Marsquakes location and 1-D seismic models of Mars from InSight data

By using body waves arrival times recorded by the seismometer from the InSight mission, we build an improved average model of the crust and mantle of Mars. We simultaneously relocate ten marsquakes and invert for the structure of the upper planet, by incorporating constraints from the gravity field and the topography. Our approach is based on two different formulations of the 1-D seismic profiles: a parametrization using splines and a parametrization of the quantities that govern the 4.5 Gyr thermochemical evolution of the planet, which allows to map the thermal structure to seismic velocities. On Earth, the rate of magma emplacement and volcanic output is largely controlled by ocean-ridge magmatism, and the volume of intrusive rocks is on average five to ten times larger than the volume of extrusive ones in oceanic and continental settings. We investigate if the depth of the transition between intrusive and extrusive rocks could be detected using InSight data. We find that the quakes are shallower than 40 km, and that eight of the ten are located in the Cerberus Fossae graben system. The two other quakes are located at a greater distance, to the SE of InSight at the Martian dichotomy and to the NW of InSight in Utopia Planitia. Our results evidence that the crust is stratified and point to a marked seismic velocity increase in the lower crust, with its top at 30 km, corresponding to the limit of the transition depth between intrusive and extrusive types of rocks. Because the crust structure is compatible with marsquakes located in different directions, we suppose that the transition is present at least regionally. The final interface in the crust located at 55 km, the petrological Moho, is associated to the depth of the smaller velocity increase, because of the mafic to ultramafic and/or cumulate nature of the lower crust. Our result show, that the thickness ratio between lower and upper crust is smaller than that found in oceanic and continental settings on Earth. This finding could explain why primary mantle melts may have reached the surface more often, as suggested by observations obtained with the Gamma Ray Spectrometer on board the Mars Odyssey Spacecraft. Finally, our models of the geodynamical evolution show that a crust of such a thickness requires an initial mantle state comparable to previous studies based on InSight data.