Monthly Volcanic Activity Report (May, 2011)

Japan Meteorological Agency

Nikko-Shiranesan [Warnings around the crater] After "The 2011 off the Pacific coast of Tohoku Earthquake" on 11th March, seismicity became higher at a depth of about 5 kilometers of W and NW foot, and 5-10 kilometers of E and SE foot of Mt. Nikko-Shiranesan. In the reporting period, two felt earthquakes were observed at NW foot region with maximum JMA Seismic Intensity of 1. After these events, seismicity gradually lowered. No volcanic tremor was observed.

Yakedake[ Alert Level : 1] Just after "The 2011 off the Pacific coast of Tohoku Earthquake" on 11th March, seismicity became higher in the

shallow parts beneath the summit and at NW foot of Mt. Yakedake, but it turned to the background level. No volcanic tremor and low-frequency earthquake were observed.

Fujisan[ Alert Level : 1]

After a M6.4 earthquake on 15th March at 22:31 at around south of the summit, increased number of earthquakes were observed in the area from focal point of the main shock to right below the summit. Seismicity is becoming at lower level. Number of deep low-frequency earthquakes around at a depth of 15km stayed at low level. No volcanic tremor and low-frequency earthquake in the shallow parts were observed.

Miyakejima [Alert Level : 2]

Gas-and-steam plumes rose to a height of 100-400m above the crater rim. The sulfur-dioxide flux was 600-900 t/d in May 2011 (cf. 700 t/d; April;Fig.1). According to the report from Miyake village, high SO2 concentration was sometimes recorded in some inhabited areas.

Seismicity has stayed at low level. There was no geomagnetic change reflecting inner heat state beneath Miyakejima. GPS observation shows continuous deflation of the volcano originated from the shallow source.

Fig.1 Emission rate of SO2 at Miyakejima. Ioto [Near-crater Warning]

Seismicity in shallow parts in Ioto has remained at high level since February 2011. According to GPS observation by GSI, land wide upheaval, which was first observed from August 2006,

slowed down since middle November to December 2010, but has been on the increase since late January 2011. The extension of the baseline in NS direction is still continuing and southward displacement has been observed in the south end.

Fukutoku-Oka-no-Ba [Near-sea-area Warning]

According to the information from JCG(Japan Coast Guard), JMSDF (Japan Maritime Self-Defense Force), and JMA, discolored water has frequently been observed around Fukutoku-Okano-Ba in recent years. Asosan [Alert Level : 1→2]

Field surveys on 6th and 9th of May revealed a small scale of mud spout at the height of 5-10m from the hot water lake (locally called "Yudamari"). A small amount of volcanic ash remaining inside the crater had been observed by visual camera installed by the Aso Volcano Museum since 1200JST of May 13th. According to a field survey on May 13th, the temperature at the bottom of crater was getting higher. A small volcanic flame has been observed at nights at the crater pits in the center of Nakadake No.1 crater since May 14th through video observation from the Museum. A small-scale eruption was observed in the morning on May 15th, followed by a small amount of ashfall extended to NE 2km from the crater. A field survey on May 15th revealed relatively high temperature of approximately 370 degrees centigrade at a part of the bottom of the crater (Photo 1).

Photo 1. Infrared images in Nakadake No.1 crater by field surveys from 21st April to 15th May 2011.

Maximum temperature was approximately 370 degrees centigrade (white circle on the upper left

photo).

15th May, 2010

13rd May, 2011

6th May, 2011

21st April, 2011

Asosan [Alert Level : 1→2；Continued!!]

A small eruption occurred on May 16th around 1000JST, and grayish plume rose to a height of 500m above the crater rim (Photo 2). Recognizing the transition of the state of the volcano into a period of relatively high activity, the alert level for Mt Aso was raised from 1 to 2 at 1100JST on May 16th. (Target area had been changed from “the area inside the crater” to “the area around the crater”). A filed survey in the morning on May 16th revealed grayish plume ejecting from the center at the bottom of Nakadake No.1 crater, and small amount of ashfall around the crater rim. Grayish plume height rose as high as 800m (maximum height in this period) above the crater rim at 1650JST. Small-scale eruptions have intermittently occurred since May 17th. The amount of water of the lake was 10-20% (cf. 10% in April) of its full volume. Surface temperature at Yudamari was not measured due to the decrease on hot water (cf. 67 degrees centigrade in April).

Seismicity, including isolated-pulse events, has remained at relatively low level. Hypocenters were located at a shallower parts of Nakadake No,1 crater. Continuous tremors with small amplitudes were observed from 3rd to 10th of May.

