HR: 15:10h
AN: P43C-06    [Abstracts]
TI: How Tides Control Some Individual Deep Moonquake Nests
AU: * Frohlich, C
EM: cliff@ig.utexas.edu
AF: University of Texas at Austin, 10100 Burnet Rd (R2200), Austin, TX 78758-4445, United States
AU: Nakamura, Y
EM: yosio@ig.utexas.edu
AF: University of Texas at Austin, 10100 Burnet Rd (R2200), Austin, TX 78758-4445, United States
AB: Seismic stations emplaced during the Apollo missions operated from 1969 to 1977, and the most numerous signals identified in these data were deep moonquakes (DMQ). These occurred mostly at depths of 700-1200 km within a few hundred discrete clusters or nests, and within individual nests DMQ occurrence times correlated strongly with the gravitational tide generated by the Earth. However, the relationship with tides differs markedly at different nests, and we still don't understand either the mechanical origin of DMQ or why the tides exercise such strong control on their occurrence. We here investigate what controls episodes of DMQ occurrence in about a dozen DMQ nests where the temporal pattern is distinct and highly repeatable. In these nests, plots of occurrence times vs anomalistic phase exhibit a very regular drift pattern influenced by both the anomalistic and draconic periods, and often modulated by a roughly sinusoidal component having a period of about seven months. We show that extrema of the normal component of the tidal stress often exhibit a nearly identical pattern. In several of these nests there is a linear relationship between the normal and shear components of the tidal stress when DMQ occur. If we use a Mohr circle analysis to interpret these observations they suggest the static friction coefficient is extraordinarily low, about 0.1 or less. This is lower than ordinary geological materials, and may indicate that trapped fluids or other exotic phases play a role in the faulting process.
DE: 5460 Physical properties of materials
DE: 5475 Tectonics (8149)
DE: 8163 Rheology and friction of fault zones (8034)
SC: Planetary Sciences [P]
MN: 2007 Fall Meeting