The first recorded historical eruption at Anatahan began on 10 May 2003. Activity through early October 2003 was [previously] reported [(BGVN 28:04, 28:05, 28:06, and 28:09)]. This report, in large part contributed by scientists of the Commonwealth of the Northern Mariana Islands Emergency Management Office (CNMI/EMO) discusses Anatahan during October 2003 to early May 2004. During October-February, Anatahan's tremor and volcanic seismicity were consistently low, with no apparent eruption signals or precursory events. Later, in March, April, and May clear emissions began, and in April-May 2004 small dome extrusions occurred. There were several peaks in seismicity, such as 2-8 February and shorter episodes on 13-14 February. Much larger peaks in seismicity, the largest in 2004, took place in late April. Many details on the dome extrusions remain undisclosed; however, near the end of this report there are a variety of data from several contributors, including one clear Modis alert (28 April) and a VAAC reports based on satellite observations of a thin plume (24 April).

During overflights in early October 2003 observers saw no ash emissions. The crater vents continued to emit steam and SO₂, and the floor of the crater was in great flux. During periods of rain, the crater floor was covered by sediment-laden water and (or) debris flows. The E crater continued to have an active geothermal system that consisted of mud pots, mini-geysers, and steam jets from the crater walls. In general, emissions continued at a low level.

During early November 2003, regional seismicity was low. For the week of 9-15 December, numerous tiny long-period earthquakes (LPs) were recorded only on the station near the crater, at a rate of ~ 1 every 10 sec. All of these LP events were much smaller than M 0.5, with dominant frequencies of 4-5 Hz. After several months of only very low magnitude LP events, on 1 February 2004 larger ones again began to occur. After increasing slowly in size, they reached M ~ 2, and they took place several times per hour.

Anatahan's seismicity peaked just before 1600 local time on 7 February 2004, with a swarm consisting of up to 15 events occurring per hour. The seismicity then decreased dramatically but remained well above levels of the previous few months. By 1000 on 8 February the maximum magnitude of volcanic earthquakes had diminished greatly from their peak magnitude two days before, but the earthquakes were occurring more often, sometimes as frequently as 1 per minute. The amplitude of low-frequency tremor had also increased considerably. The combined effect was that the level of seismic energy release around 7-8 February, averaged over hours, remained nearly constant at its peak.

By 9 February the magnitude of the volcanic earthquakes decreased significantly and were no longer visible on the records. The amplitude of low-frequency tremor remained high but decreased some from its peak on 8 February. The total daily seismic energy release decreased somewhat from its peak during the previous few days. From 10 February to 29 March 2004, Anatahan volcanic seismicity, tremor, and energy release were all very low, with no apparent eruption signals.

Volcanic seismicity occurred again 13 February during a 6-hour period, the first such seismicity since the episode of 2-7 February. Short tremor episodes began at 0543 and occurred every 8-12 minutes until 1130 on 14 February, the largest being approximately equivalent to a magnitude M 2 earthquake. From 15 February through 30 March 2004, Anatahan tremor and volcanic seismicity stood at low background levels, with no apparent eruption signals or precursory events.

A seismic swarm began beneath Anatahan Island on 31 March, the third such swarm since the eruption of May-June 2003. The largest earthquakes in the swarm were all smaller than those that occurred during the previous swarm in early February 2004. During 2-3 April the swarm intensified significantly. Most earthquakes were followed by long, tremor-like signals that CNMI scientists believed indicated small emissions of steam, possibly bearing ash and thought to rise to altitudes much less than 1,000 m, though they had no visual confirmation of such emissions at the time.

On 6-7 April, Anatahan volcanic seismicity was the highest since the eruption of May-June 2003, with events as large as M ~ 2.5 usually followed by tremor-like signals. That swarm's level of seismic activity remained high through 23 April.

With regard to the rise in seismicity during early 2004, Scientists of the Commonwealth of the Northern Mariana Islands Emergency Management Office (CNMI/EMO) opined that these events were likely the result of magma degassing and/or moving beneath the recently active crater. The Office of the Governor, CNMI, placed Anatahan Island off-limits and concluded that, although the volcano was not currently dangerous to aircraft, pilots should exercise due caution in Anatahan's vicinity.

