The use of deep moonquakes for constraining the internal structure of the Moon

Details The use of deep moonquakes for constraining the internal structure of the Moon The use of deep moonquakes for constraining the internal structure of the Moon

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.u51b0037w&link_type=abstract

American Geophysical Union, Fall Meeting 2010, abstract #U51B-0037

Other

[5494] Planetary Sciences: Solid Surface Planets / Instruments And Techniques, [6250] Planetary Sciences: Solar System Objects / Moon, [7299] Seismology / General Or Miscellaneous

Scientific paper

The installation of seismometers on the Moon’s surface during the Apollo era provided a wealth of information that transformed our understanding of lunar formation and evolution. Seismic events detected by the nearside network were used to constrain the structure of the Moon’s crust and mantle down to a depth of about 1000 km. The presence of an attenuating region in the deepest interior has been inferred from the paucity of farside events, as well as other indirect geophysical measurements. Recent re-analyses of the Apollo data have tentatively identified this region as a lunar core, although its properties are not yet constrained. Here we present new modeling in support of seismic missions that plan to build upon the knowledge of the Moon’s interior gathered by Apollo. Of the many types of Apollo events, deep moonquakes were the most numerous. They were found to originate in distinct nearside clusters, at depths between approximately 700 and 1200 km. Each cluster produced its own unique waveform, occurring with both monthly and 6-year periodicity as dictated by the lunar orbit. We predict that these clusters are still active today. By taking advantage of this periodicity we are therefore able to project the times of their occurrence into the future. Thus planned missions can rely on these events as known seismic sources. For most seismic methods used to determine structure, recorded events must be located. Traditional event-location techniques require a minimum of four stations. Due to cost constraints, new missions may not be able to deploy that many. Fortunately, future landers will be able to operate in a virtual network with the Apollo instruments, as deep moonquake source locations are already constrained. We have devised a method in which individual events can be linked to a known cluster using the observed S-P arrival time differences and azimuth to only two stations. Events can be further identified using each cluster’s unique occurrence time signature. As we expect future seismometers to be able to identify deep moonquakes, we can determine the ideal landing sites to detect the Moon’s core. Although current works have made progress in the recognition of core-reflected phases in the Apollo data, such phases typically arrive in the coda of the main P and S arrivals, hampering their identification on individual seismograms. We thus focus on the detection of PKP, a seismic compression wave that travels through the Moon’s core. Our method takes into account the predicted ray density, arrival amplitudes, and level of seismicity from the known distribution of deep moonquakes. At large epicentral distances, PKP is predicted as a first arrival, and hence should be easily identifiable on future seismograms. This method can be adapted to any core-interacting phase.

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