Moment Tensor Estimation of Event S1222a and Implications for Tectonics
Near the Dichotomy Boundary in Southern Elysium Planitia, Mars
Abstract
On May 4th, 2022 the InSight seismometer SEIS recorded the largest
marsquake ever observed, S1222a, with an initial magnitude estimate of
Mw 4.7. Understanding the depth and source properties of this event has
important implications for the nature of tectonic activity on Mars.
Located ~37 degrees to the southeast of InSight, S1222a
is one of the few non-impact marsquakes that exhibits prominent ratio
surface waves. We use waveform modeling of body waves (P and S) and
surface waves (Rayleigh and Love) to constrain the moment tensor and
quantify the associated uncertainty. We find that S1222a likely resulted
from dip-slip faulting in the mid-crust (source depth
~18 – 28 km) and estimate a scalar moment of 3.51015 –
5.01015 Nm (magnitude Mw 4.3 – 4.4). The best-fitting focal mechanism
is sensitive to the choice of phase windows and misfit weights, as well
as the structural model of Mars used to calculate Green’s functions. We
find that an E-W to SE-NW striking thrust fault can explain the data
well, although depending on the choice of misfit weighting, a normal
fault solution is also permissible. The orientation of the best-fitting
fault plane solutions suggests that S1222a takes place on a fault system
near the martian crustal dichotomy accommodating relative motion between
the northern lowlands and southern highlands. Independent constraints on
the event depth and improved models of the (an)isotropic velocity
structure of the martian crust and mantle could help resolve the
ambiguity inherent to single-station moment tensor inversions of S1222a
and other marsquakes.