is equipped with a feedback system and outputs the relative gravity value in microGal at the 2-Hz sampling rate and 1-microGal resolution. We also estimated a response coefficient (in microGal/mV) to convert the G31's observed value (in mV) to the gravity value (in microGal), by comparing the tidal gravity change of the G31's observed data with that predicted by the D58's tidal parameters. We then converted the G31's voltage data to the gravity data by using the response coefficient, and subtracted the predicted tidal gravity change from the G31's gravity data. We finally extracted the long-period component of the gravity data by applying a low-pass filter with a cutoff frequency of 0.005 Hz, and compared the gravity data with the pressure data collected at Kyoto University.
The response coefficient of the G31 gravimeter on January 15, 2022 was estimated to be approximately 0.4 microGal/mV, so the resolution of the gravity change is 0.4 microGal/mV * 0.1 mV = 0.04 microGal, which is smaller than the gravity signal observed by the superconducting gravimeter during the Tonga eruption (Imanishi, 2022). The long-period component of the G31's gravity data was found to decrease by approximately 0.6 microGal simultaneously with the ~2.0 hPa increase of atmospheric pressure associated with the passage of the Lamb wave. The response factor of the gravity change to the pressure change was -0.2791 microGal/hPa in the case of the Lamb wave, which is consistent with typical response factors (~ -0.3 microGal/hPa). These results indicate that minute gravity signals of less than 1 microGal can be detected in the voltage time series readout from common LaCoste relative gravimeters.
Therefore, we verified whether one of the spring-type relative gravimeters detected a small gravity change due to the atmospheric pressure change caused by the big eruption of Hunga Tonga-Hunga Ha'apai Volcano on January 15, 2022. The gravity change of approximately 0.5 microGal associated with the pressure change had been detected by a superconducting gravimeter installed in Matsushiro, Nagano Prefecture (Imanishi, 2022). Here we analyzed the continuous relative gravity data collected by the G31's LaCoste gravimeter at Kyoto University; the gravimeter outputs its spring position in voltage from its readout socket, and the voltage value is continuously recorded at the 1-Hz sampling rate and 0.1 mV voltage resolution. We first performed a tidal analysis using the Baytap08 software (Tamura and Agnew, 2008) on the continuous gravity data collected by the D58's LaCoste gravimeter at Kyoto University; the gravimeter is equipped with a feedback system and outputs the relative gravity value in microGal at the 2-Hz sampling rate and 1-microGal resolution. We also estimated a response coefficient (in microGal/mV) to convert the G31's observed value (in mV) to the gravity value (in microGal), by comparing the tidal gravity change of the G31's observed data with that predicted by the D58's tidal parameters. We then converted the G31's voltage data to the gravity data by using the response coefficient, and subtracted the predicted tidal gravity change from the G31's gravity data. We finally extracted the long-period component of the gravity data by applying a low-pass filter with a cutoff frequency of 0.005 Hz, and compared the gravity data with the pressure data collected at Kyoto University.
The response coefficient of the G31 gravimeter on January 15, 2022 was estimated to be approximately 0.4 microGal/mV, so the resolution of the gravity change is 0.4 microGal/mV * 0.1 mV = 0.04 microGal, which is smaller than the gravity signal observed by the superconducting gravimeter during the Tonga eruption (Imanishi, 2022). The long-period component of the G31's gravity data was found to decrease by approximately 0.6 microGal simultaneously with the ~2.0 hPa increase of atmospheric pressure associated with the passage of the Lamb wave. The response factor of the gravity change to the pressure change was -0.2791 microGal/hPa in the case of the Lamb wave, which is consistent with typical response factors (~ -0.3 microGal/hPa). These results indicate that minute gravity signals of less than 1 microGal can be detected in the voltage time series readout from common LaCoste relative gravimeters.