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Magmatic, Eruptive and Tectonic Processes in the Aleutian Arc, Alaska

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Magmatic, Eruptive and Tectonic Processes in the Aleutian Arc, Alaska MAGMATIC, ERUPTIVE AND TECTONIC PROCESSES IN THE ALEUTIAN ARC, ALASKA PBO Working Group for Alaskan Volcanoes: J. Power1, M. Johnston2, J. Freymueller3, M. Lisowski4, T. Murray1, Zhong Lu5, J. Eichelberger3, D. Dzurisin4, S. McNutt3, S. Moran1, M. Wyss3, D. Mann3, C. Wicks2, C. Nye6, W. Thatcher2, J. F. Larson3, P. Haeussler7 1Alaska Volcano Observatory, USGS, 4200 University Dr. Anchorage, AK 99508 2U.S. Geological Survey, Menlo Park, CA 94025 3Alaska Volcano Observatory, Geophysical Institute, UAF, Fairbanks, AK 99775 4U.S. Geological Survey, 5400 MacArthur Blvd., Vancouver, WA 98661 5EROS Data Center, Sioux Falls, SD, 57198 6Alaska Volcano Observatory, Alaska Division of Geological and Geophysical Surveys, Fairbanks, AK 99775 7U.S. Geological Survey, 4200 University Dr. Anchorage, AK 99508 Introduction The Aleutian Arc contains roughly ten percent of the world’s active volcanoes and is the only place within the Pacific-North American plate boundary system where eruptions are frequent. Individual volcanoes along the 3,000-mile long arc represent the entire spectrum in eruptive style, size, volume, and magma composition commonly found along convergent plate boundaries. The four most active volcanoes, Pavlof, Akutan, Shishaldin, and Okmok, have produced a total of 82 eruptions during the past century. More silicic volcanoes near the eastern end of the arc are also highly active. Augustine, Redoubt, and Spurr have all erupted within the last 15 years, and the 1912 Katmai eruption on the Alaska Peninsula was the largest eruption on earth during the 20th century. Presently six volcanic centers, Augustine, Trident, Fisher, Westdahl, Akutan, and Okmok, are known to be actively deforming. Only by instrumenting Aleutian volcanoes will we be assured of making observations that can be used to study eruption triggering and dynamics; elsewhere along the plate boundary, based on the historical record, eruptions are less likely to occur. Major earthquakes also occur frequently along the Aleutian Arc (eg. The 1964 Great Alaskan earthquake (M 9.2), the 1965 Rat Islands earthquake (M 8.7), the 1957 Andreanof Islands earthquake (M 9.1), and numerous other events M > 7). More than 90% of the historic seismic moment release in the Pacific – North American plate boundary has occurred along the Alaska – Aleutian trench. The Aleutians thus form an ideal and unique natural laboratory in the U.S. for studying fundamental issues such as 1) the location and evolution of magma bodies, 2) eruption dynamics, 3) eruption triggering, 4) triggered seismicity, 5) relations between deformation, LP and VLP events, 6) interaction between active faulting and volcanoes 7) how the plates are coupled, and 8) provides the most likely opportunity in North America for near-field observations of a M~8 earthquake. While there is not a clear link between major earthquakes and eruptions, observations made by the Alaska Volcano Observatory (AVO) seismic 2 monitoring networks demonstrate that seismic swarms at Aleutian Arc volcanoes have been triggered by significant earthquakes, and volcanically induced stresses interact to control deformation and perhaps other processes at these volcanoes. We propose to install GPS, strain, and seismic instrumentation at five selected volcanic centers along the eastern half of the Aleutian arc. The proposed volcanoes include Augustine, Pavlof, Unimak Island (Shishaldin Volcano, Fisher Caldera, and Westdahl Volcano), Akutan Volcano, and Okmok Volcano (Figure 1). Taken together these volcanoes represent the most active volcanoes in North America in a variety of compositional and eruptive styles. Based on the historical record we would expect to observe an eruption at one of these 5 centers every 2 years. The suggested instrumentation, along with proposed PBO backbone stations and existing networks operated by the AVO, provides for minimal instrumentation along about half of the Aleutian subduction zone. The tectonic setting and history also varies in each of these portions of the arc. Different fundamental geophysical questions can be posed in each subregion. Figure 1. Volcanoes proposed for additional instrumentation in the Aleutian Arc. Rupture zones of the 1964, 1938, 1946, and 1957 earthquakes are also shown. Design Strategy The suggested strategy at Aleutian volcanoes is to deploy smaller, yet still adequately dense networks of borehole strain/seismometers (BHSM/BHS) and continuous GPS instruments at each target volcano. The basic deployment would consist of four BHSM/BHS and 8 GPS receivers. This level of instrumentation would allow us to locate 1 BHSM/BHS and 2 GPS receivers in each quadrant at four of the selected volcanic centers. The borehole seismometers would become part of the Earthscope permanent network. Because of the need to understand the relationship between volcanic and tectonic strain we have also included a few GPS receivers as far-field references for each of the volcanic centers. In areas where 3 volcanic deformation is more distributed spatially, such as Unimak Island, we would only deploy GPS instruments. Although some of these volcanoes are remote, AVO has installed and successfully operated seismic nets and conducted campaign GPS surveys on most of them, and we are confident that instruments can be installed and operated at all of these centers. Campaign surveys of GPS, deployment of USARRAY instruments, as well as geologic investigations would provide greater detail in areas of special interest. In areas where borehole water level data are available campaign gravity surveys would also be pursued. Clearly, the numbers of instruments are not adequate for solution of all questions posed, but tight instrumentation in a few critical areas and broader coverage in others should provide a guide to future studies. As a first step AVO plans to deploy a proto-type installation during the summer of 2002 on Akutan Volcano in the central Aleutian Islands (Figure 1). Akutan is a 1100-meter high cone shaped stratovolcano that is topped by a 2-km-diameter caldera. The volcano is the fourth most active volcano in Alaska with 27 reported historical eruptions. Additional eruptive events have likely gone unreported as a result of the remote location and persistent cloud cover at the volcano. A cinder cone within the summit caldera has been the site of all historic activity. Large ash plumes, basaltic lava flows, strombolian activity at the summit cinder cone, and lahars and mudflows characterize historic eruptive activity at Akutan. During March 1996 Akutan was the center of two large earthquake swarms that consisted of more than 200 shocks greater than magnitude 3.5. The total estimated seismic energy release for these two swarms is 2.7 X 1018 N m. InSAR measurements spanning the swarm suggest that the NW flank of the volcano was uplifted by as much as 60 cm, while portions of the eastern side of the island subsided. The proposed instrumentation for 2002 at Akutan consists of borehole strain/seismometers and telemetered GPS at 5 locations surrounding the volcano (Figure 2). Scientific Opportunities With instrumentation of this quality surrounding the five proposed volcanic centers we will be well positioned to address the following issues: • Obtain real-time data on the size and location of magma chambers, the dynamics of magma transport, and the geometry of intrusive bodies as they evolve before, during, and after eruptions. • Determine the inter-relation between volcanic and tectonic deformation using GPS and BHSM. • Develop a suite of continuous deformation measurements at a number of active volcanoes that cover the full range of eruptive styles and magma chemistry. • Combine BHSM and BHS data to isolate the mechanics, geometry and implications of long-period (LP) events, very long period (VLP) events, volcanic tremor, and other volcanic seismicity and strain transients. • Obtain real-time data on volume/pressure changes of magma chambers. 4 Figure 2. Red dots indicated locations of borehole strain/seismometers and GPS instruments on Akutan Island proposed for deployment by AVO during 2002. Green triangles and blue squares show locations of existing AVO seismometers and campaign GPS monuments. • Determine the degree of remote and static triggering of volcanic intrusions and earthquake swarm activity on active volcanoes using BHSM and BHS data. • Determine the relative frequency and importance of both static and dynamic triggered seismicity at instrumented volcanoes. • Measure regional tectonic strain and its variations through each region and on a broader scale using continuous GPS. • Determine regional tectonic stress within the surrounding the target areas using shorter term deployment of higher quality seismic instrumetation. • Estimate coupling, slip partitioning, and arc wide deformation along the eastern half of the Aleutian subduction zone. • Provide the most likely opportunity in North America to make near field measurements of a M~8 or larger earthquake. .
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