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COOK INLET VOLCANOES
The seismic swarm that began on 1 August 1996 beneath Mount Iliamna persisted during the past week at a level of 3 to 20 earthquakes located per day ranging in magnitude from 0.1-2.5. Two additional seismic stations were installed on the volcano this week, allowing AVO to increase the detection sensitivity and improve earthquake locations. Located earthquakes ranged in depth from within the volcanic edifice to 9 km below sea level. All earthquakes seen so far are volcano-tectonic (VT); no long-period earthquakes or tremor that usually precede volcanic eruptions have been observed.
Two flights to measure volcanic gas emissions have been conducted and will be repeated as weather permits. SO2 flux was measured at the background rate of approximately 40 tonnes per day; CO2, often indicative of magmatic degassing, was detected.
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 past 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.
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 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.