Ellipticity and crustal corrections for seismic body wave paths: application to Mars and Moon

Abstract:

Forward modeling of seismic body wave travel times and ray parameters for a given density and seismic velocity model is an important tool to investigate the interior structure of planets. The popular toolbox TauP by Crotwell et al. (1999) facilitates application to planets other than Earth, but does not consider a planet's ellipticity nor its surface topography. Due to their ellipticity, smaller radii and larger relative surface topography, these corrections become more significant in predicting seismic observations for celestial bodies like the Moon and Mars. In preparation for NASA's INSIGHT discovery mission (launch in March 2016), we include ellipticity corrections, geometrical spreading and topography corrections into TauP. The respective TauP extensions, as well as Lunar and Martian applications are presented: Previously, Lunar and Martian seismic velocity models have been proposed based on mass, moment of inertia, Love numbers and estimated bulk composition, and in case of the Moon also based on seismic data acquired during the Apollo Program (1969-1977). Due to the lack of direct seismic evidence, current Martian seismic velocity models vary widely and exhibit large travel time excursions, as well as considerable variations in epicentral distance ranges for which a given body wave is predicted to arrive. We discuss the effects of Lunar and Martian ellipticity and crustal structure on seismic travel times for a set of seismic velocity models and compare these to variations observed between the different 1D models. This comparison demonstrates the relevance of modeling the effects of ellipticity and crustal thickness during interpretation of seismic data acquired on planets like Mars or Moon.