The North American continent contains vastly diverse geology, from the volcanoes and mountains in the west to the interior plains that anchor the central United States and the broad Appalachian hills that roll into the Atlantic Ocean. This topography reflects the many episodes of continental collision, breakup, and modification that the land mass has experienced over billions of years. The movements of the tectonic plates at Earth?s surface are inextricably linked to the movement of rocks in Earth?s mantle (mantle convection). Understanding the connection between the Earth?s interior and surface is essential to addressing fundamental questions about how continents are created, how they evolve over time, and how they are destroyed. This study will use seismic waves generated by earthquakes to image the properties of the crust and mantle beneath North America, with particular emphasis on imaging in three dimensions the energy loss (attenuation) experienced by seismic waves. While it is generally acknowledged that seismic attenuation has the potential to be a valuable source of information about the Earth?s interior, it is difficult to isolate attenuation from other wave-propagation phenomena, which has historically limited its use by researchers. Integrated research and education activities will utilize data recorded by the NSF EarthScope USArray seismometers, which have recorded what is likely the best data set in existence to confront these challenges.
This project will investigate, using new data analysis and seismic-wavefield simulations, the three-dimensional variations in shear attenuation beneath North America. This study will also develop a 3-D model of shear velocity in the upper mantle and, through joint interpretation with the attenuation model and laboratory measurements of attenuation and wave speed, place constraints on the variations in temperature, composition, partial melt, and water in the North American upper mantle. The attenuation model will be derived from measurements of Rayleigh wave amplitude and body-wave attenuation at USArray stations throughout the contiguous United States, and the velocity model will be derived from Rayleigh wave travel-time and local site-amplification observations. Separating the competing effects of focusing by elastic structure and anelastic decay on the Rayleigh wave amplitudes is the primary obstacle to imaging small-scale attenuation variations; spectral-element simulations with high-resolution elastic models for the U.S. that have been made possible by USArray will be used to compute focusing effects with greatly improved accuracy. The educational activities are targeted at high-school and undergraduate students. Educational seismometers will be installed at two Providence, RI area high schools. Data recorded by these seismometers and USArray stations will be used to engage the high-school students in earthquake science, fundamental properties of waves, and the tectonic history of North America. Teaching modules utilizing these data that address the Next Generation Science Standards (NGSS) will be developed and disseminated more widely. At the undergraduate level summer interns will carry out research projects at Brown during three summers, and a new field course in Environmental Geophysics will be developed.
