Infrasonic Observation near Active Volcanoes and its Results in Japan Hitoshi YAMASATO*, Takayuki SAKAI* and Yoshiaki FUJIWARA** *Meteorological Research Institute, Japan Meteorological Agency ** Japan Meteorological Agency (present : Geographical Survey Institute) Usu, Hokkaido-Komagadake, Asama, Miyake, Unzen, Sakurajima Infrasound Technology Workshop 2007 (ITW2007) 14 November 2007, at Tokyo

Volcano Observation of JMA 傾斜計 High-sensitive Visual Camera Seismometer Infrasonic Microphone GPS Tiltmeter Volcano Observation and Information Centers (Sapporo, Sendai, Tokyo, Fukuoka) Real-time telemeter Mobile Observation (Thermal, Geodetic, Geomagnetic etc.)

Infrasonic Observation of JMA Infrasonic microphone: ACO type7144/3348 Response : Hz, dB Telemeter : 100Hz, 12-16bit Meakan Tokachi ② Tarumae Usu ② Hokkaido-Komagadake ③ Iwatesan Adatara Aduma Bandai ③ Nasu Kusatsu-Shirane Asama ③ Ontake Fuji Izu-Tobu Izu-Oshima ③ Miyakejima ⑧ Kuju Aso ③ Unzen ② Kirishima ② Sakurajima ⑤ Satsuma-Iojima Kuchinoerabujima Suwanosejima Active volcanoes in Japan and infrasonic stations of JMA (as of 2007). Encircled figures represent the number of microphones. Detected infrasounds by JMA Sakurajima Tokachidake Unzen Aso etc Usu, Miyake etc Suwanosejima etc Fuji

Infrasonic Observation at Sakurajima An example of C type (harmonic) tremor, infrasonic signals and their spectra. (Sakai et al., 1996) Typical seismic and infrasonic signal from an explosive eruption at Sakurajima volcano. Monthly number of explosions at Sakurajima. Tokyo VAAC VOIC Narita Aviation Weather Center VAA

Results at Unzen eruption Nov Phreatic eruption May 1991 Lava dome and PFs Infrasonic observation by JMA 1 station (1991-) 4 station (1992-) Dome collapse Pyroclastic flow Low frequency earthquake Seismic and infrasonic observation at Unzen volcano by JMA (1992-) ● ： seismographs △： infrasonic microphone ◆： visual camera

Dome collapse An example of seismic (left) and infrasonic (right) signals from a dome collapse. SPs are seismograms obtained by short period seismographs. LP is by a long period seismograph. Seismic records are for the vertical component. Infrasonic records were obtained by low frequency microphones. The records are arranged according to the horizontal distance from the source. The phases, A and B, are considered to correspond respectively to the start of dome collapse and to the fall of lava blocks onto the slope. (Yamasato et al., 1993)

Infrasonic signal from pyroclastic flow (Left) An example illustrating the migration of a medium sized pyroclastic flow front and the corresponding seismic and infrasonic signals. (Flow distance=3km) Seismic and infrasonic waves were excited while the pyroclastic flow front ran down the slope. (Right) Source locations of infrasonic sources from pyroclastic flows. after Yamasato (1997; JPE) Dome

Waveform correlation of PF signals ●A-course (05:34 May 21, 1993) ●B-course (00:38 April 4, 1993) ●C-course (10:31 June 22, 1992) C K8(+8s) C K8(+8s) C K8(+8s) Examples of infrasonic signals from pyroclastic flows that ran to different three directions. Good correlations are observed among the waveforms of different stations. The source location of the infrasonic signal can be estimated from the travel time differences of phases in the signal, assuming the sound velocity is 340 m/s and the source is on the ground surface.

