Seismic Efficiency and Seismic Moment for Small Craters on Mars Formed in the Layered Uppermost Crust

• Published in: Journal of geophysical research. Planets Vol. 128; no. 4
• Main Authors: Rajšić, A., Miljković, K., Wójcicka, N., Collins, G. S., Garcia, R. F., Bredemeyer, C., Lagain, A., Daubar, I. J., Lognonné, P.
• Format: Journal Article
• Language: English
• Published: 01.04.2023
• Subjects: impact cratering; impact seismicity; InSight mission; Mars; seismic efficiency; seismic moment
• ISSN: 2169-9097; 2169-9100
• DOI: 10.1029/2022JE007698

Seismic activity generated by impacts depends on impact conditions and properties of the impact site. Here, we combined mapping of the regolith thickness with numerical impact simulations to better estimate the seismic efficiency and seismic moment generated in small impact events in the uppermost crust on Mars. We used mapping of crater morphology to determine the regolith thickness that craters formed in. We found that local regolith thickness in the late Amazonian units is between 4 and 9 m. Combined with previous estimates for the NASA InSight landing site, we composed a more realistic uppermost crust analog and implemented it in numerical impact simulations. We estimated the seismic efficiency and seismic moment for small craters on Mars impacting a non‐porous or fractured bedrock overlaid by 5, 10, or 15 m thick regolith. Seismic energy showed more dependence on target properties. Three orders of magnitude more energy were produced in stronger targets. The seismic moment does not depend on target properties, and we confirm that seismic moment is almost proportional to impact momentum. The resulting seismic moment is in agreement up to a factor of 4 between different target types. We improved the scaling relationships developed from numerical simulations used in seismic moment approximations by constraining its dependence on more realistic target properties. Plain Language Summary Small impacts form in the top few meters of the crust of Mars. From the NASA InSight mission data, we learned that the uppermost crust of Mars is layered. Here, we mapped and used rocky ejecta craters as a proxy to estimate the thickness of this top layer. We demonstrated that a younger, late Amazonian geological unit, has a thinner top regolith layer (4–9 m) compared to an older, Hesperian unit (3–17 m). We modeled several target scenarios, constrained by mapping results, and performed a suite of numerical simulations of small impacts. From these simulations, we determined a relationship between impact‐generated seismic energy and the seismic moment for different types of Martian upper‐crust analogs. Our models show that impact‐generated seismic energy strongly depends on target properties (three orders of difference among different analogs investigated here). On the contrary, seismic moment shows agreement up to a factor of 4 in different targets. We discussed our modeling results to the newly detected impacts on Mars, and confirmed that our models are reliable for predicting seismic moments of small impacts. Our results contribute to the understanding of the size and energy of impact‐generated seismic sources in different target analogs. Key Points Using rocky ejecta craters, we mapped regolith thickness on Mars and found that in the late Amazonian volcanic unit, it is 4–9 m Seismic efficiency shows larger sensitivity on target properties (up to three orders of magnitude) than the seismic moment (factor of four) A comparison of numerical simulations with the new seismic detections of craters on Mars showed agreement with our models