The Incorporated Research Institutions for Seismology (IRIS), with cooperation and support from the Alaska Earthquake Center and University of Alaska-Fairbanks Geophysical Institute, is in the process of deploying 261 seismometers in Alaska as part of EarthScope’s USArray observatory. 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, originally migrating a footprint of 400 seismometers across the contiguous United States. TA stations are sited in a wide variety of environments, typically operate for ~2 years, provide data with real time telemetry, and produce high quality recordings with an average return of over 98%. In October 2013 that footprint, encompassing nearly 1700 individual stations, completed its movement 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 and 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.
Figure 1: Planned TA Deployments by Year
Figure 2: Alaska TA Deployment Status Map
The TA stations in Alaska will be 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. Several pilot (TOLK, TCOL, POKR, EPYK, HDA) and wider footprint (A36M, C36M) stations were installed in 2011-2013 and continue to operate. Eighteen new TA and upgraded existing stations came online during 2014. The majority of new TA stations will be deployed from 2015-2017. Most TA stations will be deployed for at least 2 years, and there is currently no plan to remove the stations.
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) though not fully characterized 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. Therefore, IRIS is working 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. Also, in concurrence with recommendations contained within the Autonomous Polar Observing Systems Workshop Report, 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).
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 likely 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. This hut will contain a seasonally dependent power system, with a high energy density air-cell/lithium iron phosphate battery cluster providing power over winter and a solar power system recharging these and providing power during the summer. This setup will power a Quanterra Q330 datalogger connected to a three-component broadband seismometer (STS-4B/5A, T120PH, CMG-3T, etc.) residing in an augered hole of several meters depth or hammer drilled directly into bedrock. In some cases, where access and site conditions permit, a traditional subsurface TA tank containing a vault instrument (e.g. T240, STS-2) may be used. The standard TA atmospheric sensor package containing a MEMS state-of-health barometer, NCPA infrasound sensor, and SETRA microbarograph will be included with each station. Additional sensors that have been suggested for deployment include a meteorological pack, strong-motion sensor, and/or soil temperature profiler. The footprint of each site will be around 10 by 20 feet. Our minimum goal is to maintain near-real time (12-24 hours, many within seconds) state-of-health telemetry and low-sample-rate waveforms via Inmarsat BGAN telemetry, but in at least some circumstances complete, high sample-rate data may only be obtained during in-person servicing of stations once or twice per year. With a datalogger capacity of up to 32 GB storage, the stations are capable of maintaining a complete record from a lengthy deployment.
Figure 4: Schematic for an Alaska TA station, click to enlarge.
Installed in August 2011, TOLK is located at Toolik Lake Research Station, north of the Arctic Circle (Figure 5). 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 5: TOLK - exterior (top left), vault interior (top right), and sensor hole (bottom)
In October 2012 four new stations were deployed across Alaska and Yukon in order to evaluate different sensor emplacement strategies and understand seasonal variations in station performance as continued preparation for the main Alaska TA deployments. These stations operate in a variety of environments and their performance further refined site construction and sensor emplacement strategies. All stations are telemetered with data being delivered in real-time and accessible through IRIS. Station EPYK has been deployed with the support and cooperation of Yukon Geological Survey and Natural Resources Canada. As with TOLK, several of these stations produce data at least equal to, and in most cases considerably better than, the average performance of the original TA sites.
An unprocessed seismogram of a M4.5 earthquake recorded at EPYK, ~171 km away.
Map - Rescoped TA Deployment Plan (.pdf) - v. 3/17/2015
Map - TA Deployment Plan (.pdf) - v. 8/5/2014
Map - TA Status Map with Station Names (.pdf) - v. 8/5/2014
Map - Proposed Sites with Landowner Data (.pdf) - v. 4/10/2014
Map - Proposed Sites with 20km Radius Buffers (.pdf) - v. 4/10/2014
Google Earth Points - Proposed Sites (.kmz) - v. 8/5/2014
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.
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.
Servicing TOLK for the winter, October 2011