S1222a—The Largest Marsquake Detected by InSight.

NASA's InSight has detected a large magnitude seismic event, labeled S1222a. The event has a moment magnitude of MWMa ${\mathrm{M}}_{\mathrm{W}}^{\text{Ma}}$4.7, with five times more seismic moment compared to the second largest event. The event is so large that features are clearly observed that were not seen in any previously detected events. In addition to body phases and Rayleigh waves, we also see Love waves, minor arc surface wave overtones, and multi‐orbit surface waves. At long periods, the coda event exceeds 10 hr. The event locates close to the North‐South dichotomy and outside the tectonically active Cerberus Fossae region. S1222a does not show any evident geological or tectonic features. The event is extremely rich in frequency content, extending from below 1/30 Hz up to 35 Hz. The event was classified as a broadband type event; we also observe coda decay and polarization similar to that of very high frequency type events. Plain Language Summary: After 3 years of seismic monitoring of Mars by InSight Seismic Experiment for Interior Structure instrument, we detected a marsquake largest ever observed during the mission. The event is larger by factor of 5 in seismic moment compared to previously detected events. With such an energetic event, we discovered various seismic features that was never observed before. For the first time, we were able to detect body waves and surface waves with their overtones. The large variety of detected seismic phases will enable us to probe the internal structure of Mars. Second, the event was located outside a well‐known seismically active region of Cerberus Fossae. This might indicate that event do not come from the same fault system with other major marsquakes. Finally, this event shows simultaneously features of marsquakes that were previously classified into different types. S1222a is classified as a broadband event with a wide frequency range of seismic energy. At the same time, the coda shape and decay at high frequency resembles that of very high frequency type events. It was an open question how different types of marsquakes are excited of what makes such differences and such event will be a key to uncover such mystery of marsquakes. Key Points: InSight detected on 4 May 2022 a MWMa ${\mathrm{M}}_{\mathrm{W}}^{\text{Ma}}$4.7 marsquake, S1222a, which is the largest seismic event detected so farThe exceptional signal‐to‐noise allows multiple phases to be identified, with a rich collection of surface wavesS1222a was located 37° southeast of the InSight landing site and close to the Martian dichotomy boundary [ABSTRACT FROM AUTHOR]