The Seismically Active Lobate Scarps and Coseismic Lunar Boulder Avalanches Triggered by 3 January 1975 (M_W 4.1) Shallow Moonquake

Abstract

On 3 January 1975, the largest shallow moonquake (MW 4.1) occurred at Laue impact crater on the Moon. The fault responsible for the moonquake and origins of coseismic boulder avalanches are unknown. Our study reveals a set of previously unreported, seismically active, young lobate scarps near the epicenter. In addition, hundreds of boulder falls are observed on the interior walls of two impact craters on either side of the lobate scarps. The varying preservation levels and crater size‐frequency distributions of impact craters superimposed on the boulder falls indicate their episodic origins at 1.6 Ma and during the 1975 shallow moonquake. Our ground motion simulations confirm that the MW 4.1 moonquake along the lobate scarp at 1‐ to 5‐km focal depths produced strong ground shaking that triggered the boulder avalanches. Also, the fault slip along the Lorentz basin wall beneath the Laue crater floor produced the lobate scarps and the shallow moonquakes.

Impacts drive lunar rockfalls over billions of years

“…However, recently acquired, high-resolution imagery has revealed a startling feature on lunar slopes: ubiquitous mass-wasting features. These include granular flows 4,5 , slides, slumps, and creeps 1,6 , as well as rockfalls 1,[7][8][9] , a process where boulders are released or ejected from topographic highs and fall, roll, bounce, and slide to topographic lows. Rockfalls carve tracks into the lunar surface, which provide a record of the dynamic emplacement process.…”

“…During the Apollo 17 mission, astronauts Jack Schmitt and Eugene Cernan directly sampled a rockfall at station 6 in Taurus Littrow valley and returned multiple samples to Earth. Using these data in combination with orbital imagery, rockfall track survivability has been estimated to range from~1.55 to 35 Ma 2,[8][9][10][11][12] with an upper limit of~150-300 Ma 13 , meaning that the tracks observed today are the expression of geologically recent (late Copernican)-and potentially current-erosion processes.…”

“…Past studies have mapped lunar rockfall features for selected regions across the lunar equatorial and polar highland, mare, pyroclastic, and permanently shadowed regions [7][8][9]14,15 . These studies have hypothesized that the main drivers of lunar rockfalls are shallow and deep moonquakes, impact-induced shaking, and thermal fatigue caused by the extreme temperature variations on the Moon 7,16 .…”

“…These studies have hypothesized that the main drivers of lunar rockfalls are shallow and deep moonquakes, impact-induced shaking, and thermal fatigue caused by the extreme temperature variations on the Moon 7,16 . Here we use the term rockfall driver to represent both the long-term cause (also called precondition) and shortterm trigger, as observations in satellite imagery only allow for an explicit distinction of rockfall triggers and causes in some cases 8,9 . Other studies have suggested that major basin-forming impact events can cause intense and global seismic shaking that results in erosion of steep slopes through large-scale mass wasting 17,18 .…”

“…Thus the current assumption is that these ancient, (pre-)Nectarian terranes represent the final evolutionary stage of the lunar surface as they have shallow slopes 18,19 and may not host mass-wasting features. However, these past studies have focused on small, isolated regions 8,9 and/or applied coarse spatial sampling 7 .…”