Aso Volcano, Central Kyushu

(Nakadake, 1,506 m, 32o52'44"N, 131o05"58"E)

(Jan. 17, 2004)
Fukuoka Cener of JMA issued Volcanic Advisory on Aso Volcano stating that a mud eruption occurred in the crater 1 of Aso Volcano at 15:41 on 14 January, being associated with volcanic tremor. Small amount of very fine ash issued in this eruption was observed in Takamori Town about 10 km ESE of the crater.

Thermal activity had risen since last year, such that hot-water level in the crater decreased by about 40% of usual. The latest mud eruption was last July. Activity Level of this volcano, adoption of which started by JMA last November together with other four active volcanoes, changed from 2 to 3. Receiving this report, Aso Town announced forbiddance for tourist to enter into the area 1 km from the crater.

(July 28, 2003)

JMA issued Volcano Observation Information # 13 on Aso Volcano on the afternoon of 28 July 2003. Continuous volcanic tremor event started around 2 p.m. 27 July. The same event had not been observed after November 1995. Though the daily number of volcanic quakes ranges around 10, that of isolated tremor around 100 since 23 July. This day, water of crater lake in the Cater 1 was gray in color, and the temperature was 76 C. Boiling was observed in the center.

JMA had observed seismic signals implying small phreatic eruptions 5 times during 12-14 July.

Geologists, Kazunori Watanabe (Kumamoto University) and others, surveyed the deposit of the 10 July eruption on the following day, and estimated the total mass of eject as about 130 t. Ash was deposited as distant as 14 km EWE of the crater. They confirmed small amount of fresh vesicular glass particles in the ejecta under the microscope. They posed a possibility that the juvenile material may be related to the eruption.

Information contact: Kazunori Watanabe, Kumamoto University;, Hitoshi Yamasato, JMA;

(July 11, 2003)
Volcano Observation Report on Aso volcano, issued from JMA on July 11, 2003 informed falling of small tephra at Aso volcano in the morning of this day. The report saids "a tremor event with an intermediate amplitude was recorded at 17:18 (JST), July 10. The staffs of the Aso Weather Station inspected the area around the first crater in Nakadake, this day, and confirmed small amount of tephra newly deposited at Hakoishi-Toge about 6 km ENE of the crater. However, neither the detail of eruption nor the reason is known, because the first crater is covered thickly by cloud. No abnormal is found in seismicity (volcanic earthquakes and tremors)".

According to Dr. Yasuaki Sudo of Aso Volcanological Laboratory, Kyoto University, who also inspected the crater area, the eruption was very small pheatic one (mud eruption). Thickness of tephra deposit consisting of wet ash aggregates is around 1 mm even at the crater rim. Spray of mud was blown off by strong wind as much as 10 km from the crater. Lake surface color of the first crater turned into dark gray from green of two days ago, probably due stirring by the eruption. Sound of the eruption was recorded with a microphone of crater live-camera monitored by the Aso Volcano Museum.

The latest tephra fall had been observed in September 1994.

Information contact: Dr. Yasuaki Sudo, Aso Volcano Observatory, Kyoto University,

According to the Observation Report No. 4 on Aso volcano that was issued at 1140, August 21, the numbers of isolated tremor events at the Crater #1 had decreased. The daily number was peaked at 340 on August 15. The temperature of the southern inner rim of the crater, measured with infrared camera, showed still high value at 314 degrees C. The temperature of the pool surface was 57 degrees C.

Daily number of isolated volcanic tremor events (JMA)

August 14


August 15


August 16


August 17


August 18


August 19


August 20


August 21

37 as of 11 a.m.


3,968 since August 5

JMA issued the Volcano Observation Reports #2 and 3 on Aso Volcano on August 9 and 14 respectively. In the crater #1 of Nakadake, one of central cones of Aso Volcano, isolated volcanic tremor events had occurred since August 5, 2002.

Daily number of isolated volcanic tremor events (JMA)

August 5


August 6


August 7


August 8


August 9


August 10


August 11


August 12


August 13


August 14

221 (as of 16:00)

The infrared camera measurement shows the temperature of the southern crater wall as high as 307 degrees C on August 14; thermally high condition continuous since early April, 2002. The water-pool temperature in the crater was 60 degrees C, had not largely changed together with the fumaloric activity, muddiness of water pool, and its level.
This is the first time that isolated volcanic tremor events had continued over 300 times a day since June 19 to July 2 in 1992, when effusion of mud and sand took place within the crater.

Information contact: JMA-Fukuoka center, N. Uchida;

(November 25, 1997)
According to newspapers and TV news, volcanic gas from Aso Volcano killed two tourists on 23 November. Around 10 a.m. two tourists (62 and 51 years old men) collapsed after inhaling volcanic gas about 100 m south of the rim of active crater (#1) in the Nakadake crater group on the volcano. The crater area 4 km around and 100 m deep is accessible by cable and automobile, or on foot. They were members of two separate tour groups.

Town officials of Aso have monitored volcanic gas around the crater, using a sensor installed by the Environment Agency this April. The view point of crater area was closed in the early morning of this day because of high content of surfurous acid gas (around 5 PPM), but opened at 9 a.m. because the content lowered down to <2.5 PPM. They announced the warning to tourists with poor health. After the first accident, they decided to close again at 10:49 am because the content reached at about 8 PPM. Two men collapsed around the same location at 9:45 and 10:40 respectively. In this morning, there was a north wind over 5 m/s.

The number of persons killed by poisonous volcanic gas amounts to seven in this volcano since 1980. A total of 71 people have been hospitalized with the poisoning since 1980, including two of this day. In June 1994, five children on junior high school excursion collapsed near the crater #1. The Aso Weather Station of JMA did not find any abnormal condition in the volcanic activity this day.

Dr. Tomoki Tsutsui, the Aso Volcano Laboratory, Kyoto University, commented the recent activity of Aso Volcano on 12 November, as follows. New fumarolic vent, about 10 m across, was formed on the southern wall of crater #1 in the early-November. Crater #1 had been quiet since 1993; greenish gray-colored, hot water is stored in the crater bottom. TV images of the Aso Volcano Museum, which monitors inside crater #1, was showing emission of mud pieces with white fumes from the new vent, being associated with noise of bubbling. Small mud mounds were formed around the vent. Volcanic tremors has been low in levels.

Aso Volcano is one of the most active volcanoes in Japan. The eruption of 553 AD is the oldest recorded one in Japanese history and the volcano has been active since then. Activity of this volcano started about 300 ka. Four huge plinian eruptions, forming Aso-1 to Aso-4 pyroclastic flows, formed a large caldera of 25 km long and 18 km wide. More than 10 central cones of basalt to rhyolite line up in the east-west on the caldera floor. Only Nakadake cone among them repeated historical eruptions of basaltic magma. Nakadake has a group of craters (1.1 km long) including crater #1 at the summit. Hot-water is stored in the crater #1 bottom when the activity is low. Strombolian, phreatic to phreatomagmatic eruptions are common style. The activity is monitored mainly by JMA and Kyoto University.

Information contact: Dr. Tomoki Tsutsui, Aso Volcano Laboratory, Kyoto University, e-mail:, and Volcano Research Center, ERI, U-Tokyo, e-mail:

Link to real time activity report (in Japanese, views of the Nakadake crater area, and crater #1 bottom with fumaloric vent) by Aso Volcano Museum . Another like to photo showing bottom of Nakadake Crater #1 by Aso Volcano Laboratory, Kyoto University (in Japanese).

To eruption menu