The Incorporated Research Institutions for Seismology (IRIS) is deploying 261 seismograph stations in Alaska and Yukon as part of EarthScope’s Transportable Array. The project is sponsered by the National Science Foundation and has cooperation and support from the Alaska Earthquake Center, University of Alaska-Fairbanks Geophysical Institute, as well as the Yukon Geological Survey and Canadian Hazard Information Service. EarthScope explores the structure and evolution of North America using continuous recordings from broadband seismometers and surface deformation measured with precision GPS sensors. IRIS has operated the USArray Transportable Array (TA) since 2004, a migrating footprint of 400 seismometers that roamed completely across the contiguous United States from the Pacific to the Atlantic. TA stations were sited at 42 mile intervals, typically operated for ~2 years and produced high quality recordings with an average return of over 98%. In October 2015 that deployment, encompassing nearly 1700 individual stations, completed its traverse across the lower-48 U.S. With the successful funding of the IRIS SAGE Proposal, the TA is being redeployed as a single, static footprint in Alaska and northwestern Canada through 2018 (Figures 1-2). The Transportable Array Advisory Committee (TAAC) advises on the operation and technical performance of the TA, in the context of EarthScope's science goals and changes in annual budgets. Many lower-48 TA stations remain as part of a separately funded project, CEUSN and continue to be available from the IRIS Data Management Center with network code N4.
Figure 1: Planned TA Deployments by Year. "TA" sites (circles) are new seismic stations, and "Existing" sites (squares) are either upgraded or contributing seismic stations.
Figure 2: Currently active stations under the _US-TA virtual network. Click on the map for station information at the IRIS DMC.
The TA stations in Alaska are arranged in a grid-like pattern spaced at ~85 km, covering all of interior Alaska and parts of the Yukon, British Columbia, and the Northwest Territories. A handful of early pilot stations were installed in 2011-2013 and continue to operate. At the end of the summer 2015, 46 new stations are operating and 65 existing stations are cooperating in the array. The majority of new TA stations will be deployed from 2016-2017, about 70 each summer. TA stations are usually temporary and will be removed beginning in 2019, although some stations may be adopted and operate more permanently. Data from the TA is routed to the IRIS Data Management Center for public distribution, and is utilized by the USGS National Earthquake Information Center in Golden, CO and NOAA Tsunami Warning Center for real-time hazard monitoring. Cooperation with the Canadian Hazards Information Service (CHIS), and Yukon Geological Survey (YGS), and Natural Resources Canada (NRCan) has been vital to extending TA stations across the border.
As outlined by the main report and associated white papers stemming from a recent Alaska-themed workshop, there are numerous well-defined scientific motivations for shifting the TA to Alaska. For instance, Alaska's rate of earthquakes is significantly higher than the entire lower-48 U.S. combined, and that seismicity is spread across much of the state (Figure 3). The TA hopes to better characterize seismicity patterns in regions with currently sparse placement of seismometers. The investment of EarthScope directly benefits other seismic network operators in Alaska, many of which are focused on characterizing seismic, volcanic, and tsunami hazards. IRIS is works with the Alaska Earthquake Center, Alaska Volcano Observatory, and the Alaska Tsunami Warning Center to upgrade and leverage existing seismic infrastructure and permitting wherever possible. IRIS is keen to collaborate with members of the Arctic science community during the development and implementation of USArray in Alaska.
Figure 3: Seismicity from 1970-2012 for Alaska and vicinity from the Alaska Earthquake Center and USGS PDE catalogs, click for pdf. Figure courtesy of Natasha Ruppert (AEC).
The world’s second largest earthquake ever recorded was a M9.2 that ruptured along the Alaskan Subduction Trench in 1964. The video below and to the left explains the significance of the 1964 earthquake and is provided by IRIS EPO. A simulation of this earthquake using recorded seismograms, tsunami data, and uplift data from Inchinose et al. (2007) along with a global tomography model is shown in the video, below and left. Earth structure determined using the data collected with the TA will be able to refine this ground motion model. Yellow dots are Alaska's three major cities of Anchorage, Fairbanks, and Juneau. Green dots are broadband seismic stations. White lines are active faults and plate boundaries. This visualization is provided by Carl Tape at the University of Alaska Fairbanks. Both videos can be expanded to full screen by clicking the expanded rectangle in the bottom right corner.
