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The increase in seismicity that began on 1 August 1996 beneath Iliamna Volcano continued during the past week with an average rate of about 27 earthquakes per day. However, the number of events has decreased in the last two days. All events were within or just below the volcanic edifice. Magnitudes of most earthquakes were less than 1.0; the largest event during the past week was 3.2. All earthquakes seen so far are volcano-tectonic (VT); no long-period earthquakes or tremor that usually precede volcanic eruptions have been observed.
Our interpretation at this time is that magmatic intrusion is occurring beneath Iliamna Volcano. This intrusion likely began during a seismic swarm between May 11-28 and is continuing during the current swarm. Such an intrusion does not mean an eruption is imminent.
Iliamna Volcano is a deeply dissected stratovolcano located 225 km southwest of Anchorage. The frequency of eruptions from Iliamna Volcano is much less than other Cook Inlet volcanoes. Lahar (mudflow) deposits, perhaps only 300 years old, occur in several major river valleys near the volcano, in addition to large noneruption-induced landslide deposits. There is evidence of a major volcanic explosive eruption, perhaps 3,000 to 4,000 years ago. No historic eruptive activity has been documented at Iliamna Volcano. However, a prominent fumarole field located near the summit produces steam plumes commonly visible from Anchorage and the Kenai Peninsula on clear days.
Seismic activity at Spurr, Redoubt, 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.