Rayleigh Wave Ellipticity Modeling and Inversion for Shallow Structure at the Proposed InSight Landing Site in Elysium Planitia, Mars

Abstract

The SEIS (Seismic Experiment for Interior Structure) instrument onboard the InSight mission will be the first seismometer directly deployed on the surface of Mars. From studies on the Earth and the Moon, it is well known that site amplification in low-velocity sediments on top of more competent rocks has a strong influence on seismic signals, but can also be used to constrain the subsurface structure. Here we simulate ambient vibration wavefields in a model of the shallow sub-surface at the InSight landing site in Elysium Planitia and demonstrate how the high-frequency Rayleigh wave ellipticity can be extracted from these data and inverted for shallow structure. We find that, depending on model parameters, higher mode ellipticity information can be extracted from single-station data, which significantly reduces uncertainties in inversion. Though the data are most sensitive to properties of the uppermost layer and show a strong trade-off between layer depth and velocity, it is possible to estimate the velocity and thickness of the sub-regolith layer by using reasonable constraints on regolith properties. Model parameters are best constrained if either higher mode data can be used or additional constraints on regolith properties from seismic analysis of the hammer strokes of InSight's heat flow probe HP 3 are available. In addition, the Rayleigh wave ellipticity can distinguish between models with a constant regolith velocity and models with a velocity increase in the regolith, information which is difficult to obtain otherwise.

Atmospheric Science with InSight

“…The trajectory of the vortex would be a fair indication of the regional and large-scale winds in the Elysium region, as was shown by Reiss et al (2014) who compared remote-sensing estimates with global climate modeling. Moreover, the amplitude of the tilt signal depends on the distance and pressure drop of the dust devil (Lorenz 2016), and on the elastic properties of the subsurface; the latter will be determined from active and passive experiments (see Kedar et al 2017;Knapmeyer-Endrun et al 2016;, hence it will be possible to estimate the core-pressure drop from the amplitude of the seismic disturbance, even for vortices not passing right over the station.…”

A Pre-Landing Assessment of Regolith Properties at the InSight Landing Site

“…This estimate is based on using the Reuss average, as in Delage et al (2017), and assuming rock properties of v P = 3000 m/s, v S = 1700 m/s and ρ = 2760 kg/m 3 derived from terrestrial data obtained for fractured basalt (Planke et al 1999;Vinciguerra et al 2005;Stanchits et al 2006;Fortin et al 2011) as well as a negligible influence of compression on the rocks within the upper 5 m of the regolith. An example of extending the velocity model to greater depths to include the coarse ejecta layer and the transition from fractured to pristine basalt can be found in Knapmeyer-Endrun et al (2017).…”

Preparing for InSight: An Invitation to Participate in a Blind Test for Martian Seismicity

“…The grading of finer grained regolith into coarser, blocky ejecta is exactly what would be expected for a surface impacted by craters with a steeply dipping negative power-law distribution in which smaller impacts vastly outnumber larger impacts that would excavate more deeply beneath the surface (e.g., Shoemaker and Morris, 1969;Hartmann et al, 2001). Knapmeyer et al (2016) used this stratigraphy, along with laboratory measurements (Delage et al, 2017), to develop a model of elastic properties with a stepwise increase in seismic velocity and seismic attenuation Q .…”