Uncovering Shallow Crustal Structures of Mars Beneath the InSight Landing Site Using P-Wave Particle Motions

Abstract

Studying the shallow crustal structure of Mars can offer valuable insights into the planet's geological evolution and climate changes. We designed a novel scheme to extract P-wave particle motions from low-frequency seismic events recorded by the InSight mission. By analyzing the frequency dependence of these motions, we inverted for the S-wave velocity structure in the top few kilometers beneath the landing site. Our results reveal a distinct discontinuity at a depth of approximately 0.75 km, with a 68% confidence interval of ±0.25 km, marked by an increase in S-wave velocity from around 1.34 km/s (±0.37 km/s) above to approximately 1.90 km/s (±0.18 km/s) below. The discontinuity is characterized by a sharp transition, around 0.1 km thick, rather than a broad zone. These observed features are robust with minimal influence from data processing and prior assumptions about shallower structures (within the top 0.2 km), and are consistently resolved in both multi-event stacking and single-event inversions. Together with previous observations, our findings suggest that the imaged velocity-increasing discontinuity may signify the top of relatively intact basaltic bedrock (less altered or modified by surface weathering or impact fragmentation), while the overlying structures are interpreted as a combination of Noachian-to early Hesperian-aged sediments with Hesperian and Amazonian basalts.