Mapping the Moho
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Evelyn Webb conducted research using seismic noise interferometry to map the crust-mantle boundary of the moon. She analyzed seismic data from lunar stations to identify arrival times that could indicate the depth of the lunar Mohorovičić discontinuity. Her analysis found arrival times around 2.5 and 4.5 seconds, suggesting a lunar Moho depth range of 43-58 km, consistent with previous estimates. However, the scattered data indicated that the lunar Moho is a transition zone rather than a distinct boundary like Earth's.
Mapping the Moho
- 1. Presenter: Evelyn Webb Advisors: Dr. Pierre Arroucau Dr. Gordana Vlahovic Dr. Jonathan Bennett MAPPING THE TERRESTRIAL AND LUNAR CRUST-MANTLE BOUNDARY VIA SEISMIC NOISE INTERFEROMETRY
- 2. Research Goal: To confirm and further develop a method of determining the depth of a planetary body’s Mohorovičić Discontinuity using seismic noise rather than a typical signal. Hypothesis: Seismic noise can be auto correlated to produce a signal from which information about the Moho depth can be extracted. OBJECTIVES & MOTIVATION Importance Understanding of Earth Plate Tectonics Non-plate boundary quakes Exploration/ Resource Mapping Applications Low-seismicity areas Extraterrestrial bodies Mars Europa Asteroids
- 3. Seismic Waves P-waves: Pressure waves, travel through entire body S-waves: Shear waves, travel through entire body Surface waves: travel through upper crust (not examined in study) Seismic Noise: Readings taken when an earthquake or other seismic event is not taking place BACKGROUND http://www.exo.net/~pauld/summer_institut e/summer_day10waves/pswaves_lrg.gif
- 4. Mohorovičić discontinuity Reflection boundary between the crust and the mantle Wave Velocities Vp / Vs ratio (velocity of a p wave / velocity of a s wave) increases across the boundary BACKGROUND Umino et. al, 2013 http://www.geocaching.com/geocache/GC2XF1J_coveracks- moho?guid=eaad2ae8-28a5-4596-b7bf-b50e3f9a3f62
- 5. Controversy about structure Researchers unsure of lunar layer structure (see figure) Previous studies Vp/Vs ratio is approximately 1.9(upper crust) to 1.7(lower crust) Pwave velocity is 5.1km/s in the upper crust and 6.8km/s in the lower crust Moho depth estimates for the moon range from 35-65 km of depth, on average Large variation is caused by the difficulty of taking precice measurements on the lunar surface BACKGROUND Nakamura 1983
- 6. Seismographs: 3- axial measurement Study is using Z-axis data Data Sources Incorporated Research Institutions for Seismology Data Management Center (IRIS-DMC) Geoscope DATA RETRIEVAL http://visual.merriam- webster.com/earth/geology/earthquake/seismographs_1.php
- 7. Stations placed during Apollo missions, no longer operative Data taken from Geoscope online public database Lunar Stations S12 S14 S15 S16 DATA RETRIEVAL http://www.iris.edu/dms/nodes/dmc/
- 8. Programs/Languages Used Fortran csh script SAC (Seismic Analysis Code) Autocorrelation of signals Transformation via: AGC (Automatic Gain Correlation) Normalizes resulting autocorrelation Band Pass - Butterworth Filter Isolates signals within frequency range Graphing and Interpretation DATA ANALYSIS http://www.physiome.org/jsim/models/webmodel/NSR/AutoCovar iance/Fig1.png
- 9. Our noise interferometry analysis of Geoscope’s lunar data shows two clear arrival times: one at 2.5s and another at 4.5s. There was also an interesting feature at 17 s, where the smooth data suddenly became scattered. LUNAR DATA RESULTS
- 13. This range is close to previous estimates of 45-65km for lunar Moho depth. The data are scattered with no consistent arrival times where the Moho should be. Therefore we conclude that the lunar Moho is not a distinct boundary as Earth’s, but rather a transition range. LUNAR DATA ANALYSIS
- 14. Lunar Findings The basalt layer beneath station S12 is approximately 6.4 km deep. Moho depth range under that station is approximately 43-58 km. CONCLUSIONS
- 15. Future Research Testing of autocorrelation method on Earth Run more data from lunar stations Attempt method on other planets CONCLUSIONS
- 16. Dr. Pierre Arroucau, North Carolina Central University Dr. Jonathan Bennett, North Carolina School of Science and Mathematics Dr. Gordana Vlahovic, North Carolina Central University Dr. Sarah Shoemaker, North Carolina School of Science and Mathematics NASA/ NSF Summer Research Grant NCCU Summer Research and Science Program ACKNOWLEDGEMENTS
- 17. Tibuleac, Ileana M., and David von Seggern, 2012. Crust- mantle boundary reflectors in Nevada from ambient seismic noise autocorrelations, Geophysics Journal International. Nakamura, YosIo, 1983. Seismic Velocity Structure of the Lunar Mantle, Journal of Geophysical Research Umino, Susumu, Kenneth Nealson, and Bernard Wood, 2013. Drilling to Earth’s Mantle, Physics Today. REFERENCES