Background and outline:
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
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.
of Tokyo), SEVO (Kyushu Univ.), and Emergency Management Office (CNMI)
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.
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 (email@example.com; office +81-3-5841-5695)
Prof. Takeshi Matsushima, Institute of Seismology and Volcanology
(SEVO), Graduate School of Science, Kyushu University (firstname.lastname@example.org)
Dr. Mitsuhiro Yoshimoto, Volcano Research Center, Earthquake Reserch
Institute (ERI), University of Tokyo (email@example.com)
Nakada on 30 Aug 2005)
(short revision on 4 Oct 2005)