Seismology is a powerful tool for understanding planetary interiors. On Earth, the seismicity is dominated by tectonic earthquakes, with the atmosphere protecting us from meteorite impacts. However, on Mars, impacts are one of the key processes generating seismic waves. Despite nominal pre-landing estimates of 1–3 impacts detected per Earth year, recognising impacts in the InSight seismic data has proven challenging. This thesis focuses on filling the current gaps in knowledge of impact-generated seismic waves, specifically in the context of the InSight mission, using numerical modelling.
Firstly, 13 small impacts onto martian regolith, forming craters 1–30 m, are simulated using iSALE-2D. Their seismic source properties are characterised in terms of impactor properties. In this size range scalar seismic moment increases almost linearly with impact momentum. Seismic efficiencies are ∼10−6, dependent on target material properties and impact velocity. The relatively low seismic efficiency and moment suggest that impact detectability on Mars is lower than previously assumed.
Secondly, a momentum-based amplitude-distance scaling relationship is derived based on a dataset containing the artificial impacts on the Moon, the Carancas impact on Earth, and impacts detected by InSight on Mars. Impacts producing craters <30m in diameter are less detectable that pre-landing estimates suggested, whilst larger impacts are more detectable. A subset of marsquakes recorded by InSight is used to derive a new estimate of the impact rate on Mars to be ∼3–5 times higher than the estimates based on orbital observations, but consistent with crater chronology models.
Finally, a further suite of iSALE-2D simulations is used to record displacement seismograms generated by impacts and compute their power spectra. The frequency content of the signals decreases with increasing impactor size and velocity, and target porosity. The low-frequency content of seismic waves generated scales well with impactor momentum, but is also sensitive
to target material properties.