4:45 PM - 5:00 PM
[SVC41-36] Spatiotemporal gravity variations from the 1970s in Sakurajima Volcano, revealed by repeated relative gravity measurements
Keywords:Sakurajima Volcano, relative gravity, crustal deformation, mass variation, magma, hydrology
On the other hand, gravity measurements at leveling points in and around Sakurajima Volcano were started in 1975 (Tajima et al., 1975). Campaign relative gravity measurements have been repeated 19 times as of February 2018, and continuous gravity data have also been collected at a few gravity points these days. However, long-term (> 10 years) gravity variations in the volcano have never been discussed until now, while gravity changes within 10 years have modeled by some researchers (e.g., Ishihara et al., 1986; Okubo et al., 2013; Kazama et al., 2016). Long-term volcanic mass variations should be discussed from the past gravity data, because it can help to forecast future volcanic activities of Sakurajima Volcano.
We therefore estimated the long-term gravity variations in Sakurajima Volcano by reprocessing the relative gravity data collected from 1975 to 2017, in order to understand the long-term mass redistributions under the volcano in the same period.
We first collected a set of original gravity data stored in Sakurajima Volcanic Observatory, and computerized gravity values in the data set which were measured from the 1970s to the 1990s. We then corrected gravity changes due to instrumental height, tide, and instrumental drift from the newly computerized gravity data, along with the already computerized gravity data collected in the 2000s. We finally estimated gravity variation rate for each gravity site from the least square method, using the corrected gravity values.
Arrows and ellipses in the figure (a) indicate the gravity variation rates and those standard deviations from the 1970s to 1990s in Sakurajima Volcano. A large gravity increase of up to +8.0 microGal/yr is clearly seen at gravity sites located in the western side of the volcano. The spatial pattern of the gravity variation rates can be modeled by a point mass of +3.0 E9 kg/yr at 1300 m below sea level (b.s.l.) of the western Sakurajima with a deflation source of -1.2 E6 m3/yr at 1800 m b.s.l. of the Minami-dake crater (Hirayoshi, master thesis).
The figure (b) also shows the gravity variation rates from the 2000s to the 2010s in the volcano. The gravity increase in the western Sakurajima during this period is smaller than that from the 1970s to 1990s by about one third, and the largest gravity change of +6.3 microGal/yr is seen at S423, located in the northern hillside of Sakurajima. These results imply that mass redistribution systems under the ground of Sakurajima Volcano changed around 2000. We will model mass redistributions in Sakurajima Volcano physically, by considering gravity changes outside the volcano and vertical ground deformations obtained from leveling surveys.