Seismicity unveils tectonics in Cerberus Fossae, Mars

Kurzfassung

Seismic measurements of the InSight lander confirm tectonic activity in an extraterrestrial geological system for the first time: the large graben system Cerberus Fossae (Giardini et al., 2020). In-depth analysis of available marsquakes thus allows unprecedented geophysical characterization of an active extensional structure on Mars, using the epicenter locations, depths, magnitudes, focal mechanisms and spectral character from marsquake data. In summary, InSight seismic data show: Both major families of marsquakes, characterized by low and high frequency content, LF and HF events respectively, can be located on central and eastern parts of the graben system (Zenhäusern et al., 2022). This is in agreement with the decrease in structural maturity towards the East as inferred from orbital images (Perrin et al., 2022). Specifically, we find that the distance distribution of the larger LF marsquakes peaks near Zunil crater and the Cerberus Mantling Unit, which has been hypothesized to be of volcanic origin (Horvath et al., 2021). The two event families correspond to two depth regimes: LF marsquake hypocenters are located at about 15-50 km, based on identification of depth phases (Durán et al., 2022; Stähler et al., 2021), while the HF marsquakes are likely much shallower and at 0-5 km depth (van Driel et al., 2021). Estimated magnitudes are between 2.8 and 3.8 (Böse et al., 2021; Clinton et al., 2021), resulting in a total seismic moment release within Cerberus Fossae of 1.4-5.6×1015 Nm/yr, or at least half of the observed seismic moment release of the entire planet. Estimated focal mechanisms of deep marsquakes (Brinkman et al., 2021; Jacob et al., 2022) show primarily extensional normal faulting, compatible with the image-based interpretation as a graben system. The deeper LF marsquakes are “slow” compared to terrestrial quakes, i.e. lack high frequency energy in the seismic body waves. This can be explained by low stress drop and a weak, potentially warm source region. We propose a geological model that integrates these observations: The deep LF quakes are caused by the large-scale extensional stress pattern, while fractures occur in this specific location only due to the presence of a dike from Elysium Mons. The shallow seismicity is caused in a brittle region near the surface, potentially on the subsurface continuation of the graben flanks. This could potentially explain the seasonality of the HF event rate, which peaks at the times of maximum solar illumination of the bottom in the Cerberus Fossae (Knapmeyer et al., 2021). While a small number of large endogenic marsquakes have been observed in other regions on Mars, specifically Southern Tharsis (Horleston et al., 2022), Cerberus Fossae represents a uniquely active seismic setting. Current day tectonic activity seems to be driven by volcanic processes, and furthermore, we find no trace of seismic activity on compressional thrust faults on Mars, as opposed to the models of seismicity driven by secular cooling and lithospheric contraction.