Possible evidence for variation in magnitude for marsquakes from fallen boulder populations, Grjota Valles, Mars

Following observations of mobilized boulder trail populations from Cerberus Fossae, Mars, that have been interpreted as possible evidence of large-magnitude marsquakes rupturing for distances of similar to 207 km along exposed active faults, additional boulder trail populations were measured along shorter faults within the region of Grjota Valles (50- to 150-km length) to test the hypotheses that (1) these faults are also candidate locations for marsquakes and (2) that marsquake magnitude might be smaller, limited by fault dimensions available for rupture. For a region containing two en echelon graben, boulder trail data define two anomalies with maxima in (a) boulder trails per kilometer and (b) maximum width of boulder trails, one that is similar to 116 km in length along strike and the other similar to 70 km in length along strike. Values for the maxima are 45 trails per kilometer and 5-m mean trail width for the 70-km-long anomaly and 115 trails per kilometer with 5.3-m mean trail width for the 116-km-long anomaly, above background values measured elsewhere along these faults of zero trails per kilometer with zero boulder trail widths. If combined with published data from Cerberus Fossae with an similar to 207-km-long anomaly in boulder trails per km (125 trails per kilometer maxima) and maximum mean boulder trail width (8.5-m maximum trail width), the three data sets suggest correlations between the (a) along-strike length of boulder trail anomalies, (b) boulder trails per kilometer, and (c) maximum boulder trail width. If interpreted as due to single marsquakes, and if the dimensions of these anomalies are a proxy for rupture length, when combined, one interpretation of this is that boulders have been mobilized by marsquakes and that the marsquake magnitude is proportional to the along-strike length of the anomalies. In other words, the data suggest that marsquake magnitude, if that is the cause of the anomalies, is limited by fault length as expected for terrestrial seismically active faults. Such findings suggest that the Martian surface may have been shaken, in the very recent past, by large-magnitude marsquakes. We discuss this in terms of the seismicity of Mars. Plain Language Summary Boulder trail populations were measured along faults on Grjota Valles, Mars, to test the hypotheses that these faults are locations for possible marsquakes. If the boulder trail populations are due to single marsquakes, one theory is that the boulders were moved in the very recent past by large-magnitude marsquakes. The area we studied showed that there are coincident maxima in boulder trail density and boulder trail widths along the strike of the faults in Grjota Valles, Mars. We also saw that the boulder count values decrease away from the locations of most boulder trails. Our results show us that geographically coincident maxima in boulder trail density per kilometer and boulder trail widths along the fault exist. This suggests that a plausible mechanism to mobilize such populations of boulders is through seismic shaking associated with palaeomarsquakes. Our results point to this possibility because boulders mobilized by seismic shaking display a particular pattern: the number of boulder falls and boulder sizes decrease away from the epicenter. Such research suggests that marsquakes not only occurred on Mars but maybe occurred in the recent past, too. With the Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSIGHT) mission now on Mars, its seismometer may well pick up seismic vibrations.