09:15 〜 09:30
[SVC28-02] 2021 Pyroclastic flow at Semeru volcano, East Java, Indonesia induced by a collapse of lava dome
キーワード：火砕流、スメル火山、溶岩ドーム崩壊
Semeru volcano (elevation 3676 m) in East Java, Indonesia, has been experiencing small-scale Vulcanian eruptions dozens to one hundred and dozens of times a day since 1967 at the Jonggring Seloko crater on the summit. In addition to the quasi-stationary Vulcanian eruptions, PDCs sometimes occurred due to the collapse of the lava dome at the crater. Since 1967, PDCs with flow distances of >9 km occurred in 1977 (10 km), 1981 (10 km), 1994 (11.5 km), and 2002 (9 km). The collapse volume of the lava dome is estimated to be 6-7 Mm3.
A PDC was induced by the collapse of the lava dome on December 4, 2021. The PDC reached a distance 16 km down the southeast slope. This is the longest distance since 1967. The number of people killed or missing due to the PDC reached 50. According to CVGHM, the collapse volume was estimated to be about 6 Mm3.
The number of VT earthquakes increased and harmonic tremors occurred continuously prior to the PDCs in 1994 and 2002. In contrast, the 2021 PDC was preceded by low seismicity and no harmonic tremors. Within one year prior, 118 VT earthquakes occurred, and only 23 have been observed since May 2021. There was no significant change in the activity of explosion earthquakes (50-80 /day) and low-frequency earthquakes (10-20 /day). In addition, the number of Rock-fall (RF) events from the lava dome is small.
For Merapi volcano, we have obtained an empirical equation between the volume of the lava dome and the seismic energy of VT earthquakes during the year before the collapse of the lava dome (Iguchi et al., 2019). Applying this to Semeru’s 118 VT earthquakes in the year before the 2021 PDC, and assuming an average energy of 2.56 MJ per VT earthquake (M1.1), the seismic energy would be 302 MJ. Based on the empirical equation, the upper limit of the ejecta volume is only on the order of a thousand m3.
The lava dome started to grow in August 2009. There have been 1312 VT earthquakes since then. RF events increased synchronously with the increase in VT earthquakes. This indicates that increased magma intrusion rate (MIR) increased stress in the subsurface as appeared by VT earthquakes and increase in RF events due to increased growth rate of the lava dome on the surface.
The seismic energy for the 12 years since August 2009 is 3.4 GJ, and the upper limit of the ejecta volume is 0.23 Mm3, which is still two orders of magnitude smaller than the volume (17 M m3) of the lava dome. The empirical equation is based on the assumption that magma intrusion causes VT earthquakes, but this is valid only if the MIR is fast enough to cause VT earthquakes simultaneously. The MIR of Merapi volcano, on which the empirical equation is based, was 0.38 M m3/day in the 2006 eruption and 3.16 M m3/day in the larger 2010 eruption (Aisyah et al., 2018). On the other hand, in the case of Semeru volcano, the volume of the lava dome was only 17 M m3 over 8 years (2009-2017), so the MIR is 2 to 3 orders of magnitude smaller at 0.006 M m3/day. Therefore, the small MIR allows ascent of magma without the occurrence of VT earthquakes.
As of November 2017, the volume of the lava dome had reached about 17 M m3. Then, it is estimated that it had reached about 25 M m3 just before the occurrence of the 2021 PDC. The collapse volume estimated to be 6 M m3 corresponds about 1/4 of the dome.
Noteworthy points are listed considering from the 5 PDCs of >9 km distance since 1967. 1) The 1977 PDC occurred after the dome growth over a period of 10 years, similarly to the 2021 PDC. The growth of the lava dome at Semeru is slower than that of Merapi and Sinabung volcanoes. 2) Volume of 6 M m3 can be used as a threshold for the occurrence of PDCs that reached more than 9 km. 3) In both the 1994 and 2002 PDCs, an increase in VT seismicity and harmonic tremors were observed. In contrast, the 2021 PDC occurred without these precursors. 4) All of these PDCs occurred during the rainy season, suggesting that the intrusion of large amounts of rain into the lava dome may have triggered its collapse. 5) The distance of the 2021 PDC was significantly longer than that in the previous year, even though the collapse volume was the same. A possible mechanism for the longer distance is the decrease in the friction coefficient of PDC. When a hot PDC touches a sedimentary layer rich in pore water, and pore water evaporate instantly, blowing upward the interparticle spaces of the pyroclastic flow. This may have exerted an upward force on the particles of the PDC and reduced the apparent friction.
