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COOK INLET VOLCANOES
The seismic swarm that began on 1 August 1996 beneath Mount Iliamna continued during the past week at a level of about 20 earthquakes located per day ranging in magnitude up to 1.9. Earthquakes occurred within and below the volcanic edifice to 9 km below sea level. The apparent increase in earthquakes this week is in part due to improved data acquisition as the two recently installed seismic stations were brought on line. All earthquakes seen so far are volcano-tectonic (VT); no long-period earthquakes or tremor that usually precede volcanic eruptions have been observed.
The interpretation of data at this time is that magmatic intrusion has occurred beneath Mount Iliamna, beginning with the May 11-28 swarm and continuing with the current swarm. Such an intrusion does not mean an eruption is imminent.
The frequency of eruptions from Mount Iliamna is much less than other Cook Inlet volcanoes. An AVO geologic mapping team in late summer clarified some of the little known eruptive history of the volcano. Lahar (mudflows), perhaps as recent as 300 years old, were mapped in several major river valleys near the volcano, as were large noneruption-induced landslide deposits. Evidence of a major volcanic explosive eruption, perhaps 3,000 to 4,000 years ago, was also detected.
Mount Iliamna is a deeply dissected stratovolcano located 225 km southwest of Anchorage in the Aleutian Range. The volcano is mostly covered with perennial snowfields and valley glaciers. No historic eruptive activity has been
documented at Mount Iliamna. However, a prominent fumarole field located near the summit frequently produces small steam plumes commonly visible from Anchorage and the Kenai Peninsula on clear days.
Seismic activity at Spurr, Redoubt, and Augustine
volcanoes remains at normal background levels.
AVO maintains a computerized alarm system capable of
notifying AVO seismologists during non-business hours
should unusual seismic activity occur at Spurr, Redoubt,
Augustine, or Akutan volcanoes.
AVO continues to test new seismic networks at Pavlof, Dutton, Makushin, Akutan, and Katmai area volcanoes. Reports on these volcanoes will be added to the AVO weekly update as data acquisition and analysis become reliable. We anticipate that this will occur by mid-November.