1993 Fiscal Year Final Research Report Summary
A SEISMOLOGICAL STUDY ON THE MAGMA CHAMBER OF UNZEN VOLCANO
| Project/Area Number |
04452066
|
|---|
| Research Category |
Grant-in-Aid for General Scientific Research (B)
|
| Allocation Type | Single-year Grants |
|---|
| Research Field |
固体地球物理学
|
|---|
| Research Institution | KYUSHU UNIVERSITY |
|---|
Principal Investigator |
SHIMIZU Hiroshi KYUSHU UNIV.F.OF SCI., ASSOCIATE PROFESSOR, 理学部, 助教授 (50178985)
|
|---|
| Co-Investigator(Kenkyū-buntansha) |
OHATA Kazuya KYUSHU UNIV.F.OF SCI., PROFESSOR, 理学部, 教授 (10037202)
MATSUSHIMA Takeshi KYUSHU UNIV.F.OF SCI., RESEARCH ASSOCIATE, 理学部, 助手 (40222301)
MATSUWO Norimichi KYUSHU UNIV.F.OF SCI., RESEARCH ASSOCIATE, 理学部, 助手 (70108645)
UMAKOSHI Kodo KYUSHU UNIV.F.OF SCI., RESEARCH ASSOCIATE, 理学部, 助手 (30232888)
|
|---|
| Project Period (FY) |
1992 – 1993
|
| Keywords | Unzen Volcano / Mt. Fugen / Eruption / Seismic activity / Magma chamber / Ascending pass of magma / Lava dome / Low-frequency earthquake |
|---|
| Research Abstract |
We tried to reveal the location and movement of magma beneath Unzen Volcano by analyzing volcanic earthquakes.
An earthquake swarm occured near the center of the Chijiwa bay, about 20 km west of Mt. Fugen, about a year before the eruption. The souce region of the swarm moved toward Mt. Fugen, and the eruptive actiity started. One remarkable feature of the hypocenter distribution is the possibility that itis delineating a ring-fault corresponding to the caldera wall in the Chijiwa bay, which suggests the existence of magma chamber beneath the bay. Another notable feature is alignment of hypocenters between the bay and Mt. Fugen, and it may represent the ascending path of magma. Geodetic measurements after the lava dome emergence showed that a depression source located at the lower border of the hypocenter alignment. Furthermore seismic S-waves passing beneath the hypocenter region were strongly attenuated. These are the evidence that supports our model for the magma feeding system of Unzen Volcano.
The pattern of seismicity drastically changed in association with the laba dome emergence. Swarm activity of low-frequency (LF) earthquakes began to occur beneath the summit craters. By contrast, seismic activity around the volcano declined extremely after the dome appearance. This is probably because the excess pressure of magma decreased owing to the lava extrusion. The LF eartquakes were activated before the extrusion of new lobes and during the endogeneous growth of the lava dome. Our seismic observation in the vicinity of the dome revealed that the LF earthquakes were generated by stick-slips of the lava blocks due to magma intrusion into the dome.
|
