Newly Discovered Shallow Moonquakes: General Characteristics and Source Parameters
Authors: Onodera Keisuke 
Published: March 2024

Seismic observation is a powerful tool to investigate the Earth&#8217;s geological activities and internal structure and has also been applied to the Moon and Mars [e.g., Latham et al., 1969; Banerdt et al., 2020]. For the Moon, a seismic network was constructed on the nearside during the Apollo missions, and nearly eight years of observation provided us with more than 13,000 seismic events [Nakamura et al., 1981]. These events have contributed to understanding the seismicity rate on the Moon and its internal structure [e.g., Garcia et al., 2019; Nunn et al., 2020], both of which are important to know the current geological activity level and trace back to the thermal evolution in the past.In the Apollo lunar seismic observation, two types of seismometers were installed: Long-Period (LP) and Short-Period (SP) seismometers. While the LP sensor has sensitivity at 0.2 &#8211; 1.5 Hz, the SP is sensitive at 1 &#8211; 10 Hz [e.g., Nunn et al., 2020]. In previous studies, the LP data were mainly used. In fact, all the cataloged events were detected solely using the LP data [Nakamura et al., 1981]. On the other hand, because of numerous unnatural signals and/or spikey noises, the majority of SP data remained unanalyzed after the initial description of high-frequency quakes by Duennebier and Sutton (1974a, 1974b) [e.g., Frohlich and Nakamura, 2006; Knapmeyer-Endrun and Hammer, 2015]. This fact implies that there are potential seismic events only identifiable in the SP data, and the lunar seismicity might be underestimated.Lately, Onodera (2023) denoised all the SP data and performed an automatic event detection. As a result, he discovered about 22,000 new seismic events, including thermally driven quakes (thermal moonquakes), impact-induced events, and tectonic-related quakes (shallow moonquakes). While the former two types are useful to understand the surface evolution or degradation processes, the latter type is closely related to the seismic activity level of the Moon. Here, I focus on shallow moonquakes. In the past, since only 28 shallow moonquakes were identified, it was difficult to give a detailed description of their source mechanism, regionality, and correlation with tidal force. In this study, using the newly discovered 46 shallow moonquakes, I&#8217;m trying to give new insights into this type of event.In the presentation, I will describe the general characteristics of newly discovered shallow moonquakes (e.g., waveforms and spectral features) and summarize the estimated source parameters (such as energy release, seismic moment, and body wave magnitude).&#160;ReferencesBanerdt et al. (2020), Nat. Geosci., 13(3), 183-189.
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