The Apollo Seismic Data, Recent Re-analyses, and What's Next

The seismic data that were acquired with a network of four stations established during the Apollo lunar landing missions and operated for eight years from 1969 to 1977 provided us with wealth of information, much of it quite unexpected, concerning the internal structure and dynamics of the Moon. Major findings included the existence of weak but numerous deep moonquakes, fewer but strong shallow moonquakes, and a lunar deep interior differentiated into a crust and upper mantle constituting a very thick lithosphere, underlain by a lower mantle similar to the Earth's asthenosphere. The acquired data were fully digital, and thus are allowing more sophisticated analyses using the high computer power that has become available only recently. New analyses have identified many times more deep moonquakes and their source regions than previously known, and their occurrence times are now better defined relative to tidal stress variations within the Moon. This is helping us to understand their source mechanisms and their relationship to the internal structure of the Moon. New analyses are also providing more reliable estimates of seismic velocity distribution and the possible extra-solar-system origin of shallow moonquakes. With all these old and new findings, are we ready to formulate a set of hypotheses regarding the origin and evolution of the Moon that are to be tested with future experiments on the Moon? My answer to this question is: No, we are not. The data acquired with the Apollo seismic network had many limitations: (a) the network covered only the front center of the Moon; (b) the instruments did not have broad enough frequency response to record very-long-period seismic waves often useful in inferring the internal structure; and (c) eight years of observation was simply too short to record many seismic events that occur only at longer intervals. A global coverage with many more seismic stations than what we had with the Apollo network is sorely needed to answer questions most geochemists are asking, including more reliable seismic velocity variations with depth and their regional variations. Thus, with our future experiments on the Moon, we must concentrate on both quality and quantity of the data to be acquired, surpassing what we did 30+ years ago.