Spectral characteristics and implications of located low-frequency marsquakes and impact events from InSight SEIS observations

Abstract

The frequency characteristics of recorded ground motion are crucial to understanding seismic wavefield originating in marsquakes and interior structures of Mars, such as seismic velocity and discontinuities. Based on the frequency content, the Marsquake Service (MQS) categorized marsquakes into low-frequency (LF) and high-frequency (HF) families and further classified them into subcategories. For example, the LF family consists of two types: low frequency (LF; < 1 Hz) and broadband (BB; predominantly <2.4 Hz). Previous studies have presented the frequency characteristics of a few significant marsquakes. With the recent application of new techniques, more marsquakes with a quality high enough to be characterized as “QB” have been located. These newly located marsquakes have the potential to constrain the internal structure of Mars, but seven of them occurring during sols 0–1011 and their spectral characteristics have not been discussed yet. Here, we first summarize these seven LF-type, B-quality marsquakes: S0185a, S0325a, S0407a, S0409d, S0484b, S0916d, and S0918a following the standard procedures of Marsquake Service. Additionally, we examine the characteristics of the two largest impact events, S1000a and S1094b, classified as BB events. We determined the frequency bounds in the Butterworth bandpass filter of all A- and B-quality LF-family marsquakes and the two impact events. We found that most LF-family marsquakes have an upper bound frequency no larger than 1 Hz, except for S0235b, reaching 1.1 Hz. Moreover, the P- and S-wave arrivals of marsquakes present highly varied spectral behaviors based on the analysis of the spectral ratio between P and S waves. We finally discuss the implications of the highly varied spectrum and S-to-P spectral ratio for these marsquakes.