Chronology and short summary

Eruption chronology of the 2004 eruption in Asama Volcano

(As of 5 October)
*Main Vulcanian explosions
     Three Vulcanian explosions occurred at 20:02 of 1 Sept., 19:44 of 23 Sept., and 12:20 of 29 Sept.  Heights of eruption clouds were not known because they took place in night of cloudy days or in a rainy day. Air shocks measured the JMA  Karuizawa Weather Station, about 8 km S of the summit, were 205, 75 and 29 Pa, respectively. Magnitudes of explosion earthquakes were M1.6 and M2.3 for 1 and 23 September explosions, respectively, according to JMA. Though that for the 29 September was not reported, felt quake associated with the 23 September eruption was noticed around the volcano.

*Major intermittent Strobmolian-like explosions
  Afternoon 16 Sept – Morning 17 Sept.

Tilt and gravity changes

     A series of eruptions in September 2004 were always preceded by earthquake swarm and tilt change that became significant a few days to half a day before explosive events. Tilt-meters of JMA and ERI are located about 3 km N and about 4 km E of the summit crater, respectively. The former is more sensitive than the latter, probably due to the EW trending shape of magma body (dyke) under Asama Volcano. The degree of inflation was as small as less than 10 by -6 radian at the site of  about 3 km N, and not detected by the GPS network surrounding the volcano.
     Preceding Vulcanian explosions on 1, 23 and 29 September, small inflation of the summit area was observed about one to half day before these events, which were also accompanied by frequent B-type earthquakes. On the other hand, the explosion event during 16-17 September followed deflation after three-day inflation (10-13 September). This tendency was also observed by the tilt-meter of ERI. The possibility of the 29 September explosion was released to the public by JMA before the event.
     Gravity measurement at the Asama Volcano Observatory (AVO) of ERI, which is conducted by S. Okubo of ERI, showed clearly that major explosion events occurred a few days after the gravity change from the increase to decrease. AVO is at 1,400 m above sea level, about 4 km east from the summit, while the summit is 2,568 m asl. Prof. Okubo proposed that this gravity changes reflect movement of magma head within the conduit, where gravity decreased when the magma head went higher than the observation level.

Volcanic gas

     Flux of volcanic gas before and during a series of eruptions are monitored with DOAS by the Kusatsu-Shirane Volcano Observatory (Tokyo Institute of Technology), Geological Survey of Japan, and University of Tokyo. SO2 flux in 2003 and 2004 was variable and sometimes reached 1,000-2,000 ton/d. Soon after the 1 September eruption, it decreased down to <1,000 ton/d but increased with time, reaching about 2,500 ton/day in the end of September.

Social correspondence

     JMA set the degree of volcanic activity of Asama as level 3 among 5 levels (5 is highest) soon after the 1 September explosion, and issued the extraordinary volcanic information #1. The level had been maintained as of early October. Local governments prohibit entry on the area 4 km from the summit.