A Geostationary Optical Seismometer, Proof of Concept

We discuss the possibility of imaging the propagation of seismic waves from a very large space-based optical telescope. Images of seismic waves propagating at the Earth's surface would be an invaluable source of information for investigating earthquake physics and the effect of the subsurface on earthquake ground motions. This application would require ground displacement measurements at about every 100 m, with centimetric accuracy, and temporal sampling on the order of 1 Hz. A large field of view (> 105 km(2)) is required to measure the full extent of a large earthquake in the areas of interest. A geostationary optical telescope with a large aperture appears to be the most promising system. We establish preliminary technical requirements for such a system, which lead us to consider a telescope with an angular field of view of 0.8 degrees and with an aperture greater than 4 m. We discuss and quantify the various sources of noise that would limit such a system: atmospheric turbulence, evolution of ground reflectance and solar incidence angle, and stability of the platform at 1 Hz. We present numerical simulations, which account for these sources of noise. They show that key details of the seismic wave field, hardly detectable using ground-based instruments, would indeed be imaged by such a system. At the upper limit of modern technology, data flow would be about 20-50 Gb . s(-1), and data memory would be about 50 Tb.