lo2 to the " NATIONAL AERONAUTICS AND SPACE ADMINISTRATION Contract NAS9-12334 -APOLLOPASSIVE SEISMICEXPERIMENT PARTICIPATION 'I NASA-CR-151882) LUNAR SEISMOLOGY: THE 1479-17782 INTERNAL STRUCTURE ORHE LOt Ph. Thesis (Massachusetts Inst. of Thch.) 666 p HC A9NATIF A0L AEOA, CSCD 03f Unclas 1 Januarya-19-12 -o 3.0 September .1978 G3/91' 1351 0 M. Nafi Toksbz- Principal Investigator Department of Earth and Planetar SciencesO Massachusetts Institute of Technology Cambridge, Massachusetts 02139 LUNAR SEISMOLOGY: THE INTERNAL STRUCTURE OF THE MOON by -Neal Rodney Goins Submitted to the Department of-Earth -and Planetary Sciences on May 24,.1978, in parti&l .flfillment of the requirements for the degree of Doctor of Philosophy.. ABSTRACT, A primary goal of-the Apollo missions was the exploration and scientific study of the moon. The nature of the lunar interior is of particular interest for comparison with the earth and in studying comparative planetology. The principal experiment designed to study the lunar interior was the passive seismic experiment (PSE) included as part of the science package on missions 12, 14, 15, and 16. Thus seis­ mologists were provided with a uniqueopportunity ta study the seismicity and seismic characteristics of a second planetary 'bdy and ascertain if analysis methods developed on earth could illuminate the structure of the lunar interior. The lunar seismic data differ from terrestrial data in three major respects. First, the seismic sources are much smaller than on earth, so that no significant information has -been yet obtained for the 4v/ry deep lunar interior. Second, a strong, high Q scattering layer exists on the surface of the moon, resulting in very emergent seismic arrivals, long ringing-codas that obscure secondary (later arriving)-phases., and 'the-destruction of coherent dispersed surface wave trains. Third,'the lunar seismic network consists of only four sta­ tions,- so that after locating-the natural seismic events, only a small amount of data is left for structural analyses. Thus the anaiyss Methods used are-designed to overcome these difficultesane,"i xtract as much information as possible concerning the structure of the lunar interior'. - The direct P and S wave arrival times are the primary data set that.can be measured on.the seismograms of natural lunar seismic events (meteorite imipacts, shallow moonquakes, and deep moonquakes).-, These are inverted using linearized matrix inversion and parameter search,methods to determine event locations,. origin-times, and..various structural -RFIGIAL PAGE IS OF POOR OUALTY parameters simultaneously. Polarization filtering techniques allow the enhancement of secondary body wave arrivals and record section plots are correlated with theoretical travel time curves to identify the secondary-phases and deduce structural information. Finally, shear wave amplitude vs. distance curves yield information on the location and magni­ tude of seismic velocity gradients in the interior. The results of these analyses show that the moon appears to have a two-layer crust at all four seismic stations: a 20 km upper crust that seems to be constant at all sites and a lower crust that is 40 km thick at stations 12 and 14 (mare), 55 ±10 km at station 16 (highland), and tentatively either 40 km or 70 km at station 15. (These values are dependent on vaiious assumptions used in identifying secon­ dary wave arrivals, and so should be regarded with suitable caution.-) Seismic quality factors Qs and Qp are about 5000 and 3000, respectively. Between 400 km and 480 km depth there is a transition zone with a sharply decreasing shear wave velocity and an accompanying possible small decrease in Vp. The dominant velocity drop may occur at a 480 km inter­ face. The lower mantle extends from 480 km to at least 1100 km depth which is the maximum depth of penetration of all but a few seismic waves used as data. The average velocities are Vp = 7.6 km/sec and Vs = 4.2 km/sec, and a small negative gradient may again be present. Attenuation is increased, with Qp ' 1500 and Qs a.1000. Below 1100 km there is tentative indication that the attenuation may increase still further for shear waves, with Qs dropping to a few hundreds' The velocity structure is not known although further velocity decrease is possible, and no definitive evidence for or against a lunar core exists. The above model is the result of analyzing nearly all of the seismic data from the Apollo phase of lunar explora­ tion that is useful in determining interior structure. Thus the structure above 1100 km depth is well-constrained and uncertainties on the above values are given explicitly by the analysis methods. The seismic model of the moon given above therefore serves as a strong constraint on the present­ day lunar compositional and thermal structure and on various models of lunar evolution. OF pooR ACKNOWLEDGEMENTS I owe- a-geAt debt to my advisor, M. Nafi Toks6z, for providing me with the unique opportunity to work on the lunar seismology