Different Martian Crustal Seismic Velocities Across the Dichotomy Boundary From Multi-Orbiting Surface Waves

We have observed both minor-arc (R1) and major-arc (R2) Rayleigh waves for the largest marsquake (magnitude of 4.7 +/- 0.2) ever recorded. Along the R1 path (in the lowlands), inversion results show that a simple, two-layer model with an interface located at 21-29 km and an upper crustal shear-wave velocity of 3.05-3.17 km/s can fit the group velocity measurements. Along the R2 path, observations can be explained by upper crustal thickness models constrained from gravity data and upper crustal shear-wave velocities of 2.61-3.27 and 3.28-3.52 km/s in the lowlands and highlands, respectively. The shear-wave velocity being faster in the highlands than in the lowlands indicates the possible existence of sedimentary rocks, and relatively higher porosity in the lowlands.