Journal Article

Detection of Mars Normal Modes From S1222a Event and Seismic Hum

Lognonné, P., Davis, P., Banerdt, W. B., Stutzmann, E., Kawamura, T., Schimmel, M., Drilleau, M., Panning, M. P., Sainton, G.

2023

Overview

We present the first detection of normal modes on Mars using the vertical records from InSight's broad‐band seismometer following the marsquake that occurred on sol 1222. The proposed catalog lists 60 potential eigenfrequencies between 3 and 12 mHz. Due to their low signal‐to‐noise ratio, these normal modes were detected using the phasor walkout approach. The normal modes amplitudes are consistent with the upper limit of the S1222a magnitude and with high quality factors. Additionally, we provide the first detection of a Martian hum before the quake for several of these frequencies. The proposed frequencies are at about 1‐sigma of those predicted by published models based on body wave travel time inversions. Our detection of normal modes is the first made on a terrestrial planet other than Earth and opens the way for future interior models that incorporate both normal modes frequencies, surface waves velocities and body wave travel times. Plain Language Summary The frequencies of a planet's global oscillations are closely linked to its internal structure. Thanks to the powerful magnitude 4.7 marsquake that occurred on sol 1222 and to the low long period noise of the very broad band Insight seismometer, we detected 60 normal mode frequencies. Furthermore, we discovered evidence of continuous vibrations on Mars, called Martian hum, as several eigenfrequencies were present before the marsquake occurred. Mars is now the second terrestrial planet after the Earth for which these planetary tones are observed. Key Points We present the first observational evidence of free oscillations excited by a seismic event and background oscillations on Mars We extracted normal modes hidden in low signal‐to‐noise ratio seismic record using a phasor walkout analysis Normal mode frequencies can be used to narrow down published Mars interior models obtained from body wave travel time inversions

• Publisher: John Wiley & Sons, Inc,American Geophysical Union,Wiley
• Subject: Detection / Earthquakes / free oscillations / Hum / InSight / Inversions / Mars / Modes / normal modes / Oscillations / phasor walk / Phasors / Resonant frequencies / S1222 / Sciences of the Universe / seismic hum / Seismographs / Seismometers / Surface waves / Terrestrial planets / Travel time / Vibrations