The project is focused on investigating the propagation of fundamental-mode surface waves across North America using data recorded on the EarthScope/USArray Transportable Array employing a variety of methods of analysis. A goal of the research is the development of comprehensive high-resolution maps of phase velocity across North America to help constrain the deep structure of the continent. The propagation of short- and intermediate-period surface waves depends on the elastic structure of the crust and shallow mantle, and observations of propagation characteristics of these waves are therefore useful for constraining the state, composition and dynamics of this portion of the Earth's interior. The Transportable Array of EarthScope/USArray is generating an unprecedented data set for the seismological imaging of the North American crust and mantle using surface waves, and the project aims to exploit these data using three approaches. First, the project involves the collection of a new set of observations of surface-wave polarization for earthquakes recorded on USArray stations. Owing to the uniform calibration of USAarray station, and the state-of-the-art technology used to orient USArray stations, this data set is ideally suited for polarization studies. Second, a recently developed method of analyzing cross spectra of ambient noise recorded on two seismic stations for the determination of inter-station surface-wave dispersion is applied systematically to USArray stations. The method, which is based on Aki's seminal 1957 paper, allows Love and Rayleigh wave phase velocities to be mapped automatically across the footprint of USArray at a spatial resolution smaller than 100 km, and at periods as short as 5 s. Third, the phase velocity maps derived from short-period ambient noise and intermediate-period earthquake signals are used, together, to determine the elastic structure of the crust and uppermost mantle of the North American Continent.
The project will lead to an improved characterization of surface-wave propagation across North America and will facilitate more specific interpretation of the composition and thermal state of the Earth's interior. Several documented data sets that can be useful for other investigators wishing to constrain the elastic structure beneath North America will result from the research.
The objective of this research was to analyze seismic data collected by the EarthScope/USArray Project in novel ways to investigate and describe how seismic surface waves propagate across North America. The primary outcome of this research is several sets of maps representing the geographical variations in seismic properties, which can be interpreted in terms of shallow and deep geological structure, and tectonic evolution of the North American continent. Measurements were made of surface-wave travel times of seismic surface waves generated by approximately 2000 earthquakes distributed worldwide and recorded on more than 1000 USArray Transportable Array stations installed temporarily in the western and central US. The measurements were analyzed using a two-station method and tomographic techniques to determined maps of Love and Rayleigh surface-wave propagation speed for wave periods between 25 and 100 seconds. To complement these measurements and models, a second set of measurements was derived from cross-correlation of continuous data recorded on pairs of USArray stations. After analysis, these measurements yielded maps of Love and Rayleigh surface-wave propagation speed at shorter periods, 5-40 seconds. Systematic measurements were also made of surface-wave amplitudes at all recording USArray stations, and maps of local amplification of Rayleigh waves were constructed at several periods. These types of measurements, which have not previously been made, provide complementary constraints on the geologic structure beneath each recording station. Secondary outcomes of the research are improved understanding of how seismic waves propagate across heterogeneous geologic structures and a more detailed characterization of the factors that limit the resolution of structure using seismic surface waves. The accompanying figure shows one of the maps resulting from the project, with the colors representing the variations in seismic wave speed of 5-second Rayleigh waves across the conterminous USA. Blue represents faster-than-average speed and yellow and brown represent slower-than-average speed. Slow speeds are characteristic of thick layers of softer sedimentary rocks, common in basins and along the coastal areas bordering the Gulf of Mexico.
