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Volcano Watch — Anatahan Volcano's ash clouds reach new heights

July 21, 2005

The U.S. Geological Survey (USGS), in cooperation with the Emergency Management Office (EMO) in the Commonwealth of the Northern Mariana Islands, continues to monitor Anatahan volcano. We last "visited" Anatahan through a Volcano Watch article in April 2004, while it was experiencing its second major eruptive episode. The current eruptive activity began on January 4, 2005.

Ash cloud rising from volcano
Ash cloud from Anatahan volcano. (Public domain.)

Anatahan volcano lies 120 km (80 miles) north of the island of Saipan and about 2,200 km (1,400 miles) south of Tokyo, Japan. In May 2003, Anatahan came to life after hundreds of years of quiescence, resulting in airport closures, the contamination of catchment water supplies on Saipan, and declaration of Anatahan Island as unsafe.

The eruption unleashed an ash cloud that billowed, at times, about 10,000 meters (32,000 feet) over the volcano. 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 normal background levels, and ash emission ceased from the east crater. During the eruption, the floor of the volcano's east crater was lowered by almost 60 meters (200 feet), and the east crater widened by almost 40 centimeters (about 15 inches).

On April 6, 2004, Anatahan reawakened for a period of nearly four months with a considerably less vigorous eruption. This phase was characterized by the growth of a spatter cone within the volcano's east crater. During its peak, the explosions from the volcano were recorded at a rate of once a minute, and ash reached several thousand feet into the atmosphere. By the end of July, earthquake and eruptive activity had stopped, and a USGS field crew was dispatched to repair seismic equipment and make measurements to determine changes in the volcano's shape.

The most recent series of eruptive episodes began around January 5, 2005, as volcanic tremor (continuous vibrations of the ground) was recorded and low-level ash expelled. This eruptive period lasted for about a month, after which activity became sporadic. During this episode, the inner crater of the volcano filled with fresh material and ejecta, including lava bombs over a meter in diameter.

On April 6, 2005, a very large explosion emitted an estimated 50 million cubic meters of ash during Anatahan's most energetic activity, to date, blowing out the growing spatter cone in the east crater. Over a five-hour period, the eruptive column and the amplitude of harmonic tremor grew, peaking around 0300 local Anatahan time on April 6, when the ash from the volcano reached a height of 15,000 meters (50,000 feet), higher than the altitude of commercial airline flight paths.

In mid-May, USGS and EMO personnel again repaired the seismic and acoustic stations on Anatahan that were rendered inoperable by the April eruption. The acoustic sensors can detect the compressional sound waves generated by volcanic explosions. Thereafter, activity has continued almost unabated, with ash and steam generally reaching at least 3,000 meters (10,000 feet), and numerous bursts of activity sending ash near commercial air paths, above 9,000 meters (30,000 feet).

Ongoing volcanic activity at several volcanoes around the world has pressed USGS personnel involved with the Volcano Disaster Assistance Program (VDAP) into service for several eruption response efforts. As a result, HVO's role in the continued monitoring of volcanoes in the CNMI will increase in the coming months, with staff taking on additional monitoring responsibilities. The recent field repairs have already been compromised by subsequent volcanic activity, as just one of the three instrumented sites on Anatahan is now operational.

With current high levels of activity and associated hazards to aircraft, daily updates are being provided through the HVO-hosted CNMI website ( To date, the duty personnel involved with the Anatahan response have contributed nearly 400 updates to chronicle the ongoing eruption.

Volcano Activity Update

Eruptive activity at Pu`u `O`o continues. On clear nights, glow is visible from several vents within the crater and on the southwest side of the cone.

Lava continues to flow through the PKK lava tube from its source near Pu`u `O`o to the ocean. Thus, the amount of lava visible on the surface is very low, with only a handful of flows scattered from above the top of Pulama pali to the ocean. Two ocean entries, at East Lae`apuki and East Kamoamoa, were active as of July 21. The East Lae`apuki bench has suffered partial collapses three times in the last three weeks. Access to the ocean entries and the surrounding area has been closed due to significant hazards. No easily accessible surface flows are currently present. If you visit the eruption site, check with the rangers for current updates, and remember to carry lots of water when venturing out onto the flow field.

This was a good week for earthquake lovers. During the week ending July 21, three earthquakes were felt on Hawai`i Island. At 5:49 a.m. on July 15, a magnitude-5.2 moderate earthquake located 49 km (30 miles) northeast of `O`okala occurred at a depth of 8 km (5 miles); this earthquake was felt in much of Hawai`i island and parts of Maui. Two days later, on July 17, another magnitude-5.2 moderate earthquake occurred at 9:15 a.m. 24 km (15 miles) west of Lo`ihi seamount at a depth of 28 km (18 miles); it was also felt throughout Hawai`i island and parts of Maui and O`ahu. At 4:32 p.m. that same day, a magnitude-3.9 minor earthquake aftershock located 21 km (13 miles) west of Lo`ihi seamount occurred at a depth of 27 km (17 miles); it was felt throughout the eastern half of Hawai`i island.

Mauna Loa is not erupting. During the week ending July 20, nineteen earthquakes were recorded beneath the summit area. Of these, 16 were deep and long-period in nature. This is the first increase in seismicity this year beneath Mauna Loa. Inflation has slowed over the last few weeks.

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