The american astronauts of lunar missions Apollo 12, 14, 15 and 16 installed from 1969 to 1972 a network of seismometers on the lunar surface, to gain more knowledge about the interior structure of the moon. Within its working period of nearly 8 years the seismographs recorded 12558 Events that were later classified into several categories. The events that were the easiest to detect had been artificial impacts, parts of the lunar modules and rocket stages that were directed to the surface close to the landing sites. Furthermore thousands of meteorite impacts were registered. The actual moonquakes, events which had occured inside the moon, were divided into three different classes. The weakest, the thermal moonquakes, were events which resulted from thermal fluctuation in times of sun rise and sun dawn and were registered only near the stations and occured close to the surface. The second category included the shallow moonquakes, which occured in regions below the crust and were assumed to have a tectonic origin. The deep moonquakes, by far the biggest group of events, had been detected in a depth of 700 km to 1200 km and were related to the tidal forces between Sun, Earth and Moon. Those events occured periodically at certain places, and because of their similar appearance on the seismograms, it was possible to allocate a certain number of events to so called nests. These nests were defined regions inside of the moon, in which a group of events had their origin. In recent years data processing methods have greatly improved, so the deep moonquakes were newly classified and assigned to nests by Yosio Nakamura of the University of Texas, Galveston. At present another attempt has been started to locate the nests with new methods again. There is hope, that some of the the newly detected nests are located at the far side of the moon. That would help to receive more information about the moons deepest interior.
Figure 2: Location of Nest number 1. Source: Till Sonnemann, Dr. Martin Knapmeyer.
Figure 1: Lunar seismograms that refer to Nest A001. Clearly visible is the good correlation. Source: Till Sonnemann.
To determine Far Side Candidates the method by Martin Knapmeyer named LOCSMITH is used, which was designed to find all possible hypocenters for as sparse data as available from the Apollo missions. Preliminary results were published at the EGU in Vienna, 2008. To achieve good new locations, the events of one nest firstly have to be freed from errors that are in the seismograms due to noise and transmission errors, which was done in a certain order of steps. Noise was reduced by a frequency filter, and the resulting events of one nest were stacked to reduce the signal to noise ratio. After this, the arrival times of the seismic waves of the events were picked. By now a couple of nests have been relocated.