According to the Saipan Tribune, the United States Geological Survey has reported a swarm of volcano-tectonic earthquakes (between magnitude 0.5 and 1.75) were occurring beneath or within a few kilometers of Sarigan Island. Volcano-tectonic events occur when stress changes happen to solid rock due to the injection or withdrawal of magma. The injection of magma into surrounding rock produces long-period earthquakes and can be an indicator that a volcanic eruption can occur at anytime.
Sarigan Island is a volcano, one of many in the Marianas Island chain. But it is volcano with no previously known eruptive history. It is located about 95 nautical miles north of Anatahan, another volcano that recently has entered a more violent eruptive phase. Prior to May of 2003, when a large explosive eruption took place forming a new crater inside the eastern caldera, there had been no previous recent historical eruption on Anatahan Island either. Both Sarigan and Anatahan are Holocene age volcanoes. Could the detected activity on Sarigan be the precursor of that volcano’s first activity since the Holocene? Could this be an indicator that general volcanic activity is increasing along this section of that island arc system? Only time will tell.
While authorities keep their seismographs tuned to the new activity under Sarigan Island, the volcano on Anatahan continues to eject huge ash plumes forcing aviation authorities to warn aircraft to keep their distance by at least 10 nautical miles. On Saturday, agencies reported that an ash and steam plume extended about 100 nautical miles west of the island, and that thin ash and volcanic smog extended another 700 nautical miles west, producing an extended risk to high-altitude jet aviation. During Anatahan’s strongest eruption April 6, that volcano expelled about 50 million cubic meters of ash, which it spewed out to 50,000 feet.