地球物理学的多項目観測から見た噴火課程

Abstract

This paper reviews the eruption processes, as well as the processes leading up to the eruption, as revealed by multiparameter geophysical observations. First, we briefly describe the history and development of geophysical observational methods in Japan. A prototype of multi-parameter geophysical observation had already been established at a few volcanoes in Japan 50 years ago. To obtain high-quality geophysical data, underground tunnels and boreholes had been constructed around volcanoes, and seismometers, tiltmeters, and strainmeters had been installed in them during the 1980 s and 1990 s. Broadband seismometers and a data logger with a GPS clock that are small and lightweight, were introduced in the mid-1990 s. Combined withlarge data storage capacity in the data loggers and efficient data transmission using IP protocols, simultaneous multi-parameter geophysical observations have been intensively and continuously conducted since the early 2000 s in order to reveal the eruption processes. Since then, various analysis methods suitable for multi-parameter observations have been developed. Although the time scales of volcanic eruptions vary widely, from several seconds to several decades, most of these scales are covered by multi-parameter geophysical observation. We review the advances and challenges for understanding not only the eruption processes, but also the precursory processes leading up to eruptions, which have been revealed by multi-parameter geophysical observations during the past decade. The explosive eruption processes, which include Plinian, Vulcanian, and Strombolian eruptions, and effusive eruptions, which include Hawaiian and dome-forming eruptions, are also reviewed. Physical models for these types of eruptions have been developed experimentally and theoretically, and examined later using geophysical observations. For each type of eruption, we briefly introduce the proposed physical models and describe their progress, mainly from the viewpoint of multi-parameter geophysical observations. The eruption processes of Plinian eruptions have been revealed mainly by worldwide seismic and infrasound observation networks and satellite images. Vulcanian and Strombolian eruptions are most intensively studied by multi-parameter geophysical observations, because of their high-frequency nature and accessibility to the proximity of active craters in deploying monitoring instruments. For example, the precursory processes of Vulcanian eruptions are characterized by inflation, relatively stable, and slight deflation stages of the volcano edifices, which are inferred by continuous crustal deformation measurements. Strombolian eruptions are understood as repeating strong gas bursts at the surface of liquid magma. Recently, new monitoring techniques such as high-speed cameras and portable radar units have been introduced to observe the Strombolian surface activities. Hawaiian eruptions are characterized by explosive lava fountains and large lava flows traveling more than several kilometers. Multi-parameter observations are quite useful for monitoring the locations of dike intrusions and lava fountains. Dome-forming eruptions have the longest time-scales among these eruption styles. Temporal fluctuations of dome growthare well monitored by geodetic and photogrammetric observations. Recent seismic observations of dome-forming have revealed the characteristic repeating earthquakes and provided new insights into the physical mechanisms of dome growth. Our understanding of the physical process of phreatic eruptions is quite limited relative to magmatic eruptions. Recently, tilt changes associated with tremors were sometimes observed before phreatic eruptions at several volcanoes, and may be forerunners of phreatic eruptions. We need much more data to understand the processes leading up to phreatic eruptions. Erupted volumes and rates are inferred from multi-parameter geophysical observations, including both seismic and geodetic methods, and confirmed by satellite data and ground-based surveys. The volumes and rates are related to eruption energy. The energy is partitioned into thermal, kinetic, gravity potential, strain, seismic, acoustic, and fracture energies, most of which are directly measured or interpreted using multi-parameter geophysical observations.