2007 Fiscal Year Final Research Report Summary
Research on Magma Migrating and Plumbing Process in the Precursory Period of Volcanic Eruptions
| Project/Area Number |
17340128
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| Research Category |
Grant-in-Aid for Scientific Research (B)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
Solid earth and planetary physics
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| Research Institution | The University of Tokyo |
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Principal Investigator |
MORITA Yuichi The University of Tokyo, Earthquake Research Institute, Associate professor (30220073)
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| Co-Investigator(Kenkyū-buntansha) |
NAKAO Shigeru Kagoshima University, faculty of Science, Associate professor (90237214)
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| Project Period (FY) |
2005 – 2007
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| Keywords | Volcanic activity / Precursor to volcanic eruption / Magma emplacement / Magma accumulation / Dike intrusion / Volcanic earthquakes / Volcanic ground deformation / Earthquake swarm |
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| Research Abstract |
We analyzed seismic and geodetic data simultaneously to reveal the magma emplacement and plumbing process for the volcanoes in the Precursory P to the following eruptions in future. We mainly analyzed the process at Eastern Cost of Izu Peninsula and Izu-Oshima volcano. The former is characterized by repeated earthquake swarms, which were accompanied with a submarine eruption. The later is in the step of preparing the eruption in future because summit inflations accompanied with increase of seismicity are observed every a few years recently.
From the systematic analysis of earthquake swarms at the eastern coast of lzu Peninsula, we reveal that the swarm activities are composed of upward migration of two kinds of magma driven by buoyancy, because the hypocenters distinctly separate in the depth. The deeper activity ranged from 7 to 9 km in depth otherwise shallower one did from 3 to 7km. The earthquakes are generated by stress concentration at the tip of the dikes and the hypocenters in t
… More
he swarms reflect the size of dikes On the other hand, the ground deformation shows the excess pressure in the dike. The joint analysis of the seismic and geodetic data, we found out that the excess pressure in deeper dikes have systematically higher than that of the shallower one. The systematic difference in the excess pressure at the same region has not been found in the previous studies, and it is one of the unique results in this study We discussed also on reasons on the systematic difference, and conclude that the condition of the tip cavity of the dike controls the condition of the dike intrusion process and it leads the systematic difference in the excess pressure.
From joint analysis of seismic and geodetic data at Izu-Oshima volcano, we found out that seismic activities are categorized into two groups. One is the seismicity occurring beneath the Izu-Oshima caldera, and the other is occurring around the coast of the island. The seismic activity beneath caldera is well synchronized with volcanic edifice inflation and deflation. The observed ground deformation in the inflation and deflation matches well with the model of point source with spherical model. In the previous studies, the ground deformation was thought to be caused by volume increase of magma chamber whose shape is approximately a sphere located beneath the caldera. From precise hypocentral distribution, and focal mechanisms of the earthquakes, we proposed the other magma sources that are composed of two dikes with different azimuth, which well fits with the data of ground deformation. Our model has several advantages in observational facts because it is supported not only by seismic and geodetic data, but also by a model of stress field caused by wide-ranged tectonic stress and heterogenous structure around the summit vent. Our model is more realistic and helpful to understand the precursory process to volcanic eruptions.
As mentioned above, we got valuable results in this study. The image of magma migration and plumbing in the precursory process of volcanic eruptions becomes more concrete and animation-like in our model. We would like to develop the study on the magma emplacing and accumulation based on the results in this study. Less
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