Energy Envelope and Attenuation Characteristics of High-Frequency (HF) and Very-High-Frequency (VF) Martian Events

Since its deployment at the surface of Mars, the Seismic Experiment for Interior Structure (SEIS) instrument of the InSight mission has detected hundreds of small-magnitude seismic events. In this work, we highlight some features of two specific families: high-frequency (HF) and very-high-frequency (VF) events. We characterize the shape of the energy envelopes of HF and VF events with two parameters: (1) the delay time td between the onset and the peak of the dominant arrival; and (2) the quality factor Qc, which quantifies the energy decay rate in the coda. We observe that the envelope of HF and VF events is frequency independent. As a consequence, a single delay time suffices to characterize envelope broadening in the 2.5-7.5 Hz band. The typical coda decay time is also frequency independent, as attested by the close to linear increase of Qc with frequency. Finally, we use elastic radiative transfer theory to perform a series of inversion of seismogram envelopes for the attenuation properties of the Martian lithosphere. The good fit between synthetic and observed envelopes confirms that multiple scattering of elastic waves released by internal sources is a plausible explanation of the events characteristics. We quantify scattering and attenuation properties of Mars and highlight the differences and similarities with the Earth and the Moon. The albedo, that is, the contribution of scattering to the total attenuation, derived from VF events is very high, which we interpret as a signature of a mostly dry medium. Our results also suggest a stratification of the scattering and attenuation properties.