Low-Frequency Marsquakes and Where to Find Them: Back Azimuth Determination Using a Polarization Analysis Approach

National Aeronautics and Space Administration’s Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport (InSight) mission on Mars continues to record seismic data over 3 yr after landing, and over a thousand marsquakes have been identified, to date. With only a single seismic station, the determination of the epicentral location is far more challenging than on the Earth. The Marsquake Service (MQS) produces seismicity catalogs from data collected by InSight, and provides distance and back azimuth estimates when these can be reliably determined; when both are available, these are combined to provide a location. Currently, MQS does not assign a back azimuth to the vast majority of marsquakes. In this work we develop and apply a polarization analysis method to determine the back azimuth of seismic events from the polarization of observed P- and S-wave arrivals. The method is first applied to synthetic marsquakes and then calibrated using a set of well-located earthquakes that have been recorded in Tennant Creek, Australia. We find that the back azimuth is estimated reliably using our polarization method. The same approach is then used for a set of high-quality marsquakes recorded up to October 2021. We are able to estimate back azimuths for 24 marsquakes, 16 of these without MQS back azimuths. We locate most events to the east of InSight, in the general region of Cerberus Fossae.