Alaska is home to most of North America’s earthquakes, including the second largest ever recorded (Mw 9.2). It is also a place where subducting plates traverse the upper mantle, driving abundant magmatism in an arc nearly 3000 km long. From now through 2018, the Transportable Array will site, deploy and operate stations throughout Alaska, completing the coverage of the continental United States. Likely the TA will be supplemented by portable seismic deployments and by other related activities onshore and offshore. All of these build on knowledge from a small number of past experiences in the area, which provide clear evidence of the wealth of scientific opportunities and special challenges working in this harsh environment. I will provide an overview of some past experiments and projects, highlighting several for which I have personal experience.
A couple lessons emerge. First, seismicity is remarkably abundant, and constitutes the vast majority of earthquakes in the U.S. Besides the signals, such high seismicity levels present operational challenges. Second, the subduction zone produces considerable structure within the upper mantle, precisely where broadband arrays can provide the most information about the Earth. In two past experiments (BEAAR and MOOS) we mapped out the subducting crust and plate interface zone from near the trench to 130 km depth, over several hundred km, and observe hints of many other structures. At this scale, the TA is guaranteed to image a great deal of interesting structure, and focused experiments have even more promise. Third, logistics are challenging but not insurmountable if projects are well designed and logistical constraints are taken into account in their design. Road systems are sparse but exist in some parts of the state, and airstrips exist in many others, although helicopters are clearly necessary in many regions. Finally, many problems associated with great earthquakes and volcanism will require coordinated marine programs, since the plate boundary system does not stop at the coastline and the Aleutians are small islands. Ocean-bottom seismometers can play a critical role in both the thrust zone and around remote segments of the volcanic arc.
Overall, the arrival of USArray has tremendous potential to reveal fundamental properties of the nature of subduction beneath continents.
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