In terms of the geographical distribution of high-quality seismographic stations, the continental US is not well covered in comparison with some other large continents, and is not as well covered today as it was in the days of the WWSSN. As a component of a broader initiative to instrument the North American continent (USArray) we explore the need for, and opportunities afforded by, an array of permanent seismic observatories.
A permanent array of broadband seismograph stations in the US would provide new and different data for regional scale studies of the upper mantle, as well as for studies targeted at the Earth's deep interior (for example, the core and the CMB). Many of the most significant advances in mantle seismology have come not through global studies but small-region studies, often involving seismic arrays (i.e., lower mantle anisotropy, the D'' discontinuity, and the ultralow velocity zone at the core-mantle boundary). A continent-sized array would provide an unprecedented window into the interior of the Earth. An excellent location for such a window is the US because of the ideal source-receiver distance from the large amounts of western Pacific seismicity, as well as the lower cost involved. Tomography of the upper mantle in North America would benefit greatly from station spacing on the order of 500 km. Given the existing distribution of sources and receivers, such spacing would allow much improved sampling of the continental interior and would enable tomographic images of the North American upper mantle to achieve true 5 by 5 degree resolution. Five hundred km should be seen as the maximum station spacing. This would allow one to work with station spacings on the order of single wavelengths for body waves at periods of 60 s (P waves) to 120 s (S waves), or surface waves in the period range 125 to 150 s.
This expanded network of stations, contributing data in near-real time to the USGS NSN, would improve the detection, location, and source characterization capabilities of the NEIS for earthquakes and other seismic events in the US and surrounding areas. With this expanded network, moment tensor estimation of earthquake parameters from regional waveforms could include earthquakes to smaller magnitudes (approximately magnitude 3.5) anywhere in the continental US, useful for studies of seismic hazard and tectonic state of stress.
A denser network of high-quality stations would be useful as fixed reference observation points for future portable deployments aimed at detailed mapping of the crust and upper mantle structure beneath the US (e.g., EarthCUBE, Humphreys et al., 1997). Current efforts to combine tomographic images from a variety of portable experiments are hampered by the lack of a common baseline between studies (e.g., Dueker, 1998).
The permanent component of USArray can be used as a true seismic array, to allow beam-forming and high resolution imaging of deep Earth structure. An example of the power of large scale array studies includes the work by Vidale and Benz (1991), who used a combination of dense local arrays in the western US for imaging details of mantle discontinuity structure in the vicinity of Pacific subduction zones. The Vidale and Benz work used short period arrays, and this kind of analysis will be further enhanced by the existence of a continent scale permanent broad band array, which could then incorporate shear wave arrivals.
USArray and its permenent component would be a natural avenue for pursuing IRIS education and outreach goals in seismology. IRIS, in cooperation with educational institutions (e.g., IRIS members) and government entities throughout the USA, would be working to create a continental-scale Earth observatory. USArray could readily incorporate aspects of the successful IRIS-University GSN program (cooperative efforts between IRIS member institutions and the IRIS GSN program) as well as provide a natural extension of some of the science and education goals of the PEPP project.