"Spatio-temporal Gravity Variation During the 2000 Volcanic Activity of the Miyakejima Volcano"

(presented by the ERI Gravity Group: Shuhei Okubo (okubo@eri.u-tokyo.ac.jp), Masato Furuya, Sun Wenke, Yoshiyuki Tanaka, Hidefumi Watanabe, Jun Oikawa, Tokumitsu Maekawa)

The figures below show gravity change at four epochs with reference to the gravity in June 1998. Steadily increasing gravityin the west coast suggests a channel through which magma is flowing out from the volcano to the swarm zone off the west coast of the island. Gravity increase in the south-southeastern part draws our attention. Gravity decrease in the central part is mostly due to the collapse of the summit caldera. The decrease in Figs. (c) and (d) appears to be less spectacular than that in Fig. (b). But this is more apparent than real because we could not observe gravity data any longer at points lost forever by the caldera collapse. If they were still available, we would have observed much more prominent gravity decrease.

"From a Supernova to a Vacuum Cleaner: Interpretation"

・ Magma is sucked in by the collapse at the summit of Miyakejima volcano, and it spreads the wave of gravity change in all directions. It is just like to see a birth of a SUPERNOVA, which is discharging gas with an imploding core at the last stage of star's life.

・Why is magma sucked out? The key to the answer is held in the generation of innumerable cracks in the swarm region stretching from the Miyakejima Island toward the Kozushima and Niijima Islands. At the end of the crack, solid-fluid (magma of hydrothermal?) interaction drives a positive feedback; expansion of cracks induces fluid flowing in and more fluid at the crack tips promotes further growth of cracks. Vast space of continuously generated cracks should remain vacuum. Magma may be passively sucked out toward that space (vacuum cleaner model).The model is based on the following grounds.

  1. Steady gravity increase on the southwest coast suggests continuous charging of fluid (Fig. d).
  2. Steady shrinking* of the Miyakejima Island observed with GPS indicates steady flow-out of magma.
  3. If sucked out magma is presumed to flow back to just beneath the Miyakejima Island, we meet difficulty in explaining the gravity value at Abs.G installed in the north of the island.
  4. Order estimate about the volume: effective dimension of cracks is presumed to be 50km long and10km wide with 2m tensile opening. That makes the volume to 10 E8 m^3 , and it can easily contain 4 E8 m^3of collapse volume of the summit of Miyakejima volcano.

The figures below show differentials in between the five epochs.

Explanation

  1. Gravity decrease at the summit (approx. - 140) was due to the creation of cavity (depth 1.5km, capacity > 6E8 m^3) responsible for ever growing summit crater.
  2. Gravity increase in southwest of the island (+111) was due to intrusion of dyke (tensile opening 1.7 meter).
  3. Gravity increase (approx.+60) along the coast was due to the subsidence of the whole island caused by deflation of the deep magma chamber.

The collapse of summit had continued to develop: capacity was 5 E7 m^3 on July and 1.5E8 m^3 on July 11.

  1. Gravity decrease at the summit (- 1135 microgals！) was due to the mass deficiency associated with the large-scale growing collapse of the caldera.
  2. Gravity increase from approx. 50 to 130 microgals along the inner circular path. In this area the crater is above the eye level. The loss of pulling-up Newton attraction leads to the gravity increase.

Cocentric gravity change suggest cylindrical conduits beneath the summit. If collapse and drain-back/flow out of magma will continue, gravity at the center of the island (high elevation) will decrease and it will increase around the coast (low elevation).

Steam explosions occurred on July 14, and 15. The summit crater became deeper (450m) with a capacity of 3 E8 m^3.

  1. At the site with +129 microgals in Fig. (b), gravity decrease of -118 microgals is observed. This is because the center of gravity of the summit crater descended below the eye level of seismic station due to the growing collapse (depression).
  2. The center of collapse is above the eye level at the loop coastal road, and so gravity increases.

Gravity change is spreading as the cocentric circles.

Steam explosions occurred on August 10, 14, and 15. The crater became deeper (500m) and a capacity increased.(the value unknown)

  1. Around the summit gravity decreased as much as 680 microgals. This is because the crater rim was approaching to the observation stations.
  2. Gravity increase of 58 to 91 microgals in the east area indicates the caldera is extending to the east.
  3. As the center of collapse is descending, white region (neutral change) is also descending (approaching to the coastal area).
  4. The center of mass of the crater is above the eye level along the coast, so gravity increases there.