Dome, ejecta, and lavas. On 12 April, the presence of a new, rather flat lava dome within a crescent-shaped crater lake was confirmed, as was the occurrence of fresh ejecta within the lowest reaches of the crater. The Hawaii Institute of Geophysics and Planetology (HIGP) web site for MODIS hot-spot satellite imagery showed a thermal anomaly for the volcano on 12 April at 1545 UTC. High seismicity through 23 April suggested continuing effusion of relatively small volumes of new lava onto the crater floor.

At 1052 on 24 April, Anatahan's seismic activity increased abruptly, rising to levels unseen since summer 2003 (3 to 4 times as high as those observed in April 2004). About that time a low-level eruption began producing steam and ash to ~ 600 m and an overflight reported incandescent cracks in a fresh lava flow or dome within the inner crater. The presence of a "cow-pie-shaped" dome within the inner crater was verified later.

The seismicity level increased slowly and fairly constantly on 24-25 April to a level similar to that of the eruption of mid-June 2003. During an overflight on 26 April between 1030 and 1100, Juan Camacho (CNMI/EMO) and Erik Hauri (Carnegie, Margins group) observed regular puffs of yellow-brown steam and ash every 1-2 min, a rate almost identical to that of seismic events recorded during that time. The maximum height of the steam and ash plume was estimated at ~ 600 m.

According to the Washington Volcanic Ash Advisory Center (VAAC), at 0725 UTC on 24 April 2004 a thin plume from Anatahan was visible on satellite imagery ~ 1 km above the volcano and extending ~ 460 km NW; this plume was not observable on satellite images taken at 1502 UTC, later in the same day. The HIGP web site for Modis hot-spot satellite imagery showed a thermal anomaly for the volcano on 28 April at 1545 UTC. On 28 April the seismicity level increased still further, a level ~ 25% more energetic than the previous high of 25 April 2004.

In accord with the elevated seismicity, the dome increased in size, and explosions also apparently increased in size and rate. The seismicity then slowly decreased ~ 25% over the next 5 days until 3 May, when it dropped off suddenly but smoothly by another 40%. Subsequently, over several days, the seismicity recovered somewhat to ~ 50% of the highest level of 28 April. On 5 May 2004 small explosions continued to occur every minute or two, and steam and ash still rose hundreds of meters.

Geological Summary. The elongate, 9-km-long island of Anatahan in the central Mariana Islands consists of a large stratovolcano with a 2.3 x 5 km compound summit caldera. The larger western portion of the caldera is 2.3 x 3 km wide, and its western rim forms the island's high point. Ponded lava flows overlain by pyroclastic deposits fill the floor of the western caldera, whose SW side is cut by a fresh-looking smaller crater. The 2-km-wide eastern portion of the caldera contained a steep-walled inner crater whose floor prior to the 2003 eruption was only 68 m above sea level. A submarine cone, named NE Anatahan, rises to within 460 m of the sea surface on the NE flank, and numerous other submarine vents are found on the NE-to-SE flanks. Sparseness of vegetation on the most recent lava flows had indicated that they were of Holocene age, but the first historical eruption did not occur until May 2003, when a large explosive eruption took place forming a new crater inside the eastern caldera.

Information Contacts: Hawaii Volcano Observatory (HVO), U.S. Geological Survey (USGS), Hawaii National Park, HI 96718, USA (URL: https://volcanoes.usgs.gov/nmi/activity/)update.html); Juan Takai Camacho and Ramon Chong, Commonwealth of the Northern Mariana Islands Emergency Management Office (CNMI/EMO), Saipan, MP 96950 USA (URL: http://www.cnmihsem.gov.mp/); Frank Trusdell, U.S. Geological Survey, Hawaiian Volcano Observatory (HVO), PO Box 51, Hawaii National Park, HI 96718, USA (URL: https://volcanoes.usgs.gov/nmi/activity/); Washington Volcanic Ash Advisory Center, Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ssd.noaa.gov/); MODIS Thermal Alerts team, Hawaii Institute of Geophysics and Planetology (HIGP), School of Ocean and Earth Science and Technology, University of Hawaii at Manoa (URL: http://modis.hgip.hawaii.edu/); Erik Hauri, Carnegie Institution of Washington, 5241 Broad Branch Road, NW, Washington, DC 20015-1305, USA (URL: http://dtm.carnegiescience.edu/).