Iliamna volcano is an andesitic stratovolcano in the southern portion of Cook Inlet, Alaska. Although it has no known historic eruptions, large fumarole fields and frequent seismicity confirm that it should still be considered active
Since 1994, at least four small debris avalanches have occurred on Iliamna. The large number of mass-wasting events is likely due to the fact that hydrothermal alteration of rocks near the summit results in weakening of the edifice. Furthermore, Iliamna is heavily glaciated and heat from the fumaroles promotes failure of the ice.
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All of these debris avalanches have been recorded by the 6-station seismic network on Iliamna. They all have extremely similar signatures indicating that the failure process is similar between events. Each event begins with a sequence of highly repetitive, discrete earthquakes. This signal degrades into a continuous rumbling signal that may endure for over an hour. Finally, the seismometers record a broadband (1-25 Hz) signal that often consists of several strong pulses and saturates stations within a few km of the event. | |
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The spectrogram to the left shows that the spectral content of the diffuse rumbling signal is very similar to that of the discrete earthquakes. One possibility is that this signal results from a nearly continuous sequence of tiny earthquakes with the same source characteristics as the discrete events. | |
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Although the individual discrete earthquakes are tiny, I made use of the waveform similarity between events to get high-resolution locations for them. The map to the left shows the topography of Iliamna with earthquake epicenters in red (each cluster represents between 40 and 80 events). These locations are exactly where we found the headwall of the 2004 debris avalanches. | |
The fascinating thing about these avalanches is the fact that each exhibits at least an hour, if not several, of precursory seismicity. This type of onset hints at the possibility of warning for future events. However, most snow avalanches and rockfalls do not exhibit the precursory activity. One major exception was an ice avalanche that occurred on Mt. Baker in 1977. This sequence also had a long seris of discrete earthquakes followed by a more diffuse, continuous ground shaking.
My goal with these data is to determine the source of each of the three portions of the seismic signal and to evaluate why Iliamna debris avalanches have a precursory sequence while others do not. Stay tuned for results.
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