The influence of tidal stresses on deep moonquake activity

Abstract

Over the period 1970–1977 the Apollo seismic network detected many deep moonquakes from relatively few discrete hypocentres on the near side of the Moon. The occurrence times of moonquakes from each hypocentre showed tidal periodicities. By calculating the tidal stress variations in five hypothesized spherically symmetric models of the Moon, and comparing with a homogeneous model, we have investigated the influence of tidal stresses on deep moonquakes. For a variety of lunar models the phases of the calculated temporal variations in tidal stresses were found to be independent of the model. Unfortunately, this means that moonquake occurrence times cannot be used to place constraints on radial variations in lunar structure. The results of this analysis contribute to our understanding of the causes of deep moonquake activity, which are briefly reviewed. The focal mechanisms of events at the most active hypocentres change with time. Nevertheless, it seems unlikely that deep moonquakes are caused by the release of tidal strain energy alone at such a large lithostatic confining stress. The tidal stresses must be superimposed on much larger secular stresses, and the most plausible mechanism that might give rise to such secular stresses seems to be thermal contraction.