SEISMIC ACTIVITY AND EARTH
From seismographs left on its surface we know that seismic events on the moon follow patterns different from those here on the earth. Moonquakes, rare meteorite impacts, and artificially produced vibrations (grenade explosions and crash landings of discarded spacecraft) are transmitted very slowly through the lunar material. They build gradually and then take up to an hour to subside.
Some seismic disturbances have been traced to geologic movements in the rilles; others, to occasional impacts of meteoroid swarms. Moonquakes frequently coincide with tidal stresses triggered by the varying distance between moon and earth. They occur at depths of 600 to 900 kilometers, much deeper than earthquakes. About 80 sou rces for these deep moonquakes have been discovered so far. But compared with the earth's seismic activity, the moon's is fairly subdued; the whole moon releases less than one-tenbillionth of the earth's earthquake energy.
About 35 shallow quakes, presumably tectonic events, have been detected. Thus if the moon is expanding or contracting, it is doing it extremely slowly. In the last 3 billion years thermal and geological activity has been relatively rare. As we have mentioned, most volcanic activity appears to have ceased about 3 billion years ago, but some minor activity may still be going on.
