Lunar Seismic Velocity and Crustal Thickness Inversions Using Constraints from Apollo and GRAIL Data

Abstract:

We present results from new Markov Chain Monte Carlo inversions of (i) 1-D lunar crustal and upper mantle velocity models and (ii) 3-D lateral crustal thickness models anchored by crustal thicknesses under the Apollo stations and the artificial and natural impact sites. These new generation models are constrained by both the Apollo impact event seismic data arrival times and by the more recent GRAIL gravimetric data. In all models, 1-D seismic velocities are parameterized using C¹ Bézier polynomials, using two independent sets to represent the crust and the underlying mantle. Other parameters of the inversions include the depth and velocity amplitude of the Bézier control points, the depth of the crust-mantle discontinuity, the thickness of the crust under each Apollo station and impact epicenter, the v_p/v_s ratio, as well as location-specific time delays. Inverting for station-specific crustal thicknesses and velocity delays highlights geology-related differences between stations (e.g. contrasts in megaregolith thickness, in shallow subsurface composition and structure). These differences have already been observed by other analytical methods in the past, as detailed in the literature. We also test the possibility of having a dual-layered crust. However, some of the finer structural elements might be difficult to observe with the available data and might fall within the inherent uncertainty of the dataset. We use the more precise LROC-located epicentral locations for the lunar modules and Saturn-IV booster artificial impacts, reducing that way some of the uncertainty observed in past models. Natural impact epicentral locations are relocated during the inversions. Constraints from deep and shallow moonquakes will be included in future inversions to potentially refine the velocity and crustal models. This work falls within the NASA InSight mission to Mars seismic investigation (SEIS). Accordingly, the method and analytical software developed for this study will be applied to the Martian data as soon as the first seismic data is available (late 2016).