# Volcano Watch — The Anatahan eruption: what might happen down the road

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The eruption of Anatahan Volcano in the Commonwealth of the Northern Mariana Islands (CNMI) continues. It has generated much interest in the volcanologic community and among the residents of Saipan and Guam. It has, unfortunately, also generated exaggerated accounts on the Internet.

Eruption cloud, from the east crater of Anatahan Volcano. View from the NE side of the island, looking in a southwesterly direction.

(Public domain.)

Anatahan is located 120 km (80 miles) north of Saipan Island and 320 km (200 miles) north of Guam. The island is about 9 km (5.6 miles) long and 3 km (2 miles) wide. Anatahan is a stratovolcano containing the largest known caldera in the Northern Mariana Islands.

In March-April 1990, USGS-HVO scientists responded to an earthquake swarm that alarmed Anatahan residents. This swarm and accompanying inflation indicated that magma had moved into the volcano, which had not been active historically. The activity prompted installation of a seismic station and geodetic networks to monitor the reawakening volcano. Owing to economic hard times, support for the monitoring fizzled, and the seismometer was not operating when the eruption began on May 10, 2003.

The eruption column initially attained a plume height of about 10 km (33,000 feet), an altitude deemed potentially dangerous to jet aircraft by the Federal Aviation Administration. For the next several weeks, a smaller but nearly continuous eruption column rose from the east crater. The resulting eruption clouds generally remained below about 6 km (20,000 ft). On May 23-24, typhoon Chan-hom shifted the prevailing east winds southward, blowing the eruption column toward Saipan and Guam. Light ash fall resulted in flight cancellations at the Saipan and Guam airports. Residents of Saipan reported a rotten egg smell associated with the ash fall.

The ongoing explosive activity excavated a deep crater within Anatahan's east crater. Scientists estimated the inner crater was nearly at sea level by about May 20; before the eruption, the floor of the east crater was 68 m above sea level. The spiny surface of a lava flow or dome was first observed in the inner crater on June 4.

In early June, personnel of the USGS, including two from HVO, and the CNMI Emergency Management Office (EMO) reestablished seismic monitoring of the volcano. For the past 3 weeks, signals from a seismometer have been transmitted by radio to the EMO on Saipan. Initially, alternating high- and low-amplitude volcanic tremor was recorded. In general, the higher the tremor amplitude, the more energetic the eruptive activity. High-amplitude periods were characterized by ash-laden plumes and eruption columns about 2.4-3 km (8,000 to 10,000 ft) high. Low-amplitude periods were reflected by feeble ash emissions and lower column heights. Low-amplitude tremor has dominated the seismograms for the past 2 weeks.

Seismologists from EMO and USGS continue to closely monitor activity on the island. The future course of the eruption is uncertain. It could continue at its present, relatively low level for weeks or months, perhaps punctuated by explosive activity sending eruption columns higher than 6 km (20,000 ft). Depending on prevailing winds, such explosions could result in minor ash fall on nearby islands. A higher level of activity could result in continued growth of the lava dome, more explosions, and attendant greater volcanic hazards on Anatahan Island.

The potential hazards from the present activity, which consists of sporadic extrusion of lava in the east crater and a nearly continuous eruption column punctuated by stronger explosive activity, are limited to Anatahan Island and to aircraft passing downwind of Anatahan below about 8 km (25,000 ft).

Moderate explosive eruptions accompanied by a tall eruptive column and pyroclastic flows are possible from Anatahan. Such activity would result in far-reaching eruption clouds and ash fall. There would be a very small chance of a local tsunami on Anatahan from pyroclastic flows entering the water or from flank landslides into the sea.

An unlikely scenario is a massive explosive eruption or a large landslide. This activity would pose a major problem to aviation and/or produce a tsunami with effects felt throughout the Marianas and beyond. We emphasize the unlikely nature of such events, the stuff of wild Internet accounts.

### Volcano Activity Update

Effusive eruptive activity at the Puu O`o vent of Kīlauea Volcano continued unabated during the past week. Three distinct surface flows are observed on Pulama pali, and a strong glow comes from breakouts farther upstream above the top of the pali. Numerous surface breakouts also occur in the coastal flats from the base of Paliuli to the Highcastle sea cliff where lava cascades down to the delta. The cascading lava does not spreading far across the Highcastle delta, but lava is pouring into the water from at least three spigots off the point of the delta.

The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying sudden collapses of the new land. The steam clouds are highly acidic and laced with glass particles. The National Park Service has put warning signs in critical places. Do not venture beyond these signs and onto the lava deltas and benches.P> No earthquakes were reported felt on the island during the past week.

Mauna Loa is not erupting. The summit region continues to inflate slowly. Seismic activity remains low, with only one earthquake located in the summit area during the last seven days.