The Need &Utility of Creating a Global Lunar Seismic Network

Details The Need &Utility of Creating a Global Lunar Seismic Network The Need &Utility of Creating a Global Lunar Seismic Network

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.u44b..06n&link_type=abstract

American Geophysical Union, Fall Meeting 2006, abstract #U44B-06

Mathematics

Logic

5410 Composition (1060, 3672), 5430 Interiors (8147), 5475 Tectonics (8149), 6250 Moon (1221), 7294 Seismic Instruments And Networks (0935, 3025)

Scientific paper

The Apollo Passive Seismic Network (PSN) showed the Moon was seismically active on a scale similar to intraplate seismicity on Earth, but the small area covered by the PSN has prevented definitive conclusions being made about the deep lunar interior. For example, increased velocities at depths >500 km have large errors associated with them so mineralogical interpretations are poorly constrained; an increased proportion of Mg-rich olivine or the presence of garnet could explain the current data. Lunar Prospector data estimated a metallic lunar core with a radius of 340 km. However, existing geophysical data are consistent with either a metal or ilmenite core. Four types of lunar seismic events were documented: 1) Thermal moonquakes (smallest magnitude event) - associated with stresses induced by surface diurnal temperature changes. 2) Deep moonquakes (magnitude 2 or less) - >7,000 having been recognized. These events occur 700-1,200 km within the Moon and are associated with Earth's tidal pull. 3) Meteoroid impacts exhibit characteristic seismic waveforms; >1,700 events representing masses of 0.1-1,000 kg were recorded. 4) Shallow moonquakes (high frequency teleseismic events) have hypocenters of 50-200 km, but exact depths and locations are unknown because all recorded events were out¬side the Apollo PSN. Shallow moonquakes are the strongest type of event, with 7 of the 28 events being magnitude 5 or greater. The causes of such quakes are not known. Shallow moonquakes are a potential hazard to a long-term lunar habitat. Applying earthquake models directly to the Moon, estimates of epicentral ground acceleration for a mb 5.7 moonquake are 0.20-0.25 m s^{- 2} (100 km focal depth). Ground motion (amplitude) estimates are ~3 cm at 2 Hz to ~0.75 cm at 8 Hz, ~1 km from the epicenter. However, moonquakes are significantly different from earthquakes: 1) shallow moonquakes contain more energy at high frequencies than earthquakes of comparable total energy; 2) coherent lunar materials have an extremely high seismic Q (4000-7000+) resulting in low damping of seismic energy, resulting in efficient transmission and allowing the maximum signal from a moonquake to last up to 10 minutes and diminishing over several hours. Also there are few data that can be used to determine small strain properties of the lunar regolith; no recordings of moonquakes that were closer than 15° from the site (resulting in large scaling factors), and little attenuation data at smaller distances. Such data are critical in predicting moonquake ground motion. Therefore, applying earthquake-engineering models to the Moon is difficult. A global seismic network (active for a minimum of 6 years) should be established early in the next era of lunar exploration (i.e., before the lunar habitat). It would gather important engineering (and safety) data as well as answer many fundamental scientific questions that still remain almost 35 years after our last visit to the Moon. In tandem with this, a concerted effort is required to produce codes applicable to the Moon to quantify ground motion associated with larger moonquakes.

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