|
Back to list of UTIG abstract submissions, Fall Agu 2003 Search for Far-Side Deep Moonquakes: Progress Report IIYosio Nakamura Institute for Geophysics, University of Texas at Austin All but one of the nests of deep moonquakes found during and immediately following the Apollo missions were on the near (Earth-facing) side of the Moon. A possibility remains, however, that there are many more deep moonquake nests on the far side but they escaped our detection because of the high attenuation of shear waves through the lunar lower mantle; without clear shear-wave arrivals it was nearly impossible to identify them visually on seismograms. To pursue this possibility, we have recently started a computer search to identify far-side deep moonquakes among 9000+ events that remained unidentified at the end of our earlier analysis. This involves several steps: (1) waveform cross-correlation of every possible pair of previously unidentified events to find formerly unrecognized groups of events with matching waveforms; (2) stacking of waveforms to enhance signal-to-noise ratio; (3) picking of seismic phases on the stacked seismograms; and (4) location of newly identified nests. So far, we have completed the first two steps and are now preparing to perform step (3). We have identified nearly 250 new nests, of which 88 show finite-amplitude signals at three or more stations. Preliminary examination of the stacked waveforms reveals that there are at least 15 new nests for which the waveforms are similar to those of the lone far-size deep moonquake nest, A33, found earlier, with no distinct shear-wave arrivals at some stations, and thus constituting potential candidates for far-side deep moonquakes. In addition, at least 13 of the newly identified seemingly near-side nests are clearly in the southeast quadrant of the Moon, where there was a large gap in deep moonquake distribution earlier. Reliable picking of P- and S-wave arrivals must be done with greatest care. We are now looking into possible use of receiver functions to perform this task.
|