Inspection of Anatahan on 24 and 26 August 2005

Smoking Anatahan was seen from Northwest airline (NW78) from Saipan to Nagoya at 9:30 a.m. (local time), 26 August 2005 (Figs. 1,2 and 3). Ash cloud issued, like boling over the active crater, though still heavily hanging over the island. The condition of eruption probably was higher than when we inspected on 24 August, as below.

Fig. 1
Fig. 2
Fig 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
ERI (Univ. of Tokyo), SEVO (Kyushu Univ.), and Emergency Management Office (CNMI)

Background and outline:
     Volcanic eruption of Antahan that started as its first historical one in May 2003 (Hilton et al., 2005) and has been activated these months, had been monitored geodetically and geologically by the Japanese team in cooperation with the EMO staffs (Watanabe et al., 2005; Nakada et al., 2005b). Continuation of the monitoring is inevitable to understand the present state of Anatahan and to evaluate the future potential of eruption, as well as seismological monitoring by USGS-EMO. The first aim of the campaign this time was collection of the GPS and tiltmeter data and to repair the instruments partially or wholly covered by ash deposit. The data are expected to cover the important period from last year that are essential to know what happened in the Anatahan magma system these years. The additional geological inspection and petrological sampling are also important to know the eruption and magma processes.
     Seismic phase of the Anatahan changed continuously high in level to intermittent pulsation around the morning of 23 August and a large LP event (M4.8?) occurred at 20:45 (local time), 23 August. Judging from this sudden seismological change and the LP event, the USGS scientists (Lockhart, White, Choue, etc) suggested suspension of visiting on the Anatahan island for, at least, a few weeks. After discussion among us and with Andy Lockhart we decided to fly over the island without landing in order to analyze the eruptive condition soon after these seismic anomalies from the air and the burial condition of our geodetic observation site. The inspection was carried out around the island from about 10 to 11 am, 24 August 2005 by Nakada, Matsushima, Yoshimoto, and Camacho with the pilot Cummingham. The wind direction was NE. Ash cloud from the active crater (East crater), which hung over the southwestern rim of the summit caldera and swept over the sea southeast and east of the island, was very vigorously issued. According to impression and photographs by Juan Takai Camacho and pilot Mike in July and May, this state was more active that when they visited before. The island was completely covered with thick layers of wet (dark and probably very fine) ash deposit and new dry ash was deposited in southern slope of the island under the ash cloud drifted from the crater. Green was very limited on outerslopes of the volcano. Many gullies were started to develop on the surface of thick ash deposit.

Eruption cloud: Eruption cloud was dark in color (densely ash-laden) and vigorously smoked from the active crater, like heaviy ash clooud hung on the island (Figs. 4 and 5). The height of eruption cloud was up to about 800 m just above the crater, and it increased up to about 2000 m over the western part of the island, diminishing darkness of the could (becoming lighter in color). Darker vigorous eruption cloud came from the western (or northwestern) side of the east crater and less frequently also from the eastern side of the crater. Though this may reflect the complex circulation of air within the eastern crater, one possibility is that the active crater was widened especially to the east recently; two ash emission points may exist inside the large active crater.

Relationship to seismicity: During our flight, seismic signals were like intermittent pulses with intervals from 5 to 20 minutes and weaker amplitudes than in the afternoon of 23 August. The manner of eruption cloud generation was not like strombolian explosions, such that we felt simply based on seismic signals after the morning of 23 August. No projectiles were observed even near the base of the eruption cloud, though the real base was very hard to look due to drifting of steamy cloud. Abundant ash-laden cloud was vigorously and continuously issued from the active crater, though the power was pulse-like with interval roughly of 5 to several tens of minutes, presumably like that of seismic signals during flight.

Ash cloud surge and ash falling: Abundant ash falling was observed from the ash cloud drifted to the southwest of the crater. Around 10 a.m. a ring of ash cloud surge expanded on the southern rim of crater and moved slowly to the southwest. As well, small many lobes of ash cloud moved slowly on the sea surface like the surges observed at Tar River Valley delta, Montserrat in the Soufriere Hills volcano eruption (Figs. 5, 6 and 7). Also, the view of ash cloud surge is very similar to that observed in the Miyakejima 2000 eruption (Nakada et al., 2005a), in which low-temperature ash cloud surge moved slowly from the summit crater on 29 August 2000. These state of ash cloud likely reflects strong contribution of water (sea water) to eruption; phreatomagmatic eruption rather than pure magmatic eruption. The happening of ash cloud surge during this flight was limited mainly when we approached to the island. Probably this corresponds to the higher level in seismicity during our flight; the seismicity had declined later.

Ash deposition: Layer of ash deposits in the village about 7 km west of the active crater was as thick as 1.5 m. Close up view of the photograph taken from helicopter showed that the antenna of GPS with the pillar of 50 cm high was recognized under the ash layer and its cable to the computer box in partly collapsed hut could be detected. Near future we may retrieve the data from the computer box. View of another GPS station in the easternend points, about 1.5 km east of the active crater, showed the ash layer as thick as about 1 m, under which the GPS antenna was recognized on the edge of a small artificial abandoned pond.

Thermal image: As the eruption cloud was too dense to catch the temperature distribution of the crater area, the images which we took with the thermal camera, Thermo Tracer TH9100MV, NEC San-kei Instruments Ltds., show only the temperature distribution of the eruption cloud (Fig. 8).

Our flight on 25 August was cancelled because it is unlikely that the present condition will change drastically soon, and accepting your recommendation. Unfortunately, in geologic point of view, we could not settle any reasons of either the difference of seismic phases before and after 22 August early morning or the meaning of a large LP event of the evening of 23 August. As we could not look inside the crater, we cannot give you any support to the idea of crater-wall collapse at the time of the large LP event. Comparison of photographs of different times may give us a good solution.

Successive geodetic monitoring such that with GPS, tiltmeter and EDM is very important. With only seismometers, the understanding of the present eruptive condition is very limited. Retrieving our GPS and tiltmeter data before complete their destruction and reinstallation (replacement) of the instrument should be harried.

This time the discussion with the USGS seismologists was very timely and useful for us. So that more intimate interaction between Japanese team, USGS and EMO should be recommended.

Hilton et al., 2005: Introduction to the special issue on the 2003 eruption of Anathan Volcano, Commonwealth of the Northern Marian Islands (CNMI). Jour. Volcanol. Geotherm. Res., 146, 1-7.
Nakada et al., 2005a: Chronology and products of the 2000 eruption of Miyakejima Volcano, Japan. Bull. Volcanol., 67, 205-218 (file of about 1 MB)
Nakada et al., 2005b:
Geological aspects of the 2003-2004 eruption of Anatahan Volcano, Northern Mariana Islands. Jour. Volcanol. Geothermal. Res. 146, 226-240.(file of about 1.2 MB)
Watanabe et al., 2005: Geodetic constraints for the mechanism of Anathan eruption of May 2003. Jour. Volcanol. Geothermal. Res. 146, 77-85.

Our first visiting to Anatahan was reported in the site;
The results of the second and third visits were involved in Nakada et al. (2005b).

Participants from Japan:
Prof. Setsuya Nakada, Volcano Research Center, Earthquake Research Institute (ERI), University of Tokyo (; office +81-3-5841-5695)
Prof. Takeshi Matsushima, Institute of Seismology and Volcanology (SEVO), Graduate School of Science, Kyushu University (
Dr. Mitsuhiro Yoshimoto, Volcano Research Center, Earthquake Reserch Institute (ERI), University of Tokyo (

(revised by Nakada on 30 Aug 2005)
(short revision on 4 Oct 2005)