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Volcano Watch — Anatahan Volcano reawakens

April 29, 2004

On April 6, 2004, Anatahan volcano in the Commonwealth of Northern Marianas Islands (CNMI) awakened from five months of slumber since its first historic eruption waned in October 2003.

Earthquakes indicating fresh magma movement beneath the volcano's east crater began to rumble at least once every minute for two weeks in early April.

A few days ago, scientists from the CNMI's Emergency Management Office (EMO) flew over the volcano in a small airplane and confirmed what the earthquakes and other observations had suggested: new lava and ash had been erupting from the volcano. Later overflights revealed that a small dome of lava was growing in Anatahan's east crater, similar to that which formed in Mount St. Helens' crater after the catastrophic 1980 eruption.

Last weekend, the eruption escalated. Ash and rocks were ejected hundreds of feet skyward from the east crater. The swarms of small earthquakes increased, and continuous volcanic tremor began. Overflights this past week revealed ongoing ejection of ash and incandescent material from a small crater within the dome.

Anatahan volcano lies 120 km (80 miles) north of the island of Saipan and 320 km (200 miles) north of Guam. In May 2003, Anatahan roared to life after hundreds of years of quiescence, letting loose an eruption that resulted in airport closures, the contamination of catchment water supplies in Saipan, and declaration of Anatahan island as unsafe. The eruption unleashed an ash cloud that billowed about 10,000 meters (32,000 feet) over the volcano in one eruptive episode. In another episode, the collapse of the eruption column fed speeding clouds of hot ash down the slopes of the east crater, incinerating and scalding everything in their path.

By October 2003, seismic activity at Anatahan returned to a normal background level, and ash emission ceased from the east crater. Overflights and deformation surveys conducted by USGS and EMO scientists revealed that the floor of the volcano's east crater had been lowered by almost 60 meters (200 feet) and the diameter of the east crater lengthened almost 40 centimeters (about 15 inches).

Since Anatahan's summer 2003 eruption, activity remained low until the volcano began to rumble in early February. On February 1, earthquakes began to occur at shallow levels beneath the volcano's crater. On February 7, activity peaked at around 15 earthquakes per hour, then dropped to a background level.

A second seismic swarm occurred March 30-31, with over 100 earthquakes recorded. These earthquake swarms probably resulted from the percolation of a small amount of magma upward into the volcano, which was either degassing or fracturing the crust as it moved beneath Anatahan's east crater.

On April 2, 2004, scientists at CNMI-EMO and USGS determined that small amounts of ash and steam were being erupted from Anatahan to low altitudes of less than 1,000 meters (3,000 feet), in contrast to the major eruption cloud of May 10, 2003, which reached upward to 10,000 meters (32,000 feet) above sea level and posed a severe threat to nearby aircraft.

The earthquake activity reached a crescendo on April 6 and 7, leading to the discovery of the lava dome. Presently, volcanic tremor and smaller ash and steam eruptions continue from the volcano, allowing for the possibility of a more significant eruption if activity continues to escalate.

When the eruption ceases and landing on Anatahan becomes safe again, scientists from the USGS will return to the island. They will join CNMI-EMO staff in updating and installing additional monitoring equipment to enhance their ability to understand the volcano and forecast future activity. Plans include placing tiltmeters and an explosion detector on Anatahan, and re-measuring Electronic Distance Measurement (EDM) lines across the east crater.

Despite the recent renewed activity at Anatahan, a repeat of the May 2003 eruption has not occurred. By returning to Anatahan to repair and enhance their monitoring equipment, USGS scientists will have a better understanding of how Anatahan Volcano works ? a crucial step toward ensuring the safety of everyone in the CNMI and of the thousands of people who fly over the volcano every month.

Volcano Activity Update

Eruptive activity at Pu`u `O`o continues. A lava flow that breaks out of the Mother's Day tube a short distance above Pulama pali has descended to about the 1600-foot elevation on the pali. Distant viewing of the lava is excellent from the end of the Chain of Craters Road. No lava is entering the ocean, and there is no eruptive activity in Pu`u `O`o's crater except for sporadic minor spattering.

No earthquakes were reported felt on the island during the week ending April 29.

Mauna Loa is not erupting. The summit region continues to inflate slowly. Seismic activity remains low, with 4 earthquakes located in the summit area during the past week.

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