5:45 PM - 6:00 PM
[G03-5-06] Continuous time variations in relative gravity and tilt, observed by a CG-3M gravimeter during the inflation event at Sakurajima Volcano on August 15, 2015
Continuous gravity observation is one of the most powerful methods to monitor mass redistributions in volcanoes. In Japanese volcanoes, absolute gravimeters have detected gravity changes of less than 10 microGal originating from volcanism, with those time period of more than a few days (e.g., Kazama et al., JGR, 2015). However, absolute gravimetry cannot precisely detect short-period (< one day) gravity changes due to the low signal-to-noise ratio in the high frequency domain. On the other hand, broadband volcanic phenomena have been monitored by other geodetic observations at many active volcanoes (e.g., Iguchi et al., JVGR, 2008). If the short-period volcanic gravity signals can be detected by continuous gravity observations other than absolute gravimerty, volcanic phenomena will be minutely discussed in terms of mass redistributions.
We were thus motivated to monitor continuous relative gravity changes at high time resolution in Sakurajima Volcano, one of the most active volcanoes in Japan. We installed a CG-3M relative gravimeter (serial number: 9403248) in September 2010, and started collecting relative gravity values at one-minute interval. After the corrections of gravity disturbances such as instrumental drift and tidal gravity changes, we succeeded in detecting a rapid gravity decrease of -5.86 microGal during the rapid inflation event on 15 August 2015. This gravity change is smaller than the typical observation error of relative gravimeters (~10 microGal), but the high-frequency measurements of relative gravity contributed to the detection of the small gravity change in the case of Sakurajima Volcano. We also found that the gravity change was consistent with one of the dike intrusion models provided by Geospatial Information Authority of Japan (2015) if the density value of 0.97 +/- 0.37 g/cm3 was assumed, which implies the drastic foaming of the intruded magma.
We were thus motivated to monitor continuous relative gravity changes at high time resolution in Sakurajima Volcano, one of the most active volcanoes in Japan. We installed a CG-3M relative gravimeter (serial number: 9403248) in September 2010, and started collecting relative gravity values at one-minute interval. After the corrections of gravity disturbances such as instrumental drift and tidal gravity changes, we succeeded in detecting a rapid gravity decrease of -5.86 microGal during the rapid inflation event on 15 August 2015. This gravity change is smaller than the typical observation error of relative gravimeters (~10 microGal), but the high-frequency measurements of relative gravity contributed to the detection of the small gravity change in the case of Sakurajima Volcano. We also found that the gravity change was consistent with one of the dike intrusion models provided by Geospatial Information Authority of Japan (2015) if the density value of 0.97 +/- 0.37 g/cm3 was assumed, which implies the drastic foaming of the intruded magma.