High-Frequency Receiver Functions With Event S1222a Reveal a Discontinuity in the Martian Shallow Crust

The shallow crustal structure of Mars records the evolutionary history of the planet, which is crucial for understanding the early Martian geological environment. Until now, seismic constraints on the Martian crust have come primarily from the receiver functions (RFs). However, analysis of the Mars RFs did not focus on the shallow structure (1-5 km) so far due to the limitation of the signal-to-noise ratio at high frequencies for most events. Here, we take advantage of the S1222a and six other marsquakes, which exhibit high signal-to-noise ratios, to probe the shallow structure of Mars. We observe a converted S-wave at approximately 1 s after the direct P-wave in the high-frequency P-wave RFs. This suggests a discontinutity at 2-km depth between highly fractured and more coherent crustal materials.