Infrasound Energy from PF Relation between the flow distance and the energy ratio of the infrasonic waves to the seismic waves from pyroclastic flows. Only the pyroclastic flows that flowed eastward (B-course) and have clear infrasonic signals were taken. The flow distances are horizontal ones as observed by Unzendake Weather Station. Purple circles indicate the pyroclastic flows for which the flow fronts were out of the range of the video camera, therefore, those distances might be larger than those shown here. (Yamasato, 1997) Energy of the infrasonic waves Energy of the seismic waves Es=CP, P=square sum of the waveform C is calculated using the relation of Es(J)=1.5M+4.8 for low frequency earthquakes.

Low frequency earthquakes in the lave dome An example of seismic (left) and infrasonic records from a low frequency earthquake beneath the dome. An infrasonic signal was excited at the time of the earthquake occurrence. Source locations of the infrasonic pulses from low frequency earthquakes (red circles) and dome collapses (crosses). Arrows indicate the direction of pyroclastic flows. The 9-th lobe appeared in Dec. 4. after Yamasato (1998) Dome

Volcanic eruption at Usu, 2000 Distribution of infrasonic microphones and source location of infrasonic signals from eruptions at Usu volcano. N-B, K-A, K-B are active craters. (Yamasato et al., 2002) Photo taken from north of the craters. An example of the infrasonic record. N-B K-B K-A

Sequence of eruptive activity Numbers and amplitudes of infrasonic signals since mid-April (Yamasato et al., 2002) An example of the time-sequence of activity at each crater. (Upper) Sequence since mid-April to May 2000 (Yellow in the left figure) (Lower) Since May to July, 2000 (Blue in the left figure)

Eruption at Miyake volcano, 2000 Date Plume Infrasonic signal July mNo observation July ,500 mContinuous (8 Pa) Aug. 10 8,000 mContinuous (16 Pa) Aug. 1814,000 m Continuous (>20 Pa) Aug Impulsive (8 Pa) Aug. 29 8,000 mContinuous (14 Pa) 2001-<1,000 mImpulsive or continuous (<8 Pa) Infrasonic record in 03h-06h, 29 August 2000 at Miyake volcano

Eruption at Hokkaido-Komagadake, 2000 Volcanic tremors and infrasound associated with phreatic eruptions at Hokkaido-Komagadake volcano in :13, 4 September :42, 28 October x10 -6 m 5 Pa 2x10 -6 m 60sec 5 Pa

Relation between Infrasonic and Seismic Amplitude Relation between seismic and infrasonic amplitudes of signals associated with eruptions during July – September 2000 at Miyakejima. Ash plume height indicated. (Δ=2km for seismic, Δ=4km for infrasonic signals) Usu (impulsive infrasound) Hokkaido-Komagadake 2000 (continuous infrasound) Miyakejima (impulsive infrasound and tremor) Miyakejima (continuous infrasound and tremor) Height of dense ash plume (m above the caldera)

Eruption at Asama, :02, 1 Sep :44, 23 Sep :17, 29 Sep :10, 10 Oct :59, 14 Nov >30Hz 200Pa 10s ↓ Onset of explosion-quake +10s 8Pa 80Pa 40Pa 80Pa Infrasonic records from explosions at Asama in The station is 8 km distant of the summit crater. Activity in Sep First explosion 15 Sep. Lave cake appeared Sep. Successive eruptions 23 Sep., 29 Sep., 10 Oct., 14 Nov. Explosions Infrasonic records from Successive eruptions. 1min

Infrasonic wave propagating far away Distribution of infrasonic microphones of JMA Red triangles indicate ones that detected the infrasonic signal excited by explosions of Asama, 20:02, 1 September 2004 (a) and 19:44, 23 September 2004 (b). Infrasonic records from an explosion of Asama, 20:02, 1 September Asama Amplitude of infrasonic signal from an explosion of Asama, 20:02, 1 September (a) (b)

Summary Detection of volcanic eruptions and the sequence of eruptive activities Explosive infrasonic pulses (Sakurajima, Usu etc.) Continuous infrasonic waves (Miyake etc.) Determination of source locations using infrasonic network Locations of dome collapses and pyroclastic flows (Unzen) Identification of activities in plural craters (Usu) Scaling / classification of magnitudes / styles of volcanic eruptions

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