Numerical assessment of the effects of topography and crustal thickness on Martian seismograms using a coupled modal solution?spectral element method

Abstract The past 4 decades of Mars exploration have provided much information about the Mars surface, when its interior structure remains relatively poorly constrained. Today available data are compatible with a large range of model parameters. Seismology is able to provide valuable additional data but the number of seismographs will likely be quite limited, specially in the early-stage of future Mars seismic networks. It is thus of importance to be able to correctly isolate effects induced by the crust structure. Mars topography is characterized by spectacular reliefs like the Tharsis bulge or the Hellas basin and by the so-called “Mars dichotomy”: the north hemisphere is made up of low-altitude plains above a relatively thin crust when the south hemisphere is characterized by a thick crust sustaining high reliefs. The aim of this paper is to study the effects induced on seismograms by the topography of the surface and crust–mantle discontinuities. Synthetic seismograms were computed using the coupled spectral element–modal solution method, which reduces the numerical cost by limiting the use of the spectral element method to the regions where lateral variations, like the presence of a topography, are considered. Due to numerical cost, this study is limited to long period and thus focuses on surface waves, mainly on long period Rayleigh waves. We show that reliefs like the Tharsis bulge or the Hellas basin can induce an apparent velocity anomaly up to 0.5% when only the surface topography is introduced. Apparent anomalies can raise up to 1.0% when the surface topography is fully compensated by a mirror-image topography of the crust–mantle discontinuity. Travel-time of surface wave are systematically increased for seismometers in the north hemisphere of Mars and decreased in the south hemisphere. When comparing effects on seismograms by the Earth and Mars topography, we found them to be larger for the Earth. It is due to the fact that we work with a seismic velocity model of Mars with a mean crust thickness of 110 km when the crust thickness has a mean value of 50 km for the Earth. When changing the Mars model for a thinner crust with a mean thickness of 50 km, effects by the topography on Mars seismograms becomes of the same order when not larger than what is observed on the Earth.