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

Abstract

On 4 May 2022 the InSight seismometer SEIS‐VBB recorded the largest marsquake ever observed, S1222a, with an initial magnitude estimate of 4.6. Understanding the depth and source properties of this event has important implications for the nature of tectonic activity on Mars. Located ∼37° 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 × 1015–3.5 × 1015 Nm (magnitude MW 4.2–4.3). Our results suggest active compressional tectonics near the dichotomy boundary on Mars, likely due to thermal contraction from planetary cooling.

A Tectonic Origin for the Largest Marsquake Observed by InSight

“…The tectonic setting within the epicentral ellipse is very different to that of the Cerberus Fosse region where the strongest other tectonic marsquakes have occurred (Kawamura et al, 2022;Stähler et al, 2022). Initial results suggest a dip-slip fault in the mid-crust (Maguire et al, 2023), consistent with an origin between 18 and 28 km depth. Further analysis is ongoing.…”

High‐ and Low‐Frequency Waveform Analysis the Marsquake of Sol 1222: Focal Mechanism, Centroid Moment Tensor Inversion and Source Time Function

“…Although we also performed calculations in the 1D layered velocity model mentioned in Earth and Space Science 10.1029/2023EA003272 several frequency ranges, the solutions for the interval 0.028-0.072 Hz were chosen to illustrate outcome from the full-waveform MT inversion. This range was used in Maguire et al (2023).…”

“…Recently, Maguire et al (2023) used waveform fitting of body waves and surface waves to estimate the moment tensor solution of S1222a and found that either an E-W to SE-NW striking thrust fault or normal fault can explain the data. For the same structural model and frequency range used in this study and in Maguire et al (2023) for low-frequency MT inversion waveform (within the estimated error) similar solutions were found.…”

“…Recently, Maguire et al (2023) used waveform fitting of body waves and surface waves to estimate the moment tensor solution of S1222a and found that either an E-W to SE-NW striking thrust fault or normal fault can explain the data. For the same structural model and frequency range used in this study and in Maguire et al (2023) for low-frequency MT inversion waveform (within the estimated error) similar solutions were found. For example, the full-moment tensor solution we find here based on long-period waveform inversion (Figure 6c with error estimation in Figure 7a) closely resembles the reverse solution of Maguire et al (2023).…”

“…To show how the obtained solutions are affected by using only one station we calculate synthetic seismograms where we added three more fictional stations (forward simulation). Synthetics correspond to the solutions given in Maguire et al (2023). We made full-and deviatoric-MT inversion using one, two, three or four stations.…”