Spectral analyses of the Largest Marsquake S1222a: Implications for the attenuation, the source parameters and the site effect

Abstract

<p> After more than 3 years of observations, NASA InSight mission has encountered a magnitude 5+ seismic event on 2022/05/04, which is the largest seismic event ever detected on Mars. This gigantic event enables us to study spectral feature of the seismic signal as well as some local effects that may be visible in the data. In this study, we will study the seismic spectra to understand the source time function of the event, attenuation feature of the crust/upper mantle of Mars. To better constrain these features, we will also evaluate the effect of local structure or the site effect on the seismic spectrum.</p><p> Spectral feature of S1222a shows two different features at low and high frequencies. While we observe typical decay in spectra at low frequencies (<1Hz) we see characteristic peak and increase of energy at high frequencies (>1Hz). The low frequency can be well explained by a classical model with a seismic source time function and attenuation (e.g. Aki and Richards, 2022). On the other hand, the limited frequency band prevents us from distinguishing between different attenuation models and a source time function. To better understand and constrain the seismic source parameters and martian attenuation, it would be mandatory to better understand also the high frequency component of the spectrum.</p><p> The energy enhancement at high frequencies does not follow predicted models and implies there is additional effect from local structure. In this study, we will overcome this problem with two approaches. One is to use other events that has similar distance with S1222a to evaluate the site effect. Since the site effect reflects the effect of locas structure around the InSight landing site, this will be commonly seen for all events. Indeed, marsquekes that are classified as Very High Frequency(VF) Type events show characteristic spectral feature where high frequency energy is enhanced especially on the horizontal components (Clinton et al., 2021). This feature was investigated through seismic coda analyses (van Driel et al., 2021; Menina et al., 2021), however, their origin is yet to be understood. While S1222a is classified as Broadband(BB) type event, its seismic coda features show significant similarities with VF type events (Menina et al., this issue). Thus, in this study, we use other VF type events to evaluate the site effect observed in seismic spectra and will use this to correct the S1222a spectrum to remove the site effect. Secondly, we will also use the noise data to constrain the site effect, this will mainly be studied in Xiao et al. (this issue). This will give us another independent estimates on the site effect and we will also use this information to correct the S1222a spectrum.</p><p> With corrected spectrum, we will better constrain the source time function, more specifically its corner frequency, and attenuation structure or seismic <em>Q</em>. The corner frequency will give us a way to estimate the stress drop related to the seismic event and the slip velocity. This will give us some unique opportunities to study seismic excitation on Mars. Attenuation will also enable us to give constrains on the elastic/inelastic features of the crust and upper mantle. The elastic features can be understood as scattering of seismic energy and we compare our results with other studies that also focus on this effect (e.g. Menina et al., this issue). Inelastic effect is closely related with thermal structure of Mars and/or inclusion of light elements such as volatiles. By estimating the attenuation on Mars, we discuss its implication to the Martian shallow structure.<img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.ca34e4e6948268456582561/sdaolpUECMynit/2202CSPE&app=m&a=0&c=43d6d84e1ed1b66fcb7c4de35adde8b3&ct=x&pn=gnp.elif&d=1" alt=""></p><p>Figure P spectrum of S1222a event and comparison with different attenuation models. The red line shows the data and green, blue, magenda, light blue and yellow lines show model spectra with different attenuation models. The figure shows a clear change in spectral feature of S1222a below and above 1Hz. Comparison with modeled spectra shows that we cannot distinguish between different attenuation models if we limit our analyses to frequencies below 1 Hz and understanding the energy enhancement at high frequencies are mandatory to fully understand the spectral feature of this event.</p>