Focal Mechanism Determination of Event S1222a and Implications for Tectonics Near the Dichotomy Boundary in Southern Elysium Planitia, Mars

On 4 May 2022 the InSight seismometer SEIS-VBB recorded the largest marsquake ever observed, S1222a, with an initial magnitude estimate of M-W(Ma) 4.6. Understanding the depth and source properties of this event has important implications for the nature of tectonic activity on Mars. Located similar to 37 degrees to the southeast of InSight, S1222a is one of the few non-impact marsquakes that exhibits prominent surface waves. We use waveform modeling of body waves (P and S) and surface waves (Rayleigh and Love) to constrain the focal mechanism, assuming a double-couple source, and find that S1222a likely resulted from reverse faulting in the crust (source depth near 22 km). We estimate the scalar moment to be 2.5 x 10(15)-3.5 x 10(15) Nm (magnitude M-W 4.2-4.3). Our results suggest active compressional tectonics near the dichotomy boundary on Mars, likely due to thermal contraction from planetary cooling. Plain Language Summary The InSight lander's sensitive seismometer recorded over 1,000 seismic events during the course of its mission, consisting of both marsquakes and impacts. Like on Earth, it is thought that the majority of quakes on Mars result from faulting events that release stress within the planetary interior. Understanding the nature of these events, including their magnitude, depth, and faulting style, provides valuable clues into planetary deformation and its driving forces. In this study, we use seismic waveform modeling to constrain the properties of the S1222a event, the largest marsquake recorded during the InSight mission. We find that S1222a likely resulted from reverse (compressional) faulting within the crust of Mars and estimate that the event was equivalent to a 4.2-4.3 magnitude event on Earth. The compressional faulting may indicate ongoing thermal contraction due to planetary cooling, which, prior to the InSight lander mission, was expected to be one of the primary drivers of seismic activity on Mars.