Geology topics
Publications and source records attributed to M. K. Sako.
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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.
No abstract available.
Increased earthquake activity and compression of the south flank of Kilauea volcano, Hawaii, have been recognized by previous investigators to accompany rift intrusions. We further detail the temporal and spatial changes in earthquake rates and ground strain along the south flank induced by six major rift intrusions which occurred between December 1971 and January 1981. The seismic response of the south flank to individual rift intrusions is immediate; the increased rate of earthquake activity lasts from 1 to 4 weeks. Horizontal strain measurements indicate that compression of the south flank usually accompanies rift intrusions and eruptions. Emplacement of an intrusion at a depth greater than about 4 km, such as the June 1982 southwest rift intrusion, however, results in a slight extension of the subaerial portion of the south flank. Horizontal strain measurements along the south flank are used to locate the January 1983 east-rift intrusion, which resulted in eruptive activity. The intrusion is modeled as a vertical rectangular sheet with constant displacement perpendicular to the plane of the sheet. This model suggests that the intrusive body that compressed the south flank in January 1983 extended from the surface to about 2.4 km depth, and was aligned along a strike of N66°E. The intrusion is approximately 11 km in length, extended beyond the January 1983 eruptive fissures, which are 8 km in length and is contained within the 14-km-long region of shallow rift earthquakes.
No abstract available.