Research on Magma Migrating and Plumbing Process in the Precursory Period of Volcanic Eruptions

同时对地震和大地测量资料进行分析，揭示了P前兆火山的岩浆侵位和管道过程对未来喷发的影响。我们主要分析了伊豆半岛东岸和伊豆大岛火山的过程。前者的特点是地震群反复出现，并伴有海底喷发。后者正处于准备未来喷发的阶段，因为最近每隔几年就会观测到伴随地震活动增加的峰顶膨胀。通过对辽祖半岛东岸地震群的系统分析，揭示了地震群活动是由两种岩浆在浮力作用下的向上运移组成的，震源在深度上有明显的分离。较深的活动范围为7至9公里，较浅的活动范围为3至7公里。地震是由岩脉顶端的应力集中和岩脉内的震源引起的。岩脉群一方面反映了岩脉的大小，另一方面地表变形表明了岩脉内的超压。通过对地震和大地测量资料的综合分析，发现深层岩脉的超压明显高于浅层岩脉。在前人的研究中未发现同一区域的超压存在系统差异，这是本研究的独特结果之一，并对产生系统差异的原因进行了探讨，认为岩脉尖端空腔的状态控制着岩脉侵入过程的状态，导致了超压的系统差异。通过对伊豆大岛火山地震和大地测量资料的联合分析，我们发现地震活动可分为两类。一个是发生在伊豆大岛火山口下方的地震活动，另一个是发生在该岛海岸附近的地震活动。火山口下的地震活动与火山大厦的膨胀和收缩很好地同步。在膨胀和收缩过程中观测到的地面变形与点源模型和球面模型吻合较好。在以前的研究中，地面变形被认为是由岩浆房体积增加引起的，岩浆房的形状近似于位于火山口下方的球体。根据精确的震源分布和震源机制，提出了由两个不同方位的岩脉组成的其他岩浆源，与地面变形资料吻合较好。该模型不仅有地震和大地测量资料的支持，而且有大范围构造应力和非均质构造引起的应力场模型的支持，在观测事实上具有优势。我们的模型更加真实，有助于理解火山爆发的前兆过程。如上所述，我们在这项研究中得到了有价值的结果。在我们的模型中，火山喷发前兆过程中的岩浆迁移和管道的图像变得更加具体和生动。我们希望在此基础上进一步开展岩浆侵位和聚集的研究。少

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