Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.u41a..07k&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #U41A-07
Physics
[5430] Planetary Sciences: Solid Surface Planets / Interiors, [5475] Planetary Sciences: Solid Surface Planets / Tectonics
Scientific paper
Seismic activities of planets and satellites are one of the most important sources of information to know the states and dynamics inside the celestial bodies. Apollo Passive Seismic Experiment observed lunar seismic events for more than 5 years and provided us with important information of the Moon (Goins et al., 1981; Nakamura, 1983; Lognonné et al., 2003). One way to estimate the physical parameters of the seismic source is to study the spectral features of the seismic events. The previous studies of the spectral feature of lunar seismic events estimated seismic moment, stress drop, and seismic energy release of shallow and deep moonquakes (Goins et al., 1981; Gudkova et al, 2010). However, since the observed bandwidth was limited the result may contain large errors according the uncertainty of the estimations of corner frequencies. In this study, we propose a new approach to investigate the source parameters of the seismic events through spectral analyses and try to give better constraints on source mechanism on the lunar seismic events. Apollo observation had 3 axis long period (LP) seismometer (center frequency: ~0.5Hz) and one vertical short period (SP) seismometer (center frequency: ~8Hz). For better identification of spectral features, especially corner frequencies, we used both LP and SP data in our calculation. Since there were both LP and SP seismometer for vertical axis, it is possible to combine the data from two seismometers and examine the spectral features of seismic events in broader frequency range. For the quantitative evaluation of the spectral features, we compared observed result with our model. Our model takes source function and attenuation into account. This will give us cut-off frequency of the seismic events and Q-value, which defines the attenuation of the seismic wave. For some events such as large impacts, the cut-off frequency will be in the range of the observation and thus we can evaluate its value, which is as important constraints for the source mechanism. We used the method and estimated the cut-off frequency of artificial impacts. This gave us cut-off frequency of ~2Hz which is higher than the previous estimation (Goins et al., 1981). For weaker events, the cut-off frequency is out of the range, however the data can be used to estimate the Q value. Previous study reports some frequency dependence of Q value (Nakamura & Koyama, 1982) thus it is important to study Q value for broader range. The Q value is closely related to the composition, temperature, pressure and stress of the lunar interior. Better understanding of the Q value can provide us with new information of the lunar interior. By making use of both the LP and SP seismometers, we can gain more information from the Apollo seismic data set. We will present our new approach and new insight it can give us.
Bourdet M.
Kawamura Taichi
Lognonné Philippe
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