Tidal triggering of deep moonquakes

Abstract

Matching signals have previously been identified from about eighty repeating deep moonquake sources. These moonquakes clearly display tidal periodicities in their histories of origin times and signal amplitudes; they are presumably triggered by the solid-body tide in the moon, raised primarily by the earth. The A1 hypocentre has been the most active and has also produced seismograms with signals of reversed polarity. In an attempt to deduce focal mechanisms for these events, we calculated various tidal stress functions at the Al hypocentre using a homogeneous moon model, and correlated them with the origin times of events. No good correlation was found, either for tidal stress peaks of consistent polarity, or for tidal stress peaks of opposite polarity at the times of “inverted” events. This could be due to an inaccurate moon model, but it has also been noted that the relative amplitudes of signals recorded at different seismic stations vary between events from the same hypocentre. Earthquake swarms often contain events with fault-plane solutions in very different orientations. A similar variation between events from each deep moonquake hypocentre would explain the different amplitude ratios and also the “inverted” events.