Other
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.p23a0225i&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #P23A-0225
Other
5430 Interiors (8147), 6250 Moon (1221)
Scientific paper
We study source mechanisms of moonquakes by using amplitude of P and S waves from the long-period records in the special event tape (Nakamura, 1982) of the Apollo missions. In order to evaluate the effect of local structure beneath stations on the observed seismic waves, we first examine the variation of the S-wave motions depending on groups of deep moonquakes. The S-wave arrivals were determined in the seismograms by using eigenvalues and eigenvectors computed in the covariance matrix method (Matsumura, 1981) as well as a traditional technique based on ocular inspection and difference of arrival times between stations (Nakamura, 1983). We estimate the orientations of the S-wave motions in the three methods with deconvolution between horizontal components, particle motions after the S-wave arrivals, and covariance matrixes, respectively. The results from the three methods agree with each other, showing that the S-wave motions at each station vary from source group to group. Thus, the observed polarizations of S-wave motions are not strongly controlled by local structure beneath stations, and are likely to depend on the source characteristics of moonquakes. Assuming that deep moonquakes are caused by shear faulting, we then determine the geometry of the shear faulting for deep moonquakes in the groups A1, A17, A18, and A20. The previous studies only concerned A1. We work with three datasets, including observed amplitude ratios of (1) P/SV, P/SH, and SV/SH at three stations when P waves are visible, (2) SV /SH at three stations, and (3) SV/SH at two stations and SH/SH between the stations. Based on the ray theory, theoretical amplitude ratios are computed from the spherically layered velocity models recently obtained, including the effect of radiation pattern and geometrical spreading. We search for strike, dip, and rake angles of shear faulting in a grid-search technique by comparing the absolute values of the residuals between the theoretical and observed amplitude ratios. Our focal mechanism solutions, which are derived from the angles that minimized the absolute values of the residuals, tend to be characterized by a combination of low- and high-dipping planes. We separately test a hypothesis that deep moonquakes are caused by crack-shaped collapses, searching for the geometry. These methods are also applied to shallow moonquakes.
Igarashi Yuji
Kuge Keiko
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