project, thus combining my research field with an interest in the planets that I have had since childhood. His patience in listening to my complaints about the lunar data and his unfailing ability to point me in the proper direction during crises were indispensible. Finally, I would .. like to express my appreciation for >the-many opportunities that he made available to me to-broaden my, experience in .... research, speaking, -and science.. Dr. Anton -Dainty also devoted many an hour to -answering questions and puzzling with me over the vagaries of the seismic data and the computer center. Without his,clear thinking in the midst of confusion all would have been lost. His -insight and knowledge were invaluable throughout the entire project. During the course of this"wotk several undergraduates made significant contributions. Loren Shure did a prodir gious amount,of Work-iH stacking the moonquake data; Ted Andtrsdncreatedm'zasierpib6es of'data handling programs;. David -Band wrote an.iicredible number"of speciaipirpose . ray trac~ng pr6g'ams','many of'which were quarant6ledto be AeI.last 9ne; Bill Bakerstrugg4ldIwith a recalcitrant miximum entropy spectral analyzer., Dorothy Frank typed the"bulk of the thesis and deserves special credit for the large number of tables she produced. Ann Harlow rescued Chapter 3 from desperate straits and produced a beautifully proof-read final copy. Finally, Sara Brydges monitored the entire process, typed m&ny last minute corrections, numbered most of the figures, stayed late to spare me a-sleepless night, and generally kept me sane and organized in the last few weeks of effort. My .fellow graduate students deserve special credit for making the-entire grad student .experience more worthwhile. than -it-would otherwise have been-, not only in ternms of late-night research discussions) but-also because of.the­ manyfin& spQrtingoppprtunities. - None of this would,hive.been possible without the continuing support and-encouragementprovided by my parents. Their continued devotion in enabling me to obtain an excel­ lent education and their unfailing confidence in my efforts­ have been responsible for the successful completion of my educational-experience. Finally, it is impossible to adequately acknowledge Ms. Sheila Newton for willingly giving up many weekends of free Lime to help in putting together this thesis. Her continuing assistance and encouragement throughout my years in graduate school and her insistence that-there -were other aspects to life are deeply appreciated. This research was supported by NASA Contract NAS9-12334 and Grant NSG-7081. oRIG M­PO& rQFppOR QUA1 TABLE OF CONTENTS PAGE TITLE PAGE 1 ABSTRACT OR15,10NAL PAGE IS 2 ACKNOWLEDGEMENTS O " ~tQUALITY 4 TABLE OF CONTENTS ­ 6 INTRODUCTION 10 0.1 Statement of Problem and Context 10 0.2 Review of Previous Work 15 0.3 Thesis Summary 19 CHAPTER 1: SEISMIC DATA 23 1.1 Seismogram Characteristics 23 1.2 Selection of Events 32 1.3 Arrival Time Measurements 44 Tables 60 Figure Captions 65 Figures 67 -CHAPTER 2: CRUST 81 2.1 Introduction 81 2.2 Natural Event Data. 91 2.3 Results of Natural Event Studies 102 2.4 Implications of the Seismic 110 Results Tables 119 Figure Captions 127 Figures 129 PAGE CHAPTER 3: MANTLE 162 3.1 Introduction 162 3.2 Results of Arrival Time Inversions 177 3.2.1 Surface Events - Upper 178 Mantle .2.2 Moonquakes - Lower Mantle 186 3.2.3 Joint Inversion 192 3.3 Secondary Data Sets 199 3.3.1 Upper-Lower Mantle Interface 199 Reflections 3.3.2 Shear Wave Shadow Zone 206 3.3.3 Amplitude vs. Distance 213 Curves Tables 228 Figure Captions 251 Figures 252 CHAPTER 4: DEEPER STRUCTURE 270 4.1 Attenuating Zone 270 4.2 Core 279 4.3 Secondary Seismic Phases 281 Tables 285 Figure Captions 288 Figures 289 ORIGHdMAL.,PAGE -IS Or F-OOR QUALITY PAGE CHAPTER 5-: IMPLICATIONS AND CONCLUSIONS 29'6 5.1 Other Geophysical Data 296 5.2 Implications of Seismic Results 299 5.3 Conclusions and Suggestions for 307 Future Work Figure Captions 309 - Figures -310 REFERENCES 313 APPENDIX 1: DATA PROCESSING 332 Al.l GeneralConsiderations 332 A1.2 Meteorite Impact Data 337 AI.3 HFT Event Data 347 A1.4 Moonquake Data 356 Tables 383 Figuie Captions 428 Fi rues 429 ° APPENDIX 2: RAY TRACING 582 A2.1 'Travel Times for Direct P and S 582 Waves A2.2 Travel Times and Amplitudes for 593 Reflected, Refracted, and Converted Phases A2.3 Amplitudes of-Direct Waves in 603 Continuously-Varying Velocity Structure Figure Captions 605 Figures 606 PAGE APPENDIX 3: POLARIZATION FILTERING 60-7 A3.1 Theoretical Background 607 A3.2 Application to Lunar Seismograms 615 A3.3 Record Sections 621 Tables 625 APPENDIX 4: INVERSION METHODS 640 A4.1 Parameter Search Method 641 A4.2 Linearized Matrix Inversion 646 Tables 663 Figure Captions 664 Figures 665 BIOGRAPHICAL NOTE .666 10 INtR&ODUTION, - 0.i Statement of Problem and Context Traditionally, seismology and seismic methods have provided th& most detailed and well-constrained information concerning the structure and state of the earth's interior.