Volcanologic investigations in the Commonwealth of the Northern Mariana Islands, September - October 1990

Moore, Richard; Koyanagi, Robert; Sako, Maurice; Trusdell, Frank; Kojima, George; Ellorda, Renee; Zane, Sandra

U.S. Geological Survey volcanologists visited four volcanic islands (Anatahan, Alamagan, Pagan, and Agrigan) in the Northern Marianas during September-October 1990, at the request of the Office of Civil Defense (CD) for the Commonwealth of the Northern Mariana Islands (CNMI). A shallow earthquake swarm on Anatahan in March—April 1990 and reports of possible new fuming on Agrigan in August 1990 had prompted CD to evacuate all inhabited islands north of Saipan. Permanent seismic stations were installed on Anatahan, Alamagan, and Pagan; data are radio-telemetered to Saipan and recorded at Civil Defense headquarters. No significant local microearthquakes or volcanic tremor have occurred on Anatahan, Alamagan, or Pagan since installation of the permanent stations. Seismographs operated for four days at temporary stations on Agrigan recorded no sustained microearthquake activity or volcanic tremor. The team established a new EDM (electronic distance measurement) network on Agrigan. Remeasurement of EDM networks on Pagan and Anatahan showed no significant changes in line lengths, in accord with the lack of local seismicity. The team conducted geologic studies on the floor of Agrigan’s caldera and discovered a boiling hot spring, associated terrace deposits, and solfataras. These features may have started to form soon after Agrigan’s most recent eruption in 1917. Temperatures of 25 solfataras and the hot spring water were 98°C. Water from the hot spring had a pH of 2.0. Abundances of gases emitted by the solfataras were 200-320 ppm HCl, 0-3 ppm CO2, >3% CO, 1,900->2,000 ppm H2S, and >400 ppm SO2, but atmospheric dilution may have occurred. We conclude that the low levels of local volcanic seismicity, deformation, and fuming indicate that the threat of eruptions in the immediate future on Anatahan, Pagan, and Agrigan is much reduced. However, a new surge in maginatic activity, such as apparently occurred on Anatahan in March-April 1990, could begin sometime in the future and once again increase the possibility of eruption.