A Simple Physical Model for Deep Moonquakes

Details A Simple Physical Model for Deep Moonquakes A Simple Physical Model for Deep Moonquakes

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p23c1273w&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #P23C-1273

Mathematics

Probability

[6250] Planetary Sciences: Solar System Objects / Moon, [7299] Seismology / General Or Miscellaneous

Scientific paper

Deep moonquakes occur between ~750 and 1000 km depth in the Moon and originate at discrete source regions referred to as clusters. The monthly-periodic occurrence times of events from individual clusters are clearly related to tidal stress, but also exhibit departures from the temporal regularity this relationship would seem to imply. The physical process that results in moonquakes is not yet fully understood. However, a single, easily-accessible observable - the time interval I(n) between events - can be used to reveal behavior not readily observed using typical periodicity analyses. We use a delay-coordinate (DC) representation - a map of successive intervals I(n+1) versus I(n) - to characterize the dynamics of moonquake occurrence. Moonquake-like DC plots can be generated by combining sequences of synthetic events that occur with variable probability at tidal periods. Though such a model provides a reasonable qualitative match to the observed moonquake DC plots, it lacks physical content. We thus investigate a more mechanistic model to determine why moonquakes occur. We present a series of models of deep moonquake occurrence that include both tidal forcing and stress relaxation during events. We first examine the behavior of inter-event times in a delay-coordinate context, and then examine the output of our models in the same context. We find that the stress relieved by moonquakes has a non-negligible influence on their occurrence times. Our preferred model assumes that the stress function within the Moon is a zero-mean sinusoid consisting of two closely-spaced tidal frequencies - a simplified version of the real tidal stress at deep moonquake depths. At a given threshold value of the stress function, slip occurs, and stress is relieved. If a positive (negative) threshold is reached when the stress function is increasing (decreasing), slip occurs and decreases (increases) the stress function. The stress relieved determines the time interval between events, and can be adjusted such that moonquake-like DC plots result. This model implies exact reversal of the slip direction, and may explain the opposite-polarity events observed at some clusters.

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