In order to operate in the challenging conditions of these high latitudes, the construction and configuration of TA stations in Alaska requires significant changes from the established design (Figure 4). Remote stations will be contained in an insulated, above-ground enclosure similar to those developed for EarthScope’s Plate Boundary Observatory (PBO) GPS stations, which already operate in Alaska. The hut contains a power system, with a high energy density lithium iron phosphate battery cluster providing power over winter that is recharged by and providing power during the summer. The seismograph system consists of a Quanterra Q330 datalogger connected to a three-component broadband seismometer (STS-4B/5A, T120PH, CMG-3T, etc.) residing in a 6-inch diameter hole of 2.5 meters depth, usually hammer drilled directly into bedrock. The standard TA atmospheric sensor package containing a MEMS state-of-health barometer, NCPA infrasound sensor, and SETRA microbarograph is included with each station. Additional sensors that have been suggested for deployment include a Vaisala WXT-520 meteorological pack, strong-motion sensor, and/or soil temperature profiler. The footprint of each site will be around 10 by 20 feet. Our goal is to maintain near-real time telemetry utilizing InMarSat BGAN, Iridium, cellular or direct radio links. With a datalogger capacity of up to 64 GB storage, the stations are capable of maintaining a complete record from 8+ years of deployment.
Figure 4: Schematic for an Alaska TA station, click to enlarge.
While bedrock is the preferred ground for installing seismic instruments, it is not always available such as in the north part of Alaska that is dominated by unconsolidated sediment (tan color in map below). With the station locations overlaying the newly released geologic map of Alaska (Wilson et al., 2015), it shows that rock formations were used when possible to reduce site noise and preserve the even spacing of stations.
Figure 5. Planned TA Deployments by Year Overlaying Geologic Map. Click on map to view version with smaller symbols. All map units follow the generalized geologic classification of Wilson et al. (2015). Black lines indicate faults and calderas. More information about the geologic map is available at the USGS.
Installed in August 2011, TOLK is located at Toolik Lake Research Station, north of the Arctic Circle (Figure 6). Its performance yielded important findings on how to auger a sensor hole beneath the active layer of the permafrost to resist its seasonal freeze-thaw cycle and exposed necessary design changes needed for the vault design and power setup. Initial measurements of the seismic power-spectra show that the TOLK station is remarkably quiet compared to other Transportable Array stations in the original footprint.
Figure 6: TOLK - exterior (top left), vault interior (top right), and sensor hole (bottom)
Map - Rescoped Deployment Plan (.pdf) - v. 4/22/2016
Map - Status with Station Names (.pdf) - v. 4/22/2016
Google Earth Points - Proposed Sites (.kmz) - v. 4/22/2016
Robert Woodward, Director of Instrumentation Services, 1-202-682-2220 ext. 206
Bob Busby, Transportable Array Manager, 1-800-504-0357
In May 2011, NSF-EarthScope held a workshop dedicated to discussing scientific opportunities in Alaska that could be explored specifically with the Transportable Array and Plate Boundary Observatory. White papers and a workshop report highlight the geologic background of Alaska and outline the key questions in solid earth science that can be addressed through EarthScope in Alaska.
In September 2011, NSF-GeoPRISMS held a workshop to determine research focuses in Alaska and coordinate with EarthScope investigators to maximally leverage the planned deployment of EarthScope’s facilities in the region.
A workshop to discuss current Earth Sceince research in northwestern Canada is in the early planning stages for March 2016 in Whitehorse Yukon.
Plafker, G. and Berg, H.C. (1994) An overview of the geology and tectonic evolution of Alaska, in Plafker, G. and Berg, H.C. (Eds.), The geology of Alaska: Boulder, Colorado, Geol. Soc. of Am., The Geology of North America, v. G1, p. 989-1021. (likely only available in print)
Freymueller, J.T., P.J. Haeussler, R.L. Wesson, and G. Ekstrom (Eds.) (2008), Active Tectonics and Seismic Potential of Alaska, Geophys. Monogr. Ser., vol. 179, 431 pp., doi:10.1029/GM179, AGU, Washington, D. C.
Wilson, F.H., Hults, C.P., Mull, C.G, and Karl, S.M, comps. (2015) Geologic map of Alaska: U.S. Geological Survey Scientific Investigations Map 3340, pamphlet 196 p., 2 sheets, scale 1:1,584,000, http://dx.doi.org/10.3133/sim3340.
Newly installed TA equipment at Alaska Earthquake Center's station at the Knik Glacier (5/12/2015)