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. Plain Language Summary The Martian shallow crustal structure is essential for understanding the geological evolution of Mars. The InSight lander successfully deployed a seismic station on Mars in late 2018, aiming to investigate the internal structure of Mars. Since most marsquakes detected previously have a low signal‐to‐noise ratio (SNR) at high frequencies, most seismic analyses do not focus on the shallow structure of Mars (1–5 km). However, when the InSight seismometer was near the end of its observational lifetime, a large marsquake occurred on sol 1222 with significant high‐frequency energy, far more than the noise level, allowing us to study the Martian shallow structure. We calculate the high‐frequency P‐wave receiver function (RF) of S1222a and extract a converted S‐wave at approximately 1 s after the direct P‐wave. To confirm the result, we also compute P‐wave RFs for high SNR events that occurred before. We observe this ∼1‐s signal in the high‐frequency P‐wave RFs of two additional large events as well. Combined with the geological analysis adjacent to the InSight lander, we attribute this 1‐s converted S‐wave to a discontinuity at approximately 2 km depth, probably corresponding to the bottom of highly fractured crustal materials beneath the InSight landing site. Key Points We calculate high‐frequency P‐wave receiver functions (RF) from InSight seismic data of seven marsquakes with high signal‐to‐noise ratios The high‐frequency RFs exhibit a converted S‐wave at approximately 1 s The ∼1‐s converted S‐wave suggests a discontinuity at a depth of approximately 2 km beneath the InSight lander