Possible extra-solar-system cause for shallow moonquakes

Reanalysis of lunar seismic data collected during the Apollo program indicates that 23 of the 28 rare events known as high-frequency teleseismic (HFT) events or shallow moonquakes occurred during one-half of the sidereal month when the seismic network on the Moon's near side faced approximately towards right ascension of 12 hours on the celestial sphere. Statistical analysis demonstrates that there is about a 1 per cent probability that this pattern would occur by chance. In a coordinate system fixed to the celestial sphere, the 26 located HFT events tend to cluster around a direction approximately between the constellations Leo and Cancer (L-C); in this coordinate system the clustering is significant at about the 97% level. With respect to this celestial coordinate system and direction L-C, 13 of 26 events are situated near the Moon's limb (between 70° and 110° from the point on the lunar surface closest to L-C). Although there is a possibility that the observed clustering of HFT events is purely a statistical coincidence, if it isn't a random phenomenon it has great scientific importance and thus we here seriously consider other possibilities. HFT events are clearly a seismic phenomenon and not an artifact of the data collection process. Most are observed on all four stations of the Apollo lunar network, exhibiting onset times of P and S that are separated by intervals appropriate for seismic wave transmission through the Moon. These observations cannot be explained by, for example, a shower of cosmic rays that interferes with seismograph electronics or data transmission. We also show that HFT events are not caused by ordinary meteoroid impacts or by tidal stresses. One possibility is that high-energy objects from a fixed source outside the solar system trigger or even cause the HFT events. High-energy astrophysicists have proposed that exotic particles such as nuggets of strange quark matter (SQM) might cause or trigger HFT events and this has motivated so-far-unsuccessful searches for terrestrial seismic signals produced by SQM passage. Recent estimates show that the seismically quiet lunar surface provides a better environment than Earth to observe the signals produced by SQM nuggets. The observations of HFT events occurring preferentially near the Moon's limb as observed from direction L-C might indicate that these exotic particles release residual seismic stresses as they pass horizontally through the Moon's lithosphere.