5:15 PM - 7:15 PM
[SVC32-P15] Re-evaluation of dike volume change under Asama Volcano during the 2004 eruptions
Keywords:Asama Volcano, eruption, dike, baseline length, GNSS, conduit
Mt. Asama (2568 m above sea level) is an andesitic stratovolcano located on the border of Gumma and Nagano prefectures. The magma plumbing system of Asama Volcano consists of the vertical magma conduit under the crater, the dike located about 1000 m below sea level under the west side of the crater, and the deeper magma chamber (Takeo et al., 2006). Among various geophysical observations being conducted around the volcano, GNSS observations have been used to quantify the volume change of the dike by monitoring the temporal variation in the length of the baselines crossing the dike (Aoki et al., 2005). Continuous observations of absolute gravity were also conducted repeatedly in the 2000s, and the data were used to monitor the temporal variation of the magma head in the volcanic conduit (Okubo et al., 2005).
For the 2004 Asama eruptions, Kazama et al. (2015) showed the temporal variation of the magma head in the volcanic conduit from the continuous absolute gravity data as follows. The magma head began to rise around September 10, about 9 days after the first moderate eruption on September 1. The magma head passed the altitude of the absolute gravity station (1406 m above sea level) around September 15, and reached the summit around September 16. The magma head variation is consistent with the first observation of the lava dome in the summit crater on September 16 (Oki et al., 2005) and the continuous eruptions from September 14 to 18 (Nakada et al., 2005). The daily variation of the magma head was obtained continuously from the absolute gravity data as described above, but the time variation of the dike volume is not fully understood on short time scales. Since the temporal variation of the dike volume must be closely related to that of the magma head, the 2004 eruptive activity can be explained in a more unified way if the short-term temporal variation of the dike volume is estimated.
Therefore, we estimated the continuous variation of the dike volume on a short time scale, by using the displacement data obtained at five GEONET stations around Asama Volcano. We first determined five baselines connecting the GEONET stations, and calculated the time variation of the baseline length L_i(t) for each baseline i from the F5 solutions. We also used the Fortran program DC3D (Okada, 1985) to compute the baseline length change a_i for each baseline i in the case of a 1 m dike opening. We then estimated the time variation of the dike opening as u_i(t) = L_i(t)/a_i for each i, and estimated its weighted average U(t) by setting the weight for each u_i(t) to a_i. In addition, we applied the running mean to U(t) with a time window of 11 days, to observe the time variation of dike opening with smaller amplitudes and shorter time scales.
According to the 11-day running mean of U(t), the dike began to inflate around May 2004 and continued to inflate until around September 10. The dike turned from inflation to deflation around September 11, which is consistent with the fact that the magma began to rise around September 10 (Kazama et al., 2015). The dike continued to deflate until around September 26, and its deflation volume during the 15 days reached 8.0 E5 m3. The deflation volume is consistent with the volume of the lava dome observed in the crater (9 E5 m3; Oki et al., 2005), but is about two orders of magnitude smaller than the magma volume in the conduit (Kazama et al., 2015). This discrepancy can be explained by the direct ascent of magma from the deeper magma chamber to the shallower conduit without significant dike inflation, if the dike forms part of the conduit connecting the deeper chamber to the crater (Kazahaya et al., 2015).
For the 2004 Asama eruptions, Kazama et al. (2015) showed the temporal variation of the magma head in the volcanic conduit from the continuous absolute gravity data as follows. The magma head began to rise around September 10, about 9 days after the first moderate eruption on September 1. The magma head passed the altitude of the absolute gravity station (1406 m above sea level) around September 15, and reached the summit around September 16. The magma head variation is consistent with the first observation of the lava dome in the summit crater on September 16 (Oki et al., 2005) and the continuous eruptions from September 14 to 18 (Nakada et al., 2005). The daily variation of the magma head was obtained continuously from the absolute gravity data as described above, but the time variation of the dike volume is not fully understood on short time scales. Since the temporal variation of the dike volume must be closely related to that of the magma head, the 2004 eruptive activity can be explained in a more unified way if the short-term temporal variation of the dike volume is estimated.
Therefore, we estimated the continuous variation of the dike volume on a short time scale, by using the displacement data obtained at five GEONET stations around Asama Volcano. We first determined five baselines connecting the GEONET stations, and calculated the time variation of the baseline length L_i(t) for each baseline i from the F5 solutions. We also used the Fortran program DC3D (Okada, 1985) to compute the baseline length change a_i for each baseline i in the case of a 1 m dike opening. We then estimated the time variation of the dike opening as u_i(t) = L_i(t)/a_i for each i, and estimated its weighted average U(t) by setting the weight for each u_i(t) to a_i. In addition, we applied the running mean to U(t) with a time window of 11 days, to observe the time variation of dike opening with smaller amplitudes and shorter time scales.
According to the 11-day running mean of U(t), the dike began to inflate around May 2004 and continued to inflate until around September 10. The dike turned from inflation to deflation around September 11, which is consistent with the fact that the magma began to rise around September 10 (Kazama et al., 2015). The dike continued to deflate until around September 26, and its deflation volume during the 15 days reached 8.0 E5 m3. The deflation volume is consistent with the volume of the lava dome observed in the crater (9 E5 m3; Oki et al., 2005), but is about two orders of magnitude smaller than the magma volume in the conduit (Kazama et al., 2015). This discrepancy can be explained by the direct ascent of magma from the deeper magma chamber to the shallower conduit without significant dike inflation, if the dike forms part of the conduit connecting the deeper chamber to the crater (Kazahaya et al., 2015).