Analysis of SH and SV-to-P reflections on Mars

The crustal structure of Mars reveals how it differentiated and evolved over geologic time. After the landing of NASAs InSight mission in November 2018, data from the Seismic Experiment for Interior Structure (SEIS) have been collected and hundreds of events have been recorded. Recent receiver functions (P-to-s and S-to-p) and autocorrelation analyses provided constraints on the crustal thickness and structure beneath the InSight landing site. It was shown that the crust is characterized by interfaces at depths of 8 2 km and 20 5 km. A third interface may also be present at 39 8 km depth, implying two possible crustal thicknesses. Although the methods employed were effective at providing the first seismic model of the Martian crust, finding additional seismic phases would help narrow down the number of crustal models, reduce the trade-offs between the model parameters (thickness of layers and their seismic wave velocities), and help further constrain the Moho depth. In this study, we analyzed several deglitched events identified by the Mars Quake Service and found two new phases that will help constrain the crust at the lander site. Specifically, so far we found coherent signals for five events that appear to be independent of the focal depth and that are consistent with SH wave and SV-to-P wave reflections off the first crustal interface. These two phases confirm the existence of and the large wave speed (or impedance) contrast across the ~ 8 km interface in the crust and are consistent with autocorrelation observations. We are now performing joint inversions of these SH and SV-to-P reflections together with the existing receiver function and autocorrelation datasets to obtain more robust models of the Martian crust. We are also analyzing the entire event catalog to find more SH reflections that can help constrain the deeper crustal layers and Moho depth.