Thousands of Thrust Faults!

High resolution view of the central portion of Mandelshtam scarp — Prominent lobate thrust fault scarp in the Mandel’shtam cluster, one of the thousands discovered in Lunar Reconnaissance Orbiter Camera (LROC) images. The fault scarp or cliff is like a stair-step in the lunar landscape formed when the near-surface crust is pushed together, breaks, and is thrust upward along a fault; note the two craters overridden by the fault. Image width 1430 meters, north is towards the top, NAC M103460280LR [NASA/GSFC/Arizona State University].

One of the many surprises made possible by the Lunar Reconnaissance Orbiter Camera (LROC) is the discovery of thousands of small, young lobate thrust fault scarps, cliff-like landforms that result when crustal materials are pushed together, break, and are pushed upward. These fault scarps are easily detected in high-resolution Narrow Angle Camera (NAC) images. Essential to the study of these features are NAC stereo-derived Digital Terrain Models (DTMs) that provide highly accurate details about the shape and relief of the young fault scarps.

Digital Terrain Models (DTM) of lobate scarps have made it possible to model the characteristics of the underlying thrust faults, such as the geometry of the fault and the depth to which it extends into the lunar crust. An example is the fault scarp near the Mandel’shtam impact crater. The NAC stereo-derived DTM indicates the scarp has about 70 m of relief relative to the surrounding terrain. Modeling the fault using the DTM allows an estimate of the seismic moment, a measure of the size of the moonquake generated by the slip on the fault needed to make the scarp.

NAC topography overlain on lobate scarp — A digital elevation model of the Mandel’shtam lobate scarp (6.9° N, 161° E) was generated from NAC stereo images (M191895630 and M191909925). The scarp (white arrows) has a maximum relief of ~70 m. The digital elevation model has a horizontal spatial scale of 5 m per pixel (NAC stereo images have a resolution of ~1 m per pixel) and a vertical precision of ~0.5 m [NASA/GSFC/Arizona State University/Smithsonian].

An estimate of the seismic moment, a shake map, or a map of the expected ground motion can be made from a fault-related shallow-depth moonquake. The shake map gives a sense of the area affected by significant seismic shaking and its distance from the Mandel’shtam scarp. The strength of the shaking depends on the size or magnitude of the moonquake. What would a strong, shallow moonquake feel like if you were standing on the Moon near one of the fault scarps? The Moon’s lower gravity means it takes less ground motion to exceed the pull of gravity and so you might get knocked off your feet. The lower gravity also means weaker moonquakes have stronger ground motion than earthquakes.

Modeled shock wave emanating from lobate scarp fault plane — Shallow moonquakes, like earthquakes, are expected to cause significant ground motion as seismic waves radiate outward from the source. This seismic shakemap shows the expected ground motion for a slip event on a thrust fault (red line) associated with the Mandel’shtam scarp. The shakemap indicates peak vertical acceleration for a moonquake of magnitude 6.36 M_w hypocenter at a depth of 350 m. Shades of blue, green, and white indicate strong shaking. The regional topography is shown by the white contour lines (500 m contour interval) [University of Maryland/Smithsonian].

Read more about lobate scarps and their connection to moonquakes in a published paper from the LROC team:

Watters, T. R., Weber, R. C., Collins, G. C., Howley, I. J., Schmerr, N. C., and Johnson, C. L. (2019) Shallow seismic activity and young thrust faults on the Moon. Nature Geoscience. 12 (6):411. https://doi.org/10.1038/s41561-019-0362-2

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Published by Mark Robinson on 14 February 2023