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Volcanologic investigations in the Commonwealth of the Northern Mariana Islands, May 1992

1993; United States Department of the Interior; Linguagem: Inglês

10.3133/ofr93541

2332-4899

R.B. Moore, Robert Y. Koyanagi, M. Sako, Frank A. Trusdell, R. L. Ellorda, George Kojima,

Botany and Geology in Latin America and Caribbean

U.S. Geological Survey volcanologists examined the ten volcanoes in the active Mariana Arc north of Saipan in May 1992, at the request of the Governor and the Disaster Control Office of 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 the evacuation of all CNMI islands north of Saipan.None of the volcanoes in the chain erupted during our visit.Five uninhabited islands (Farallon de Pajaros, Maug, Asuncion, Guguan, and Sarigan) were inspected only from the air, while the other four were studied in more detail.The previously installed seismic stations on Anatahan, Alamagan, and South Pagan were upgraded.A new station was established at the southwestern base of the intermittently active Mount Pagan, on the northern end of Pagan Island.Portable seismometers were operated on Anatahan, Alamagan, Pagan, and Agrigan.The seismometers on Anatahan, Alamagan, and Agrigan recorded no local shallow earthquake swarms nor volcanic tremor indicative of shallow magmatic movement.On Mount Pagan, intermittent low-amplitude tremor indicated the continuing possibility of occasional small ash eruptions, which prior to our visit had been witnessed in April 1992.Ash eruptions resumed in February 1993.Electronic distance measurement (EDM) lines were remeasured on Agrigan, Mount Pagan, and Anatahan.Line length changes were generally insignificant, in accord with the absence of significant shallow seismicity.Hot spring waters were collected on Agrigan, Pagan, and Anatahan, and fumarole temperatures were measured on Agrigan, Pagan, Alamagan, and Anatahan.The temperature data showed no indication of an impending significant change in the state of these volcanoes.We mapped the geology of Alamagan, collected charcoal to date eruptions of Alamagan and Mount Pagan, and collected rocks on Agrigan, Pagan, Alamagan, and Anatahan for petrographic and chemical studies.We conclude that the low shallow seismicity, lack of significant deformation, and low fumarole temperatures suggest that no eruption is likely soon on Agrigan, Alamagan, or Anatahan.The persistent low-level volcanic tremor on Mount Pagan suggests that intermittent small explosive eruptions may continue to occur.The normally populated islands Agrigan, Alamagan, and Anatahan were evacuated in August 1990 as a result of a shallow earthquake swarm on Anatahan in March-April 1990 and reports, which later proved erroneous (Moore and others, 1991), of increased fuming on Agrigan in August 1990.Except for brief visits by the authors, fishermen, and CNMI personnel, Pagan Island has been evacuated since the major Plinian eruption of Mount Pagan on May 15,1981 (Banks and others, 1984).About ten residents returned to Agrigan during May 1992.The volcanic islands that form part of the active Mariana Arc north of Saipan (fig. 1) are built 2-3 km above the East Mariana Ridge, which in turn rises about 2-4 km above the ocean floor.The >6-km-deep Mariana Trench adjoins the East Mariana Ridge on its east side, and the Mariana Trough, partly filled with young lava flows and volcaniclastic sediments, lies to the west of the East Mariana Ridge (Tanakadate, 1940;Hess, 1948;Karig, 1971; Karig and others, 1978; Banks and others, 1984).The submarine West Mariana Ridge, Tertiary in age (Kroenke and others, 1981), bounds the western side of the Mariana Trough.The Mariana Trench and East Mariana Ridge overlie an active subduction zone where the Pacific Plate, moving northwest at about 10.3 cm/year, is passing beneath the Philippine Plate, moving west-northwest at 6.8 cm/year (Simkin and others, 1989).Beneath the Northern Mariana Islands, earthquake hypocenters at depths of 50-250 km identify the location of the west-dipping subduction zone, which farther west becomes nearly vertical and extends to 700 km depth (Katsumata and Sykes, 1969).During this century, 36 earthquakes of magnitude 6.5-7.7 have occurred beneath the Mariana Trench.From January 1990-February 1992, nearly 400 earthquakes of magnitude >3.6 occurred below the Trench and East Mariana Ridge (National Earthquake Information Center, unpublished data, 1992).The Mariana Islands form two parallel, concentric, concave-west arcs (fig.1).The southern islands constitute the outer arc and extend northward from Guam to Farallon de Medinilla.They consist of Eocene to Miocene volcanic rocks and uplifted Tertiary and Quaternary limestone (G.Corwin, unpublished manuscript, 1971).The nine northern islands extend from Anatahan to Farallon de Pajaros and form the active inner arc.The inner arc extends south from Anatahan, where volcanoes, some of which are active, form seamounts west of the older outer arc (fig.1;Karig, 1971;Stern and Bibee, 1984).Other volcanic seamounts of the active arc surmount the East Mariana Ridge north of Farallon de Pajaros (Karig, 1971).The ten volcanoes that form nine islands along the active inner arc are spaced an average of about 42 km apart; distances between volcanoes range from 10 km between South Pagan and Mount (North) Pagan to 92 km between Agrigan and Asuncion.No volcanoes along the active inner Mariana Arc erupted during our visit.Five (Farallon de Pajaros, Asuncion, Agrigan, Mount Pagan, and Guguan) have erupted during this century (Tanakadate, 1940).Oral reports and photographs by CNMI personnel indicate that Mount Pagan resumed intermittent eruptive activity on February 13, 1993.These small ash eruptions apparently are continuing at this writing (July 1993).This report summarizes the results of our visit and includes several recommendations to the CNMI government about monitoring seismic activity.A condensed summary of this report was published in Moore and others (1992).