2005 Volume 50 Issue Special Pages S77-S100
Recent advances in the volcano-seismology largely depend on the advances of the technology in the field of seismic observation at volcanoes. Many new insights about the volcanic phenomena and quantitative measurements of magma emplacement are brought about by the modern seismic observation. In this paper, we mention key technologies that have brought a progress in volcano-seismology, some results and interpretations on precise hypocentral distributions around volcanoes and those on long period ground motion observed at volcanoes. Modernizations of seismic observation around volcanoes are summarized into the following two points. 1) The modern observation system has higher precision and wider dynamic range even it consumes less electric power than old-fashioned system did. The timing is synchronized to GPS timing in modern systems. These advances make us easily to construct more densely and evenly spaced seismic network with high performance even around volcanoes where there are often no commercial services of electricity and telephone. 2) The practical use of mobile broadband seismometer close to the vent of many volcanoes presents us that many kinds of long period ground motion occur around volcanoes and brings us many observational evidences of ground motions caused by magma and/or gas emplacement. The seismograms with high sampling rate, high resolution and wide dynamic ranges from densely and evenly arranged seismic network improve the precision in hypocentral distributions around volcanoes drastically. With the help of the modern hypocentral determination method, hypocenters that distribute dimly like clouds are relocated on a thin penny-shaped distribution, sharply aligned pipe, several parallel aligned planes, and so on. These systematic distributions in hypocenters are strong evidence that earthquakes around volcanoes are caused by simple volcanic processes like magma intrusion, slipping on the pre-existing fault, and so on. Several examples are introduced in this paper. Long-period ground motions (LP motion) observed in the vicinity of volcanoes are well-known in many previous studies, but systematic understanding on this vibration has not been examined enough until the last 10 years. Prevailing usage of mobile broadband seismometers provides us many observational evidences of LP motion around volcanoes and they give us much progress in interpreting their sources. Recent researches show many kinds of possible sources models that generate LP motion, and they present realistic images of magma emplacement. In many cases introduce in this paper, the generation of LP motion is related to the motions of the volcanic fluid like magma and/or volcanic gases. The feature in the wave forms of LP motion gives us important information on the physical property of volcanic fluid trapped in crack and/or magma reservoir. The advance in seismic observations around volcanoes will play more important role to better understand the volcanic process quantitatively. Further development in the seismic observation is needed to progress in volcanic seismology.
