Monitoring System of a Large Rockslide in Heisei-Shinzan Lava Dome, Mt. Unzen, Japan

Yasuyuki HIRAKAWA, Nobuhiro USUKI, Kouji FUJITA, Toshiaki TANAKA, Masafumi KANEKO, Toshiyasu UENO, Hidenori EGUCHI and Kazuhiro SHIMOKUBO

It is concerned that a large rockslide is possible to occur at lobe 11 in Heisei-Shinzan lava dome, Mt. Unzen. Predicted collapse block of 107 m3 was delimited based on geological structure and distribution of groundwater and superficial fractures. 6 types of in-situ and long-distance monitoring instruments have been installed. Surface of lobe 11 has moved to SE-ESE at rate of 2.45-5.77 cm/year in last decade, similar to other European rockslides or a little smaller. Results of the monitoring and other investigations indicate that creep deformation of subsurface pyorclastic-flow deposits induces slide and settlement of lobe 11, leading to toppling movement at the back-crack. For emergency managements, thresholds and evaluation procedure were established based on measurement. Threshold consists of three categories (i.e. "immediate", "short-term" and "long-term") according to length of lead time before the final collapse. A new index-value ISSV was adopted for threshold of seismometer. In the evaluation procedure, a flowchart using each instrument's status was prepared to mitigate either overestimation or underestimation.

2018/1 363-372