Kirishimayama（Shinmoedake）[Alert Level : 3]

No explosion(*) was observed in the reporting period at Shinmoedake. White plume height was about 200m on average (maximum; 500m) above the crater rim.

Seismicity has been decreasing, but stayed at relatively high levels. Seismic events observed 1,784 times in this period (cf. 3,840 times in April). Hypocenters were located at a depth of 0-2km above the sea level near Shinmoedake. Small-scale tremors were sometimes occurred. Duration of tremors was 01h09m in total (cf. 42h13m in April) (Fig 2).

Aerial observations were conducted in cooperation with JGSDF (Japan Ground Self-Defense Force), JASDF (Japan Air Self-Defense Force) and JMSDF on 2nd, 13th, 17th and 31st of May. According to these surveys, the diameter of lava accumulated inside the crater remains about 600m, as same as in April. White plume rose from the SE parts of the margin of lava, with a height of 50-100m above the crater rim. Infrared observation showed no major change regarding the heat distribution in the reporting period, and high-temperature area corresponded to comparatively plume area (Photos 3 and 4).

The sulfur-dioxide flux of 200 t/d was observed on May 13th (cf. 100-200 t/d in April). (*) Explosion at Shinmoedake is defined as the one which accompanies air shock more than 20 Pa with

explosive earthquake.

Photo 2. Nakadake No.1 crater by field surveys from 21st April to 16th May 2011.

A grayish ash plume was ejected from the white circle on the left photo.

21th April, 2011 16th May, 2011

Fig.2 Seismicity and plume activity at Shinmoedake from 2003 to May 2011.

Photo.3 Aerial surveys from 2nd to 31st of May 2011. Red and yellow circles show relatively high temperature, corresponding to plume area. Courtesy of JGSDF, JASDF and JASDF.

Photo.4 Infrared images by aerial surveys from 2nd to 31st May 2011. Courtesy of JGSDF, JASDF and JASDF.

11:21, 2nd May, 2011 11:05. 13rd May, 2011

10:58, 31st May, 2011

11:10, 2nd May, 2011

10:53 31st May, 2011

10:56 17th May, 2011

11:24, 13rd May, 2011

Sakurajima[Alert Level : 3]

Number of eruptions including explosive ones at Showa-crater has remained at a high level. In this period, eruptions observed 113 times (cf. 109 in April), 76 cases of which were explosive ones (cf. 92 in April). Ballistics reached areas about 800-1300m from the Showa-crater. No pyroclastic flow was observed.

There was no eruption at Minamidake summit crater. Volcanic seismicity has remained at relatively low level. According to field surveys on 9th and 30th of May, sulfur-dioxide flux was 900-1,100 t/d on average (cf.

400-1,700 t/d in May). According to GPS measurement, slight contraction inside Sakurajima Island has been observed in this period. The water-tube tiltmeter by MLIT (installed at 2.5km SE of Minamidake summit crater) did not observe any

change (Fig.3). According to GPS measurement by GSI, extension of the baselines that traverse the Aira-Caldera (at closed-off section of Kagoshima bay) has been observed.

An aerial observation was conducted in cooperation with JMSDF on May 31st and hot lava with a diameter of 50-60m was newly observed inside the Showa crater. According to an infrared observation, high thermal area was observed in the center of lava (Photo 2).

Without apparent change in both volume of ejecta and crustal deformation, it is possible neither a large amount of lava has been moving into just beneath Sakurajima, nor lava spews outside the Showa crater. However, continuous monitoring is still needed.

Fig. 3 Tilt change observed by water-tube tiltmeter at station Arimura from January 2009 to May 2011, eliminated tidal response and eruptions. Summit upheavals correspond to positive tilts. In the upper figure, red bars denote monthly frequencies of explosions from the Showa-Crater.

Satsuma-Iojima [Alert Level : 2] White-plume activity at Iodake summit crater remained above background levels, and plumes rose less than 200

meters (maximum; 400m) above the crater rim in this period. Seismicity has stayed at low level. A volcanic tremor with small amplitude and short duration occurred in this period.

Suwanosejima [Alert Level : 2]

An explosive eruption occurred on May 3rd in this period (cf. two eruptions in April). Plume rose 400 meters in maximum (cf. 1,000m in April) above the crater rim in this period. Volcanic glows were sometimes observed at night with a high-sensitivity camera.

Activity of volcanic earthquakes and tremors remained relatively high.