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Volcano Watch — Volcanic hazards assessment and monitoring continue at Anatahan, Northern Marianas Islands

September 18, 2003

On May 10, 2003, Anatahan Volcano in the Northern Marianas Islands awoke for the first time in reported history and sent eruption cloud to heights reaching 10 km (approximately 6 miles). The unrest heralded by this event -- described in Volcano Watch reports of May 15 and June 26 -- continues.

For over 20 years, the U. S. Geological Survey (USGS) has assisted the Emergency Management Office (EMO) of the Commonwealth of the Northern Marianas Islands (CNMI) in establishing volcano monitoring networks and assessing volcanic hazards. This weekend, volcanologists from USGS offices across the western United States will gather with their CNMI counterparts on Saipan to assess the impact of the activity thus far, to evaluate the types of activity that may occur if the unrest continues, and to study the potential impacts and hazards that such activity would pose.

Current monitoring, telecommunications, and computer technology have allowed the responsibilities of continuous monitoring of Anatahan to be truly shared among USGS scientists and their CNMI EMO counterparts. Seismographic monitoring equipment was installed in 1990 to offer basic warning capabilities, such as the ability to observe earthquake swarms or volcanic tremor exceeding quiet background monitoring levels. Although far fewer in number, the field equipment is virtually identical to those operated in our U. S. volcano monitoring networks.

The seismic data are recorded on seismographic drum recorders operated at EMO headquarters. The seismic data are also converted into digital format and automatically analyzed and stored on a computer installed at EMO in late June. The computer runs a seismic data acquisition and processing system called Earthworm, developed by USGS scientists and used at many seismographic network operations centers.

At EMO in Saipan, the Earthworm computer is configured to acquire data sent from Anatahan. While the small number of stations in CNMI does not allow the calculation of an earthquake location, Earthworm automated processing creates seismic records that are accessible over the Internet and sends the data to computers at the Hawaiian Volcano Observatory (HVO) and the Cascades Volcano Observatory (CVO) in Vancouver, WA. If seismic levels of volcanic unrest increase above specified levels, USGS volcanologists will receive computer-issued notifications.

Because of the need for continuous monitoring of the unrest at Anatahan, and due to the time differences between Saipan and our USGS offices in Hawaii and on the US mainland, round-the-clock means were established to notify USGS staff whenever necessary. Volcanologists at HVO or CVO are able to receive notifications via wireless messaging and internet services, view Anatahan data on handheld PC phones, and use these phones to communicate with CNMI. Since late June, volcanic activity at Anatahan has continued at low levels, exhibiting principally volcanic tremor. The level of activity could quite possibly increase abruptly, and there is a remote chance that Anatahan could erupt again in the near future with a moderate explosive eruption that could result in far-reaching eruption clouds and ash fall. This underscores the need to closely monitor the volcano.

There will be new aspects learned about Anatahan's eruption through the joint efforts of the USGS and CNMI EMO in the next few weeks. Anatahan eruption updates can be found on the web at:

Volcano Activity Update

Eruptive activity at the Pu`u `O`o vent of Kīlauea Volcano continued unabated during the past week. Surface activity is visible on Pulama pali as well as the coastal plain. The Kohola arm of the Mother's Day flow, on the western flow field, has some incandescent patches high on Pulama pali, as well as some small, sluggish breakouts on the western side of coastal plain. The east-side lobe of the Mother's Day flow has apparently stagnated, as it is no longer incandescent. The August 9 breakout, which starts high up the Mother's Day tube system on the east side of the flow field, has now descended Pulama pali to just above the gentle slope below. Much of the August 9 flow on the pali is incandescent. No lava is entering the ocean.

Two earthquakes were felt on the island during the past 7 days. One, of magnitude 2.7, was felt from Captain Cook to Kailua at 7:56 a.m. September 17; it was located 2 km (1 mile) southeast of Holualoa at a depth of 16 km (10 miles). The second felt earthquake occurred on September 17 at 8:34 p.m., 17 km (11 miles) north-northwest of Kailua, near the western tip of the island, at a depth of 45 km (28 miles). It had a magnitude of 3.4, and was felt at Captain Cook, Honoka`a, Kailua, Kapa`au, Kapulena, Kawaihae, Waikoloa, and Waimea. Mauna Loa is not erupting. The summit region continues to inflate slowly. Seismic activity remains low, with no earthquakes located in the summit area during the last seven days. Visit our website ( for daily volcano updates and nearly real-time earthquake information.

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