A PDC was induced by the collapse of the lava dome on December 4, 2021. The PDC reached a distance 16 km down the southeast slope. This is the longest distance since 1967. The number of people killed or missing due to the PDC reached 50. According to CVGHM, the collapse volume was estimated to be about 6 Mm3.
The number of VT earthquakes increased and harmonic tremors occurred continuously prior to the PDCs in 1994 and 2002. In contrast, the 2021 PDC was preceded by low seismicity and no harmonic tremors. Within one year prior, 118 VT earthquakes occurred, and only 23 have been observed since May 2021. There was no significant change in the activity of explosion earthquakes (50-80 /day) and low-frequency earthquakes (10-20 /day). In addition, the number of Rock-fall (RF) events from the lava dome is small.
For Merapi volcano, we have obtained an empirical equation between the volume of the lava dome and the seismic energy of VT earthquakes during the year before the collapse of the lava dome (Iguchi et al., 2019). Applying this to Semeru’s 118 VT earthquakes in the year before the 2021 PDC, and assuming an average energy of 2.56 MJ per VT earthquake (M1.1), the seismic energy would be 302 MJ. Based on the empirical equation, the upper limit of the ejecta volume is only on the order of a thousand m3.
The lava dome started to grow in August 2009. There have been 1312 VT earthquakes since then. RF events increased synchronously with the increase in VT earthquakes. This indicates that increased magma intrusion rate (MIR) increased stress in the subsurface as appeared by VT earthquakes and increase in RF events due to increased growth rate of the lava dome on the surface.
The seismic energy for the 12 years since August 2009 is 3.4 GJ, and the upper limit of the ejecta volume is 0.23 Mm3, which is still two orders of magnitude smaller than the volume (17 M m3) of the lava dome. The empirical equation is based on the assumption that magma intrusion causes VT earthquakes, but this is valid only if the MIR is fast enough to cause VT earthquakes simultaneously. The MIR of Merapi volcano, on which the empirical equation is based, was 0.38 M m3/day in the 2006 eruption and 3.16 M m3/day in the larger 2010 eruption (Aisyah et al., 2018). On the other hand, in the case of Semeru volcano, the volume of the lava dome was only 17 M m3 over 8 years (2009-2017), so the MIR is 2 to 3 orders of magnitude smaller at 0.006 M m3/day. Therefore, the small MIR allows ascent of magma without the occurrence of VT earthquakes.
As of November 2017, the volume of the lava dome had reached about 17 M m3. Then, it is estimated that it had reached about 25 M m3 just before the occurrence of the 2021 PDC. The collapse volume estimated to be 6 M m3 corresponds about 1/4 of the dome.
Noteworthy points are listed considering from the 5 PDCs of >9 km distance since 1967. 1) The 1977 PDC occurred after the dome growth over a period of 10 years, similarly to the 2021 PDC. The growth of the lava dome at Semeru is slower than that of Merapi and Sinabung volcanoes. 2) Volume of 6 M m3 can be used as a threshold for the occurrence of PDCs that reached more than 9 km. 3) In both the 1994 and 2002 PDCs, an increase in VT seismicity and harmonic tremors were observed. In contrast, the 2021 PDC occurred without these precursors. 4) All of these PDCs occurred during the rainy season, suggesting that the intrusion of large amounts of rain into the lava dome may have triggered its collapse. 5) The distance of the 2021 PDC was significantly longer than that in the previous year, even though the collapse volume was the same. A possible mechanism for the longer distance is the decrease in the friction coefficient of PDC. When a hot PDC touches a sedimentary layer rich in pore water, and pore water evaporate instantly, blowing upward the interparticle spaces of the pyroclastic flow. This may have exerted an upward force on the particles of the PDC and reduced the apparent friction.