Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p42a..01w&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P42A-01
Other
5400 Planetary Sciences: Solid Surface Planets, 5430 Interiors (8147), 6200 Planetary Sciences: Solar System Objects, 6250 Moon (1221), 7200 Seismology
Scientific paper
Changes in tidal stress within the lunar interior, induced by Earth's varying relative position, may influence deep moonquake activity, as suggested by the monthly periodicities observed in event occurrence times. In addition, the typically large S- to P-wave arrival amplitude ratios observed on deep moonquake seismograms indicate the occurrence of shear failure at depth, despite the prohibitive temperature and pressure conditions. Transformational faulting, in which shear failure is induced by mineral phase changes, clearly occurs at depth within the Earth and may also occur within the Moon. We investigate the relationship between tidal stress and deep moonquake occurrence by searching for a linear combination of the normal and shear components of tidal stress that best approximates a constant value, when evaluated at the times of moonquakes from each of 39 different moonquake clusters. We compute the stresses, resolved onto a suite of possible failure planes, and determine which orientation performs best. While our failure criterion adequately describes deep moonquakes for some clusters, we see three categories of additional complexity: (1) For some clusters, the best-fitting linear combination of shear and normal stress is not strongly dependent on plane orientation, suggesting that the process responsible for generating moonquakes has two components: one that is dependent on plane orientation, and another that is independent of orientation. (2) Some clusters have best-fitting linear combinations of stresses for which the relative contribution of normal stress is larger, which may be indicative of transformational faulting since the phenomenon is known to originate as anti-cracks that form perpendicular to the maximum compressive stress. (3) For some clusters, the fault plane can be better constrained when the failure criterion is expanded to include the influence of shear and normal stress rates. This rate-dependence may reflect a delay between the stress state that marks the onset of anti-crack formation and the eventual coalescence of anti-cracks that leads to failure.
Bills Bruce G.
Johnson Clifton L.
Weber Renee C.
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