The Effect of Mars Impact Crater Topography on Seismic Signals

Meteorite impacts on Mars result in seismic recordings with relatively small surface wave amplitudes compared to body wave amplitudes, unlike what is generally observed in earthquakes. Here, we test with numerical modeling whether Mars' topography could be responsible for the preferential attenuation of surface waves. We use a spectral element method to simulate 3D seismic wave propagation from impact events. The model topography is based on Mars' digital elevation model, and the crustal structure is based on published works. Our results show that surface waves encountering a crater lose energy proportionally to the crater depth via scattering because the surface waves are partially converted to body waves and scatter out of the original direction of propagation. The energy loss from scattering can be recovered through multiple scattering, but the result is a net loss. The multiply scattered surface waves from distant craters arrive late, adding energy to the recorded coda waves. Body waves excited by impacts, after reflection in the subsurface, arrive at steep incidence angles on the seismometer and therefore, are almost unaffected by topography. The difference between topographic scattering of body and surface waves could explain the weak or total lack of surface waves recorded by InSight's seismometer.