volcano using seismoacoustic cross correlation<\/strong><\/p>\n
SUMMARY
On 2013 November 20, a submarine eruption started close to Nishinoshima island, which
lies \u223c1000 km south of Tokyo. Real-time monitoring of the eruption is crucial for understanding\u00a0the formation processes of the new volcano island and related disaster prevention. In situ\u00a0monitoring, however, is difficult in practice because the closest inhabited island, Chichijima,\u00a0is 130 km away from Nishinoshima. This study presents an infrasonic monitoring method that\u00a0uses cross-correlating records at a pair of online stations on Chichijima. One is the horizontal\u00a0ground velocity recorded at a permanent seismic station operated by the Japan Meteorological\u00a0Agency (JMA). The seismic records were corrected for atmospheric pressure using\u00a0an empirical ground response to infrasound. The other is the air pressure recorded at the\u00a0JMA Meteorological Observatory. For each station, we divided the whole records into 3600-s\u00a0segments. To suppress outliers, each segment was normalized by the envelope function. We\u00a0then calculated cross-correlation functions between the pair of stations using the fast Fourier\u00a0transform. They present clear successive arrivals of infrasound coming from Nishinoshima.
We also conducted an offline tripartite-array observation using three low-frequency microphones\u00a0with a station spacing of \u223c50 m installed in 2013 May. The array analysis supports\u00a0the results obtained from the online stations. The typical root-mean-squared amplitude is on\u00a0the order of 0.01 Pa, and the typical duration is several days. The amplitudes were primarily\u00a0controlled by the effective sound velocity structure from Nishinoshima to Chichijima. The\u00a0infrasonic observations together with the meteorological observation at Chichijima suggest\u00a0that infrasonic activity was not present in the first two weeks in 2015 January. With the help\u00a0of a more quantitative estimation of the meteorological effect, we could infer eruptive activity\u00a0in real time. Now many online seismic stations are available worldwide. This study shows that\u00a0installation of another infrasonic sensor close to the existing seismic station (\u223c1 km) is useful\u00a0for monitoring a remote island volcano in real time.<\/p>\n![\"(a)](\"http:\/\/www.eri.u-tokyo.ac.jp\/old\/en\/wp-content\/uploads\/sites\/3\/2015\/12\/fig.1.jpg\")
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a range of 20s; one can see propagation with a phase velocity of \u223c350 m s\u22121<\/sup>. Ground velocity records at JMA station CHIJI3 were converted to atmospheric
pressure using the empirical response function. The vertical axes show the distance from Nishinoshima<\/figcaption><\/figure>\n","protected":false},"excerpt":{"rendered":"