Published on JGR Planets
Volume 126, No 10
Cite: Kim, D., V. Lekić, J. Irving, N. Schmerr, B. Knapmeyer-Endrun, R. Joshi, M. Panning, B. Tauzin, F. Karakostas, R. Maguire, et al. (2021). Improving constraints on planetary interiors with PPS receiver functions, J. Geophys. Res., doi: 10.1029/2021JE006983.
Seismological constraints obtained from receiver function (RF) analysis provide important information about the crust and mantle structure. Here we explore the utility of the free-surface multiple of the P-wave (PP) and the corresponding conversions in RF analysis. Using earthquake records, we demonstrate the efficacy of PPs-RFs before illustrating how they become especially useful when limited data is available in typical planetary missions. Using a transdimensional hierarchical Bayesian deconvolution approach, we compute robust Ps- and PPs-RFs with InSight recordings of five marsquakes. Our Ps-RF results verify the direct P-to-S (Ps) converted phases reported by previous RF analyses with increased coherence and reveal other phases including the primary multiple reverberating within the uppermost layer of the Martian crust. Unlike the Ps-RFs, our PPs-RFs lack an arrival at 7.2 s lag time. Whereas Ps-RFs on Mars could be equally well fit by a two- or three-layer crust, synthetic modeling shows that the disappearance of the 7.2 s phase requires a three-layer crust, and is highly sensitive to velocity and thickness of intra-crustal layers. We show that a three-layer crust is also preferred by Sp-RFs. While the deepest interface of the three-layer crust represents the crust-mantle interface beneath the InSight landing site, the other two interfaces at shallower depths could represent a sharp transition between either fractured and unfractured materials or thick basaltic flows and pre-existing crustal materials. PPs-RFs can provide complementary constraints and maximize the extraction of information about crustal structure in data-constrained circumstances such as planetary missions.
Plain Language Summary
Most of our geophysical understanding about the interior of other planets and moons comes from indirect, remote measurements. Other than Earth, only the Moon and Mars have been directly investigated with seismometers, by the Apollo and InSight missions, respectively. The ground vibration measurements on Mars have revealed much of the interior structure and dynamics of the red planet. A widely-used tool for analyzing ground vibrations is the so-called receiver function technique, which allows us to extract constraints on subsurface structure directly beneath the seismometer. Already, receiver functions have constrained the overall crustal structure of Mars. Our study explores the utility of one of many underused seismic phases from a seismic source, the P-wave bouncing off the planet’s surface (called PP), when studying planetary crustal structures with receiver functions. We show that using PP waves to compute receiver functions provides complementary information, to more commonly used direct P and S seismic arrivals and maximizes the amount of information extracted from limited data, which is particularly helpful in the context of planetary missions. Using data from the five best-quality marsquakes, we find Mars’ crust beneath the InSight lander in Elysium Planitia likely consists of three distinct layers.