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International audience; In anticipation of the upcoming 2018 InSight (Interior exploration using Seismic Investigations, Geodesy and Heat Transport) Discovery mission to Mars, we calibrate magnitude scales for marsquakes that incorporate state-of-the-art knowledge on Mars interior structure and the expected ambient and instrumental noise. We regress magnitude determinations of 2600 randomly distributed marsquakes, simulated with a spectral element method for 13 published 1D structural models of Mars' interior. The continuous seismic data from InSight will be returned at 2 samples per second. To account for this limited bandwidth as well as for the expected noise conditions on Mars, we define and calibrate six magnitude scales: (1) local Mars magnitude M-L(Ma) at a period of 3 s for marsquakes at distances of up to 10 degrees; (2) P-wave magnitude m(b)(Ma); (3) S-wave magnitude m(bS)(Ma) each defined at a period of 3 s and calibrated for distances from 5 degrees to 100 degrees; (4) surface-wave magnitude M-s(Ma) defined at a period of 20 s, as well as (5) moment magnitudes M-FB(Ma); and (6) M-F(Ma) computed from the low-frequency (10-100 s) plateau of the displacement spectrum for either body waves or body and surface waves, respectively; we calibrate scales (4)-(6) for distances from 5 degrees to 180 degrees. We regress stable calibrations of the six scales with respect to the seismic moment magnitude at M-w 5.5 by correcting filtered phase amplitudes for attenuation with distance and source depth. Expected errors in epicentral distance and in source depth (25% and 20 km, respectively) translate into magnitude errors of 0.1-0.3 units. We validate our magnitude relations with an independent test dataset of 2600 synthetic marsquakes (1.0 15 degrees are expected to be hidden in the Mars background noise and will likely not be detectable.

The InSight mission to Mars is well underway and will be the first mission to acquire seismic data from a planet other than Earth. In order to maximise the science return of the InSight data, a multifaceted approach will be needed that seeks to investigate the seismic data from a series of different frequency windows, including body waves, surface waves, and normal modes. Here, we present a methodology based on globally-averaged models that employs the long-period information encoded in the seismic data by looking for fundamental-mode spheroidal oscillations. From a preliminary analysis of the expected signal-to-noise ratio, we find that normal modes should be detectable during nighttime in the frequency range 5–15 mHz. For improved picking of (fundamental) normal modes, we show first that those are equally spaced between 5–15 mHz and then show how this spectral spacing, obtained through autocorrelation of the Fourier-transformed time series can be further employed to select normal mode peaks more consistently. Based on this set of normal-mode spectral frequencies, we proceed to show how this data set can be inverted for globally-averaged models of interior structure (to a depth of $\sim 250~\mbox{km}$ ), while simultaneously using the resultant synthetically-approximated normal mode peaks to verify the initial peak selection. This procedure can be applied iteratively to produce a “cleaned-up” set of spectral peaks that are ultimately inverted for a “final” interior-structure model. To investigate the effect of three-dimensional (3D) structure on normal mode spectra, we constructed a 3D model of Mars that includes variations in surface and Moho topography and lateral variations in mantle structure and employed this model to compute full 3D waveforms. The resultant time series are converted to spectra and the inter-station variation hereof is compared to the variation in spectra computed using different 1D models. The comparison shows that 3D effects are less significant than the variation incurred by the difference in radial models, which suggests that our 1D approach represents an adequate approximation of the global average structure of Mars.