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A Policy for Rapid Mobilization of USGS OBS (RMOBS)


Introduction
Potential Sites
Alaska Volcanoes
Continental Borderland
Mendocino Triple Junction
Policy and Resources
Obligations of RMOBS Participants
Criteria for Support
Contact Information
Acknowledgements

Alaska Volcanoes

Written by John Power


The Aleutian volcanic arc is comprised of at least 75 volcanic centers that extend more than more than 2500 km from Mount Spurr to Kiska (Fig. 1).  The arc is the result of the subduction of the Pacific plate beneath the North American plate.  Within historic times 40 of these volcanoes have produced more than 265 documented eruptions (Miller et al., 1998).  Based on historic rates of activity over the last 100 years1 to 2 eruptions within the Aleutian arc each year.  The high rate of volcanic activity is thought to be related to the rapid rate of plate convergence (~7 cm/year maximum) along much of the arc.  Aleutian arc volcanoes cover the entire range in eruptive style and magma composition.  The principal hazards from Aleutian volcanic eruptions are airborne volcanic ash, which is extremely dangerous for aircraft.  The Aleutian volcanoes lie beneath the heavily traveled North Pacific air routes, Local communities, oil production facilities and land based sea food processing plants are also exposed to hazards from mudflows, ash fall, and pyroclastic flows.

Map of historically active volcanoes in Alaska

Figure 1. Historically active volcanoes in Alaska.


The Alaska Volcano Observatory (AVO) a joint program of the U.S.G.S., the Geophysical Institute of the University of Alaska, and the Alaska Division of Geological and Geophysical Surveys is presently involved in monitoring Aleutian arc volcanoes and provides warnings to local communities and affected industries.  AVO presently monitors seismic activity in real-time at 25 Aleutian arc volcanoes in real-time and plans to extend seismic monitoring to all 41 historically active centers within the next 10 years. 

Many of the Aleutian volcanoes are located on small islands that greatly restrict the geometry and aperture of AVO’s seismographic networks.  Location of seismic stations is often further restricted by strong mircoseismic noise generated by ocean waves.  The effects of mircoseismic noise are often amplified by the unconsolidated pyroclastic deposits that frequently makeup the flanks of many Aleutian arc volcanoes.  The limited extent of many of AVOs seismographic networks often restricts the accuracy of earthquake hypocenter determination and frequently precludes the detection and tracking of volcano induced seismicity in the mid- to lower crust.  The addition of Ocean Bottom Seismometers (OBS) would greatly strengthen AVOs existing seismic networks and would allow improved locations of volcano related seismicity such as Long-Period events and Volcano-Tectonic earthquakes as well as improved determination of velocity and attenuation structures using tomographic techniques.  These measurements would provide for a greatly improved understanding of the subsurface components of the magmatic system and the processes that proceed and lead to eruptions.

The Aleutian arc volcanoes that are most likely to benefit from deployment of OBS systems would be Augustine, Westdahl, Akutan, Makushin, Okmok, Great Sitkin, Kanaga, Tanaga, and Gareloi (Fig. 2).  These volcanoes have a combined total of 64 eruptions within the last 100 years, suggesting that an eruption could be expected at one or more of these volcanoes within the next 5 years.  Other Aleutian volcanoes could certainly benefit from OBS deployments but these are not currently monitored by AVO.  Volcanic eruptions are frequently preceded by weeks to months of increased earthquake activity.  Once the AVO seismic network identified significant unrest we would deploy OBS systems surrounding the target volcano within 1 to 2 weeks.  The OBS would be deployed from a contract vessel based in Homer, Dutch Harbor or Adak (Fig. 2).

Possible target volcanoes for OBS deployment

Figure 2. Possible target volcanoes for OBS deployment.



References

Miller, T.P., McGimsey, R.G., Richter, D.H., Riehle, J.R., Nye, C.J., Yount, M.E., Dumoulin, J.A., 1998, Catalog of the active volcanoes of Alaska, U.S. Geological Survey Open-file Report 98-582.

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