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By John Ravelo

SAIPAN, CNMI (Saipan Tribune, June 20) – A swarm of volcano-tectonic earthquakes occurred on Sarigan Island, while Anatahan's rumbling volcano displayed small explosions every 10-20 seconds in the Commonwealth of the Northern Mariana Islands on Saturday.

The U.S. Geological Survey and the Emergency Management Office reported Saturday that a swarm of 10 volcano-tectonic events with magnitudes between 0.5 and 1.75 had occurred beneath or within a few kilometers of Sarigan Island.

Sarigan, which is located about 95 nautical miles north of Saipan and is the next island north of Anatahan, is actually a volcano with no known historic eruption.

Volcano-tectonic events occur when stress changes happen to solid rock due to the injection or withdrawal of magma. Volcano-tectonic earthquakes can cause land to subside and can produce large ground cracks. These earthquakes can occur as rock is moving to fill in spaces where magma is no longer present.

Volcano-tectonic earthquakes may indicate that volcanic eruption can occur at anytime. EMO director Rudolfo Pua could not be reached yesterday for an interview about the Sarigan situation.

In a joint report, the USGS and the EMO also said Saturday that seismicity on Anatahan had reached its highest since early May.

"Within the tremor, frequent small long-period events and explosions occur as often as every 10-20 seconds," the agencies said.The injection of magma into surrounding rock produces long-period earthquakes. They result from pressure changes during the unsteady transport of the magma and indicate a volcanic eruption.

Meanwhile, the agencies also reported that Anatahan's volcano continued emitting dense ash plume that rose to 10,000 feet in the air. In Saturday's report, the agencies said that ash and steam plume extended about 100 nautical miles west. Thin ash and volcanic smog extended another 700 nautical miles west beyond that, and then turned north-northeast.

The agencies maintained that aircraft should take extra precaution within 10 nautical miles of Anatahan, advising them to pass upwind of the island or beyond 10 nautical miles downwind. They pointed out that conditions could change rapidly, and volcanic activity could just suddenly escalate.

The volcano vented out its fury two Saturdays ago, spewing out thick clouds of ash to 45,000 feet, its second strongest eruption in recorded history. The volume of ash emitted by the volcano in that eruption has yet to be estimated.

During its strongest eruption last April 6, the agencies said the volcano expelled about 50 million cubic meters of ash, which it spewed out to 50,000 feet.

June 20, 2005

Saipan Tribune

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