5:15 PM - 6:45 PM
[SGD01-P03] Continuous relative gravity change in 1-10 mHz band associated with the 2022 Tonga volcanic eruption
Keywords:LaCoste relative gravimeter, continuous gravity observation, Tonga volcanic eruption, atmospheric pressure change, Earth’s free oscillations
The Hunga Tonga-Hunga Ha’apai volcano erupted at around 13:00 JST on January 15, 2022. Atmospheric waves were excited due to the eruption, and their propagation was observed as air pressure changes around the world. Imanishi (2022) analyzed the continuous gravity data collected by the superconducting gravimeter iGrav-028 at Matsushiro, and found the gravity change of ~0.5 microGal associated with the air pressure change due to the eruption. He also modeled the gravity change by applying the theory of acoustic-gravity wave proposed in Zürn and Wielandt (2007), and showed that the inertial effect due to atmospheric loading was clearly detected. However, he applied the low-pass filter with a cutoff frequency of 2.5 mHz to the original gravity data, so he did not discuss any gravity changes in the higher frequency band. Since the amplitude of the inertial effect is proportional to the frequency, inertial gravity signals can be detected in the frequency band higher than 2.5 mHz more pronouncedly.
Oda and Kazama (2023) analyzed the 1-Hz continuous data of relative gravity and air pressure, which have been collected by the LaCoste-G31 relative gravimeter and a BME280 pressure sensor, respectively, at Kyoto University. After applying the 1.0-mHz low-pass filter to each data obtained on January 15, 2022, they found that the gravity changed by ~0.6 microGal as the air pressure changed due to the propagation of acoustic-gravity wave. They also showed that the relative gravity responded to the air pressure change with a factor of about -0.3 microGal/hPa, mainly due to the temporal variation in the Newtonian attraction caused by the spatiotemporal changes in atmospheric mass distribution. However, they did not discuss any gravity signals in the higher-frequency band, nor did Imanishi (2022). Since LaCoste gravimeters can observe broadband gravity changes, the inertial effect described above will be assessed in more detail by extracting the higher-frequency gravity changes from the gravity data collected by the G31 gravimeter.
Therefore, we investigated the characteristics of the gravity changes in the frequency band of 1-10 mHz associated with the Tonga volcanic eruption, by analyzing the continuous relative gravity data collected by the G31 gravimeter on January 15, 2022. We first extracted the millihertz band component of gravity data by applying a band-pass filter with a frequency band of 1-10 mHz to the original gravity record. We also calculated the spectrum for the 12-hour record of the continuous gravity data just after the eruption. We found that the gravity in the 1-10 mHz band started changing around 14:00 JST and reached a maximum amplitude of 0.64 microGal at 21:10 JST. In addition, we detected spectral peaks corresponding to 0S29, 0S30, and 0S39 of the solid Earth normal modes in the spectrum of the gravity time series. Ringler et al. (2023) reported that the long-period Rayleigh waves were excited with unique frequencies of ~3.7 mHz and ~4.6 mHz during the Tonga volcanic eruption, so our result indicates that the G31 gravimeter observed the normal mode oscillations of the Earth associated with the Rayleigh wave propagation.
Next, we created synthetic gravity waveforms using the air pressure data collected from 20:00 to 21:30 JST on January 15, 2022, according to Zürn and Wielandt (2007). We found that the synthetic gravity change agrees with the observed one in the 1-10 mHz band if we use the elastic constants of the upper crust in PREM (Dziewonski and Anderson, 1981) for the calculation of synthetic waveforms. We also confirmed based on the theory of Zürn and Wielandt (2007) that the inertial effect has the amplitude of 0.49 to 4.89 microGal/hPa at 1-10 mHz, which is significantly larger than the amplitudes of the Newtonian and free-air effects. These results indicate that the gravity change in the 1-10 mHz band observed by the G31 gravimeter can be explained mainly by the inertial effect of the atmospheric loading.
Oda and Kazama (2023) analyzed the 1-Hz continuous data of relative gravity and air pressure, which have been collected by the LaCoste-G31 relative gravimeter and a BME280 pressure sensor, respectively, at Kyoto University. After applying the 1.0-mHz low-pass filter to each data obtained on January 15, 2022, they found that the gravity changed by ~0.6 microGal as the air pressure changed due to the propagation of acoustic-gravity wave. They also showed that the relative gravity responded to the air pressure change with a factor of about -0.3 microGal/hPa, mainly due to the temporal variation in the Newtonian attraction caused by the spatiotemporal changes in atmospheric mass distribution. However, they did not discuss any gravity signals in the higher-frequency band, nor did Imanishi (2022). Since LaCoste gravimeters can observe broadband gravity changes, the inertial effect described above will be assessed in more detail by extracting the higher-frequency gravity changes from the gravity data collected by the G31 gravimeter.
Therefore, we investigated the characteristics of the gravity changes in the frequency band of 1-10 mHz associated with the Tonga volcanic eruption, by analyzing the continuous relative gravity data collected by the G31 gravimeter on January 15, 2022. We first extracted the millihertz band component of gravity data by applying a band-pass filter with a frequency band of 1-10 mHz to the original gravity record. We also calculated the spectrum for the 12-hour record of the continuous gravity data just after the eruption. We found that the gravity in the 1-10 mHz band started changing around 14:00 JST and reached a maximum amplitude of 0.64 microGal at 21:10 JST. In addition, we detected spectral peaks corresponding to 0S29, 0S30, and 0S39 of the solid Earth normal modes in the spectrum of the gravity time series. Ringler et al. (2023) reported that the long-period Rayleigh waves were excited with unique frequencies of ~3.7 mHz and ~4.6 mHz during the Tonga volcanic eruption, so our result indicates that the G31 gravimeter observed the normal mode oscillations of the Earth associated with the Rayleigh wave propagation.
Next, we created synthetic gravity waveforms using the air pressure data collected from 20:00 to 21:30 JST on January 15, 2022, according to Zürn and Wielandt (2007). We found that the synthetic gravity change agrees with the observed one in the 1-10 mHz band if we use the elastic constants of the upper crust in PREM (Dziewonski and Anderson, 1981) for the calculation of synthetic waveforms. We also confirmed based on the theory of Zürn and Wielandt (2007) that the inertial effect has the amplitude of 0.49 to 4.89 microGal/hPa at 1-10 mHz, which is significantly larger than the amplitudes of the Newtonian and free-air effects. These results indicate that the gravity change in the 1-10 mHz band observed by the G31 gravimeter can be explained mainly by the inertial effect of the atmospheric loading.