Moonquake-Triggered Mass Wasting Processes on Icy Worlds

Intense tectonism is evident on many outer solar system satellites with some surface regions exhibiting ridge-and-trough structures which have characteristics suggestive of normal faulting. In some cases, topographic lows between subparallel ridges are sites of smooth material displaying few craters. We consider whether such smooth material can be generated by mass wasting triggered from local seismic shaking. We hypothesize that debris would flow from topographic highs into lows, initially mobilized by moonquake-induced seismic shaking during formation of local tectonic ridges, covering and infilling older terrain. We analyze the feasibility of seismicity to trigger mass movements by measuring fault scarp dimensions to estimate quake moment magnitudes. Seismic moment (M_o) is defined as the energy release caused by a fault rupture and subsequent quake, and moment magnitude (M_w) is a logarithmic scaling of M_o, a function of shear modulus µ (here adopted as 3.5 GPa for ice), A_b, the area of the rupture block face in m², and p, the resulting scarp slip in m. Given that p is currently unknown for icy satellites, we consider a range of assumed values in our calculations. The resulting magnitude range is 5.3–8.6, and we use numerical modeling to estimate seismic accelerations resulting from such quakes.

Magnitude ranges are used to model resulting seismic accelerations. Interior models to create the synthetic seismograms were generated using Planetprofile, based on current constraints of spacecraft data. Synthetic seismograms were then reconstructed for arbitrarily placed receivers and a seismic source within the generated satellite interior models. The seismic source strength is set to be within our calculated magnitude range.

We adopt surface gravitational acceleration as the criterion which, if exceeded, implies that coseismic mass wasting is expected. Modeled seismic accelerations can exceed satellite gravitational accelerations, particularly near quake epicenters. Thus, seismic events could feasibly cause mass wasting of material to form some fine-scale smooth surfaces observed on at least three icy satellites: Ganymede, Europa, and Enceladus.

Currently, existing image resolution, areal extent, and stereo coverage are severely constrained. A better understanding of tectonic and coseismic mass wasting processes will be possible when the Europa Clipper and JUICE missions provide high-resolution surface imaging, including stereo imaging, along with subsurface radar sounding, for both Europa and Ganymede.