A key element of the EarthScope seismic observation system is a dispersed permanent network that provides a long-term reference frame for comparison of observations made with the denser, but transient, Transportable Array as it crosses the country. This Reference Network was developed in close collaboration with the USGS as an augmentation to the backbone component of the USGS Advanced National Seismic System (ANSS). EarthScope contributed the installation and upgrading of 39 stations to the ANSS Backbone (this was referred to as the Permanent Array component of USArray and was completed September 30, 2006). The Transportable Array also installed 20 stations to improve the uniformity of the Reference Network coverage (these are often referred to as the Advanced Deployed TA stations), and these stations will be operated for the duration of USArray.
As a result of these various efforts the Reference Network now consists of more than 100 stations, at approximately 300-km spacing, across the conterminous United States and 10 stations in Alaska. EarthScope and IRIS are exploring ways to work with regional operators and the USGS to fill some gaps in coverage that continue to exist, especially in the north-central United States.
Each of the reference stations includes broadband sensors capable of recording seismic waveforms, from long-period surface waves to high-frequency body phases, for use in studies of structure from the lithosphere to the core. A subset of the stations are equipped with ultra-long-period sensors, extending instrument response to thousands of seconds, making them capable of capturing Earth's free oscillations excited by the largest earthquakes. Data from these stations are transmitted in real time to the USGS National Earthquake Information Center in Golden, CO to support their mission of national and global earthquake monitoring. These data, along with those from all other ANSS Backbone stations, are also transmitted to the IRIS Data Management Center (DMC) for archiving and distribution.
The USGS and its affiliated regional network operators have the operations and maintenance responsibility for the majority of the Reference Network stations.